DEVELOPMENT AND EVALUATION OF A URETHANE JACKETED TAIL ROLLER 

FOR CONTINUOUS MINING MACHINES 

Adam K. Smith 

NIOSH - PRL 

626 Cochrans Mill Road 

Pittsburgh, Pennsylvania 15236 

Email: ASmith9@cdc.gov 

David S. Yantek 

NIOSH - PRL 

626 Cochrans Mill Road 

Pittsburgh, Pennsylvania 15236 

Email: DYantek@cdc.gov 

J. Shawn Peterson 

NIOSH - PRL 

626 Cochrans Mill Road 

Pittsburgh, Pennsylvania 15236 

Email: JPeterson@cdc.gov 

ABSTRACT 

 Occupational noise-induced hearing loss continues to be 

one of the most pervasive health problems in the mining 

industry, despite over 25 years of regulation. One of the 

loudest pieces of equipment used in underground mining is the 

continuous mining machine. Noise sample data collected by 

the Mine Safety and Health Administration indicate that 42% of 

noise overexposures between 2000 and 2005 involved 

continuous mining machine operators. Previously conducted 

field and laboratory tests have determined that the continuous 

mining machine conveyor system is a dominant noise source. 

Loud impacts occur as the conveyor chain flight bars, used to 

move the mined aggregate, traverse their path from the top to 

the underside of the conveyor deck. Various noise control 

treatments have been applied to abate noise caused by the 

conveyor system. A durable polyurethane coating has been 

developed for the conveyor flight bars, resulting in a timeweighted average reduction of 3 dB(A) for an eight-hour work 

shift. In an attempt to further reduce continuous mining 

machine operator overexposures, a similar urethane coating has 

been applied to the tail roller component of the conveyor 

system. Laboratory results showed a 2 dB(A) reduction in 

sound power levels, but the component failed during 

underground durability testing. An outer steel sleeve has been 

added to the urethane coating of the tail roller to enhance wear 

resistance during mining. The urethane jacketed tail roller is 

the latest effort, combined with previous noise treatments, to 

bring the continuous mining machine into compliance with 

federal noise regulations. 

NOMENCLATURE 

 

Lw - sound power level 

Lp - sound pressure level 

a - acceleration 

La - acceleration level 

ω - frequency 

H(ω) - complex frequency response function 

X (ω) - frequency force input response 

Y(ω) - frequency acceleration output response 

H (ω) - FRF magnitude response 

Φ(ω) - FRF phase response 

Subscripts 

r - referring to the reference sound source 

n - number of measurement locations 

i - measurement number 

rms - root mean square average 

∗ - complex conjugate 

T - combined response 

This material is declared a work of the U.S. Government and is not subject to copyright protection in the United States.

Approved for public release; distribution is unlimited.
INTRODUCTION 

Hearing loss caused by exposure to occupational noise 

results in a disability that is particularly severe in the mining 

industry. Previous studies conducted by the National Institute 

for Occupational Safety and Health (NIOSH)∗

 have shown that 

approximately 90% of coal miners and 49% of metal/non-metal 

miners develop a hearing impairment by age 50 (based on an 

average hearing threshold level of 25 dB or greater for the 

1000, 2000, 3000, and 4000 Hz frequencies), while only 10% 

of those who are not exposed to occupational noise experience 

a hearing loss by the same age [1]. In addition, NIOSH studies 

of 17,260 audiograms for 2871 coal miners show that 80% of 

coal miners have a moderate to profound high-frequency 

hearing loss by age 64. 

The Mine Safety and Health Administration (MSHA) 

evaluated the effects of past legislation and issued an 

occupational noise exposure rule to help reduce hearing 

impairment in the mining industry [2,3]. This legislation, 

among other things, defined a Permissible Exposure Level 

(PEL), gave no credit for the use of personal hearing 

protection, and gave engineering and administrative controls 

supremacy to reduce worker exposure. Since the inauguration 

of this legislation the overall annual median noise dose for all 

underground coal mine workers has decreased by 24% [3]. 

However, the annual median noise dose for continuous mining 

machine (CMM) operators has remained unchanged. 

CMMs are one of the principal pieces of equipment used in 

underground mining, and they are also one of the loudest. 

Sound levels around these machines can range from 78 to 109 

dB(A), and operator noise dose can range from 44 to 347% [4]. 

Sixty-five percent of workers who were overexposed to noise 

operated one of seven types of equipment, according to MSHA 

Coal Noise Data collected from 2000 to 2002 [5]. Accounting 

for 35% of noise overexposures shown in Figure 1, CMMs 

produced the most noise overexposures of all surveyed mining 

equipment. A recent examination of this data shows that 42 % 

of all noise overexposures from 2000 to 2005 involved 

continuous mining machine operators [6]. 

 ∗

 The finding 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 

Figure 1: MSHA coal noise Sample Data - Percentage of 

Equipment Operators that Exceed 100% Dose 

BACKGROUND 

CMMs are used to cut, gather, and remove material from 

underground mining facilities. These machines have a series of 

rotating drums that are fitted with carbide cutting bits, which 

gouge the mine seam (known as the face). The cutting drum 

advances into the face and material falls to the floor. 

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 coal 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 aggregate to the rear of the machine. 

As the mined product is dumped off the end of the conveyor, 

the chain and flight bars are sent 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 mining facility for processing. 

CMM noise is generated by the combined contribution of 

several “independent” noise sources. These sources can be 

categorized by machine operation and are a result of the 

cutting, dust collection, and conveying systems. Past studies 

conducted under the Bureau of Mines (BOM) examined the 

relative contributions of cutting and conveying noise to the 

overall sound level, concluding that cutting was the dominant 

noise source [7]. However, in these studies the average sound 

level was examined in the reverberant field (to simulate inmine conditions) and did not include the sound level at the 

operator’s position. There have also been advances in 

designing more efficient cutting heads since the time of these 

studies. The dust collection system is a relatively new CMM 
component that generates high sound levels. Recent studies 

have examined treatments applied to the vane-axial fan of the 

dust collection system that reduced the overall A-weighted 

sound level by 8 dB in a laboratory environment [8]. 

Of all machine operation noise sources, the conveyor 

system is the largest contributor to CMM noise. A CMM with 

an unloaded conveyor produces sound levels between 104 dB 

and 108 dB under dry conditions, depending on chain tension 

[9]. This noise source is also of particular interest because of 

its close proximity to where the operator is typically positioned. 

Numerous research efforts conducted in the past have 

concentrated on reducing conveyor system noise. This noise is 

predominantly caused by the interaction that occurs between 

the chain flight bars and conveyor deck at the tail roller and 

gear sprocket regions. The low damping of the conveyor flight 

bars causes them to ‘ring’ when impacted. In response, a 

urethane coating has been developed that adds damping to the 

flight bars and reduces the energy transmitted to the conveyor 

deck. This noise control reduced the overall sound level by 5 

dB(A) and exposure by a time-weighted average (TWA) of 3 

dB(A) over an 8-hour work shift [9,10]. Constrained layer 

damping treatments have also been applied to the top, bottom, 

and sides of the conveyor deck to reduce impact and scraping 

noise caused by the flight bars [11]. Initially the constrained 

layer damping treatments showed a promising reduction of 2 

dB(A), but after one year of operation no significant reduction 

was observed. 

Treatments have also been applied to the tail roller of the 

CMM. Impact forces exerted on the tail shaft by the conveyor 

chain are transmitted as vibrations through the rest of the 

structure. Resilient materials have been placed between the 

chain and roller in an attempt to reduce these forces. Studies 

conducted under the Bureau of Mines investigated an isolated 

tail roller design, where an elastomer was bonded to the tail 

shaft and protected with a steel tube [11,12]. This treatment 

achieved a 2 dB(A) reduction in sound level at the operator’s 

position, but was never tested underground. Recently, a 

urethane coating, similar to the coating used for the coated 

flight bar design, was applied to the outer diameter of the tail 

roller [13]. This design also achieved a 2 dB(A) reduction, but 

failed underground due to high point contact loading from 

material under the chain. 

EXPERIMENTAL DESIGN AND ANALYSIS 

The CMM used for this study was a 14CM-9 manufactured 

by Joy Mining Machinery, shown in Figure 2. By design, the 

tail section of CMMs can swing 90° to the left or right off 

center in order to deposit mined material strategically during 

the mining of crosscuts. This machine arrangement is usually 

louder due to chain tension and the flight bars contacting the 

conveyor side plates. Thus, only the straight conveyor 

configuration is examined for each tail roller for comparison 

purposes. The chain tension was made constant for all cases by 

measuring the slack in the chain. The gear sprocket that drives 

the conveyor chain is invariable, making the chain speed 

constant. Also, a water spray of approximately 1 gal/min was 

applied to the conveyor to simulate the wet environment 

encountered during mining. 

Figure 2: 14CM-9 Continuous Mining Machine in 

Reverberation Chamber 

The urethane jacketed tail roller used in this study 

consisted of a standard tail roller machined down to 

accommodate for a 3/8-inch-thick layer of urethane that was 

bonded to a 1/4-inch-thick steel outer tube. The overall 

diameter of the urethane jacketed tail roller is one inch larger 

than the standard tail roller, which required the slide plate to be 

slightly cut. The properties of the urethane material can be seen 

in Table 1. 

Table 1: Material Properties of Urethane 

Polyurethane Material (PO 650) 

Durometer 84A 

Max Elongation 550% 

Compression 45% Max 

Ultimate Tensile Strength 6000 psi 

Sound power levels, vibration levels, and Frequency 

Response Functions (FRFs) were examined in this study to 

quantify the performance of the standard and urethane jacketed 

tail rollers. Sound power measurements were conducted in a 

large reverberation chamber. At the same time, vibration 

measurements were taken using 54 accelerometers mounted at 

various locations on the CMM tail section. The FRF data were 

obtained separately on only the tail roller component using a 

roving impact hammer and accelerometers. 

Sound pressure levels that CMM operators are exposed to 

result from the sound power radiated by the machine and the 
acoustic properties of the environment. Sound power, which is 

independent of the acoustic environment, can be used to 

compare noise treatments directly. Sound power testing for this 

study was conducted in the reverberation chamber at the 

Pittsburgh Research Laboratory [14,15]. All sound power 

measurements were in accordance with ISO 3743-2 using the 

comparison method with the equation: 

( r ) r pp ww −+= LLLL , (1) 

where is the sound power level of the reference sound 

source, and 

wr L

Lp is the space-averaged sound pressure level for 

the source under test. The sound pressure data were collected 

with a Bruel & Kjaer Pulse acquisition system using 15 

measurement channels. The results were A-weighted and 

calculated in one-third octave bands. 

Vibration levels were examined at 54 measurement 

locations on the CMM tail section for both tail rollers. The tail 

section was divided into six measurement areas: left flex plate, 

right flex plate, left side, right side, top deck, and return deck. 

Nine accelerometers were mounted at each measurement area 

using cyanocrylate gel. For the top deck measurement area, 

accelerometers were embedded into the deck by drilling 6-mmdiameter holes approximately 6 mm deep. In order to run the 

cables to the outside of the machine, grooves were made in the 

underside of the top deck. The acceleration levels were 

digitized, A-weighted, and filtered in one-third octave bands 

using a LMS PimentoTM analyzer. The RMS acceleration for 

each measurement area was calculated from the acceleration 

levels using the equation: 

∑=

=

n

i

i a

n

a rms

1

1 2 , (2) 

where is the acceleration at the individual measurement 

locations. The RMS acceleration levels were converted 

logarithmically using the equation: 

ai

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ ⋅= g

a L rms

a 1 log20 10 , (3) 

where is the average acceleration in one-third octave bands. 

Imp

La

act testing was performed on the both the standard and 

urethane jacketed tail rollers to gain insight about the structural 

modes of vibration. The tail roller component was 

disconnected from the CMM tail section and Styrofoam was 

used to simulate free boundary conditions during testing, as 

shown in Figure 3. Two accelerometers were installed on each 

end of the tail roller shaft using cyanocrylate gel. A PCBTM 

model 086C03 impulse hammer was used to excite the system 

and data were collected using the same LMS PimentoTM 

analyzer as mentioned above. The frequency response between 

the force inputs and accelerometer locations were found using 

the relationship [16]: 

( ) ( )( )

( ) ωω )(

ωω

ω

XX

YX H ∗

∗

= , (4) 

where (ω) ∗ X is the complex conjugate frequency response of 

the imp mer, and act ham Y(ω) is the frequency response of the 

accelerometers. The m ude and phase responses were 

translated from the complex FRF data using the equations 

[16,17]: 

agnit

( ) [ ] ( ) [ ] ( ) 2 2 H ω = Re H ω + Im H ω

and 

( ) [ ] ( )

[ ] ( ) ⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ =Φ −

ω

ω

ω

H

H

Re

Im

tan 1 , 

(5) 

Three impact locations were utilized on the tail roller at: the 

midpoint of the outer sleeve, four inches to the left of the 

midpoint, four inches to the right of the midpoint. The impact 

locations were chosen based on where the conveyor chain is in 

contact with the tail roller. The FRF magnitude data were 

combined logarithmically using the equation: 

HT (ω) ⋅= log20 10 ( ) Hi( ) ω , (6) 

where (ω) HT is the average response for the tail rollers. 

Also, coherence between force inputs and acceleration outputs 

were also examined to ensure linearity between signals. 
Figure 3: Impact Test Setup for Tail Roller Component 

RESULTS AND DISCUSSION 

ed to determine the modal 

shap

The FRF data were examin

es and frequencies of the treated and untreated tail rollers. 

The magnitude response from the impact testing is shown in 

Figure 4. The resonant peak of the urethane jacketed tail roller, 

just below 6 kHz, is 6 dB lower than that of the standard tail 

roller. While the magnitude frequency response at 7 kHz has 

been eliminated, the energy appears to have shifted to the next 

higher mode for the treated tail roller. A similar phenomenon is 

observed at the lower frequency mode below 2 kHz. 

Figure 4: Magnitude Frequency Response of Tail Roller 

he results of the sound power tests conducted in the 

reve

asurements were taken during the sound 

pow

Components 

T

rberation chamber are displayed in Figure 5. The results 

show levels above 100 dB(A) at the one-third octave band 

frequency between 400 Hz and 3,150 Hz. This accounts for 

85-90% of the sound energy emitted by the CMM. The 

urethane jacketed tail roller sound power test shows modest 

reductions of around 1 dB(A) for all one-third octave band 

frequencies except at 160 Hz, which exhibited a 2 dB(A) 

increase. The overall sound power level of the CMM with 

standard tail roller is 116 dB(A), and 115 dB(A) with the 

urethane jacketed tail roller installed. This translates to an 

overall reduction of just 1 dB(A) in sound power level when 

the urethane jacketed tail roller was installed, which will be 

explained below. 

Vibration me

er testing in the reverberation chamber. The measurements 

were divided into 6 component areas on the CMM and are 

displayed in Figures 6-11. The results show a similar pattern to 

the sound power level spectra, with the highest acceleration 

levels occurring between 400 Hz and 5 kHz. A possible 

resonant peak is observed at the 800 Hz band for the left and 

right side measurement locations, shown in Figures 6 and 7. 

The top deck, shown in Figure 8, demonstrates the largest 

overall reduction in acceleration level of 3 dB(A) with the 

urethane jacketed tail roller installed. The return deck 

measurement area shown in Figure 9 displays highest overall 

acceleration level of 19 dB(A) with the standard tail roller 

installed. 

Figure 5: A-weighted Sound Power Level Comparison of 

CMM Standard and Urethane Jacketed Tail Rollers 

Figure 6: A-weighted Acceleration Level at CMM Left 

All vibration measurement areas on the CMM show an 

over

Side 

all reduction in A-weighted acceleration levels of at least 1 

dB using the treated tail roller, except the left flex plate. Figure 

10 shows how the acceleration levels in each one-third-octave 

frequency band at this measurement area increased after the 

jacketed tail roller was installed. At the left flex plate 

measurement segment an overall increase of 5 dB acceleration 

is observed. This is not seen at the right flex plate 

measurements displayed in Figure 11, where an average 
reduction of 4 dB is observed in one-third octave band 

frequencies greater than 2 kHz. 

Figure 7: A-weighted Acceleration Level at CMM Right 

Side 

Figure 8: A-weighted Acceleration Level at CMM Top 

Deck 

To better quantify the performance of the urethane jacketed 

tail roller, the FRF, sound power, and vibration measurements 

can be analyzed together. The higher frequency reductions 

shown in the magnitude response of Figure 4 are annulled by 

modal shifts and increases at lower frequency. This somewhat 

explains the slight increases in the sound power levels at the 

frequencies below 500 Hz. However, this study only examines 

A-weighted levels, which should limit low frequency 

contributions to the overall level. As stated above, the left flex 

plate measurement area showed an increase in acceleration 

levels. As the chain travels down the conveyor the flight bars 

make contact with the flex plates. These 1⁄4-inch-thick steel 

plates guide the chain along the conveyor deck on each side. 

Although chain tension was consistent for all tests, there is no 

mechanism to ensure that the chain is traveling straight down 

the conveyor deck. It appears from the vibration data shown in 

Figure 10 and sound power data shown in Figure 5 that the 

chain flight bar contacting the left plate is radiating noise. This 

would explain why reductions in sound power levels are not 

achieved at the higher frequencies with the urethane jacketed 

tail roller installed. 

Figure 9: A-weighted Acceleration Level at CMM Return 

Deck 

Figure 10: A-weighted Acceleration Level at CMM Left 

Flex Plate 
Figure 11: A-weighted Acceleration Level at CMM Right 

Flex Plate 

CONCLUDING REMARKS 

A treated tail roller has been developed in order to reduce 

sound emission from a CMM. The urethane jacketed tail roller 

was developed based on previous noise control treatments and 

consists of a standard tail roller with a layer of urethane bonded 

to a steel outer shell. This noise control is the latest attempt to 

reduce noise exposure of CMM operators. Sound power levels, 

FRF, and vibration data were examined to quantify the 

performance of the treated tail roller. A modest overall 

reduction of 1 dB(A) in sound power level was achieved with 

the urethane jacketed tail roller. However, the urethane 

jacketed tail roller could be combined with proven noise 

controls to further reduce noise exposure caused by CMMs. 

Examining FRFs and acceleration levels gives insight as to why 

higher reductions were not achieved. The magnitude response 

from impact hammer testing indicated insufficient reductions 

and shifting of modes toward low frequencies. The urethane 

layer could be made with a lower durometer that may improve 

low frequency performance, but could also reduce durability. 

The accelerometers positioned on the CMM left flex plate 

reveal that the chain flight bars may be making contact with the 

plate, causing this component to radiate noise. Further 

investigation is needed to determine how this possible noise 

source is contributing to the overall conveyor system noise. 

Future studies will examine flex plate noise and using different 

materials for the resilient layer between the tail roller and steel 

outer shell. 

REFERENCES 

[1] J. R. Franks, “Analysis of audiograms for a large cohort of 

noise-exposed miners,” National Institute for Occupational 

Safety and Health, Internal Report, Cincinnati, OH, 1996, 

7 pp. 

[2] Federal Register, Health Standards for Occupational Noise 

Exposure in Coal, Metal, and Nonmetal Mines; 

Final Rule. 30 CFR Parts 56 and 57 et al., Vol. 64, No. 

176. U.S. Department of Labor, Mine Safety and 

Health Administration, September 13, 1999, pp. 49548-

49634. 

[3] G. J. Joy and P. J. Middendorf, “'Noise Exposure and 

Hearing Conservation in U.S. Coal Mines - A Surveillance 

Report,” Journal of Occupational and Environmental 

Hygiene, 4(1), 26-35 (2007). 

[4] 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, December 

2006, 77 pp. 

[5] Title 30 CFR Part 62, 2000-2002, U.S. Department of 

Labor, Mine Safety and Health Administration, 

Information Resource Center, Denver Co. 

[6] Title 30 CFR Part 62, 2000-2005, U.S. Department of 

Labor, Mine Safety and Health Administration, 

Information Resource Center, Denver Co. 

[7] Bartholomae, R. C., John Kovac, and Jack Robertson, 

“Measuring Noise from a Continuous Mining Machine,” 

Information Circular 8922, United States Dept. of the 

Interior, Bureau of Mines, 1983, 71 pp. 

[8] B.A. Carter. Noise Reduction of a Vaneaxial Fan for a 

Continuous Mining Machine. Thesis (M.S.) Pennsylvania 

State University, 2003, 200 pp. 

[9] P. Kovalchik, M. Johnson, R. Burdisso, F. Duda, and M. 

Durr, “A Noise Control for Continuous Miners,” 

Proceedings from the 10th International Meeting on Low 

Frequency Noise and Vibration and its Control, York, 

England, September 11-13, 2002, pp. 299-306. 

[10] A.K. Smith, E.R. Spencer, L.A. Alcorn, P.G. Kovalchik, 

“Underground Evaluation of Coated Flight Bars for a 

Continuous Mining Machine, “ Proceedings of InterNoise 2006, Honolulu, Hawaii, December 3-6, 2006, 8 

pp. 

[11] A. Galaitsis, R. Madden, and D. Andersen, “Noise 

Reduction of Chain Conveyors, Volume II,” A mining 

research contract report, no. H0155133, U.S. Dept. of the 

Interior, Bureau of Mines, Washington, D.C 1983. 

[12] A. Galaitsis, R. Madden, and D. Andersen, “Noise 

Reduction of Chain Conveyors, Volume I,” A mining 
research contract report, no. H0155133, U.S. Dept. of the 

Interior, Bureau of Mines, Washington, D.C 1983. 

[13] David S. Yantek, “Estimated Sound Power Radiated by 

Surfaces on a Continuous Miner Tail Section Using 

Vibration Measurements,” Proceedings of Noise-Con 

2003, Cleveland, Ohio, June 23-25, 2003, 9 pp. 

[14] J.S. Peterson and R.C. Bartholomae, “Design and 

instrumentation of a large reverberation chamber,” 

Proceedings of Noise-Con 2003, Cleveland, Ohio, June 

23-25, 2003, 8 pp. 

[15] 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. 

[16] A. Barber, Handbook of Noise and Vibration Control, 6th 

ed., Elsevier Science, Oxford, United Kingdom, 1992. 

[17] M. Norton and D. Karczub, Fundamentals of Noise and 

Vibration Analysis for Engineers, 2nd ed, Cambridge 

University Press, New York, New York, 2003.