EVALUATION METHODS FOR VIBRATION EFFECT PART 3 ...
Transcript of EVALUATION METHODS FOR VIBRATION EFFECT PART 3 ...
Ind. Health, 1967, 5. 213.
EVALUATION METHODS FOR VIBRATION EFFECT
PART 3. MEASUREMENTS OF THRESHOLD AND EQUAL
SENSATION CONTOURS ON HAND FOR VERTICAL
AND HORIZONTAL SINUSOIDAL VIBRATIONS
Toshisuke MIWA
National Institute of Industrial Health, Kizuki-Sumiyoshi, Kawasaki.
(Received November 27, 1967)
The levels of threshold and equal sensation on hand for vertical and horizontal
vibrations (3-300 c/s) were determined by the same method in the previous reports and
their characteristics did not differ between both vibrations. The equal sensation curves
of hand above 10 c/s showed good agreement with those of whole body.
Then, the sensation of hand for horizontal vibration was equated to that of verti-
cal one and it was observed that the sensation for both vibrations was equal at the
same vibration acceleration level and frequency.
The vibration greatness levels (VGL) for vertical and horizontal vibrations
and the conversion of sensation for horizontal vibration into that for vertical vib-
ration were determined for the whole body in Part 1 and 2. In this report, the
same problems were studied for human hand. Because various kinds of portab-
le vibrating tools (rock drill, chipping hammer, pneumatic hammer, air grinder, ty-
tamper, chain saw and hand tractor etc) have induced various injuries in worker's
hands, for example, Raynaud' s phenomenon, degenerative change of joint bones
etc. These portable tools are being used in many industries, because of facility of
carying and setting, and of high productivity. Moreover, the new types of them
have been devised by development of midget engines.
Under such circumstances, the safty limit for vibration to handle portable vi-
brating tools without injury is needed in the industrial hygiene. Although many
predecessors in this field have investigated about this problem by various medical
method, the clear solutions have not been obtained owing to the complexity of
these injuries. The psychological evaluation method described in the previovs re-
port is considered to contribute to this problem.
With the hand, the levels of threshold and equal sensation for vertical and
horizontal vibrations were measured. Equalization of sensation for horizontal vibra-
tion to that for vertical one was also examined. The reports dealing with these
problems have not yet been published. This paper reports about these problems.
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6 7
8 9
Fig. 1 Photographs of postures of subjects.
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SUBJECTS AND POSTURES
Ten subjects tested were the same crew as previous reports. On the posture
of the subjects, their single hand was directly pushed to the vibration table and
joints of hand and elbow were bent at right angle with each other in order to im-
itate an actual working posture and to prevent the vibration transmission from
hand to head.
For the vertical vibration, the hand pressed the vibration table with the force
of 5 kg continuously. This static pressure was controlled by the subject himself
watching the strain-meter which was made by the amplifier and the strain gages
stuck on the upper leaf spring supporting the vertical vibration table.
For the horizontal vibration, the hand was vibrated to the finger direction on
the palm. In this case, the force pressing the vibration table could not be meas-
ured because of difficulty of instrumentation. Therefore, subjects were requested
to press the table in the same sensation strength as in the vertical vibration.
In preliminary experiments on three subjects, the following various cases were
tested, namely, when the subject' s hand pressed the vibration table by means of
an iron handle of a rock drill which was set on the both vibration tables, when
the both hands directly pressed the vibration table, when the horizontal vibration
was excited in the rectangular direction on the finger, and when the static pressure
of single hand added to the vibration table was changed from 5 to 10 kg. However,
the observed results of these cases were in good agreement with the data deter-
mined in the condition described above. Therefore, actual experiments were done
under the above simplified conditions and subject's postures were shown in Fig 1.
MEASURING METHOD
The same vibration tables as described in the report 1 and 2 were used for
vertical and horizontal vibrations. The frequency range examined was selected
from 3 to 300 c/s, because, with the various kinds of actual portable vibrating
tools, the spectrum affecting mainly on human hand seems to be this range. The
amplitude of vibration was taken as widely as possible by using three kinds of the
Fig. 2 Block diagram of this experiment and photographs of the measuring
apparatus.
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vibration tables. Namely, the maximum vertical vibration was 80dB (VAL) at
100 c/s, and the maximum horizontal vibration 70dB (VAL) at 100 c/s.
The measurement of threshold and equal sensation was carried out by the
method of paired comparisons and vibration was given the subjects in the ascending
and descending series. The detailed procedures have been specialized in Part 1.
The equal sensation at the highest level of both vibrations was observed only by
ascending series. Because the sufficient higher level of vibration at starting of de-
scending series could not be given the subject owing to the amplitude limit of the
vibration tables. The standard frequency in the equal sensation was also 20 c/s.
The block diagram of the experiment is shown in Fig 2.
RESULTS AND DISCUSSIONS
The contours of threshold and equal sensation with hand for vertical and hor-
izontal vibration are shown in Fig 3 and 4. These show the average and the stan-
Fig. 3 Contours of threshold and equal sensation for vertical vibration with hand.
In Fig. 3 and 4 ,ordinate indicates vibration acceleration levels (VAL) (dB) (201og10(a/aref)
a; rms acceleration value, (g), aref; 10-3g, 1g=980cm/sec2) and abscissa frequencies (c/s).
The solid curves are contours connecting the average values of threshold and equal sensation
with hand. The dotted curves are contours of equal sensation curves for the vertical and
horizontal vibrations with whole body (Fig. 13 in Part 1) respectively. The group of three
circles with dots arranged lengthwise shows the standard deviations and the average values
calculated with 10 men. The center of this group indicates the average value and outside
of them the range of standard deviation.
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Fig. 4 Contours of threshold and equal sensation for horizontal vibration with hand.
dard deviation of their pooled data with 10 men.
Threshold contours: The threshold curves for vertical and horizontal vibrations
are in good agreement with each other. These threshold contours consist of three
lines with the slope of velocity, jerk and displacement, and their bending points
are 15 and 100 c/s. Furthermore, the fact that these contours are similar to the
mechanical impedance curve1) of hand for vertical vibration attracts our attention,
though the bending points of the impedance curve are somewhat higher than those
of threshold curves.
The standard deviation of threshold is uniformly distributed within about •}3
dB independent of vibration frequency.
Equal sensation contours: The solid curves in Fig 3 and 4 which were approx-
imately connected with the average value of equal sensation level are composed of
three lines according with acceleration, velocity and displacement and two bending
point, 6 and 60 c/s. Above 5 c/s, the contours of the equal sensation of hand forboth vibrations well accord with each other, that is, they have good individual
congruity and group congruity whose meaning was already defined in Part 1. Besides,
above 5 c/s, the equal sensation curves on hand for both vibrations are in good
agreement with those on whole body as shown in Fig 13 in Part 1. Below 5 c/s,
the dashed curves in Fig 3 and 4 show the equal sensation contours on whole body
for vertical and horizontal vibrations respectively. The frequency characteristics
in this frequency range with hand and whole body did not agree with each other.
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In order to compare the equal sensation curves of hand for both vibrations with
each other, these two figures (Fig 3 and 4) are superposed as Fig 5. From this
figure, the equal sensation contours of hand have no special different frequency cha-
racteristics for both vibrations.
The standard deviations of equal sensation show frequency dependency, that
is, they are small between 5 and 60 c/s but, except this range, they become grad-
ually larger in proportion to frequency difference between this and outside range
(•}max. 5 dB).
Fig. 5 Contours obtained by superposition of Fig. 3 and Fig. 4.
In the additional experiment, the horizontal vibration was equated in sensation
to the vertical vibration by three subjects, This equalization was made at the
same frequency of both vibrations and the measured frequency was changed at 10,
20, 60, 100, 200 and 300 c/s. One hand was set on the vertical vibration table and
another hand on the horizontal one. At this time, the joints of hand and elbow
of both hands were bent at right angle shown in Fig 1-(5). The subject was asked
to keep his posture constant as strictly as possible, because the result was consi-
derably affected by the posture. The vibrations of two directions were simultan-
eously started and stopped in both tables every 3 sec by the tester. The vertical
vibration was fixed at a certain level as the standard and the procedure giving the
subject the horizontal vibration was done in the ascending and descending series.
The level of horizontal vibration was controlled with the attenuator by the tester
untill the subject answered to obtain the equal sensation. In this experiment,
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both hands were interchanged each other on the vibration tables to check the
difference of sensibility of both hands at each frequency.
The same vibration acceleration level of both vibrations induces equal sensa-
tion at each frequency. This fact may be forecasted by the result that there was
almost no difference in the threshold with hand for the vertical and horizontal vi-
brations. It is also mentioned from this result that the sensation between vertical
and horizontal vibrations with the same VAL is equally convertible with hand.
Consequently, by Fig 5, vibration greatness level is defined for the vibration
sensation of hand. This graph is commonly used in all of the following cases, nam-
ely, both vibrations (vertical and horizontal), single and both hands, the pressing
force of hand to the vibration table from 5 to 10 kg, various shapes of the handle
and every direction of horizontal vibration on the palm.
REFERENCE
1) Miwa, T. (1964). Ind. Health, 2.95.
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