Determining the viscous splash losses in the housing of a hydraulic motor through CFD-simulations : A master thesis in collaboration with Bosch-Rexroth in Mellansel AB

Abstract: One possible way of solving future energy shortages is by the optimization of our current energy consumption. These optimizations must span all possible fields of consumption. In the mechanical field radial piston hydraulic motors may show some margin of improvement. The radial piston hydraulic motor is driven by a pressure difference in hydraulic oil. These motors are commonly found in heavy industrial equipments such as drills and conveyor belts. The advantage with these motors in comparison with electric motors is the high torque and ability to absorb shock loads that may cause damage to electrical motors. The effectiveness of these motors are determined both by the motor and by the drive system as a whole consisting of hydraulic pump driven by a electric motor, hydraulic hoses, motor and possible external coolers. If the effectiveness of the motor is low the whole drive system will be affected thus amplifying the total losses. The losses in the motor can be both mechanical and derived to the viscosity of the oil. One region in the motor where there are viscous losses are in the housing. The housing is filled with oil, that both aids in the cooling and acts as a lubricant for the motor. Pistons and rollers are some of the components found in the housing. These components rotates around the centre line axis while having a pulsating radial motion following a cam ring. This rotating and pulsating motion will push oil in and out of a volume between two consecutive pistons and rollers. This will create viscous losses and regions with a enhanced risk of cavitation. This study investigates if the flow of oil in the housing can be simulated accurately. The study also examine what are the main problems regarding the flow of oil in the housing and the factors affecting the size of the viscous losses. The study also examines the correlation between viscosity and viscous losses. Finally two different optimizations with the intention of decreasing the viscous losses are compared. The study found that the majority of the viscous losses in the housing can be derived to the flow of hydraulic oil in and out of the volume between two consecutive pistons and rollers. The oil will pass a sharp edge around the cylinder block and a narrow passage under the spacing between the cylinder rows in a two cam ring configured motor. This will create regions with a enhanced velocity and risk of cavitation. The stroke of the motor will greatly affect the effectiveness of the motor especially at a high rotational speed. The viscous losses will be transformed into internal energy, heat, thus increasing the temperature of the oil. A increased temperature will decrease the viscosity and the viscous losses. The viscous losses will vary with 17 % if the viscosity is varied between 20 and 100 cSt. The developed model is not sufficient to determine the viscous losses accurately since the geometry had to be considerably simplified, but can act as a way of comparing different optimizations of the motor. The viscous losses can be decreased with 25 % in the CCe motor at 150 rpm by milling material of the cylinder block between the piston holes. This is an expensive optimization and needs to be justified from a cost-benefit perspective.