EHL film thickness and load dependent power loss of cycloid reducers

Abstract

Robotic arms, space joints and micro-medical devices demand high positioning accuracy, high efficiency, lightweight reducers which often are required to be lubricated for life. Cycloid drives have been gaining popularity in recent years due to their compact form, high reduction ratio and stiffness. Energy loss minimization and ensured good lubrication are vital for those applications. Total power loss is commonly computed as sum of two parts. The first is the zero or minimum-load, which is caused mainly by oil churning and drag as well as contact seals friction. The second part is load dependent and is related to the transmitted power. It is dominated by the friction in all sliding contacts. Geometry parameters of the meshing surfaces affect the load distribution as well as the rolling and sliding velocity of the contacting bodies. The study utilizes meshing equations and an ideal load distribution model to calculate pressure, rolling and sliding at the contacts of a typical 1-disc cycloid reducer. The conditions are characterized by severe slip and counter-turning. A comparison with established EHL models revealed high discrepancy at areas of counter-turning. The ultimate aim of the present study is to calculate the local film thickness and friction. Due to the shortcomings of the aforementioned models, an advanced numerical EHL solver was employed. The determined conditions of the contact points were used as input for the solver. In order to demonstrate the method, calculations were performed for four different load cases as a way of showing the effect of input speed and torque on the efficiency.