Page 302 - Biomedical Engineering and Design Handbook Volume 1, Fundamentals

VIBRATION, MECHANICAL SHOCK, AND IMPACT 279

Included in Table 11.2 are the metric used for assessing the exposure, the frequency or weighting of
the stimulus, or model, and a representative value for the response or health effect under consideration.
As already noted, there are large variations between individuals in response, and susceptibility, to
vibration, shock, and impact.

Vibration Perception. The perception of vibration depends on the body site and on the stimulus
frequency. The thresholds in Table 11.2 are typical of those for healthy adults, and are expressed as
instantaneous RMS accelerations [i.e., with T ≈ 1 s in Eq. (11.2)]. The value for whole-body
vibration is given in terms of a frequency-weighted acceleration, and so is applicable to vibration at
frequencies from 1 to 80 Hz. The values for hand-transmitted vibration are for sinusoidal stimuli
applied to the fingertips of males (M) and females (F) at the specified frequencies.

Thresholds for Health Effects. Thresholds for the onset of health effects have been estimated for
regular, near-daily exposure to hand-transmitted and to whole-body vibration. The metrics employed,
however, differ. For hand-transmitted vibration, the assessment is in terms of the magnitude of the
8-h, energy-equivalent, frequency-weighted, RMS, vector acceleration sum, a WAS(8) . This metric
employs values of the RMS component accelerations averaged over 8 h [i.e., T = 28,800 s in
Eq. (11.2)] which have been frequency-weighted using W in Fig. 11.1. The components are determined
h
for each of the directions of the basicentric, or biodynamic, coordinate system shown in Fig. 11.2,
a a and a respectively. Thus:
X,RMS(8), Y,RMS(8), Z,RMS(8),
a = [a 2 + a 2 2 + a 2 2 1/2 (11.12)
]
WAS(8) X,RMS(8) Y,RMS(8) Z,RMS(8)
A reduction in the metric will occur for a given acceleration magnitude if the duration of the
exposure is reduced, as is illustrated by the following example (see also “Vibration Exposure” in
Sec. 11.1.1).
EXAMPLE 11.2 A worker uses a percussive rock drill for 3 h daily. Measurement of the handle vibra-
2
tion indicates a frequency-weighted RMS acceleration of 18 m/s along the drill axis, and a
2
frequency-weighted RMS acceleration of 5 m/s perpendicular to this axis. Estimate the 8-hour daily
exposure.
Answer: From the (limited) available information, we obtain the following approximate values:
a = 18 a = a = 5
Z,RMS(3) X,RMS(3) Y,RMS(3)
2 1/2
2
2
So a = (5 + 5 + 18 ) = 19.3
WAS(3)
Now an exposure for 3 h (T ) can be expressed as an equivalent 8-h exposure using Eq. (11.5):
(3)
/ 12
⎛ T ⎞
3
a WAS(8) = a WAS(3) ⎜ () ⎟
T ⎠
⎝ ()8
12
/
⎛ 10 800, ⎞
.
a WAS(8) = 19 3 ⎜ ⎟
so that ⎝ 28 800, ⎠
a WAS(8) = 11..8 m/s 2
The assessment of whole-body vibration employs the VDV averaged over 8 h [i.e., T = 28,800 s
in Eq. (11.6)], with frequency weighting W for vertical vibration and frequency weighting W for
k d
horizontal vibration. It is believed that the higher-power metrics, as recommended here, better repre-
sent the hazard presented by motion containing transient events, particularly when these become
small-magnitude shocks or impacts. The most appropriate metric, however, remains a subject for
research.