The evolution of drill bit and chuck 

isolators to reduce roof bolting 

machine drilling noise

D. Yantek and J. Peterson

Research engineers, National Institute for Occupational Safety and Health 

(NIOSH), Office of Mine Safety and Health Research (OMSHR), Pittsburgh, PA

R. Michael

Engineering manager, Corry Rubber Corporation, Corry, PA and Penn State Erie,

The Behrend College, Erie, PA

E. Ferro

President, Corry Rubber Corporation, Corry, PA

Abstract

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

To address this problem, the U.S. National Institute for Occupational Safety and Health Office of Mine 

Safety and Health Research (NIOSH OMSHR) conducts research to reduce the noise emission of 

underground coal mining equipment, an example of which is the roof bolting machine. Field studies 

show that, on average, drilling noise is the loudest noise that roof bolting machine operators would be 

exposed to, and it contributes significantly to the operators’ noise exposure. OMSHR has determined that 

the drill steel and chuck radiate a significant amount of noise during drilling. OMSHR and the Corry 

Rubber Corporation (CRC) have developed a bit isolator that breaks the steel-to-steel link between the 

drill bit and drill steel and a chuck isolator that breaks the mechanical connection between the drill steel 

and the chuck. This effectively reduces the noise radiated by the drill steel and chuck and in turn reduces 

the noise exposure of roof bolter operators. This paper documents the evolution of the bit isolator and 

chuck isolator. Laboratory testing confirms that production bit and chuck isolators reduce the A-weighted 

sound level generated during drilling by 3.7 to 6.6 dB.

Introduction

Hearing loss prevention is one of 21 priority research areas 

listed in the the U.S. National Institute for Occupational Safety 

and Health (NIOSH) national occupational research agenda 

(NIOSH, 1996). Further, the U.S. Department of Labor’s Mine 

Safety and Health Administration (MSHA) collects noise 

sample data that assists OMSHR in selecting equipment whose 

operators are most likely to be overexposed to noise (MSHA, 

2005). Sample data collected from 2000 to 2005 show that 

seven types of machines compose the bulk of the equipment 

whose operators exceed 100% noise dose per the MSHA permissible exposure level (PEL) (Table 1). Among operators of 

all equipment used in underground coal, the table shows that 

roof bolting machine (RBM) operators were the second most 

numerous group of miners to be overexposed to noise.

To develop effective noise controls, it was important to determine the tasks and noise levels associated with significant noise 

exposure for roof bolting machine operators. The objective was 

to reduce operators’ noise exposure to a time-weighted average 

(TWA) of 90 dB(A) or less for an eight-hour shift. This would 

correspond to a noise dose of 100%—the maximum allowed 

per the MSHA PEL. OMSHR performed noise dosimetry and 

time-motion studies to determine the noise doses and sound 

levels associated with the typical tasks required of RBM operators and the time spent conducting these tasks. Post-processing 

of this data revealed the tasks that operators devoted the most 

time to, the noise dose accumulated during each task and, of 

course, the tasks which are the primary contributors to the 

operators’ noise exposure. This information helped OMSHR 

to determine where noise control research should be focused.

Tables 2 and 3 summarize data collected during a timemotion study of a dual-boom J.H. Fletcher roof bolting machine 

(Peterson and Alcorn, 2007). The tasks shown are those for 

which the operators were engaged at least 1% of their shift. 

The ‘other’ category of Tables 2 and 3 includes time spent 

riding the elevator, when the operators were bolting alone, 
both drilling or both bolting, and other times where the activity could not be documented. Table 2 shows that the operators spent a significant amount of time drilling and bolting 

simultaneously—roughly 3.5 hours of the shift. As shown in 

Table 3, Operator 1 accumulated 52% PEL dose during this 

time, or two-thirds of his daily noise exposure, 78% PEL dose. 

Operator 2 accumulated 68% PEL dose during this time, or 

greater than half of his full-shift exposure of 127%. Further 

analysis showed that drilling sound levels far exceeded those 

associated with the bolting portion of the duty cycle (Table 4).

Table 1 — Percentage of noise overexposures by machine.

Machine %

Continuous mining machine

Bulldozer

35

24

Roof bolting machine

Front end loader

17

8

Shuttle car 6

Auger miner

Truck

5

5

Table 2 — Time spent by task, minutes.

Task Operator 1 Operator 2

Both drilling and bolting 218 218

Drilling alone 7 10

Riding mantrip 28 28

Tramming 63 63

Lunch 54 54

Prep time 108 108

Other 241 239

Table 3 — PEL dose by task, %.

Task Operator 1 Operator 2

Both drilling and bolting 52 68

Drilling alone 2 4

Riding mantrip 8 21

Tramming 11 18

Lunch 0 2

Prep time 5 9

Other 0 7

Total 78 127

Table 4 — Equivalent TWA sound level, dB(A).

Task Operator 1 Operator 2

Drilling 90 95

Bolting 70 86

With ordinary drilling times and relatively short bolting 

times for resin-type roof bolts, the operators spent significantly 

more time drilling than bolting. These observations confirmed 

the initial expectation that for the RBM, the drilling portion 

of the operators’ duty cycle exposed them to the highest noise 

levels and was the most significant contributor to the operators’ noise dose. Thus, if noise controls were to be effective at 

reducing RBM operator noise exposures, then drilling noise 

must be reduced.

In percussive rock drilling, one notable source of noise 

generation is drill steel vibration (Hawkes and Burks, 1979; 

Carlvik, 1981). There are three fundamental ways to reduce 

these vibrations and, thus, noise: reduce the source of the vibration, attenuate the vibration or attenuate the noise. To this end, 

OMSHR first sought to quantify the vibration levels of the RBM 

components associated with drilling. These included the drill 

head, slinger plate, drill guide, drill steel and the drill media.

Figure 1 shows a slip ring assembly to interface drill steel 

acceleration signals with data acquisition equipment. 

Figure 1 — Drill steel accelerometers and slip ring assembly.

Figure 2 

shows acceleration levels measured on a hexagonal drill steel 

while drilling with a 35-mm-diameter bit during a five-second 

sample (rotation speed of 200 rpm and thrust of 9.4 kN). OMSHR 

used granite as the drill media to represent high compressive 

strength roof and because this compressive strength should 

remain consistent throughout the drilling, which helps ensure 

test repeatability. Levels peaked at more than 500 g, confirming 

significant drill steel vibration during drilling.

To locate noise sources near the drill steel area, OMSHR 

used a 42-microphone Bruel and Kjaer beamforming array and 

associated data acquisition system and analysis software. To 

measure operator ear sound levels, an additional microphone 

was placed near the RBM operator’s head. Granite was used 

as the drill media for the aforementioned reasons.
A jaw-type shaft coupling with a 58 shore D durometer spider was used to test the premise that reducing the mechanical 

coupling between the drill steel and the chuck would reduce 

sound levels at the operator’s ear position (Fig. 3). 

Figure 2 — Typical drill steel acceleration, hexagonal drill steel, 35-mm-diameter bit, granite 

drill media.

Figure 3 — Jaw-type coupling with 58 Shore 

D urethane spider.

Using the 

jaw-type coupling reduced the sound level at the operator’s 

position from a baseline of 100 dB(A) to 96 dB(A) for 9.4 kN 

of thrust and from 104 dB(A) to 100 dB(A) for 22 kN of thrust.

Additional testing using the beamforming array showed 

that the steel radiates a significant amount of noise during 

drilling. Figure 4 shows baseline data collected using a thrust 

of 9.4 kN. 

Figure 4 — Beamforming results for baseline drilling, 100 

dB(A) at the operator’s position.

The noise sources are centered roughly 100 to 

200 mm (4 to 8 in.) below the drill steel-media interface and 

above the chuck. As the drill steel advances during drilling, 

the lower noise source moves with the chuck, while the upper source essentially remains just below the media. Figure 5 

shows beamforming results with the inclusion of the jaw-type 

coupling with the same operating conditions. In Figs. 4 and 5, 

light colors indicate areas of high noise radiation while dark 
colors indicate areas of lower noise radiation. Given these 

results, vibration isolation is a viable option to reduce noise 

generation during RBM drilling. 

Further development of a vibration isolator for drilling with 

RBMs required the aid of a collaborating partner proficient in 

the design and production of vibration isolators. Therefore, 

OMSHR began working with Corry Rubber Corporation (CRC) 

on bit and chuck isolators. 

Figure 5 — Beamforming results for drilling with the jawtype coupling, 96 dB(A) at the operators’ position.

Design overview

The final isolator designs are shown in Fig. 6. 

Figure 6 — Final production designs of isolators for use 

with 35-mm-diameter drill bits.

These designs 

evolved from several prototype designs that were evaluated for 

noise attenuation and durability. There are three main parts to 

each: the inner steel member, the outer steel member and the 

elastomer. The elastomer, which is between the inner and outer 

steel members, provides the vibration isolation. Some of the 

earlier designs consisted of a bonded joint (elastomer bonded 

to an inner member), which was subsequently assembled into 

an outer member. These precompressed, bonded joint designs 

utilize a postvulcanization (PV) bond between the outer surface of the elastomer and the inner surface of the outer steel 

member to react cutting forces. Advantages of precompressed 

bonded joint designs include improved elastomer durability 

(after assembly, the elastomer is in compression) and ease of 

manufacture, since the mold has to accommodate only one 

metal and, therefore, one bond.

While these designs offered sufficient noise attenuation, 

they did not meet the load carrying requirements, due to the 

relatively low bond strength and variability associated with 

PV bonds. The current production designs are fully bonded 

designs, which means the elastomer is bonded to the metal 

components during the molding operation, thus eliminating 

the need for a PV bond. Based on lab testing, the production 

designs provide outstanding noise attenuation while meeting 

all load requirements. 

A typical cross section of the production isolator design is 

shown in Fig. 7. 

Figure 7 — Cross section of production design chuck isolator.

The outer and inner members are machined 

out of 4130/4140 steel and heat-treated to 350 BHN. Sound 

level measurements were conducted with various elastomers 

to examine the attenuation as a function of elastomer properties. However, the production elastomer will more than likely 

be a 50 to 55 durometer (Shore A) natural rubber (NR) blend. 

The bonded length of 254 mm is consistent across all designs 

and necessary to safely withstand the drilling forces applied 

to the bonds between the elastomer and the metal members. 

An 11.4-mm gap protects the isolator from overload, since 

the outer member will bottom out against the inner member 

shoulder at roughly 45 kN. Any load above 45 kN will then 

be reacted by the metal components instead of the elastomer. 
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.

The stiffness requirements were based on sound level 

measurements using prototype isolators. The sound level at 

the operator’s ear was measured during “normal” drilling and 

also with several prototype isolator parts of varying stiffness. 

The production designs meet the requirements listed above. 

Extensive lab testing and analysis were performed on the 

production design to verify these requirements.

Elastomer compound evaluation

Several isolators were made with various elastomer compounds (Fig. 8). 

Figure 8 — Isolators bonded with elastomers from Table 5.

These elastomers were selected to provide a 

broad range of dynamic and static properties for sound level 

evaluation while still satisfying the requirements outlined 

above. A description of each elastomer compound evaluated 

is shown in Table 5.

Table 5 — Evaluated elastomer compounds listed by CRC compound number and description.

100 Modulus (MPa) Elongation (%) Tensile (MPa) Durometer (Shore A) Damping loss factor a

M-350-40 (45 durometer NR)

1.6 569 22.4 45.3 0.09

M-460-2 (58 durometer NR)

2.7 438 20.8 59.2 0.10

M-370-35 (68 durometer NR)

4.1 360 25.8 67.7 0.17

M-660-1 (50 durometer butyl rubber)

1.1 807 16.8 48.3 0.44

a (tan del), 10 Hz / + 20% strain

The natural rubber compounds used for the isolators have a 

range of tensile modulus values from 1.6 MPa (45 durometer 

compound) to 4.1 MPa (68 durometer compound). The butyl 

compound provides approximately the same static stiffness 

as the 45 durometer natural rubber, with four times as much 

damping. Furthermore, the 58 durometer and 45 durometer 

natural rubber compounds have nearly the same damping but 

different stiffness. The goal was to explore the effects that 

damping and stiffness have on noise attenuation.

Figure 9 — MTS810 test system used to measure axial load 

capacity and static and dynamic stiffness.

Figure 10 — Apparatus used for testing torsional load 

capacity and static torsion stiffness.

Lab testing

Isolators bonded with the elastomers detailed in Table 5 
were tested in the lab both in the torsion and axial directions. 

The machines used for this testing are shown in Figs. 9 and 

10. The purpose of this testing was to verify that stiffness and 

load requirements were met. A summary of all the testing 

completed with results follows.

1. Axial static stiffness – record load up to 7.6 mm deflection (see Fig. 11.)

Figure 11 — Axial static load-deflection curves.

2. Axial static stiffness– load in the axial direction to 18 kN.

3. Dynamic stiffness at 5 +/- 0.5 mm – perform frequency 

sweep from 1 Hz to 30 Hz in 3-Hz increments. Record 

dynamic stiffness, K’, damping stiffness, K”, and loss 

factor (tan del). (See Fig. 12 for dynamic stiffness, K’, 

versus frequency.)

Figure 12 — Dynamic axial stiffness, K’, vs. frequency.

4. Torsion test to approximately 410 N-m (the design 

torque requirement) and record torque versus angular 

deflection curve (see Fig. 13.)

Figure 13 — Static torque vs. angle of twist.

5. Torsion test one sample to failure to determine failure 

torque.

Summary table and discussion of results

Axial static and dynamic stiffness and torsional stiffness for 

all the isolators tested are summarized in Table 6. 

Table 6 — Stiffness results summary. Elastomers listed by compound number (CRC designation) 

and description.

Axial static (secant) 

stiffness at 7.6 mm 

(kN/m)

Axial static (secant) 

stiffness at 18 kN 

(kN/m)

Torsional (secant) 

stiffness at 410 N-m 

(N-m/deg)

Axial dynamic stiffness 

10 Hz, + 0.5 mm (kN/m)

M-350-40 (45 durometer NR)

4,200 3,100 5,600 9.5

M-460-2 (58 durometer NR)

5,600 3,400 8,600 10.2

M-370-35 (68 durometer NR)

6,100 4,000 10,200 11.9

M-660-1 (50 durometer butyl rubber)

4,500 3,300 9,600 TBD

From these 

results, some general observations can be made:

1. Measured stiffness (both axial and torsional) is consistent with the durometers specified in Table 1 for each 

compound. In other words, the 68-durometer NR has 

the highest torsional and axial stiffness, followed by the 

58-durometer and then the 45-durometer compounds.

2. All measured stiffness values meet design requirements 

and were verified with finite element analysis (FEA) 

and hand calculations.

3. The 50-durometer, heavily damped butyl is shown to 

be highly frequency sensitive (see Fig. 12). While the 
static stiffness is nearly the same as for the 45-durometer natural rubber, the dynamic stiffness, K’, at 30 

Hz is more than twice as high. This sort of behavior 

(frequency sensitivity) is typical of heavily damped 

elastomer compounds. The noise and vibration benefits 

from high damping may be offset by the higher dynamic 

stiffness at higher frequencies.

4. Load requirements of 45 kN axial and 410 N-m torsion were met by all isolators. The metal components 

showed no evidence of yielding and the elastomers 

showed no evidence of tensile failure or bond failure 

at these loads. Additionally, two isolators were loaded 

to failure on the torsion machine, with failure torques 

of 730 N-m and 760 N-m, which are nearly twice the 

torque requirement.

Laboratory sound level measurements

A computer-controlled drilling apparatus (see Fig. 14) was 

used to test the bit and chuck isolators made with the natural 

rubber compounds. 

Figure 14 — Test setup for sound level measurements at 

the operator’s ear.

The butyl rubber parts could not be tested, 

because the vacuum system on the test apparatus failed prior 

to testing the butyl parts. This caused the drill steel to become 

clogged and the drill head to stall. Table 7 shows the tests 

performed with and without drilling. 

Table 7 — Test configurations for sound level 

measurements.

Test Bit isolator Chuck isolator

BG not applicable not applicable

HYD/VAC not applicable not applicable

BASE not applicable not applicable

T1 45 durometer NR none

T2 45 durometer NR 45 durometer NR

T3 58 durometer NR none

T4 none 58 durometer NR

T5 58 durometer NR 58 durometer NR

T6 68 durometer NR none

T7 68 durometer NR 68 durometer NR

An optical tachometer 

was used to check the rotational speed for each test. A Bruel 

& Kjaer Type 4188 microphone was positioned roughly 1 m 

from the drill steel at a height of 1.5 m. An LMS Pimento 

portable data acquisition system was used to record the sound 

pressure using 24-bit resolution at 20,000 samples/second. 

The microphone data were postprocessed to calculate the 

A-weighted 1/3-octave-band sound levels according to ANSI 

standards (ASA, 2009).

For all testing, 35-mm-diameter drill bits were used with 

hexagonal drill steels to drill into a granite block. A 1.4-mlong drill steel was used for baseline measurements. When the 

bit and chuck isolators were tested, shorter drill steels were 

used to maintain a total length of 1.4 m. Measurements with 

either a bit isolator or a chuck isolator were performed using 

a 1.0-m-long drill steel, while measurements with both a bit 

isolator and a chuck isolator were completed with a 0.7-m-long 

drill steel. All tests were performed with a rotational speed of 

270 RPM, a penetration rate of 28 mm/sec and a maximum 

thrust of 22 kN. These parameters were selected based on prior 

experience showing that lower rotation speeds yield improved 

bit life when drilling into high compressive strength material, 

such as granite. For each configuration, two holes were drilled 

and the data for each hole were averaged. For the two measurements of a given configuration, the minimum and maximum 

differences in the overall A-weighted sound levels were 0.3 

dB and 2.2 dB, respectively.

Figure 15 — A-wtd sound level in 1/3-octave bands for 

hydraulics and baseline drilling conditions.

Figure 16 — A-wtd sound level in 1/3-octave bands for 

baseline and with bit isolators.

Prior to performing measurements with drilling, the sound 

pressure was measured with only the hydraulic and vacuum 

systems operating. Figure 15 shows the A-weighted sound 

levels in 1/3-octave bands with the hydraulics and vacuum 
systems and for drilling with a 1.4-m-long hex drill steel. The 

figure clearly demonstrates that noise from the hydraulics 

and vacuum are insignificant compared to drilling noise. The 

figure shows that the levels in the 400 Hz through 10 kHz 

1/3-octave-bands increase by 10 dB or more with drilling. In 

addition, the levels at and below 315 Hz change very little, 

suggesting that the noise at these frequencies can be attributed 

to the hydraulics and vacuum rather than drilling. Finally, the 

figure shows that the highest in-band levels are in the 2,500, 

3,150 and 4,000 Hz 1/3-octave bands. The remaining figures 

will focus on the levels above 250 Hz, because drilling noise 

does not have significant low frequency content.

Figure 16 shows the A-weighted sound level spectra for 

the tests with bit isolators compared to the baseline test case. 

The figure shows that each of the bit isolators significantly 

reduced drilling noise above 1,600 Hz. The 45-durometer bit 

isolator achieved the highest reductions while the 68-durometer bit isolator yielded the least reductions. The 45-, 58- and 

68-durometer bit isolators reduced the overall A-weighted 

sound level by 6.6, 5.9 and 3.9 dB, respectively. Because 

the reduction increased as the durometer decreased, and the 

stiffness of a rubber component decreases as the durometer 

decreases, the data suggest that decreasing the stiffness of the 

bit isolator could further reduce the sound level. However, 

reducing the durometer could also have a negative impact on 

the life of the part.

A single test was conducted with a chuck isolator only. 

Figure 17 shows the resulting spectra for the baseline compared to the test with a 58 durometer chuck isolator. 

Figure 17 — A-wtd sound level in 1/3-octave bands for 

baseline and with only a 58 duro NR chuck isolator.

The figure 

shows that the chuck isolator substantially reduced the levels 

above 1,600 Hz and reduced the overall A-weighted sound 

level by 3.7 dB.

Figure 18 shows the resulting 1/3-octave band spectra for 

drilling with a 58 durometer bit isolator and with both a 58 

durometer bit isolator and chuck isolator. 

Figure 18 — A-wtd sound level in 1/3-octave bands: baseline; 58 duro NR BISO; and 58 duro NR BISO with 58 duro 

NR CISO.

The figure shows a 

marginal additional reduction of the in-band levels for some 

frequencies. However, the changes are so small this could be 

test-to-test variation.

Combinations of bit and chuck isolators of the same durometer were tested to explore their effect on the noise generated 

by drilling. Figure 19 shows the results for these combinations 

along with the results for baseline conditions. 

Figure 19 — A-wtd sound level in 1/3-octave bands for 

baseline; 45 duro NR BISO and CISO; 58 duro NR BISO 

and CISO; and 68 duro NR BISO and CISO.

Figure 20 — A-wtd sound level in 1/3-octave bands for 

baseline and best three tests: 45 duro NR BISO; 58 duro 

NR BISO; and 58 duro NR BISO with 58 duro NR CISO.

The figure shows 

that the levels with the 58 durometer isolators are lower than 

those with either the 45 or 68 durometer isolators. This is some-
what surprising, since isolation usually improves as stiffness 

is reduced. However, the figure shows that the combination 

of the least stiff isolators—the 45 durometer parts—resulted 

in higher levels. This could be a result of the lower stiffness 

chuck isolator allowing the drill steel to drill an out-of-round 

hole. This would tend to cause the sides of the drill steel to rub 

against the drill medium, which would in turn increase noise. 

Future testing should explore using a low stiffness bit isolator 

in combination with a wide range of chuck isolators to fully 

examine this phenomenon.

Figure 20 shows the results for the best three tests with isolators along with the baseline results. The figure shows that using 

a 45-durometer bit isolator resulted in the greatest reductions. 

However, each of these tests resulted in substantial sound level 

reductions in the 1/3-octave bands at and above 1,600 Hz.

Conclusions

The current designs of production bit and chuck isolators 

evolved from relatively simple isolation devices that were only 

suitable for laboratory testing. The production designs were 

verified to meet design targets for spring rate, torque and load 

capacity and sound level reduction. Sound level measurements 

collected using an automated drilling apparatus showed that bit 

and chuck isolators with natural rubber elastomers reduced the 

sound level near the drill steel. While all the tested isolators 

reduced the A-weighted sound level whether used singly or 

in combination, the 45-durometer natural rubber bit isolator 

achieved the highest reduction, 6.6 dB, while the 58-durometer natural rubber bit isolator yielded a reduction of 5.9 dB. 

Measurements with a combination of a bit isolator and chuck 

isolator revealed that the sound level could increase compared 

to isolator-only values if the stiffness of the chuck isolator is 

low. This suggests that there is an optimal stiffness for the 

chuck isolator.

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. 

References

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