Vibrometry - Fundamentals and Assessment Tools for Occupational Ergonomics - page 652

z

y

x

x

z

z

y

y

a x , a y , a z = acceleration in the

x

directions of the x, y, z axes
x axis = back-to-chest
y axis = right-to-left side
z axis = foot (or buttocks)-to-head

FIGURE 33.1 Whole-body vibration measurements coordinate system. (ANSI S3.18, ISO 2631, ACGIH-TLV
(WBV), EU).

“X axis” motion is in the front-to-back direction (through the sternum) for WBV, and for HAV measure-
ments the motion is through the palm of the hand.
Having defined the directions of motion, the vibration magnitude or intensity parameter(s) must be
specified. We can choose between three mathematically interrelated quantities: displacement, velocity, or
acceleration. Displacement is merely the distance moved away from some reference position. Velocity (or
speed) is the time-rate-of-change of displacement. Acceleration is the time-rate-of-change of velocity.
Acceleration is usually the magnitude intensity parameter of choice for several reasons which include

ease of measurement and the belief that acceleration is both a hard and soft tissue stressor. Acceleration
is expressed in units of meters

/

sec.
The “peak” acceleration or maximum values are not usually evaluated, rather an average acceleration par-
ameter called root-mean-squared or rms acceleration is measured and evaluated and is relatable directly to

/ sec / sec or in terms of gravitational g units, where 1g ¼ 9.81 m / sec /

z h

z h

x h

Biodynamic
coordinate system
Basicentric
coordinate system

y h

FIGURE 33.2 Hand-arm vibration measurements coordinate systems. (ANSI S3.34, ACGIH-TLV (HAV), EU,
NIOSH #89-106).