Investigation on the ball screws no-load drag torque in presence of lubrication through MBD simulations

The no-load drag torque is a crucial factor in the design phase to estimate the power required by the electric motor in electro-mechanical actuators (EMA). This paper focuses on the analysis of the drag torque contribution generated in preloaded double nut ball screws of EMAs. In literature its value is commonly estimated with a linear relationship with respect to the preload level. However, many other parameters play a role in determining the friction torque, such as operating speed, temperature and geometry. A sensitivity analysis on these parameters is presented by means of simulation results obtained from a high-fidelity multibody dynamic (MBD) model, capable of describing the full three-dimensional dynamics of each subcomponent and the presence of full film, mixed or boundary lubrication between mating elements. The results have been interpreted with a physics-based approach highlighting a strong dependency of the drag torque on temperature basing on the lubricant's viscosity. This parameter also influences the drag torque speed dependence, which resulted to be more pronounced for low operating temperatures. An experimental test bench is being constructed to validate the model results.