Verification of cylindrical interference fits under impact loads with LS-Dyna

Interference fits are a commonly used means to couple shafts and wheels for example. The usual dimensioning is performed by a static verification. As long as the system geometry is not too complicated and the deformation is assumed to be linear elastic, the interference pressure can easily be calculated with the more familiar solutions of the equations of elasticity. The maximum static contact forces can be calculated together with an assumed coefficient of static friction. In order to investigate whether a cylindrical interference fit provides sufficient stability against slip the real loads have to be known. However in various applications this is not the case and the interference fit is subjected to dynamic loads, especially to impact loads. We simulate a model interference fit that is first axially mounted and later also axially loaded. This is a typical case in hydraulic systems. Similar problems occur in gears, e.g. worm gears, especially if there are reverse torques as in many applications. The crucial number a design engineer seeks is the safety against slip, S. A dimensional analysis shows that S is dependent on the length l of the interference fit, its interference Z, the velocity v and the mass of the impacting body m and the static friction coefficient μ. Altogether we find: S ~ Z l μ v-1 m-0.5. Numerical experiments have shown that the easiest way is to vary the velocity of the impacting body to find the design with minimum safety S = 1. The desired safety can then be achieved by simply changing the parameters. We investigate the influence of different contact types, and find the OSTS contact as optimal for the shaft-hub contact. The same way we consider the NTS contact as optimal for the shaft-impacting body contact. The results also show that the forces due to an impact are huge and that it is not possible to make an appropriate design without a numerical or experimental analysis.

application/pdf L-I-02.pdf — 4.5 MB