Dynamic response of the hand-arm system to a sinusoidal input

Abstract

The mechanical impedance measuring technique was used to obtain data necessary to determine the vibration characteristics of the human hand. From this data, by means of a standard systems identification method, the values of the masses, springs and dampers were derived for an equivalent mass-spring-damper system which simulates the dynamic response of the hand. The influence of grip tightness, hand pressure upon the tool handle and arm position on the vibration response of the hand were examined. For normal arm positions used for operating vibrating power tools, the arm position had little influence upon the vibration response of the hand. However, grip tightness and hand pressure upon the tool handle influenced the vibration response of the hand, especially above a forcing frequency of 60 Hz. In the frequency range studied (20–500 Hz) for a specified arm position, grip tightness and handle pressure, the hand-arm system can be modeled as a six degree of freedom system, two degrees of freedom in each of the principle orthogonal directions. With proper restrictions, the hand was modeled as a three degree-of-freedom system, one degree of freedom in each direction. For a specified arm position, grip tightness and hand pressure on the tool handle, the hand response to a sinusoidal input was similar to that of a linear system. The hand impedances were independent of the amplitude of force input to the hand. The results of this investigation yielded information necessary for determining meaningful hand vibration limit standards. To establish acceptable standards, it is necessary to monitor the vibration limit levels as a function of forcing frequency, the tightness of grip and the pressure of the hand on the tool handle. For, even though the vibration levels may decrease with a tighter grip or firmer hand pressure, the amplitude of the force transmitted through the hand may increase, resulting in an increased probability for injury.