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 I/"* QOC1 BUREAU OF MINES 
 
 |^ y^DI INFORMATION CIRCULAR/1990 
 
 Frictional Ignition With Coal 
 Mining Bits 
 
 By Welby G. Courtney 
 
 #+ >W X UiS> BUREAU OF MINES 
 
 (80) 
 
 1910-1990 
 
 \ years g THE MINERALS SOURCE 
 
 ^U OF ^ 
 
Mission: As the Nation's principal conservation 
 agency, the Department of the Interior has respon- 
 sibility for most of our nationally-owned public 
 lands and natural and cultural resources. This 
 includes fostering wise use of our land and water 
 resources, protecting our fish and wildlife, pre- 
 serving the environmental and cultural values of 
 our national parks and historical places, and pro- 
 viding for the enjoyment of life through outdoor 
 recreation. The Department assesses our energy 
 and mineral resources and works to assure that 
 their development is in the best interests of all 
 our people. The Department also promotes the 
 goals of the Take Pride in America campaign by 
 encouraging stewardship and citizen responsibil- 
 ity for the public lands and promoting citizen par- 
 ticipation in their care. The Department also has 
 a major responsibility for American Indian reser- 
 vation communities and for people who live in 
 Island Territories under U.S. Administration. 
 
Information Circular 9251 
 
 Frictional Ignition With Coal 
 Mining Bits 
 
 By Welby G. Courtney 
 
 UNITED STATES DEPARTMENT OF THE INTERIOR 
 Manuel Lujan, Jr., Secretary 
 
 BUREAU OF MINES 
 T S Ary, Director 
 

 > 
 
 q^ 
 
 \ 
 
 TW 
 
 Library of Congress Cataloging in Publication Data: 
 
 Courtney, Welby G. 
 
 Frictional ignition with coal mining bits / by Welby G. Courtney. 
 
 p. cm. — (Information circular / Bureau of Mines; 9251) 
 
 Includes bibliographical references. 
 
 Supt. of Docs, no.: I 28.27:9251. 
 
 1. Coal-cutting bits-Testing. 2. Friction. 3. Methane-Combustion. I. Title. 
 II. Series: Information circular (United States. Bureau of Mines); 9251. 
 
 TN295.U4 [TN813] 622 s--dc20 [622\334] 89-600350 CIP 
 
CONTENTS 
 
 Page 
 
 Abstract 1 
 
 Introduction 2 
 
 Experimental technique 5 
 
 Results 6 
 
 Rectangular bits 6 
 
 Kennametal K-100 6 
 
 Carmet TC3 10 
 
 AMS THRU-FLUSH 12 
 
 Hydra Tools HP74ISR 13 
 
 Conical bits 14 
 
 GTE 14 
 
 Anti-ignition-modified cutter drum for Simmons Rand 265 continuous mining machine 16 
 
 Kennametal K-178DC 17 
 
 Discussion 18 
 
 Conclusions 24 
 
 References 25 
 
 ILLUSTRATIONS 
 
 1. Coal mine explosion 2 
 
 2. Frequency of frictional ignitions in U.S. coal mines 2 
 
 3. Shearer drum that caused frictional ignition of coal mine explosion in Nova Scotia 3 
 
 4. Hot streak formed on surface of sandstone with worn bit 3 
 
 5. Conventional- and mushroom-tipped conical bits 3 
 
 6. Conventional- and dovetail-tipped rectangular bits 4 
 
 7. Geometry of conical bit tip 4 
 
 8. Effect of bit attack angle and initial tip angle on frictional ignition with conical bits 4 
 
 9. Frictional ignition chamber 5 
 
 10. Bit lacing of Joy ILS shearer drum 6 
 
 11. Kennametal K-100 bit 6 
 
 12. Effect of bit wear on frictional ignition with Kennametal K-100 bit 7 
 
 13. Effect of bit velocity on frictional ignition with 0.45-cm-worn Kennametal K-100 bit 8 
 
 14. Anti-ignition back-spray and bit geometry with Kennametal K-100 bit 8 
 
 15. Hot streak cooled by back spray 9 
 
 16. Artificial coal block containing sandstone slab 9 
 
 17. Back sprays with fully laced Joy ILS shearer drum 9 
 
 18. Carmet TC3 bit 11 
 
 19. AMS THRU-FLUSH bit and test bit 12 
 
 20. AMS THRU-FLUSH bit with fan back spray at 100 psig 12 
 
 21. Hydra Tools HP74ISR bit 13 
 
 22. Hydra Tools HP74ISR bit with cone back spray at 100 psig 13 
 
 23. Hydra Tools HP74ISR bit with jet back spray at 100 psig 13 
 
 24. Back spray and conical bit located at end of cutter drum 15 
 
 25. New and 0.5- and 0.75-cm-worn GTE bits 15 
 
 26. Anti-ignition-modified Simmons Rand 265 drum 17 
 
 27. Modified Simmons Rand 265 drum in operation 17 
 
 28. Kennametal K-178DC bit 17 
 
 29. Kennametal K-178DC bit with solid-cone back spray at 150 psig 17 
 
 30. Kennametal K-178DC bit with jet front spray at 150 psig 18 
 
 31. Cutting processes with worn conical bit 20 
 
 32. Spotty hot streak formed on surface of sandstone 21 
 
 33. Theoretical temperatures inside conical bit and on wear-flat surface on conical bit 22 
 
 34. Area of wear flat formed on steel-tipped conical bit versus number of cuts 22 
 
 35. Geometry of linear wear distance with conical bit 23 
 
 36. Linear wear distance versus cutting distance with conical bits 23 
 
 37. Dust cloud formed by bit cutting sandstone 24 
 
TABLES 
 
 Page 
 
 1. Effect of bit wear on frictional ignition with Kennametal K-100 bit during dry cutting 
 
 2. Effect of bit speed on frictional ignition with 0.45-cm-worn Kennametal K-100 bit during dry cutting 
 
 3. Effect of Spraying Systems GG3004 back spray on frictional ignition with worn Kennametal K-100 bit 
 
 4. Effect of Spraying Systems GG3004 back spray on frictional ignition with fully laced drum using 
 
 0.54-cm-worn Kennametal K-100 bit 
 
 5. Ignition results with Carmet TC3 bit and Senior Conflow back spray 
 
 6. Ignition results with AMS THRU-FLUSH bit using fan-type back spray 
 
 7. Ignition results with Hydra Tools HP74ISR bit with back spray 
 
 8. Ignition results with GTE conical bit and Spraying Systems back spray 
 
 9. Ignition results with Kennametal K-178DC bit 
 
 7 
 7 
 9 
 
 10 
 11 
 13 
 14 
 16 
 18 
 
 
 UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT 
 
 °c 
 
 degree Celsius 
 
 
 
 mm 
 
 millimeter 
 
 cal/cm 2 • 
 
 s calorie per square centimeter 
 
 per 
 
 second 
 
 mm 2 
 
 square millimeter 
 
 cm 
 
 centimeter 
 
 
 
 /im 
 
 micrometer 
 
 cm 2 
 
 square centimeter 
 
 
 
 ms 
 
 millisecond 
 
 cm/min 
 
 centimeter per minute 
 
 
 
 pet 
 
 percent 
 
 cm/r 
 
 centimeter per revolution 
 
 
 
 psig 
 
 pound per square inch gauge 
 
 cm/s 
 
 centimeter per second 
 
 
 
 rpm 
 
 revolution per minute 
 
 deg 
 
 degree 
 
 
 
 rps 
 
 revolution per second 
 
 gpm 
 
 gallon per minute 
 
 
 
 s 
 
 second 
 
 gpm/cm 
 
 gallon per minute per square 
 
 centimeter 
 
 
 
FRICTIONAL IGNITION WITH COAL MINING BITS 
 
 By Welby G. Courtney 1 
 
 ABSTRACT 
 
 This publication reviews recent U.S. Bureau of Mines studies of frictional ignition of a methane-air 
 environment by coal mining bits cutting into sandstone and the effectiveness of remedial techniques to 
 reduce the likelihood of frictional ignition. Frictional ignition with a m inin g bit always involves a worn 
 bit having a wear flat on the tip of the bit. The worn bit forms hot spots on the surface of the sandstone 
 because of frictional abrasion. The hot spots then can ignite the methane-air environment. A small 
 wear flat forms a small hot spot, which does not give ignition, while a large wear flat forms a large hot 
 spot, which gives ignition. 
 
 The likelihood of frictional ignition can be somewhat reduced by using a mushroom-shaped tungsten- 
 carbide bit tip on the mining bit and by increasing the bit clearance angle; it can be significantly reduced 
 by using a water spray nozzle in back of each bit, which is carefully oriented to direct the water spray 
 onto the sandstone surface directly behind the bit and thereby cool the hot spots formed by the worn 
 bit. A bit replacement schedule must be used to avoid the formation of a dangerously worn bit. 
 
 Supervisory research chemist, Pittsburgh Research Center, U.S. Bureau of Mines, Pittsburgh, PA. 
 
INTRODUCTION 
 
 The frictional use of quartzitic and pyritic materials to 
 make fire was known to Neanderthalers (I), 2 and frictional 
 ignition has been called "about the second oldest profes- 
 sion" (2). Frictional ignition of a violent coal mine ex- 
 plosion such as shown in figure 1 was reported in 1675 (i). 
 
 Field reports by the Mine Safety and Health Ad- 
 ministration (MSHA) have indicated a general increase in 
 the frequency of frictional ignitions in U.S. coal mines 
 during the past years (fig. 2). While no miners have been 
 killed in the United States because of frictional ignition 
 since the early 1970's, injuries have occurred. The coal 
 mine explosion in Nova Scotia in 1979 that led to 14 fa- 
 talities was caused by a frictional ignition with the shearer 
 drum shown in figure 3 (5). The increase in frictional 
 ignitions in the United States thus is alarming. 
 
 The MSHA field reports indicate that frictional ig- 
 nitions in the United States almost always involve a metal 
 bit cutting into sandstone. 3 Field samples of sandstone 
 involved in frictional ignitions in Great Britain indicated 
 that the sandstone must contain at least 20 pet quartz and 
 usually contains 40 pet or more quartz (5). Early studies 
 of frictional ignition reviewed in 1969 by Powell (6) 
 indicated that frictional ignition with a metal bit cutting 
 sandstone (1) involved a luminous hot streak that formed 
 on the surface of the sandstone because of abrasion by the 
 
 2 Italic numbers in parentheses refer to items in the list of references 
 at the end of this report. 
 
 Very occasionally frictional ignitions have appeared to involve a bit 
 cutting into pyritic material. Also, a few incidents have involved 
 sandstone-sandstone abrasion during the fall of a sandstone roof, a 
 hand pick striking sandstone, and a drill bit striking sandstone or steel. 
 One possible exception involved the cutting of "hard coal" (4). 
 
 bit and did not involve hot, airborne, moving sparks, 4 
 (2) always involved a worn bit that had formed a wear flat 
 on the tip of the bit and was never obtained with a new, 
 sharp bit, (3) could be obtained with almost any abrading 
 material but was, e.g., much easier to obtain with steel 
 than with a tungsten-carbide cermet (9), and (4) was 
 apparently less likely to occur with a lower bit veloc- 
 ity (5, 9-10). Figure 4 shows the hot streak formed on the 
 
 4 Ignition by sparks can occur when the sparks adhere to a surface. 
 However, references 7 and 8, e.g., conclude that ignition can be 
 obtained with airborne, moving sparks. 
 
 Figure 1.-Coal mine explosion. 
 
 I00 
 
 80 
 
 Q 
 
 u 60 
 
 o 
 a. 40 
 
 UJ 
 
 m 
 
 3 
 
 20 
 
 KEY 
 Continuous miner 
 
 Shearer 
 
 Roof bolter 
 
 Other 
 
 A 
 
 L 
 
 / 
 
 y_J£ 
 
 n 
 
 V\ V\ 
 
 '/, 
 
 m 
 
 Z 
 
 2 
 
 n 
 
 2 
 
 V2 
 
 
 
 M 
 
 2 
 
 
 
 V\ 
 
 i i 
 
 I * 
 
 I97I I972 I973 I974 I975 I976 I977 I978 I979 I980 I98I I982 I983 I984 I985 I986 I987 I988 
 Figure 2.-Frequency of frictional ignitions in U.S. coal mines. 
 
Figure 3.-Shearer drum that caused frictional ignition of coal 
 mine explosion in Nova Scotia (3). 
 
 Figure 4.-Hot streak formed on surface of sandstone with 
 worn bit 
 
 surface of a sandstone block by a downward-moving, worn 
 rectangular bit having a wear flat. 
 
 All mining bits presently use tungsten-carbide tips to 
 reduce bit wear, but the carbide tips usually are small. 
 The U.S. Bureau of Mines (11) designed large mushroom- 
 shaped carbide tips for conical bits and dovetail-shaped 
 carbide tips for rectangular (radial) bits to physically 
 protect the steel shank and thereby reduce the likelihood 
 of frictional ignition due to abrasion by the incendive steel 
 shank. Figure 5 shows conventional- and mushroom- 
 tipped conical bits. Figure 6 shows conventional plug- and 
 peak-tipped rectangular bits and a mushroom-type 
 dovetail-tipped bit. Laboratory tests indicated that the 
 mushroom- and dovetail-tipped bits had longer wear lives 
 than the conventional-tipped bits before the incendive steel 
 shank became exposed (12). An in-mine test with conical 
 bits supported these laboratory conclusions (13). 
 
 Bureau tests have indicated, however, that frictional 
 ignition can eventually be obtained with the wear flat 
 formed on the tungsten-carbide tip of a nonrotating 
 (frozen) conical bit without exposure of the steel shank. 
 The likelihood of frictional ignition was significantly 
 reduced with an increased initial bit clearance angle 9 C , 
 
 Conventional tip Mushroom tip 
 
 Figure 5. -Conventional- and mushroom-tipped conical bits. 
 
 e.g., with conical bits, with an increased initial bit attack 
 angle 9 A and/or a decreased internal bit tip angle T as 
 shown in figure 7, where 6 A is the angle between the bit 
 axis and the sandstone surface and 6 C = A - 6 T /2 (14). 
 Figure 8 shows that the number of cuts to give ignition 
 with a new, tungsten-carbide-tipped, frozen conical bit 
 cutting into a sandstone block increased by a factor of 
 
Peak tip 
 
 Dovetail tip 
 
 Figure 6.-Conventlonal- and dovetail-tipped rectangular bits. 
 
 Bit axis 
 
 Bit motion 
 
 • Sandstone- 
 
 Figure 7,-Geometry of conical bit tip. (e A = attack angle; 
 = initial clearance angle; 8j. = initial tip angle.) 
 
 45 50 55 60 65 70 
 BIT ATTACK ANGLE (0J,deg 
 
 75 
 
 Figure 8.-Effect of bit attack angle and initial tip angle on 
 frictional Ignition with conical bits. 
 
 about 3 if 6 A increased by 10° or T decreased by 10°. An 
 in-mine test by Jim Walter Resources (JWR) (15) support- 
 ed these laboratory conclusions. In a section prone to fric- 
 tional ignitions because of a sandstone parting, no ignitions 
 occurred in mining 50,000 tons of coal with mushroom bits 
 having a Bureau-recommended bit attack angle of 57°, 
 while two ignitions then occurred in mining 10,000 tons 
 using conventional-tipped bits having the normal mine at- 
 tack angle of 50°. 
 
 While both of these features (mushroom tip and in- 
 creased bit clearance angle) are valuable in reducing the 
 early ignition hazard associated with conical bits, both fea- 
 tures merely postpone the time when the bits have been 
 worn to a dangerous condition likely to cause frictional 
 ignition. A replacement schedule for worn bits is there- 
 fore required. E.g., the bits in the JWR field test (15) 
 were replaced after every shuttle car was loaded if the 
 mining machine was cutting into a sandstone parting. 
 
 Other laboratory studies (16-17) have indicated that a 
 solid-cone water spray impacting onto the surface of the 
 freshly cut sandstone directly in back of the cutter bit very 
 significantly reduced the likelihood of frictional ignition, 
 even with very worn bits, by promptly cooling the hot 
 streak formed by the worn bit cutting the sandstone. The 
 water requirement to reduce ignition was about 0.5 gpm 
 for each bit and thus was somewhat high but not excessive. 
 
the use of water that impinges onto the front of the bit to 
 reduce airborne respirable dust has become a standard 
 mining practice in Great Britain. However, laboratory 
 tests indicate that it had negligible effect in reducing 
 frictional ignition (18). 
 
 As part of its program to enhance the safety of workers 
 in the mining industry, the Bureau has studied the problem 
 of frictional ignition and especially the use of water sprays 
 in back of the cutter bits to reduce the likelihood of 
 frictional ignition. This report reviews several Bureau- 
 funded studies conducted from 1979 to 1988. A detailed 
 laboratory study with a rectangular bit and several types of 
 water sprays was conducted by Bituminous Coal Research 
 (now, Bituminous Coal Research National Laboratory 
 (BCRNL)) from 1979 to 1981 under Bureau contract (18). 
 
 In-house Bureau studies of a second back spray and 
 rectangular bit system and a back spray and conical bit 
 system were conducted in 1983. and a study of a front 
 spray and conical bit system was conducted in 1987. Brief 
 Bureau studies of several commercial back-spray systems 
 with rectangular and conical bits were conducted in 1983 
 and 1987. Results of these studies have not been 
 previously published. A field test with back sprays and 
 conical bits on a wet-head-modified cutter drum on a 
 Simmons Rand 265 5 continuous mining machine was com- 
 pleted in late 1988. These studies generally had a practical 
 orientation. Recent Bureau work having a more funda- 
 mental orientation is briefly presented in the "Discussion" 
 section. 
 
 EXPERIMENTAL TECHNIQUE 
 
 Previous laboratory techniques to investigate frictional 
 ignition have involved (1) a worn field bit making a single 
 centimeter-deep spiral cut into new locations on a rotating 
 block of sandstone (5), (2) a metal tool (9) or a worn 
 rectangular field bit (10) making multiple shallow cuts at 
 the same location on a stationary sandstone block, and (3) 
 miscellaneous techniques such as a metal tool held against 
 a grinding wheel, drill bit, falling weight, hand pick, and 
 high-speed projectile (6). 
 
 The miscellaneous techniques were used mainly to indi- 
 cate qualitatively whether frictional ignition could occur. 
 Techniques 1 and 2 were used mainly to indicate the 
 effectiveness of a remedial technique in reducing the 
 likelihood of frictional ignition. E.g., in investigating the 
 effect of bit velocity, a worn bit making a deep spiral cut 
 in a sandstone block (5) gave ignition in 3 s with a bit 
 velocity of 150 cm/s and 0.3 s with a bit velocity of 
 450 cm/s. The metal tool making multiple shallow cuts at 
 the same location in a sandstone block (9) gave ignition 
 with about 175 cuts with a tool velocity of 150 cm/s and 
 75 cuts with a tool velocity of 450 cm/s. The worn bit 
 making multiple shallow cuts at the same location (10) 
 gave ignition with about 350 and 240 cuts with bit velocities 
 of 100 and 280 cm/s, respectively. The investigators re- 
 commended that a lower bit velocity be used to reduce 
 the likelihood of frictional ignition. 
 
 However, the relationship between these cutting 
 techniques and field cutting is not clear. It was thought 
 advisable to simulate field cutting insofar as reasonably 
 possible. A new laboratory frictional ignition apparatus 
 was developed by BCRNL (18). A single field bit was 
 mounted onto a field cutter drum rotating downward in 
 the vertical plane at a typical field drum rotation speed. 
 A 51-cm-high block of sandstone containing 78 pet quartz 
 (from Cleveland Quarries, Berea, OH) was positioned on 
 a cart, with the bedding plane horizontal, and was moved 
 horizontally across the rotating drum. A series of slanted 
 downward cuts thus were made in new parts of the sand- 
 stone block. The block was precut to excavate the circular 
 arc in the block and was positioned so that a 0.6-cm-deep 
 
 cut was made by the bit along the entire length of the arc. 
 The block and drum were enclosed in a plywood chamber 
 containing a 7-pct methane-air environment. The chamber 
 included plastic blowout panels to relieve the chamber 
 pressure when ignition occurred. Figure 9 shows the igni- 
 tion chamber. 
 
 The number of cuts made by the bit to ignite the 
 methane-air environment was counted. The number of 
 cuts during 1 pass of the block depended upon the cart 
 speed and drum rotation speed and usually was about 20. 
 If no ignition occurred during passage of the block, the 
 block usually was redressed, repositioned, and again pass- 
 ed across the rotating drum. The total number of cuts 
 to obtain ignition was taken as a measure of the ease of 
 
 s Reference to specific products does not imply endorsement by the 
 U.S. Bureau of Mines. 
 
 Figure 9.-Frlctional ignition chamber. 
 
ignition. The number of cuts to obtain ignition when a 
 remedial technique was used, such as a lesser drum 
 rotation speed or a water spray, then was measured. The 
 increase in the number of cuts required to obtain ignition 
 with the remedial technique was taken as a measure of the 
 effectiveness of the remedial technique. 
 
 The present cutting technique is similar to the tech- 
 nique reported later in reference 19, in which multiple 
 deep cuts with a worn field bit mounted on a field shearer 
 drum rotating at a field speed were made in a sandstone 
 block being moved horizontally below the drum. 
 
 The initial laboratory study described below used a wet- 
 head shearer drum fabricated for a Joy Technologies, Inc., 
 ILS shearer. Subsequent laboratory studies used a seg- 
 ment of the cutter drum from a used Joy 12 ripper-type 
 continuous mining machine, which was modified here to be 
 a wet head by using a small-diameter water seal attached 
 to the end of the drum shaft. The field study used a wet- 
 head cutter drum on a Simmons Rand 265 ripper machine 
 that was designed and fabricated by Simmons Rand and 
 used large-diameter water seals from Cannings Seals in 
 Great Britain. 
 
 RESULTS 
 
 Six laboratory studies with water sprays were conducted, 
 four with different rectangular bits and two with different 
 conical bits. A third study with conical bits and back 
 sprays mounted on the wet-head Simmons Rand 265 con- 
 tinuous mining machine was conducted in the field. 
 
 RECTANGULAR BITS 
 
 Kennametal K-100 
 
 In 1979, the Bureau initiated a contract with BCRNL 
 (18) (1) to conduct a laboratory study of frictional ignition 
 and especially the effectiveness of water sprays in reducing 
 the likelihood of frictional ignition with rectangular cutting 
 bits on a shearer drum and (2) to initiate and monitor a 
 subsequent field study. A 76-cm-wide wet-head cutter 
 drum for a Joy ILS shearer expected to be available for 
 laboratory tests at the Bureau's Bruceton facility was 
 designed by BCRNL and AMS Technology and fabricated 
 by AMS. The drum used 43 bits and a tight lacing pat- 
 tern (fig. 10) designed for difficult cutting conditions. 
 Scheduling difficulties prevented use of the Joy ILS shearer 
 at Bruceton, so the test chamber described earlier was 
 used with the ILS drum. 
 
 Kennametal, Inc., K-100 bits having a 2.5-cm-wide tip 
 were used here. Figure 11 shows the side view of the 
 K-100 bit. The bit had the conventional, peak-shaped, 
 tungsten-carbide and cobalt cermet tip and a shank made 
 of AISI Type 4140 steel. New bits and bits that had been 
 shortened by 0.04 to 0.56 cm and ground flat to simulate 
 a worn field bit were used. Bit wear during a test was 
 negligible. The tip-to-tip diameter was 137 cm. A drum 
 speed of 47 rpm was used in most of this study, giving a 
 tip velocity of 337 cm/s. A cart speed of 41 cm/min was 
 usually used, giving about 60 cuts during 1 pass of the 
 51 cm-wide sandstone block, with one-third of the 2.5-cm- 
 wide bit tip cutting into fresh sandstone and two-thirds 
 of the tip abrading the previously cut portion of the 
 sandstone. 
 
 Initial work used a single bit. The hot streak formed on 
 the surface of the sandstone by a worn K-100 bit cutting 
 
 into the sandstone block was shown in figure 4. Airborne 
 hot sparks were also formed, but motion picture and video 
 
 672 mm 
 
 762 mm 
 
 90 mm 
 
 Figure "lO.-Bit lacing of Joy ILS shearer drum. Circled x's 
 denote mining bits. 
 
 Figure 1 1 .-Kennametal K-100 bit 
 
photography indicated that ignition, with one exception, 6 
 occurred at the bottom part of the hot streak. 
 
 The number of cuts to give frictional ignition depended 
 upon the amount of wear of the bit tip. The average num- 
 ber of cuts to obtain ignition with worn bits is given in 
 table 1 and shown in figure 12. Raw data are given in the 
 footnotes in table 1. Results were rather scattered. Data 
 considered to be probable "outliers" due to cutting into 
 damp sandstone (see section on GTE bits) are enclosed 
 in brackets in table 1 and were ignored in calculating the 
 average number of strikes. Excluding outliers, the results 
 still had considerable scatter; i.e., the coefficients of 
 variation of the sets of data in the footnotes of table 1 and 
 also other sets of data presented later in this report ranged 
 from 25 to 80 pet. 
 
 Table 1 .-Effect of bit wear on frictional ignition with 
 Kennametal K-100 bit during dry cutting 
 
 Bit shortening, Av number of cuts 
 
 cm for ignition 
 
 *>217 
 
 0.08 2 >293 
 
 0.16 3 45 
 
 0.27 4 5.8 
 
 0.32 V.O 
 
 0.40 6 3.3 
 
 0.45 7 5.5 
 
 0.48 8 3.6 
 
 0.54 9 2.2 
 
 x No ignition with 150, 180, 230, 230, 250, 260. 
 
 2 No ignition with 205, 255, 304, 306, 310, 312, 312, 312, 318. 
 2, 3, 9, 14, 17, 28, 40, 50, 69, 79, 82, 91, 100, 
 [143]; no ignition with [207, 328]. 
 
 4 4, 4, 7, 8, [22, 120]. 
 
 5 2, 3, 3, 3, 3, 4, 4, 5, 5, 6, 6, 6, 9, 9, 11, 12, 12, 12, 13, 16, 
 [34, 38, 39, 50, 58, 60, 61]. 
 
 6 2, 2, 4, 5, [14, 42, 57]. 
 
 7 4, 4, 6, 8, [13]. 
 
 8 2, 2, 2, 4, 4, 4, 5, 6, [14, 91]. 
 
 9 1, 1, 1,3,3,4, [17,22]. 
 
 No ignition was obtained with 217 dry cuts with a new 
 bit or 293 cuts with a 0.08-cm-worn bit. Ignition was 
 obtained with a slightly more worn bit, e.g., with an aver- 
 age of 45 dry cuts with a 0.16-cm-worn bit and 5.8 dry cuts 
 with a 0.27-cm-worn bit. The steel shank, which became 
 exposed when the bit was worn 0.14 cm, presumably was 
 the major contributor to ignition. The physical differ- 
 ence between a "safe" 0.08-cm-worn bit and a "dangerous" 
 0.27-cm-worn bit was only barely discernible in the 
 laboratory, and a bit replacement schedule in the field 
 probably could not be based on visual observation of the 
 extent of bit wear or exposure of the steel shank. 
 
 The likelihood of frictional ignition with a worn bit was 
 not decreased with a lower drum rotation speed in the 
 
 ^n one test, ignition occurred below the block and presumably was 
 due to the hot sparks adhering to the floor of the chamber. 
 
 
 JUU 
 
 
 • 1 1 1 
 
 1 1 
 
 
 z 
 
 
 - 
 
 
 
 - 
 
 o 
 
 II 
 
 
 
 
 1- 
 z 
 
 200 
 
 - 
 
 Tungsten j 
 
 Steel shank 
 
 - 
 
 o 
 
 
 
 carbide | 
 
 exposed 
 
 
 DC 
 
 
 
 1 
 
 
 
 £ 
 
 50 
 
 
 1 
 
 
 - 
 
 C/5 
 
 
 
 
 KEY 
 
 
 h- 
 
 40 
 
 
 
 • No ignition 
 
 _ 
 
 O 
 
 
 
 
 o Ignition 
 
 
 Ll_ 
 
 
 
 
 
 
 o 
 
 30 
 
 
 
 
 - 
 
 rr 
 
 
 
 
 
 
 UJ 
 
 
 
 
 
 
 cu 
 
 ?0 
 
 
 
 
 — 
 
 :> 
 
 
 
 
 
 
 -} 
 
 
 
 
 
 
 ?• 
 
 
 
 
 
 
 
 10 
 
 
 
 0"* 1 * 2 — ^_ 
 
 
 
 
 
 1 1 1 
 
 1 ^1 ° 
 
 
 
 0.2 0.3 0.4 
 
 BIT WEAR, cm 
 
 0.5 
 
 0.6 
 
 Figure 12.-Effect of bit wear on frictional ignition with 
 Kennametal K-100 bit 
 
 range considered to be of practical interest. Results ob- 
 tained with a 0.45-cm-worn bit during dry cutting are given 
 in table 2 and plotted in figure 13. Ignition was obtained 
 with about the same number of cuts when the drum was 
 operated with a bit velocity of 108 or 337 cm/s (15 or 
 47 rpm). The likelihood of frictional ignition did rapidly 
 decrease (the number of cuts for ignition rapidly 
 increased) with a still lower drum speed, but such a low 
 drum speed may be impractical for underground mining. 
 
 Table 2.-Effect of bit speed on frictional ignition 
 
 with 0.45-cm-worn Kennametal K-100 
 
 bit during dry cutting 
 
 Drum speed, Bit speed, Number of cuts 
 rpm cm/s for ignition 
 
 47 337 ^.S 
 
 38 272 12 
 
 27 194 4, 6 
 
 21 150 14 
 
 17 122 8 
 
 15 108 7 
 
 13 93 >117 
 
 9 65 >32 
 
 ^e table 1. 
 
 The effect of water sprays from various water spray 
 nozzles mounted in front or in back of a worn bit was 
 investigated. With a water jet impinging onto the front of 
 a 0.32-cm-worn bit, a 0.2-gpm jet was ineffective in 
 preventing ignition; i.e., ignition occurred with nine cuts 
 during wet operation versus seven dry cuts (table 1). A 
 1.1-gpm front jet was more effective, in that ignition 
 occurred with 69 wet cuts versus seven dry cuts. 
 
 A back-mounted spray nozzle whose spray impacted the 
 back of a 0.48-cm-worn bit was not very effective. With 
 
120 
 
 h- 110- 
 
 QC 40 
 O 
 
 O 20 
 
 (T 
 UJ 
 
 2 10 
 
 KEY 
 
 • No ignition 
 o Ignition 
 
 See table 1 
 
 1 
 
 200 300 
 
 BIT VELOCITY, cm/s 
 
 400 
 
 Figure 13.-Effect of bit velocity on frictional ignition with 
 0.45-cm-worn Kennametal K-100 bit 
 
 a water jet, ignition was obtained with 10 wet cuts, while 
 ignition was obtained with an average of 3.6 dry cuts (ta- 
 ble 1). A fan-type spray (Spraying Systems Co. W1502) 
 was effective when the fan spray nozzle was operated at 
 100 psig because the spray splashed off the back of the bit 
 and impacted the sandstone surface within about 5 cm 
 behind the leading edge of the tip of the bit. However, the 
 fan spray was ineffective if the nozzle was operated at 
 140 psig because the splash impact zone then was further 
 behind the bit; i.e., impacting the hot streak with the water 
 spray within about 5 cm behind the bit tip appeared to be 
 required to avoid ignition. A solid-cone spray that im- 
 pinged onto the back of the bit similarly gave only a 
 modest anti-ignition benefit; i.e., ignition occurred with 
 13 wet cuts compared with 3.6 dry cuts. 
 
 A fan spray impinging the freshly cut sandstone surface 
 directly behind the bit, with the fan oriented parallel to 
 and directed onto the bit path, was somewhat effective. 
 E.g., a 0.3-gpm fan spray did not permit ignition with a 
 0.48-cm-worn bit with 120 wet cuts, while ignition was 
 obtained with 3.6 dry cuts. However, ignition was obtained 
 with 90 wet cuts with a 0.80-cm-worn bit. Also, the fan 
 spray was ineffective when oriented perpendicular to the 
 bit path. Since the fan spray appeared to be of only 
 modest benefit with a very worn bit and especially since 
 maintenance of a parallel orientation in the field was 
 expected to be difficult because of nozzle misorientation, 
 no additional work was done with fan sprays. 
 
 A cone spray was selected for further work to avoid the 
 nozzle orientation problem. A solid-cone spray was pre- 
 ferred to a hollow-cone spray in order to give better 
 cooling of the hot streak; i.e., the solid circle of drops at 
 the impaction plane obtained with a solid-cone spray was 
 expected to give better cooling than the annular ring of 
 drops obtained with a hollow-cone spray. Spray nozzles 
 giving a mean number drop size of about 200 /jm, con- 
 sidered to be optimum for impaction coverage of a target 
 surface (20), were chosen. Only Spraying Systems nozzles 
 
 GG3004 
 nozzle 
 
 Figure 14.- Anti-ignition back-spray and bit geometry with 
 Kennametal K-100 bit 
 
 were considered in this study since the manufacturer 
 provided information on mean drop size. The Spraying 
 Systems nozzles considered included the GG3 nozzle, with 
 an internal spray angle of 60° (0.9 gpm at 100 psig) and 
 the GG3004 nozzle, with an internal spray angle of 30° 
 (0.5 gpm at 100 psig). 
 
 The GG3 spray at 70 psig did not give ignition with 
 115 wet cuts with a very worn bit, while ignition occurred 
 with an average of 3.6 dry cuts. Water consumption was 
 0.7 gpm and thus somewhat high. The GG3004 nozzle was 
 selected for detailed work since it involved a lesser water- 
 flow rate and was considered to be more practical for field 
 mining operations. Also, a spray with a 30° internal angle 
 gives a greater water density (e.g., gallon per minute per 
 square centimeter) at the impaction plane on the sand- 
 stone surface than a 60° spray with a similar flow rate and 
 thus should give better cooling of the hot streak. 
 
 The likelihood of frictional ignition with a very worn bit 
 was significantly reduced when the spray from the GG3004 
 nozzle located 13 cm behind the bit tip impinged onto the 
 freshly cut sandstone surface directly in back of the bit. 
 Figure 14 shows the spray-bit geometry used here, with the 
 spray nozzle oriented 50° to the surface of the drum. At 
 the impaction plane, the spray had a diameter of about 
 6.4 cm and an area of 33 cm 2 . Impingement of about 
 10 pet of the spray water onto the back of the bit was 
 purposely selected here in the event that a smaller bit may 
 be used in the field. The spray nozzle was recessed in a 
 steel housing to protect the nozzle from damage by broken 
 coal. 
 
 Laboratory results with a GG3004 nozzle, showing the 
 effect of waterflow rate and pressure on the number of 
 
cuts to give ignition with worn bits, are given in table 3. 
 Ignition did not occur with 100+ wet cuts using 0.4 to 
 0.6 gpm (70 to 110 psig). Figure 15 shows the effect of the 
 back spray on cooling the hot streak and is to be com- 
 pared with figure 4. The hot streak extended about 2 cm 
 behind the bit and thus existed for about 7 ms before be- 
 ing cooled by the back spray. Ignition did occur in four 
 out of six tests (table 3, footnote 4) when the nozzle pres- 
 sure was 40 psig (0.3 gpm), but this low-pressure spray 
 was poorly formed and gave a poor impingement pattern 
 on the sandstone surface. 
 
 Table 3.-Effect of Spraying Systems GG3004 back spray on 
 frictional ignition with worn Kennametal K-1 00 bit 
 
 Bit wear, 
 
 
 Spray 
 
 
 
 Av number of cuts 
 
 cm 
 
 Row rate, gpm 
 
 Pressure, 
 
 psig 
 
 for ignition 
 
 0.40 
 
 Dry 
 
 0.44 
 
 .50 
 
 
 
 Dry 
 70 
 90 
 
 
 '3.3 
 2 >114 
 3 >130 
 
 0.45 
 
 Dry 
 .34 
 .47 
 .56 
 
 
 
 Dry 
 40 
 80 
 
 110 
 
 
 2 5.5 
 4 >65 
 5 >98 
 6 >119 
 
 0.54 
 
 Dry 
 .44 
 
 
 
 Dry 
 70 
 
 
 '2.2 
 7 >120 
 
 1 See table 1. 
 
 2 No ignition with 102, 109, 131. 
 
 3 No ignition with 97, 114, 178. 
 
 4 15, 15, 43, 76; no ignition with 119, 122. 
 
 5 30; no ignition with 119, 121, 121. 
 
 6 No ignition with 115, 117, 120, 120, 124. 
 
 7 No ignition with 108, 119, 121, 121, 121, 121, 121, 123, 126. 
 
 The above tests used a single bit. With a fully laced 
 drum, broken coal from nearby bits may interfere with the 
 spray pattern and reduce the anti-ignition effectiveness of 
 the back spray. This aspect was investigated using the 
 drum fully laced with 43 bits making a sump-type cut into 
 a 152-cm-high, 152-cm-long, 76-cm-wide block of artificial 
 
 coal containing a 51-cm-high, 76-cm-long, 76-cm-wide 
 sandstone slab in the bottom part of the block (fig. 16). A 
 GG3004 spray nozzle was located behind each of the 
 43 bits as shown in figure 14. Figure 17 shows several of 
 the bits with their back sprays. Total water consumption 
 was 22 gpm when the 43 sprays were operated at 100 psig. 
 Bits worn 0.54 cm were used to subject the anti-ignition 
 back sprays to a severe test. The drum and block were 
 again enclosed in a wooden box containing a 7-pct 
 methane-air mixture. The drum was operated at 47 rpm. 
 Depth of cut usually was 0.079 cm/r. The time to give ig- 
 nition was taken as a measure of the ease of ignition. 
 
 Artificial coal 
 
 Sandstone 
 slab 
 
 CJ1 
 
 ro 
 
 o 
 
 3 
 
 Figure 16- Artificial coal block containing sandstone slab. 
 
 ********) 
 
 Figure 15.-Hot streak cooled by back spray. 
 
 Figure 17.-Back sprays with fully laced Joy ILS shearer drum. 
 
10 
 
 No ignition occurred during 50 s of dry cutting into the 
 artificial coal part of the block. Results obtained with the 
 drum cutting into the sandstone part of the block are given 
 in table 4. Raw data are again given in the footnotes, with 
 tests considered to be outliers given in brackets. The 
 17 "dry" tests in brackets in footnote 1 were conducted 
 shortly after a wet test and thus involved cutting into damp 
 sandstone. The bracketed test in footnote 3 involved bits 
 that were severely worn on their sides because of abrasion 
 of the bit on the side of the sandstone slab; this test was 
 considered to be an outlier since the back spray would not 
 impact and cool the hot streak formed on the side of the 
 sandstone slab. 
 
 Table 4.-Effect of Spraying Systems GG3004 back spray on 
 frictional ignition with fully laced drum using 
 0.54-cm-worn Kennametal K-100 bit 
 
 Depth 
 
 Spray 
 
 Av time 
 for igni- 
 
 Nozzles 
 
 of cut, 
 
 Row rate, Pressure, 
 
 plugged 
 
 cm/r 
 
 gpm psig 
 
 tion, s 
 
 
 0.079 . . 
 
 Dry Dry 
 
 > 
 
 
 
 
 0.37 40 
 
 , 2 53 
 
 
 
 
 .41 60 
 
 3 >75 
 
 9 
 
 
 .47 80 
 
 4 >73 
 
 
 
 
 .47 80 
 
 5 >65 
 
 4 
 
 
 .53 100 
 
 6 >63 
 
 
 
 
 .53 100 
 
 7 >69 
 
 9 
 
 
 .67 170 
 
 8 >68 
 
 
 
 
 .82 240 
 
 9 >92 
 
 9 
 
 0.19 ... 
 
 Dry Dry 
 
 10 13 
 
 
 
 
 .53 100 
 
 9 >54 
 
 
 
 0.58 .. . 
 
 .53 100 
 
 9 >74 
 
 
 
 '3, 6, 8, 9, 10, 12, 22, 30, [6, 10, 10, 11, 12, 14, 14, 16, 19, 24, 32, 
 34, 34, 38, 42, 42, 48]. 
 2 38, 68. 
 
 3 [22]; no ignition with 72, 78. 
 4 No ignition with 72, 74. 
 5 No ignition with 50, 80. 
 6 No ignition with 48, 52, 60, 60, 66, 70, 70, 78. 
 7 48; no ignition with 75, 83. 
 8 No ignition with 61, 74. 
 9 1 test. 
 10 12, 14. 
 
 Ignition occurred with an average of 13 s during dry 
 cutting into the sandstone slab. Ignition was not observed 
 in 54+ s during wet cutting if the water pressure was 
 60 psig or higher. Ignition was observed in 53 s when the 
 water pressure was reduced to 40 psig. 
 
 No ignition occurred (>65 s) when four adjacent spray 
 nozzles were plugged, indicating that a back spray provided 
 considerable anti-ignition protection in cooling the hot 
 streaks formed by nearby bits whose spray nozzles became 
 clogged. When nine adjacent spray nozzles were plugged, 
 no ignition occurred in five of six tests (averaging >75 s), 
 again indicating the considerable protection provided by a 
 back spray to nearby bits. 
 
 The depth of cut did not appear to be significant, in 
 that ignition was also obtained in 13 s of dry cutting when 
 the depth of cut was 0.19 cm/r. 
 
 Since 47 rpm corresponds to 0.8 rps, a 13-s test 
 corresponds to 10 cuts by a bit. These multibit results may 
 be compared with the earlier single-bit results, which gave 
 ignition with 2.2 dry cuts, and suggest that frictional 
 
 ignition is less likely with a multibit system. However, the 
 anti-ignition protection provided by back sprays with both 
 systems appears to be very significant. Briefly summarized, 
 the BCRNL results were— 
 
 1. In dry cutting, 
 
 a. No frictional ignition occurred using new or 
 slightly worn bits but ignition readily occurred with a few 
 cuts using slightly more worn bits; 
 
 b. Decreased bit velocity (in the range considered 
 to be of practical interest) had negligible effect on reduc- 
 ing the likelihood of frictional ignition using very worn bits. 
 
 2. In wet cutting with very worn bits, 
 
 a. Frictional ignition readily occurred when the wa- 
 ter spray impinged onto the bit; 
 
 b. The likelihood of ignition was significantly re- 
 duced if the spray from a Spraying Systems GG3004 nozzle 
 impinged onto the freshly cut sandstone surface directly 
 behind the bit when the nozzle was operated at 80 to 
 240 psig (0.47 to 0.82 gpm), but ignition occurred when the 
 nozzle was operated at 40 psig; 
 
 c. The broken coal did not significantly interfere 
 with the anti-ignition effectiveness of the back spray, 
 
 d. No ignition occurred when four adjacent nozzles 
 were plugged, and the likelihood of ignition was signifi- 
 cantly reduced when nine adjacent nozzles were plugged. 
 
 BCRNL recommended that a Spraying System GG3004 
 spray nozzle giving a solid-cone spray be located behind 
 each cutter bit, be carefully oriented so that the spray 
 water impinged onto the freshly cut mineral surface directly 
 behind the bit, and be operated at 80 to 100 psig (0.5 gpm) 
 at the nozzle. 
 
 An early field test with an anti-ignition-modified 
 Joy ILS drum using back sprays that extended 03 cm 
 beyond their steel protective housings failed because of 
 breakage of the spray nozzles. Field tests with the anti- 
 ignition-modified Joy ILS drum using the recessed spray 
 nozzle shown in figure 14 were cancelled because of the 
 unavailability of a field shearer. 
 
 Carmet TC3 
 
 A mining company purchased anti-ignition shearer 
 drums with Carmet (Allegheny Ludlum Industries Co.) 
 TC3 rectangular bits and specified back-mounted Senior 
 Conflow, Inc., 280INC spray nozzles instead of the Spray- 
 ing Systems GG3004 nozzles because of easier nozzle re- 
 placement. The 280INC nozzle gives a solid-cone spray 
 having about the same spray geometry and waterflow rate 
 (0.4 gpm at 100 psig) as the GG3004 nozzle and was ex- 
 pected to provide similar anti-ignition protection. 
 
 However, several frictional ignitions occurred in the 
 field with the anti-ignition-modified drums. The mine op- 
 erator had experienced nozzle clogging and had attempted 
 to avoid clogging by (1) modifying half of the 280INC 
 nozzles so that they delivered a hollow-cone spray instead 
 of the solid-cone spray and (2) increasing the water 
 pressure to 300 psig instead of using the 80- to 100-psig 
 
11 
 
 range recommended by BCRNL. Furthermore, the opera- 
 tor planned to use Senior Conflow 777NC nozzles 
 operated at 500 psig in the future. While the general spray 
 characteristics of the 777NC nozzle are similar to those of 
 the 280INC and GG3004 nozzles and should provide simi- 
 lar anti-ignition protection at 100 psig, the anti-ignition 
 performance of these nozzles at 300 and 500 psig was 
 unknown. 
 
 Numerous in-mine tests have indicated that nozzle 
 clogging due to dirty water entering the mining machine 
 can he easily avoided with a Bureau-designed nonclogging 
 system (21), which involves a Y-type strainer, a hydro- 
 cyclone, and a polishing filter. Rust and scale particles 
 that form in the water channels inside the machine can be 
 eliminated with screens installed in each spray nozzle. 
 Otherwise, regarding modification 1, the anti-ignition 
 performance of a hollow-cone spray had not been 
 investigated but was expected to be less than that provided 
 by a solid-cone spray because of lesser cooling of the hot 
 streak by the annular drop-impaction pattern instead of the 
 solid-circle pattern. Regarding modification 2, a nozzle 
 operated at high pressure tends to give a fine mist instead 
 of the coarse spray obtained when the nozzle is operated 
 at lower pressure. Misting is undesirable from an anti- 
 ignition viewpoint since the small mist drops rapidly lose 
 their momentum and would be less likely to impact the 
 sandstone surface and cool the hot streak. 
 
 Laboratory tests with the Carmet TC3 bit were 
 conducted by the Bureau to investigate the anti-ignition 
 performance of the 280INC and 777NC nozzles, especially 
 when operated at high pressure. Figure 18 shows the 
 1.3-cm-wide Carmet TC3 bit. TC3 bits were shortened by 
 0.32 and 0.43 cm and flattened to simulate field bits that 
 had become worn in order to expose the incendive steel 
 
 Carbide tip- 
 
 shank and subject the back sprays to a severe test. 
 Previous work (with the GTE bit described later) had 
 indicated that ignition with sandstone that had been 
 dampened by a previous wet test was more difficult, i.e., 
 required more cuts for ignition. Therefore, a sequence of 
 only two ignition tests were made per day. The first test 
 was conducted dry to confirm that the sandstone was 
 suitably dry. The second test was conducted wet. Tests 
 were then stopped, and a space heater was installed in the 
 chamber to dry the sandstone block overnight. A new dry- 
 wet sequence of two tests was conducted the next day. 
 
 Results are given in table 5. Frictional ignition with 
 the 0.32-cm-worn TC3 bit was obtained with an average of 
 4.0 dry cuts but was not obtained with about 65 wet cuts 
 using either the 280INC nozzle operated at 100, 300, and 
 500 psig (0.4, 1.1, and 1.1 gpm) or the 777NC nozzle op- 
 erated at 100 and 300 psig (0.4 and 0.8 gpm). However, 
 with the 0.43-cm-worn bit, ignition was obtained with 
 19 and 9 wet cuts when the 777NC nozzle was operated at 
 100 and 300 psig and in 1 of 2 tests performed at 500 psig 
 versus the average of 2.4 dry cuts for ignition. 
 
 Table 5. -Ignition results with Carmet TC3 bit 
 and Senior Conflow back spray 
 
 Bit 
 
 Nozzle 
 
 Spray 
 
 
 Av number 
 
 wear, 
 
 Flow rate, 
 
 Pressure, 
 
 of cuts for 
 
 cm 
 
 
 gpm 
 
 psig 
 
 Ignition 
 
 0.32 . . 
 
 Dry 
 
 Dry 
 
 Dry 
 
 U.o 
 
 
 280INC 
 
 0.4 
 
 100 
 
 2 >66 
 
 
 280INC 
 
 1.1 
 
 300 
 
 3 >65 
 
 
 280INC 
 
 1.1 
 
 500 
 
 4 >63 
 
 
 777NC 
 
 .4 
 
 100 
 
 5 >68 
 
 
 777NC 
 
 .8 
 
 300 
 
 6 >69 
 
 0.43 . . 
 
 Dry 
 
 Dry 
 
 Dry 
 
 7 2.4 
 
 
 777NC 
 
 .4 
 
 100 
 
 8 19 
 
 
 777NC 
 
 .8 
 
 300 
 
 9 9.0 
 
 
 777NC 
 
 1.3 
 
 500 
 
 10 >36 
 
 Figure 18.-Carmet TC3 bit 
 
 *2, 2, 2, 3, 3, 3, 4, 4, 5, 7, 8. 
 No ignition with 65, 68. 
 3 No ignition with 62, 64, 68. 
 4 No ignition with 62, 64. 
 s No ignition with 68, 68, 68. 
 6 No ignition with 68, 70. 
 n Z, 2, 2, 2, 2, 3, 3, 3. 
 8 1 test 
 9 5, 9, 12. 
 
 10 
 
 4; no ignition with 68. 
 
 As with the Kennametal K-100 bit, the physical differ- 
 ence between the "safe" 0.32-cm-worn bit and the "danger- 
 ous" 0.43-cm-worn bit when using back sprays was barely 
 discernible in the laboratory; e.g., the lengths of the ex- 
 posed steel shanks were 1.7 and 2.2 cm, respectively. 
 
 Since the areas of the exposed steel shanks of the worn 
 Carmet TC3 and the Kennametal K-100 bits are com- 
 parable and the worn K-100 bit was easily protected with 
 the Spraying Systems GG3004 nozzle operated at 0.5 gpm 
 and 100 psig, the Senior Conflow 280INC and 777NC 
 sprays as used here appear to be somewhat less protective 
 than the Spraying Systems GG3004 nozzle. 
 
 Operation of either the 280INC or 777NC nozzle at 
 300 psig and especially at 500 psig would appear to offer 
 
12 
 
 no anti-ignition advantage and would definitely be dis- 
 advantageous in terms of high waterflow rate and the usual 
 expense and inconvenience of operating at high water 
 pressure. 
 
 It may be that the several field ignitions with the anti- 
 ignition drums involved nozzle clogging. 
 
 AMS THRU-FLUSH 
 
 Sometimes there may be insufficient physical space on 
 the cutter drum to locate a spray nozzle on the drum 
 directly behind each bit as shown in figure 14. The back- 
 mounted anti-ignition spray nozzles then must be located 
 to one side of the bit, or some other physical change in the 
 nozzle-bit geometry must be considered. 
 
 One approach is to install the spray nozzle directly in 
 the back part of the bit shank. Such a bit is commercially 
 available from AMS Technology and is called a THRU- 
 FLUSH bit. AMS offers either a slot-type spray nozzle, 
 which gives a fan spray whose orientation is fixed and 
 directed onto the bit path, or a solid-cone spray. AMS 
 reported that field tests indicate that the fan spray gives a 
 significant reduction in respirable dust compared with both 
 the AMS cone spray and a conventional boom-mounted 
 water spray system. 
 
 The anti-ignition performance of the fan-spray version 
 of the THRU-FLUSH bit was investigated in the labora- 
 tory. In initial work, the THRU-FLUSH bit was cut down 
 1.3 cm and flattened to expose the steel shank behind the 
 carbide section in the front part of the bit tip but still 
 protect the recessed spray nozzle, thereby simulating a 
 worn field bit. However, reproducibility was poor because 
 the trailing steel section of the flattened tip became worn 
 in an irregular manner. The front carbide section of the 
 tip was cut down another 0.6 cm so that only the flattened 
 steel shank was abrading the sandstone surface. Figure 19 
 shows the 2.5-cm-wide THRU-FLUSH bit on the left and 
 the modified bit used in the present ignition tests on the 
 right. Figure 20 shows the THRU-FLUSH bit with the fan 
 back spray operated at 100 psig. The water density in the 
 spray shown in figure 20 was less near the bit tip than 
 farther behind the tip. This spray inhomogeneity may have 
 been due to a faulty spray nozzle, but this aspect was not 
 examined. 
 
 A 38-cm-wide, 51-cm-high sandstone block was used in 
 these tests. The block was moved at 23 cm/min across 
 the drum being rotated at 39 rpm, giving about 65 cuts 
 during 1 pass of the block. About one-fourth of the bit tip 
 cut into a fresh part of the sandstone block and three- 
 fourths of the tip abraded the previously cut part of the 
 block. 
 
 Results are given in table 6. The outlier in footnote 2 
 occurred when the side of the bit abraded the side of the 
 sandstone block. Frictional ignition was obtained with an 
 average of 3.7 dry cuts and was not obtained with 63+ wet 
 
 Carbide tip 
 
 Water 
 channel 
 
 New bit 
 
 Test bit 
 
 Figure 19.-AMS THRU-FLUSH bit (left) and test bit (right). 
 
 Figure 20 -AMS THRU-FLUSH bit with fan back spray at 100 
 psig. 
 
13 
 
 cuts. The AMS fan spray thus significantly reduced the 
 likelihood of frictional ignition with the THRU-FLUSH 
 bit, although the water consumption of 1 gpm was some- 
 what high. 
 
 Table 6.-lgnition results with AMS THRU-FLUSH 
 bit using fan-type back spray 
 
 The anti-ignition performances of the Hydra Tools 
 spray systems were investigated. The Hydra Tools 
 tungsten-carbide tip was replaced with a steel tip having 
 the same geometry in order to investigate the effectiveness 
 of the back spray under severe conditions. A new steel tip 
 
 
 
 Spray 
 
 
 
 Av 
 
 number of cuts 
 
 Row rate, 
 
 gpm 
 
 
 Pressure, 
 
 psig 
 
 for ignition 
 
 Dry 
 
 
 
 Dry 
 
 
 
 l 3.7 
 
 0.9 
 
 
 
 50 
 
 
 
 2 >64 
 
 1.1 
 
 
 
 100 
 
 
 
 3 >67 
 
 1.1 
 
 
 
 300 
 
 
 
 4 >64 
 
 1.1 
 
 
 
 500 
 
 
 
 S >63 
 
 H, 2, 2, 2, 2, 2, 3, 3, 3, 3, 3, 3, 4, 4, 5, 7, 8, 10. 
 
 2 [64]; no ignition with 64, 64. 
 3 No ignition with 65, 68, 68, 68. 
 4 No ignition with 62, 64, 67. 
 5 No ignition with 62, 64. 
 
 Hydra Tools HP74ISR 
 
 A rectangular bit that incorporates a water spray nozzle 
 mounted on the cutter drum directly behind the bit, with 
 the spray passing through a channel in the bit block and 
 impinging onto the mineral surface in back of the bit, is 
 commercially available from Hydra Tools International, 
 Inc. (HP74ISR). Spray nozzles giving solid-cone and jet 
 sprays are offered. Figure 21 shows the 2.5-cm-wide bit. 
 Figure 22 shows the cone version of the Hydra Tools back 
 spray, and figure 23 shows the jet version, with both spray 
 nozzles operated at 150 psig. 
 
 Figure 22.-Hydra Tools HP74ISR bit with cone back spray at 
 100 psig. 
 
 Carbide tip- 
 
 Figure 21. -Hydra Tools HP74ISR bit 
 
 Figure 23.-Hydra Tools HP74ISR bit with jet back spray at 100 
 psig. 
 
14 
 
 was used in each test. The 51-cm-wide sandstone block 
 was moved at 84 or 127 cm/min across the drum being 
 rotated at 42 rpm, giving 26 or 17 cuts during 1 pass of the 
 block, with 80 and 100 pet of the width of the cuts, respec- 
 tively, made in fresh sandstone. The total number of cuts 
 to obtain ignition with a new steel tip was counted. 
 
 Results (table 7) did not depend upon cart speed. The 
 three bracketed tests in footnote 1 involved cutting into 
 damp sandstone. Ignition was obtained with an average of 
 5.7 dry cuts. No ignition was obtained with the cone or jet 
 spray with an average of 132 or more cuts. The cone 
 spray perhaps could be considered less protective than the 
 jet spray, in view of the two ignitions in three tests at 100 
 psig, but additional replicate tests are required. The jet 
 spray definitely involves considerably greater water 
 consumption. 
 
 Table 7.-lgnition results with Hydra Tools HP74ISR 
 bit with back spray 
 
 Nozzle 
 
 
 Spray 
 
 
 
 Av number of cuts 
 
 
 Row rate, 
 
 gpm Pressure, 
 
 psig 
 
 for ignition 
 
 Dry . . . 
 
 Dry 
 
 
 Dry 
 
 
 '5.7 
 
 Cone . . 
 
 1.3 
 
 
 100 
 
 
 2 >132 
 
 Do ... 
 
 1.6 
 
 
 150 
 
 
 3 >203 
 
 Jet 
 
 3 
 
 
 100 
 
 
 4 >155 
 
 Do ... 
 
 5 
 
 
 150 
 
 
 4 >182 
 
 l 1, 2, 2, 2, 2, 2, 2, 3, 3, 5, 5, 5, 5, 6, 6, 6, 7, 8, 9, 11, 12, 13, 14; 
 no ignition with [16, 17, 18]. 
 2 61, 136; no ignition with 199. 
 
 ..,.._ ignition with 199 
 3 No ignition with 102, 303, 
 4 1 test 
 
 The present results indicate that the Hydra Tools back- 
 spray system is effective in reducing the likelihood of 
 frictional ignition with a worn version of their bit. Of 
 course, ignition with only a few dry cuts with the present 
 steel-tipped bit does not indicate the ignition performance 
 of the commercial Hydra Tools bit with its tungsten- 
 carbide tip. 
 
 CONICAL BITS 
 
 About 75 pet of the frictional ignition incidents in U.S. 
 coal mines (fig. 2) involved ripper-type continuous mining 
 machines. Application of the back-spray concept to a 
 ripper machine involves (1) the design and fabrication of 
 a wet-head ripper drum using durable large-diameter 
 water seals and (2) careful installation of properly 
 engineered water spray nozzles behind each cutter bit. 
 
 In the early 1970's several Bureau-funded field stud- 
 ies (22) investigated the effect of drum-mounted water 
 spray nozzles on respirable dust with wet-head ripper 
 machines, using state-of-the-art water seals. Results 
 indicated that water sprays mounted on the ripper drum in 
 the general vicinity of the cutter bits reduced respirable 
 dust compared with conventional boom-mounted water 
 sprays and did not interfere with visibility or splash onto 
 
 the machine operator. However, seal life was severely 
 limited. A durable, large-diameter water seal was recently 
 developed by Cannings Seals in Great Britain and has 
 been successfully used in mining equipment in Europe and 
 elsewhere for several years. A wet-head cutter drum for 
 a Simmons Rand 265 continuous mining machine using 
 Cannings water seals was designed and fabricated by 
 Simmons Rand under Bureau contract J0395040. 
 
 However, a ripper drum usually uses conical bits with 
 a small bit attack angle while a shearer drum usually uses 
 rectangular bits with a large attack angle. In addition, the 
 bit and bit block of a conical bit may physically interfere 
 with the back spray impacting the mineral surface immedi- 
 ately behind the bit to a greater extent than occurs with a 
 rectangular bit system such as described previously. Also, 
 there may be insufficient physical space on the ripper 
 drum to locate an anti-ignition spray nozzle directly behind 
 each bit, and the back-mounted spray nozzle for a ripper 
 drum may have to be located to one side of the bit and 
 further away from the mineral surface. The design of an 
 anti-ignition back-spray system for a conical bit on a ripper 
 drum thus may be more difficult than for a rectangular 
 (radial) bit on a shearer drum because of spray-bit 
 geometry. 
 
 A laboratory study of the anti-ignition performance of 
 several types of back-spray nozzles with a typical conical 
 bit using the wet-head segment of a Joy 12CM drum is 
 described in the following section. A recent field study of 
 a specific back-located spray nozzle with several types of 
 conical bits using the fully laced Simmons Rand 265 wet- 
 head drum is described later. 
 
 GTE 
 
 A commercial "double-carbide" GTE Corp. conical bit 
 being used in a coal mine in which a future field test was 
 expected to be performed was tested with the wet-head 
 segment of the Joy 12CM drum. The commercial bit 
 block gave a bit attack angle of 45°. The bit and bit block 
 involved considerable interference with the back spray im- 
 pacting the mineral surface immediately behind the bit. 
 This interference was especially severe for a bit mounted 
 on the end of the drum. 
 
 The effect of spray parameters for a single conical bit 
 mounted in the middle or at the end of the drum was in- 
 vestigated. Water spray nozzles were located on the cutter 
 drum in back of and, because of space limitations with the 
 Joy 12CM as laced, to one side of the bit. The spray 
 nozzles were about 10 cm from the tips of the bits. It was 
 expected that a field drum would channel water through a 
 scroll mounted on the outer surface of the drum. The 
 spray nozzles were mounted 2 cm above the surface of the 
 drum to simulate a field drum with scroll. A universal 
 joint was used to facilitate nozzle orientation. Figure 24 
 shows the end bit with its spray nozzle. 
 
15 
 
 Figure 24. -Back spray and conical bit located at end of cutter 
 drum. 
 
 Figure 25.-New and 0.5- and 0.75-cm-worn GTE bits, left to 
 right 
 
 The base of the bit was cut with a slot that fit over a 
 tongue in the bit block to prevent the bit from rotating 
 during cutting. The tip of the bit was shortened by 0.5 or 
 0.75 cm and then cut to have a flat surface to simulate a 
 field bit that had become frozen and had worn a wear flat. 
 The flattened bit purposely exposed part of the steel shank 
 in order to submit the anti-ignition back spray to a severe 
 test. The wear flat was about 2.5 cm wide and 3 cm long. 
 Figure 25 shows a new bit on the left and 0.5- and 
 0.75-cm-worn bits for the end location in the middle and 
 on the right. A newly fabricated flattened bit was "worn 
 in" with 100 cuts to match the flat on the bit to the exca- 
 vated arc in the 51- by 51- by 51-cm block of sandstone. 
 With the typical cart speed of 28 or 71 cm/min and 
 the drum speed of 39 rpm used in the field, about 70 or 
 
 29 cuts were respectively made in the block during 1 pass. 
 With the slower cart speed, about 33 pet of the bit cut 
 into sandstone and 67 pet abraded the previously cut sur- 
 face of the sandstone. With the faster cart speed, about 60 
 pet of the bit cut into sandstone and 40 pet abraded the 
 previously cut surface. 
 
 The sandstone used in these studies readily absorbed 
 water, and frictional ignition with a damp sandstone 
 surface that had been moistened by weather or a previous 
 wet test was more difficult (required more cuts) than with 
 a dry sandstone surface. In four instances involving a 
 sequence of dry-wet-dry tests with a 0.75-cm-worn middle 
 bit, the second "dry" test involved cutting into the 
 dampened sandstone and required an average of 22.5 cuts 
 for ignition instead of the average of 4.7 cuts in the first 
 dry test using sandstone that had been dried overnight. 
 Later tests therefore involved only one wet test per day, 
 with the sandstone block then dried overnight with a space 
 heater. 
 
 The present wet results were very sensitive to the 
 precise orientation of the back spray nozzle, in that 
 changing the aim of the nozzle relative to the bit by only 
 a few degrees significantly decreased the anti-ignition 
 effectiveness of the back spray. This decrease appeared to 
 be due to water impacting the bit and bit block; i.e., such 
 water again seemingly was wasted in terms of anti-ignition 
 performance. The spray nozzles were oriented initially by 
 visually observing the spray impaction pattern on a plastic 
 sheet held to simulate the impaction plane on the 
 sandstone and later by using a template. 
 
 Results are given in table 8 and reported in refer- 
 ence 23. Bracketed tests in the footnotes were considered 
 to be outliers due to cutting into damp stone or to a poor 
 nozzle orientation. Ignition with worn bits again was 
 readily obtained with only a few dry cuts. There was no 
 drastic difference at the middle and end locations; e.g., 
 with a 0.75-cm-worn bit, ignition was obtained with an 
 average of 4.7 dry cuts with the middle bit and 5.0 dry cuts 
 with the end bit. Results did not depend upon cart speed. 
 
 As before, the likelihood of frictional ignition during dry 
 cutting was not appreciably reduced when the drum speed 
 was decreased; i.e., with the 0.75-cm-worn middle bit, 
 ignition was obtained with 5 and 7 dry cuts in 2 tests when 
 the drum was operated at 20 rpm compared with the 
 average of 4.7 cuts when the drum was operated at 
 39 rpm. 
 
 Wet cutting with a carefully oriented back spray again 
 significantly reduced the likelihood of frictional ignition. 
 With 0.5-cm-worn middle and end bits, ignition was ob- 
 tained with an average of 6.9 and 2.8 dry cuts and was not 
 obtained with 60+ and 40+ wet cuts. Similarly, with 
 0.75-cm-worn middle bits, ignition was obtained with 
 4.7 dry cuts and was not obtained with 39+ wet cuts. 
 
 The planned field test was canceled by the coal mine 
 operator. 
 
16 
 
 Table 8.-lgnition results with GTE conical bit and Spraying Systems back spray 
 
 
 
 Spray 
 
 
 Bit wear, 
 cm 
 
 Av number of cuts 
 
 Nozzle 
 
 Flow rate, 
 gpm 
 
 Pressure, 
 psig 
 
 Internal 
 angle, deg 
 
 for ignition 
 
 
 Middle bit End bit 
 
 Dry 
 
 GG3 . . . . 
 3002.5 . . 
 3004 . .. 
 
 TTD3-56 
 TTD2-56 
 
 Dry 
 Dry 
 Dry 
 0.7 
 .8 
 .9 
 .3 
 .3 
 .4 
 .4 
 .5 
 .5 
 .5 
 .5 
 .6 
 .4 
 .5 
 
 Dry 
 
 Dry 
 
 Dry 
 
 60 
 
 80 
 
 100 
 
 60 
 
 80 
 
 110 
 
 60 
 
 80 
 
 80 
 
 110 
 
 100 
 
 150 
 
 100 
 
 150 
 
 Dry 
 Dry 
 Dry 
 60 
 60 
 60 
 35 
 35 
 35 
 30 
 30 
 30 
 30 
 25 
 25 
 20 
 20 
 
 New 
 0.50 
 .75 
 .75 
 .75 
 .75 
 .75 
 .75 
 .75 
 .50 
 .50 
 .75 
 .75 
 .50 
 .50 
 .50 
 .50 
 
 z 6.9 
 4 4.7 
 
 '>69 
 
 8 41.7 
 >>47 
 l >72 
 2 >60 
 ! >39 
 ( >54 
 
 5 > 67.2 
 
 5 > 59.3 
 
 '>47 
 3 2.8 
 5 5.0 
 6 29 
 
 s >42 
 
 5 >24 
 
 °>59 
 } >20.5 
 
 '>24 
 
 16 
 
 19 
 
 14 
 '34 
 
 39.5 
 l >69.5 
 
 *24; no ignition with 70. 
 
 2 3, 4, 5, 5, 6, 7, 10, 11, 11, [17, 23, 23, 27, 44, 59, 72]; no ignition with [50, 72, 76]. 
 
 3 1, 1,2,7. 
 
 4 1, 1, 2, 2, 2, 2, 2, 3, 3, 3, 3, 3, 3, 4, 4, 4, 4, 4, 5, 5, 5, 5, 5, 5, 6, 6, 6, 6, 7, 7, 7, 7, 8, 8, 8, 8, 9, [14, 14, 16, 16, 26, 32, 35]; no ignition with [36]. 
 
 5 1, 1, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 3, 4, 4, 4, 5, 5, 6, 6, 7, 7, 7, 8, 8, 9, 9, 9, 10, 10, 14, [14, 14, 26, 28]; no ignition with [61]. 
 
 6 1 test. 
 
 7 66; no ignition with 69, 72 
 
 8 28, 35, 62. 
 
 9 [7], 29; no ignition with 65. 
 
 10 15; no ignition with 26. 
 
 u No ignition with 70, 72, 72, 73, 73. 
 
 12 27; no ignition with 74, 80. 
 
 13 23, 26, 26; no ignition with 81. 
 
 14 14, no ignition with 24, 33, 58, 74, 75, 75. 
 
 15 [3, 4, 6]; no ignition with 24. 
 
 16 [3], 14. 
 
 [6], 34. 
 
 No ignition with 62, 67, 67, 67, 73. 
 
 [5], 24, 27, 47, 60. 
 ^No ignition with 33, 67, 67, 70. 
 21 No ignition with 69, 70. 
 
 NOTE.-Dashes indicate no tests were performed under these conditions. 
 
 17 
 
 19 
 
 Anti-Ignition-Modified Cutter Drum for 
 
 Simmons Rand 265 Continuous 
 
 Mining Machine 
 
 The bit lacing for the wet-head Simmons Rand 265 rip- 
 per machine was designed by AMS for moderate cutting 
 conditions occurring in a specific mine that had a history 
 of frictional ignitions and was expected to provide a field 
 test site. The drum was laced with a total of 76 AMS 
 conical bits, 72 bits in the center part of the drum and 2 
 bits in the middle part of each end of the drum. The bit 
 blocks gave a bit attack angle of 52°. Sufficient space was 
 available to install water spray nozzles about 18 cm behind 
 the tip of each of the 72 center bits. Figure 26 shows 
 several of the center bits and their spray nozzles. No spray 
 nozzles were used behind the four end bits because of 
 engineering difficulties in channeling water to the end 
 locations. While a channeling technique could un- 
 doubtedly be devised, the present effort was limited by 
 financial restrictions, and conveyance of the water to the 
 end locations was postponed to a future effort if seal 
 durability in the field could be demonstrated. 
 
 The AMS solid-cone spray nozzle (06-900870-000) had 
 a 20° internal angle and delivered a well-formed spray at 
 0.3 gpm and 50 psig. Each nozzle was carefully oriented 
 using a template so that about 75 pet of the spray water 
 impacted the mineral surface directly behind the bit and 
 25 pet of the water impacted the bit and bit block. This 
 spray orientation led to wastage of the water impacted on 
 the bit and block but was selected in view of the sensitivity 
 of the anti-ignition performance to the spray-bit orienta- 
 tion and the possibility that other bits might be used in the 
 field. No laboratory ignition tests were done with this 
 spray-bit system. 
 
 The wet-head-modified Simmons Rand 265 machine 
 was installed in the ignition-prone mine in mid- 1987 in a 
 section having a 30- cm-high sandstone parting. Figure 27 
 shows the modified drum in operation in the mine. Re- 
 sults indicated no water leakage with the Cannings water 
 seals during the mining of 150,000 tons, despite several 
 early shifts of dry operation with no water being passed 
 into the seals. Miscellaneous conical bits were used. No 
 frictional ignitions have occurred, airborne respirable dust 
 was dramatically reduced according to visual observation, 
 
17 
 
 Spray 
 nozzle 
 
 i^ii'iiiiiiifpN. 
 
 Figure 26.-Anti-ignition-modified Simmons Rand 265 drum. 
 
 Scale, cm 
 Figure 28.-Kennametal K-178DC bit 
 
 X 
 
 Figure 27.-Modified Simmons Rand 265 drum in operation 
 
 bit life increased 50 pet, and the wet-head drum was well 
 received by face personnel because of increased visibil- 
 ity (24). The test was completed in mid- 1988 when the 
 mining machine was removed for routine maintenance 
 overhaul. Inspection of the seals indicated that they were 
 still in good condition. 
 
 Kenna metal K-178DC 
 
 A conical bit that incorporates water spray nozzles 
 directly in the bit block behind and/or in front of the bit 
 is commercially available from Kennametal (K-178DC) 
 (fig. 28). The bit attack angle is 55°. Kennametal offers 
 either solid- or hollow-cone or jet spray nozzles. Figure 29 
 shows the bit with the Kennametal solid-cone back spray 
 (Senior Conflow 2801) operated at 150 psig. Figure 30 
 shows the Kennametal jet front spray, also operated at 
 150 psig. 
 
 Figure 29.-Kennametai K-178DC bit with solid-cone back 
 spray at 150 psig. 
 
 The anti-ignition performance of the Kennametal back- 
 and front-located spray systems was investigated. The 
 anti-ignition benefit of a front-located high-pressure jet 
 using a Bureau-designed 0.06-cm-diam nozzle orifice also 
 was tested during other work investigating the enhance- 
 ment of cutting (25). The Kennametal tungsten-carbide 
 tip was replaced with a steel tip having the same geometry 
 in order to subject the Kennametal sprays to severe test 
 conditions. A new steel-tipped bit was used in each test. 
 
18 
 
 Figure 30.-Kennametal K- 178 DC bit with jet front spray at 
 150 psig. 
 
 Results are given in table 9. Ignition was obtained 
 with an average of 4.2 dry cuts and was not obtained with 
 97+ wet cuts with the hollow- or solid-cone spray mounted 
 in back of the bit. With the front-mounted Kennametal 
 and Bureau jet sprays, ignition was obtained in about half 
 of the tests at each of the cited pressures. 
 
 The Kennametal back spray thus significantly reduced 
 the likelihood of frictional ignition. Again, the present re- 
 sults giving ignition with only a few dry cuts with the steel- 
 tipped bit do not indicate the ignition performance of the 
 commercial Kennametal bit with its tungsten-carbide tip. 
 
 Table 9.-lgnition results with Kennametal K-178DC bit 
 
 
 Nozzle 
 
 Spray 
 
 Av number 
 
 Location 
 
 Type 
 
 Row rate, 
 
 Pressure, 
 
 of cuts for 
 
 
 
 gpm 
 
 psig 
 
 ignition 
 
 Dry . . . 
 
 Dry 
 
 Dry 
 
 Dry 
 
 U.2 
 
 Back . . 
 
 Hollow cone 
 
 1 
 
 150 
 
 2 >97 
 
 Do . . . 
 
 Solid cone. . 
 
 1 
 
 100 
 
 3 >138 
 
 Front . . 
 
 Jet 
 
 .9 
 
 150 
 
 4 >18 
 
 Do . . . 
 
 Bureau jet. . 
 
 .75 
 
 2,000 
 
 s >65 
 
 Do . . . 
 
 . . do 
 
 .90 
 
 3,000 
 
 6 26 
 
 Do . . . 
 
 . . do 
 
 1.15 
 
 5,000 
 
 7 >65 
 
 ! 1, 1,3,3,3,4,6,8,9. 
 
 2 No ignition with 28, 107, 117, 137. 
 
 3 1 test. 
 
 4 9; no ignition with 28. 
 
 5 10; no ignition with 121. 
 
 6 1 7, 21,39. 
 
 7 10, 13, 24, 35, 37; no ignition with 103, 122, 179. 
 
 DISCUSSION 
 
 Frictional ignition with the cutter bits on a mining 
 machine can occur when the mining machine is cutting 
 into sandstone (or pyrite). Conversely, if the mining 
 machine is cutting into sandstone, the mine operator 
 probably should expect frictional ignitions. 
 
 Remedial techniques to reduce the likelihood of 
 frictional ignition include 
 
 1. Mushroom-shaped tungsten-carbide bit tips; 
 
 2. Increased bit clearance angle, e.g., with conical bits, 
 increased bit attack angle and/or decreased internal tip 
 angle; and 
 
 3. A carefully oriented water spray nozzle in back of 
 each cutter bit. 
 
 With a shearer drum, the preferred remedial technique 
 is to use drum-mounted back sprays. Fabrication of an 
 anti-ignition shearer drum involves moving the variously 
 located antidust water spray nozzles on the commercially 
 available wet-head shearer drum so that they are located 
 directly behind each of the bits and are carefully oriented 
 so their sprays impact the mineral surface directly behind 
 the bits. Several anti-ignition back spray and bit systems 
 are commercially available (e.g., AMS, Hydra Tools, 
 Kennametal) and have been used in the field. An antidust 
 benefit has been reported with the anti-ignition back spray. 
 
 A wet-head cutter drum for the Simmons Rand 265 
 continuous mining machine that incorporates anti-ignition 
 drum-mounted back-located spray nozzles was developed 
 by Simmons Rand and AMS. However, wet-head cutter 
 drums for other continuous mining machines are not 
 commercially available at present. In the interim, the 
 operator can use the mushroom tip and increased 
 clearance angle features for conical bits presently being 
 used by JWR (15). Mushroom-shaped tips for conical bits 
 are commercially available. 
 
 The mine operator is cautioned that no anti-ignition 
 remedial technique will be a panacea and that a systematic 
 schedule must be used to replace bits before they have 
 worn to a dangerous condition. The JWR schedule of 
 replacing bits after every shuttle car is loaded when 
 sandstone is being cut is inconvenient and expensive but 
 is probably as important in avoiding frictional ignition 
 as the use of mushroom tips and increased bit clearance 
 angle (15). 
 
 Ignition seemingly can be obtained with almost any tip 
 material but appears less likely to occur (requires more 
 cuts) with a harder, more durable material. With conical 
 bits, ignition was typically obtained here with 5 cuts using 
 a medium-hard steel tip and 50 cuts using a hard tungsten- 
 carbide tip. Similar results were obtained with the more 
 massive rectangular bits. Ignition with the Carmet TC3 bit 
 
19 
 
 required about 10 cuts with a steel tip but was not 
 obtained here with 200 cuts with a tungsten-carbide tip. A 
 small, 1-mm 2 wear flat did form on the carbide tip, and 
 ignition presumably would eventually have been obtained 
 with the larger wear flat formed with additional cuts. The 
 formation of a similar small wear flat on a synthetic 
 diamond tip was observed here and also reported by 
 Roepke (26). The wear rates of diamond- tipped and 
 tungsten-carbide-tipped rectangular bits were not mea- 
 sured here because of the low wear rates of the massive 
 tips. However, field tests (27) indicated that diamond- 
 tipped rectangular bits were significantly more durable 
 than comparable tungsten-carbide-tipped bits and thus may 
 be less likely to cause frictional ignition. 
 
 The present observation that a lower bit velocity did not 
 appreciably decrease the likelihood of frictional ignition 
 with a worn bit until a very low velocity was used does not 
 agree with previous studies (5, 9-10). This disparity 
 perhaps is due to differences in bits or test procedures. 
 The present procedure involved a moving bit that inter- 
 mittently made deep cuts into new locations of a sandstone 
 block, while the study reported in reference 5 used a 
 stationary bit making a continuous deep cut into a new 
 location on the moving sandstone block and the studies in 
 references 9 and 10 used moving bits making shallow cuts 
 into the same location on a stationary sandstone block. 
 However, the present results indicate that a lower bit 
 velocity probably is not a reasonable alternative to avoiding 
 frictional ignition with worn bits in a practical mining 
 operation. 
 
 Improved ventilation is often proposed as a remedial 
 technique to reduce the likelihood of frictional ignition. 
 While improved ventilation reduces the severity of the 
 methane explosion that results from a frictional ignition, it 
 probably increases the likelihood of a frictional ignition. 
 Methane is discharged from the face into the entry and 
 mixes with the ventilation airstream in the entry. A 
 boundary layer automatically forms at the face, wherein 
 the local methane concentration is high at the face and 
 decreases away from the face. A combustible methane- 
 air zone therefore must be formed near the face. If the 
 ventilation is increased, the total amount of methane in the 
 boundary layer is decreased but the combustible methane- 
 air zone is moved closer to the ignition source, the hot 
 streak at the face. The net result is that the likelihood of 
 a frictional ignition should increase as the ventilation 
 increases, but the severity of the resulting methane ex- 
 plosion should decrease; i.e., small pops occur instead of 
 a big bang. 
 
 The anti-ignition effectiveness of a back spray depends 
 upon the severity of the hot streak, the spray density at 
 the impaction plane on the sandstone, the cooling effec- 
 tiveness of the spray drops, and the spray-bit geometry. A 
 minimum water density at the spray impaction plane of 
 about 0.015 gpm/cm 2 appears to be required to cool the 
 hot streak formed by the rectangular Kennametal K-100 
 bit during severe test conditions. With the spray and 
 conical bit system used with the GTE bits, a minimum 
 water density of about 0.007 gpm/cm 2 was required for the 
 
 middle bit but 0.015 gpm/cm 2 was required for the end 
 bit. With the AMS spray used on the Simmons Rand 
 265 drum, the water density also was 0.007 gpm/cm 2 . Such 
 water densities can be readily achieved with commercial 
 water spray nozzles using modest flow rates and pressures, 
 e.g., with the GG3004 nozzle operated at 0.5 gpm and 
 100 psig or with the AMS nozzle operated at 0.3 gpm and 
 50 psig. The use of a high waterflow rate of 1 or 2 gpm is 
 commercially popular but does not appear to be required 
 from an anti-ignition viewpoint. Powell and Billinge (17) 
 recently reported that 90-^m drops were twice as effective 
 as 200-/im drops in cooling the hot streak, and a future 
 study of improved water sprays will include spray nozzles 
 selected to give 90-/im drops. 
 
 However, continued bit wear may give a hot streak that 
 is not adequately cooled by this water density; therefore, 
 a bit replacement schedule still should be used with a 
 back-spray technique, but a less frequent bit replacement 
 would probably be required. Also, although all back spray 
 systems tested here successfully reduced the likelihood of 
 frictional ignition by all bits tested here by a very signi- 
 ficant degree, other bit geometries might prevent the hot 
 streaks from being promptly cooled by back sprays and 
 thereby allow frictional ignitions. These aspects have not 
 yet been examined. 
 
 The present observation that frictional ignition is less 
 likely when the sandstone contains water is not surprising 
 and was reported by Titman (28). However, the natural 
 water in the sandstone does not appear significant; i.e., the 
 ignition frequency in the field in the wet summer months 
 was comparable to the frequency in the dry winter months. 
 Infusion of the coal seam with water in order to reduce 
 respirable dust should have an anti-ignition benefit, but the 
 degree of benefit remains to be determined. 
 
 From a more fundamental viewpoint, the ignition of a 
 methane-air mixture by a hot surface depends upon the 
 temperature and area of the hot surface and the exposure 
 time. With an electrically heated surface and a 1-s 
 exposure time, Rae (29) reported that a surface tempera- 
 ture of about 1,400° C was required for ignition with a 
 10-mm 2 surface while 1,250° and 1,200° C were required 
 with 50- and 100-mm 2 surfaces. Similar results were re- 
 ported by other investigators (30-33), but the required sur- 
 face temperature increased as exposure time decreased; 
 e.g., with a 10-ms exposure time, 1,900° and 1,850° C were 
 required with 50- and 100-mm 2 surfaces. 
 
 When a metal bit cuts into a material such as sand- 
 stone, frictional abrasion leads to the formation of a wear 
 flat on the tip of the bit and the generation of heat by the 
 wear flat. Following Osburn (34), cutting involves a 
 sequence of chipping-crushing-abrading processes such as 
 shown in figure 31, where the tip of the wear flat cuts off 
 a chip of the sandstone, and the wear flat, which has a 
 wear angle, then crushes the remaining layer of the sand- 
 stone and also abrades the newly formed sandstone parti- 
 cles against the sandstone substrate. It is generally 
 thought that the surfaces of the wear flat and the sand- 
 stone become heated by abrasion between asperities until 
 the melting temperature of the lower melting material is 
 
20 
 
 -Bit axis 
 
 i,,. Hi-*. i.iijwM.'-I'.-I'ium'.BISvMU-'' ?><~- 
 
 "■■■■■■ ^ ^Particles 
 
 Wear angle 
 
 Figure 31 -Cutting processes with worn conical bit 
 (0 A = attack angle; 8 C = initial clearance angle; 6^ = initial tip 
 angle.) 
 
 reached. A hot smear of the molten material then forms 
 on the surface of the sandstone. Further heat generation 
 due to abrasion is considered to be minor since the contact 
 area is lubricated by the molten material. With a steel tip, 
 the smear is considered to be molten steel at 1,450° C. 
 With a high-melting tip material such as tungsten carbide, 
 the smear is usually considered to involve molten silica 
 (Si0 2 ) at 1,710° C. Such temperatures are sufficiently high 
 to ignite a methane-air mixture if the area and exposure 
 time of the hot smear are adequate. 
 
 Blickensderfer (9) investigated the frictional ignition of 
 a methane-air mixture using a rectangular, 1-cm-wide tool 
 to make a series of 0.005-cm-deep cuts at the same loca- 
 tion in a 10-cm-long sandstone block. Ignition with a steel 
 tool typically required about 75 cuts with a tool velocity of 
 450 cm/s and 175 cuts with a tool velocity of 150 cm/s. A 
 steel smear was left on the sandstone. A temperature of 
 about 1,400° C was measured directly behind the tool for 
 about 2 ms after passage of the tool, which then decreased 
 to 1,200° C in an additional 2 ms. The steel smear was 
 about 20 /im thick, and the top 0.5-/im layer of the smear 
 was oxidized. 
 
 Blickensderfer (35) developed a theoretical model of 
 frictional ignition with a steel tool cutting into sand- 
 stone. He assumed that (1) abrasion between the wear 
 flat on the tool and the sandstone rapidly heated the 
 surface of the wear flat on the steel tool to its melting 
 point of 1,450° C, (2) a smear of molten steel was 
 deposited on the sandstone and was maintained at its 
 
 melting point until the tool passed on and exposed the 
 molten smear, and (3) the exposed molten steel smear 
 then cooled by heat conduction into the sandstone sub- 
 strate. The steel smear theoretically was at 1,450° C for 
 1.7 ms after exposure because of the heat released during 
 solidification of the molten steel smear and 2.7 ms if the 
 heat of oxidation of the 0.5-/xm oxide layer on the steel 
 smear was included. With a tool velocity of 450 cm/s, the 
 length of the molten smear was 12 mm and the area thus 
 was 120 mm 2 . Blickensderfer concluded that such a hot 
 surface (1,450° C for 2.7 ms with a 120-mm 2 area) 
 reasonably agreed with the requirements noted earlier for 
 ignition of a methane-air mixture with electrically heated 
 surfaces. 
 
 Blickensderfer considered that the likelihood of fric- 
 tional ignition decreased as the area of the molten smear 
 decreased. When the tool velocity decreased to 150 cm/s, 
 the theoretical area of the exposed molten smear 
 decreased to 40 mm 2 . He concluded that this decrease in 
 molten area explained the increase in the number of cuts 
 experimentally required to get ignition with the lower tool 
 velocity. 
 
 Cutler (37) similarly measured a streak temperature of 
 about 1,400° C behind a tungsten-carbide-tipped mining bit 
 cutting into sandstone. 
 
 In the present study, the nature of the hot streak 
 formed by a mining bit appears to be complex. Figure 4 
 shows that the streak directly behind the bit initially is very 
 hot: i.e., the film was overexposed for about 20 ms (5 cm) 
 after passage of the bit because of the high local 
 temperature. 7 The streak then cooled to show a series of 
 discrete luminous hot spots about 1 cm or so apart on the 
 surface of the sandstone, although luminous strips about 
 5 cm long occasionally formed. The hot spots usually were 
 about 100 mm 2 in area. Figure 32 gives another example 
 of the series of hot spots formed by the bit, which in this 
 case had passed downward out of the field of view. The 
 hot spots at the bottom and top of figure 32 were about 
 20 and 100 ms old, respectively, and were still hot enough 
 to be luminous but did not give ignitions. 
 
 The BCRNL study (18) indicated that water that im- 
 pacted the sandstone surface further than 5 cm (20 ms) 
 behind the bit was ineffective in preventing frictional igni- 
 tion. Ignition therefore presumably involves the very hot 
 spots in the hot streak within about 5 cm behind the bit, 
 because the spots further than 5 cm behind the bit seem- 
 ingly had cooled to a safe temperature. 
 
 The hot spots formed here during an early nonigniting 
 dry cut visually were similar to the hot spots formed during 
 the next cut that gave ignition. Also, ignition always oc- 
 curred during the bottom part of the igniting cut. Thus, a 
 worn bit readily generates a nonigniting hot spot but then 
 undergoes a small change and generates an igniting hot 
 spot. 
 
 Photometric measurement here of streak temperature was unsuc- 
 cessful in that a streak temperature of about 1,100° ± 300° C was mea- 
 sured behind the bit with both steel and tungsten-carbide tips. 
 
21 
 
 Figure 32.-Spotty hot streak formed on surface of sandstone. 
 
 The heat-generation process (or processes) taking place 
 when a metal bit cuts into sandstone therefore must (1) be 
 intermittent (to generate hot spots) and (2) be sensitive to 
 small changes in the bit (to generate the igniting hot spot). 
 A spotty type of hot streak and the qualitative similarity 
 between nonigniting and igniting hot spots do not appear 
 to have been reported by other investigators. 
 
 Intermittent metal smears were left on the sandstone 
 surface. X-ray analysis of the smears indicated that a tip 
 made of Type 4140 steel left a steel smear while a cobalt- 
 bonded tungsten-carbide tip left a cobalt smear. The 
 metal smear was intermittent and did not appear to match 
 the more regular sequence of luminous hot spots such as 
 shown in figure 32. The formation of a metal smear 
 implies melting of the metal tip. 
 
 The luminous spotty hot streak formed on sandstone by 
 a steel or carbide tip in a nitrogen environment visually 
 was similar to the luminous streak formed in an air 
 environment, and any exothermic oxidation of the molten 
 metal smear appears to be of secondary importance. 8 
 
 Frictional ignition due to sandstone-sandstone abrasion (35-55) is not 
 addressed here. 
 
 Heat generation during the cutting of sandstone with a 
 metal bit presumably involves mechanical friction and 
 depends upon the normal (perpendicular) force exerted by 
 the wear flat of the bit on the sandstone substrate during 
 the cutting process. With the present cutting technique, a 
 tangential force is being applied to the bit by the rotating 
 cutter drum in order to cut the sandstone, but no external 
 normal force is being applied to push the bit against the 
 sandstone target, as in rubbing-friction or drill-bit tests. A 
 local normal force during the cutting operation does occur 
 if a wear angle forms on the tip of the bit as shown in 
 figure 31. 
 
 An intermittent heat-generation process might be 
 the formation of chips, where the partly formed chip 
 (fig. 31) presses the wear flat against the sandstone sub- 
 strate and thereby momentarily generates a high local 
 normal force on the bit. Screen analysis of the particles 
 formed during a 1-cm-deep cut indicated that 30 pet of 
 the mass of the particles were large flakes with a nominal 
 flake diameter greater than about 0.1 cm, indicating that 
 chipping does occur. The normal force exerted by the 
 bit during the present cutting operation has not yet been 
 measured. 
 
 While melting of the steel tip implies that the tempera- 
 ture of the wear flat on the bit during cutting is about 
 1,450° C, the temperature of the surface of the wear flat 
 does not appear to have been directly measured. A tem- 
 perature of about 400° C was measured with a thermo- 
 couple 0.5 mm below the surface of the wear flat on a 
 rectangular bit continuously cutting into sandstone (39). 
 A similar low temperature of about 300° C was measured 
 here with a thermocouple 1.2 mm below the wear flat on 
 a conical bit intermittently cutting sandstone and also was 
 measured with a thermocouple inside a drill bit (40). 
 However, extrapolation of single values to the surface of 
 the wear flat is of course impossible. 
 
 A theoretical model of the temperature-time history of 
 a conical steel bit having a wear flat and intermittently 
 cutting into a sandstone block was developed by Edwards 
 at the Bureau's Pittsburgh Research Center. The model 
 involved (1) a constant rate of heating of the interface 
 between the wear flat on the bit and the sandstone surface 
 during the cutting part of the drum rotation due to fric- 
 tional abrasion combined with cooling by heat conduction 
 into the bit and the sandstone and (2) heat conduction 
 into the bit and convective cooling of the bit during 
 the remainder of the rotation cycle. A sawtoothed 
 temperature-time curve resulted. The model used the bit 
 geometry and drum rotation speed corresponding to 
 the temperature-measurement test noted above and a 
 constant-area wear flat of 0.7 cm 2 obtained in a separate 
 ignition test that gave ignition in the seventh cut with this 
 wear-flat area. The theoretical temperatures 1.2 mm 
 inside the bit and at the surface of the wear flat on the bit 
 depended linearly upon the heating rate. The heating rate 
 at the surface of the wear flat was varied until the 
 theoretical temperature 1.2 mm inside the bit matched 
 the experimental temperature. When a heating rate 
 of 90 cal/cm 2 • s was used, the theoretical internal 
 
22 
 
 
 600 
 
 
 500 
 
 
 400 
 
 
 300 
 
 o 
 
 
 
 
 200 
 
 Ld 
 
 
 cc 
 
 
 1- 
 
 100 
 
 < 
 
 
 or 
 
 
 LJ 
 
 
 
 Q. 
 
 
 ^ 
 
 400 
 
 UJ 
 
 
 - Theoretical 
 
 ~i 1 r 
 
 -| i | 1 1 r 
 
 j I i I I L 
 
 Bit surface 
 
 J I I I I I I L 
 
 300 
 200 
 100 
 
 ^ i ' i ' r 
 Experimental — ^Jt ^ i 
 /\ ' 
 
 Figure 33.-Theoretical temperatures inside conical bit and on 
 wear-flat surface on conical bit 
 
 temperature matched the experimental temperatures of 
 200° C at the end of the 7th cut and 300° C after the 
 20th cut (fig. 33). 
 
 The corresponding theoretical temperatures of the 
 surface of the wear flat were only 400° and 500° C after 
 the 7th and 20th cuts, while the observed melting of the 
 steel wear flat implies that the temperature of the surface 
 of the wear flat was about 1,450° C. However, Ueda (41) 
 recently reported that the temperature inside a grinding 
 wheel abrading a steel target measured with a thermo- 
 couple was considerably lower than the temperature 
 measured with a pyrometric technique, and the validity of 
 thermocouple measurements thus is uncertain. 
 
 The formation of the wear flat would seem to be a 
 critical feature of the ignition process. With a conical bit, 
 when the bit attack angle decreased, ignition required 
 fewer cuts with a new bit (fig. 8) and involved a larger 
 area wear flat on the used bit. 
 
 The wear rate of a steel-tipped conical bit was in- 
 vestigated by measuring the area of the wear flat formed 
 on a new bit in an air environment versus the number of 
 cuts in the sandstone block. The area of the wear flat 
 formed after a few cuts was measured by inking the wear 
 flat, pressing the inked flat against graph paper, and 
 counting the number of inked squares. A few additional 
 cuts were then made, and the area of the wear flat was 
 
 CM 
 
 E 
 
 < 
 
 UJ 
 
 cr 
 < 
 
 cr 
 < 
 
 <!.D 
 
 
 — i — 
 
 1 ' 
 
 1 ' 1 
 D 
 
 ' 1 i 
 
 KEY 
 
 Attack angle (9 A ) 
 
 2.0 
 
 
 
 
 / U 
 
 □,■ 45° 
 
 o 53° 
 
 A,A 70° 
 
 1.5 
 
 
 
 
 
 - 
 
 10 
 
 
 
 /V^ 
 
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 ▲ 
 
 0.5 
 
 
 
 o^ 
 
 
 - 
 
 
 mZ 
 
 1 
 
 A 
 
 1 i 
 
 1 , 1 
 
 1 
 
 20 40 60 80 
 
 NUMBER OF CUTS (N) 
 
 100 
 
 Figure 34.-Area of wear flat formed on steel-tipped conical bit 
 versus number of cuts. (Solid and hollow symbols for the same 
 e A indicate repeat tests.) 
 
 remeasured, and so forth. The effect of bit attack angle 
 8 A was investigated. A constant internal bit tip angle 6 T 
 of 80° was used. 
 
 Figure 34 plots the area of the wear flat a versus the 
 number of cuts N for three attack angles (9 A = 45°, 53°, 
 and 70°). The area increased approximately parabolically 
 with N. The area also increased as 6 A decreased; e.g., 
 after 20 cuts, the area was 0.5 cm 2 if 6 A = 70° and 1 cm 2 
 if 6 A = 45°. Similar results were observed with brass and 
 tungsten-carbide tips. 
 
 The linear wear length £, perpendicular to the sand- 
 stone surface, is related to the wear flat and the bit geo- 
 metry 6 A and 6 T for a conical bit as shown in figure 35. 
 The wear flat is oval in shape because of the slanted wear 
 of the conical tip. Approximating the oval wear flat with 
 a circle and assuming the wear flat is parallel to the sand- 
 stone surface, £ is related to the wear-flat area a by sim- 
 ple geometry, with £ = J a • { sin^yj /[tan (9 T /2) -J*]}. 
 The parabolic a - N relations for various © A 's in figure 34 
 convert to a single parabolic-type £ - N relation for the tip 
 material, which is independent of 6 A . The large wear flat 
 obtained with a small 8 A compared with the small wear 
 flat obtained with a large 6 A thus appears to be a simple 
 geometric effect and does not involve a variation of 
 £ with 6 A . The value of £ does depend upon the nature 
 of the tip material and the total length of cut L. Since the 
 109-cm-diam drum is cutting into a 51-cm-high sandstone 
 block, each cut is 54 cm long and L = 54 -N (in centi- 
 meters). Figure 36 plots £ versus L for brass, steel and 
 carbide tips. The wear rate of a tip is the tangent of an 
 £ - L curve. Initial wear rates were high and depended 
 upon the bit material. The wear rates then slowed and 
 were similar for these various materials. 
 
23 
 
 distance _il*y 
 
 Sandstone 
 
 Figure 35.-Geometry of linear wear distance with conical bit 
 (e A = attack angle; 8j. = initial tip angle.) 
 
 1.0 
 
 i — r 
 
 200 400 600 800 
 TOTAL LENGTH OF CUT (L),cm 
 
 ,000 
 
 Figure 36.-Unear wear distance versus cutting distance with 
 conical bits. 
 
 The rapid early wear of a drag-type drill bit was pre- 
 viously reported (42-43). Neville and Crone (43) similarly 
 observed that the area of the wear flat formed on a drill 
 bit increased if the bit attack angle 6 A decreased and that 
 the linear wear rate of the bit tip was independent of 6 A . 
 They concluded that (1) wear was independent of the ex- 
 ternal normal force (thrust) exerted on the drill bit 
 because the bit is mainly crushing and not rubbing the 
 rock and (2) any temperature effect due to heating of the 
 drill bit was minor. 
 
 The variation in the early wear rates of the present 
 three materials and the similarity of the later wear rates 
 suggest that different aspects of the wear process are 
 important in the early and the later stages of bit wear, 
 which remain to be explained. 
 
 However, the area of a wear flat formed in a methane- 
 air environment during an ignition test was about three 
 times larger than the wear-flat area formed in an air 
 environment with the same number of cuts; i.e., the wear 
 length £ in a combustible methane-air environment was 
 about two times the wear length in an air environment 
 (fig. 36). The simplest explanation for this increase in 
 wear rate in a methane-air environment perhaps is that a 
 hot spot ignites the methane-air mixture to form a micro 
 flame kernel that heats the surface of the bit (and also the 
 sandstone) but is quenched by the bit and sandstone sur- 
 faces and does not ignite the bulk methane-air environ- 
 ment. The hot surface of the wear flat then wears faster 
 than the warm surface of the wear flat because of heat 
 generated only by friction; this process forms a large-area 
 wear flat, which then forms a macro flame kernel that ig- 
 nites the bulk environment. 
 
 A working hypothesis for the mechanism of frictional 
 ignition with a new frozen conical bit cutting sandstone 
 thus might be as follows: 
 
 1. The bit tip initially abrades very rapidly to form a 
 small wear flat. The initial linear wear rate of the tip is 
 slower with a harder (more durable) tip material. 
 
 2. The tip of the wear flat forms chips of the sandstone 
 target, and the wear flat abrades the remaining layer of the 
 sandstone and the resulting sandstone particles. During 
 chip formation, the normal force exerted by the bit on the 
 sandstone target is momentarily very high, leading to the 
 intermittent formation of a small hot spot on the surface 
 of the sandstone. 
 
 3. The small hot spot on the sandstone is about 
 1,450° C and ignites a small flame kernel. The small flame 
 kernel heats the wear flat and increases its wear rate 
 but is quenched by the bit and the sandstone and does 
 not propagate into and ignite the bulk combustible 
 environment. 
 
 4. The area of the wear flat on the bit tip increases 
 because of frictional abrasion. The rate of increase of the 
 wear-flat area depends upon the linear wear rate of the 
 bit tip and the tip geometry. The linear wear rate is 
 increased in a combustible environment because of heating 
 of the tip by the flame kernel. Eventually, a large flame 
 kernel ignited by a large hot spot formed by a large-area 
 wear flat propagates into and ignites the bulk combustible 
 environment. 
 
 A similar mechanism should apply to frictional ignition 
 with a rectangular-shaped bit except that the geometrical 
 relation between the area of the wear flat and the linear 
 
24 
 
 wear of the tip of the rectangular bit will be different from 
 that shown in figure 35. 
 
 Major features requiring clarification include hot-spot 
 formation by chipping, quenching of the flame kernel, and 
 cooling of the hot spots by the back spray. E.g., the bit 
 geometry presumably should have a significant effect on 
 kernel quenching and in "protecting" the hot spot from 
 being cooled by the spray water drops. Detailed investi- 
 gation of these features is planned. 
 
 It should also be noted that an anti-ignition back spray 
 may also achieve an antidust benefit. The dust cloud 
 formed when a conical bit cuts into sandstone is shown in 
 figure 37. With an antidust wet-head cutter drum such as 
 discussed in reference 22, the drum-mounted water spray 
 nozzles usually were located in the general vicinity of the 
 bits and the water drops merely filled the cavity between 
 the surface of the drum and the face. With an anti- 
 ignition wet-head drum, the spray nozzles must be located 
 behind the bit and must be carefully aimed to impact the 
 mineral surface immediately behind the bit. The anti- 
 ignition sprays thus are pointed directly into the dust cloud 
 and should be more likely to capture the dust particles. 
 
 Figure 37.-Dust cloud formed by bit cutting sandstone. 
 
 CONCLUSIONS 
 
 1. In the United States, frictional ignition usually 
 involves a metal bit cutting into sandstone to form a hot 
 streak on the surface of the sandstone. 
 
 2. Frictional ignition always involves a worn bit having 
 a wear flat on the tip of the bit; it seemingly can be ob- 
 tained with almost any bit material but is much more likely 
 to occur with a steel tip than with a tungsten-carbide tip. 
 
 3. The likelihood of frictional ignition decreases with 
 (1) a mushroom-shaped tungsten-carbide bit tip and (2) an 
 increased bit clearance angle. A field test with conical- 
 shaped bits in a coal mine supports these laboratory 
 conclusions. Commercial versions of mushroom-shaped 
 tungsten-carbide-tipped conical bits are available. 
 
 4. The likelihood of ignition is not significantly 
 decreased with a lower bit velocity in the range considered 
 to be of practical interest. 
 
 5. The likelihood of ignition with very worn bits 
 significantly decreases if water spray nozzles are located in 
 back of each bit and are carefully oriented so that the 
 water spray impacts and cools the hot streak on the 
 sandstone surface immediately behind the bit. A mini- 
 mum water density at the impaction plane of about 
 0.007 gpm/cm 2 is apparently required and can readily be 
 achieved with commercial spray nozzles operated at 
 
 moderate conditions (0.5 gpm, 50 to 100 psig). The like- 
 lihood of ignition with all bits tested here was reduced by 
 using a carefully designed back spray. Commercial 
 versions of back spray and bit systems that use a high 
 waterflow rate (1 gpm) are available. 
 
 6. Replacement of worn bits must be scheduled. The 
 frequency of bit replacement will depend upon the reme- 
 dial technique but may range from every shuttle car load 
 (if a mushroom bit and increased bit clearance angle are 
 used) to every week (if a back spray is used). 
 
 7. Formation of the hot streak appears to involve com- 
 plex cutting-abrasion phenomena. Formation of local hot 
 spots may be due to a momentary high normal force on 
 the bit because of chip formation during the cutting pro- 
 cess. The hot spot perhaps forms a micro flame kernel 
 at the wear flat and sandstone interface, which heats the 
 bit but does not propagate into the bulk environment. The 
 heated bit would then wear more rapidly to form a large 
 wear flat, which forms a macro flame kernel that ignites 
 the bulk environment. 
 
 8. An anti-ignition back spray may also achieve an anti- 
 dust benefit because the back spray is aimed directly into 
 the dust cloud formed by the cutter bit. 
 
25 
 
 REFERENCES 
 
 1. Wynn, A. H. A. The Ignition of Firedamp by Friction. Paper in 
 Seventh International Conference of Directors of Safety in Mines 
 Research. SMRE Rep. 42, 1952, 23 pp. 
 
 2. Eisner, H. S. Discussion of paper by F. Powell and K. Billinge, 
 " The Frictional Ignition Hazard Associated With Colliery Rocks." Min. 
 Eng. (London), v. 134, July 1975, pp. 527-533. 
 
 3. Elfstrom, R H. Explosion in No. 26 Colliery, Glace Bay, 
 Nova Scotia, on February 24, 1979. Can. Dep. Labour, Apr. 1980, 
 122 pp. 
 
 4. Hartmann, I. Frictional Ignition of Gas by Mining Machines. 
 BuMines IC 7727, 1955, 17 pp. 
 
 5. Powell, F., and K. Billinge. Ignition of Firedamp by Friction 
 During Rock Cutting. Min. Eng. (London), v. 134, May 1975, 
 pp. 419-426. 
 
 6. Powell, F. Ignition of Gases and Vapors. Ind. and Eng. Chem., 
 v. 61, No. 12, 1969, pp. 29-37. 
 
 7. Lobejko, A. The Ignition of Methane-Air Mixtures by Frictional 
 Sparks. Health and Saf. Exec. Transl. Serv. Harpur Hill, Bunton, 
 Great Britain, Transl. 8999, May 1980, 8 pp. 
 
 8. Bartknecht, W. Preventive and Design Measures for Protection 
 Against Dust Explosions. Paper in Industrial Dust Explosions, ed. 
 by K. L. Cashdollar and M. Hertzberg. ASTM STP 958, 1987, 
 pp. 158-190. 
 
 9. Blickensderfer, R, D. K. Deardorff, and J. E. Kelley. Incendivity 
 of Some Coal-Cutter Materials by Impact-Abrasion in Air-Methane. 
 BuMines RI 7930, 1974, 20 pp. 
 
 10. Larson, D. A, V. W. Dellorfano, C. F. Wingquist, and 
 W. W. Roepke. Preliminary Evaluation of Bit Impact Ignitions of 
 Methane Using a Drum-Type Cutting Head. BuMines RI 8755, 1983, 
 23 pp. 
 
 11. Liebman, I. and L. Cheng. Anti-Incendive Coal Cutting Bits. 
 Pat. Appl. 319, 862, Nov. 1981. 
 
 12. Cheng, L., I. Liebman, A. L. Furno, and R W. Watson. Novel 
 Coal-Cutting Bits and Their Wear Resistances. BuMines RI 8791, 1983, 
 15 pp. 
 
 13. Cheng. L. Design of Anti-Incendive Conical Cutting Bits. Pres. 
 at SME-AIME Annu. Meet., Las Vegas, NV, Feb. 27-Mar. 2, 1989. 
 Trans. Soc. Min. Eng. AIME, in press. 
 
 14. Cheng, L., A. L. Furno, and W. G. Courtney. Reduction in 
 Frictional Ignition Due to Conical Coal-Cutting Bits. BuMines RI 9134, 
 1987, 10 pp. 
 
 15. NcNider, T., E. Grygiel, and J. Haynes. Reducing Frictional 
 Ignitions and Improving Bit Life Through Novel Pick and Drum 
 Designs. Paper in Proceedings of the Third Mine Ventilation 
 Symposium. Soc. Mining Eng. AIME, Littleton, CO, 1987, pp. 119-125. 
 
 16. Kocherga, N. G. Prevention of Methane Ignitions by the Use of 
 Water Spraying During Operation of the Cutting Tools of Mining 
 Machines. Health and Saf. Execu., Transl. Serv., Harpur Hill, Bunton, 
 Great Britain. Transl. 7507, Nov. 1977, 7 pp. 
 
 17. Powell, F, and K. Billinge. The Use of Water in the Prevention 
 of Ignitions Caused by Machine Picks. Min. Eng. (London), v. 141, 
 Aug. 1981, pp. 81-85. 
 
 18. Agbede, R O, K. L. Whitehead, R L. Mundell, and R D. 
 Saltsman. Frictional Ignition Suppression by the Use of Cutter-Drum 
 Mounted Water Sprays. BuMines contract J0395040, Bituminous Coal 
 Res., 1982; for inf., contact W. G. Courtney, BuMines, Pittsburgh, PA. 
 
 19. Lewis, W. T, P. R Smith, and F. Powell. Circumstances 
 Generating Incendive Sources in Rock Cutting. Paper Dl in 20th 
 International Conference of Safety in Mines Research Institutes. 
 SMRE, 1983, 11 pp. 
 
 20. Cheng, L. Dynamic Spreading of Drops Impacting onto a Solid 
 Surface. Ind. & Eng. Chem. Process Des. and Dev., v. 16, No. 2, 1977, 
 pp. 192-197. 
 
 21. Divers, E. F. Nonclogging Water Spray System for Continuous- 
 Mining Machines. BuMines IC 8727, 1976, 10 pp. 
 
 22. Strebig, K. C. Wet-Head Tests on Miners Concluded. Coal Min. 
 & Process., v. 12, Apr. 1975, pp. 78-88. 
 
 23. Courtney, W. G. Prevention of Frictional Ignition With Ripper- 
 Type Continuous Mining Machines Using Water Sprays. Paper in 
 Proceedings of the 3rd Mine Ventilation Symposium. Soc. Min. Eng. 
 AIME, Littleton, CO, 1987, pp. 126-131. 
 
 24. Merritt, P. C. The Wet-Head Miner Comes Back. Coal Age, 
 v. 92, Nov. 1987, pp. 44-46. 
 
 25. Taylor, C. D., and A. L. Furno. Evaluation of High-Pressure 
 Front-Mounted Water Jets for Frictional Ignition Suppression. 
 BuMines RI 9237, 1989, 8 pp. 
 
 26. Roepke, W. W., B. D. Hanson, and C. E. Longfellow. Drag Bit 
 Cutting Characteristics Using Sintered Diamond Inserts. BuMines 
 RI 8802, 1983, 30 pp. 
 
 27. Collin, W. D., and J. A. Kornecki. The Development and Use 
 of Diamond Picks for Longwall Shearers at Secunda Collieries. Paper 
 in Mining 85 Conference, Mining Productivity Through Reliability and 
 Control. Inst. Min. Eng., Doncaster, England, v. 1, 1985, pp. 153-163. 
 
 28. Titman, H. A Review of Experiments on the Ignition of 
 Inflammable Gases by Frictional Sparking. Trans. Inst. Min. Eng. 
 v. 115, 1955-56, pp. 535-557. 
 
 29. Rae, D., B. Singh, and R Danson. The Size and Temperature of 
 a Hot Square in a Cold Plane Surface Necessary for the Ignition of 
 Methane. SMRE Res. Rep. 224, 1964, 35 pp. 
 
 30. Cutler, D. P. The Ignition of Gases by Rapidly Heated Surfaces. 
 Combust, and Flame, v. 22, 1974, pp. 105-109. 
 
 31. . Further Studies of the Ignition of Gases by Transiently 
 
 Heated Surfaces. Combust, and Flame, v. 33, 1978, pp. 85-91. 
 
 32. Laurendeau, N. M. Thermal Ignition of Methane-Air Mixtures 
 by Hot Surfaces: A Critical Examination. Combust, and Flame, v. 46, 
 1982, pp. 29^9. 
 
 33. Laurendeau, N. M., and R N. Caron. Influence of Hot Surface 
 Size on Methane-Air Ignition Temperature. Combust, and Flame, v. 46, 
 1982, pp. 213-218. 
 
 34. Osburn, H. J. Wear of Rock-Cutting Tools. Powder Metall., 
 v. 12, 1969, pp. 471-502. 
 
 35. Blickensderfer, R Methane Ignition by Frictional Impact Heat- 
 ing. Combust, and Flame, v. 25, 1975, pp. 143-152. 
 
 36. Burgess, M. J., and R V. Wheeler. The Ignition of Firedamp by 
 the Heat of Impact of Rocks. SMRE Rep. 46, 1928, 29 pp. 
 
 37. Nagy, J., and E. M. Kawenski. Frictional Ignition of Gas During 
 a Roof Fall. BuMines RI 5548, 1960, 11 pp. 
 
 38. Rae, D. The Role of Quartz in the Ignition of Methane by the 
 Friction of Rocks. SMRE Res. Rep. 223, 1964, 50 pp. 
 
 39. Wingquist, C. F, and B. D. Hanson. Bit Wear-Flat Temperature 
 as a Function of Depth of Cut and Speed. BuMines RI 9112, 1987, 
 15 pp. 
 
 40. Whitbread, J. E. Bit Temperatures in Rotary Drilling. Colliery 
 Eng., (London), v. 37, Jan. 1960, pp. 25-59. 
 
 41. Ueda, T., A. Hosokawa, and A. Yamamoto. Measurement of 
 Grinding Temperature Using Infrared Radiation Pyrometer With 
 Optical Fiber. J. Eng. Ind., v. 108, 1986, pp. 247-251. 
 
 42. Fish, B. F, G. A. Guppy, and J. T. Ruben. Abrasive Wear 
 Effects in Rotary Rock Drilling. Bull. Inst. Min. and Metall., No. 630, 
 1959, pp. 357-383. 
 
 43. Neville, H. F. C, and J. G. D. Crone. Wear of Rotary Drag Drill 
 Bits in Granitic Rock. Trans. Inst. Min. and Metall., v. 71, 1961-62, 
 pp. 249-263. 
 
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