Comparison of seat designs for underground mine 

haulage vehicles using the absorbed power and 

ISO 2631-1(1985)-based ACGIH threshold limit 

methods 

Alan G. Mayton 

National Institute for Occupational Safety and Health, 

Pittsburgh Research Laboratory, 

P.O. Box 18070, PA 15236, Pittsburgh, USA 

Fax: 412 386 6764 E-mail: amayton@cdc.gov 

Farid Amirouche* 

Department of Mechanical and Industrial Engineering, 

University of Illinois at Chicago, 

2027 Engineering Research Facility, 

842 West Taylor St., IL 60607, Chicago, USA 

Fax: 312 413 0447 E-mail: amirouch@uic.edu 

*Corresponding author 

Christopher C. Jobes 

National Institute for Occupational Safety and Health, 

Pittsburgh Research Laboratory, 

P.O. Box 18070, PA 15236, Pittsburgh, USA 

Fax: 412 386 6764 E-mail: cjobes@cdc.gov

Abstract: Based on prior mine vehicle studies of operators’ exposure to 

whole-body vibration, researchers from the National Institute for 

Occupational Safety and Health (NIOSH) evaluated four seat designs on mine 

haulage vehicles, with regard to roadway-induced jarring/jolting and operator 

comfort. Investigators collected objective and subjective data from vehicle 

operators on two existing and two NIOSH seat designs. This study included 

time and frequency response data using accelerometers and a data recorder, 

operator perceptions of jarring/jolting and discomfort levels using a linear 

visual analogue scale, and data from a questionnaire developed for this study. 

Results from the analysis of subjective data show that operators generally 

favoured the NIOSH-designed seats over the existing seats. The results of 

analyses from the absorbed power method and the threshold limit method, 

based on ISO 2631-1 (ISO, 1985), support the premise that NIOSH seat 

designs are superior to the existing vehicle seat designs on both models of mine 

shuttle cars used in the study. The NIOSH seat designs, featuring viscoelastic 

foam padding, were more effective in reducing vibration energy for 

operators exposed to vehicle jarring/jolting. In this paper, the performances of 

the NIOSH and existing seat designs are compared relative to the 

operator’s exposure to vehicle vibration (mainly jarring/jolting). 
 

 

 

 226 A.G. Mayton, F. Amirouche and C.C. Jobes 

 

 

 

 

Keywords: seat designs; human body vibration; comfort analysis; absorbed 

power; mine operators. 

Reference to this paper should be made as follows: Mayton, A.G., 

Amirouche, F. and Jobes, C.C. (2005) ‘Comparison of seat designs for 

underground mine haulage vehicles using the absorbed power and ISO 

2631-1(1985)-based ACGIH threshold limit methods’, Int. J. Heavy Vehicle 

Systems, Vol. 12, No. 3, pp.225–238. 

Biographical notes: Alan Mayton, a registered Professional Engineer, works 

as Mining Engineer in the Ergonomics and Machine Safety Groups at the 

NIOSH – Pittsburgh Research Laboratory, Pittsburgh, Pennsylvania. He 

received his BS degree in Mining Engineering from West Virginia University, 

Morgantown. His research interests include human body vibration control, 

ergonomic design of seats and operator compartments, and machine-mounted 

lighting systems. He has published papers in mining and illumination journals 

and a variety of conference proceedings. He serves on the International 

Minerals Association, Ergonomics Task Force, to provide technical expertise 

on human body vibration control and ergonomic seat design for heavy vehicle 

operators. 

Farid Amirouche is a Professor, Mechanical and Bioengineering Department, 

College of Engineering and Adjunct Professor, Orthopedics Department, 

College of Medicine at the University of Illinois in Chicago (UIC). He is also 

Director, Biomechanics Research Laboratory and Director, Industrial Advisory 

Board, Mechanical Engineering Department, UIC. He received his PhD in 

Mechanical Engineering at the University of Cincinnati, Ohio. His research 

interests include biomechanics – human joints, spine modelling and 

osteoporosis, multi-body dynamics – rigid and flexible systems, human body 

vibration control. He has received several prestigious awards, is the author of 

several books, and also serves as Associate Editor for two journals. 

Dr. Jobes is a Mechanical Engineer for the National Institute for Occupational 

Safety and Health Pittsburgh Research Centre. After receiving two Bachelor 

degrees from Geneva College, he worked as a Software Consultant for a year. 

During his graduate studies at West Virginia University he developed a 

variable speed robot controller for his Masters and a remotely controllable 

residential circuit breaker for his Doctorate. He worked in control and 

navigation of a computer-assisted mining machine from 1987 through 1997. 

Dr. Jobes has since been working in the fields of Programmable Electronic 

Systems Safety and Jolting and Jarring Abatement in Mining Machinery. 

1 Introduction 

In designing a comfortable seat, it is important to understand the vibration environment to 

which individuals are exposed and how well they can tolerate this environment. 

Moreover, human sensitivity to low-frequency whole-body vibration (WBV) has pointed 

to ride quality as an important need in seat design (Amirouche et al., 1997). This is 

especially true in the mining industry. Shuttle car haulage vehicles are among the major 

sources of exposure to WBV, which includes vehicle jarring/jolting, in underground coal 

mines. Remington et al. (1984) showed that WBV was severe for these vehicles, as well 

as for load-haul-dumps (LHDs) or scoops. These circumstances have not changed much 
 

 

 

 Comparison of seat designs for underground mine haulage vehicles 227 

 

 

 

 

since 1984. The evidence for this includes data from injury reports gathered by the Mine 

Safety and Health Administration and testimonials from operators of these vehicles. 

During the 7-year period from 1993 to 1999, 13% of 10,393 powered haulage injuries in 

coal mining were attributed to vehicle jarring/jolting. Sixty-seven percent of the total 

(1,330) powered haulage injuries concerned underground operations. The LHDs/scoops, 

shuttle car, and mantrip are consistently the top three machine types cited for powered 

haulage-related injuries and accounted for 95% of the powered haulage incidents 

attributed to jarring/jolting for this period. 

Mayton et al. (1999) previously studied a low-coal mine shuttle car haulage seat 

design that included limited underground mine field trials. The current seat design 

comparison study was a more comprehensive evaluation of the low-coal shuttle car seat 

design and included the evaluation of seat designs for a mid-coal shuttle car. With a 

larger sample of shuttle car operators, researchers were able to support earlier findings 

that NIOSH seats, with unique viscoelastic foam padding, better isolate shuttle car 

operators from vehicle jarring/jolting (Mayton et al., 2003). 

Griffin (1990) defines absorbed power as the “power dissipated in a mechanical 

system as a result of an applied force”. He continues that “the vibratory power dissipated 

in the human body has variously been advocated as an indicator of discomfort from 

whole-body vibration or injury from hand-transmitted vibration”. Pradko et al. (1965) 

first introduced the concept of absorbed power as a way to measure the human response 

to WBV. Lee and Pradko (1968) discussed the analytical use of absorbed power to 

determine how humans would respond to vibration in the time and frequency domains for 

periodic and random environments. They concluded that the method of absorbed power is 

important in that it has physical significance; it can be measured and computed 

analytically. Moreover, absorbed power is a scalar quantity and thus can be summed to 

assess the human response in complex systems with multiple degrees of freedom. 

More recently, Amirouche et al. (1994) and Tong et al. (1999a) reported on analytical 

computer models for optimising the energy absorption during exposure to human body 

vibration and for evaluating the distribution of absorbed power and the reaction of the 

body to roadway-induced vibration. Furthermore, Tong et al. (1999b) discuss how energy 

is transmitted to different parts of the body and what happens when input conditions 

change. Their model was developed to study the energy absorption and work done by the 

human body’s muscles (represented as springs and dampers) during a rough ride. They 

assert that understanding the energy flow among the body’s parts can provide valuable 

input for the design of a seat and its suspension. 

According to Griffin (1990), the standard ISO 2631 (ISO, 1985) does not define a 

precise analysis method and requires some judgement in defining the optimum procedure 

according to its content. In a more recent assessment, Griffin (1998) concludes that it is 

possible to interpret ISO 2631 (ISO, 1997) as consistent with BS 6841 (1987), but adds 

that other interpretations are possible. Moreover, the American Conference of 

Governmental Industrial Hygienists (ACGIH, 2002) points out that the ANSI/ISO 

standard is not adequate for evaluating a vibration environment characterised by 

high-amplitude mechanical shocks (jars or jolts). It will ‘underestimate the effects of 

WBV ... when crest factors exceed 6’. However, ISO 2631 (ISO, 1997) has revised the 

crest factor upwards to a value of 9. 
 

 

 

 228 A.G. Mayton, F. Amirouche and C.C. Jobes 

 

 

 

 

2 Method 

Time and frequency response data were gathered with a digital data recorder, triaxial 

accelerometers, signal conditioning amplifiers, and in-line, 150 Hz, low-pass filters. 

Researchers collected data to measure levels of vehicle jarring/jolting experienced by the 

seated operators of shuttle car haulage vehicles. Triaxial accelerometers (PCB models 

356B18 and 356B40) were placed on the floor of the operator’s compartment near the 

base of the seat (frame measurement) and on the seat at the subject/seat interface 

(seat measurement). Because of muddy conditions, the frame accelerometers were 

mounted on to the frame of the shuttle car, above the control panel. During the field 

trials, mine roadway conditions were noted as smooth, pothole-riddled, debris-strewn, 

rutted, dry, wet, or water-filled. 

In gathering subjective data, researchers interviewed shuttle car operators with an 

interview guide consisting of seven basic questions. It was administered at the end of 

each trial for each seat and took about 10 minutes to complete. The questions were 

concerned with rating the seat in terms of comfort and perceived level of shock and 

vibration, operator likes and dislikes about seat features, how to improve the seat, and 

comparing the seats with each other. 

The remainder of the subjective data was collected using a linear visual analogue 

scale (VAS). It was used to obtain the operators’ immediate impressions of shock, 

vibration, and discomfort levels for the vehicle ride on each of the seats. The VAS, a 

horizontal line about 10 cm long, was labelled ‘zero or none’ at one end and ‘maximum’ 

at the other end to indicate the extremes in levels of jarring/jolting and discomfort. 

Operators were asked to mark the line at a point that represented their discomfort and 

jarring/jolting levels. The shuttle car operator marked this scale after travelling with a full 

load of coal and with no load on the first, third, and sixth round trip of the trials for each 

seat. A round trip consisted of travelling to the coal face with no load and returning to the 

load discharge location with a full load of coal. The ratings were later translated into 

decimal values less than 1 and tabularised. 

Field trials were conducted on shuttle car haulage vehicles operating at mid-coal 

seam and low-coal seam mines. Approximate specifications of the shuttle cars 

in the study included: weight – 13, 517 kg (29, 800 lb), overall length – 8.5 m (28 ft), 

width – 3 m (10 ft), wheel base – 2.7 m (9 ft), tire diameter – 0.8 m (2.7 ft), and 

travel speed – 8 km/hr (5 mph). Side-saddle style describes the mid-coal seam vehicle 

(the JOY 10SC) and how the vehicle operator is positioned in the vehicle cab. In this 

case, the operator remains in one seat and is perpendicular to, instead of facing, the 

direction of travel. The low-coal vehicle (the JOY 21SC) operator changed seats to face 

the direction of travel. 

Four basic seat designs were compared on the shuttle cars. Mid-coal seam vehicle 

seats were designated as M1 (existing) and M2A and M2B (NIOSH) (Figure 1(a)). 

Low-coal seam vehicle seats, shown in Figure 2(a), were designated as L1 (existing) and 

L2A, L2B, and L2C (NIOSH). The viscoelastic foam padding arrangements 

distinguishing the different NIOSH seats are shown in Figures 1(b) and 2(b). 
 

 

 

 Comparison of seat designs for underground mine haulage vehicles 229 

 

 

 

 

Figure 1 (a) Mid-coal seam shuttle car seats – existing (left) and NIOSH/Ergonomic (right): 

(b) Viscoelastic foam padding M2A (left – without pudgee) and M2B 

(right – with pudgee) 

(a) 

(b) 

Figure 2 (a) Low-coal seam shuttle car seats – existing (left)and NIOSH/Ergonomic (right): 

(b) Viscoelastic foam padding arrangements from left to right, L2A, L2C, and L2B 

(without pudgee) 

(a) 

(b) 
 

 

 

 230 A.G. Mayton, F. Amirouche and C.C. Jobes 

 

 

 

 

The existing seats were in service for some time and thus were more worn than the 

NIOSH seats, which were like new. The seats (after the NIOSH design) for the mid-coal 

seam shuttle cars were designated according to viscoelastic foam arrangements as 

follows: 

• seat M2A: included a total thickness of 13 cm (5 in) of Sun-Mate Extra-Soft (XSS) 

foam padding 

• seat M2B: included padding with a combination of Pudgee (PU) and XSS and a total 

thickness of 13 cm (5 in). 

For the low-coal seam shuttle car, the seats (after the NIOSH design) were designated 

according to the viscoelastic foam arrangements as follows: 

• seat L2A: included padding with a combination of PU and XSS and a total thickness 

of 8 cm (3 in) 

• seat L2B: included a total thickness of 13 cm (5 in) of XSS foam padding 

• seat L2C: included padding with a combination of PU and XSS and a total thickness 

of 13 cm (5 in). 

Eight shuttle car operators participated in the study; five operated the JOY 10SC and 

three operated the JOY 21SC. The operators were all males from 24 to 58 years old 

and averaged about 39 years. They ranged in height from 175 to 185 cm (69–73 in), an 

average of 180 cm or 71 in, and ranged in weight from 73 to 91 kg (160–200 lb), an 

average of about 87 kg or 191 lbs. The subjects’ experience in operating a shuttle car 

varied from 2 to 24 years and averaged about nine years. Similarly, their underground 

mining experience varied from 2 to 37 years and averaged 14 years. Before participating 

in the study, the shuttle car operators were briefed and asked to sign informed consent 

and photo release forms. 

3 Results 

3.1 Subjective data 

In summarising the results from the questionnaire data, the ratings reflect how the seats 

felt to the operator. For the mid-coal seam shuttle car, seat M2A is the favourite. Seat 

padding rated well for both seats M2A and M2B. Seat M1 is the least favourite in all 

ratings. Adding armrests is the improvement most often suggested for any of the seats. 

For the low-coal seam shuttle car, seat L1 is the least favourite in all ratings. Seat 

padding, lumbar support, and seat-pan tilt are rated better in seat L2B than any other seat. 

The reclining back is better on seat L2B and surprisingly favoured on seat L1. Making 

the seat a better fit for the operator compartment is a suggested improvement. This could 

improve clearance between operator and controls and allow for better operator 

adjustability and visibility. 

Average ratings from VAS responses indicated that the NIOSH-designed seats were 

superior to the existing shuttle car seats. For both no-load and full-load conditions, 

average ratings of mid-coal seam shuttle cars operators showed lower jarring/jolting and 

discomfort levels with the NIOSH seats using the two different 13-cm (5-in) viscoelastic 
 

 

 

 Comparison of seat designs for underground mine haulage vehicles 231 

 

 

 

 

foam padding arrangements. Seat M2A with 13 cm (5 in) of XSS foam padding was most 

preferred by operators of the mid-coal seam shuttle car. Similarly, for shuttle car no-load 

and full-load conditions, average ratings of low-coal seam shuttle car operators showed 

lower jarring/jolting levels with the NIOSH seat, using three different viscoelastic foam 

pad arrangements. The seats and viscoelastic foam padding arrangements, in order of 

operator preference, were seat L2B with 13 cm (5 in) of XSS foam, seat L2A with 8 cm 

(3 in) of PU/XSS foam, and seat L2C with 13 cm (5 in) of PU/XSS foam. Nevertheless, 

with regard to levels of discomfort, the average low-coal seam shuttle car operator rating 

favoured the existing seat slightly better than the NIOSH seat with the three different 

viscoelastic foam pad arrangements under full-load and no-load conditions. 

The explanation for this is that the closer proximity of the NIOSH seats to the control 

panel made the shuttle car operators feel awkward and slightly cramped. Researchers had 

to use existing bolt holes when mounting the NIOSH seats in the shuttle car. In addition, 

the NIOSH seats L2B and L2C with 13-cm (5-in) thick foam padding, elevated the 

operators higher, i.e. nearer to the canopy. 

3.2 Objective data 

Data segments showing greatest peak accelerations (most severe incidences of 

vehicle jarring/jolting) were selected from each vehicle operator data set for analysis. 

Figures 3 and 4 are examples of the frequency spectra for the operator/seat interface 

(output acceleration) and the frame (input acceleration) for the NIOSH seat 2C during 

full-load and no-load vehicle operation. 

Figure 3 Frequency spectra input (frame) and output (operator/seat interface) for NIOSH Seat 

L2C when vehicle operated during full-load travel 
 

 

 

 232 A.G. Mayton, F. Amirouche and C.C. Jobes 

 

 

 

 

Figure 4 Frequency spectra input (frame) and output (operator/seat interface) for NIOSH Seat 

L2C when vehicle operated during no-load travel 

3.3 Threshold limit method based on ISO 2631-1 (1985) 

Exposure limits and total overall RMS accelerations (3-directions vector sum) for the 

low- and mid-coal shuttle cars are shown in Tables 1 and 2 with average values for each 

variable. The exposure limit times were computed from the daily exposure 

time-dependent curves, whereas total overall RMS accelerations were computed as the 

square root of the sum of the squares of the weighted accelerations for the x, y, and z axes 

(ACGIH, 2002). It should also be noted that higher exposure limit values indicate better 

isolation performance of the seat, whereas the reverse is true for the vector sum, i.e., the 

lower the value, the better the seat performance. 

When comparing the average exposure limit values for the NIOSH seats L2A, L2B, 

and L2C, under full-load conditions their performance is 31–88% better (i.e., a worker 

can be safely exposed to the vehicle jarring/jolting environment measured for 61–175 

additional minutes). Similarly, comparing the vector sum values, the NIOSH seats L2A, 

L2B, and L2C show a 15–19% reduction in total overall RMS accelerations. 

The no-load conditions provided the more severe levels of jarring/jolting for the 

shuttle car operators. When comparing the average exposure limit values for the NIOSH 

seats L2A, L2B, and L2C, under no-load conditions their performance is 61–198% better 

than the existing L1 (i.e., a worker can be safely exposed to the vehicle jarring/jolting 

environment measured, for 77–249 additional minutes). Comparing the vector sum 

values, the NIOSH seats L2A, L2B, and L2C show a 19–46% reduction in total overall 

RMS accelerations. 
 

 

 

 Comparison of seat designs for underground mine haulage vehicles 233 

 

 

 

 

Table 1 Exposure limits and total overall RMS accelerations (3-directions vector sum) for the 

low-coal seam shuttle car with average values according to seat and vehicle operator 

Low-coal seam shuttle car 

 Full-load – ISO 2631 No-load – ISO 2631 

Seat 

Vehicle 

operator 

Exposure limit, 

min

Vector sum, 

m/sec2 Exposure limit, min

Vector sum, 

m/sec2

L1 1 240 1.81 138 2.55 

 2 180 1.84 150 2.23 

 3 180 2.16 90 3.35 

 Average 200 1.93 126 2.71 

L2A 1 243 1.73 144 2.51 

 2 240 1.55 225 2.03 

 3 300 1.64 240 1.68 

 Average 261 1.64 203 2.07 

L2B 1 360 1.45 150 2.63 

 2 360 1.49 228 1.89 

 3 330 1.74 240 2.09 

 Average 350 1.56 206 2.20 

L2C 1 ND ND ND ND 

 2 600 1.15 360 1.36 

 3 150 2.12 390 1.59 

 Average 375 1.63 375 1.47 

ND: No data. 

Table 2 Exposure limits and total overall RMS accelerations (3-directional vector sum) for the 

mid-coal seam shuttle car with average values according to seat and vehicle operator 

Mid-coal seam shuttle car 

 Full-load – ISO 2631 No-load – ISO 2631 

Seat 

Vehicle 

operator 

Exposure limit, 

min

Vector sum, 

m/sec2 Exposure limit, 

min

Vector sum, 

m/sec2

M1 1 240 2.22 144 2.65 

 2 540 1.37 300 0.92 

 3 900 1.00 240 1.98 

 4 450 1.48 180 2.20 

 5 330 2.06 300 2.16 

 Average 492 1.62 233 1.98 

M2A 1 90 3.10 330 1.63 

 2 440 1.49 660 1.13 

 3 ND ND ND ND 

 4 200 2.06 400 1.48 

 5 ND ND ND ND 

 Average 243 2.21 463 1.41 
 

 

 

 234 A.G. Mayton, F. Amirouche and C.C. Jobes 

 

 

 

 

Table 2 Exposure limits and total overall RMS accelerations (3-directional vector sum) for the 

mid-coal seam shuttle car with average values according to seat and vehicle operator 

(continued) 

Mid-coal seam shuttle car 

 Full-load – ISO 2631 No-load – ISO 2631 

Seat 

Vehicle 

operator 

Exposure limit, 

min

Vector sum, 

m/sec2 Exposure limit, 

min

Vector sum, 

m/sec2

M2B 1 180 2.44 147 2.43 

 2 300 1.58 1140 0.97 

 3 150 2.39 465 1.27 

 4 210 2.06 420 1.48 

 5 165 2.45 345 1.49 

 Average 201 2.19 503 1.53 

ND: No data. 

For the mid-coal seam vehicle, Table 2 shows that the existing seat M1 performed better 

than the NIOSH seats for full-load conditions in terms of exposure limit and total overall 

RMS accelerations. Researchers suspect that this lower performance is attributable to a 

frame-mounted horizontal spring on which the viscoelastic foam padding rested in the 

NIOSH seats. This additional spring layer would increase accelerations at the 

operator/seat interface. However, under the more severe no-load conditions, the NIOSH 

seats performed better than the existing seat M1. When comparing the average exposure 

limit values for the NIOSH seats M2A and M2B under no-load conditions, their 

performance was 99–116% better (i.e., a worker can be safely be exposed to the vehicle 

jarring/jolting environment measured for 230–270 additional minutes). Comparing the 

vector sum values, the NIOSH seats M2A and M2B show a 23–29% reduction in total 

overall RMS accelerations. 

Absorbed power 

Lee and Pradko (1968) first introduced absorbed power as a way to measure the 

energy being transferred from the seat to the body. It is used to measure fatigue, 

endurance, and critical limits in human tolerance. The absorbed power, however, was 

never used in assessing the actual energy transferred between different body segments. 

Amirouche et al. (1994) introduced this concept by modelling the human driver as a 

lumped mass modelled with the muscles and inter-connective forces used as a 

combination of linear and nonlinear elastic springs and damping functions. The models 

are usually validated using accelerometers mounted on drivers’ different body segments. 

In the model used to obtain the results presented in Tables 3 and 4, the human body is 

divided into five different body parts: legs, lower torso, middle torso, upper torso, and 

head. Each body part has its own mass and is connected to the corresponding body parts 

through springs and dampers. Each body part has only one degree of freedom, which is 

the vertical displacement. As input, the seat acceleration data about the z-axis were 

used to compute the results about the whole body. Absorbed power levels and RMS 

(vertical or z-direction) accelerations are for all vehicle operators using the different 

seats. 
 

 

 

 Comparison of seat designs for underground mine haulage vehicles 235 

 

 

 

 

Table 3 Average absorbed power analysis for main body segments of vehicle operators 

driving a low-coal seam shuttle car. (The values below correspond to the average of 

the vertical accelerations obtained for each of five body parts: legs, 

lower torso, middle torso, upper torso and head) 

Low-coal seam shuttle car 

 Full-load No-load 

Seat 

Vehicle 

operator 

RMS vertical 

acceleration, m/sec2 Absorbed 

power, watts 

RMS vertical 

acceleration, m/sec2 Absorbed power, watts

L1 1 1.87 1.00 3.58 1.40 

 2 0.52 0.30 1.85 0.95 

 3 0.85 0.45 4.61 2.03 

 Average 1.08 0.58 3.35 1.46 

L2A 1 0.52 0.33 2.52 1.57 

 2 0.30 0.22 1.75 1.04 

 3 0.39 0.24 1.77 0.97 

 Average 0.40 0.26 2.01 1.19 

L2B 1 0.19 0.15 3.36 1.85 

 2 0.30 0.15 2.01 1.07 

 3 1.93 0.97 2.25 1.22 

 Average 0.80 0.42 2.54 1.38 

L2C 1 ND ND ND ND 

 2 0.23 0.13 1.31 0.69 

 3 0.24 0.21 1.79 0.99 

 Average 0.24 0.17 1.55 0.84 

ND: No data. 

Table 4 Average absorbed power analysis for main body segments of vehicle operators 

driving a mid-coal seam shuttle car. (The values below correspond to the average of 

the vertical accelerations obtained for each of five body parts: legs, 

lower torso, middle torso, upper torso and head) 

Mid-coal seam shuttle car 

Full-load No-load 

Seat 

Vehicle 

operator 

RMS vertical 

acceleration, m/sec2 Absorbed 

power, watts 

RMS vertical 

acceleration, m/sec2 Absorbed power, watts 

M1 1 0.36 0.17 0.32 0.32 

 2 0.30 0.14 0.30 0.16 

 3 0.08 0.07 0.49 0.14 

 4 0.18 0.13 0.14 0.14 

 5 0.25 0.15 0.37 0.17 

 Average 0.23 0.13 0.13 0.18 
 

 

 

 236 A.G. Mayton, F. Amirouche and C.C. Jobes 

 

 

 

 

Table 4 Average absorbed power analysis for main body segments of vehicle operators 

driving a mid-coal seam shuttle car. (The values below correspond to the average of 

the vertical accelerations obtained for each of five body parts: legs, 

lower torso, middle torso, upper torso and head) (continued) 

Mid-coal seam shuttle car 

Full-load No-load 

Seat 

Vehicle 

operator 

RMS vertical 

acceleration, m/sec2 Absorbed 

power, watts 

RMS vertical 

acceleration, m/sec2 Absorbed power, watts 

M2A 1 0.16 0.21 0.18 0.13 

 2 5.04 0.10 36.35 0.10 

 3 ND ND ND ND 

 4 1.34 ND 2.28 0.00 

 5 ND 0.00 ND ND 

 Average 5.04 0.10 12.94 0.08 

M2B 1 0.15 0.16 0.18 0.18 

 2 0.09 0.11 0.08 0.07 

 3 0.11 0.15 0.07 0.07 

 4 0.11 0.14 0.13 0.10 

 5 0.28 0.15 0.12 0.09 

 Average 0.15 0.14 0.12 0.10 

ND: No data. 

As input, the vertical acceleration of the seat was used to compute its vertical 

displacement. Researchers supplied the displacements computed for the spring and 

damper connecting the seat to the lower torso. The programme, consequently, provides 

the acceleration of each body part which is used to compute the RMS acceleration for 

each body. Using the acceleration and the forces applied to each body part (the weight of 

the higher body parts), the labour is obtained. Integrating the labour with regard to time, 

the absorbed power is obtained for each body part. Next, the average absorbed power for 

each body part is computed with regard to the time. Using these averages, the average 

absorbed power representing the driver’s body is computed. RMS acceleration is the sum 

of the RMS accelerations for each body part. 

For both shuttle car models, the sum of the energy during the selected operator 

exposure times (absorbed power) varied from 0.00 to 1.00 watts during full-load travel 

and from 0.00 to 2.03 watts during no-load travel. In general, for the more severe no-load 

travel, the NIOSH seats showed lower levels of absorbed power, i.e., the vibration energy 

absorbed by the body. These seats, with the viscoelastic foam padding, exhibited higher 

damping and better energy dissipation during exposure to vibration, thus indicating better 

comfort. Indeed, absorbed power is the actual energy being circulated at the body level 

and is, in most cases, less than the actual energy transferred from the seat. This result 

occurs from the body’s ability to dissipate the transmitted energy as a form of heat 

(sweat), perspiration, and muscle fatigue. What remains in the body is usually given back 

through the muscles (springs). 
 

 

 

 Comparison of seat designs for underground mine haulage vehicles 237 

 

 

 

 

4 Discussion 

The preceding results, using both the ISO 2631-1 (1985)-based threshold limit method 

from ACGIH (2002) and absorbed power method of analysis, support the notion that 

NIOSH seat designs are superior to the existing vehicle seat designs on both models of 

mine shuttle cars used in this study. The NIOSH seat designs, featuring viscoelastic foam 

padding, were more effective in reducing vibration energy for operators exposed to 

vehicle jarring/jolting. 

Furthermore, the unique contribution of this paper stems from its in-depth 

analysis using experimental data and analytical tools such as the RMS acceleration, the 

ISO 2631-based protocol, and absorbed power to evaluate different seating designs and 

seats with different padding conditions. It is quite remarkable how the seats at low 

frequency favour the padding with high damping and viscoelastic materials more, 

whereas high accelerations with random jolting seem to be unresponsive to cushion and 

padding when the body is in an excited vibration mode, where much more human effort 

is expended to tolerate the energy, vibration, and exposure time. In most cases, the 

primary suspension of the seat must be designed to reduce the vibration so that the 

secondary suspension (the seat padding) can then play this critical role. This paper is a 

good illustration of such a development. 

Acknowledgements 

The authors thank Dean H. Ambrose, N. Kumar Kittusamy, Sc.D., and Timothy J. Matty 

for their contributions to this work. In addition, the authors thank the following 

engineering technicians at the NIOSH Pittsburgh Research Laboratory: Albert H. Cook, 

George F. Fischer, and Mary Ellen Nelson. The authors also express their gratitude to 

Wayne Persinger, Safety Director for the Laurel Alma and Black King Mines of Elk Run 

Coal Co. (an affiliate of A.T. Massey Corp.). Lastly, the authors thank Ron Dickey of 

JOY Mining Machinery, a subsidiary of Harnischfeger Industries, and Charles Yost of 

Dynamic Systems, Inc., for their help and support in this work. 

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