Attenuation of hand-held machine vibrations. Application of non-linear tuned vibration absorber

Licentiate thesis, 2017

Vibration injury in the hand-arm system from vibrating hand-held machines is one of the most common occupational health injuries and causes severe and often chronic nerve and vascular injuries to the operator. The objective with this study is to contribute to reduce hand-arm vibration injuries, first by developing methods of redesigning machines that will lower their vibration level and second to study how high frequency vibrations (HFV) above 1250 Hz effect the finger tissue which are not included in the current standard for risk evaluation. Vibrating hand-held machines can be divided into two main groups depending of the source of the vibration. The first and largest group is machines where a reciprocating piston creates a vibration, e.g. breakers and hammer drills. The second group is rotating machines, e.g. grinders and polishing machines, where the vibration is generated by the unbalance force from the grinding disc or the rotating force from the friction between the abrasive pad and the work piece. This study has mainly been focused on reciprocating machine vibrations by development of a method for attenuation of these vibrations based on tuned vibration absorbers (TVA). The TVA comprises of a spring and an additional mass connected to the main vibrating body that creates a counterforce to attenuate system vibration. The limitation of conventional TVA is that they are only effective in a narrow frequency span and thereby have a limited practical use. However by introducing appropriate gap and spring preload into classical TVA makes the absorber dynamics nonlinear. It was shown that this kind of nonlinearity can lead to frequency auto tuning vibration attenuation. The efficiency of the developed technology called Auto Tuning Vibration Absorber (ATVA) was proven by simulations and the results were validated both in a dedicated test rig as well as in prototype machines. The results from a prototype machine showed that the vibration could be reduced by approx. 80 % and at the same time reduce the weight with 50 % compared to the original machine without affecting the efficiency. The study of the mechanical effects from HFV on finger tissue was made by, first: development of measurement technique based on Micro Electrical Mechanical Systems (MEMS) accelerometers to be able to accurately measure vibrations up to 50 kHz, second: create a Finite Element (FE) model for simulating the pressure wave propagation into the finger tissue, and third: development of methods to reduce the HFV content in machines. Measurements show that there are high frequency high amplitude accelerations from impact tools and that these accelerations create a pressure wave that propagates into the tissue. It was also shown that there is a large potential to effectively reduce the HFV content in machines. On the study of effects on finger tissue from HFV clearly indicated that there is potentially an increased risk that HFV will cause vibration injuries on humans and on biological tissues and that the current standard ISO 5349 underestimates the associated risks.