Evaluations of a noise control
� for roof bolting machines
�

by A.S. Azman, D.S. Yantek and L.A. Alcorn 

Abstract In collaboration with Kennametal Inc. and Corry Rubber Corporation, the U.S. 

National Institute for Occupational Safety and Health (NIOSH) developed a drill bit isolator to 

address noise overexposures associated with roof bolting machines in underground coal mines. 

NIOSH laboratory studies confirmed that the drill bit isolator reduces noise during drilling. 

Field studies were needed to confirm that a noise reduction could be obtained under working 

conditions and that the device was sufficiently durable. This paper reports results of field tests 

of the device conducted at five underground coal mines. Noise reduction was assessed by 

comparing the operator’s noise exposure during drilling with and without the drill bit isolator. 

Durability was assessed by recording the number of holes and total feet drilled with each bit 

isolator until either the test period ended or the device failed. The results from these tests 

showed that the device is an effective noise control in a mine environment. The field-tested 

drill bit isolators provided a noise reduction of 3-5 dB(A). Of nine devices tested for durability, 

five exceeded 610 m (2,000 ft) drilled and two exceeded 762 m (2,500 ft) drilled before failure. 

Durability issues found in the field tests led to final production optimizations that have resulted 

in a commercially available product for drilling with 35-mm- (1.3-in.-) diameter roof bits and 

hexagonal drill steels. 

Introduction 

The mining workforce experiences 

high levels of noise exposure and, in 

turn, suffers from high rates of noiseinduced hearing loss (NIHL). In fact, 

the mining sector has the highest prevalence of hazardous workplace noise 

exposures (76%) among all industrial 

sectors (Tak and Calvert, 2008). Despite 

engineering and administrative controls 

implemented to reduce noise, miners 

continue to exhibit a high prevalence 

(24%) of hearing difficulty (Tak et al., 

2009). Overexposure to noise is a significant concern for roof bolting machine 

(RBM) operators. The overall Aweighted sound level at the RBM operator’s location often exceeds 100 dB 

when drilling (NIOSH, 2006). At this 

sound level, RBM operators can quickly become overexposed to noise, reaching the U.S. Mine Safety and Health 

Administration (MSHA)’s permissible 

exposure level (PEL) in two hours and 

the U.S. National Institute of Occupational Safety and Health (NIOSH)’s 

recommended exposure limit (REL) 

in 15 minutes. Prior NIOSH research 

has shown that the noise radiated by 

the drill steel during the drilling process 

is the most significant contributor to the 

noise exposure of RBM operators (Peterson and Alcorn, 2007). To address 

this noise issue, NIOSH, Corry Rubber Corporation and Kennametal Inc. 

developed a drill bit isolator to reduce 

drill-steel-radiated noise when drilling 

with 35-mm- (1.3-in.-) diameter drill 

bits (Michael et al., 2010). 

The drill bit isolator was designed to 

control noise by limiting the transmission of vibrations from the drill bit to 

the drill steel.The device consists of two 

hollow steel cylinders with a rubber layer between them. For the current study, 

a preproduction prototype version of 

the device was tested. A schematic of 

the device is shown in Fig. 1. Figure 2 is 

a photograph of the device installed on 

a drill steel with a drill bit attached to 

the end. The rubber layer between the 

inner and outer cylinders isolates vibrations at the drill bit from the drill steel, 
thereby reducing the noise radiated from the drill steel. This 

layer is chemically bonded to the steel components to yield a 

high strength bond that can withstand the axial and torsional 

loads experienced during drilling. Figure 3 is a closeup view 

of the inner and outer members and the rubber layer that 

separates them.

Figure 1 

Diagram of the prototype drill bit isolator detailing the various 

components. 

Figure 2
	

Drill bit isolator installed with 35-mm (1.3-in.) drill bit and 

hexagonal drill steel. 

Figure 3 

Closeup of inner and outer members of the drill bit isolator, 

showing the location of the isolating rubber layer. 

The drill bit isolator has a drill steel coupling 

on one end and a bit coupling on the other. These couplings 

are welded to the ends of the inner and outer cylinders.There 

is a 10-mm (0.4-in.) gap at the end of the outer cylinder that 

is designed to allow for a small amount of relative movement 

between the layers as axial thrust loads are applied and removed.The gap acts as a safety, preventing axial overload by 

closing if the applied thrust exceeds the design limit of the bit 

isolator and rebounding to the original position when thrust 

is removed. Minor modifications based on the results of the 

field studies detailed here were incorporated into the final 

production version of the device. 

The drill bit isolator went through several design and 

testing iterations to achieve greater noise reduction and 

durability (Michael et al., 2011). The first prototype tested 

in the laboratory was approximately 25 cm (9.8 in.) long. 

The rubber layer in this first version was bonded to the inner cylinder during the molding process and bonded to the 

outer cylinder with a post-vulcanization bond. While this 

first version achieved noise reduction in laboratory testing, 

the bond between the rubber and the outer cylinder failed 

under torque and thrust loads during drilling in laboratory and field tests. In the next prototype, the length of the 

cylinder was extended to approximately 30 cm (12 in.) to 

spread the torque and thrust loads over a larger area, but the 

prototypes still experienced excessive failures of the postvulcanization bonds. Changing the type of bond in the next 

prototype iteration, so that the rubber layer was bonded to 

both the inner and outer cylinders during the molding process,solved the bond failure problem.This version of the drill 

bit isolator then began the first round of field testing for noise 

reduction and durability in a production setting. Although a 

noise reduction was measured, the outer cylinders on some 

of the devices cracked due to brittleness of the hardened 

metal. The hardness was reduced from 60 Rockwell C to 35 

Rockwell C to improve resistance to cracking. Reducing 

the hardness increases the fracture toughness, reducing the 

brittleness and likelihood of the device cracking. The prototype in the current tests combines the design modification 

of a 30-cm (12-in.) cylinder length, inner and outer molded 

bonds, and 35 Rockwell C hardness. 

Prior NIOSH research has shown that the drill bit isolator is effective at reducing drill-steel-radiated noise under 

limited controlled testing (Michael et al., 2010; Yantek et al., 

2011). Analysis of the 1/3-octave-band A-weighted sound 

level data acquired during this laboratory research revealed 

that the bit isolator primarily reduced noise above 1,600 Hz. 

However, additional field testing under a range of actual 

mining conditions was needed to demonstrate to potential 

manufacturers and mining operations that the device is a viable solution. Specifically, evidence of sustained noise reduction and durability was needed over a time period that would 

represent miners’ actual use of the device. These data, when 
combined with the previously obtained laboratory data, provide a solid case for the effectiveness of the drill bit isolator 

as a practical noise control. 

Methodology

This study’s objectives were to evaluate durability and 

noise reduction of the drill bit isolator across a range of mining conditions. Consequently, a multisite field methodology 

was used, in which the devices were deployed in working 

mines and operated by the sites’ miners in the course of their 

normal production work.Tests for noise reduction and durability were conducted over time to assess changes over the 

useful life of the device. 

Evaluations were conducted at five underground coal 

mines, designated as Mines A, B, C, D and E. Depending on 

access and agreements with the participating mine, evaluations at each site included either noise reduction or durability 

or both noise reduction and durability. The drill bit isolators in this study were preproduction models assembled by 

the manufacturer. Before use, the researchers inspected the 

devices and measured them to identify any deviations from 

design specifications, including axial runout that would indicate misalignment of the assembled components. 

Noise dosimetry. Roof bolting machine operator noise 

exposure was measured with a Larson Davis Spark noise 

dosimeter. Noise dosimetry data can be used to determine 

sound levels at any point in time or to determine noise exposure over a specific period of time. The dosimeter was set 

up to continuously record A-weighted sound levels at onesecond intervals.The one-second samples could then be compiled over any time period of interest to evaluate exposure. 

Prior to the start of the shift, the dosimeter microphone was 

attached to the midpoint of the operator’s shoulder that was 

closest to the drill steel. Exposure calculations derived from 

the dosimeter data were based on the MSHA permissible 

exposure level (PEL) of an eight-hour, time-weighted average (TWA) sound level of 90 dB(A) with a threshold of 90 

dB(A) and a 5-dB exchange rate. 

It is difficult to compare daily exposures due to drilling, 

because other factors can affect how much drilling an operator accomplishes. For instance, the number of holes an operator can drill during a shift can vary considerably from mine 

to mine and from shift to shift within a mine. Consequently, 

the same number of holes (100) was used as a consistent 

basis for computing TWAs, following the MSHA standard 

of a 5-dB exchange rate and 90-dB threshold. The 100-hole 

TWAs represent the full-shift sound level for a worker who 

drills 100 holes and has no other noise exposure. A worker 

who drills fewer holes will have less exposure and a worker 

who drills more holes will have higher exposure. To avoid 

contamination from other noise sources, data used for the 

exposure analysis came only from tests conducted with no 

other drilling or machines operating nearby. 

Time-motion study. To separate drilling from other activities, the NIOSH investigators conducted time-motion studies. 

A time-motion study consists of observations of workers’ 

tasks. The activities being performed and the times when 

they started and stopped were written into observation logs 

for later correlation with the recorded noise exposure data 

acquired with the dosimeter. The main task of interest during this study was the drilling of bolt holes. It was necessary to determine the drilling times that correlated with the 

dosimeter-collected data, so that a correct analysis of the 

noise from drilling could be attained. The researchers noted 

whether the drilling was performed with or without the drill 

bit isolator and whether there were any extraneous noise 

sources that would have affected the dosimetry measurements. In addition to the time-motion data, the height of each 

entry and changes in drilling conditions were documented. 

For instance, longer times required to drill each hole indicate 

drilling through harder roof strata. 

The NIOSH investigators instructed the machine operators on proper use of the bit isolator prior to testing. Few 

instructions were needed, because installation of the device 

was performed by the investigator and no changes were required to standard drilling procedures. The operators were 

cautioned not to strike the body of the isolator against hard 

surfaces, as is sometimes done with standard drill steels to 

dislodge a dust blockage, and the investigators monitored 

compliance with this instruction. Where possible, the researchers measured maximum thrust for the roof bolting 

machines used for bit isolator testing at the mines. Although 

it is not feasible to measure actual thrust applied during drilling, the maximum thrust measurement indicates the upper 

limit of thrust that can be applied. 

Durability evaluation. To determine the drill bit isolator’s 

true operational life, following the use of the device over time 

in working conditions is necessary. The durability evaluation 

of the device is based on the amount of time it remains operational and continues to reduce noise.The drill bit isolator 

was tested for durability on the RBMs by observing its functional life.The main durability indicators were the number of 

holes and feet drilled with each device before it sustained so 

much wear or damage that the operator could not continue 

drilling with the device installed. NIOSH has established durability goals of drilling at least 762 m (2,500 ft) or 500 holes 

before failure of the device.This goal was set to be equivalent 

to the expected life of the drill steel as reported by the initial 

test sites. Durability was assessed through visual inspection 

of the isolator for cracks, excessively worn surfaces, separated 

components (inner steel tube, elastomer layer, outer steel 

tube) and damage to the hexagonal drill steel and drill bit 

couplers. Nine drill bit isolators were followed over repeated 

use in the field to determine their functional lives at Mines B, 

C, D and E (Mine A was used only to collect the dosimetry 

and time-motion study data). The durability evaluation for 

each device continued until the device failed or until the end 

of the testing time allotted through the agreement with the 

participating mine. 

At Mines C and D, tests were performed at several points 

in time to determine if the drill bit isolator was maintaining its noise reduction properties throughout its operational 

life. At each of these test times, a series of holes was drilled 

with the isolator installed, then a series of holes was drilled 

without the isolator. These series of alternating holes were 

repeated across several days of testing at Mines C and D, using the same device to confirm that the noise reduction was 

consistent.Time-motion data were noted only for this limited 

number of holes, rather than all of the holes drilled during 
	 	

	 	

	 	

the shift. To minimize differences in mining conditions, these 

holes were drilled in the same entries and the tests were conducted with no other machines, including drills, operating. 

At the end of each testing session, the researchers retrieved and retained the drill bit isolator so its use could 

be accurately monitored and recorded. The test plan varied 

based on the intent of the evaluation (noise reduction or 

durability) and by mine-related factors such as type of machine in use, number of concurrent roof bolting activities and 

individual machine operator variations.Although evaluations 

occurred at five different mines across multiple days at each 

mine, some data were excluded from the analysis because of 

inconsistent test conditions. For the noise reduction assessments to be accurate, the test conditions need to be consistent 

for data acquired with and without the device. Specifically, 

large differences in drill penetration rates were noted that 

could have confounded the sound level data. These changes 

in penetration rate indicate changes in roof conditions, specifically the hardness of the strata through which the drill bit 

was cutting. Therefore, the reported data were limited to the 

comparison tests where mining conditions were consistent. 

Table 1
	

Dose per hole and reduction in the time-weighted average sound level (TWA) by mine and test number. 

Data are based on 100 holes drilled per device. 

Mine Test # Dose/hole without 

isolator (% of PEL) 

Dose/hole with 

isolator (% of PEL) 

Relative dose/hole 

reduction (%) with isolator 

PEL TWA 

reduction (dB(A)) 

Mine A Test 1

Device 1 0.50 0.34 32 2.8

Test 2

Device 2 0.43 0.21 51 5.2

Test 3

Device 2 0.80 0.60 25 2.1

Mine B 1.83 1.25 32 2.8

Mine C 0.69 0.45 35 3.1

Mine D 0.90 0.66 27 2.2

Test sites. The test sites included five underground coal 

mines in four states across the United States. A variety of 

sites were selected to evaluate the drill bit isolator under different mine entry sizes, degrees of roof hardness, bolt depths 

and operating techniques. Because of varying conditions and 

bolting techniques at the different mines, the RBM operators progressed at different speeds, taking more or less time 

to drill individual holes and to advance from one section to 

the next. 

Mine A was a longwall mine in the western United States 

with entries that averaged 3 m (10 ft) high by 6 m (20 ft) 

wide. An entry is an underground horizontal passage used 

for haulage, ventilation or as a main passageway. Data were 

collected in the longwall development section.The mine was 

using a dual-head RBM to drill 2.4-m- (8-ft-) deep bolt holes 

by stacking two 1.2-m- (4-ft-) long drill steels with the drill 

bit isolator installed on the first drill steel. Ventilation tubing 

was being installed as the drilling progressed through each 

entry. The left operator assisted with installing the ventilation tubing whenever he had completed his portion of the 

drilling and there was a time interval before more drilling 

was needed. Dosimetry and time-motion study data were 

collected at this mine. A durability evaluation of the drill bit 

isolator was not conducted at this mine. 

Mine B was a longwall operation in the eastern United 

States with entries that averaged 1.8 m (6 ft) high by 5 m 

(16.5 ft) wide. At this mine, the device was tested in entries 

as well as crosscuts.A crosscut is passageway driven between 

the entry and its parallel air course for ventilation purposes. 

At this mine, roof channels were used in entries and header boards were used in crosscuts. A roof channel is a steel 

strap secured across the roof of the mine with roof bolts as 

a temporary or permanent solution for roof deterioration. A 

header board is a wooden block that is held in place by roof 

bolts and increases the area over which the roof bolt forces 

are carried. When roof channels are used, additional noise 

can be generated whenever the bit or drill steel makes metalto-metal contact as they pass through the metal strap.The use 

of wooden header boards, on the other hand, does not generate additional noise, because they are installed with the roof 

bolt and not during the drilling process.The mine was drilling 

2.4-m- (8-ft-) deep bolt holes by stacking two 1.2-m- (4-ft-) 

long drill steels with the isolator installed on the first drill 

steel. The data at this mine were collected by one NIOSH 

researcher simultaneously observing two RBM operators. 

Noise dosimetry, time-motion study and durability data were 

collected at this mine during two separate evaluation trips. 

Mines C and D were both longwall mines in the eastern 

United States with entries that averaged 2.4 m (8 ft) high 

by 4.9 m (16 ft) wide. Both mines are owned by the same 

mining company and are geographically in close proximity. 

Data were collected in entries and crosscuts.A single drill bit 

isolator was evaluated at both mines.The mines were drilling 

2.4-m- (8-ft-) deep bolt holes by stacking two 1.2-m- (4-ft-) 

long drill steels, with the isolator installed on the first drill 

steel. Noise dosimetry, time-motion study and durability data 

were collected at these mines. 

Mine E was a room-and-pillar mine in the midwestern 
United States with entries that averaged 2.4 m (8 ft) high by 

6 m (20 ft) wide. Roof conditions varied greatly within this 

mine, at times requiring the installation of different length 

bolts in a single section. The bolt hole depths varied from 

1.2 to 1.8 m (4 to 6 ft). The variable conditions rendered 

any comparison of noise reduction performance unreliable; 

therefore, only durability data were collected at this mine, 

along with a time-motion study to document the number of 

holes and drilling conditions. 

Results 

Data collected at the five sites yielded results showing an 

initial noise reduction, sustained noise reduction over time 

and mixed durability performance. Results are separated by 

mine site because of differences in conditions and types of 

data collected. 

Initial noise reduction. The results of the noise dosimetry 

with time-motion study are shown in Table 1. The reported 

data represent the percentage of the overall PEL and relative 

changes in acquired dose. For consistency, the data are based 

on 100 holes drilled per test.At Mine A,two drill bit isolators 

were evaluated across three days. One set of data regarding 

noise reduction was acquired for one of the devices and two 

sets of data regarding noise reduction were acquired for the 

other device. For the device that was evaluated once, the dose 

per hole was reduced from 0.5% to 0.34% of the PEL (32% 

reduction), and the 100-hole TWA for drilling was reduced 

by 2.8 dB(A). For the first of two evaluations of the other 

device, the dose per hole was reduced from 0.43% to 0.21% 

of the PEL (51% reduction) and the TWA for drilling was 

reduced by 5.2 dB(A). The second evaluation of this device 

showed the dose per hole was reduced from 0.8% to 0.6% 

of the PEL (25% reduction) and the TWA for drilling was 

reduced by 2.1 dB(A). 

One drill bit isolator was evaluated at Mine B. Although 

data were collected in entries and crosscuts, only the data collected in entries are shown, because the crosscut conditions 

were too variable for accurate comparison of holes drilled 

with and without the isolator. When the isolator was used, 

the dose per hole was reduced from 1.83% to 1.25% of the 

PEL (32% reduction) and the TWA for drilling was reduced 

by 2.8 dB(A).Although the dose per hole at these two mines 

varied considerably before installation (from 0.43% of the 

PEL in Mine A Test 2 to 1.83% at Mine B), installing the 

isolator consistently achieved a reduction that ranged from 

25% to 51%. 

Noise reduction over time. Data collected at the beginning and end of testing at Mines C and D showed that the 

drill bit isolator maintained its noise reduction properties 

over time.The dose per hole measured during the initial tests 

at Mine C was 0.69% of the PEL without the isolator and 

0.45% of the PEL with the device installed (35% reduction), 

and the TWA for drilling was reduced by 3.1 dB(A). By the 

end of the time allotted for testing at Mine C, only 276 m 

(906 ft) had been drilled, so testing continued with the same 

device at Mine D. By the end of the test period at Mine D, 

the device had been used to drill a total of 253 holes and 628 

m (2,060 ft) and it was still fully functional.The dose per hole 

for measurements taken at the end of testing was 0.9% of the 

PEL without the drill bit isolator and 0.66% of the PEL with 

the isolator (27% reduction), and the TWA for drilling was 

reduced by 2.2 dB(A). 

Durability evaluation. Nine drill bit isolators in this study 

were evaluated for durability at four of the mines.Two of the 

devices exceeded the 762-m (2,500-ft) drilling goal before 

failure, two were still functional when testing was ended short 

of completing 762 m (2,500 ft) and five failed before reaching 762 m (2,500 ft). Table 2 shows the number of holes and 

feet drilled with each of the nine devices. 

Table 2
	

Number of holes and distance drilled for drill bit isolator durability evaluation.
	

Isolator Mine Holes Meters (ft) Status 

1 C&D 253 629 (2,060) Functional 

2 B 301 815 (2,674) Failed 

3 B 314 841 (2,760) Failed 

4 B 291 710 (2,328) Failed 

5 B 291 710 (2,328) Functional 

6 E 88 107 (352) Failed 

7 E 243 296 (972) Failed 

8 E 62 76 (248) Failed 

9 E 143 174 (572) Failed 

Drill bit Isolator 1 

was evaluated for durability at Mines C and D. After drilling 

253 holes and 628 m (2,060 ft), the time allotted for testing 

the device at the mines ended, although the 762-m (2,500-ft) 

drilling goal had not yet been reached. It is unknown whether 

the device would have completed 762 m (2,500 ft) if testing 

had continued. Isolators 2 through 5 were evaluated at Mine 

B. Isolators 2 and 3 were used on separate machines, while 

Isolators 4 and 5 were used concurrently on the same dual-
boom machine with different operators. Isolator 2 failed at 

301 holes and 815 m (2,674 ft) and Isolator 3 failed at 314 

holes and 841 m (2,760 ft). Isolator 4 failed at 291 holes and 

710 m (2,328 ft). Isolator 5 was used for drilling on the other 

side of the same machine as Isolator 4. Testing of Isolator 

5 was discontinued when Isolator 4 failed, at which time it 

had drilled the same 291 holes and 710 m (2,328 ft). It is unknown whether Isolator 5 would have reached 762 m (2,500 

ft) if drilling had continued.The three devices that eventually 

failed at Mine B (including the two that only failed after exceeding the goal of 762 m (2,500 ft) drilled) all suffered failure of the rubber layer.When the rubber layer fails,the inner 

and outer cylinders are no longer fixed in position and the 10-

mm (0.4-in.) gap at the base of the outer cylinder closes up, 

allowing the metal of the inner and outer portions to touch 

(Fig 4).

Figure 4
	 Gap between inner and outer cylinder of the drill bit isolator for normal function (left) and after bottoming out from high axial 

loading (right).
	

 Once these two parts come into contact, vibrations 

can no longer be isolated and noise reduction will cease, and 

the device should be replaced. 

Four devices (Isolators 6 through 9) were evaluated at 

Mine E. Observations at Mine E noted that the roof height 

and conditions were more variable than at the other mines. 

Drilling times were also more variable and longer than at the 

other mines, indicating the presence of harder roof strata. 

Observers also noted that operators at this mine often misaligned the drill steel several inches from the hole. Maximum 

thrust of one of the machines used for the drill bit isolator 

testing was measured. The measured maximum thrust exceeded the design limit of the bit isolator, but it is unknown 

whether the thrust applied during drilling with the isolator 

approached the maximum setting. RBM manufacturers recommend aligning the drill with the hole to prevent excessive 

side loads and keeping torque within design limits to prevent 

damage to drilling components. Isolator 6 failed at 88 holes 

and 107 m (352 ft); Isolator 7 failed at 243 holes and 296 m 

(972 ft); Isolator 8 failed at 62 holes and 76 m (248 ft) and 

Isolator 9 failed at 143 holes and 174 m (572 ft). 

Discussion 

The drill bit isolators tested in this study achieved noise 

reductions across a range of actual mine conditions and over 

time. Two of the devices exceeded the goal of 762 m (2,500 

ft) drilling and eventually failed due to a failure of the rubber bond in the device. A third device experienced the same 

rubber bond failure 52 m (172 ft) short of the 762-m (2,500-

ft) goal. Two other devices were still operating at the end of 

the time period allotted under the testing agreement with 

the mine. Although both had exceeded 610 m (2,000 ft) of 

drilling, it is unknown whether they would have reached 762 

m (2,500 ft). 

Four of the nine devices failed at welded joints between 

components. All of these failures occurred at Mine E, at 

which none of the devices reached 305 m (1,000 ft) drilled. 

Observers at Mine E noted slower drilling times, indicating 

the presence of harder strata, and that one operator misaligned the drill steel with the mine roof, which would have 

exerted moment loads on the drill steel and the connected 

isolator. Maximum thrust measurements indicated that the 

roof bolting machine used for testing at this mine was capable of applying thrust that would exceed the design limit 

of the drill bit isolator. Maximum thrust is typically set to a 

value at or below the maximum for the drilling components 

used. It is unknown to what extent these factors resulted in 

the specific device failures that occurred at this mine. 

Based on the results of the field tests detailed in this 

study, the manufacturer implemented a design modification 

to make the bit isolator more robust, while maintaining the 

vibration isolation performance. Inspection of the drill bit 

isolators that failed at Mine E indicated that failure occurred 

at the welded joints. The prototypes tested were hand-built 

out of machined components, some of which were welded 

together.When parts are joined, the joints can be a potential 

source of weakness, and misalignment of the components 

can occur. This appears to have happened in some of the 

preproduction samples, which were found to have excessive 

runout when measured in the laboratory. If the welding material is not a consistent thickness around the joint, the device 

will deviate around its axis of rotation. For production, the 

manufacturer eliminated most of the welds by machining 

components from continuous metal pieces wherever possible. 

The only remaining weld is between the outer cylinder and 
the bit coupling, which offers a large surface area for greater 

strength and easier alignment than could be attained at the 

smaller welded joints. 

Conclusions

Field testing of the drill bit isolator showed that it is an 

effective noise control and that the prototype version was 

sufficiently durable for use in some mines, but had correctable failures in other mining conditions. In three separate 

underground coal mines, the drill bit isolator achieved a noise 

reduction of 2-5 dB(A) and reduced the operator’s noise exposure per hole by 25-51%. These reductions are significant 

and useful for avoiding overexposure to noise. For example, 

a roof bolter operator who reaches a 100% MSHA PEL 

dose by 100 holes drilled could reduce his or her noise dose 

to 49-75% by using the drill bit isolator. The same operator 

could continue to drill 133-204 holes before reaching 100% 

noise dose. Noise exposure measurements over the life of 

the device showed that the noise reductions continued as 

long as the device remained functional. The amount of noise 

reduction varied from mine to mine and between different 

areas of a mine, possibly due to differences in the roof bolting 

machines, operator techniques and roof conditions.Although 

there will be some costs associated with implementing the 

drill bit isolator, reduced noise can be expected based on the 

results from this series of evaluations. 

That the drill bit isolator provided noise reduction across 

varying mining conditions is evidence that it is an effective 

noise control for a variety of underground coal mining conditions. The in-mine tests showed that the isolator maintained 

its noise reduction properties after drilling over 610 m (2,000 

ft), indicating that it is durable enough for use in the harsh 

conditions found in underground coal mines. However, the 

durability evaluation also revealed that failures could occur 

under certain mining conditions and operating techniques, 

which led to changes in the final design to make the isolator 

more robust. These changes included reducing the number 

of welds in favor of machining sections out of solid metal, inverting the device to reduce moment loads on the narrowest 

segments and reducing the axial runout and other tolerances 

through a more mechanized assembly process. 

Commercialization of the drill bit isolator has progressed 

based on the results of these field evaluations and subsequent 

design modifications. Modifications to the design of the isolator based on NIOSH laboratory and field testing results 

have been implemented. The modifications are intended to 

improve the durability of the device without any negative 

impact on the noise reduction capability. While the current 

design functions with 35-mm- (1.3-in.-) diameter roof bits 

and hexagonal drill steels, the manufacturing partners are 

also developing a version for use with 25-mm (1-in.) roof 

bits.Additionally,the manufacturers intend to provide a brief 

instructional document illustrating the correct use of the drill 

bit isolator, indicating that overthrusting, drilling off center 

and striking it against other hard objects may reduce the 

life of the device. The culmination of laboratory tests, field 

tests and modifications has led to a commercially available 

product from Corry Rubber Corporation and Kennametal 

that can reduce the noise exposure of roof bolting machine 

operators in underground coal mines. 

Disclaimer

The findings and conclusions in this report are those of 

the authors and do not necessarily represent the views of 

the National Institute for Occupational Safety and Health. 

Mention of any company or product does not constitute endorsement by NIOSH. 

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