EXTRA-SOFT lowers the isolation frequency to 5 Hz, whereas
the PUDGEE lowers it to 4 Hz. Either of these selections
supplies substantially improved isolation performance of the
seat in the limited space available compared to the standard
foam-padded, shuttle car seat. Although, feedback from a
shuttle car operator indicates the PUDGEE composite, because
of its higher damping, may lessen (more so than the EXTRA-
SOFT) the displacement the operator experiences when jarred.

ACKNOWLEDGMENTS

‘The authors thank J. R. Bartels, J. P. DuCarme, R. L. Unger,
and R. S. Fowkes for their contributions in the design and
construction of the ergonomic seat.

REFERENCES

Remington, P.J., Andersen D.A., and Alakel, M.N., 1984.
Assessment of whole body vibration levels of coal miners.
Vol. II, Whole-body vibration exposure of underground coal
mining machine operators (contract JO308045, Bolt, Beranek,
and Newman, Inc.), Bureau of Mines OFR 1B-87, NTIS PB
87-144-119; 114 pp.

Cross, J., and Walters, M., 1991. Analysis of joint coal board
accident statistics: head, neck, and back injuries due to jarring in
vehicles. Suppl. to vibration related back injuries to operators
of mobile equipment in NSW coal mines; 13 pp.

Magid, E.B., and Coermann, R., 1960. The reaction of the
human body to extreme vibrations. Proc Inst Envir Sci, pp. 135.


DYNAMIC MODELING

Using a lumped-parameter analytical model, investigators
analyzed the foam isolation system dynamically. The model,
which features a seven degree of freedom spring-mass-damper
system, simulated the dynamic interaction of the vehicle,
isolation system, and operator. A variable mass atop a
maximum six layers of viscoelastic foam (with variable
thickness) constituted the model.

The ergonomic seat was chosen as the baseline seat in the
model because of its known foam configuration and properties.
Modulus of elasticity and damping values were applied to the
model for 10% pre-strain on each sample of viscoelastic foam.
‘The entire depth of the foam was assigned a temperature of
21.1° C (70° F) for the assumed typical day. Several designs
were also evaluated with a material temperature of 4.4° C (40°
F) to look at the suitability of the configuration to colder
environments.

Using the model, investigators applied a shock impact (from
the field data collected) to the isolation system and determined
the operator's response for each foam configuration. Responses
were generated at the operator/seat interface (seat accelerometer
location) and at the operator’s torso. These responses were
‘compared to the response of the ergonomic seat measured
underground and to the operator's response predicted by the
model. An analytical transfer function between the input force
and the torso was also generated to show the isolation frequency
and the amplification at resonance. In iterative fashion,
investigators changed the foam configuration to optimize the
seat,

DISCUSSION OF RESULTS

EXTRA-SOFT, SOFT, and PUDGEE foams exhibited
characteristics that make them the best three of the viscoelastic
foams evaluated in isolating the shuttle car operator from jolts
and jars. 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 temperature and have
similar damping properties. Across the temperature range
tested, SUN-MATE PUDGEE shows less than an order of
magnitude change in modulus of elasticity. Across the
frequency span and temperature range, the damping properties
of PUDGEE are also fairly uniform. In on, PUDGEE has
the lowest modulus of elasticity values of the seven foams
tested. PUDGEE'’S higher damping than the SUN-MATE
EXTRA-SOFT could, however, restrict its ability to expand
from a compressed state during a jolt or shock.

‘The measured properties of the foams were integrated into the
spting-mass-damper model used to represent the seat and
operator. A single mass, placed atop the springs and dampers
(layers of viscoelastic foams) represented the analytical model of
the ergonomic seat tested at the eastern Kentucky mine.
Investigators selected this representation because the reclined
position of the driver reduces the torso to a single mass. The 50
percentile male, with upper torso weight of 50.6 kg (112 Ibs),
was used for the evaluations since the exact weight distribution
between the seat pad and the back pad was not known. In-mine
measurements of the shuttle car with a full load provided

displacement data of the operator deck that was used as the input
forcing function to the analytical system.

Investigators evaluated a .127-m (5-in) composite of one-inch
layers of EXTRA-SOFT and PUDGEE. In this composite, the
PUDGEE was sandwiched between .076 m (3 in) of EXTRA-
SOFT on the bottom and .025 m (1 in) of EXTRA-SOFT on top.
‘This arrangement 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
combination also permits a mine or manufacturer the convenience
of using only one supplier of foam materials in designing a seat.
‘The isolation characteristics of the seat are improved to
approximately 4 Hz (Figure 5). A small amplification in the 3 Hz
region appears due to the lowering of the driver/seat resonance.

Another composite evaluated included a single foam material,

1

‘Compance in}

” o 5 wm 6 20 8 BD 8B 4 eo

Figure 5 - Comparison of shock isolation:
composite (Ergonomic) versus EXTRA-SOFT/PUDGEE
(Case 2).

127 m (5 in) of EXTRA-SOFT. This design provides the best
single viscoelastic foam, in terms of low modulus of elasticity,
low and consistent damping, and low temperature sensitivity.
With the .127-m (5-in) EXTRA-SOFT composite,
shock isolation is achieved at 5 Hz using the model. Moreover,
when analyzing the EXTRA-SOFT composite for the 95% male
and 5% female weights, a slight change in the resonance location
and in the slope of the mass line occurs, as expected. The lighter
operator weight will experience larger displacements at higher
frequencies, whereas a heavier operator will experience the
opposite effect

SUMMARY

‘The dynamic model provides an analytical tool to design and
‘optimize shock and vibration isolation systems for use on a
variety of seating configurations in underground mines. It will
aid investigators at NIOSH providing mining companies and
manufacturers with guidelines for the construction of
ergonomically-designed seats to reduce the shock and WBV
exposure for operators of underground mobile equipment.
Moreover, the seat padding designs showing the best shock
isolation properties for the shuttle car operator are the SUN-
MATE EXTRA-SOFT and PUDGEE foam configurations. The


SHOCK REDUCTION FOR LOW-COAL SHUTTLE CAR
OPERATORS USING VISCOELASTIC SEATING FOAM

A. Mayton
NIOSH
Pittsburgh, PA
R. Merkel
Roush Anatrol
Livonia, Ml
S. Gallagher

NIOSH
Pittsburgh, PA


ABSTRACT

The prolonged exposure of equipment operators to shock and
whole-body vibration (WBV) is linked to cumulative back,
neck. and abdominal disorders, In low-coal mines. space
restrictions make seat suspensions difficult to use in isolating
‘operators from shock and WBV. Researchers at NIOSH.
Pittsburgh Research Labatory, are responding to these issues by
investigating viscoelastic foams. For the full-load case, an
ergonomic seat with viscoelastic foams isolated the shuttle car
operator from shock at 15 Hz. Researchers used results from
additional foam testing with an analytical model to identify
viscoelastic foams that provide shock isolation at a frequency
below 5 Hz.

INTRODUCTION,

Nearly one-third of equipment operators in underground coal
mines experience adverse levels of exposure to whole-body
vibration (WBV) (Remington et al., 1984). Major sources of
WBV exposure are shuttle cars. Operators are exposed to WBV
and shock when the shuttle car travels over rough mine floor
characterized by numerous bumps, ruts, and potholes. In low-
coal mines, seat suspension systems are difficult to implement
because of space restrictions. The seriousness of prolonged
exposure to WBV and shock is demonstrated by the connection
with cumulative back. neck, and abdominal disorders. In New
South Wales, Australia, back injuries from vehicle jarring equal
those from overexertion among underground equipment
operators (Cross and Walters, 1991).

The focus of this study was set with the input of underground
shuttle car operators. Their most common complaint concerned
the jolts and jarring they experienced when traveling over rough
botiom. Consequently, the research objectives were to develop
a more ergonomically-designed seat and identify how well
viscoelastic foams of various types and thicknesses reduce the
shock impact experienced by shuttle car operators.

FIELD DATA COLLECTION

Ata drift mine in easter Kentucky, NIOSH researchers
recorded data on the original seat of a JOY 2)SC shuttle car
(Figure 1) and a redesigned, version of this seat (Figure 2). The
coal seam at the mine is .889 m (35 in) thick with an operating
height of approximately 1.09 m (43 in). These conditions
require shuttle car operators to assume a reclining posture when
traveling in their vehicles. The original seat provided little
lower back support and adjustability. Considering the
shortcomings of the original seat, NIOSH researchers modified
it to include the following ergonomic features: an easily
adjustable lumbar support, fore-aft seat pan movement, and seat
padding composed of viscoelastic foam. Seat padding included
six .013-m (¥4-in) layers of the following
configuration from top to bottom:' EXTRA-SOFT, PUDGEE.
BLUE. YELLOW, SOFT, and GREEN. Figure 3. The BLUE.
YELLOW, and GREEN layers were CONFOR medium-density,
open-celled polyurethane foams from E-A-R Speciality

Reference to specific products does not imply
endorsement by NIOSH

Composites Corporation, Newark. DE. The EXTRA-SOFT and
SOFT are SUN-MATE polyurethane foams with organic com-
position of more than 50% plant derivatives. SUN-MATE



pea

Figure 3. Initial viscoelastic foarn composite for ergonomic
seat.

PUDGEE is a viscoelastic gel-foam with a unique composition
and soft dough-like consistency.

Data were recorded with the shuttle car operating empty and
with a full load along a typical stretch of roadway. The resulting
data analysis showed improved isolation for the ergonomic seat
in the full load case. Isolation was achieved at 15 Hz.

Although this configuration of viscoelastic foams provided
significantly improved isolation for the shuttle car operator,
NIOSH researchers were interested in further reducing the
isolation frequency considering the effects of shock and WBV
‘on the human body, Figure 4. Consequently, additional testing
of the viscoelastic foams was arranged with a company that
specializing in noise and vibration engineering, Roush Anatrol.

Pheryrs ttanons
Recpieton compat
ast pe
eck in
doin pi
~ conta rg rite nd teeta

"—~werasead muse tonaton

Figure 4 - Complaints according to stimutated vibration
frequencies, Magid and Coermann (1960).

FORCED OSCILLATION METHOD.

Investigators at Roush Anatrol employed the forced oscillation
technique to evaluate the six foams above and a seventh (SUN-
MATE MED-SOFT). They examined each prospective foam to
quantify the influence of static pretoad, dynamic strain
amplitude, and temperature on modulus of elasticity and damping
to serve as input for the analytical model. With the forced
oscillation method. a sinusoidal displacement (strain) was applied
to the specimen as the resulting force (stress) and input
displacement were measured. Input displacement and output
force are complex values with the complex stiffness and phase
angle used to define the dynamic properties of viscoelastic
materials. The size of each specimen was .013 m (1/2 in)
diameter with a 013 m (1/2 in). Three test temperatures [44°C
(40° F), 21.1° € (70* F), and 37.8 °C (100° F)] were selected
based on actual temperature readings and knowledge of the mine
environment. The chamber temperature was held constant at
these temperatures for testing at pre-strain levels of 0%, 10%, and
456, At discrete frequencies (from | Hz to 100 Hz in 25 Hz
steps), data was collected for each temperature. The input

strain was controlled to provide the prescribed dynamic strain
levels of .1%, 1%, and 2%. Force and displacement data during
the first several oscillations were collected in the time (instead of
the frequency) domain. ‘This was necessary to avoid changes in
foam properties due to internal heating of the foam over repeated
cycling. The dynamic properties of the material were then
calculated using the amplitudes of the force and displacement
responses with phase angle in the following equations:

E' = 1K] F,(h/S, ) cosé

E’=E tané

Damping = tan = E"/ E’

Where,

[K] = stiffness modulus (N/m) - measured

6 = loss angle (degrees) - measured

h = height of test piece (m)

S. = excited surface area on test piece (m*)

S, = lateral surface area of test piece (m*)

F, = corrective factor, test piece geometry dependent

Fal /(14+2(8,/8, )