This paper presents an investigation of the tribological films formed in hydraulic motors. Hydraulic motors convert the fluid power energy produced by positive displacement pumps into rotary motion. Earlier research found that the efficiency of this energy transformation can be enhanced by reducing boundary friction. In order to study the nature of the boundary films formed in an orbital motor, a prototype ashless hydraulic fluid was evaluated in a low-speed high-torque dynamometer. The resulting tribofilm was probed via Energy Dispersive X-Ray Spectroscopy. The results reveal that increasing the hydraulic system temperature raised the relative phosphorus level of the tribofilm.

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