Results of noise measurements from underground testing of 

a roof bolting machine duty cycle 

Jeffrey Shawn Peterson 

Lynn Alcorn 

National Institute for Occupational Safety and Health 

Pittsburgh Research Laboratory 

626 Cochran's Mill Road 

Pittsburgh, Pennsylvania 15236 

Disclaimer: The findings 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 

1. ABSTRACT 

Data collected by the Mine Safety and Health Administration (MSHA) has shown that roof 

bolting machine (RBM) operators are the second most likely type of underground mining 

machine operator to be are over exposed to noise, per the MSHA Permissible Exposure Limit. In 

response to this, the Pittsburgh Research Laboratory of the National Institute for Occupational 

Safety and Health (NIOSH) has initiated a project to develop engineering noise controls for roof 

bolting machines. Key to this research is an evaluation of the duty cycle of a typical roof bolting 

machine operation and this paper presents the results of such an exercise for data collected at two 

cooperating coalmines. The evaluation entails a time motion study of an operators' shift, logging 

each activity for time (e.g., tramming the machine, drilling holes, installing the roof bolt, etc.) 

and the noise levels associated with each task with a Larson Davis Spark 705+ dosimeter. 

NIOSH post processed the data to determine which tasks the operator devotes the most time to 

and the noise dosage accumulated during those tasks. These results provide insight into which 

portion of the operators' duty cycle is the most prominent contributor to noise exposure and thus, 

where research activities for the development of engineering noise controls should be focused. It 

was determined that the task of drilling holes is the most significant source of the operators' 

noise exposure and project research has focused on engineering noise controls to reduce the 

noise emissions generated during the drilling portion of the machine duty cycle. 

2. INTRODUCTION 

NIOSH's National Occupational Research Agenda lists hearing loss among its 21 Priority 

Research Areas and its Pittsburgh Research Laboratory maintains a Hearing Loss Prevention 

Branch to investigate reducing Noise Induced Hearing Loss (NIHL). MSHA coal noise sample 
 

 

 

data collected from 2000 to 2005 show that only seven types of machines compose the bulk of 

the equipment whose operators exceed 100% noise dosage, per the MSHA Permissible Exposure 

Level (PEL). Of these machines, the roof bolting machine operator was the second most likely 

to be over exposed among operators of all equipment used in underground coal.2 

Underground coal mining requires a roof control plan to ensure the safety of its workforce. After 

the removal of coal or rock, the remaining strata may be subject to fall, either from overhead or 

from the side. One method used in underground coalmines to prevent roof failures requires the 

installation of roof bolts, of sometimes-varying lengths, at roughly 1.2 m (four-foot) intervals. A 

RBM installs roof bolts by tramming to the required location, drilling a hole, and then installing 

the bolt. 

Commonly used roof bolts employ either mechanical anchors, resin, rebar, or a combination of 

both to secure the bolt. There are other types of roof bolts available as well. Examples of resin 

rebar roof bolts are shown in Figure 1. Drill steels are either round or hexagonal in shape and are 

available to mate with either 2.54 cm (one inch) or 3.49 cm ( 1 3/8 inch) drill bits (Figure 2). 

Drill bits are available for vacuum drilling i.e., dry, and for mist or wet drilling (Figure 3). The 

operator installs a bit on a drill steel, places the drill steel in the RBM chuck and then drills the 

hole for the roof bolt. The operator removes the drill steel from the chuck, replaces it with a 

wrench holding the roof bolt, inserts the resin into the hole, if used, drives the roof bolt into the 

hole with the RBM, then rotates the bolt for a predetermined length of time and thrust until the 

resin sets. The operator repeats this exercise as needed to meet the requirements of the mines 

roof control plan. 

Figure 1: Resin bar roof bolts 

Figure 3: Drill bits, vacuum (top), wet or mist (bottom) 

Figure 2: Drill steels, hex (top), round (bottom) 

Essential to developing engineering noise controls to reduce the occurrences of NIHL for roof 

bolting machine operators, was to document the noise doses and sound levels associated with the 

typical tasks required of a RBM operator, and the time spent conducting these tasks. A time 

motion study of an operators shift comprised logging each activity for the time spent conducting 

a particular activity (e.g., tramming the machine, drilling holes, installing the roof bolt, etc.) and 

the noise levels associated with these tasks. Post-processing of this data reveals the tasks the 

operator devotes the most time to, the noise dosage accumulated during each task and of course 

then, the tasks which are the primarily contributors to the operators’ noise dose exposure. This 
 

information would then guide NIOSH in determining which areas of the RBM should be 

addressed when developing engineering controls to reduce noise emissions. 

Over the course of three days, project personnel conducted three time-motion studies at 

cooperating coalmines and documented the noise dose exposures and activities of four RBM 

operators and two operators of a bolter-miner, a machine that serves as a RBM and a continuous 

mining machine in combination. Results of these studies confirmed our initial expectation that 

for the RBM, the drilling portion of the operators’ duty cycle exposed the operator to the highest 

noise levels and was a significant contributor to the operators’ noise dose, relative to other tasks. 

3. TIME-MOTION STUDY 1 

NIOSH personnel conducted a dosimeter time-motion study of dual-boom Fletcher roof bolting 

machines at two portals, A & B, of an underground coalmine. A Larson Davis Spark 705+ can 

set used as four separate dosimeters, each configured differently. Table 1 lists the configuration 

settings for this time study. Dosimeter 1 was set up to collect data per the MSHA PEL and 

dosimeter 4 for all noise, or wide range. Table 2 lists the noise dose accumulation and 

equivalent time weighted average sound pressure levels for the four RBM operators. At each 

portal, the RBM’s were configured for vacuum drilling and 2.54 cm (one inch) bits. 

Table 1 – Larson Davis Spark 705+ dosimeter settings 

Dosimeter Threshold 

level (dB) 

Exchange 

rate (dB) 

Criterion 

level (dB) Weighting Response 

1 – MSHA PEL 90 5 90 

A slow 

2 85 5 90 

3 85 3 90 

4 - wide-range 40 3 85 

Table 2 – Operator noise dose and equivalent time weighted 

average 

Portal A -

Operator 

Portal B - 

Operator 

A1 A2 B1 B2 

Noise dose (%) 78 127 43 65 

Noise dose, 8 hrs (%) 52 85 29 43 

TWA (dB) 85 89 81 84 

TWA, 8 hrs (dB) 88 92 84 87 

Both operators for each machine were observed during shift exercises and the tasks were 

manually logged on a second-by-second basis. Primary tasks logged include; the drilling of the 

bolt hole, installing the bolt; tramming of the RBM; riding the man-trip to the section; prep time; 

and riding the elevator down the shaft. For each of these tasks, accumulated times and their 

associated noise dosages were compiled. Shown in Figures 4-7 are the noise accumulations over 
time and in Figures 8 and 9 are the tasks for which the operator was engaged at least 1% of his 

shift. Included in the ‘other’ category of Figures 8 and 9 includes time spent riding the elevator, 

when the operators were bolting alone, or both drilling, or both bolting, and other times where 

the activity could not be documented. At portal B, there was an equipment malfunction near 

midday and the section to shut down for the day. The effects of these ‘other’ conditions on the 

noise dose are minimal, as 93% of the operator noise dose exposures for portals A & B are 

attributable to those activities shown in Figures 8 and 9. 

Figure 4: Noise dose accumulation, operator A1, portal
 

A
 

Figure 7: Noise dose accumulation, operator B2, portal
 

B
 

Figure 5: Noise dose accumulation, operator A2, portal
 

A Figure 8: Time spent by task, portal A 

Figure 6: Noise dose accumulation, operator B1, portal 

B 

Figure 9: Time spent by task, portal B 
 

 

Figures 10 and 11 show that periods of time when both operators were drilling and bolting 

contribute a minimum of one-half of the operators accumulated exposure while no other task 

contributes as much as one-fifth. Further analysis of the data indicated that the bulk of the time 

spent either drilling or bolting were in situations where is was difficult to discern between the 

operators’ drilling and bolting activities (Table 3). Thus, over 90% of this data involves the 

operators drilling and bolting simultaneously. 

Figure 10: Dose by task, portal A Figure 11: Dose by task, portal B 

Table 3 – Percentage of time spent drilling and bolting1 

Portal A – 

Operator 

Portal B – 

Operator 

A1 A2 B1 B2 

Drilling & bolting simultaneously 96 95 94 94 

Drilling alone 3 4 5 5 

Bolting alone 02 02 1 1 

Drilling together 02 02 ----

3 ----3 

Bolting together 02 02 ----

3 ----3 

1

 – Considering only time spent drilling and/or bolting. 

2

 – Less than 1%. 

3

 – No observations of this activity. 

However, it was necessary to determine the noise levels and exposures the operators were 

exposed to when drilling or bolting singly during the duty cycle. NIOSH documented multiple 

instances of the drilling/bolting cycle to determine the time spent drilling a hole and then to 

install the bolt into the associated hole. In all, 27 drilling/bolting cycles were observed and mean 

values for these for each of three operators are given Figure 12. Researchers were able to 

document multiple instances where Operator A1 drilled alone, but only one complete 

drilling/bolting cycle. Thus, Operator A1 data is not included in Figure 12. Clearly, drilling of 

the hole requires significantly more time than the associated installing of the roof bolt. This was 

expected, as the roof bolt resin hardens very quickly. Further, the operator is exposed to greater 
 

sound pressures during drilling than bolting (Figure 13), and thus accumulates dose at 

significantly faster rate when drilling than bolting (Figure 14). From this, the operator would be 

expected to accumulate significantly more dose exposure during drilling than bolting. 

Figure 12: Mean drilling and bolting time, portals A & Figure 13: Equivalent time weighted average sound 

B pressure level, portals A & B 

Figure 14: Mean drilling and bolting dose per hour, portals A & B 

Both operators accumulated noise dose at significant rates when drilling and/or bolting, 12-24 % 

per hour at portal A and 8-16 % per hour at portal BA. Figures 15 and 16 show the RBM 

operators dose accumulation per hour by task for this time study. During the ride on the man trip 

from the elevator to the section and back, the operators accumulate noise exposure at a 

significant rate, 44% per hour for Operator A2, and 18% for Operator A1. However, for both 

portals, man trip time was only roughly 30 minutes (Figures 8 and 9) and its contribution to noise 

exposure relatively small compared to drilling and bolting (Figures 10 and 11). 
Figure 15: Dose per hour by task, portal A Figure 16: Dose per hour by task, portal B 

These results suggest that if the noise dose exposure for an RBM operator is to be reduced, then 

the drilling portion of the operators’ should be evaluated and engineering noise controls 

developed to reduce drilling noise emissions. Laboratory investigations in NIOSH’s Hemianechoic Chamber (HAC) also support the initial premise that drilling noise should be addressed. 

Sound pressure level measurements made at the RBM operator ear location confirm that drilling 

generates higher noise levels than bolting (Table 4). 

Table 4 – Operator ear sound pressure levels 

Roof bolting machine 

condition Level (dBA) 

idling 72 

+ vacuum 80 - 90 

+ vacuum + bolting 92 - 95 

+ vacuum + drilling 97 - 108 

3. TIME-MOTION STUDY 2 

NIOSH personnel conducted a dosimeter time-motion study of a dual-boom Voest-Alpine bolterminer, type 14ABM, at a second cooperating coalmine. The Larson Davis Spark 705+ dosimeter 

settings shown in Table 1 were again used for this study. The bolter miner was configured for 

vacuum drilling and 3.49 cm (1 3/8 inch) bits. Both bolter operators for the machine (Operator 

C1 and Operator C2) were observed during shift exercises and again, the tasks were manually 

logged on a second-by-second basis. Here, in addition to the two roof bolter operators, NIOSH 

collected noise dose exposure data for the continuous miner operator (CM1) and the loader 

operator (L1), who carried coal away from the section. 

Figure 17 shows the noise dose accumulation over time for these four miners and Table 5 lists 

the overall dose accumulation and the equivalent time weighted average sound pressure levels. 

The addition of the coal cutting, conveying, and loading at this location contributed greatly to the 

complexity of the operation. Thus, the activities of the four individuals were not documented as 

before, but the general activity of the section was logged. Primary tasks logged included those 

from portals A & B; the drilling of the bolt hole; installing the bolt; tramming of the RBM; riding 
 

the man-trip to the section, prep time; and riding the elevator down the shaft; but also include 

the addition of the cutting, conveying and loading of the coal. 

Table 5 – Operator noise dose and equivalent time 

weighted average sound pressure levels 

Figure 17: Noise dose accumulation, portal C 

C1 C2 CM1 L1 

Noise dose (%) 200 256 115 65 

Noise dose, 8 hrs 

(%) 133 171 77 43 

TWA (dB) 92 94 88 84 

TWA, 8 hrs (dB) 95 97 91 87 

Figure 18 illustrates the tasks representing at least 1% of the time utilized during the shift. In 

this case, included in the ‘other’ category are times spent in the elevator, for maintenance, coal 

conveying only, when both RBM operators were bolting, or drilling or bolting singly, and lunch. 

Figures 19 and 20 shows the dose accumulation for each task. Operators C1 and C2 

accumulated a significant portion of their exposure during cutting and conveying as well as while 

coal was being loaded, as these tasks accounted for roughly 45% of their dose exposure. As 

expected, CM1 was exposed to the most noise during cutting and conveying and L1 during 

loading. During the ‘other’ time, the RBM operators accumulated one-fifth of their noise dose, 

the continuous miner operator one-seventh his dose, and the loader operator accumulated none. 

Figure 18: Time spent by task, portal C Figure 19: Dose by task, RBM operators, portal C 
Figure 20: Dose by task, CM and loader operators, portal C 

Operators C1 and C2, however, accumulated dose at significants rate while both were drilling 

(Figure 21), 61% dose per hour for C1 and 81% per hour for C2. They also rapidly accumulated 

exposure during cutting and conveying (56 and 68% per hour), loading (47 and 56% per hour) 

and cutting (41 & 54% per hour). It appears the roof bolting machine operators noise exposure is 

adversely affected by the continuous mining and loading activity in the immediate vicinity. 

Simultaneous cutting and conveying, cutting alone, and loading were, as expected, the tasks 

during which CM1 was exposed to the most noise and loading most affected L1 (Figure 22). 

The equivalent time weighted average for the tasks and an overall value are given in Figures 23 

and 24. Many are in excess of 100 dBA, indicating that exposure to that activity for only 2 hours 

would exhaust the miners’ permissible 100% dosage. 

Figure 21: Dose per hour by task, RBM operators, Figure 22: Dose per hour by task, CM and loader 

portal C operators, portal C 
Figure 23: Equivalent time weighted average sound Figure 24: Equivalent time weighted average sound 

pressure level by task, RBM operators, portal C pressure level by task, CM and loader operators, portal 

C 

4. CONCLUSIONS 

The time-motion study conducted with the dual-boom Fletcher RBM supports NIOSH’s premise 

that roof drilling is a significant contributor to the RBM operators’ noise dose. During the time 

in which these operators accumulated greater than 90% of their noise exposure, a significant 

portion of that time was devoted to roof drilling and bolting. Additional analysis indicated that 

the operators spent four to nine times longer drilling than bolting during a drilling/bolting cycle 

and accumulated dose three to twelve times more quickly when drilling than bolting. This has 

narrowed the focus of the engineering noise control research to drilling noise and NIOSH is 

currently focusing on the drill steel, drill bit, and chuck areas of the RBM. 

The second time motion study on the dual-boom Voest-Alpine miner-bolter (portal C) appears to 

indicate that the addition of cutting and conveying by the continuous miner and the addition of 

the coal loading complicated matters significantly when attempting to discern task noise dose 

contributions for the roof bolting machine operators. The RBM operators at portals A & B, in 

general, were not exposed to cutting, conveying, or loading noise. The miner bolter operators 

did accumulate significant noise dosages (200 and 255%) and very quickly when they were 

drilling simultaneously (61 and 81% per hour). Noise controls reducing drilling noise emissions 

should help reduce operator noise dose exposure. However, for this particular case, additional 

reductions in cutting and conveying noise may well be warranted if significant reductions in 

operator exposure are to be achieved. 

REFERENCES 

1

 National Institute for Occupational Safety and Health, 1996. National Occupational Agenda, 

DHHS Publication No. 96-115. 

2

 Title 30 CFR Part 62, 2000-2005, U.S. Department of Labor, Mine Safety and Health 

Administration, Information Resource Center, Denver, Co.