Underground evaluation of coated flight bars 

 for a continuous mining machine. 

Adam K. Smitha, Ellisworth R. Spencerb, Lynn A. Alcornc, 

Peter G. Kovalchikd

NIOSH/Pittsburgh Research Laboratory 

626 Cochrans Mill Road 

Pittsburgh, Pennsylvania 15236 

USA 

ABSTRACT 

Noise induced hearing loss (NIHL) is the most common occupational disease among workers 

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 have a hearing impairment by age 50. The Mine Safety and Health 

Administration (MSHA) has determined that continuous mining machines rank first among all 

equipment in underground coal mining whose operators exceed 100% noise dosage. The 

conveying system is one of the principal noise sources on continuous mining machines, due to 

metal on metal impacts that occur between chain flights and the conveyor deck. A highly 

durable polyurethane coating has been developed for the chain flights to decrease noise 

generated by these impacts. A continuous mining machine retro-fitted with coated flight bars 

has achieved overall sound level reductions of 5-7 dB in a laboratory setting. This paper 

evaluates the effectiveness of this engineering noise control in reducing the noise exposure of 

continuous mining machine operators in an underground coal mine environment. The results 

show that the operator of a continuous mining machine utilizing the coated flights receives a 

predicted 35% reduction in total work shift noise dose. 

1 INTRODUCTION 

Noise-induced hearing loss is the second most common form of sensorineural hearing deficit, 

after presbycusis (age-related hearing loss) [1]. Hearing loss caused by exposure to occupational 

noise results in devastating hearing disability that is virtually 100 percent preventable. 

Occupational noise-induced hearing loss (NIHL) has been recognized by the National 

Occupational Research Agenda (NORA) as the most common job-related disease in the United 

States. An analysis of audiograms conducted by NIOSH shows that nearly 90% of coal miners 

and 49% of metal/non-metal miners 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 age 

[1]. 

Overexposure to noise remains a health concern in the mining industry, in spite of 25 years 

of regulation. Sixty five percent of workers who were overexposed to noise operated one of 

a

 Email address: Asmith9@cdc.gov 

b

 Email address: efs9@cdc.gov 

c

 Email address: lyn3@cdc.gov 

d

 Email address: pdk0@cdc.gov 
seven types of equipment, according to MSHA Coal Noise Data collected from 2000 to 2002 [1]. 

Accounting for 35% of noise overexposures shown in Figure 1, the continuous mining machine 

produced the most noise overexposures of all surveyed mining equipment. Federal regulations 

require that a mine operator enroll workers in a hearing conservation program if they are exposed 

to an average sound level of 85 dB(A) or more during an eight-hour period. Furthermore, if a 

worker is exposed to more than the Permissible Exposure Level (PEL) of 90 dB(A), then the 

mine operator is required to implement all feasible engineering and administrative controls [2, 

3]. 

Past research conducted at NIOSH’s Pittsburgh Research Laboratory (PRL) has concentrated

on developing engineering noise controls to abate unwanted sound caused by continuous mining 

machines. A coated flight bar design has been developed to suppress sound emissions caused by 

the continuous mining machine conveying system [4]. This paper presents results from

underground case studies that examined the effectiveness of this noise control. Specifically, this 

paper evaluates the reduction of sound exposure during continuous mining machine operations 

using MSHA defined exposure limits. An underground comparison of A-weighted sound levels 

before and after the coated flight bar chain installation will also be examined. 

Figure 1: MSHA coal noise sample data - Percentage of equipment operators that exceeded 100% dose.

2 BACKGROUND

Continuous mining machines are one of the fundamental machines used to cut, gather, and 

remove coal and non-coal minerals from underground room and pillar mining operations. Each 

of these processes generates sound, which is the result of three subsystem machine components: 

the cutting-head drum, the conveying system, and the dust collection system. NIOSH, through 

its partnerships, has developed a novel engineering noise control to attenuate sound levels that 
are emitted from the conveying system. Steel bars that are perpendicular to the conveyor chain, 

called flights, span across the width of the conveyor and move the coal aggregate to the rear of 

the machine. A urethane coated flight bar has reduced the overall sound levels by 5-7 dB(A) in a 

laboratory setting [4]. Past underground investigations have evaluated the durability and sound 

level reductions of the urethane-coated conveyor flight bars [5]. However, this control has never 

been evaluated to determine the underground reductions in noise exposure to the machine 

operator. This study examines the underground performance of this noise treatment using 

dosimetry, as well as sound pressure level measurements. 

Underground working environments create complex sound fields that make it challenging to 

determine the effectiveness of noise controls. Mine geometry, rock composition, and other 

working machines are variables that are difficult to quantify, and influence the observed overall 

sound levels. Thus, dosimetery can be used to evaluate the effectiveness of noise treatments in 

an underground environment. MSHA defines that 100% noise dosage is equivalent to a 

continuous noise exposure of 90 dB(A) for 8 hours, also known as the PEL. Using the MSHA 5 

dB exchange rate, an average exposure of 95 dB(A) for four hours or 100 dB(A) exposure for 

two hours would yield a 100 % dose. Whereas, NIOSH defines a Recommended Exposure Limit 

(REL) for an 8 hour work shift as a Time-Weighted Average (TWA) of 85 dB(A) using a 3 dB 

exchange rate. 

 Underground mine workers, specifically continuous mining machine operators, are exposed 

to large amounts of acoustical noise. The source(s) of the noise that the workers are exposed to 

can originate from a host of different underground machinery. Worker exposure is also timevarying depending on the operation/task that is being performed. The contributions to an 

operators overall dose from multiple tasks can be estimated by determining the dose accumulated 

during specific operations. This approach is known as sound exposure profiling [6, 7]. Once the 

equivalent sound exposure levels have been determined for each task, the TWA can be estimated 

for an 8-hour work shift. This study utilizes a similar procedure to model a typical work shift for 

a continuous mining machine operator, based on the length of time the operator is performing a 

specific task. A model is computed and compared for both non-coated and coated conveyor 

flights to evaluate the performance of the noise treatment. 

3 PROCEEDURE 

Dosimetry determines the amount of acoustic noise that a worker is exposed to during a work 

shift. A Quest TechnologiesTM Q-400∗

 dosimeter was chosen for this research because of its 

durability and underground permissibility. The dosimeter parameter settings were set to NIOSH 

and MSHA specifications, and are displayed in Table 1 [8]. The dosimeter determines the 

equivalent continuous A-weighted sound pressure level for a sample interval of 10 seconds. 

Then, knowing the allowed MSHA exposure time for the equivalent sound level, the incremental 

noise dose is determined. The incremental dose and TWA for the sample interval are stored in 

the device memory. The NIOSH settings were used for comparison with other past underground 

field investigations, since they capture sound levels below 90 dB(A). 

 

∗

 Product(s) mentioned are not endorsed by NIOSH or authors 
Table 1: Internal dosimeter settings. 

- Dosimeter 1 Dosimeter 2

Parameters Settings Designation Settings Designation 

Weighting A A 

Threshold Level 40 dB 90 dB 

Exchange Rate 3 dB 5 dB 

Criterion Level 85 dB 90 dB 

Response Slow Slow 

Upper Limit 140 dB 

NIOSH 

 Wide-Range 

Exposure Level 

140 dB 

MSHA 

Permissible 

Exposure Level 

(PEL) 

Insight can be gained as to how the operator accumulates dose by utilizing time motion 

analysis. The subject is fitted with a dosimeter at the beginning of the work shift, and his activity 

is recorded on an event basis. When a task or phenomenon occurred, the observer logged the 

start and stop time, which was synchronized with the internal dosimeter clock. A Personal 

Digital Assistant (PDA) could not be used to facilitate this process as done with past NIOSH 

time motion studies [9], due to underground test equipment permissibility. Note that this method 

allows more than one event to occur and be logged at the same time. The total time spent for 

each operation was summed throughout the day, and averaged with other comparable days. The 

time motion data was used to compute the average dose from each task that the continuous 

mining machine operator was exposed to during the work shift. 

4 RESULTS 

All measurements were obtained at a single underground coal mine. Prior to the work shift, 

the continuous mining machine operator was fitted with a dosimeter. The dosimeter was 

calibrated prior to and after each shift measurement, and was attached to the midpoint of the 

operator’s shoulder with the microphone diaphragm pointing up [8]. The dosimeter, machine 

type, manufacturer, model, and serial number were documented using hand written notes. The 

same continuous mining machine and operator were used for all underground measurements. 

The time that the dosimeter was installed and removed was also documented. Digital pictures 

were taken to document the installation of the coated conveyor chain flights, as shown in Figure 

2. 

Prior to the dosimeter survey, sound level measurements were taken to further assess the 

performance of the treated flight bars. A Quest TechnologiesTM 2900 sound level meter∗

 with Aweighting and a slow time constant was used to compute the sound levels over ten-second 

averages. A 16 by 3 meter grid was constructed in middle of a 45 meter underground mine entry 

way. Measurements were taken on the grid at 1 meter intervals, at a horizontal plane 30 

centimeters above the continuous mining machine. This was done for conveyor chain flight bars 

with and without the urethane coating. The continuous mining machine was run empty for both 

cases (no coal collection). The first 8 by 3 meters of the grid are displayed in Figure 3, and 

 

∗

 Product(s) mentioned are not endorsed by NIOSH or authors 
demonstrate a sound level reduction of 5-7 dB(A) between non-coated (top plot) and coated 

(bottom plot) conveyor flight bars. 

Figure 2: Coated conveyor flight bars on a continuous mining machine. 

An underground mining facility is a harsh environment with many variables that must be 

taken into consideration when attempting acoustic measurements. Production, safety, roofbolting operation, and unexpected machine failure all had to be taken into consideration for this 

study. Due to variable conditions in the underground mining environment, only three surveys 

could be completed. Each noise survey comprised a full work shift that began as soon as the 

machine operator was fitted with the dosimeter. Two days of non-coated and one day of coated 

conveyor flight bar data were collected at the underground mining facility. Mechanical failure of

the conveyor chain (not caused by the noise treatment) during the coated flight bar survey further 

complicated a direct portal to portal worker dose comparison. The accumulated noise dose of the 

continuous mining machine operator for each work shift can be seen in Figure 4. For the first 

work shift (solid-line) the continuous mining machine was performing overcast and floor cutting 

maintenance to an already established part of the mine. Data on the second (dashed-line) and 

third (dotted-line) work shifts was obtained at the working face of the mine. The coated flight 

bar conveyor (dotted-line) shows a promising initial reduction in noise dose compared to the 

non-coated flight bars (dashed-line and solid-line). However, sound exposure profiling must be 

used to objectively evaluate any reductions that are the result of the coated flight bars. 

A time motion study was conducted in conjunction with the dosimeter measurements to 

evaluate the reduction in exposure achieved by the coated flight bars. Hand written notes were 

used to categorize different continuous miner operator tasks, as explained above. Table 2 shows 

the results of the sound exposure profiling for both non-coated and coated flight bars. 

Events/Tasks of the continuous mining machine operator were divided into five categories: man 

trip down, preparation time, cutting/conveying, lunch, and man trip out. For each work shift 

category the average task time, dose rate, and accumulated dose were computed. The time spent 

performing each task was averaged between all sampled work shifts to obtain an average task 

time. The dose rate was found by summing the noise dose associated with each operator task 
using the time motion notes, and dividing by the total time for that task. The dose rate was then 

used to determine the amount of accumulated dose the operator receives for each task based on 

the average task time. 

-4 m -3 m -2 m -1 m 0 m 1 m 2 m 3 m

0 m

-1 m

1 m

Non-Coated Conveyor Flight Bars

Coated Conveyor Flight Bars

104

107

102

105

108

105

106

108

105

106

106

105

104

104

105

103

103

102

102

102

102

101

101

101

97

102

95

97

101

97

97

103

98

97

100

98

96

98

97

96

97

96

95

95

95

94

94

94

0 m

-1 m

1 m

-4 m -3 m -2 m -1 m 0 m 1 m 2 m 3 m

Figure 3: Non-coated vs. coated A-weighted sound levels. 

Figure 4: Accumulated operator work shift noise dose. 

Table 2: Results of Sound Exposure Profiling.
Operator Task Average Task Time

(min.) 

Dose Rate

 (%/min) 

Accumulated Dose

(%) 

Man-Trip Down 43.3 0.15 6.34 

Prep. Time 105.0 0.05 5.30 

Non-Coated 

Cutting\Conveying 

234.0* 0.50 117.15

Coated 

Cutting\Conveying 

177.2** 0.30 51.89

Lunch 34.2 0.03 1.10

Man-Trip Out 37.2 0.24 9.00 

 * Average for two shifts

** Total cutting\conveying time for one shift until equipment breakdown

The results from sound exposure profiling were used to “model” reductions achieved by the 

coated conveyor flight bars. The average time spent performing each task was multiplied by the 

dose rate and combined to simulate the total dose that the continuous mining machine operator 

would receive during a typical work shift. A “model” to compare sound exposure of the 

continuous mining machine operator between coated and non-coated flight bar conveyors is 

shown in Figure 5. The projected accumulated dose of the non-coated conveyor chain case 

(solid-line) was found to be 178%, while the projected accumulated dose of the coated conveyor 

chain case (dashed-line) was reduced to 114%. This corresponds to a predicted 3 dB exposure 

in a full shift TWA using the MSHA exposure criteria. 

Figure 5: Projected operator work shift noise dose. 
5 CONCLUSIONS 

An underground investigation of the coated flight bar noise control was conducted on a 

continuous mining machine using sound pressure level and dosimeter measurements. A grid was 

constructed to measure sound levels at\around the continuous mining machine. These 

measurements showed a significant reduction of 5-7 dB near the cutting face of an underground 

coal mine. Dosimetry analysis was used to evaluate the noise exposure of continuous mining 

machine operators. The data was also used as a means to evaluate the performance of the coated 

flight bars. The dose rate of the operator was computed for various work tasks. The average 

amount of time spent doing these tasks was calculated, and an average work shift profile was 

formulated to compare exposure levels for both the coated and non-coated conveyor flight bar 

cases. Results demonstrated a 35% reduction in total work shift dose when the coated flight bar 

conveyor was used. The predicted eight hour TWA reduction in underground noise exposure 

was found to be 3 dB, using MSHA criteria. Use of the coated flight bar conveyor on a 

continuous mining machine will reduce exposure levels of the continuous mining machine 

operator, as well as all underground workers that are in the vicinity of the machine. 

6 REFERENCES 

[1] J.R. Franks. Analysis of Audiograms for a Large Cohort of Noise-Exposed Miners. 

Internal Report, National Institute for Occupational Safety and Health, Cincinnati, OH. 

October 1997, pp. 1-7. 

[2] Title 30 CFR Part 56 and 57, 2000-2002, U.S. Department of Labor, Mine Safety and 

 Health Administration, Information Resource Center, Denver Co. 

[3] Title 30 CFR Part 62, 2000-2002, U.S. Department of Labor, Mine Safety and Health 

 Administration, Information Resource Center, Denver Co. 

[4] P.G. Kovalchik, F.T. Duda, and T.M. Durr, “Noise controls for continuous miners,” 

Proceedings of 10th International Meeting on Low Frequency Noise and Vibration and ts 

Control. York, England, September 2002, pp. 299-306. 

[5] M.T. Durr, P.G. Kovalchik, and E. Kwait, “Evaluation of Engineering Noise Controls for a 

Continuous Miner Conveyer System,” Proceedings of Noise-Con 03. Cleveland, Ohio, 

June 2003, pp. 1-11. 

[6] E. Berger, L. Royster, J. Royster, D. Driscoll, and M. Layne (Editors), “The Noise 

 Manual,” Fifth Edition, American Industrial Hygiene Association, Akron, OH, 2000. 

[7] ANSI S12.19-1996, Noise – Measurement of Occupational Noise Exposure. 

[8] Quest Technologies [1997]. Instructions for Q-400 and Q-500, 56-253 Rev. F. 

[9] E. Spencer, “Heavy Construction Equipment Noise Study Using Dosimetry and Time 

 Motion Studies,” Proceedings of Noise-Con 05. Minneapplis, MN, Oct. 2005, pp. 1- 8.