Impact wrench is a popular hand tool whose mechanism is featured with impact characterized by short time duration and large contact torque. The nonlinear impact problem has drawn researchers’ interests for a long time and the investigation is still undergoing. Fully and accurately modeling impact can facilitate the understanding and improving the performance of impact wrench. Under certain circumstance, not only coefficient of restitution is interested but also the whole impact process. Hence, discrete impact model cannot fulfil these requirements and dynamic modeling is indispensable. At the same time, sufficient knowledge about contact parameters such as contact stiffness is necessary for accurately impact modeling. But these parameters are usually not readily available. Moreover, researchers mainly focused on translational impact problem while rotational impact problem is ignored. In this paper, Hunt-Crossley nonlinear contact model is applied and extended in torsional impact dynamic modeling. Based on this model, an experimental method is developed to evaluate the distribution of inertias in lumped parameter models. Contact stiffness is measured based on spectrum results. To obtain reliable experimental data, impact wrench is driven by a servo motor to generate a controllable torque impulse. Contact stiffness is acquired under different impact speed. Results show the nonlinear relationship between contact stiffness and impact speed.

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