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| Illustration 1 | g00396177 |
(1) Housing for actuator (2) Actuator 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 pump for the 914G Wheel Loader and the IT14G Integrated Toolcarrier is a variable displacement piston pump with a maximum displacement of 90 cc (5.5 cu in/rev). The pump is driven from the engine flywheel by a coupling that is fastened to the spline of the pump input shaft.
When the engine is running, the shaft (8) and the barrel assembly (3) are rotating. There are nine pistons (12) in the barrel assembly (3). The port plate (6) and the swashplate (11) are fastened to housing (4). The port plate (6) and the swashplate (11) do not rotate. The spring (13) keeps a force on the barrel assembly (3) in order to make a high pressure seal between the barrel assembly (3) and the port plate (6). When the barrel assembly (3) is rotating, each piston (12) 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 from the charge pump lubricates the pump components. Charge oil from the charge pump 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 actuator piston (2). The solenoid control valve (5) receives an electrical signal from the electronic control module when the direction control lever or the direction control switch is moved to the forward or the reverse direction. The solenoid control valve (5) directs signal oil to one side of the actuator piston (2). Signal oil from the speed sensing valve moves actuator piston (2). The actuator piston (2) controls the angle of the swashplate.
Oil flows from the pump to the drive motor and back to the pump passage (9). The position of the swashplate (11) determines the direction of flow in the two passages of the main loop (9) and (14) .
The swashplate (11) upstrokes. As pistons (12) follow the swashplate, the pistons move in and out of the barrel (3). As pistons (12) move out of the cylinder, oil is supplied behind the pistons. 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 passages 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 (13) compressed against the ring. The compression of the spring is the force which holds the face of the barrel (3) 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.
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| Illustration 2 | g00815353 |
Schematic of the Hydrostatic Piston Pump (11) Swashplate (15) Pressure override valve (16) Resolver (17) Charge relief valve (18) Crossover relief valve (19) Speed sensing valve (20) Hydraulic oil filter (21) Pressure switch (22) Forward/Neutral/Reverse valve (23) Charge pump (24) Hydrostatic pump (25) Hydrostatic pump group (26) Line to the Motor signal control valve | |
Charge Pump
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| Illustration 3 | g00808203 |
The charge pump (23) is a gerotor pump that is located at the rear of the hydrostatic pump. The charge pump (23) 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 (19) to provide signal oil to the pump and the motor controls. The charge oil that is not used by the hydrostatic system flows into the implement pilot system and the line to the motor signal control valve (26) .
Charge Relief Valve
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| Illustration 4 | g00808207 |
The charge relief valve (17) limits the pressure of the charge oil that is not sent through the speed sensing valve (19). The charge relief valve (17) also controls the maximum pressure for the implement pilot system.
Speed Sensing Valve
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| Illustration 5 | g00808216 |
The speed sensing valve (19) detects the change in the oil flow of the charge pump (23) as the engine speed changes. The orifice in the speed sensing valve (19) creates a pressure differential which is proportional to the engine speed. The pressure differential is multiplied by the speed sensing valve (19), which creates the signal oil. The signal oil is used to control the pump and the small motor.
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| Illustration 6 | g00808223 |
Charging of the Speed Sensing Valve (A) Oil flows to the Hystat Motor and to the Brakes. (B) Oil flows to the Forward/Neutral/Reverse Valve. (C) Oil flows to the Crossover Relief Valves. (D) Oil flows from the Charge Pump. | |
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| Illustration 7 | g00778291 |
Neutral or Accelerating of the Speed Sensing Valve (A) Oil flows to the Hystat Motor and to the Brakes. (B) Oil flows to the Forward/Neutral/Reverse Valve. (C) Oil flows to the Crossover Relief Valves. (D) Oil flows from the Charge Pump. | |
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| Illustration 8 | g00778297 |
Decelerating of the Speed Sensing Valve (A) Oil flows to the Hystat Motor and to the Brakes. (B) Oil flows to the Forward/Neutral/Reverse Valve. (C) Oil flows to the Crossover Relief Valves. (D) Oil flows from the Charge Pump. | |
The position of the speed sensing valve varies depending on the following conditions of the machine:
- Charging
- Neutral or Accelerating
- Decelerating
The signal oil from the speed sensing valve (19) flows to the Forward/Neutral/Reverse valve (22) on the pump and to the displacement control valve on the small motor and brakes. All oil that is not used by the speed sensing valve (19) flows to the following components: the remainder of the charge oil circuit, the implement pilot system and the line to the motor signal control valve (26) .
An orifice in the passage after the speed sensing valve (19) allows the hydrostatic brake valve to function correctly. The orifice also allows the pressure override valve (POR valve) (15) to function correctly. If the orifice is not installed, the speed sensing valve (19) will compensate for the loss of the flow of signal oil through the hydrostatic brake. 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 (15) will not correctly limit the maximum pressure of the drive loop.
Crossover Relief Valves
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| Illustration 9 | g00808236 |
There are two crossover relief valves (18). There is one in each loop of the hydrostatic circuit. Crossover relief valves (18) limit the spikes of high pressure and the crossover relief valves are not capable of passing a large flow of oil. The maximum pressure of each loop line of the closed circuit is limited to 45500 kPa (6600 psi) by the crossover relief valves (18). 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.
The crossover relief valve (18) can function as a towing valve. Both crossover relief valves (18) can be adjusted in order to hold the relief valve open. This allows the loops of the low pressure and the high pressure to be connected together. The machine may be towed for short distances. For the 914G Wheel Loader, refer to Operation and Maintenance Manual, SEBU6868, "Towing the Machine". For the IT14G Wheel Loader, refer to Operation and Maintenance Manual, SEBU6869, "Towing the Machine".
Pressure override valve (POR valve)
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| Illustration 10 | g00808266 |
The POR valve (15) limits the maximum system pressure for both sides of the hydrostatic loop. The relief pressure of the POR valve (15) is set lower than the crossover relief valves. The POR valve (15) is set at 43000 ± 1000 kPa (6240 ± 145 psi). The POR valve (15) prevents an excessive oil flow through the crossover relief valves in the pump.
Resolver (16) sends oil from the loop with the highest pressure to the POR valve (15). When the POR valve (15) 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 motors are upstroked, the machine speed is reduced. The ECM will upstroke the large motor due to the slower drive shaft speed.