This paper was produced for the 2019 NAFEMS World Congress in Quebec Canada

Resource Abstract

Stick-slip effects are observed in mechanical systems at relative motion interfaces. In MBS tools, root functions are used for an exact simulation of these effects. With root functions, the solver can determine the exact time corresponding to the state change at the cost of dramatically increasing simulation time. Nevertheless, in many cases the exact behaviour of the friction elements is of lesser interest for the user as compared to their effects on the entire system.



This work establishes a preliminary comparison between different strategies for modelling stick-slip effects. Each strategy refers to a suitably parameterized variation of a force element, which was either selected from the existing library or custom defined. All considered force elements can be broadly classified into two types. In the first, the stick-slip effect is modelled as a continuous function of a velocity dependent friction coefficient. In the second, the stick-slip effect is modelled as a combination of two force laws, wherein the state changes depending on a predefined threshold parameter, such as the velocity or force.



Furthermore, for each strategy, the force element was implemented for three different applications: the hydraulic actuation mechanism of an excavator, a vertical coulomb friction damper of a railway bogie and the sliding plate bearing the car body in a different railway bogie. Finally, the strategies were rated with respect to the accuracy of the results (as compared to an FEM benchmark simulation or real measured values where available) and computational time. The modelling and simulation were done in the MBS tool SIMPACK.



Initial results show that it is recommendable to use the two-force law elements with limited time steps, thereby avoiding the need for root functions. The dynamic friction elements are efficient, but highly sensitive to chosen parameters.