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| Illustration 1 | g00396177 |
(1) Housing for servo (2) Servo piston (3) Barrel (4) Housing (5) Solenoid control valve (6) Port plate (7) Head (8) Input Shaft (9) Passage for the main loop (10) Passage (11) Swashplate (12) Piston (13) Spring (14) Passage for the main loop | |
The 908 Compact Wheel Loader is equipped with a variable displacement piston pump with a maximum displacement of 71 cc (4.33 in3). The pump is driven from the engine flywheel by a coupling that is fastened to the splines of the pump input shaft.
When the engine is running, the shaft (8) and the barrel (3) are rotating. There are nine pistons (12) in the barrel assembly. The port plate (6) and the swashplate (11) are fastened to the housing (4). The port plate and the swashplate (11) do not rotate. The spring (13) keeps a force on the barrel (3) in order to make a high pressure seal between the barrel and the port plate. When the barrel is rotating, each piston follows the angle of the swashplate.
If the swashplate angle is at zero, the pistons do not move in and out of the barrel and there is no oil flow. Charge oil from the charge pump maintains oil pressure in the pump in order to keep the barrel full of oil. Charge oil also lubricates the pump components and charge oil makes up for the normal, internal loss of oil that is due to leakage.
The position of the swashplate (11) is controlled by the solenoid control valve (5) and the servo piston (2). The solenoid valve receives an electrical signal from the direction control switch. The solenoid valve directs oil to one side of the servo piston. Signal oil from the speed sensing valve moves the servo piston. The servo piston then changes the angle of the swashplate.
Oil flows from the piston pump to the piston motor and back to the pump through the main loop. The position of the swashplate (11) determines the direction of flow in the two passages of the main loop (9) and (14) .
As the pistons (12) follow the swashplate the pistons move in and out of the barrel (3). As pistons move out of the cylinder, oil is pushed into the cylinder behind the head of the piston. This oil is supplied under pressure from the charge circuit through passage (10) .
Oil that is pushed ahead of pistons (12) goes through the outlet passage of the port plate (6). The oil leaves the pump through the passage for the loop line (14). Inlet oil is sealed from the outlet oil by a seal that is metal to metal. The seal is between the spherical faces of the port plate (6) and the barrel (3) .
The spring (13) seats against a ring on shaft (8). The ring on the barrel (3) holds the spring compressed against the ring. The compression of the spring is the force which holds the face of the barrel against the port plate (6) and the head (7) .
The length of the stroke of the pistons (12) is changed when the swashplate (11) is turned on the axis. At the neutral position, the stroke of the piston and the delivery of the oil is zero. At maximum inclination of 15°, piston stroke is at the maximum.
Charge Pump
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| Illustration 2 | g00640834 |
Charge pump | |
The charge pump is a gerotor pump that is located at the rear of the piston pump. The charge pump provides oil for the hydrostatic system at start-up. Charge oil is also makeup oil that is used for lubrication and cooling of the pump and the motor.
The charge oil is used by the speed sensing valve to provide signal oil to the pump and the motor controls. Charge oil that is not used by the hydrostatic system flows into the pilot system.
Charge Relief Valve
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| Illustration 3 | g00641253 |
Location of the charge relief valve | |
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| Illustration 4 | g00640835 |
Charge relief valve | |
The charge relief valve limits the pressure of the charge oil that does not flow through the speed sensing valve. The charge relief valve controls the pressure in the low pressure side of the hydrostatic loop. The charge relief valve also controls the maximum pressure for the pilot system. The charge relief valve is set at 3200 kPa (460 psi).
Speed Sensing Valve
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| Illustration 5 | g00641253 |
Location of the speed sensing valve | |
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| Illustration 6 | g00640836 |
Speed sensing valve | |
The speed sensing valve detects the change in the oil flow of the charge pump as the engine speed changes. The orifice in the speed sensing valve creates a pressure differential which is a proportional amount to the engine speed. This signal oil is used to control the pump and the motor.
The signal oil from the speed sensing valve flows to the Forward/Neutral/Reverse valve on the pump and to the displacement control valve on the motor. All oil that is not used by the speed sensing valve flows to the remainder of the charge oil circuit and flows to the pilot system.
An orifice in the passage after the speed sensing valve allows the master cylinder valve to function correctly. The orifice also allows the POR valve to function correctly. If the orifice is not installed, the speed sensing valve will compensate for the loss of the flow of signal oil through the master cylinder. The loss of flow would prevent the pump from destroking and the motor from upstroking in order to help the braking of the machine. If the orifice is not installed, the POR valve will not correctly limit the maximum pressure of the drive loop.
Crossover Relief Valves
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| Illustration 7 | g00640837 |
Crossover relief valves | |
There are two crossover relief valves. There is one crossover relief valve for each side of the hydrostatic circuit. The crossover relief valves limit spikes of high pressure. The relief valves are not capable of passing a large amount of oil. The maximum pressure for each direction of the closed circuit is limited to 48200 kPa (6990 psi) by the crossover relief valves. The maximum pressure is determined by the combination of the spring in the crossover relief valve and the amount of charge pressure. The oil from each valve flows into the charge oil circuit.
There is also a makeup valve in each relief valve. The makeup valve allows the charge oil to flow to the low pressure side of the hydrostatic circuit. This makeup oil replenishes the oil that is lost to lubrication and cooling in the case drain circuit.
Also, the crossover relief valve can function as a towing valve. Both crossover relief valves can be adjusted in order to hold the relief valve open. This allows the free flow of oil through the relief valves. The machine may be towed for short distances. Refer to Operation and Maintenance Manual, "Towing the Machine".
POR Valve
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| Illustration 8 | g00640839 |
Pressure override valve | |
The POR valve limits the maximum system pressure for both sides of the hydrostatic loop. The relief pressure of the POR valve is set lower than the maximum pressure of the crossover relief valves. The POR valve is set at 45000 kPa (6526 psi). The POR valve prevents an excessive oil flow through the crossover relief valves in the pump.
A shuttle valve ball allows oil to flow from the high pressure side of the loop to the POR valve. When the POR valve is opened, signal oil is drained to the hydraulic tank. As this signal oil is drained, the pump will destroke. When the pump is destroked and the motor is upstroked, the machine speed is reduced.