GENERAL INFORMATION 2.3 Variable displacement piston pump Clockwise input shaft rotation 2 Case drain High pressure Charge pressure inlet Fixed displacement drive motor Case drain C4447 Brake Line The drive shaft of the piston pump is rotated by the engine. The piston block which is splined to the drive shaft also turns. The piston block, rotating group, consists of 9 piston assemblies which have free swiveling shoes swagged on the ball end of each piston assembly. The shoe end of the piston rides against the smooth machined surface of the swashplate. With the swashplate in the neu- tral position, the piston assemblies do not reciprocate in the piston block, but are rotating.
No oil is drawn into or discharged from the pump. The pump is in a zero dis- placement position and the loader remains stationary. With the swashplate in the neutral position the pressure of the charge oil, which ranges from 200 to 280 psi (13.8 - 19.3 bar), is able to unseat both check valves and supply oil to both sides of the pump because of the balance in pressure. Very little charge oil volume is required in the neutral position so the excess oil is bypassed over the charge pressure relief valve and recirculated back to the reservoir.
The oil that leaks internally in the pump and motor collects in their body housings and is returned to the reservoir by external case drain in the pump and motors. This leakage oil is the only oil the charge check replenishes. This makes the design a closed loop system. As the steering lever is moved forward, or reverse, the loader starts a directional movement. As the swashplate begins to move, the piston assemblies start to reciprocate in the piston block. As the steering lever continues further movement the cam angle increases, the pistons recipro- cate further, more oil is pumped and the speed of the loader is increased.
When the swashplate begins to move the check valve on the discharge, or pressure, side seats because of the high- er pressure differential. The other check valve remains open on the intake or low pressure side to continue sup- plying the closed loop system with charge oil. The drive motor, which is a fixed displacement type, delivers a constant output torque for a given pressure throughout the speed range of the motor. The movement of the pump swashplate, forward or reverse, controls the direction of the drive motor rotation.
The function of the pressure relief valve is to relieve the pressure side of the system of excessive high pressure when the loader encounters a heavy load or stalls out. When the relief valve senses an over load it unseats, allowing excess pressure and volume to flow into the low pressure side of the pump. A small volume of oil starts to flow across to the other relief valve. This relief valve is exposed to the low pressure on the intake side of the pump and is seated by the spring tension within the relief valve body. The small volume of oil being bypassed is enough to unseat the relief valve and let it recirculate back into the inlet side of the pump.
As the pressure continues to build on the pressure side, a larger volume of oil flows and at a greater speed through the drilled orifice in the relief valve cartridge, causing a pressure drop inside the relief valve. The surrounding pressure is now able to unseat the relief valve and bypass maximum volume of oil. The system reliefs function the same for both sides of the system. 2-5