Ergonomic seat with viscoelastic
foam reduces shock on underground
mobile equipment

Alan Mayton and Sean Gallagher

National Institute for Occupational Safety and Health, Pittsburgh Research Center, P.O. Box 18070, Pittsburgh, PA 15236

Ron Merkel

Roush Anatrol, 11916 Market Street, Livonia, MI 46150

Operators of underground mobile
equipment, particularly shuttle cars,
are often exposed to significant levels
of whole-body vibration (WBV) and
shock. The Human Factors group at
the NIOSH-Pittsburgh Research
Center has investigated the use of
viscoelastic foam to reduce shock for
the equipment operator and improve
seats on mine shuttle cars. In-mine
data were recorded on a JOY 21 SC
shuttle car for the original seat and
an ergonomic version with viscoelas-
tic foam. A review of the data shows
improved isolation across the driver/
seat pad interface. For the full-load
case, the ergonomic seat decreased
transmissibility and isolated the
shuttle car operator down to 15 Hz.
Additional testing of the foam
materials with the development and
use of a lumped-parameter analytical
model showed different composites of
the foam materials can reduce the
isolation frequency to below 5 Hz.
This paper describes the under-
ground mine trials and the testing
done to evaluate properties of the
viscoelastic foams. The paper also
discusses the development of an
analytical model using the data from
underground trials and the foam
testing.

Introduction

Studies sponsored by the U. S

Bureau of Mines (USBM) have
reported that as many as one-third of
the equipment operators in under-
ground coal mines may be exposed
to adverse levels of whole-body

vibration (WBY) (Remington et al.,
1984). Shuttle cars are identified as a
primary source of this adverse
exposure. The operator experiences
WBY and shock when the shuttle car
travels over rough mine floor marked
by bumps, ruts, and potholes. In
low-coal mines (< 1.25 m or 50 in),
restricted space makes seat suspen-
sion systems difficult to use in
isolating operators from WBY and
shock. Miners log 25 million hours
of work annually in these mines.

Interviews with underground
mobile equipment operators revealed
complaints mostly about shocks or
jolts they received when operating
shuttle cars. Hence, the feedback
from equipment operators made the
case for research to reduce shock.
Specifically, this research focuses on
a more ergonomically designed seat
and on identifying bow well various
types and thicknesses of viscoelastic
foams reduce the shock experienced
by shuttle car operators.

Underground mine trials
Researchers at the Pittsburgh
Research Center (PRC) modified
the original seat in a JOY 218C
shuttle car used at an eastern
Kentucky mine. The coal seam at
this mine is 0.889 m (35 in) thick
with an operating height of
approximately 1.09 m (43 in)
Owing to the low mining height,
Operators must adopt a reclining
Posture to operate the shuttle car.
From observing and talking with the
shuttle car operators, it became

obvious that the original seat
provided little adjustability or lower
back support. Modifications to the
original seat resulted in a more
ergonomically designed version with
an easily adjustable lumbar support,
fore-aft seat pan movement, and
viscoelastic foam padding. The
padding included six layers, each
.013 m (5 in) in thickness, of the
following order from top to bottom:
EXTRA-SOFT, PUDGEE, BLUE,
YELLOW, SOFT, and GREEN. The
BLUE, YELLOW, and GREEN layers
are CONFOR medium-density,
open-celled polyurethane foams
from E-A-R Specialty Composites
Corp., Newark, DE. The remaining
materials are manufactured by
Dynamic Systems, Inc., Leicester,
NC. EXTRA-SOFT and SOFT are
SUN-MATE polyurethane foams with
organic composition of more than
50% plant derivatives. SUN-MATE
PUDGEE is unique among the
materials as a viscoelastic gel-foam
with a soft dough-like consistency.
Data were recorded for a typical
shock with the shuttle car operating
empty and with a full load. In
terms of transmissibility across the
driver/seat pad interface, the
ergonomic design shows consider-
ably improved isolation for the full
load case. Below 15 Hz, transmis-
sion increases across the seat due
to the improvement in isolation
frequency. The design objective was
to lower the isolation frequency and
improve the damping characteristics
of the seat so an increase in



transmissi-bility is minimized.

Although the seating foam
composite above provided substan-
tially improved isolation for the
shuttle car operator, PRC research-
ers were interested in further
reducing the isolation frequency.
Consequently, additional testing of
the viscoelastic foams was arranged
with a company specializing in
noise and vibration engineering,
Roush Anatrol.

Material property
evaluation

Using the forced oscillation
technique, investigators at Roush
Anatrol evaluated the six foams
above plus SUN-MATE MED-SOFT.
This method was used to quantify
the influence of static preload,
dynamic strain amplitude, and
temperature on the modulus of
elasticity and damping properties of
the foams. Test temperatures were
selected from actual readings and
knowledge of the mine environ-
ment. The chamber temperature
was held constant at these tempera-
tures for testing at pre-strain levels
of 0%, 10%, and 45%.

Analytical model

The isolation system provided by
the foam was analyzed dynamically
using a lumped-parameter, analyti-
cal model. The dynamic interaction
of the vehicle, isolation system, and
driver was simulated with the
model using a seven degree of
freedom spring-mass-damper
system. Six material layers with
variable material thickness were
included in the model.

The ergonomic seat was chosen
as the baseline seat in the model
because of its known material
configuration and properties.

A shock input, taken from
in-mine data recordings, was
applied analytically to the system to
determine the driver's response for
each material configuration.
Responses were generated at the

driver/seat interface (seat acceler-
ometer location) and at the driver's
torso. The responses were then
compared to the experimentally
measured, modified seat response
and to the analytically predicted
driver response. An analytical
transfer function between the input
force and the torso was also
generated to show the isolation
frequency and the amplification at
resonance. Accordingly, investiga-
tors optimized the seat material
configuration using an iterative
process.

Analytical mode! results
For isolating the shuttle car
operator, EXTRA-SOFT, SOFT, and
PUDGEE foams exhibited character-
istics that make them the best of
the materials evaluated. At 4.4° C
(40° F) and 21.1° C (70° F), the
SOFT and EXTRA-SOFT have lower
modulus of elasticity values than
the YELLOW with EXTRA-SOFT the
lowest. The SOFT and EXTRA-SOFT
are relatively stable with tempera-
ture and have similar damping
properties. Across the temperature
range tested, PUDGEE shows less
than an order of magnitude change
in modulus of elasticity. Across the
frequency span and temperature
range, the dampmg properties of
PUDGEE are also fairly uniform.
Moreover, PUDGEE has the lowest
modulus of elasticity values of the
seven foams tested. PUDGEE’S
higher damping than the
EXTRA-SOFT could, however.
restrict its ability to expand from a
compressed state during a jolt or
shock.

The EXTRA-SOFT and PUDGEE
composite was evaluated as Case
#2. This provided the maximum
depth of a low modulus foam as
well as one that maintains its
stiffness and damping properties
over the expected operating.
temperature range. The isolation
characteristics of the seat are
improved to approximately 4 Hz. A

small amplification in the 3 Hz
region appears due to the lowering
of the driver/seat resonance.

Five inches of EXTRA-SOFT
constituted Case #6. For a low-coal
application, this design represents
nearly the maximum amount of seat
padding that might be used. The
design also employs the best single
material m terms of low modulus
of elasticity, low and consistent
damping, and low temperature
sensitivity. This foam configuration
provides analytical isolation
frequency of 5 Hz.

Conclusions

The shuttle car seat design that
shows the best isolation properties
corresponds to foam configurations
of Case #2 or #6. Bither of these
selections maximizes the isolation
performance of the seat in the
limited space available. The
analytical model provides a usable
tool to design and optimize shock
and vibration isolation systems for
use on a variety of seating configu-
rations in underground mines. It
will greatly aid investigators at the
NIOSH - PRC in providing mining
companies and manufacturers with
guidelines for the construction of
ergonomically designed seats to
reduce the shock exposure of
underground mobile equipment
operators.

Acknowledgments:

The authors thank JR. Bartels, J.P. DuCarme,
RL. Unger, and RS. Fowkes for their
contributions in the design and construction
of the ergonomic seat

References

[1] Remington, Pj. DA. Andersen, and HN.
Alakel. Assessment of whole body
vibration levels of coal miners. Volume
I: Whole-body vibration exposure of
underground coal mining machtne
operators (contract 10308045, Bolt,
Beranek, and Newman. Inc.) CS. Bureau of
Mines OFR 18-87. Mar. 1984, 114 pp.; NTIS PB
87-144-119.

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