Avoiding early failures in conjugate cam mechanism by means of different design strategies

Abstract

To solve the indetermination of forces existing in a form-closed cam mechanism formed by conjugate cams, where the contact between the cams and the follower rollers is constantly ensured by only the geometry of the elements, dynamic models that consider the elasticity of the elements must be proposed. Because the stiffness of the main elements is associated with the elasticity of the solids, tight variations in manufacturing and assembly errors modify the effective interference fit, which significantly affects the expected fatigue life of the mechanism, leading to a premature failure of the elements due to surface fatigue. Based on a real industrial application of a conjugate cam mechanism and using lumped-parameter models, the objectives of this paper are: first, to show that it is difficult to achieve a pure form-closed conjugate cam mechanism, with the expected fatigue life of the mechanism, by using only standard tolerance specifications; second, to compare the expected fatigue life and motor torque with other cam mechanism design strategies such as force-closed and the combination of force-closed and form-closed strategies, known as force-closed conjugate cam strategy. This paper based on simulation results demonstrates that this latest strategy can, thanks to a better control of the preload, easily achieve results very similar to the theoretical ones of a form-closed conjugate cam mechanism. A prototype of the mechanism of the force-closed conjugate cam strategy is also built.