Operating speed assessments of 

underground mining equipment
�

J.P. DUCARME, A.J. KWITOWSKI 

AND J.R. BARTELS 

J.P. Ducarme, A.J. Kwitowski and J.R. Bartels, are 

research scientist, research engineer and research engineer, 

respectively, with the National Institute for Occupational Safety 

and Health (NIOSH), Pittsburgh, PA. 

Abstract

Numerous fatal/nonfatal accidents involving underground 

workers struck by powered machinery occur yearly, with 

continuous miners and roof bolters involved in the majority 

of these accidents. In an attempt to reduce these accidents, 

researchers at the the National Institute for Occupational 

Safety and Health (NIOSH) Pittsburgh Research Laboratory conducted studies of operator interactions with the motions of continuous mining machines and roof bolter boom 

arms.These operators generally perform their tasks in close 

proximity to the equipment in confined workspaces, often 

employing awkward postures. Since experiments with human subjects using the actual equipment were not feasible 

due to safety concerns, researchers opted to conduct controlled experiments in laboratory settings. Utilizing motion 

capture technologies and computersimulations,the studies 

collected data on equipment and operator movement in a 

range of seam heights and working postures. This article 

details the results of these studies to examine operating 

speeds based on usage and seam height. 

Introduction	 The U.S. Mine Safety and Health 

Administration (MSHA) reports 

that both fatal and nonfatal remotecontrol continuous miner accidents 

from 1999 to 2007 averaged 243 per 

year during routine mining activities, with the majority of accident 

victims working within the turning 

radius of moving continuous miners. 

The mining industry uses an educational aid called “red zones are no 

zones” (MSHA 2006) to help operators of continuous 

miners to understand which areas around the machine 

to avoid. However, fatalities and injuries continue to 

occur. 

During the same period, MSHA data shows an average of nearly 600 accidents per year related to roof 

bolting, accounting for nearly one-third of powered machinery accidents. There are currently no regulations or 

data on determining safe velocities for roof bolter boom 

arms operating in close proximity to workers in underground mines (MSHA, 1994 and Turin et al., 1995). 

Other industries, such as robotics, have conducted 

studies and implemented guidelines for safe machine 

velocities. The U.S. Department of Occupational Safety 

and Health Administration (OSHA, 

1987) and the U.S. Department of 

Energy (DOE, 1998) recommend 

velocities be limited as low as 152 

mm/sec (6 in./sec) for manufacturing robots during programming. 

Another study, (Etherton, 1987), 

recommends 254 mm/sec (10 in./ 

sec) as a safe velocity for robots operating in proximity to humans. 

To address these issues, the National Institute for Occupational 

Safety and Health (NIOSH) conducted studies to investigate safe operating speeds for these two machines. 

To minimize risk to the human subjects, the studies used 

motion capture technologies and computer simulations 

in a controlled laboratory setting. 

For the continuous mining machine study, the researchers attempted to determine how quickly subjects 

could escape from the perceived danger of a machine 

rotating towards them. Continuous mining machines 

move in a straight line relatively slowly, but their rotational speed can be appreciably faster when the tracks 

move in opposite directions. In that case, the machine 

pivots around the center point of its tracks. The front 

and back of the machine are far enough from this center 

to result in high velocities in the turning direction. Researchers conducted numerous motion capture sessions 

with human subjects in controlled laboratory environments. This data was used to create a virtual environment to analyze factors influencing struck-by accidents 

during the tramming of a continuous mining machine. 

The roof bolter boom arm motion studies had United Mine Workers of America (UMWA) volunteers operate a model of a Fletcher Ranger II left-hand roof 

bolter arm while recording the motions of the bolter 

boom arm and subjects. While this manufacturer has 

the majority of the underground roof bolter market, 

the use of this particular manufacturer’s design should 

not limit the application of the results of these studies 

because other manufacturers’ articulating boom arms 

use similar designs. NIOSH’s model is of the common 

1,830 mm (72 in.) length arm and all testing was done 

with full sump extension. For the vertical boom speed 

research, a virtual environment with digital humans was 

created to run simulations.A level of randomness added 

to the motion of the digital operator during the drilling 

and bolting cycle enabled the simulation to realistically 
represent the operator’s motions while performing the 

bolting task. Volberg and Ambrose (2002) discuss in 

detail the development of these random motions. The 

simulator output produced data on the number of contacts between the operator and moving boom arm. This 

data was analyzed to show the effects of vertical speed 

on the risk of boom arm contact with the operator. For 

the horizontal boom speed study, it was only necessary 

to complete analysis of motion capture data from the 

human subjects testing to determine if an operator’s 

safety could be compromised by excessive speeds. 

Procedures 

The continuous mining machine investigation analyzed factors influencing struck-by accidents during 

tramming by using a digital human model (DHM) with 

simulations driven by captured human motion data with 

a variety of subjects, postures, facing orientations, environmental constraints and machine characteristics. The 

DHM used MSHA fatality information to validate the 

model parameters relating to operator position, which 

could pose a threat to operator safety. Some of these 

positions were in the MSHA red zone (MSHA, 2006). It 

should be noted that the results from this study make a 

case supporting or even expanding the red zone strategy. 

The human subjects recruited from local mines were 

asked to perform realistic movements in a laboratory 

setting (Fig. 1) that mimic escaping from the path of a 

moving machine. 

FIGURE 1 

Motion capture for continuous miner speed study.	�

The motion data was obtained using 

various operator work postures and escape paths (directions) typical for tramming operations of continuous 

miners. Complete details of these tests, the development 

of the DHM and data analysis are contained in Bartels 

et al. (2008). 

The DHM, shown in Fig. 2, was developed using the 

motion capture data to provide the means to measure 

parameters that would be used to predict struck-by 

events when the operator tries to move out of the way 

of the moving machine. 

FIGURE 2 

Digital human/continuous mining machine model. 

The digital human operator’s 

movements were constrained by using motion capture 

data of test subjects as discussed in (Bartels et al., 2007). 

To present a realistic operator response to the moving 

machine, researchers programmed the operator’s movement using a delayed start in accordance with reaction 

times reported by Drowatzky (1981). This delay ranged 

from 0.19 to 24 seconds. The DHM output parameters 

included: 

• The time when the machine first begins to move. 

• The time when the operator first begins to move. 

• The time when the operator is struck by an object. 

• Name of the object that struck the operator. 

• The operator’s distance from the start position 

 when struck by an object. 

MSHA’s fatality reports (Dransite and Huntley, 

2005) provided information to help validate the model 

regarding objects that struck the operator and the operator’s distance and location from the machine at the 

time of being struck. 

FIGURE 3 

Digital human/roof bolter model. 

A similar method was used to investigate the vertical 
	 											 			 							

		 		 	

	 	

speed for the roof bolter arm. 

Researchers developed a three-dimensional computer model that uses virtual human simulation software 

as the primary means to gather contact data when the 

boom arm touches the operator’s hand, arm, head or 

leg. Human subject tests using UMWA volunteers with 

a full-scale working mockup of a boom arm were used 

to collect motion data that helped determine parameters 

for building this model. The computer model, shown in 

Fig. 3, contains a virtual mine environment that includes 

a roof bolter boom arm and a virtual human operator. 

The complete development of this model and data analysis is available in (Ambrose et al., 2005). 

The study of the horizontal speed of roof bolter 

boom motion examined the relationship between operator and boom arm motion, with the goal of increasing 

the safety of bolting machine operators during lateral 

boom (swing) operations. Swing motion usually occurs 

when the operator is repositioning the boom arm to a 

new bolt insertion location; this action requires that the 

operator properly actuate the correct control(s) and 

simultaneously reposition his/her body in coordination 

with the moving boom arm. Figure 4 depicts the motion of an operator during a boom swing. In low seam 

heights, operators may perform this task from kneeling 

positions, which further hinders their ability to keep 

pace with the boom arm. The fundamental issue is that 

it is not known what boom swing velocities will minimize the operators’ risk of injury while still allowing 

the operators to perform bolting functions effectively. 

Roof bolter manufacturers set the rate of boom swing 

to a known velocity, but these settings may subsequently 

change from repair or rebuild. 

With the 1,830 mm (72 in.) bolter arm mock-up at 

full sump extension, the operator’s station moves about 

1,220 mm (48 in.) during a full swing. This mock-up 

bolter arm has adjustable operating speeds and various safety features to minimize risk to test subjects. The 

operator control layout used in the mock-up (Fig. 4) is 

a layout that is representative of older units used in the 

field.

FIGURE 4 

Operator movement during boom swing.

 Several versions of controls are in use because 

manufacturers alter machines to meet their customers’ 

needs. Figure 5 shows a subject at the start of a test in 

the kneeling posture. 

The tests used velocities of 300, 410 and 610 mm/ 

sec (12, 16 and 24 in./sec) when moving away from the 

operator (swing-in) and 300, 350 and 410 mm/sec (12, 

13.7 and 16 in./sec) when moving toward the operator 

(swing-out). The lowest velocities for both directions of boom arm testing were the manufacturer’s 

base velocity. The fast velocities were arrived at 

through preliminary testing using NIOSH test subjects. Kwitowski and Ducarme (2007) provide complete descriptions, data analysis and details on these 

tests. The medium velocities used were calculated 

to be half way between the fast and slow velocities 

selected. Since the radius of the motion is preset, 

all velocities were converted to a time base for full 

swing travel. The times for full swing and velocities 

that were used are listed in Table 1. 

Table 1 

Times and velocities used for UMWA trials.

Swing-in Swing-out

Target	 Actual Target Actual

(avg.)	 (avg.)

Normal

Time, sec.	 4.0	 3.7	 4.0	 3.8

Velocity, cm/sec	 30.5	 33.0	 30.5	 32.1

Medium

Time, sec	 3.0	 2.7	 3.5	 3.2

Velocity, cm/sec	 40.6	 45.2	 34.8	 38.1

Fast

Time, sec	 2.0	 2.2	 3.0	 2.7

Velocity, cm/sec	 61.0	 55.4	 40.6	 45.2

Both actual and 

target times and velocities are listed in the table 

because the method used to set the velocities did 

not result in the exact target swing times. The actual swing times achieved were used in the analysis 

process to ensure accuracy. A small PVC tube attached 

to the boom arm simulated the obstruction created by 

the actual roof bolting machine’s protective canopy and 

was adjusted appropriately for each test to correspond 

with the seam heights used. The sequence of velocities, 

seam heights and posture combinations were randomized. Several markers were placed on the boom arm to 

record its motion in addition to the standard marker set 

on the subject. 

Results 

The studies, as expected, show that speed had an 

impact on the possibility of worker injury. In addition, 
operator posture and position were factors in the frequency of struck-by events in the studies. Since the 

studies involved a complex relationship between the 

influential factors, seam height and proximity to the machine could affect the frequency of struck-by events and 

the significance of the other variables. 

FIGURE 5 

UMWA subjet kneeling posture. 

For the evaluation of continuous miner speeds, frequency and cross-tabulation analyses were completed on 

14,308 simulations. Of these, 10,254 exhibited struck-by 

events between the operator and the continuous miner 

equipment. Figure 6 shows the simulation at the instant 

a struck-by event occurs.

FIGURE 7 

Effect of operator distance to CM on contacts. 

Cross tabulations showed that 

two major factors had an effect on the operator’s ability 

to avoid being struck by the machine: the speed of the 

machine and the operator’s distance from the machine. 

The effect of the operator’s distance from the machine 

can be seen in Fig. 7.

FIGURE 6 

Simulation showing operator struck by machine. 

 The differences in incident rates 

between a distance of 300 mm (1 ft) and 610 mm (2 ft) 

is small but, at 910 mm (3 ft), the incident rate is significantly reduced. Figure 8 shows the effect of machine 

rotational speed on struck-by events.

FIGURE 8 

Effect of continuous miner speed on contacts. 

 Both figures show 

an almost linear reduction in struck-by rates as machine speed is reduced or operator distance is increased. 

These results are not surprising but do indicate that recommendations on speed and operator distance from 

the machine could reduce operator injuries. What did 

stand out was that there were no rotational speeds that 

showed a dramatic reduction in incidents. A maximum 

reduction of 20% occurred between the maximum and 

minimum speeds. Position, on the other hand, showed a 

sudden 40% decrease in incidents at the 910 mm (3 ft) 

distance compared to the 610 mm (2 ft) distance. This 

would indicate that position may be the most significant 

parameter in preventing accidents. 

Additional data was collected from the simulations 

such as the parts of the body most frequently struck 

(Fig. 9). Upper body parts tend to be struck most often 

due to the shape of the mining machine, particularly the 

tail, that can only contact the operator’s upper body. 

This also explains why stature did not have a significant 

influence on incidents. 

The simulation results for roof bolter boom vertical speed were evaluated using frequency distribution, 

cross-tabulation and survival analyses. These analyses 

were conducted using only the occurrences for the operator with slow reactions that included one contact per 

simulation execution. The difference between slow and 

fast reaction times of the operator did not significantly 

affect the outcome because the number of contact incidents between slow and fast reaction times was less 

than 1%. 

Results from the frequency distribution analysis 

showed: 
• The 1,520 mm (60 in.) seam height had 59% of 

the total number of contacts. 

• The combined right and left one knee work posture had 49% of the contact incidents. 

• The boom speeds of 41 and 560 mm/sec (160 

and 22 in./sec) combined for 43% of the contact 

incidents (Fig. 10). 

• Of the total number of contacts, 76% occurred 

during boom up motion. 

FIGURE 9 

Frequency of continuous miner contact by body part. 

FIGURE 10 

Frequency distribution of vertical boom speed incidents. 

The cross-tabulation analysis had the following results: 

• Regardless of boom speed, the boom-up direction had the majority of the contacts, with most 

occurring at the highest two speeds of 41 cm/sec 

(16 in./sec) (22%) and at 560 mm/sec (22 in./sec) 

(21%). 

• The both-knees work posture resulted in more 

contacts than the other postures and had the 

largest number of contacts (23%) at the boom 

speed of 410 mm/sec (16 in./sec). 

• The machine part that had the most contacts was 

the boom arm and those contacts occurred during 

the highest speeds of 410 and 560 mm/sec (16 and 

22 in./sec). 

• The body part that had the most contacts was the 

hand, with the largest number of contacts (24%) 

occurring during the boom speed of 410 cm/sec 

(16 in./sec). 

• The 1,520 mm (60 in.) seam had more contacts 

than the other seam heights, with most (22%) 

occurring at the 410 mm/sec (16 in./sec) speed 

(Fig. 11). 

The survival analysis results were: 

• Boom speed is the most important factor in 

determining the chance of an operator making 

contact. 

• Boom speed was the most influential variable for 

explaining the time to a contact occurring. 

• Increases in boom speed resulted in an increased 

chance of a contact throughout the period of the 

simulation. 

• The chance of being contacted at the two highest boom speeds was generally two to four times 

greater than at 33 mm/sec (13 in./sec) and four 

to eight times greater than at 180 mm/sec (10 in./ 

sec). 

• A boom speed greater than 410 mm/sec (16 

in./sec) resulted in a substantial increase in the 

chance of a contact, while speeds less than or 

equal to 330 mm/sec (13 in./sec) had much lower 

probability of a contact occurring. 

Horizontal roof bolter boom swing speed was evaluated based on the difference in operator acceleration 

and the speed of the approaching bolter boom. Figure 

12 shows the results for the 1,830 mm (72 in.) standing 

posture, which are typical results for all postures except 

the 1,220 cm (48 in.) kneeling results, which are shown 

in Fig. 13. The slope values were calculated as the averages of all 12 test subjects. When the value of slope is 

positive, the subject to boom arm distance was increasing. Conversely, a negative slope indicates the subject to 

boom arm distance was closing. As the absolute value 

of a slope increased, the velocity between operator and 

boom arm also increased. 

The swing direction must be taken into account when 

deciding whether a positive or negative slope is desired. 

During boom swing-out, the boom is moving towards 

the operator and a positive slope value indicates the 

operator is moving faster than the boom. This situation 

should be a benign case and may indicate boom swing 

velocity could be increased. However, a negative slope 

value during boom swing-out indicates the operator to 

boom arm distance is decreasing, meaning the operator 

could not keep up with the boom arm motion and a 

reduction in swing velocity should be considered. This 

is the more hazardous situation, as the possibility of operator to boom arm contact and exposure to pinch point 

and crushing hazards increased. The most hazardous 

situations occur when the slopes are more negative than 

-0.5 during swing-out. At this slope, the closing velocity 

between the boom arm and the operator is 50% of the 

arm’s velocity. As the actual fast swing-out velocity was 

about 440 mm/sec (17.5 in./sec), the closing velocity between operator and boom arm would be about 230 mm/ 

sec (9 in./sec). This velocity exceeds the limiting velocities from Etherton, 1987 and OSHA, 1987. 

For boom swing-in events, the boom arm is moving 

away from the operators, forcing them to follow it. If the 

boom arm is moving faster than the operator can follow, 

the distance between the arm and operator will naturally increase. However, this situation could increase 

tripping hazards. 
FIGURE 10 

Frequency distribution of vertical boom speed incidents. 

FIGURE 11 

Cross tabulation results of vertical boom speed and seam 

height. 

FIGURE 12 

1,830 mm (72 in.) standing posture, horizontal boom speed results. 

FIGURE 13 

1,220 mm (48 in.) kneeling posture, horizontal boom speed results. 

For all postures except one, the trend is clear.As the 

boom swing velocity increased, the velocity of the boom 

arm approach or recession relative to the operator increased. The kneeling 1,220 mm (48 in.) results show 

that the slowest velocity tested, ~30 mm/sec (12 in./sec), 

appears to be excessive for both swing directions, as the 

operators were unable to keep pace with the moving 

boom arm. During swing-out motion, the slope value 

was always negative, meaning the operator to boom arm 

distance decreased at all velocities. 

Conclusions 

The data obtained in these three studies revealed 

a complex interaction of factors that affect the risk of 

struck-by accidents when miners operate mining machines in an underground mining environment. However, the increased understanding of these relationships 

should ultimately result in recommendations that reduce the risk of potentially fatal accidents to machine 

operators. 

Continuous miner rotational speed was one the most 

influential variables in terms of explaining the struck-by 

event occurring. Increases in machine speed resulted in 

increased chance of being struck and the increased risk 

associated with higher speeds was constant throughout the times investigated in the study. In general, compared to the 4.77 deg/s condition, the 9.75 deg/s speed 

increased risk of being struck by 20%. The distance of 

the operator from the machine, at the start of the test, 

also had a significant influence.The relative risk of being 

struck by the machine, while working within 0.3 m (12 

in.), was the greatest at the beginning of the simulation. 

However, the struck-by incidents drop 40% at an operator position 0.9 m (36 in.) from the machine, indicating 

position may have a greater influence on incidents than 

machine speed. It is important that continuous miner 

operators constantly keep aware of their position in 

relation to the machine and maintain a safe distance to 

reduce their risk of being struck. 

Data analysis of roof bolter vertical boom speed 

simulations shows that the speed of the boom arm is 

the most important factor in determining the risk of an 

operator making contact with it. Regardless of other 

variables, contact incidents were always greater when 

the bolter arm was moving up, greater on the hand and 

greater for the boom arm part of the machine. The reason why the subject experiences more contacts when the 

boom arm is moving up rather than down is that more 

risk occurs during drilling and bolting when the boom 

arm is ascending. Based on the data collected, boom 

speeds greater than 330 mm/sec (13 in./sec) result in a 

substantial increase in risk to the roof bolter operator 

for making contact. Speeds less than or equal to 330 

mm/sec (13 in./sec) are associated with a more modest 

relative risk of making contact, which represents a decrease in potential hazard. 

For all the studies of horizontal roof bolter arm 

swing, velocity appears to be a primary factor deter-
mining operator safety in relation to pinch point and 

crush hazards during the boom positioning phase of the 

bolting sequence. 

For the tests conducted with the boom swing-out velocity set to the average of 440 mm/sec (17.5 in./sec), the 

absolute values of slopes were equal to or greater than 

0.5 and, therefore, should be considered too fast for all 

the postures tested. The medium rate tested for swingout averaged 380 mm/sec (14.9 in./sec) and resulted in 

satisfactory slope values for all postures tested except 

for the 1,220 mm (48 in.) kneeling posture. 

The results presented indicate that operators should 

be able to keep pace with the manufacturer’s standard 

boom swing rate of 300 mm/sec (12 in./sec) for all the 

tested postures except for the 1,220-mm (48-in.) kneeling posture. Additional research needs to be conducted to provide adequate data for setting swing velocity 

guidelines at the 1,220 mm (48 in.) working height. 

The data shows that operators are able to keep pace 

with horizontal boom swing velocities in excess of the 

manufacturer’s standard in other working heights and 

working postures. However, caution must be used with 

utilizing the information obtained from these studies 

because the test trials were of a limited duration and 

the subjects were permitted rest periods as needed. Operator fatigue could be a factor associated with higher 

velocity settings. Also, floor conditions and lighting in 

the laboratory were ideal during testing, where as actual 

mine environments seldom provide such conditions. In 

summary, operator distance from the continuous miner 

is a more important factor than rotational speed in reducing risk of injury during tramming. By maintaining 

a minimum of 910 mm (3 ft) distance to the machine, 

continuous miner operators can substantially reduce 

their risk of being struck. For roof bolter operators, a 

vertical boom operating speed of less than 330 mm/ 

sec (13 in./sec) results in a decreased risk of contacting 

the machine, and a horizontal operating speed of 30 

mm/sec (12 in./sec) or less results in a decreased risk of 

contact in all cases except in the 1,220 mm (48 in.) seam 

height with the kneeling posture. In that case, additional 

research needs to be conducted to determine safe boom 

speeds. 

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