This article describes a model used to predict the friction in a ball joint of a hydraulic motor operating under unique conditions. The ball joint transfers very high loads and rotates at very low speeds (including zero speed twice per motor revolution), which means that it operates in the mixed and boundary lubrication regimes. The model includes the influence of a squeeze film. It also shows the dependence of friction on the various surface qualities and is used to predict overall motor performance. The model was validated using hydraulic motors with various surface finishes on the ball joints. Measurements show that the model is a good predictor of mechanical performance and that the key variables highlighted by the model do lead to improved mechanical efficiencies. [PUBLICATION ABSTRACT]
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