Table 4.—Rebullt 600/575, PMR = 13.1 peak transmissibility, attenuation frequency,
and overall acceleration RMS for various cushions

Cushion Density, Frequency = Transmissibility Attenuation Arms: m/s?
kg/m peak, Hz peak frequency, Hz
40.0 23 2.66 69 072
80.1 27 2.20 89 87
320.4 25 217 9.3 65

Ams Acceleration root mean square.
OEM — Original equipment manufacturer.

New 6000/575 and PMR = 13.1

The new 6000/575 seat was tested with the OEM, Sun-
Mate, and Pudgee cushions for fixed preload and mass of
889.5 N (200 Ibf) and 68.0 kg (150 Ib), respectively (PMR
= 13.1). The transmissibility plots for the new 6000/575
are shown in figure 13, Table 5 and figures 14 and 15 give
peak transmissibilities and their corresponding frequencies,
attenuation frequencies, and overall acceleration RMS’s.

Table 5 and figure 14 show that peak transmissibilities
in the range of 2 to 3 Hz are similar to those for the re-
built 6000/575 scat, i.c., seat cushions with increasing foam
densities have decreasing peak values of transmissibility.

From the transmissibility plots (fig. 13), as the cushion
density increases, the peak transmissibility bandwidth in-
creases. For the Pudgee cushion, the peak transmissibility
spreads over a frequency range of 2.0 to 3.0 Hz. A sec-
ond peak transmissibility occurred in the range of 5.8 to
7.9 Hz. The levels of transmissibility for the second peak
are in the range of 2.1 to 2.3. The frequency at which
the second peak occurs increases with increasing cushion
density. Attenuation in the OEM cushion was obtained
at 7.9 Hz, However, in the frequency range of analysis
(0.7 to 10 Hz), the Sun-Mate and Pudgee cushions never
achieved attenuation.

3.0 T Tt T T
255

2.0-

1.0 r

TRANSMISSIBILITY, dimensionless
uw
T

1 1 1 L

0 1 2 3 4
FREQUENCY, Hz

5 6 7 8 9

Figure 11.—Transmissibility of seat-base with differing seat cushions,


w
ro

N
u

N
°

TRANSMISSIBILITY, dimensionless

KEY

Human
Dummy
51.8 kg mass with 13.7 PMR
68.0 kg mass with 9.8 PMR

1.55
1.0-
5b
1 1 1 it
o 1 2 3 4 5 6 7 8 9 10
FREQUENCY, Hz

Figure 3.—Transmissibility of seat-base using human, anthropomorphic dummy, and inert mass with new 6000/
575 mechanical suspension. Inert mass of 51.8 kg was tested with PMR = 13.7; inert mass of 68.0 kg was tested

with PMR = 9.8.

One sees from table 2 that the PMR’s of 9.8 produce
similar overall RMS acceleration values. The dummy and
68.0-kg (150-Ib) inert mass produced high transmissibilities
near 1,0 Hz; however, a peak transmissibility at 1 Hz was
absent for the human. The PMR of 13.7 produced trans-
missibilities that exceeded all cases of the 9.8 PMR except
at very low frequencies (f < 1 Hz) and very high fre-
quencies (f > 9.5 Hz). The human attained attenuation at
the lowest frequency. The dummy never attained attenua-
tion, There appear to be two major effects of the legs.
For low frequency (f < 3.0 Hz), long-stroke displacement,
the legs resist relative motion between the top of the seat
suspension and floor, hence decreasing the transmissibility
level. This resistance to relative motion is due to the
coupling between seat and floor by the legs. At higher
frequencies (f > 4.0 Hz), human legs act as a vibration
energy absorber, resulting in lower transmissibility levels.
The reverse effect is present for the dummy. At higher
frequencies, the dummy’s legs act as a vibration transmis-
sion path to the seat cushion, resulting in higher trans-
missibility levels.

With the exception of initial comparison of results
between human, dummy, and inert mass, for the tests con-
ducted, the seat was loaded with a mass, not a person or
dummy. The driver’s mass supported by the seat is taken
to be (1) equal to and (2) 5/7 of the total human body
mass (7-8), assuming 2/7 is supported by the legs.

Effect of Preload and Inert Mass

The effect that preload and inert mass have on the
vibration attenuation characteristics of the seat for the new
Isringhausen 6000/575 mechanical seat suspension was
investigated. Changing the PMR is equivalent to changing
the static displacement height of the seat suspension. This
increases the possibility that the seat will hit the com-
pression limit stop for decreasing preload or hitting the
extension limit stop for an increasing preload. If the seat
hits a limit stop, the limit stop will contribute to an
increase in the stiffness in the seat suspension, hence
changing the vibration attenuation characteristics of the
seat.


10

KEY

an D oem 7
® ak & Sun—Mate 4
Li Pudgee
oak 4
z
& 6+ 4
<=
a2 sbt 4
a
ow #7 7
D
3 | |
= 2- 4

TF 4

Oo

Frequency, Peak transmis— Attenuation

a Hz sibility frequency, Hz

Figure 15.—Second transmissibility peak of seat-base and corresponding frequency and attenuation frequen-
cy with differing seat cushions for new 6000/575 mechanical suspension, PMR = 13.1.

2.5 T T T T T T T T T

thd
fo)
T

w
T

°
T

TRANSMISSIBILITY, dimensionless
w
T
i

1 i L i i 1 L 1 1
0 1 2 3 4 5 6 7 8 9 10

FREQUENCY, Hz

Figure 16.—Transmissibility of seat-base with differing seat cushions for new 6000/575 mechanical suspen-
slon, PMR = 9.8.



14,

15.

16.

17.

18.

19,

oe

. Transmissil

ILLUSTRATIONS—Continued

First transmissibility peak of scat-base and corresponding frequency and acceleration RMS with differing
seat cushions for new 600/575 mechanical suspension, PMR = 13.1............... beet eeeeeee
Second transmissibility peak of seat-base and corresponding frequency and attenuation frequency with
differing seat cushions for new 6000/575 mechanical suspension, PMR = 13.1
Transmissibility of seat-base with differing seat cushions for new 6000/575 mechanical suspension,
PMR = 98 ........0065 Seno eee e eee wees Deen e cece eee eeneee err a
Transmi: ity peaks and frequency of seat-base and overall acceleration RMS of seat with differing
seat cushions for new 6000/575 mechanical suspension, PMR = 9.8 ........0. 0. cece cece cece
Transmissibility of seat-base for human, anthropomorphic dummy, and inert mass for new 6500/57 air
suspension, 503 kPa .............005 settee nee e cette eeeneeeceeenseeeeaes bees
Transmissibility peaks and frequency of seat-base and overall seat acceleration RMS for human,
orphic dummy, and inert mass for new 6500/577 air suspension, 503 kKPa.............+
ity of seat-base with air pressures of 393.0, 448.2, and 503.3 kPa for new 6500/577 air

suspension

ity of seat-base with air pressures of 503.3, 551.6, 620.5, and 689.5 kPa for new
6500/57 air suspension ..... beac ee eee eee e ee eeee eens beeen cece eee eee bee e ee eeee
Transmissibility peaks and frequency of seat-base, attenuation frequency, and seat RMS acceleration with
differing air pressures for new 6500/577 air suspension, 68.0-kg mass .....
Transmissibility peaks and frequency of seat-base and overall RMS acceleration with ditering air
pressures for new 6500/577 air suspension, 68.0-kg mass .......
Transmissibility of seat-base with differing seat cushions for new 6500/: 75 ani 6500/57 air suspension

TABLES

Slope-intercept values for acceleration test spectrum ....... 0... e cece eee eect eee e eee eneeeee
Comparison of transmissibility peak values and frequencies, attenuation frequency, and overall
acceleration RMS between human, anthropomorphic dummy, and inert mass for new 6000/575 ......
New 6000/575 peak transmissibilities and frequencies, attenuation frequency, and overall acceleration
RMS value for various PMR’s Deca eee cent ence enee
Rebuilt 6000/575, PMR = 13.1 peak transmissibility, attenuation frequency, and overall acceleration
RMS for various cushions ......... see Pease eee e eee cane teeneees
New 6000/575, PMR = 13.1 peak transmissibility, attenuation frequency, and overall acceleration RMS”
for various cushions .
New 6000/575, PMR = ‘98 pe:
for various cushions .............. POC n eee e eee e cence ene e renee eee eee nett ene eeees
Comparison of transmissibility peak values and frequencies and overall acceleration RMS between
human, anthropomorphic dummy, and inert mass for new 500/577
6500/577 peak transmissibility and overall acceleration RMS for various test pressures
6500/575 and 6500/577 peak transmissibility, Frequency, and overall acceleration RMS for various
cushions ........ see

transmissibility, attenuation frequency, and overall acceleration RMS

Page

17
18
18
19

19

14
15
16

7
7

2


Table 5.—New 6000/575, PMR = 13.1 peak transmissibility, attenuation frequency,
‘and overall acceleration RMS for various cushions

Cushion Density, Frequency —Transmissibility Attenuation Arma: TM/8?
kg/m peak, Hz peak frequency, Hz
OEM esse 40.0 26 24)
7 .

Pr a 9 0.78
Sun-Mate ..... 80.1 27 22)

a4 3a >10 at
Pudgee ...... 320.4 2.0-<3.0 >1.75

79 Zot 710 7
‘Arms Acceleration root mean square.

OEM — Original equipment manufacturer.
New 6000/575 and PMR = 9.8

The new 6000/575 seat was tested with the OEM, Sun-
Mate, and Pudgee cushions for fixed preload and mass of
666.4 N (150 Ibf) and 68.0 kg (150 Ib), respectively. The
transmissibility plots for the new 6000/575 are shown in
figure 16. Table 6 and figure 17 give peak transmissibil-
ities and their corresponding frequencies, attenuation
frequencies, and overall acceleration RMS’s.

From tables 5 and 6 and figures 14, 15, and 17, the fre-
quencies at which peak transmissibilities occur are highly

dependent on cushion type but nearly independent of
PMR. The general trends in transmissibilities appear to
be independent of PMR but highly dependent on cushion
type. These most obvious trends are that (1) attenuation
is never attained in the seat for the Pudgee and Sun-Mate
cushions, (2) the OEM cushion had higher transmissibil-
ities for f < 3.0 Hz but lower values for f > 6.0 Hz, (3)
the Sun-Mate cushions generally had higher transmissibil-
ities than the Pudgee cushions, and (4) as the PMR ratio
increases, the transmissibility levels and overall RMS
acceleration increase.

10
KEY
OF 4
O OEM

wn Bb 63 Sun—Mate 4
oO Pudgee
wot J
—
3
=z 6r 4
<
o Sr 4
[ay L 4
a 4
a
D 3r 7
n
& 2+ 4
=

ib . 4

° Go)

Frequency, Peak transmis— Arms, Attenuation
Hz sibility m/s2 frequency, Hz

Figure 12.—Transmisaibility peak and frequency of s

ase, acceleration RMS (A,,..), and attenuation frequen-

cy with differing seat cushions for rebuilt 6000/575 mechanical suspension, PMR = 13.1.


9
N

Pres:

TRANSMISSIBILITY, dimensionless
a
T

1 1 i 1 1 i

KEY
sure, kPa
503.3
551.6
620.5

9 1 2 3 4 5 6 7 8
__ FREQUENCY, Hz

Figure 21.—Transmissibility of seat-base with
577 air suspension.

9 10

Pressures of 503.3, 551.6, 620.5, and 689.5 kPa for new 6500/

45
KEY

4.0F Pressure, kPa
n 0 393.0
rH] 357 BH 448.2 7
= 2 a
fo 30k 503.3
=
<=
ce 25+ 4
<=
7 20
a
@
© 15- 4
n
sg OF 4
= x4

bb Ba [] 4
<
a ZZ.

Frequency Transmis— Frequency Transmis— Attenuation

1, Hz sibility 1 2, Hz sibility 2

Frequency

A

rms,

Figure 22.—Transmissibility peaks and frequency of seat-base, attenuation frequency, and seat RMS accelera-

tion with differing air pressures for new 6500/577 air suspension, 68.0-kg mass.

m/s?


Table 7.Comparison of transmissibility peak values
and frequencies and overall acceleration RMS
between human, anthropomorphic dummy,
and Inert mass for new 6500/57

Object Mass, Frequency Transmissibility A,,,, m/s”
kg __poak, Hz peak
Human. 63.6 16 0.78 0.19
Dummy 726 1.6 66 M4
Inert mass .. 68.0 1.8 58 16

‘Arms_ Acceleration root mean square.

From figures 18 and 19 and table 7, the peak trans-
missibility is greatest for the human. The general shape of
the transmissibility curves is similar for the human, dum-
my, and inert mass. At frequencies greater than 5 Hz, the
dummy produces the lowest transmissibility. This is in
contrast to the results for the dummy on the mechanical
suspension seat (see figure 3). As was the case for the
mechanical suspension seat, the air suspension seats were
tested with the inert mass of 68.0 kg (150 Ib).

Effect of Air Pressure

The Isringhausen 6500/577 air suspension spring was
tested for varying air pressures of 393, 448, 503, 552, 621,

and 690 kPa (57, 65, 73, 80, 90, and 100 psi, respectively).
The resulting transmissibility plots are presented in fig-
ures 20 and 21. The summaries of the tests are presented
in table 8 and figures 22 and 23.

Table 8—6500/577 peak transmissibility and overall
acceleration RMS for various test pressures

Pressure Frequency ‘Transmis- Attenuation Ans,
peak, Hz —sibilitypeak —_ frequen m/s?
ey
15 44
a 3 40 0.61
448. 12 1
23 2aal 23 0
503. 9 F a
18 so ® 6
552 .... 1.0 A
es sy () 16

9 a

Ww oy 0 1
a
Ame Acceleration root mean square.

MTransmissibility < 1, 80 there is no attenuation frequency.

3.0
KEY

256 D oEM J
7.)  Sun—Mate
ao Pudgee
tw
5 2.0F 4
=
<
a
<o15- 4
oa
a
lJ
& 1.0 4
=)
Yn
a
Ss SF ‘ 4

0 “
Frequency, Peak transmis— Arms,

ee Hz ____ sibility m/s2

Figure 14.First transmissibility peak of seat-base and corresponding frequency and acceleration RMS (A,,,,)
with differing seat cushions for new 600/575 mechanical suspension, PMR = 13.1.


KEY
7 UO Frequency, Hz 4
RY Peak transmissibility

FREQUENCY AND PEAK TRANSMISSIBILITY
+
T
1

9.9 11.3 13.1 16.4 19.6
____ PMR, _N/kg

Figure 8.—Third transmissibility peak and corresponding frequencies of seat-base for various PMR’s with
new 6000/575 mechanical suspension.

10

OF 4

FREQUENCY, Hz

7.1 9.9 11.3 13.1 16.4 19.6
___ PMR, N/kg

Figure 9.—Attenuation frequency for various PMR’s with new 6000/575 mechanical suspension.



Dynamic Systems, Inc. through Carter Mining Co. The
cushions provided by Dynamic Systems were developed for
high-energy impact absorption and to provide uniform
orthopedic support. The two trade names tested were
Sun-Mate cushion and Pudgee cushion.

The Sun-Mate cushion’s foam is a high-density 100-pet

The Sun-Mate cushion is a visco-elastic foam that contours
to give uniform pressure distribution and soft springback.

The Pudgee cushion is a visco-clastic gel-foam with a
100-pct open-cell structure. The Pudgee cushion’s dough-
like consistency contours to a body’s outline with low
tension-shear force. The Pudgee cushion possesses a high

open-cell elastomeric foam with a density of 80.1 kg/m? density of 320.4 kg/m? (20 Ib/ft?).
(5 lb/ft). It possesses high impact energy absorption.
VIBRATION TESTING

TESTING HARDWARE AND INSTRUMENTATION

The seat was tested using a MB Dynamics Model C50
electrodynamic shaker over the frequency range of 0.7
to 10.0 Hz. The C50 shaker was limited to a 2.5 cm
(1 in) peak-to-peak stroke and a maximum force rating of
22,241 N (5,000 Ibf).

The acceleration input to the base of the seat was
measured using a Bruel and Kjaer (B&K) 4370 acceler-
ometer. The response of the seat was obtained using a
B&K 4322 triaxial seat accelerometer. The B&K 4322
consists of a B&K 4321 triaxial accelerometer mounted
inside a semi-rigid rubber pad. The B&K 4322 was at-
tached to the center of the seat cushion using a generous
amount of duct tape.

A study was made comparing transmissibility measure-
ments on scat cushions using three different seat interface
transducers (9). The seat interface transducer measures
the vibrations on top of the seat. The seat interface
transducer is mounted between the seat surface and hu-
man buttocks. The three different seat interfaces were a
semi-rigid rubber pad, an aluminum bar, and a rigid nylon
disk. It was shown that good agreement for the transmis-
sibilities was obtained for the semi-rigid rubber pad and
the rigid nylon disk. In this study, the semi-rigid rubber
pad was used for measuring the acceleration on top of the
seat cushion,

The acceleration signals from the base and cushion
accelerometers were conditioned and amplified using a
Timewave Technology 4050 two-channel portable charge-
icp (internal circuit preamplifier) amplifier. ‘

Shaker control problems caused by 60- and 180-Hz line
noise were eliminated by using notch filters. Each channel
employed a Frequency Device 781R1Q2 60- and 180-Hz
notch filter.

‘Reference to specific products does not imply endorsement by the
US. Bureau of Mines.

COMPUTER CONTROL

The shaker was controlled using a Genrad 2514 vibra-
tion control system. The Genrad 2514 is a computer-
based, software-controlled closed-loop vibration test and
shaker control system. The Genrad 2514 is a Digital
Equipment Corp. LSI-11 microcomputer interfaced with
analog-to-digital and digital-to-analog subsystems. Soft-
ware programs for product testing available for the Gen-
rad 2514 are random, swept sine, shock, random-on-
random, and sine-on-random,

Initial testing revealed that random vibration testing
could not be performed at sufficiently high vibration levels
because of the limited stroke of the C50 shaker. Random
vibration tests superimpose individual frequencies simul-
taneously. This superimposition contributes to a large
overall vibration test level with low individual frequency
levels. A swept sine vibration test was chosen over a ran-
dom vibration test because of the ability to subject the seat
to an individual forcing frequency at any instant of time.
By subjecting the seat to a swept sine test, the vibration
level for an individual discrete frequency could be set at a
significantly greater level than could be accomplished by
using a random test.

REFERENCE SPECTRUM

The tests on the seats were performed using an accel-
eration spectrum. The sine test program can define a test
spectrum in units of acceleration only. The spectrum was
constructed to produce accelerations at levels as great as
possible without having the C50 shaker exceed its stroke
limits. For programming purposes, the acceleration refer-
ence spectrum was defined using a combination of con-
stant slope segments.


1.1 Hz, 16 to 3.2 Hz, and 5.8 to 6.9 Hz. The mechanical
scat dynamic response is dependent on the PMR or oper-
ator’s weight adjustment. As the PMR decreases from
19.6 to 7.1 N/kg (2 to 0.72 Ibf/Ibm), the vibration char-
acteristics of the seat improve. For decreasing PMR, the
RMS of the seat’s peak acceleration (Aja,) decreased from
1.03 to 0.48 m/s? (3.37 to 1.57 ft/s). For decreasing
PMR, the peak transmissibility levels are also decreased.
The new and rebuilt mechanical suspension seats were
tested using three different seat cushions, The rebuilt seat
was tested with a PMR equal to 13.1 N/kg (1.33 Ibf/Ibm).
The new mechanical suspension seat was tested with a
fixed PMR of 13.1 and 9.8 N/kg (1.33 and 1 Ibf/lbm).
The seat cushions tested for both the new and rebuilt seats
were (1) OEM, (2) Sun-Mate, and (3) Pudgee. The den-
sities of the seat cushions in ascending order are OEM,
Sun-Mate, and Pudgee. For a PMR equal to 13.1, the new
and rebuilt seats compared similarly for peak acceleration
RMS levels and peak transmissibility levels in the 1.6- to
3.2-Hz frequency band. For ascending cushion density, the
Avs Value for the rebuilt mechanical seat decreased from
0.72 to 0.65 m/s? (2.36 to 2.13 ft/s*). For the new me-
chanical seat the OEM cushion had lower A,,,, than the
Sun-Mate cushion but greater than the Pudgee cushion.
The difference in transmissibility and A,,,, values between
the rebuilt and the new seat may be due to differences in
preload setting, suspension spring stiffness, and shock
absorber damping. The peak transmissibilities for all
mechanical seats and cushions were greater than 1; hence,
vibration was not generally attenuated for peak trans-
missibilities. Moderate attenuation was achieved at higher
frequencies (f > 8.0 Hz). For the new mechanical seat
and PMR equal to 9.8, the OEM cushion had the lowest
overall RMS acceleration level of 0.56 m/s? (1.84 ft/s’),
while the Sun-Mate cushion had the largest with a value of
0.68 m/s? (2.23 ft/s). The Sun-Mate and Pudgee cushions
had lower transmissibilities than the OEM cushion for
frequencies less than 3.5 Hz. The mechanical seat sus-
pension exhibited greater transmissibility than expected.
The seats were tested at acceleration levels below the ISO
2631’s FDP, 4-h exposure time. Hence, if an operator was
exposed to the test vibrations levels while working an 8-h
shift, he or she could complete work with little degradation
of work performance due to vibration exposure. Because
of the nonlinear nature of the response of the chair, the
transmissibility could change due to differing vibration
levels. Hence, the results appear to give the trends in
issibility of the seat at low vibration levels. The
transmissibility of the seat was higher than expected;
possible contributing factors are as follows: (1) at the ac-
celeration levels tested, the legs of a person may limit

the vertical motion of the seat, prevent rocking, and pro-
vide damping, while an inert mass provides none of these;
and (2) the suspension may have been constructed to
provide attenuation at acceleration levels that can cause
serious health risks. Hence, at the acceleration levels
tested, which were generally low, the suspension system
may operate differently than when acceleration levels are
present that can cause harmful affects to the operator.
The seat should be adjusted to a preload setting of the
operator’s weight. This setting would position the chair at
its mid-ride position. A chair positioned at the mid-ride
position reduces the operator’s risk of receiving high accel-
eration levels due to the seat’s hitting the limit stops dur-
ing large shocks.

AIR SUSPENSION SEATS

The 6500/57 air suspension seat was initially tested
with varying levels of air pressure from 393 to 690 kPa
(57 to 100 psi). For the pressure range of 393 to 503 kPa
(57 to 73 psi), the response of the air suspension scats
appears to be extremely sensitive to air pressures. For
increasing air pressures of 393 to 503.3 kPa (57 to 73 psi),
Ajnue decreased nearly an order of magnitude from 0.61 to
0.16 m/s? (2.0 to 0.52 ft/s*). In addition, the seats pres-
surized at 393 and 448.2 kPa (57 and 65 psi) exhibited two
large transmissibility peaks in the frequency bands of 1.2
to 15 Hz and 23 to 3.1 Hz. For seats pressurized from
503.3 to 690 kPa (73 to 100 psi), the transmissibility never
attained a value greater than 1. Hence, the 6500/57
pressurized greater than 503.3 kPa (73 psi) attenuated
vibrations for all frequencies. There was little difference
in overall acceleration RMS level due to increasing air
pressure from 503.3 to 690 kPa (73 to 100 psi).

As with the mechanical seats, the 6500/575 and 6500/
577 air suspension seats were tested with different seat
cushions. For the cushion comparison tests, all seats were
pressurized to a level of 503.3 kPa (73 psi). The con-
ducted tests were 6500/575 with OEM, Sun-Mate, and
Pudgee cushion and backrest, and 6500/57 with OEM air
chamber seat cushion and longitudinal suspension. For all
cushion combinations tested the maximum transmissibility
occurred for frequencies between 1.0 and 2.0 Hz. For
cushions of increasing density, the peak transmissibility and
Axms level decreased.

The performance of the 6500/577 seat quickly deterio-
rates as the air pressure decreases. An air suspension seat
inflated to pressures greater than 503.3 kPa (73 psi) pro-
vides a stiffer ride, hence decreasing the chances of the
seat’s hitting limit stops when installed for actual use in an
off-road vehicle. The response of a simple single degree


VIBRATION TESTING OF OFF-ROAD VEHICLE SEATS

By John C. Gagliardi’ and Walter K. Ut?

ABSTRACT

The U.S. Bureau of Mines, in cooperation with Carter Mining Co., conducted vibration tests of four
off-road vehicle seats. The purpose of the tests was to determine which seat provided the best vibration
attenuation under laboratory conditions. Laboratory tests were constructed to simulate the mining vibra-
tion environment within the limitations of the test equipment. The acceleration test levels and corre-
sponding response of the seats were low compared to ISO 2631’s fatigue-decreased-proficiency time lim-
its. Two of the seats employed mechanical suspensions and two seats employed air suspensions. The
seats were tested using a 22,241-N (5,000-Ibf) electrodynamic shaker. Mechanical suspension seats were
tested with various preload-to-mass ratios (PMR’s) and cushion densities. Air suspension seats were
tested with various air pressure levels and seat cushion densities. Air suspension seats provided good
vibration attenuation if pressurized greater than 552 kPa (80 psi). Mechanical suspension seats’ attenua-
tion performance decreased if the PMR exceeded 9.8. Seat cushions of lower density provided less
vibration damping.

‘Mechanical engineer.
2Blectrical engineer.
‘Twin Cities Research Center, U.S. Bureau of Mines, Minneapolis, MN.

~


UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT

cm centimeter Ibf pound force

ft/s? foot per square second Ibf/lbm pound force per pound mass
h hour Ib/ft® pound per cubic foot

Hz hertz min minute

Hz/s hertz per second mn millimeter

in inch m/s? meter per square second
kg kilogram N newton.

kg/m? —_ kilogram per cubic meter N/kg newton per kilogram

kPa kilopascal pet percent

Ib pound psi pound per square inch



1.00

N

n

S

E

Zz 75h 4

9

a

<

oe

lJ

a sok 4

cS)

ro)

<

g

@ .25- 4

4

—

<x

ia

lJ

5 0

o 7 9.9 11.3 13.1 16.4 19.6
PMR, N/kg

Figure 10.—Overall RMS acceleration for various PMR’s with new 6000/575 mechanical suspension.

Effect of Cushions

To determine the effect of the seat cushion on seat
dynamics, the rebuilt 6000/575 mechanical seat was tested
with a test mass of 68.0 kg (150 Ib) and a preload of
889.5 N (200 Ibf) (PMR = 13.1). The new 6000/575 me-
chanical suspension seat was tested with a test mass
of 68.0 kg (150 Ib) and preloads of 666.4 N (150 Ibf)
(PMR = 9.8) and 889.5 N (200 Ibf) (PMR = 13.1).

Rebuilt 6000/575 and PMR = 13.1

The transmissibility plots for the rebuilt 6000/575 are
shown in figure 11, The peak transmissibilities and their
corresponding frequencies are given in table 4. Also ‘in-
cluded in table 4 are the attenuation frequencies and the
overall acceleration RMS values. A graphical summary of
table 4 is presented in figure 12, As shown by the data in
table 4 and figure 12, as the density of the cushion mate-
tial increases, the transmissibility decreases. Assuming
that the stiffness, damping, and density are all directly
related, these results correspond similarly to a previous

study (7). In that work, an analytical model of an
Isringhausen mechanical suspension seat was developed to
aid in determining optimal suspension parameters. The
peak transmissibility value decreases with increasing
cushion density (fig. 12). This compares similarly with the
study (7), which found that stiffer cushions suppress res-
onant peaks. Table 4 and figure 12 show that transmis-
sibility attenuation is achieved (transmissibility < 1) at
lower frequencies for less dense seat cushions. This trend
was also reported in the CONCAVE (Concordia Com-
puter Aided Vehicle Engincering) study, which stated that
acceleration transmissibility increases at higher frequencies
for stiffer cushions (7). From figure 12, the overall ac-
celeration RMS value decreases with increasing cushion
density.

A sharp decrease in transmissibility occurs at 5 Hz for
the 6000/575. This phenomenon occurs to some extent for
all seats tested under every test condition. The sudden
drop in transmissibility at 5 Hz is due to a fore-aft res-
onance from the asymmetrical mounting of the shock ab-
sorber relative to the scissor suspension (10).


Using the measured parameters, Rakheja developed a gen-
eral analytical model of a mechanical seat (7).

In the present study, four off-road vehicle seats were
tested using an acceleration test spectrum that was swept
over the frequency band of 0.7 to 10 Hz. Two seats em-
ployed mechanical suspensions and two seats employed air
suspensions. The mechanical seats were tested to deter-
mine the effect of PMR. The preload is an adjustment in
the seat that loads the suspension spring in tension to
allow constant static deflection of the seat for various
operators’ weight. The mass is simply the weight on the
seat. PMR is the amount of preload force put into the
mechanical suspension divided by the mass of the oper-
ator. If the operator adjusts the preload to less than his
or her weight, when the operator sits on the seat, the seat
suspension will compress beyond the mid-ride static de-
flection. In contrast, if the preload is set greater than the

operator’s weight, when the operator sits on the seat, the
compressive static deflection of the seat will be less than
the mid-ride position. In either case, the operator will
increase vibration levels transmitted to him or her by
hitting the compression or extension limit stops during
high-level shocks because of improper static deflection
position of the seat. Mechanical seats were also tested
with foam seat cushions of different densities and mate-
rials. The air suspension seats were tested at various air
Pressures. Air suspension seats were also tested with foam.
seat cushions of different densities. Comparisons among
the seats are given in terms of peak transmissibilities
and their corresponding frequencies, overall frequency-
weighted RMS acceleration, and attenuation frequency.
The attenuation frequency is defined as the largest fre-
quency at which the transmissibility decreases to a value
of 1,

OFF-ROAD VEHICLE SEATS

Four off-road vehicle seats were tested. All seats were
manufactured by the German company Istinghausen and
provided through a cooperative agreement between the
Bureau and Carter Mining Co. The seat models were two
Isringhausen 600/575, mechanical suspension seats, one
6500/575 air suspension seat, and one 6500/57 air sus-
pension seat. The two model 6000/575’s were a rebuilt
seat and an unused new seat. The 6500/575 and 6500/577
were both unused new seats. All seat types are commer-
cially available. The major elements of all seats consisted
of a suspension, seat cushion, and backrest cushion. Var-
ious types of seat suspensions and cushions were tested;
they are described below.

MECHANICAL SUSPENSION

The 6000/575 mechanical suspension consists of a scis-
sors suspension linkage. The seat is mechanically sprung
by two-coil steel springs mounted horizontally to the top
of the scissor suspension linkage. A one-coil spring is an
extension spring connected to a preload deflection linkage.
The operator’s weight adjustment dial is connected to the
preload linkage. As the dial is turned for increasing op-
erator weight, the preload linkage extends the coil spring.
This extension loads the spring in tension. When the op-
erator sits, the scat lowers, causing the coils to further
extend. The sum of the preload tension force and static
deflection force of the seat suspension equals the oper-
ator’s weight. The preload adjustment on the 6000 seats
is calibrated so that the seat’s static deflection will be the
same for all operators.

A shock absorber is mounted asymmetrically from the
top front of the scissor suspension to the rear of the seat-
base. The shock absorber is oil hydraulic with a dual set-
ting. The 6000/3575 has a permitted suspension stroke of
95 mm (3.74 in). The 600/575 also has load-independent
height and inclination adjustment. The original equipment
manufacturer's (OEM’s) seat cushion is a high-density
polyurethane foam.

AIR SPRING SUSPENSION

The 6500 series seat features the same scissor linkage
and asymmetrically mounted shock absorber as the 6000/
575 seat. The 6500 series replaces the 6000’s mechanical
spring with an air spring. The 6500 possesses an auto-
matic leveling suspension. The automatic leveling sus-
pension consists of a cam-operated valve that positions the
system independently of the driver’s weight to the mid-ride
suspension stroke location. The 6500 series integrates
pneumatic lumbar support into the seat backrest. This
feature was not examined as part of this study. The 6500/
575 consists of a standard polyurethane foam seat cushion,
while the 6500/577 contains air chambers in the seat cush-
ion for individual adjustment. The 6500/577 model also
has incorporated a suspension system for longitudinal mo-
tion that was not tested. The vertical suspension stroke on
the 6500 is 100 mm (4.0 in).

CUSHIONS FOR SEAT AND BACK

In addition to original vendor-supplied seat cushions for
the Isringhausen chairs, seat cushions were provided by


KEY
n’7P D oem 7
oF & Sun—Mate
F 6F Z Pudgee 7
[ray
z
& *y ]
a yh J
Q
id
3°) BS 7
< SS
= BS

Frequency Transmis— Frequency Transmis— Frequency Transmis— Arms,
1, Hz sibility 1 2, Hz sibility 2 3,Hz sibility 3 m/s

Figure 17.—Transmissibility peaks and frequency of seat-base and overall acceleration RMS (A,,,,) of seat with
differing seat cushions for new 6000/575 mechanical suspension, PMR = 9.8.

2

0.9

TRANSMISSIBILITY, dimensionless
>
T

L 1 1 1 L 1 1 L L
0 1 2 3 4 5 6 7 8 9 10
_FREQUENCY, Hz

Figure 18.—Transmissibility of seat-base for human, anthropomorphic dummy, and Inert mass for new 6500/
577 alr suspension, 503 kPa.


RI| 9454 | | __ REPORT OF INVESTIGATIONS/1993 =

Vibration Testing of Off-Road Vehicle Seats

By John C. Gagliardi and Walter K. Utt

__UNITED STATES DEPARTMENT OF THE INTERIOR

_ BUREAU OF MINES


ANALYSIS

For each run, the stored acceleration values of the seat-
base and cushion were transferred to a personal computer
for further processing. The computation of the various
physical measures was performed on the personal com-
puter using the commercially available interactive signal
analysis program DADiSP. The seat-base and cushion
acceleration spectra for the five runs per test were
imported to DADiSP, From within DADiSP, the five
spectra for each common location were averaged together.

Within DADISP, the reference acceleration spectrum
shown in figure 1 and defined in table 1 was generated. In
addition, a vertical weighting factor was defined (fig. 2).
This weighting factor is a continuous frequency weighting
factor that has equal values of ISO 2631 weighting factors
at the 1/3 octave center frequencies. The processing
procedure computed the following measures:

1. Normalizing of reference acceleration spectra to
measured base acceleration spectra:

NE = A,O/AO- @

1.0

2. Transmissibility of the scat, iic., ratio of cushion
acceleration spectra to the base acceleration spectra:

TA) = A(O/AO- 1}
3. Normalized weighted seat acceleration spectra:
AWD = AO WONO. 0}

4, Normalized weighted seat mean squared acceleration
spectrum ([m/s*}):

P,(D = A? (0). 6)

5. Overall mean normalized weighted acceleration lev-
el:

N
1
Aad Aw ©
Ts1
6. Overall RMS acceleration:

1s 1/2
Aims = [: Ee «| . 0)

Or

WEIGHT

1 1 L

1 1 1 L L

0 1 2 3 4
FREQUENCY, Hz

5 6 7 8 9 10

Figure 2.—Vertical vibration weighting.


A =M(-f.) +B, Q)

where A = acceleration (m/s”),
M_ = slope [(m/s”)/Hz],
f = frequency (Hz),
B = acceleration intercept (m/s),
and f. = upper frequency limit of the previous
band or the lower limit of the current
band.

The parameters of the employed acceleration test
spectrum of equation 1 are given in table 1 with the
resulting spectrum shown in figure 1. This spectrum func-
tions as a reference spectrum for the Genrad 2514 vibra-
tion control system. An error spectrum for updating the
drive control is obtained from the difference between the
reference and measured test spectrums on the shaker
table. An actual test spectrum measured at the base of
the seat is shown in figure 1.

0.8 T T T T

Table 1.—Slope-intercept values for
acceleration test spectrum
4, Hz M, B, m/s?
(rn/s*)/Hz
0.7 to <1.0... 0.0 0.1963
1.0 to <1.5... 5794 1963
1,5 to <3.0... .0693 4860
3.0 to <3.5... 31 5900
3.5 to <6.0. - 86 745
6.0 to <10. 53
B Acceleration intercept.
Frequency.
M Slope.
TEST PROCEDURE

Each seat test condition was performed five times, each
time being considered a run. A single linearly increasing
frequency sweep was employed in cach test run. The
swept frequency range was 0.7 to 10.0 Hz. The sweep dur-
ation was 10 min per run for a sweep rate of 0.01549
Hz/s. To equalize the system, a startup time of 5 min was
employed. The startup was performed at 0.7 Hz with a
gradual buildup of acceleration amplitude to full test level.
During the swept tests, a total of 450 acceleration values
were stored for both the seat-base and cushion.

7h

ACCELERATION, m/s?

KEY 4
Reference
Measured

n L

o 1 2 3 4

6
FREQUENCY, Hz

5 7 8 9 10

Figure 1.—Acceleration test spectrum.


of freedom linear spring mass system exhibits a peak trans-
missibility greater than one at the system’s natural fre-
quency. Above the natural frequency, the transmissibility
decreases. The actual air suspension measurements at
pressures greater than 503.3 kPa (73 psi) have peak trans-
missibilities that are less than 1. These results appear to

be fairly consistent with the results obtained using a per-
son, dummy, and inert mass as the load on the seat, At
the vibration levels tested, the air suspension seat pres-
surized above 503.3 kPa (73 psi) provided better vibration
attenuation than the mechanical suspension.

REFERENCES

1, Intemational Standards Organization. Guide for the Evalua-
tion of Human Exposure to Whole-Body Vibration. ISO 2631, 1978,
13 pp.

2. Society of Automotive Engineers. Measurement of Whole Body
Vibration of the Seated Operator of Off-Highway Work Machines. SAB
31013, January 1980, 3 pp.

3. Remington, P. J., D. W. Andersen, G. Redmond, and R. Barthol-
omae. Whole Body Vibration Exposure of Surface Coal Mi
chine Operators. SAE Pap. 840479, 1984, 10 pp.

4. Society of Automotive Engineers. Vibration Performance Evalua-
tion of Operator Seats. SAE J1384, May 1983, 2 pp.

5. Classification of Earthmoving Machines for Vibration Tests of
Operator Seats. SAE J1385, June 1983, 3 pp.

6. International Standards Organization. Earth-Moving Machinery-
Operator Seat-Transmitted Vibration. ISO 7096, 1982,

7. Rakheja, S., Subhash, and S, Sankar. Computer Assisted Analysis
of Vertical and Lateral Seat Suspension for Forestry Vehicles.
CONCAVE Research Centre, Montreal, Quebec, CONCAVE-06-89,
1989, 66 pp.

8. Rakheja, S., S. Sankar, and Y. Afework. Vibration Transmission
Performance of Vertical Seat Suspension System. CONCAVE Research
Centre, Montreal, Quebec, Int. Rep., 1988, 115 pp.

9. Whitham, E. M., and M. J. Griffin, Measuring Vibrations on Soft
Seats. SAE Pap. 770253, 1977, 9 pp.

10. Fairley, T. E. Predicting the Transmissibility of a Suspension Scat.
Ergonomics, v. 33, No. 2, 1990, pp. 121-135,


INTRODUCTION

A potential health risk to the operators of mobile
surface mining equipment is whole-body vibration (WBV)
exposure. The final point of transmission of vehicle
vibration to the operator is through the seat. Having a
seat that provides good vibration isolation and allows good
vehicle control is of prime importance to operator safety
and health. The U.S. Bureau of Mines performed vibra-
tion tests on four off-road vehicle operators’ seats in sup-
port of the Bureau’s goal to enhance the safety of miners.
The study determined the optimal adjustment settings (air
pressure, preload setting, etc.) for each seat tested. From
the best performance from each seat tested, comparisons
were obtained among the different seat types. The seats
were tested at maximum vibration levels that never ex-
ceeded the 4h, fatigue-decreased-proficiency (FDP) ex-
posure time defined by the International Standards Or-
ganization in its guideline ISO 2631. In addition, the
measured response of the scat never exceeded a 4-h FDP
time. Two seats employed mechanical suspensions and
two seats employed air spring suspensions. The seats were
tested using a 22,241-N (5,000-Ibf) electrodynamic shaker.
The seats were subjected to a swept sine acceleration in-
put. The frequency range tested for the seats was from 0.7
to 10 Hz. Mechanical suspension seats were tested with
varying seat cushion densities and preload-to-mass ratios
(PMR’s). Air suspension seats were tested with varying
seat cushion densities and air pressure levels. Measured
results were transmissibility and the root mean square
(RMS) of the peak accelerations of the seat. Results for
the mechanical suspension showed that seats with PMR’s
greater than 9.8 (9.8 is recommended normal ride setting)
had increased acceleration levels, Results for the air
suspension showed that seats pressurized greater than
552 kPa (80 psi) attenuated vibration over the frequency
range tested (0.7 to 10 Hz).

The effect of WBV on operators of off-road vehicles
has received considerable attention. Recommendations for
field measurement and definitions of acceptable levels of
WBV are available (1-2)

The International Standards Organization has developed
guidelines for the evaluation of human exposure to WBV
known as ISO 2631 (1). ISO 2631 provides recommenda-
tions for performing vibration measurements in the field
and analysis in the laboratory. ISO 2631 also provides
guidelines or recommendations for exposure limits for
WBV in the frequency range of 1 to 80 Hz. The standard
is specified in terms of vibration frequency, acceleration
magnitude, exposure time, and the direction of vibration
relative to the body. Three different human criteria were

3Italic numbers in parentheses refer to items in the list of references
at the end of this report.

established: human comfort, FDP, and health risk expo-
sure. The Society of Automotive Engineers (SAE) has
defined a SAE Recommended Practice known as SAE
31013 Jan80 for the measurement of WBV of seated op-
erators of off-highway work machines (2). SAE J1013 and
ISO 2631 compare similarly in terms of measurement pro-
cedures and frequency weighting,

A field study was conducted by the Bureau and its con-
tractor that determines the probability of WBV exposure
for a large class of off-road machinery operating in surface
coal mines (3). The study determined that, with the ex-
ception of loaders and graders, vertical vibration is re-
sponsible for the most severe vibration exposure. It was
found that more than 40 pct of off-road equipment oper-
ators were exposed to WBV exceeding the FDP level.

The vibration transmission path from source to oper-
ator for vehicles is given as follows: (1) tire-road and
track-terrain, (2) frame, (3) cab, and (4) operator. Cou-
pling the various transmission paths are suspensions. The
purpose of the suspension is to dissipate vibrational en-
ergy. The various suspensions are (1) primary suspension,
(2) cab mounting, and (3) seat. The primary suspension
couples road input to vehicle frame. The primary sus-
pension for vehicles such as loaders, scrapers, and rubber-
tired dozers typically consists of tire damping only. Tire
damping is provided by the dissipation of energy by the
flexing of the tires. Cab mounting is usually an elas-
tomeric material that separates frame from cab. The seat
uncouples the operator from cab floor motion. The seat
is typically the least expensive element of a vehicle’s
overall suspension to modify or redesign, and modification
of the seat would appear to be the most cost-effective way
to achieve better ride quality.

Because of the importance of the seat in protecting the
operators of off-road vehicles, guidelines have been es-
tablished for measuring vibration performance of scats
(46). SAE 31384 and J1385 and ISO 7096 provide rec-
ommendations for laboratory measurement and evaluation
of seat transmissibility and damping (4-6). SAE J1385 and
ISO 7096 define classes of earth-moving machines and
provide test parameters for each class in the form of
power spectral density (PSD). SAE J1384 specifies the
measurement of acceleration and maximum transmissibility
at the seat-buttock interface.

Laboratory and simulation studies of vibration attenua-
tion characteristics of off-road vehicle seats have been
conducted (7-8). The vibration transmission performance
of five off-road vehicle seats of different makes and manu-
facturers was investigated for varying amplitudes of excita-
tion and suspension height in the frequency range of 1 to
8 Hz (8). The static and dynamic characteristics of the
seats’ cushion and suspension were also measured (8).


Table 6—New 6000/575, PMR = 9.8 peak transmissibility, attenuation frequency,
and overall acceleration RMS for various cushions

Cushion Density, Frequency —Transmissibility Attenuation Aemsy m/s?
kg/m’ peak, Hz peak frequency, Hz

OEM... eee 40.0 0.95 2.03
23 | 78 0.56

58 1.38)

80.1 1 1.63
15 1.64! >10 68

34 1.34

78 1.80,

Pudgee ...... 320.4 94 1.69
29 ra} >10 60

79 1.57,

‘Acceleration root mean square.
OEM Original equipment manufacturer.

6500/575 AND 6500/577 AIR SPRING

SUSPENSION SEATS

Reference Settings

A comparison of transmissibilities, attenuation fre-
quencies, and overall RMS accelerations was made for an

The air pressure for the comparison test
(73 psi). Figure 18 presents the transmis
man, dummy, and 68.0-kg (150-Ib) inert mass. Table 7
presents the type of load object, its mass, and various test
measures. The test measures in table 7 are transmissibility
peaks with corresponding frequencies and overall accel-

Isringhausen 6500/577 air suspension seat loaded with a eration RMS levels. Figure 19 presents the results of ta-
human, an anthropomorphic dummy, and an inert mass. ble 7 graphically.

25 T T T T T T T
a
x
§ 2.0+
77)
c
E
6 WSF
a
mm 1.0F
no
4
2 Sun—Mate
g Sr Pudgee 1
&
=
1 1 1 1 1 1 1 1
9 1 2 4 5 6 7 8 9 10

FREQUENCY, Hz

Figure 13.—Transmissibility of seat-base with differing seat cushions for new 6000/575 mechanical suspen-

ston, PMR = 13.1,


MEASURED PARAMETERS

KEY
Pressure, kPa
D 503.3
Bi 551.6 7
620.5
689.5

Se

Transmis—
sibility 1

Frequency
1, Hz

Frequency
2, Hz

Transmis— Arms,
sibility 2 m/s?

Figure 23.—Transmissibility peaks and frequency of seat-base and overall RMS acceleration with differing alr
pressures for new 6500/577 air suspension, 68.0-kg mass.

In figure 20, for an air pressure of 393 kPa (57 psi),
there exist two extremely large transmissibility peaks. Sim-
ilar peaks exist in figure 20 for the seat pressurized to
448.2 kPa (65 psi). Although the levels of transmissibility
have significantly decreased, they are still considered ex-
tremely large. Figures 22 and 23 show the major trends
for the effect of pressure. The first and second trans-
missibility peaks occur at decreasing frequency and are of
smaller magnitude for increasing pressure. For the 6500/
577 tested at pressures greater than 503.3 kPa (73 psi), the
transmissibility never exceeds a value of 1. The overall ac-
celeration RMS level for pressures over 503.3 kPa (73 psi)
is nearly a whole order of magnitude less than for
393.0 kPa (57 psi). From figure 21, the seat behaves sim-
ilarly for pressures between 503.3 kPa (73 psi) and
690 kPa (100 psi). The gain in seat performance by in-
creasing the air pressure above 503.3 kPa (73 psi) is
modest; however, higher pressures may be required for
actual field usage.

Effect of Cushions

To test the effect of seat cushions on the dynamics of
the air suspension 6500/575 and 6500/577 seats, a fixed air

pressure of 503.3 kPa (73 psi) and test mass of 68.0 kg
(150 Ib) was employed.

The transmissibility plots for the air suspension seats
are shown in figure 24, The peak transmissibilities, their
corresponding frequencies, and overall acceleration RMS’s
are given in table 9.

Table 9.-6500/575 and 6500/577 peak transmissibility,
frequency, and overall acceleration RMS
for various cushions

Seat and Frequency Transmis- Arms m/s?
cushion peak, Hz sibility peak
500/575:
OEM ....... 1.0 075 0.16
Sun-Mate ... 1.0 55)
15 a a
Pudgee ..... 10 58)
14 for) 2
6500/57:
OEM ....... 9 ‘
18 ot 6

‘Acceleration root mean square.

OEM Original equipment manufacturer.


CONTENTS

Abstract .
Introduction .
Off-road vehicle seats ..... :
Mechanical suspension ...............
Air spring suspension .....
Cushions for seat and back .
Vibration testing
Testing hardware and instrumentation .
Computer control
Reference spectrum .
Test procedure
Analysis .
Results ... seneee
6000/575 mechanical suspension seats .
Reference settings .

Effect of preload and inert mass .
Effect of cushions............
Rebuilt 6000/575 and PMR = 13.1.
New 6000/575 and PMR = 13.1.

New 6000/575 and PMR = 9.8.

6500/575 and 6500/577 air spring suspension seats ....
Reference settings . .
Effect of air pressure .
Effect of cushions .
Conclusions ...
Mechanical seats
Air suspension seats .
References

1. Acceleration test spectrum . 5
2. Vertical vibration weighting . be eeee eens : 6
3, Transmissibility of seat-base using human, anthropomorphic dummy, and inert mass with new 6000/575
mechanical suspension .......... o 8
4, Transmissibility of seat-base for PMR’s of 7.1, 9.9, and 11.3 with new 6000/575 mechani suspension . 10
5. Transmissibility of seat-base for PMR’s of 13.1, 16.4, and 19.6 with new 6000/575 mechanical suspension 10
6, First transmissibility peak of seat-base for various PMR’s with new 6000/575 mechanical suspension ... 11
7. Second transmissibility peak and corresponding frequencies of seat-base for various PMR’s with new
6000/575 mechanical suspension ................ eee eee eee beeen eee 11
8. Third transmissibility peak and corresponding frequencies of seat-base for various PMR’s with new
6000/575 mechanical suspension .. os 12
9, Attenuation frequency for various PMR’s with new 600/575 mechani suspension 2
10. Overall RMS acceleration for various PMR’s with new 6000/575 mechanical suspension 13
11. Transmissibility of seat-base with differing seat cushions .......... weeeeeees 14
12, Transmissibility peak and frequency of scat-base, acceleration RMS, and attenuation frequency with
differing seat cushions for rebuilt 6000/575 mechanical suspension, PMR = 13.1.............0005 15
13, Transmissibility of seat-base with differing seat cushions for new 6000/575 mechanical suspension,
PMR = 13.1... . . . 16



Figures 4 and 5 present transmissibilities for the new
Isringhausen 6000/575 with OEM seat cushion, and PMR’s
of 7.1, 9.9, and 113 (fig. 4), and 13.1, 16.4, and 19.6,
(fig. 5).

The seat’s vibration characteristics appear to be heavily
dependent on PMR. Table 3 contains peak transmissibility
values and corresponding frequencies, attenuation frequen-
cies, and overall acceleration levels for the various PMR’s.
Figures 6 through 10 graphically summarize these results.

As seen in figure 6, as the PMR increases, the peak
transmissibility increases for all ratios except PMR equal
to 19.6. The first transmissibility peak values range from
1.9 to 2.6. Figure 7 shows a second transmissibility peak
for all seats in the frequency range of 1.8 to 3.2 Hz. From
figure 7, one sees that the second peak transmissibility
tends to shift to a higher frequency with increasing PMR
and also that the value of the transmissibility increases
with the exception of PMR equal to 19.6. Figure 8 shows
a third peak transmissibility occurring in the frequency

range of 5.7 to 6.9 Hz. As with the second transmissibility
peak, the trend is for the third peak transmissibility to shift
to a higher frequency and to a greater level with increasing
PMR. The attenuation frequencies for the various levels
of PMR are given in figure 9. As seen in figure 9, as the
PMR increases, the frequency at which the seat transmis-
sibility decreases to below 1 increases. Figure 10 presents
the overall RMS acceleration value as a function of PMR.
As the PMR increases, the overall acceleration RMS value
increases, These results demonstrate that seat dynamics
have significant sensitivity to PMR adjustment above and
below the operator’s weight. If an operator sets the chair
preload setting to a value greater than his or her weight,
the PMR ratio would be greater than 9.8, resulting in less
vibration attenuation of the seat. Also, if the operator sets
the chair preload setting to less than his or her weight, the
possibility of the operator receiving shocks because of the
chair bottoming out would increase.

Table 3.—New 6000/575 peak transmissibilities and frequencies, attenuation
frequency, and overall acceleration RMS value for various PMR’s

PMR Frequency —Transmissibility Attenuation Arma 1/8?
peak, Hz peak frequency, Hz
TA eeeceees 09 1.9)
18 13 13.4 0.48
5.8 12) 17
9G veeeeeee 1.0 20)
25 16 78 56
57 1.3,
WB eee 9 24
25 1.6) 84 56
60 1.3)
BA eee 8 24
26 24 7.9 81
5.8 21
164.0... 9 26)
28 3.8 a4 95
66 22
1 1.9)
17 24 >10.0 1.03
32 32
69 28

Arms Aeceleration root mean square.
1A dip occurred for transmissi

18 <1 for frequencies between 3.4 and 4.2 Hz.

*The peak transmissibility level was broad in frequency between 2.0 and 2.9 Hz.


3.0

2.5

2.0

1.0

PEAK TRANSMISSIBILITY
a

Bb 4
0
7.4 9.9 11.3 13.1 16.4 19.6
a PMR, N/kg
Figure 6.—First transmissibility peak of seat-base for varlous PMR’s with new 6000/575 mechanical sus-
pension.
= 4.0
Fra] KEY
H 3.5-+ CO Frequency, Hz 4
n BY Peak transmissibility
=
H 30fF 4
Zz
©
fe 2.54 4
=
mS 2.0F 4
a
Q 1.5- s 4
Zz
=<
> %10F 4
S)
Gi
Wo .SF |
o
wo
fF 7A 9.9 11.3 13.1 16.4 19.6
a ___PMR, N/kg a

Figure 7.Second transmissibility peak and corresponding frequencies of seat-base for various PMR’s
with new 6000/575 mechanical suspension.



Library of Congress Cataloging in Publication Data:

Gagliardi, John C.
Vibration testing of off-road vehicle seats / by John C. Gagliardi and Walter K.
Ute.

Pp. cm. — (Report of investigations; 9454)
Includes bibliographical references (p. 25).
1. Mine haulage. 2. All terrain vehicles—Vibration. 3. Vibration tests. I. Utt,

Walter K. II. Title. IIT. Series: Report of investigations (United States. Bureau of
Mines); 9454.

N

TN23.U43 629.2°042—dc20 92-32293 CIP



KEY

1.67 O Human 7
O sab & Dummy |
a” Mass
bi
Ld 1.25 4
=
<
 1.0F 4
<x
ao
a BE 4
i
SF 4
Yn
S ab |
=

2b 4

0

Frequency, Peak transmis—

Hz _ sibility

Figure 19.—Transmissibility peaks and frequency of seat-base and overall seat acc
man, anthropomorphic dummy, and Inert mass for new 6500/577 air suspension, 503 kPa.

4.5 T T T T T T T T T

4.0b KEY 4
Pressure, kPa

3.5 393.0 5

448.2
503.3

uw
°
T

N
w
T

o ow
T T

TRANSMISSIBILITY, dimensionless
a $
T T

L 1
0 1 2 3 4 5 6 7 8 9 10
FREQUENCY, Hz

Figure 20.—Transmissibility of seat-base with alr pressures of 393.0, 448.2, and 503.3 kPa for new 6500/577
air suspension.



Report of Investigations 9454

Vibration Testing of Off-Road Vehicle Seats

By John C. Gagliardi and Walter K. Utt

UNITED STATES DEPARTMENT OF THE INTERIOR
Bruce Babbitt, Secretary

BUREAU OF MINES


0.8 T T T T T T T T T
non
oO 7b KEY 4
5 OEM
nm 6b Sun—Mate 4
5 Pudgee
E st 6500/577
mol
4b
a
O 3F
a
2)
S 2b
a
Zz
took 4
e-
fe

i 1 1 1 1 1 i L 1
0 1 2 3 4 5 6 7 8 9 10

FREQUENCY, Hz _ —

Figure 24,—Transmissibility of seat-base with differing seat cushions for new 6500/575 and 6500/577 alr
suspension. OEM, Sun-Mate, and Pudgee cushions were tested on 6500/575; 6500/57 was tested us-

ing OEM cushion.

The transmissibility peak never attained a value greater
than 1 for any air spring suspension and seat cushion com-
bination. In addition, all peak transmissibilities occurred
below 1.8 Hz. All seats exhibited a peak transmissibility
near 1.0 Hz. The 6500/575 with Sun-Mate and Pudgee

cushions and the 6500/577 also had a second peak trans-
missibility between 1.4 and 1.8 Hz. Seat cushions with
increasing densities produced lower overall acceleration
RMS values.

CONCLUSIONS

Four off-road vehicle seats were vibration tested using
an acceleration test spectrum. The tests were conducted
by linearly sweeping the acceleration spectrum in time.
The levels of the tests and the measured responses on the
seats did not exceed ISO 2631’s 4-h, FDP time. The seats
tested employed a scissor linkage. Two seats employed a
mechanical-coil-spring suspension and two seats employed
an air spring suspension. All seats included an oil hy-
draulic shock absorber. The two mechanical seats tested
were of the same model, differing only in length of service.
One seat was rebuilt after extensive use in the field, while
the other seat was new and sent directly from the factory.

The 6500/575 and 6500/577 air suspension seats employed
vertical suspensions that were similar in design. The 6500/
577 air suspension seat also contained a longitudinal sus-
pension and air chamber seat cushion that was absent for
the 6500/575.

MECHANICAL SEATS

For the new mechanical suspension seat with OEM’s
seat cushion and backrest, the typical response is char-
acterized with three peak transmissibilities. The three
transmissibility peaks typically occur in the bands 0.8 to


Where —f._ = frequency (Hz), A,(f) = normalized weighted seat acceleration
spectra (m/s*),
A,() = reference acceleration spectra (m/s),
P,(f) = normalized weighted seat mean square
A) = acceleration spectra from seat cushion acceleration spectrum ({m/s?)°),
(m/s’),
. 2 N = number of acceleration values per
A,() = acceleration spectra of seatbase (m/s), spectra,
T() = transmissibility, A, = overall mean normalized weighted ac-
N(@® = normalized factor, celeration level (m/s?),
W(). = vertical weighting factor, and  Ajas = overall RMS acceleration (m/s).
RESULTS

6000/575 MECHANICAL SUSPENSION SEATS
Reference Settings

A comparison of transmissibilities, attenuation fre-
quencies, and overall RMS accelerations was made for a
new Isringhausen 6000/575 mechanical suspension seat
loaded with a human, an anthropomorphic dummy, and in-
ert masses. Table 2 presents the type of load object, its
mass, seat preload setting, PMR, and various test mea-
sures. The test measures in table 2 are transmissibility

peaks and corresponding frequencies, overall acceleration
RMS levels, and attenuation frequencies. The peak trans-
missibilities were selected as being significantly higher than
the local measures. The correct setting for a human sit-
ting on the seat is a PMR of 9.8. The value of 9.8 is the
acceleration constant of gravity. Following the assumption
of other researchers that 2/7 of thé seated body mass is
supported by the feet, one obtains a PMR of 13.7 (7, 10).
Figure 3 presents the transmissibilities of human, dummy,
and 51.8- and 68.0-kg (114- and 150-Ib) inert masses for
PMR’s of 13.7 and 9.8, respectively.

Table 2.—Comparison of tranamissibility peak values and frequencies, attenuation frequency, and overall
acceleration RMS between human, anthropomorphic dummy, and Inert mass for new 6000/575

Mass, kg Preload PMR Frequency Transmissibilty Attenuation A,,,, m/s
setting, N peak, Hz peak frequency, Hz
Human .... 63.6 623.3 98 21 1.38)
3 be) 49 0.52
Dummy .... 72.6 712.0 98 9 1.74)
27 ra} >10 60
79 1,49)
Inert mass .. 518 712.0 13.7 28 2.82)
3.2 2.65 9.0 84
69 1.80)
68.0 666.4 98 y 1.0 2.00)
25 ~ ‘sh 78 56
5.7 1.30,

Ams Aeceleration root mean square.


y
uw

KEY

‘
20r A 7.1 PMR 7

uw
T

TRANSMISSIBILITY, dimensionless
wn 3
T T
L

i L L 1 1 1 i 1 1
0 1 2 3 4 5 6 7 8 9 10

_FREQUENCY, Hz

Figure 4.—Transmissibility of seat-base for PMR’s of 7.1, 9.9, and 11.3 with new 600/575 mechanical sus-
pension. ee

4.0 T T T T T T T T T

KEY

13.1 PMR 7
16.4 PMR

19.6 PMR

3.55

3.0

2.55-

2.0

1.55

TRANSMISSIBILITY, dimensionless

1 i 1
oO 1 2 3 4 5 6 7 8 9 10
FREQUENCY, Hz

Figure 5.—Transmissibility of seat-base for PMR’s of 13.1, 16.4, and 19.6 with new 6000/575 mechanical sus-
pension.