A control suite to reduce roof bolting machine drilling noise 

M. Jenae Lowea) 

a) Email address: jli7@cdc.gov 

J. Shawn Petersonb) 

b) Email address: jnp3@cdc.gov 

David S. Yantekc) 

c) Email address: dcy6@cdc.gov 

Lynn A. Alcornd) 

d) Email address: lyn3@cdc.gov 

National Institute for Occupational Safety and Health 

Office of Mine Safety and Health Research 

626 Cochran’s Mill Road 

Pittsburgh, Pennsylvania 15236 

Robert Michaele) 

e) Email address: bmichael@corryrubber.com 

Penn State Erie — The Behrend College, Corry Rubber Corporation 

601 West Main Street 

Corry, Pennsylvania 16407 

Disclaimer: The findings and conclusions in this report are those of the author(s) and do not necessarily represent 

the views of the National Institute for Occupational Safety and Health. Reference to specific brand names does not 

imply endorsement by the National Institute for Occupational Safety and Health. 

Among underground coal miners, hearing loss remains one of the most common 

occupational illnesses. In response, the National Institute for Occupational Safety and 

Health (NIOSH) conducts research to reduce the noise emission of various underground 

coal-mining equipment, an example of which is a roof bolting machine. Field studies 

support the premise that, on average, drilling noise is the loudest noise that a roof bolting 

machine operator would be exposed to and contributes significantly to the operators’ noise 

exposure. NIOSH has determined that the drill steel radiates a significant amount of noise 

during drilling. NIOSH, in collaboration with Corry Rubber Corporation and 

Kennametal, Inc., has developed a suite of controls to reduce drilling noise which consists 

of a bit isolator, chuck isolator, and a collapsible drill steel enclosure. This control suite 

effectively reduces the noise radiated by the drill steel. Laboratory testing confirms that 

the control suite reduces sound pressure levels generated during drilling by 13 dB(A) at the 

operator's location. As a result of this reduction in drilling noise, the noise exposure of the 

roof bolting machine operator is significantly reduced. 
1 INTRODUCTION 

Hearing loss is one of the most common occupational illnesses in the United States1

. As 

such, hearing loss prevention is one of twenty-one Priority Research Areas listed in the National 

Institute for Occupational Safety and Health (NIOSH) National Occupational Research Agenda2. 

In the mining industry, hearing loss is an even more serious issue— it is 2.5-3 times greater than 

what is expected for the average of the population that is not exposed to occupational noise. 

Additionally, the same NIOSH studies have shown that by the age of 50, 90% of coal miners 

have a hearing impairment versus only 10% of the population not exposed to occupational 

noise3. Department of Labor Mine Safety and Health Administration (MSHA) noise sample data 

collected from 2000 to 2005 show that only seven types of machines compose the bulk of the 

equipment whose operators exceed 100% noise dosage per the MSHA Permissible Exposure 

Level (PEL). Of these machines, the roof bolting machine (RBM) operator was the second-most 

likely to be over-exposed among operators of all equipment used in underground coal mining4. 

A roof bolting machine is a large, electrohydraulic machine used to temporarily stabilize the 

mine roof while installing roof bolts to support it after coal has been extracted. To install the 

bolts, the roof bolter drills holes into the mine roof using 25 or 35 mm diameter drill bits 

attached to drill steels with either hexagonal or round cross-sections (Fig. 1). The lengths of the 

drill steels normally vary from approximately 0.3 meters to 1.5 meters, or longer combinations 

thereof, depending on the mine and its roof conditions. After a hole is drilled, a roof bolt is 

inserted into the hole to secure the overlying strata, thereby supporting the roof. To spread the 

load across the roof, roof bolts are inserted through a steel plate prior to inserting the bolt into the 

roof. Fig. 2 shows a roof bolting machine in an underground coal mine. 

Prior underground time-motion studies confirmed that RBM operators are exposed to the 

highest noise levels when drilling as opposed to bolting, tramming, and other tasks associated 

with their typical work day5. Therefore, an essential part of developing noise controls to reduce 

the occurrences of Noise Induced Hearing Loss (NIHL) for RBM operators is to find ways to 

reduce drilling noise. Prior research has shown that much of the noise from percussive rock 

drilling originates from bending vibrations in the drill rod6-7 . NIOSH measurements coupled 

with beamforming analysis on the rotary drilling system used on RBMs found that the highest 

sources of drill steel noise were near the chuck and the drill bit8

. 

Therefore, in order to meet our overall objective to reduce an operator’s noise exposure to a 

time-weighted average (TWA) of 90 dB(A) or less for an eight hour shift per the MSHA PEL, 

NIOSH researchers developed a suite of controls to target drilling noise along the drill steel. 

These include bit isolators, chuck isolators, and a collapsible drill steel enclosure (CDSE). 

2 TEST SETUP 

All testing was conducted outdoors in free field conditions on a Fletcher model HDDR 

RBM drilling into granite. NIOSH used granite as the drill media to represent a high 

compressive strength roof and because of its homogeneity, which helps ensure test repeatability. 

A large steel stand was used to support the granite drill media, which was isolated from the stand 

by sheets of urethane. Additional design considerations to reduce the radiation of sound energy 

included filling several members of the support stand with sand9. 

Sound pressure levels (SPL) were measured using a Brüel & Kjær (B&K) PULSE data 

acquisition system and a B&K Type 4188 microphone, positioned at the operator’s right ear 

location. Ambient noise levels were insignificant since they were over 10 dB below all SPLs 

measured with the roof bolter running. The roof bolter was run at a thrust of 12.6 kN and set at 
an average rotation speed of 230 RPM. These are the typical settings we use for drilling harder 

materials such as granite9, since the higher rotation speeds and thrust settings used for coal would 

quickly dull the bits for this media. Drilling was conducted with 1.2 m (4 ft) long hex drill steels 

and 35 mm (1 3/8”) bits. A new bit was used for each hole to ensure measurement repeatability 

and data were taken for 30 seconds of drilling. Each configuration was measured three times and 

the results were averaged. The bit and chuck isolators were tested separately and in combination 

with each other and the CDSE to determine the improvement over the baseline configuration. 

3 BIT AND CHUCK ISOLATORS 

3.1 Bit and Chuck Isolator Design 

The NIOSH bit and chuck isolators were developed in collaboration with Corry Rubber 

Corporation and Kennametal, Inc. to reduce the noise sources found near the drill bit and chuck 

in previous studies8. They consist of inner and outer steel members machined out of 4130/4140 

steel and heat treated to 350 BHN that are isolated from each other by a layer of 75 durometer 

Shore A natural rubber (Figs. 3 and 4). This durometer was chosen because the fatigue life of 

the rubber was a concern. The 75 durometer elastomer is bonded to the inner steel member, precompressed, and assembled into the outer steel member. A post-vulcanization (PV) bond 

attaches the elastomer’s outer surface to the inner surface of the outer steel member. This type of 

“pre-compressed bonded joint design” has the advantage of improved durability since the precompression of the rubber ensures that the elastomer never goes into tension during operation. 

The bit and chuck isolators are similar in design, except for having slightly different 

geometry in their steel components to connect to the chuck, drill steel, and bit. The bit isolator 

has one end machined to mate with a standard 35 mm (1 3/8”) drill bit and the other is machined 

the same as a standard drill steel coupling such that a bit isolator can fit on top of the drill steel. 

For the chuck isolator, one end is machined to the same dimensions as the bottom of a hex drill 

steel in order to fit into the chuck, and the other is the same as a standard drill steel coupling so a 

drill steel will fit on top of it. 

An 11.4 mm gap exists between the inner steel component where it begins to neck down and 

the edge of the outer steel member. This is to protect the elastomer from overload since the outer 

sleeve will bottom out on the inner member shoulder at about 45 kN, thus allowing loads above 

this threshold to be reacted by the steel components. 

Design requirements for the 35 mm isolator are summarized as follows: 

1. Max torque: 410 N-m 

2. Max axial load (thrust load): 35 to 45 kN 

3. Isolator metals will bottom out at 11.4 mm deflection or approximately 45 kN thrust load 

4. Preliminary life requirement: 2 weeks to 1 month of continuous operation 

5. Estimated static axial stiffness (for noise attenuation): 2,600 kN/m to 5,200 kN/m 

6. Estimated torsional static stiffness (for noise attenuation): 6.8 to 20.3 N-m/deg. 

All of the above requirements have been met for the bit and chuck isolators. Maximum torque 

and axial loads were determined from the maximum torque and thrust outputs a typical roof 

bolting machine can provide. The life requirement was chosen to be at least as long as that of an 
average drill steel. The stiffness requirements were developed from testing the material 

properties of earlier prototype bit and chuck isolators that were successful at reducing noise8. 

3.2 Discussion of Bit and Chuck Isolator Results 

As shown in Fig. 5, the bit isolator is effective at reducing noise above 1.25 kHz, but 

increases A-weighted SPLs in the 800 Hz – 1.25 k Hz 1⁄3-octave bands. This yields an overall 

reduction in A-weighted SPL at the operator’s ear of 4 dB. There is little to no reduction in the 

low frequencies because the RBM noise from the electric motor, hydraulic system, and vacuum 

dominates the noise in this range, making low frequency noise reductions difficult to achieve. 

Previous studies have shown that noise from these systems is prominent at the 1 kHz octave band 

and below10 . 

As seen in Fig. 6, the chuck isolator causes slight improvements over the entire frequency 

range, again with the exception of the 800 Hz – 1.25 kH z 1⁄3-octave bands. Overall it yields a 2 

dB improvement from baseline in A-weighted SPL at the operator’s ear. While not as effective 

at reducing noise as the bit isolator, the fact that it reduces some lower frequency noise suggests 

that it could be used in combination with the bit isolator to gain even greater SPL reductions 

from the baseline configuration. 

Combining the bit isolator and chuck isolator (Fig. 7), roof bolter noise at the operator’s ear 

location is lowered by a total of 5 dB(A) from baseline. As was the case with the bit isolator and 

chuck isolator on their own, the 1⁄3-octave bands that were improved were mainly in the range 

from 1.25 to 10 kHz, with degradation from 800 Hz to 1.25 kHz. 

4 CDSE 

4.1 CDSE Design 

An additional noise control is the CDSE, which serves as a barrier to block noise radiated by 

the drill steel from reaching the operator’s ear. It consists of a bellows with a spring to support 

it, a hinge that allows the CDSE to move out of the way for roof bolt installation, a cap to help 

the CDSE seal against the roof, and customized mounting hardware to install the CDSE onto a 

specific RBM (Figs. 8 and 9). 

The bellows is made of 0.864 mm-thick aluminum-coated fiberglass and has an extended 

length of 1.22 m, but lengths may vary depending on the specific needs of the application. 

Aluminum-coated fiberglass was chosen due to its heat resistance, ability to block noise, 

resistance to acids, oils, corrosive vapors, and many solvents, and its incombustibility. 

Each CDSE bellows is supported by a straight cylindrical compression spring which keeps 

the bellows upright and collapses as the drill chuck of the roof bolter rises during drilling. The 

spring is made of zinc-plated steel wire 0.635 cm in diameter with a load of 303 N. The outside 

coil diameter is 15.82 cm, but again this could be varied for specific applications. 

The top of the CDSE is made of PVC 203 mm in diameter and 31.8 mm thick. A conical 

ultra high molecular weight polyethylene (UHMW-PE) insert into a bottom made of 6061 

aluminum accommodates either 25 mm or 35 mm drill steels as shown in Fig. 10. The funnellike shape of the insert guides the drill steel into place in the chuck. A hinge connects the roof 

bolter chuck to the bottom of the CDSE. 

Before drilling, the operator installs a drill steel through the top of the CDSE and into the 

chuck. At the beginning of the drilling cycle, the CDSE is fully extended. A gap between the 

cap and the roof allows the operator to see the drill bit-to-media interface and start the hole. The 
gap can be designed to the operator’s preference to make the drill steel visible for a longer 

portion of the drilling cycle, but a larger gap will result in less noise reduction. As the drill 

chuck rises, the gap closes until the cap is flush with the roof, as in Fig. 9. The CDSE then 

collapses as drilling continues. When drilling is completed, the operator swings the CDSE to the 

side and installs the roof bolt as usual. 

4.2 Results Using the CDSE 

Previous testing has shown that the CDSE alone gives a 7 dB(A) reduction in SPL at the 

operator’s ear from baseline11 and reduces roof bolter drilling noise by 6 dB(A) during an 

operator’s 8-hour time-weighted average in the field12 . In combination with the bit isolator, it 

yields a 12 dB reduction in A-weighted SPL at the operator’s ear location as shown in Fig. 11 

and the results summary in Table 1. These improvements are seen in the 1⁄3-octave bands from 

250 Hz to 10 kHz. The advantage of the bit isolator and CDSE combination is that there is no 

degradation from baseline in the range between the 800 Hz – 1.25 kH z 1⁄3-octave bands as is 

characteristic with the bit and chuck isolators on their own. Combining the bit isolator, chuck 

isolator, and CDSE provides one more dB(A) of reduction for a total of 13 dB(A) less than 

baseline. However, adding the chuck isolator to the bit isolator and CDSE combination does add 

back in the noise in the 200-400 Hz 1⁄3-octave bands. 

5 FURTHER DEVELOPMENTS WITH BIT AND CHUCK ISOLATORS 

Realizing that fatigue life of the metal components— rather than the elastomer layer 

separating them— is the limiting factor with the chuck and drill bit isolators, the research team 

found that a pre-compressed bonded joint design was not necessary to meet life requirements. 

Further, the variability in bond strength, concentricity, and stiffness inherently associated with 

these types of designs could be improved upon by using a fully-bonded design. Reducing this 

process variation would ensure that the load-carrying requirements of the isolators would 

consistently be met. Therefore, the bit and chuck isolator designs were modified to incorporate 

chemical bonds created during the molding operation (Figs. 12 and 13). To create them, metal 

components are sandblasted, degreased, phosphatized, and coated with Chemlok® adhesive prior 

to vulcanization to yield high bond strengths between the elastomer and metal components. 

Additional studies with the new fully-bonded designs have been completed to optimize the 

durometer of the bit and chuck isolators8

, which should yield even greater noise reductions in the 

field than those discussed previously. An initial series of sound power level measurements were 

taken per ISO 3743-2 at the NIOSH Office of Mine Safety and Health Research Reverberation 

Chamber, located in Pittsburgh, Pennsylvania. Measurements were taken while drilling into 

granite for 30 seconds, and three runs for each configuration were averaged. The total length 

from the chuck to the drill bit was kept constant for this test at 1.2 m (4 ft) to eliminate potential 

differences in vacuum and drill steel noise radiation, and a new 35 mm (1 3/8”) bit was used for 

each hole. Rotation speed was kept constant at 230 RPM, with a thrust of 2.2 kN (500 lbs). 

Since we wanted to prove the concept first, we chose a lower thrust for this study to avoid 

potential mechanical failure of the lower durometer isolators while testing. Three different 

durometers of bit isolator were tested: 58 durometer natural rubber, 50 durometer butyl rubber, 

and 68 durometer natural rubber. 

All bit isolators tested showed a clear trend of noise reduction above the 630 Hz 1⁄3-octave 

band, with the lower durometer materials providing more reduction in sound power than the 

higher durometer materials (Fig. 14). The 50 durometer butyl rubber bit isolator provided the 
greatest reduction in A-weighted sound power level, which was 6 dB. This is significant because 

it suggests that the bit isolator alone could yield a SPL reduction of up to 6 dB(A) at the 

operator’s ear. As discussed earlier, the highest sources of noise on the RBM is drilling noise at 

the drill steel interface to the chuck, and at the top of the drill steel near the bit. The operator is 

positioned close to these noise sources, so changes from baseline in overall SPLs at the 

operator’s right ear location should be quite similar to changes in overall sound power levels. 

Using this same logic, the combination of bit and chuck isolator could improve from a 

current reduction in A-weighted SPL of 5 dB up to 9 dB. As seen in Fig. 15, testing the best bit 

isolator (45 durometer) from a previous study8 in combination with a 50 durometer butyl rubber 

chuck isolator yielded an overall reduction in sound power of 9 dB(A). Other combinations of 

bit and chuck isolator durometers may obtain even better results. 

6 CONCLUSIONS 

Each noise control reduces roof bolter drilling noise on its own, as summarized in Tables 1 

and 2. The single most effective noise control presented herein is the collapsible drill steel 

enclosure (CDSE). The bit isolator, chuck isolator, and CDSE reduce overall A-weighted sound 

pressure level (SPL) at the operator’s ear location by 4, 2, and 7 dB, respectively. In order to 

meet the MSHA Permissible Exposure Level (PEL), a combination of two or more controls may 

be necessary. Combining the bit and chuck isolators yields a total reduction of 5 dB(A) in 

overall SPL. The combination of two controls that is most effective is to combine the bit isolator 

and CDSE for an overall reduction of SPL by 12 dB(A). However, if a 13 dB(A) reduction is 

needed, all three controls may be combined. These reductions in sound pressure levels under 

controlled laboratory conditions may be less if drilling into softer materials when testing in the 

field. New developments in bit and chuck isolator designs focusing on durometer optimization 

for the elastomer show promise for even greater noise reductions. 

7 ACKNOWLEDGEMENTS 

The authors would like to thank Pat McElhinney for his assistance with running the roof 

bolter, Ernie Ferro from Corry Rubber Corporation for his expertise and design assistance with 

the chuck and bit isolators, Chad Swope and Doug Bise from Kennametal, Inc. for machining 

chuck and bit isolator samples and providing technical advice on roof bolting parameters, and 

Hugo Camargo for assisting with the sound power level testing. 

8 REFERENCES 

1.	 John R. Franks, Mark R. Stephenson and Carol J. Merry, “Preventing occupational hearing 

loss—A practical guide”, Technical Report No. 96-110, National Institute for Occupational 

Safety and Health, (1996). 

2.	 National Institute for Occupational Safety and Health, National Occupational Agenda, 

DHHS Publication No. 96-115, (1996). 

3.	 J. R. Franks, “Analysis of audiograms for a large cohort of noise-exposed miners”, National 

Institute of Occupational Safety and Health, Internal Report, Cincinnati, OH, 7, (1996). 

4.	 Title 30 CFR Part 62, 2000-2005, U.S. Department of Labor, Mine Safety and Health 

Administration, Information Resource Center, Denver CO. 
5.	 J. Shawn Peterson and Lynn A. Alcorn, “Results of noise measurements from underground 

testing of a roof bolting machine duty cycle”, NoiseCon06, (2007). 

6.	 I. Hawkes and J. A. Burks, “Investigation of noise and vibration in percussive drill rods”, Int. 

J. Rock, Mech, Min. Sci. & Geomech, Abstr, 16, 363-376, (1979). 

7.	 I. Carlvik, “The Generation of Bending Vibrations in Drill Rods”, Int. J. Rock, Mech, Min. 

Sci. & Geomech, Abstr, 18, 167-172, (1981). 

8.	 Robert Michael, David Yantek, J. Shawn Peterson and Ernie Ferro, “The evolution of drill bit 

and chuck isolators to reduce roof bolting machine drilling noise”, Society for Mining, 

Metallurgy, and Exploration, Inc., Annual Meeting Preprints, Littleton CO, (2010). 

9.	 J.S. Peterson, P.G. Kovalchik and R.J. Matetic, “Sound power level study of a roof bolter”, 

Transactions of Society for Mining, Metallurgy, and Exploration, Inc., 320, 171-177, (2006). 

10. D.S. Yantek, J.S. Peterson and A.K. Smith, “Application of a microphone phased array to 

identify noise sources on a roof bolting machine”, NoiseCon07 Proceedings, (2007). 

11. Rudy J.	 Matetic, Peter G. Kovalchik, J. Shawn Peterson and Lynn A. Alcorn, “A noise 

control for a roof bolting machine: Collapsible drill steel enclosure”, NoiseCon08, (2008). 

12. J. Shawn Peterson and Lynn A. Alcorn, “Collapsible drill steel enclosure for reducing roof 

bolting machine drilling noise,” NIOSH Technology News, 532, (2008). 

Table 1 – Summary of operator’s right ear A-weighted sound pressure level results. 

Configuration Overall SPL, dB(A) dB from Baseline 

Baseline 106 — 

75 Durometer Bit Isolator 102 4 

75 Durometer Chuck Isolator 104 2 

Bit Isolator and Chuck Isolator 101 5 

Bit Isolator and CDSE 94 12 

Bit Iso, Chuck Iso, and CDSE 93 13 

Table 2 – Summary of A-weighted sound power level results. 

Configuration 

Overall Sound 

Power Level, 

dB(A) 

dB from 

Baseline 

Baseline 114 — 

68 Durometer Natural Rubber Bit Isolator 111 3 

58 Durometer Natural Rubber Bit Isolator 109 5 

50 Durometer Butyl Bit Isolator 108 6 

45 Duro NR Bit Iso & 50 Duro Butyl Chuck Iso 105 9 
Fig. 1 – Hex and round drill steels for a 35­

mm diameter drill bit (upper) and 

two examples of drill bits – 25 mm 

and 35 mm (lower). 

Fig. 2 – A roof bolting machine in an 

undergroundcoal mine. 

Fig. 3 – Model of bit isolator design. The blue color indicates the inner member, red indicates 

the outer steel member, and black indicates the elastomer layer. 

Inner member with elastomer 

Outer steel member 

Fig, 4 – Picture of bit isolator design, two units disassmbled. 
Fig. 5 – A-weighted SPL in 1⁄3-octave bands for baseline vs. the bit isolator. 
	

Fig. 6 – A-weighted SPL in 1⁄3-octave bands for baseline vs. the chuck isolator. 
	
Fig. 7 – 		A-weighted SPL in 1⁄3-octave bands for baseline vs. the bit isolator, chuck isolator, and 

combination of bit and chuck isolator. 

Fig. 8 – Exploded view of the CDSE. 

Fig. 9 –	 The CDSE installed on a roof 

bolting machine. At this point in 

the drilling process, the drill chuck 

has been raised far enough that the 

top of the CDSE has come into 

contact with the mine roof. 

Fig. 10 –	 Bottom of the CDSE, showing the 

conical UHMW-PE insert used to 

guide the drill steel into place 

when attaching it to the chuck. 
Fig. 11 – A-weighted SPL in 1⁄3-octave bands for baseline vs. the bit isolator, bit isolator 

combined with the CDSE, and combination of bit isolator, chuck isolator, and CDSE. 

Chuck isolator: 

Drill Bit isolator: 

QC-19835 

QC-19836 

Fig. 12 – Final production designs of chuck 

and bit isolators for use with 35
 

mm-diameter drill bits. The gray
 

color indicates the inner member
 

and red indicates the outer
 

member.
 

Fig. 13 – Cross-section of production design 

chuck isolator. 
Fig. 14 – A-weighted sound power level in 1⁄3-octave bands for baseline vs. 50, 58, and 68 

durometer bit isolators. The 68 and 58 durometer isolators are made of natural 

rubber, while the 50 durometer isolator is made of butyl. 

Fig. 15 – A-weighted sound power level in 1⁄3-octave bands for baseline vs. 45 durometer natural 

rubber bit isolator with a 50 durometer butyl chuck isolator.