Work Study and Ergonomics

Introduction

The present chapter considers the aspects of physical stresses due to ambience, the omnipresent sensations to which worker is exposed from diverse sections of machinery or from other systems in his workplace. These ambiance pollutants mainly include vibration, the noise, the temperature, the illumination, etc. But the current chapter is confined to a brief description.

Vibration

Vibration is the most fundamental environmental factor. Therefore, it is essential to understand the definitions and parameters of structure-borne vibration, and it will help us understand the processes of other environmental parameters – particularly noise and, to a lesser extent illumination. A body is said to vibrate when it describes an oscillating motion about a fixed position. As like sound, the number of times a complete motion cycle takes place during the period of one second is referred to as the frequency, which is measured in hertz (Hz). The motion can consist of a single component occurring at a single frequency, as with a tuning fork, or of several components occurring at different frequencies simultaneously, e.g., with piston motion of an internal combustion engine.

Vibration signals in practice usually consist of many frequencies occurring simultaneously, so that it is not possible to see immediately, just by the examination of; the amplitude–time pattern, how many components there are and at what frequencies they occur. These components can be revealed by plotting vibration amplitude against frequency, the process being known, as in case of sound as frequency analysis. The graph showing the vibration level as a function of frequency is known as frequency spectrogram, and the vibration amplitude is the characteristic which describes the severity of vibration.

Frequency ranges of significance

(a) The principal hazards associated with vibration are cold body vibration and condition known as vibration induced white finger (VWF), the physiological aspects of which are discussed in another subsequent section. The human body is most sensitive to vibration in the frequency range 1–80 Hz and is principally subjected to vibration in 3 supporting surfaces, viz., the feet of a person while standing, the buttocks of a seated person, and the supporting areas of a person lying down. In the longitudinal direction, i.e., feet to head, the human body is most sensitive to the vibration in the frequency range 4–8 Hz. In the transverse direction, however, it is the most sensitive to the frequency range 1–2 Hz.