A dual sprocket chain as a noise control for a continuous mining

machine1)

1)

The finding and conclusions in this paper are those of the

authors and do not necessarily represent the views of the

National Institute of Occupational Safety and Health.

Adam K. Smitha)

a)	 National Institute of Occupational Safety and Health,

Pittsburgh Research Laboratory, P.O. Box 18070, Cochrans Mill Road, Pittsburgh PA 15236; email:

eyv7@cdc.gov.

, Peter G. Kovalchikb), Lynn A. Alcornb) and Rudy J. Mateticb)

b)	 National Institute of Occupational Safety and Health,

Pittsburgh Research Laboratory, P.O. Box 18070, Cochrans Mill Road, Pittsburgh PA 15236.



Over-exposure to noise remains a widespread, serious health hazard in the U.S.

mining industry despite 25 years of regulation. Most other categories of illnesses

and injuries associated with mining have improved, with the exception of

hearing loss. In order to reduce cases of Noise Induced Hearing Loss (NIHL) in

the mining industry, retrofit acoustic treatments and controls are being

developed to subdue noise at the source. The Mine Safety and Health

Administration (MSHA) coal noise sample data collected from 2000 to 2005 has

determined that continuous mining machines rank first among all mining

equipment whose operators exceed 100% noise dosage. The continuous mining

machine conveyor, used to move coal from the cutting face to the rear of the

machine, has been identified as a dominant noise source. A dual sprocket

conveyor chain was tested as a potential solution. Sound power level

measurements conducted at the Pittsburgh Research Laboratory (PRL)

accredited reverberation chamber showed a 3 dB reduction in the A-weighted

sound power level when the dual sprocket chain was implemented. Underground

results show an 8-hour Time Weighted Average „TWA8 hrs... reduction a 3 dB for

continuous mining machine operators. Utilizing this newly developed noise

control, along with previously proven controls, provides continuous mining

machine operators an opportunity to be within the MSHA-Permissible

Exposure Limit (MSHA-PEL).



1 INTRODUCTION	 

Hearing loss caused by exposure to occupational

noise results in devastating hearing disability that is

virtually 100 percent preventable. Occupational NoiseInduced Hearing Loss (NIHL) has been recognized by

the National Occupational Research Agenda (NORA)

as the most common job-related disease in the United

States1

. This health issue is particularly ubiquitous in

the United States mining industry. “Hearing Loss or

Impairment” accounted for 15% of the total reported

cases of illness and disease in the mining industry for

20062

. An analysis of audiograms conducted by the

National Institute for Occupational Safety and Health

(NIOSH) shows that over half of the mining population

sampled had a hearing impairment by the age of 50,

while only 10% of those who are not exposed to

occupational noise experienced a hearing loss by the

same age3,4.

Despite 25 years of regulation, overexposure to

noise remains a widespread serious health hazard in the

U.S. mining industry. In 1999, the Mine Safety and

Health Administration (MSHA) altered its rules regarding noise exposure in an effort to reduce the incidence

of NIHL5

. Among other changes, the new regulations

required mine operators to use all feasible engineering

and/or administrative controls to reduce the noise

exposure of overexposed miners. However, for many of

the machines used in underground mining no noise

controls exist to reduce worker exposure below the

MSHA Permissible Exposure Level (PEL).

Due to the relatively large size of underground

mining equipment, NIHL is common in the mining

community. Comparing equipment used in underground coal mining operations, the Continuous Mining

Machine (CMM) accounts for the most noise overex­
posures. Examination of the MSHA coal database

shows that 65% of workers who were overexposed to

noise operated one of seven types of equipment, of

these the CMM produced the most noise overexposures

to workers of all surveyed equipment6

. NIOSH underground noise surveys have observed CMM sound

pressure levels as high as 109 dB(A), and operator

noise dose levels as high as 347%7

.

2 BACKGROUND

Underground mining methods are employed when

coal deposits lie deep below the surface of the earth.

Coal mine operations employs over 80,000 workers in

the United States, and approximately 60% work

underground8

. There are two primary approaches that

are used in underground coal mining: the longwall

mining method and the continuous mining method. The

longwall mining method utilizes a mechanized rotating

shearer that moves back and forth across the coal face,

which can be over 300 m wide. The continuous mining

method utilizes a CMM to divide the mine into a series

of 6-to-9 m “rooms” where work areas are cut into the

coal seam and “pillars” are left behind for roof support.

The longwall mining method is a more efficient means

to extract coal, but is not applicable for all economic

and geological situations. The continuous mining

method accounted for 173 million tons of coal mined in

the United States in 2007, just under half of the total

underground coal produced9

. This method of mining

remains one of the fundamental methods to extract coal

during mining operations, and the CMM is one of the

primary machines used for this type of mining.

The CMM is used to remove product from the

working face of underground mining operations. These

machines were first introduced in the late 1940’s and

greatly improved the efficiency and speed of the

mining process. The CMM has an armature attached to

a rotating drum that is fitted with carbide Tungsten

cutting bits, which gouge the mine face. Particulate

matter created from the cutting process that is not

suppressed by a water spray is collected by a vane-axial

fan system, called a dust scrubber. The fractured pieces

of mined material are scooped up by the gathering

arms, which are located underneath the cutting head.

Steel bars perpendicular to the conveyor chain, called

flight bars, span the width of the conveyor and move the

material to the rear of the machine. As the mined

product is dumped off the end of the conveyor, the

chain and flight bars traverse their path to the underside

of the machine via the tail roller. The operator typically

maneuvers the machine, by remote control, close to the

end of the conveyor. Finally, the mined material is

either picked up by a loader or dumped directly onto a

shuttle car where it is hauled out of the mine for

processing.

Overall noise generated by the CMM is a result of

several operational system mechanisms. The cutting,

dust collection, and conveying systems can be operated

independently, but are often all running simultaneously

during operation. The cutting system noise is a result of

the interaction between the cutting bits on the rotating

drum and the mine seam. The dust collection system

noise is the result of the vane-axial fan used to collect

fine particulate during cutting, which is usually located

opposite of the operator. The conveying system noise is

caused by impacts that occur between the conveyor

deck and flight bars, used to move mined material to

the discharge end of the machine.

Various research efforts have concentrated on reducing noise generated by CMM component systems.

Treatments developed to reduce noise generated by the

vane-axial fan in the dust scrubber have shown

promise10. However, noise levels produced by the dust

collection system are less than other component operations, which must be reduced first in order to obtain a

reduction in overall CMM noise levels11,12. Past studies

conducted by the Bureau of Mines (BOM) concentrated on the importance of noise associated with the

cutting system13. However, this assumed that the operator controlled the continuous mining machine via on

board controls, unlike the remote machine controls

typically used by industry today. Through the use of

remote controls operators are more directly exposed to

the noise generated by the conveying system, which has

been the focus of recent investigations12,14.

Efforts to reduce the overall noise levels produced

by the CMM must concentrate on engineering noise

controls for the conveying system. Noise generated by

the impacts that occur between chain flight bars and

conveyor components is particularly severe where the

chain traverses its path; at the gear sprocket (front of

machine) and tail roller (rear of machine)14,15. Early

research conducted by BOM concentrated on treatments to reduce CMM conveyor system noise16. Some

of these treatments showed promise in the laboratory,

but were never investigated for durability in an underground working environment. Recent studies have

examined treatments at the tail roller component to

reduce noise in this area due to the close proximity to

the operator location12,15,17. However, recent research

has shown that noise control treatments must also be

considered at the gear sprocket area at the front of the

machine.

3 APPROACH

A dual sprocket chain has been developed to address

CMM conveyor system noise at the chain/sprocket
interface. A conventional CMM uses a conveyor chain

that is driven by a four or five tooth sprocket, depending on the chain manufacturer. Pictures of a Joy Mining

Machinery2)

2)Mention of company name or product does not constitute endorsement by

the National Institute for Occupational Safety and Health.

conventional four tooth sprocket chain

and a prototype dual sprocket chain used during laboratory testing are shown Fig. 1.

Fig. 1—CMM prototype dual sprocket chain (a) at 

the front foot shaft and single sprocket 

chain (b) at the rear tail roller. 

The single sprocket chain

consists of an 83 mm pitch chain with 433 mm length

flight bars and a four tooth sprocket. The prototype dual

sprocket chain is driven by two eight tooth sprockets

that have the same pitch as the single sprocket chain. To

accommodate for the extra sprocket, the conveyor

flight bars on the dual sprocket chain have a shorter

length of 370 mm. Both chains were driven at a

measured speed of 2.4 m/ s.

Acoustic evaluations were conducted in the laboratory and in an underground working environment.

Original prototype versions of the dual sprocket chain

were built by Joy Mining Machinery, and acoustic

laboratory testing was performed at the NIOSH Pittsburgh Research Laboratory (PRL). Noise generated by

various conveyor conditions was examined for each

chain type. A modified Joy Mining Machinery 12CM-9

CMM was used during laboratory testing to examine

both chain types, shown in Fig. 2.

Fig. 2—Joy Mining Machinery 12CM9 in PRL 

reverberation chamber. 

Later, a production

version of the dual sprocket chain was manufactured

and installed on a CMM in an underground coal mine.

A similar model CMM with a single sprocket chain

was used to collect baseline operator noise exposure.

Task observations were also recorded to ensure that the

CMM operator work shifts would be comparable.

Sound Power Level measurements were used to

compare noise generated by the dual and single

sprocket driven chains. These measurements were

performed in the National Voluntary Laboratory

Accreditation Program (NVLAP) reverberation

chamber at PRL18,19. All sound power measurements

were conducted in accordance with ISO 3743-2 using

the comparison method with the equation:

r ( ̄  ̄ LW = LW + LP − LPr

) (1)

where  ̄ LW is the sound power level, LP is the spaceaveraged sound pressure level, and the subscript “r”

denotes the reference source. Fifteen microphone

locations were used to create the parallelepiped

measurement grid, and the sound pressure level data

were collected with a Bruel & Kjaer Pulse acquisition

system. The sound power level was calculated for each

one-third octave band and these values were logarithmically summed to calculate an overall sound power

level. If the dual sprocket demonstrated promise in the

laboratory then the next step would be to conduct an

underground field evaluation.

Underground field evaluations were utilized to determine the amount of acoustic noise that a CMM operator is exposed to during a work shift. Dosimetry was

used to evaluate the underground noise exposures of

CMM operators using single and dual sprocket

conveyor chains. A Larson Davis Spark 703 personal

noise dosimeter was used to collect worker exposure

data. Dosimeter parameters were set according to

MSHA specifications, which are displayed in Table 1.

Table 1—Parameters used for dosimeters to monitor noise exposure. 

Parameter Setting Designation

Weighting Function

Threshold Level

Exchange Rate

A

90 dB

5 dB

MSHA Permissible

Exposure Level

(PEL)

Criterion Level 90 dB

Response Slow

Upper Limit 140 dB



The microphone on the dosimeter was placed at the

midpoint of the CMM operator’s shoulder, with the

microphone diaphragm pointing up. The dosimeter was

installed and removed from the miner before and after

the mine mantrip.

Exposure data were collected over four total days

during two separate trips to the mine site. For both field
visits, the dual sprocket and single sprocket chain were

examined for a full work shift. On one day dosimetry

data were collected on the standard chain and on the

other day dosimetry data were collected on the dual

sprocket chain. The same model CMM, Joy Mining

Machinery 14CM-15, was used for all underground

measurements. However, different operators were used

because the underground tests were conducted over two

days and the same operators were not available. Typical

underground machine orientation and operator

positioning is shown in Fig. 3.

Fig. 3—Underground orientation of continuous 

mining machine. 

Note that the CMM

operator uses a wireless remote control and would

generally stand in the area shaded red. Time motion

studies were also conducted each day to guarantee

comparable machine usage. Operator noise exposure

was calculated in accordance with ANSI S12.19 using

the equations:

100T 

D = ( )2(Li

−Lc)/Q (2) Tc 

to calculate noise dose, and

Q ( D 

LTWA(8) = Lc + log10 ) (3) log10(2) 100

to calculate the eight-hour time-weighted average

sound level. In Eqn. (3), D is the noise dose, T is the

measurement duration, Tc is the criterion duration, Li is

the A-weighted sound pressure level, Lc is the criterion

level, Q is the exchange rate, and LTWA(8) is the eighthour time-weighted average sound level.

4 RESULTS

Acoustic measurements made in the laboratory

allowed various machine conditions to be examined. In

order to place the mined product in strategic locations,

the CMM has the ability to articulate its conveyor tail

section. Three CMM conveyor conditions (left,

straight, and right) for each chain type were examined

as shown in Fig. 4.

Fig. 4—CMM conveyor configurations: left, 

straight, right. 

The one-third octave band results of

the A-weighted sound power level testing with the tail

section straight is shown in Fig. 5.

Fig. 5—Sound power level comparison with the 

CMM tail section straight. 

The results shown in

Fig. 6 and Fig. 7 show the A-weighted sound power

levels for the tail section swung fully to the right and

left, respectively.

Fig. 6—Sound power level comparison with the
 

CMM tail section swung right.
 



The solid black lines in Figs. 5–7

depict levels for the single sprocket chain and the grey

lines represent levels for the dual sprocket chain. The

overall A-weighted sound power levels for each test

condition are summarized in Table 2.

By examining the sound power level measurements,

important observations can be made about how CMM

noise is generated. For the single sprocket chain with

the tail straight shown in Fig. 4, over 90% of the

A-weighted sound power generated occurs between
400 Hz– 5 kHz. The same general trend occurs when

the tail section is swung to the right and left in Fig. 6

and Fig. 7.

Fig. 7—Sound power level comparison with the 

CMM tail section swung left. 

The highest individual one-third octave

band level for all three test conditions occurred at

2 kHz. Reductions are observed in most one-third

octave band frequencies when the dual sprocket chain

was installed, with the most significant reductions

occurring at frequencies above 2 kHz.

Table 2—Overall results of sound power level tests 

in reverberation chamber. 

Conveyor Single Sprocket Dual Sprocket

Orientation Chain Chain

Conveyor Straight 118.5 dB(A) 115.5 dB(A)

Conveyor Right 121.2 dB(A) 116.2 dB(A)

Conveyor Left 119.1 dB(A) 117.1 dB(A)

The field evaluation was conducted in an underground coal mine in western Kentucky to examine

worker noise exposure. Two of the same model

machines (Joy Mining Machine 14CM-15) were used;

one with the dual sprocket chain installed and the other

with a standard single sprocket chain. The mine seam

height was approximately 2.5 m high and entries were

2 m wide by 5 m long. Underground CMM coal

production was around 2.7 KT for each shift that was

observed. The CMM operator accumulated noise dose

for each underground field evaluation is shown in Fig.

8 and Fig. 9.

Fig. 8—CMM Underground Operator Noise Exposure for Trip 1. 

Fig. 9—CMM Underground Operator Noise Exposure for Trip 2. 

The solid line indicates a work shift where

the operator used the machine with a standard single

sprocket chain, and the dashed line designates a work

shift where the dual sprocket machine was used. Basic

CMM operator task observation are also displayed in

Fig. 8 and Fig. 9 showing the relative time spent doing

a particular operation. The end of shift operator noise

exposure results for both underground field trips are

summarized in Table 3.

Table 3—Underground CMM noise exposure results. 

Chain

Type Trip

Work Shift

Length

Accumulated

Noise Dose TWA(8 hrs)
 

Single 1 10.3 Hrs 161% 93.4 dBA

Sprocket Chain 2 10.3 Hrs 157% 93.3 dBA

Dual Sprocket 1 9.3 Hrs 131% 91.9 dBA

Chain 2 10.3 Hrs 134% 92.1 dBA

Underground noise dosimetry data shows how

miners are exposed to CMM noise. While comparing

chains in Fig. 8 and Fig. 9, it can be seen that the

continuous mining machine operator experienced a

generally uniform dose accumulation throughout the

shift with occasional periods of low exposure as shown
by the flat slopes of the cumulative dose line. These low

dose periods occurred when the worker was changing

cutting bits or when the CMM was tramming. Further

observations revealed that less dose accumulation

occurred when the miner was in the man trip. High

dose periods occurred when the continuous mining

machine was cutting and conveying. It should also be

noted that the conveyor was continuously running

during all the high dose periods. Exposure levels with

the standard chain were relatively consistent with levels

of 157% for survey one and 161% for survey two. The

dual sprocket machine exposure levels were 130% for

survey one and 133% for survey two, which were

consistently lower. This resulted in a 8 hour Time

Weighed Average (TWA) reduction of over 1.2 dB(A)

for trip 1, and 1.5 dB(A) for trip 2.

5 DISCUSSION

Results of laboratory testing conducted in the PRL

reverberation chamber were examined to determine if

the dual sprocket chain was a feasible noise control.

Noise generated by the conveyor system was on

average 3 dB(A) less when the dual sprocket chain was

installed during laboratory evaluations. A standard

CMM conveyor chain is driven by a four or five tooth

sprocket gear, which can cause impacts to occur at the

front of the machine and interference with the Continuous Loading Arms (CLA). The dual sprocket chain

provides a smooth transition at the drive shaft that

reduces impacts and interference with the CLA. Also,

tension is kept more uniform due to the contestant

contact between the chain and 16 sprocket teeth.

Chain tension is an important variable when examining noise generated by the CMM conveyor system. The

A-weighted sound power generated by the dual

sprocket was 3 dB less than the standard chain when

the conveyor was in the straight position. However,

when the conveyor was extended fully to the right the

dual sprocket displayed a 5 dB reduction and only 2 dB

in the A-weighted sound power level when extended

fully left. These differences were most likely the result

of different chain tensions due to the coil spring

take-up system. As per the chain manufacturer specifications, chain tension could only be verified when the

tail section was in the straight position15 during laboratory testing. The coil spring take-up system was

replaced with a hydraulic cylinder that provided

uniform tension when the conveyor was swung to the

left and right during underground evaluations.

The underground operation of the continuous

mining machine is subjected to many variables that can

affect the noise emissions. Some of these variables,

such as the acoustic environment, geometry and

composition of the surfaces, mine geometry, and

compressive strength of the face media, can not be

controlled. The compressive strength of the face media

being cut affects the acoustic absorptive properties of

the mine environment. Harder media, with compressive

strengths above 140 MPa, reflect more acoustic energy

than softer media, i.e. less than 70 MPa compressive

strength. The operator who is responsible for controlling the CMM can also influence the overall noise level

generated. All of these variables need to be considered

when conducting underground field evaluations.

6 CONCLUSION

A dual sprocket conveyor chain for a CMM was

examined both in the laboratory and in underground

field studies. Laboratory results show that the dual

sprocket chain has a significant influence on lowering

the sound power emissions of the chain conveyor of a

CMM when compared to a standard chain. The underground results were not as significant for the dual

sprocket chain as in the laboratory, but did show a

reduction in noise exposure to the operator. From the

time motion study it was noted that there were several

variables that could have influenced the results. Some

of these were difficult to control such as the face media

when cutting (different compressive strength), different

operators, chain tension on the standard chain, and

different sections of the mine. However, it is believed

that the dual sprocket chain could be an effective noise

control if used with another noise control; such as

urethane coated flight bars12,15,16.

In general, experimental results show that the dual

sprocket chain has potential and if used in conjunction

with other noise controls could be effective. However,

implementing a dual sprocket chain on a CMM that is

close to the MSHA-PEL may bring the operator within

compliance with federal regulations. Utilizing this

noise control on a CMM will reduce the noise generated at the front (gathering end) of the machine. If

noise produced by other CMM conveyor components is

reduced through engineering controls, the dual

sprocket chain could have a greater impact on noise

reductions. Using a dual sprocket conveyor chain in

combination or as part of a suite of noise controls will

reduce noise generated by the CMM, and reduce the

occurrence of noise overexposures in the mining industry.

7 REFERENCES

1.	 National Institute for Occupational Safety and Health, “National Occupational Research Agenda”, DHHS (NIOSH) Publication No. 96-115, (1996).

2.	 National Institute for Occupational Safety and Health, “Distribution of illnesses & diseases reported to MSHA in 2006”,

[http://www.cdc.gov/niosh/mining/statistics/pdfs/pp15.pdf],

(2008).
3.	 National Institute for Occupational Safety and Health, “Preventing occupationalhearing loss—a practical guide”, Edited by

J. Franks, M. R. Stephenson and C. J. Merry, DHHS (NIOSH)

Publication 96-110, (1996).

4.	 J. R. Franks, “Analysis of Audiograms for a Large Cohort of

Noise-Exposed Miners”, Unpublished technical report.

NIOSH, (1996).

5.	 Department of Labor, Mine Safety and Health Administration,

“Health Standards for Occupational Noise Exposure: Final

Rule”, 30 CFR Part 62. Federal Register, (1999).

6.	 Title 30 CFR Part 62, 2000–2005, U. S. Department of Labor,

Mine Safety and Health Administration, Information Resource

Center, Denver CO.

7.	 E. R. Bauer, D. R. Babich and J. S. Vipperman, “Equipment

Noise and Worker Exposure in the Coal Mining Industry”, Information Circular 9492, National Institute for Occupational

Safety and Health, (2006).

8.	 Energy Information Administration Form EIA-7A, “Coal Production Report”, and U. S. Department of Labor, Mine Safety

and Health Administration, Form 7000-2, “Quarterly Mine Employment and Coal Production Report”, [http://

www.eia.doe.gov/cneaf/coal/page/acr/table21.html]

9.	 Annual Coal Report, Energy Information Administration, Office of Coal, Nuclear, and Alternative Fuels, U. S. Dept. of Energy, Washington, DC, (2007).

10. B. A. Carter, C. B. Burroughs and D. Armour, “Noise reduction

of a vane-axial fan for a continuous mining machine”, NoiseCon03, (2003).

11. D. S. Yantek, “Estimated Sound Power Radiated by Surfaces on

a Continuous Miner Tail Section Using Vibration Measurements”, NoiseCon03, (2003).

12. A. K. Smith, J. S. Peterson and P. G. Kovalchik, “Continuous

Mining Machine Conveyor System Sound Power Levels”, NoiseCon08, (2008).

13. W. W. Aljoe and R. C. Bartholomae, “Coal Cutting Noise Control”, Longwall, (1987).

14. H. E. Camargo, A. K. Smith, P. G. Kovalchik and R. J. Matetic,

“Noise Source Identification on a Continuous Mining Machine”, ASME NCAD, (2008).

15. P. K. Kovalchik, M. Johnson, R. A. Burdisso and F. Duda, “A

Noise Control for Continuous Miners”, 10th International 

Meeting on Low Frequency Noise and Vibration and its Control,

(2002).

16. J. A. Burks and W. J. Peterson, “Noise of Chain Conveyors in

Mining”, InterNoise86, (1986).

17. A. K. Smith, D. S. Yantek and J. S. Peterson, “Development and

Evaluation of a Urethane Jacketed Tail Roller for Continuous

Mining Machines”, ASME IMECE, (2007).

18. J. S. Peterson and R. C. Bartholomae, “Design and instrumentation of a large reverberation chamber”, NoiseCon03, (2003).

19. ISO 3743-2, Acoustics—“Determination of sound power levels

of noise sources—Engineering methods for small, movable

sources in reverberant fields—Part 2: Methods for special reverberation test rooms”.