Whole-body vibration exposure 

comparison of seat designs for low- and 

mid-seam shuttle cars in underground 

coal mines 

A.G. Mayton, C.C. Jobes and D.H. Ambrose 

National Institute for Occupational Safety and Health (NIOSH) 

Pittsburgh Research Laboratory, Pittsburgh, Pennsylvania
�

N.K. Kittusamy 

(Deceased)
�

NIOSH Spokane Research Laboratory, Spokane, Washington 

Abstract 

In a systematic study, the National Institute for Occupational Safety and Health (NIOSH) 

evaluated seat designs in low- and mid-seam shuttle cars during production operations at two underground coal mines in southernWest Virginia. The purpose was to support, with additional data, earlier 

findings that NIOSH ergonomic seat designs (featuring viscoelastic foam padding and lower-back 

support) may help reduce health risks to operators of coal mine shuttle cars. Eight shuttle car operators evaluated seven seat designs (one already in use in each vehicle and five NIOSH designs) relative 

to perceived and measured whole-body vibration (WBV) exposure (including vehicle jarring/jolting) 

and discomfort. Operators’ perceptions using a visual analog scale (VAS) and questionnaire ratings 

were compared with International Standards Organization (ISO) 2631-1:1985 fatigue-decreased 

proficiency (FDP) limits and measured WBV levels on low- and mid-seam shuttle cars. Objective 

and subjective data results indicated that NIOSH seat designs (with added adjustability, lower-back 

support and improved seat padding) performed better to reduce vehicle jarring/jolting levels and that 

shuttle car operators favored them over existing seat designs. The NIOSH low- and mid-seam shuttle 

car seats showed 45 to 77 percent better performance in FDP and 9 to 60 percent better performance 

overall in operators’ ratings. Considering the VAS results for low- and mid-seam shuttle cars under 

no-load conditions, operators rated the level of jarring/jolting 18 to 89 percent lower with the NIOSH 

seats. Reductions in measured vehicle jarring/jolting were 19 to 46 percent for the three-directional 

vector sum accelerations relative to the existing seats on the low- and mid-seam shuttle cars. Questionnaire responses indicated that operators for both shuttle car models rated NIOSH seat designs as 

more comfortable overall. Vehicle operators most frequently suggested adding armrests to improve 

the seats on the mid-seam shuttle car. A suggested improvement for the low-seam shuttle car was to 

make the seat a better fit for the operator compartment, which would enhance clearance between the 

operator and vehicle controls and allow for better seat adjustment and operator visibility. 

Introduction 

Moderntransportationvehiclescontinuallyexposeindividuals to whole-body vibration (WBV) and mechanical shock. 

These include airplanes, ships, trains and a variety of industrial 

and agricultural equipment. Exposing individuals to WBVand 

mechanical shock can negatively impact their health, safety, 

comfort and working efficiency and performance.

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 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.

Figure 1 — JOY 21SC low-seam shuttle car (courtesy JOY Mining Machinery). 

Figure 2 — JOY 10SC mid-seam shuttle car (courtesy JOY Mining Machinery). 

A study by Mayton et al. (1999) reported on a low-seam 

shuttle car seat design that underwent limited, yet positive, 

undergroundminetrials. Researchhasshownthatunderground 

coal mine equipment operators experience adverse levels of 

exposure to WBV and vehicle jarring/jolting (mechanical 

shock). This exposure is identified as the higher-amplitude, 

peak components of WBV. Shuttle car haulage vehicles are 

amongthemajorsourcesofexposuretovehiclejarring/joltingin 

underground coal mines. Remington et al. (1984) showed that 

WBV was severe for these vehicles, as well as for load-hauldumps (LHDs), or scoops. These circumstances have changed 

little since 1984, as evidenced by injury statistics and operator 

testimonials about these vehicles. Injury reports (narratives) 

from the Mine Safety and Health Administration (MSHA) 

databasewereanalyzedformobileminingequipmentoperators 

in association with vehicle jarring/jolting. These injuries can 

be described as acute and chronic musculoskeletal disorders 

affecting the back, neck and head. The trend for total back, 

neck and head from MSHA-reported injuries on underground 

mine shuttle cars showed a slight decline for the period 1999 

- 2005. Nevertheless, vehicle jarring/jolting-related injuries 

averaged 75 % (253) per year of the total (338) back, neck and 

head injuries for mine shuttle car operators.

Additional evidence exists to illustrate that serious health 

effects can result from prolonged exposure of vehicle operators 

to jarring and jolting. Critical surveys of the literature have 

concluded that exposure to long-term WBV and awkward postures can adversely affect the spine and can increase the risk of 

low-back pain (Kittusamy and Buchholz, 2004; McPhee et al., 

2001; Bernard and Fine, 1997; Wikström et al., 1994; Seidel 

and Heide, 1986; Hulshof and van Zanten, 1987).

In an Australian study, Cross and Walters (1994) identified 

WBV and vehicle jarring as a contributing factor to back pain 

in the mining industry and as a significant concern to mobile 

equipment operators. They reviewed 28,306 compensation 

claims for a four-year period (July 1986 to March 1990), including surface and underground mining environments. Of the 

8,961 claims relating to the head, back and neck, 11% (986) 

were related to vehicular jarring. Underground transporters 

and shuttle cars accounted for 53% of all injuries attributed 

to vehicle jarring.

The intent of this current study was to support earlier work 

with a more systematic evaluation of the low-seam shuttle car 

seat design and a second mid-seam shuttle car seat design. 

With additional information and a larger sample of shuttle car 
 

 

 

operatorsinthisstudy,researchersaffirmedearlierfindingsthat 

NIOSH seat designs, with unique viscoelastic foam padding, 

weremoreeffectivethansomeexistingseatsinisolatingshuttle 

car operators from jars and jolts. Moreover, the results of this 

study are relevant to the mining industry, since injury data and 

research studies have identified mine shuttle cars as major 

sources of adverse WBV exposure. Improving seat designs 

is important to reduce the adverse effects of WBV exposure 

and minimize health risks to vehicle operators. 

(a) (b)
�

Figure 3 — Low-seam shuttle car seats: existing (a) and NIOSH (b).
�

 

 

 

(a) (b)
�

Figure 4 — Mid-seam shuttle car seats: example of existing (a) 

and NIOSH (b). 

Methodology

Seat design trials were conducted in two West Virginia 

mines on a JOY 21SC shuttle car (Fig. 1) operating at a lowseam mine, <122 cm (4 ft) and a JOY 10SC side-saddle-style 

shuttle car (Fig. 2) operating at a mid-seam mine, 122 to 144 

cm (4 to 4.7 ft). Different seat designs were compared on 

each shuttle car. The existing seats, shown in Figs. 3a and 

4a, were designated Seats L1 and M1 for trials with low- and 

mid-seam shuttle car models, respectively. The NIOSH seats 

shown in Figs. 3b and 4b featured viscoelastic foam padding, 

arranged as follows for the low-seam shuttle car (Fig. 5): 

Seat L2A included a combination of Pudgee (PU) and SunMate Extra-Soft (XSS) foams, 7.6 cm (3 in.) thick; Seat L2B 

included XSS foam padding, 12.7 cm (5 in.) thick and Seat 

L2C included a combination of PU and XSS foams, 12.7 cm 

(5 in.) thick. The NIOSH seats for the mid-seam shuttle car 

featured viscoelastic foam padding arranged as follows (Fig. 

6): Seat M2A’s foams included XSS foam, 12.7 cm (5 in.) 

thick and Seat M2B included a combination of PU and XSS, 

12.7 cm (5 in.) thick.

Figure 7 shows the instrumentation setup on the low- and 

mid-seam shuttle cars, respectively. Three low-seam shuttle 

car operators were tested on four seats: the existing seat, 

L1, and three NIOSH test seats, L2A, L2B and L2C. Five 

mid-seam shuttle car operators were tested on three seats: the 

existing seat, M1, and two NIOSH test seats, M2A and M2B. 

The data sets were divided into full-load and no-load conditions. 

Subjects. 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 of 

age and averaged 39 years. They ranged in height from 175 

to 185 cm (69 to 73 in.), with an average height of 180 cm (71 

in.) and they ranged in weight from 73 to 91 kg (161 to 201 

lb), with an average weight of 87 kg (192 lb). The subjects’

experience at operating a shuttle car varied from one-half to 24 

years and averaged 9 years. Similarly, their underground mining experience varied from one-half to 37 years and averaged 

14 years. Before participating, the shuttle car operators were 

briefed about the study and were asked to sign an informed 

consent and photo release forms. 

Figure 5 — Viscoelastic foam padding arrangements. 

L2A, 7.6 cm thick with two layers XSS and one PU; L2C, 

12.7 cm thick with four layers XSS and one PU; L2B, 12.7 

cm thick with five layers XSS and no PU. 

Pudgee (PU) layer 

M2A M2B

Figure 6 — Viscoelastic foam padding arrangements - 12.7 

cm thick. M2A (no PU) and M2B (one PU). 

Figure 7 — Instrument setups on the low- (a) and mid- (b) seam shuttle 

car seats. 

Procedure for vibration data collection. Objective data 

were collected using accelerometers with pre-amplifiers and 

filters connected to a data recorder. Subjective data were gathered with a visual analog scale (VAS) and a questionnaire. An 

eight-channnel digital data recorder (model PC208Ax, Sony 

Manufacturing Systems America, Lake Forest, CA) used with 

instrumentation from PCB Piezotronics, Inc., Depew, NY. 

These included triaxial accelerometers (models 356B18 and 

356B40), signal conditioning amplifiers (model 480E09) and 

in-line, 150-Hz low-pass filters (model 474M32). A floor- or 

frame-mounted accelerometer featured a frequency range of 

0.3 Hz to 5 kHz and a charge sensitivity ranging from 949 to 

1052 mV/g for the three directional axes. A seat pad accelerometer featured a frequency range of 0.5 Hz to 1 kHz and a 

charge sensitivity ranging from 97.4 to 105 mV/g for the three 

directional axes. One triaxial accelerometer was mounted on 

the frame of the shuttle car above the control panel (frame or 

chassis measurement) and one on the seat at the subject/seat 

interface (seat measurement). The frame accelerometers were 

ordinarily mounted on the floor of the operator’s compartment 

near the base of the seat, but muddy conditions dictated the 

above mounting locations; this did not affect the data. Data 

were analyzed to determine the acceleration and transmissibility of energy entering the seat from the vehicle frame 

or chassis. During the field trials, roadway conditions were 

noted as smooth, pothole-riddled, debris strewn, rutted, dry, 

wet and/or water-filled. 
 

 

 

 

 

Table 1 — Total average VAS ratings for low-seam JOY 21SC and mid-seam JOY 10SC shuttle car operators (lower 

value = better). 

Low-seam JOY 21SC 

shuttle car seat 

Mean of average 

operator ratings 

Mid-seam JOY 10SC 

shuttle car seat 

Mean of average 

operator ratings 

ID No.’s 1, 

2 and 3 

ID No.’s 1, 

2, 3, 4 and 5

 NO LOAD 

L1 - Existing seat: M1 - Existing seat:

 Level of jar/jolt 0.395 Level of jar/jolt 0.742

 Level of discomfort 0.372 Level of discomfort 0.738 

L2A - NIOSH seat (3-in. XSS/PU): M2A - NIOSH seat (5-in. XSS):

 Level of jar/jolt 0.325 Level of jar/jolt 0.085

 Level of discomfort 0.438 Level of discomfort 0.093 

L2B - NIOSH seat (5-in. XSS): M2B - NIOSH seat (5-in. XSS/ 

PU):

 Level of jar/jolt 0.170 Level of jar/jolt 0.432

 Level of discomfort 0.396 Level of discomfort 0.334 

L2C - NIOSH seat (5-in. XSS/PU):

 Level of jar/jolt 0.273

 Level of discomfort 0.301 

FULL LOAD 

L1 - Existing seat: M1 - Existing seat:

 Level of jar/jolt 0.433 Level of jar/jolt 0.652

 Level of discomfort 0.223 Level of discomfort 0.666 

L2A - NIOSH seat (3-in. XSS/PU): M2A - NIOSH seat (5-in. XSS):

 Level of jar/jolt 0.170 Level of jar/jolt 0.061

 Level of discomfort 0.377 Level of discomfort 0.087 

L2B - NIOSH seat (5-in. XSS): M2B - NIOSH seat (5-in. XSS/ 

PU):

 Level of jar/jolt 0.054 Level of jar/jolt 0.424

 Level of discomfort 0.344 Level of discomfort 0.366 

L2C - NIOSH seat (5-in. XSS/PU):

 Level of jar/jolt 0.189

 Level of discomfort 0.364 

Procedure for subjective data collection. The subjective 

data were gathered using a visual analog scale (VAS) form to 

obtain the operators’ immediate impressions of shock, vibration and discomfort levels for the vehicle ride on each of the 

seats and viscoelastic foam configurations. The VAS consisted 

of a line approximately 10 cm (4 in.) long and terminated at 

each end by a vertical hash denoting the two limits of jarring/

jolting level and level of discomfort, essentially “no” and 

“high or extreme.” The rating scale was scored by measuring 

the distance (from left to right) from the beginning of the line 

to the operator’s mark and dividing this value by the total 

length of the line. The shuttle car operator marked this scale 

after traveling with and without a full load of coal on the first, 

third and sixth round trips of the trials for each seat. A round 

trip consisted of traveling to the coal face with no load and 

returning to the load discharge location with a full load of coal. 

After the each segment of the trip, participants were asked to 

rate the vehicle ride in terms of the level of jarring and jolting experienced through the selected seat. These values were 

summed and averaged to obtain an average operator rating for 

the individual seats. In turn, the average operator ratings were 

summed and averaged to obtain a total average operator rating.

Subjective data were also obtained using a short questionnaire (Appendix A) to interview shuttle car operators. The 

interview, lasting five to 10 minutes, occurred at the conclusion 

of each trial for each seat. The seven questions asked of the 

shuttlecaroperatortojudgehisexposurerelativetocomfortand 

vibrationorshock,givehisopinionsonseatpaddingandlumbar 

support,indicatehislikes,dislikesandsuggestedimprovements 

concerning each seat and lastly, summarize comparisons of the 

seats. In order to quantify the responses, most of the questions 

included a 4- or 5-point scale to describe degrees of comfort 
Table 2 — Pearson correlations from VAS ratings relating level of jarring/jolting to discomfort for low-seam JOY 21SC and midseam JOY 10SC shuttle car seats (P ≤ 0.05). 

Low-seam JOY 21SC Pearson correlation Mid-seam JOY 10SC Pearson correlation 

shuttle car seat coefficient, r shuttle car seat coefficient, r 

NO LOAD 

L1 - Existing seat: 0.29 M1 - Existing seat: 0.89 

L2A - NIOSH seat (3-in. XSS/PU): 0.84 M2B - NIOSH seat (5-in. XSS) 0.71 

L2B - NIOSH seat (5-in. XSS): 0.88 M2B - NIOSH seat (5-in. XSS/PU): 0.94 

L2C - NIOSH seat (5-in. XSS/PU): 0.58 

FULL LOAD 

L1 - Existing seat: 0.66 M1 - Existing seat: 0.93 

L2A - NIOSH seat (3-in. XSS/PU): 0.88 M2B - NIOSH seat (5-in. XSS) 0.90 

L2B - NIOSH seat (5-in. XSS): 0.69 M2B - NIOSH seat (5-in. XSS/PU): 0.97 

L2C - NIOSH seat (5-in. XSS/PU): 0.73 

XSS Extra-soft Sun-Mate. PU Pudgee. 

 

 

 

 

Figure 9 — Seat comfort and vibration reduction ratings 

based on questionnaire responses from mid-seam shuttle 

car operators. 

Figure 8 — Ratings of seat design for comfort and vibration reduction, based on questionnaire given to low-seam 

shuttle car operators. 

and poor/good or worse/better seat performance.

Objective data were analyzed according to the American 

Conference of Governmental Industrial Hygienists (ACGIH) 

TLVs (threshold limit values), ACGIH (2006) and ISO 2631-

1:1985 fatigue-decreased proficiency (FDP) methods. Subjective data were analyzed with simple statistics for frequency 

of occurrence and arithmetic mean. The Pearson correlation 

analysis was performed on the VAS data. Owing to the small 

samplesizes,theauthorsbelievedperformingadditionalstatistical analysis on the data would not yield any meaningful results.

Theconceptsofpeakacceleration,root-mean-square(RMS) 

acceleration,crestfactorandoverallweightedtotalRMSacceleration were used in the data analysis as a means for assessing 

vehicle jarring/jolting exposure. These analytical parameters, 

presented in ISO 2631-1:1985 and by the American National 

Standards Institute (ANSI) (1979), were used by the ACGIH 

(2006) to develop the TLVs. The method assesses the effects 

of environmental vibration on the human body relative to 

health, efficiency and comfort. Human exposure to vibration 

is described relative to three broad criteria: health and safety 

(exposure limit), working efficiency (FDP, fatigue-decreased 

proficiency) and comfort (RCB, reduced-comfort boundary) 

(ISO, 1985, ANSI, 1979). The ACGIH (2006) points out 

that the TLVs may not be adequate for evaluating a vibration 

environment characterized by high-amplitude mechanical 

shocks (jars or jolts) and it may “underestimate the effects of 

WBV...when crest factors exceed 6.” The ISO 2631–1:1997 

recommends a crest factor of 9.0. The crest factor is a way of 

describing the rate of change in acceleration.

Some optional methods of analysis concerning the impact 

of jars and jolts on the body are presented in ISO 2631–1:1997 

and ISO 2631–5:2004. ISO 2631–1:1997 offers health guid-
ance caution zones based on the RMS value of the frequencyweighted acceleration. In cases where the crest factor exceeds 

9, the running RMS method featuring the maximum transient 

vibration value and the fourth-power vibration dose value 

method are suggested. On the other hand, ISO 2631–5:2004 

presents a method for evaluating vibration containing multiple 

shocks (jars and jolts) using the spinal response acceleration 

dose. Moreover, Griffin (1990, 1998) discusses the evolution of ISO 2631–1 from its inception and points out various 

shortcomings through the 1997 revision. Griffin suggests that 

ISO 2631–1:1997 may cause unneeded confusion relative to 

the measurement, evaluation and assessment of human shock 

and vibration exposures. These optional methods were not 

used for comparisons in this study, because researchers considered the FDPcriteria (included as part of ISO 2631–1:1985) 

more relevant since they were utilized in the earlier study by 

Remington et al. (1984) and the TLVs of ACGIH (2006) were 

based on ISO 2631–1:1985 and ANSI S3.18:1979. 

Table 3 — Performance comparison for no-load conditions using FDP limit values, crest factors and vehicle operator questionnaire ratings with percent better than existing seats L1 and M1.

 Percent better than seat L1 or M1 

Seat Fatiguedecreased 

proficiency 

(FDP) (min) 

Crest factor Question #2 

rating (Scale: 

1.0 to 4.0) 

Fatiguedecreased 

proficiency 

(FDP) 

Crest factor Question #2 

rating 

JOY 21SC 

(No-Load) 

L1 32 1.02 3 — — — 

L2A 59 0.88 3.3 45 16 9 

L2B 59 0.94 4 46 9 25 

L2C 139 1.02 3.3 77 0 9 

JOY 10SC 

(No-Load) 

M1 76 1.47 2.00 — — — 

M2A 176 1.13 3.75 57 30 47 

M2B 190 1.31 2.40 60 12 17 

NOTE - Crest factor (peak acceleration/RMS acceleration) is dimensionless. Higher values for “questionnaire” and “fatigue 

decreased” indicate better results, whereas lower values for “crest factor” indicate better results. 

Results 

Subjective evaluation - VAS. Table 1 illustrates the total 

average ratings for the shuttle car operators of the JOY 21SC 

low-seam and JOY 10SC mid-seam shuttle cars, respectively. 

The lowerratingsfromVASresponsesindicate thattheNIOSHdesigned seats performed better than the existing seats used 

in the shuttle cars. Accordingly, the following observations 

were made: 

• For both no-load and full-load conditions, mid-seam 

shuttle car operators observed lower levels of jarring/jolting 

and discomfort with the NIOSH seats (M2A and M2B), using 

two different 12.7-cm (5-in.) viscoelastic foam pad arrangements. 

• The viscoelastic foam arrangement (M2A) with five 2.5-

cm (1-in.) layers of XSS foam padding was the most preferred 

by operators of the mid-seam shuttle car.

• Similarly, for no-load and full-load conditions, lowseam shuttle car operators rated jarring/jolting as lower with 

the NIOSH seats (L2A, L2B and L2C), using three different 

viscoelastic foam arrangements. The viscoelastic foam arrangements, in order of operator preference, were the 12.7-cm 

(5-in.) XSS (L2B), 7.6-cm (3-in.) PU/XSS (L2A) and 12.7-cm 

(5-in.) PU/XSS (L2C) padding.

• However, under no-load and full-load conditions for 

the low-seam shuttle car, operators rated discomfort levels 

as lower with the existing seat (L1) versus the NIOSH seats 

(L2A, L2B and L2C) using three different viscoelastic foam 

pad arrangements. Because researchers had to use existing 

bolt holes to install the NIOSH seat, the seat was closer to the 

control panel and made the shuttle car operators feel awkward 

and cramped.

Pearson correlation values were calculated for the low- and 

mid-seam shuttle car seats and are displayed in Table 2. The 

following is a summary of the results obtained: a weak-tostrong positive correlation (for jar/jolt and discomfort) was 

realized for different seats tested on the low-seam JOY 21SC 

shuttle car (P ≤ 0.05). Moreover, a strong positive correlation for jarring/jolting and discomfort was determined for the 

different seats tested on the mid-seam JOY 10SC shuttle car. 

All correlation values for the mid-seam shuttle car showed a 

significant relationship between the variables of jarring/jolting 

and discomfort (P ≤ 0.05). 

Subjective evaluation - operator questionnaire. JOY 

21SC low-seam shuttle car - Figure 8 shows seat ratings regarding comfort and shock/vibration reduction. L2B was rated 

highest in comfort, vibration reduction, seat padding, lumbar 

support and seat-pan tilt. Seat L1 (the existing seat) ranked 

the lowest in seat comfort and vibration reduction. Operators 
liked the degree to which this seat reduced jars and jolts and 

one operator thought it was fairly comfortable. However, 

operators disliked its lack of durability and lack of lumbar 

support. Suggestions to improve the seats were to make the 

back support better, improve adjustments for better visibility 

and improve padding. Seat L1 did not have seat-pan tilt or 

Table 4 — Perceived discomfort and jarring/jolting levels versus measured reductions in vector sum accelerations for operators of the JOY 21SC low- and JOY 10SC mid-seam shuttle cars (no load) (Mayton et al., 2003 and 2006). 

Seat Perceived reduction Measured reduction 

Discomfort (%) Jarring/ jolting Vector sum (%) 

(%) 

JOY 21SC (No-Load) 

L1 - Existing seat — — — 

L2A - NIOSH seat (3-in. XSS/PU) 118 18 24 

L2B - NIOSH seat (5-in. XSS) 16 56 19 

L2C - NIOSH seat (5-in. XSS/PU) 19 31 46 

JOY 10SC (No-Load) 

M1 - Existing seat — — — 

M2A - NIOSH seat (5-in. XSS) 87 89 29 

M2B - NIOSH seat (5-in. XSS/PU) 55 42 23 

Vector sum (m/s2) = Overall weighted total RMS acceleration for three directions (x, y and z axes). 

1Increase instead of reduction. 

fore-aft adjustment.

Seats L2A and L2B ranked high in comfort and vibration 

reduction. Operators liked how seat L2A took the strain off 

the lower back when the shuttle car traveled across large holes. 

Also, operators liked the thick cushion on seat L2A and how 

the seat adjusts to the body. For seat L2B, operators liked 

how comfortable it felt, how it reduced shocks and its thick 

cushion. However, both prototype seats were too close to the 

controls and did not adequately fit the available space of the 

shuttle car operator workstation.

Regarding seat L2B, operators found that they drove the 

shuttle car slower to avoid being bounced into the canopy. 

Suggested improvements for seats L2Aand L2B were to make 

the lumbar support wider. One operator suggested improving 

the operators’ control panel envelope to accommodate better 

seats, such as seat L2B.

Concerning seat L2C, operators liked the padding, how 

well it reduced jarring/jolting, how well the lumbar support 

took the strain off the back and the general comfort of the seat. 

Operators, however, did not like the lumbar width. Also, the 

seat seemed too large for the shuttle car operator workstation. 

Suggested improvements for seat L2C were to make the lumbar support and seat wider and to add a scaled-down seat (by 

reducing the padding) to better fit the control layout.

Ratings in Fig. 8 show seat L2C comparable to seat L2A

in comfort, but better in reducing vibration. Seat L1 is the 

least favorite in all ratings. Seat padding, lumbar support and 

seat-pan tilt were rated better for seat L2B than any other seat. 

The reclining backrest, favored by one operator on seat L1, 

overall was better on seat L2B. Making the seat a better fit 

for the operator compartment (workstation) was a suggested 

improvement. This adjustment would improve clearance 

between the operator and the controls and allow for better 

operator adjustability and visibility.

JOY 10SC mid-seam shuttle car - Figure 9 displays the 

operators’ responses to questions about reductions in vibration (jarring/jolting) and discomfort. Seat M2A was ranked 

highest in comfort and vibration reduction, as well as seat 

padding and lumbar support. Seat M1 ranked the lowest in 

comfort, vibration reduction, seat padding and lumbar support; 

however, one operator liked its comfort and another liked the 

way the body fit the seat frame. “Partly broken,” “weak” and 

“no comfort” highlighted the descriptions for seat M1. Suggested improvements to seat M1 included adding armrests 

and removing and replacing the seat with the one originally 

installed on the vehicle. 

Seat M2A was ranked most favorable, in that operators 

liked the seat’s apparent ability to absorb vibration and jars, as 

well as its good back support and comfort. The seat seemed 

to adversely affect visibility and was somewhat close to the 

control panel. Adding armrests and improved seat positioning 

relative to vehicle operator controls were primary suggestions 

for improving seat M2A.

Operators appreciated the comfort and firmness of seat 

M2B, but viewed its poor ability to absorb jars/jolts and stiff 

back support as shortcomings. Operators offered several suggestions to improve seat M2B, such as making the seat softer, 

adding armrests and improving the lumbar support.

Seat comparison rankings reflected how the operators 

viewed the seats relative to each other. Again, Seat M2A was 

the favorite. Seat padding rated well for both Seat M2A and 

Seat M2B. Seat M1 (the existing seat) was the least favorite in 
 

 

 

 

all ratings. Adding armrests was the improvement most often 

suggested for all of the seats. 

Measured vehicle jarring/jolting and questionnaire 

data compared to ISO 2631-1:1985 FDP limits. Data were 

collected on the JOY 21SC at the low-seam coal mine in West 

Virginia. No information could be salvaged from operator 1 

on seat L2C, owing to excessive battery bounce in the data 

recorder caused by a very rough ride. Thus, from a possible 

24 data sets, researchers obtained and analyzed a total of 22.

Similarly, data were collected on the side-saddle-style 

shuttle car atthemid-seammine. Noinformationwas collected 

from operator Nos. 3 and 5 on seat M2A due to other mining 

operation duties of these test subjects. Thus, from a possible 

30 data sets, researchers obtained and analyzed a total of 26 

data sets. The results were distinguished by vehicle operation 

during full-load and no-load conditions.

FDPlimitsfromthe InternationalOrganizationforStandardization (ISO) 2631-1:1985 were compared with the objective 

and subjective data discussed above. Griffin (1990) describes 

FDP as a concept created to define vibration exposure limits 

relative to exposure duration for working efficiency. 

Considering the shuttle car operator receives the roughest 

ride when traveling with no load, acceleration levels tend to 

be higher during no-load conditions, since the shuttle car has 

less mass while maintaining the same spring rate and damping. 

The natural frequency of the vehicle shifts higher in the noload condition and lower in the full-load condition, as shown 

by the equation 

 

 (1) 

where: 

ωn = natural frequency;

k = spring constant and 

m = mass. 

FDP limits correlated with the results obtained from 

measured levels of vehicle jarring/jolting and questionnaire 

responses for the different vehicle operators and seat designs 

(Mayton et al., 2006). Table 3 provides data for the JOY 21SC 

and JOY 10SC shuttle cars when operating under no-load 

conditions. For the JOY 21SC, seat L2A showed 45% better 

performance in FDP, 16% better performance in the crest factor 

and 9% better performance overall when rated by the operators. Seat L2B showed 46% better performance in FDP, 9% 

better performance in the crest factor and 25% better overall 

performance when rated by the operators. Seat L2C showed 

77% better performance in FDP, no change in the crest factor 

and9% betteroverallperformancewhenratedbytheoperators. 

For the JOY 10SC, seat M2A showed 57% better performance in FDP, 30% better performance in the crest factor and 

47% better overall performance when rated by the operators. 

Seat M2B showed 60% better performance in FDP, 12% 

better performance in the crest factor and 17% better overall 

performance when rated by the operators. 

 

Comparison of VAS and Measured Data. Table 4 shows 

the results for the VAS and measured data using overall 

weighted total RMS acceleration or vector sum. The VAS 

ratings showed that vehicle operators overall (on average from 

three test trial ratings) rated the NIOSH-designed seats better 

than the existing seats. Regarding the low-seam shuttle car, 

under no-load conditions, operators rated the level of jarring/

jolting 18 - 56% lower with the L2A, L2B and L2C seats. In 

addition, operators rated the level of discomfort 19% lower 

with seat L2C, but 6 - 18% higher with the L2Aand L2B seats. 

Regarding the mid-seam shuttle car, under no-load (worse 

case of two) conditions, operators rated the level of jarring/

jolting 42 - 89% lower and level of discomfort 55 - 87% lower 

with the M2Aand M2B seats. Reductions in measured vehicle 

jarring/jolting are shown in terms of vector sum accelerations 

relative to the existing seats L1 and M1. For the low-seam 

shuttle car, the percent reduction in the three-directional vector 

sum accelerations ranged from 19 - 46% and for the mid-seam 

shuttle car from 23 - 29%. 

Discussion 

No known studies of mine shuttle car seat design relative to 

WBVexposurehavebeenreportedasidefromthisNIOSHstudy. 

Eger et al. (2006) evaluated vibration exposure for operators of 

a variety of heavy surface and underground mining vehicles. 

Although none of the underground vehicles were mine shuttle 

cars, the study included LHDs (two 3.5-yd and one 7-yd) which 

are similar in function to shuttle cars, yet different in vehicle 

design. They analyzed WBV exposures relative to the ISO 

2631-1:1997 Health Guidance Caution Zone (HGCZ) limits 

associated with an eight-hour daily exposure. The smaller 

capacity LHDs showed exposures above the HGCZ, whereas 

the higher capacity LHD exposures were within the HGCZ. 

Their results were comparable to exposures reported by Village et al. (1989). Furthermore, the Remington et al. (1984) 

study estimated that 30 to 40 percent of all underground mining 

machines may exceed the FDP criteria and cited that shuttle 

cars were a primary source of WBV exposure. Consequently, 

using the FDP criteria for comparison in this NIOSH study 

was deemed relevant. 

Amirouche et al. (1991) and Tong et al. (1999a) reported 

on analytical computer models for optimizing the energy 

absorption during a human body’s exposure to vibration and 

for evaluating the distribution of absorbed power and how 

the body reacts to roadway-induced vibration. Using their 

model to study energy absorption and work done by the body’s 

muscles (represented as springs and dampers) during a rough 

ride, Tong et al. (1999a) discussed how energy is transmitted 

to different parts of the body and what happens when input 

conditions change. Understanding the energy flow among 

the body’s parts can provide valuable input for the design of 

a seat and its suspension. The application of this approach to 

mining vehicle seats requires further study.

Mayton et al. (2005) compared the NIOSH seat designs 

according to the absorbed power method discussed by Amirouche et al. (1991) and Tong et al. (1999a and 1999b). The 

results concurred with the aforementioned analytical results 

of this study in showing lower energy absorption to the body.

Distinguishingtheoperationaldifferencesbetweentraveling 

with a full load versus no load is again worth noting. During full-load conditions, the foam- or air-filled tires provided 

primary damping or attenuation of jars/jolts as a result of the 
 

 

 

 

 

 

 

 

 

 

 

 

 

extra mass from the load of coal. The performance of the 

seat in providing this attenuation of jars/jolts is thus secondary. However, the reduced mass under no-load conditions 

allows for more severe levels of jarring/jolting for the shuttle 

car operators. Consequently, it is significant that the NIOSHdesigned seats performed better than the existing seats under 

no-load conditions when comparing average values for peak 

acceleration, RMS acceleration and crest factor.

The use of seat foam padding alone is not the ultimate 

answer in providing optimum isolation for vehicle operators. 

Seat foammaterials willamplify vibration atlower frequencies 

(1.7 to 5.5 Hz) as shown in investigations reported by Jobes 

and Mayton (2006). Nevertheless, the NIOSH seat designs 

showed definite improvements over the existing seat designs 

for the shuttle car models studied. 

Joy Mining Machinery has been marketing the NIOSH seat 

designs and includes the improved seat design in its current 

productline. Thecompanyindependentlytestedthenewdesign 

and affirmed the results of the NIOSH studies. In terms of the 

U.S. market for low-seam shuttle cars, an estimated 51% of 

shuttle cars are now equipped with the improved seat design. 

Since 1999, it is estimated that the improved seat designs may 

have positively impacted the health and safety of nearly 1,980 

shuttle car operators. 

Limitations 

This study provides useful results and information regarding shuttle car seat designs for two models of underground 

coal mine shuttle cars. The primary limitations of the study 

included: the small sample size of eight subject shuttle car 

operators, the constraints of conducting field trials during 

coal mining production operations, the differences in driving 

technique among individual subjects, two underground coal 

mines and no non-coal mines, the inability to drive the same 

route on every trip (the same roadway was used), the worn 

existing seat versus the virtually new NIOSH seats, the short 

period of time that subjects used the NIOSH seats and the inability to measure the durability and reliability of the NIOSH 

seat designs over time. 

Conclusions 

The objective of this study was to gather additional data 

to support earlier findings that NIOSH seats, with unique viscoelastic foam padding, are indeed improved designs for coal 

mine shuttle cars. A larger sample of shuttle car operators was 

includedinthisworkcomparedtoapriorNIOSHinvestigation.

Theresultsobtainedfromtheanalysisofquantitative(objectiveormeasured)data,qualitative(subjective)dataandtheFDP

analysis demonstrate that NIOSH-designed seats performed 

better than existing seats for both shuttle car models. This 

was particularly significant concerning vehicle jarring/jolting 

levels for the worse of the two operating conditions, no load. 

The quantitative levels of vehicle jarring/jolting for no-load 

conditions showed that the NIOSH L2A, L2B, L2C, M2A and 

M2B seats for the low- and mid-seam shuttle cars performed 

better than the existing seat in terms of overall weighted total 

RMS acceleration or vector sum acceleration and crest factor. 

The NIOSH seat designs show greater effectiveness in reducing levels of jarring/jolting and generally enhancing operator 

comfort, considering the limitations indicated with the NIOSH 

seat installations for the low-seam shuttle car. Questionnaire

responses indicated that operators for both shuttle car models 

rated NIOSH seat designs as more comfortable overall. Future 

researchshouldstudytheeffectsofcombiningviscoelasticfoam 

seatpaddingwithpassive,semi-activeoractiveseatsuspension 

system, such as that described by Tong et al. (1999a, 1999b).

These results can provide the mining industry with additional evidence that NIOSH seat designs are improvements to 

existing designs for isolating operators from vehicle jarring/

jolting. In addition, the results of this study have afforded 

the equipment manufacturer the opportunity to further refine 

and improve the NIOSH seat designs from the added input of 

shuttle car operators. Furthermore, the results of this study may 

have potential application for the seats of other heavy off-road 

vehicles used in surface mining, construction and agriculture. 

Acknowledgments

The authors thank the following engineering technicians 

with the NIOSH Pittsburgh Research Laboratory: Albert H. 

Cook for his work in constructing foam pads for the NIOSH 

seatsandassistancewithminefieldtrials;andGeorgeF.Fischer 

(retired) and Mary Ellen Nelson for their efforts in constructing 

the NIOSH seats for the shuttle car field trials. The authors 

also express their appreciation to the mine Safety Director, 

his staff and the mine management and hourly workers at the 

underground coal mines in West Virginia for their cooperation 

in the mine field trials in the late 1990s. In addition, the authors 

thank Joy Mining Machinery for their help and support in this 

work. Finally, the authors thank Dynamic Systems, Inc., for 

supplying the viscoelastic foams used as seat padding for the 

various padding configurations in the NIOSH seat designs. 

Disclaimer 

The findings and conclusions in this report are those of 

the authors and do not necessarily represent the views of the 

National Institute for Occupational Safety and Health. 

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Appendix A

Interview questions for shuttle car operators: 

1. How would you rate this seat in terms of comfort?

1 = very comfortable 2 = comfortable 3 = uncomfortable 

4 = very uncomfortable 

2. How would you rate this seat relative to reducing shock

and vibration? 

1 = very good 2 = good 3 = fair 4 = poor 

3. What do you like about this seat? 

4. What don’t you like about this seat? 

5. Rate the following: seat padding, lumbar support, reclining 

seatback, seat-pan tilt, armrest, fore-aft adjustment using

the scale 1 = poor 2 = fair 3 = good 4 = very good 5

= excellent. 

6. What would you do to improve this seat? 

7. Compare seat No. 1 with seat Nos. 2 a, b and c using

the scale 1 = much worse 2 = worse 3 = same 4 = better 

5 = much better.