Investigation of nonlinear transformation of impulses in impact units for improvement of hammer drill performance

Hammer drills are known to cause vibration induced injury. One method of alleviating this problem is to reduce the level of vibration generated by the hammer drill impact unit, so that less vibration is felt by the operator. The objective of this study was to increase the understanding of impact unit behaviour and develop theoretical models to assist the design process. The impact unit was initially simplified to a two degree of freedom vibro-impact system with impact excitation. The periodic Green's function method was used to study the system analytically. The equations of motion were solved for the initial two degree of freedom system for both sinusoidal and impulse excitation cases, without recourse to numerical methods. This is the first purely analytical solution that has been obtained for such a two degree of freedom system with impact excitation. Two solutions to the equation of motion were found but a stability analysis showed that only one was stable. The analytical solution provides a reliable basis for the development of more detailed numerical models of the impact unit. A Simulink model achieved a good agreement with the analytical solution for both sinusoidal and impulse excitation. It was found that the use of compliance in the impact surfaces was essential to avoid the accumulation of integration errors due to infinite acceleration at impact. A more complex model with a loose mass was also simulated. A two mass test rig was developed to provide data to support the development of the simplified models. Two resonances and an antiresonance were identified, confirming the modelling results. The first experimental rig to be based on an actual hammer drill was also developed, to support the development of more complex models. A laser vibrometer was used to measure the velocities of the internal parts. By varying the hammer drill speed a general understanding of its behaviour was obtained. The hammer drill showed periodic behaviour with the same period as the excitation but with some variation from cycle to cycle.