DEVELOPMENT OF ROOF-BOLTING MACHINE BIT AND CHUCK ISOLATORS FOR 

DRILLING NOISE REDUCTIONS 

Jeffrey Shawn Peterson 

NIOSH 

Pittsburgh Research Laboratory 

Pittsburgh, Pennsylvania, USA 

Peter G. Kovalchik 

NIOSH 

Pittsburgh Research Laboratory 

Pittsburgh, Pennsylvania, USA 

David Yantek 

NIOSH 

Pittsburgh Research Laboratory 

Pittsburgh, Pennsylvania, USA 

ABSTRACT 

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 (RBM). After the removal of coal or rock, 

the remaining strata may be subject to fall, either from overhead 

(the roof) or from the side (the rib). One method used in 

underground coal-mines to prevent failures requires the

installation of roof bolts. The roof bolting machine operator 

trams the machine to the required location, drills a hole into the 

strata, and then installs a roof bolt, supporting the roof or the rib, 

as the case may be. 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 is developing bit and chuck 

isolators to reduce vibration, and thus noise radiation of the drill 

steel, with the longer-term goal of reducing roof bolting machine 

operator noise exposure. Laboratory testing has shown that 

operator ear sound pressure levels may be reduced by 3 to 7 

dB(A), depending upon the test configuration and drilling media. 

INTRODUCTION 

Hearing loss prevention is one of 21 Priority Research Areas 

listed in the NIOSH National Occupational Research Agenda 1

. 

Further, the Department of Labor Mine Safety and Health

administration (MSHA) collects noise sample data that assists 

NIOSH in selecting equipment whose operators are most likely 

to be over exposed to noise 2

. Such 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) (Table 1). Of 

these machines, the roof bolting machine operator was the

second most likely to be over exposed among operators of all 

equipment used in underground coal. 

Table 1 - Percentage of noise over-exposures by 

machine 

Machine % 

Continuous mining machine 

Bull dozer 

35 

24 

Roof bolting machine 

Front end loader 

17 

8 

Shuttle car 6 

Auger miner 

Truck 

5 

5 

Commonly used roof bolts employ either mechanical 

anchors to secure the bolt, resin and rebar to serve the same 

purpose, or both. There are other types of roof bolts available as 

well. Examples of resin rebar roof bolts are shown in Figure 1. 

Drill steels are either round or hexagonal in shape and are 

available to mate with either 2.54 cm (one inch) or 3.49 cm (1 

3/8 inch) drill bits (Figure 2). Drill bits are available for vacuum 

drilling i.e., dry, and for mist or wet drilling (Figure 2). 

Underground, the operator trams the roof-bolting machine to the 

required location, installs a bit on a drill steel, places the drill 

steel in the RBM chuck and then drills the hole for the roof bolt. 

The operator removes the drill steel from the chuck, replaces it 

with a wrench holding the roof bolt, inserts the resin into the 

hole, if used, drives the roof bolt into the hole with the RBM, 

then rotates the bolt for a predetermined length of time and 

torque until the resin sets. The operator repeats this exercise, 

installing rows of bolts across the roof as specified in the mines 

roof control plan, typically every 1.2 m (four-ft). 

1 
Figure 1 - Drill steels (round & hex), bolt plate, resin and resin 

type roof bolts, shown for 2.54 and 3.49 cm bit drilling and 

bolting 

Figure 2 - Drills bits; vacuum (top), wet (bottom), 2.54 and 3.49 

cm 

 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 day 3

. Essential to developing engineering 

noise controls to reduce the occurrences of Noise Induced 

Hearing Loss (NIHL) for roof bolting machine operators is to 

develop engineering noise controls to reduce drilling noise. The 

objective is to reduce an operators’ noise exposure to a timeweighted average (TWA) of 90 dBA or less for an eight hour 

shift. This would correspond to a noise dose of 100%, the 

maximum allowed per the MSHA PEL. A suite, or combination, 

of controls, is the most promising path of success to meet the 

objective. To this end, NIOSH is pursuing several engineering 

controls to reduce drilling noise. This paper discusses research 

results for bit and chuck isolators. 

DRILLING ACCELERATION TESTING 

In percussive rock drilling, one notable source of noise 

generation is drill rod vibration 4,5. There are three fundamental 

ways to reduce these vibrations, and thus noise: reduce the cause 

of the vibration, attenuate the vibration, or attenuate the noise. 

Early laboratory testing included measuring vibration on the drill 

head, slinger plate, drill guide, and on the drill media. NIOSH 

also measured acceleration on the drill steel and employed a slip 

ring assembly to dress acceleration signals to data acquisition 

equipment (Figure 3). 

Figure 3 - Drill steel accelerometers and slip-ring assembly 

Figure 4 shows acceleration levels 

measured on a hexagonal drill steel and a 3.49 cm (1 3/8”) bit 

during a 5-second sample (rotation speed of 200 rpm and thrust 

of 9.4 kN).

Figure 4 - Typical drill steel acceleration, hexagonal drill steel, 

3.49 cm bit, granite drill media 

 NIOSH used granite as the drill media to represent 

high compressive strength roof and for the fact that this 

compressive strength should remain consistent throughout the 

drilling, helping to ensure test repeatability. Levels peaked at 

more than 500 g’s, confirming that during drilling, there is 

significant vibration in the drill steel. 

HEMI-ANECHOIC CHAMBER BEAMFORMING AND 

SOUND LEVEL TESTING 

NIOSH maintains a hemi-anechoic chamber at PRL for 

noise source identification purposes. Key to this testing is a 42­

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 2 
microphone Bruel and Kjaer beamforming array and associated 

data acquisition system and analysis software. Early chuck 

isolator laboratory testing comprised beamforming analysis to 

locate noise sources and a supplemental microphone placed near 

the RBM operators head to collect operator ear sound level 

measurements. Table 2 lists the drilling configuration for this 

testing. 

Table 2 - Drilling configurations 

Drill media granite 

Rotation speed (rpm) 230 

Thrust (kN) 9.4 or 22 

Drill bit (cm) 3.49 

Drill steel type hexagonal 

Drilling type vacuum 

A jaw-type shaft coupling used as a chuck isolator served as 

a simple device to test the premise that breaking the mechanical 

link between the drill steel and the drill chuck would reduce 

noise levels at the operator’s ear position (Figure 5).

Figure 5 - Jaw-type coupling with 58 Shore D urethane spider 

Shown in 

Figures 6 and 7 are operator ear sound level data collected using 

a typical hexagonal drill steel (baseline) and when using the jawtype coupling. Sound levels are reduced in the most relevant 

one-third octave band frequencies of 1,600 Hz through 6,300 Hz 

(shaded area of Figures 6 and 7).

Figure 6 - Operator ear sound pressure level, 9.4 kN thrust, four 

dBA reduction using a jaw-type coupling 

Figure 7 - Operator ear sound pressure level, 22 kN thrust, four 

dBA reduction using a jaw-type coupling 

 Relevant one-third octave 

frequencies are defined as those in which greater than 90% of the 

noise energy are contained. Overall, using the jaw-type coupling 

reduced the operators sound level to 96 dBA from a baseline 

measurement of 100 dBA for the 9.4 kN thrust pressure testing 

and from 104 dBA to 100 for the 22 kN thrust tests (Table 3). 

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Table 3 – Sound pressure level, A –weighted decibels 

Average of three tests 

9.4 kN thrust 22 kN thrust 

1.6 kHz 1.6 kHz 

Overall – 6.3 Overall – 6.3 

kHz kHz 

Baseline 100 A 99 104 B 104 

Jaw-type 96 B

 95 100 A 99 coupling 

Reduction 4 4 4 5 

Bit isolator 97 C

 97 ---- ----

Reduction 3 2 ---- ----

Combination 92 B

 90 ---- ----

Reduction 7 9 ---- ----

 A – standard deviation of 0.4 dBA 

B

 – standard deviation of 0.6 dBA 

C

 – standard deviation of 1.6 dBA 
Additional testing using the beamforming array showed that 

the steel radiates a significant amount of noise during drilling 6

. 

Figure 8 shows baseline data collected during testing using a 

thrust of 9.4 kN.

Figure 8 - Beamforming of baseline drilling, 100 dBA @ the 

operators’ position 

The noise sources are centered roughly 10 to 

20 cm (4 to 8 inches) below the drill bit/media interface and 

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

the lower drill steel noise source advances with it, while the 

upper source essentially remains the same. Figure 9 shows 

similar drilling but with the inclusion of the jaw-type coupling. 

Figure 9 - Beamforming of drilling with the jaw-type coupling, 

96 dBA @ the operators’ position 

Both Figures 8 and 9 show spectral data in the 1.6 kHz through 

6.3 kHz one-third octave band frequency range and are scaled 

from 75 to 85 dB. The differences between the baseline data and 

that collected using the coupling are significant. The operator 

ear sound level is reduced from 100 to 96 dBA and the 

beamforming results change dramatically. For the given 

frequency range, the chuck area drill steel noise source is 

reduced below 75 dB and the upper source is reduced to roughly 

80 dB. Given these results, NIOSH deemed chuck isolation as a 

viable option to reduce noise generation during roof bolting 

drilling. 

NIOSH also considered a bit isolation approach to reducing 

the vibration, and thus noise generation, of the drill steel at the 

steel-bit interface. The initial prototype was a modified vacuum 

chuck coated with 60 Shore D urethane, hereafter referred to as a 

bit isolator (Figure 10). 

Figure 10 – Bit isolator with 60 Shore D urethane coating 

 Shown in Figure 11 are operator ear 

sound level data collected using a typical hexagonal drill steel 

(baseline) and when using the bit isolator. As with the chuck 

isolator, sound levels are reduced in the most relevant one-third 

octave band frequencies of 1,600 Hz through 6,300 Hz (shaded 

area of Figure 11). Overall, using the bit isolator reduced the 

operators sound level to 97 dBA from a baseline measurement of 

100 dBA for the 9.4 kN thrust pressure testing (Table 3). 

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Figure 11 - Operator ear sound pressure level, 9.4 kN thrust, 

three dBA reduction using a bit isolator 

In combination, the bit and chuck isolators reduced the 

RBM operators sound level by 7 dBA (Table 3). Shown in 

Figure 12 is the one-third octave band plot of the operators 

sound level. Here, the significant reduction in the operators 

sound level is prominently shown in the frequency range of 

importance, 1.6 kHz through 6.3 kHz. 

Figure 12 - Operator ear sound pressure level, 9.4 kN thrust, 

seven dBA reduction when using a chuck and bit isolators in 

combination 

REVERBERATION CHAMBER SOUND POWER LEVEL 

TESTING 

NIOSH also maintains a reverberation chamber used to 

measure the overall noise emission of mining and construction 

equipment via sound power level testing. This testing is 

conducted per ISO 3743-2 (engineering grade and used for this 

analysis) as well as precision grade testing per ISO 3741/ANSI 

S12.51. The testing configurations were the same as listed in 

Table 2. Shown in Figure 13 is the sound power level data for 

baseline and coupling testing conducted at a thrust of 9.4 kN. 

Here, the sound power level for the baseline condition was 110 

dBA; with the inclusion of the coupling, the sound power drops 

to 107 dBA, a three-decibel reduction (Table 4). As for the 

sound pressure level testing discussed earlier, noise levels are 

reduced in the most relevant one-third octave band frequencies 

of 1.6 kHz through 6.3 kHz (shaded area of Figure 13). 

Figure 13 – Sound power level per ISO 3743-2, 9.4 kN thrust 

Figure 14 shows similar results for testing conducted at 22 

kN thrust. 

Figure 14 - Sound power level per ISO 3743-2, 22 kN thrust 

Here, the inclusion of the jaw type coupling reduced 

the sound power level from 114 dBA to 110 dBA. Table 4 

summarizes sound power levels, broken down as overall values 

and a summation of the levels in the 1.6 kHz through 6.3 kHz 

one-third octave frequency bands. This data confirms that the 

significant portion of the sound power levels is contained within 

1.6 kHz and 6.3 kHz one-third octave band frequency ranges, 

and the reductions gained by using the coupling reduce the sound 

levels in these important frequency bands. 

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Table 4 – Sound power level, A –weighted decibels 

Average of three tests 

9.4 kN thrust 22 kN thrust 

1.6 kHz 1.6 kHz 

Overall – 6.3 Overall – 6.3 

kHz kHz 

Baseline 110 C 110 114 A 113 

Jaw-type 107 D 106 110 B

 110 coupling 

Reduction 3 4 4 3 

Chuck 108 B

 107 ---- ---- isolator 1 

Reduction 2 3 ---- ----

Chuck 109 B

 108 ---- ---- isolator 2 

Reduction 1 2 ---- ----

 A – standard deviation of 0.2 dBA 

B

 – standard deviation of 0.3 dBA 

C

 – standard deviation of 0.4 dBA 

D – standard deviation of 0.6 dBA 

As an extension of this work, NIOSH developed and tested a 

chuck isolator (Figure 15) in the reverberation chamber. 

Figure 15 - Chuck isolator 1, 58 Shore D polyurethane filled 

This 

chuck isolator consists of two steel parts, a cylindrical outer 

sleeve with three drivers, located 120 degrees apart; and a center 

chuck section, also with three drivers located at 120 degrees 

apart. Between these is 58 Shore D polyurethane, bonded to all 

surfaces and served to break metal-to-metal contact between the 

drill chuck and the drill steel. Testing at a thrust of 9.4 kN, using 

the chuck isolator reduced the sound power emission from 110 to 

108 dBA (Figure 16). 

Figure 16 - Sound power level per ISO 3743-2, chuck isolators 

Moreover, as was the case with the jawtype coupling, this reduction is attributable to reductions in the 

relevant frequency range of 1.6 kHz through 6.3 kHz (Table 4). 

NIOSH also tested a second chuck isolator, this time with 

the outer sleeve and the center chuck section using four drivers 

located 90 degrees apart (Figure 17).

Figure 17 - Chuck isolator 2, 45 Shore A polyurethane filled 

The chuck section is 

“floating” in 45 Shore A polyurethane. Figure 16 also shows 

sound power levels for this chuck isolator 2 and the results 

illustrate a reduction of 1 dBA compared to the baseline data. 

Here, reductions in the sound power are in the 800 Hz through 

1.6 kHz one-third octave bands and do not adequately reduce 

levels in the preferred frequency bands. 

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FIELD TESTING OF A CHUCK ISOLATOR 

NIOSH continued testing of a chuck isolator (Figure 18), 

similar to chuck isolator 2, the same general driver design and 45 

Shore A polyurethane damping material. However, this chuck 

isolator contained an internal dust collector funnel/sleeve to 

ensure that the vacuum system maintained suction under heavy 

loading. 

Figure 18 – Chuck isolator 3, 45 Shore A polyurethane filled, 

used for field testing and dose accumulation model 

NIOSH was granted the opportunity to field test this chuck 

isolator at a co-operating coal mine. There, NIOSH conducted 

time-motion studies and dosimetry on the operators of a dualboom J.H. Fletcher roof bolting machine. The operators were 

observed drilling normally, without using a chuck isolator, for an 

entire shift. Of the two operators, one was an experienced 

operator while the other was new to operating a RBM. The more 

experienced operator drilled more smoothly and consistently and 

was the primary focus of the time motion study for this 

evaluation. During the shift, this operator drilled and bolted 106 

holes and accumulated 67.2% noise dose exposure (Table 5). 

Based on this, the operator accumulated 12.7% dose per 20 bolts. 

NIOSH did not collect a full shift of data using chuck 

isolator 3 due to maintenance considerations at the mine. But, 

NIOSH also was able to test the chuck isolator during a 

simulated drilling and bolting exercise where the operator 

installed 20 bolts using chuck isolator 3. All other test 

conditions were as close as possible to the data collected drilling 

normally, same mine section, same RBM and set-up, and the 

same RBM operator. Using chuck isolator 3, the operator 

accumulated 7.7 % dose when installing the 20 roof bolts (Table 

5). 

Table 5 – RBM operator noise dose accumulation data 

Drilling 

Drilling with a 

normally chuck 

isolator 

Number of holes 106 20 

Accumulated dose (%) 67.2 7.7 

Dose per 20 holes (%) 12.7 7.7 

For simplicities sake and for comparative purposes, a 

modeled shift was developed. In the model, a pair of operators 

completes ten rows of four bolts across, (20 bolts per operator) 

comprising a “cut” in this example. It is also assumed that the 

pair of operators completes ten “cuts” per ten-hour shift. This 

would entail installing 200 bolts per day each. Shown in Table 6 

is how the ten-hour shift is broken up, with 30 minutes travel 

time at the beginning and end of the shift, a 30 minute lunch, and 

4:15 minutes of roof bolting both before and after lunch. To 

illustrate, given five cuts before lunch, an operator accumulating 

12.7% dose per cut would accumulate 63.5% dose, or 14.9% 

dose per hour for 4:15 (Table 6).

Table 6 – Dose accumulation model 

 Dose accumulation 

rate (% / hour) 

Task Time 

(hrs) 

With 

Normally Chuck 

isolator 

Elevator and 

mantrip in 0.5 2.6 2.6

Installing bolts 4.25 14.9 9.1 

Lunch 0.5 0.0 0.0

Installing bolts 4.25 14.9 9.1 

Elevator and 

mantrip out 0.5 2.6 2.6

 Shown in Figure 19 is the 

modeled shift noise dose accumulation for a RBM operator. In 

this model, using a chuck isolator reduces the RBM operator’s 8­

hour shift exposure 106% to 67% and the time weighted average 

from 92 dBA to 89 dBA. This is important, as MSHA considers 

a 3 dBA reduction in exposure to be significant. 

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Figure 19 – RBM dose accumulation model, drilling normally 

and with a chuck isolator 

NIOSH is continuing to pursue a suite of engineering noise 

controls to reduce RBM operator noise dose exposures. Chuck 

and bit isolators are key components of this suite. The suite 

approach allows for mining companies to select the engineering 

noise control that best suits their needs. In cases where the RBM 

operator is nominally over-exposed to noise, either a chuck or a 

bit isolator may be used to bring the operators noise dose into 

compliance. Operators exposed to greater noise levels may 

require both, or additional noise controls as developed by

NIOSH. This includes a Collapsible Drill Steel Enclosure

(CDSE), which NIOSH has developed and field tested at

cooperating mines 7

. THE CDSE creates an acoustic barrier 

between the noise source (drill steel) and the receiver (operator) 

to reduce noise levels at the operators position and can be used 

singly or in combination with bit and chuck isolators. 

SUMMARY 

Past research has shown that drill steel/rod vibration is a 

common source of noise during drilling operations. Research by 

NIOSH has shown that accelerations in a roof bolting machine 

drill steel may exceed 500 g’s, suggesting that this is the cause of 

significant noise during roof bolting drilling as well. Thus, 

chuck and drill bit isolation as a means to reduce drill steel 

vibration has been investigated. Early testing using a simple 

jaw-type coupling has shown reductions in operator ear sound 

levels as well as sound power. Early prototype chuck isolators 

have proven to also reduce noise emissions (sound power level) 

but to a lesser degree that the jaw-type coupling. A field study 

and dose accumulation model using a chuck isolator showed a 

three decibel reduction in operator noise exposure, a significant 

finding. Early prototype bit isolators also showed promise in 

reducing noise emissions. Research continues in this area for 

possible inclusion in a suite of engineering noise controls that 

should reduce drilling noise during roof bolting and thus, reduce 

noise exposures to the operator. This, in turn, supports a NIOSH 

research objective of reducing noise induced hearing loss. 

ACKNOWLEDGMENTS 

The authors wish to thank Lynn Alcorn, Physical Sciences 

Technician, for his tireless efforts in fabricating and field testing 

the noise controls discussed in this paper. 

REFERENCES 

1

 National Institute for Occupational Safety and Health, 

1996. National Occupational Agenda, Department of Health and 

Human Services Publication No. 96-115. 2 Title 30 CFR Part 62, 2000-2005, U.S. Department of 

Labor, Mine Safety and Health Administration, Information 

Resource Center, Denver, Co. 3

 Jeffrey Shawn Peterson and Lynn Alcorn, “Results of noise 

measurements from underground testing of a roof bolting 

machine duty cycle”, Noise-Con 2007. 4 I. Hawkes and J. A. Burks, “Investigation of Noise and 

Vibration in Percussive Drill Rods”, Int. J. Rock, Mech, Min. 

Sci. & Geomech, Abstr, 16, pp 363-376 (1979). 5

 I. Carlvik, “The Generation of Bending Vibrations in Drill 

Rods”, Int. J. Rock, Mech, Min. Sci. & Geomech, Abstr, Vol 18, 

pp 167-172 (1981). 6 Yantek DS, Peterson JS, Smith AK [2007]. “Application 

of a Microphone Phased Array to Identify Noise Sources on a 

Roof Bolting Machine”. Noise Con 2007. 7 Peterson JS [2008]. “Collapsible Drill Steel Enclosure for 

Reducing Roof Bolting Machine Drilling Noise”, NIOSH 

Technology News No. 532, September 2008. 

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public release; distribution is unlimited.

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