An Investigation of Hydraulic Fluid Composition and Aeration in an Axial Piston Pump

Manufacturers of fluid power equipment have decreased the size of hydraulic fluid reservoirs in response to economic, environmental and performance requirements. Residence times as brief as 30 seconds in mobile equipment are not unusual. Shorter fluid residence times dictate use of hydraulic fluids with improved air release characteristics. In this investigation, hydraulic fluids of the same ISO viscosity grade but varying base oil and additive composition were evaluated in a dynamometer fitted with a reservoir that incorporated an aerator at the inlet, and a mass flow meter at the outlet. The effects of aeration on piston pump efficiency and air borne noise generation were evaluated. Fluids of the same ISO viscosity grade exhibited significantly different air release rates and as a result sustained different volume fractions of entrained air. Hydraulic oils that entrained a greater volume of air demonstrated lower volumetric efficiencies and higher sound levels. Aerated fluids of the identical viscosity grade differed in volumetric efficiency by as much as 8% and perceived sound level by as much as 50%. Models for the effect of aeration on pump performance are presented.