Abstract

Prolonged exposure to vibration transmitted to the operators of hand-held power tools has been related to symptoms of vibration white finger (VWF) disease, also known as "Raynaud's Phenomenon of Occupational Origin". Many subjective and objective epidemiological, and clinical studies have established the prevalence rates and vibration related symptoms, such as VWF disease, loss of muscle strength, injuries to bones and joints, and disorders of the central nervous system. The mechanism leading to these disorders, however, remains almost unknown. The biodynamic response characteristics of the hand-arm system, subject to handle vibration are investigated through: (i) measurement and synthesis of driving point mechanical impedance; (ii) study of hand-grip pressure distribution to gain an insight to a probable mechanism leading to VWF disease; (iii) study of electromyography of finger flexor muscles to enhance an understanding of the vibration induced loss of muscle and grip strength; and (iv) study of vibration transmissibility of the hand-arm system and the protection offered by so-called "anti-vibration" gloves. The biodynamic response of the hand-arm system is investigated for sinusoidal as well as stochastic excitations in the three orthogonal directions of vibration. Linear and nonlinear analytical models with constant and grip force dependent parameters are developed to characterize the dynamic response of the hand-arm system. A comparison of the model response to the measured data revealed that the nonlinear analytical models with grip force dependent parameters can accurately predict the hand-arm vibration response for different hand-grip forces over a wide frequency range of tool vibration. Further, the measured data reported in the literature are synthesized to propose a range of idealized values of driving point impedance in all the three directions of vibration. The hand response to impinged and transmitted vibration is investigated through measurement and analysis of distribution of dynamic forces at the hand-handle interface, and electrical activity of the finger flexor muscles. A sensing grid comprising of flexible pressure sensors was fabricated to measure the interface pressure under static and dynamic conditions. The pressure distribution is related to impaired blood flow, and thus to the probable mechanism leading to the onset of VWF disease. The primary injury mechanisms are further investigated using electromyography (EMG) of the finger flexor muscles, the muscle group responsible for exerting the grip force on the handle. Analysis of the results showed that EMG of muscle group under vibration is as high as 6 times the EMG measured under static grip condition. A methodology is proposed to measure the hand-transmitted vibration and to assess the performance of the anti-vibration gloves. Analytical models are developed to characterize the vibration transmissibility of the hand-arm and the protective gloves. The study concluded that anti-vibration gloves attenuate vibration only in a limited frequency band.