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| Illustration 1 | g00600883 |
(1) Housing for servo. (2) Actuator piston. (3) Barrel. (4) Housing. (5) Port plate. (6) Head. (7) Input shaft. (8) Passage for the main loop. (9) Passage. (10) Swashplate. (11) Pistons. (12) Spring. (13) Passage for the main loop. (14) Slipper plate. | |
The piston pump (hydrostatic) is a variable displacement piston pump. The piston pump (hydrostatic) in the 228 Skid Steer Loader has a maximum displacement of 35 cc (2.1 in3/rev). The piston pump (hydrostatic) in the 248 Skid Steer Loader has a maximum displacement of 46 cc (2.8 in3/rev). The pumps are driven from the engine flywheel by couplings that are fastened to the splines of the pump input shafts.
The input shaft (7) has external splines. The barrel (3) has internal splines. The shaft connects to the barrel via the splines. There are nine pistons (11) in the barrel. The pistons attach to the slipper plate (14). The slipper plate (14) contacts the swashplate (10). The slipper plate matches the angle of the swashplate. The pistons slide in the barrel as the slipper plate matches the angle of the swashplate. When you operate the engine, the shaft, the barrel, the slipper plate, and the pistons rotate.
The port plate (5) and the swashplate (10) are fastened to housing (4). When the engine is in operation, the port plate (5) and the swashplate (10) do not rotate.
The spring (12) seats against a ring on input shaft (7). The ring on the barrel (3) holds the spring (12) compressed against the ring. The compression of the spring is the force which holds the face of the barrel (3) against the port plate (5) and the head (6) .
The spring (12) keeps a force on the barrel (3) in order to make a high pressure seal between the barrel (3) and the port plate (5). The seal is between the spherical faces of the port plate (5) and the barrel (3). Inlet oil is sealed from the outlet oil by the metal to metal high pressure seal.
If the swashplate angle is at zero, the pistons do not move in and out of the barrel. 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 length of the stroke of the pistons (11) is changed when the swashplate (10) 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.
The position of the swashplate (10) is controlled by the signal oil from the joystick control (hydrostatic) and the actuator piston (2). Signal oil from the speed sensing valve flows to the interlocking manifold valve. The signal oil flows through the interlocking manifold valve to the joystick control (hydrostatic). When the operator moves the joystick control (hydrostatic), the signal oil flows to the actuator piston (2). The servo piston moves the swashplate.
The piston pump will destroke when both conditions occur:
- The engine rpm drop
- The hydraulic oil flows to the piston motors
The pump speed and the drive pressure also affect the control of the swashplate. Higher pump speed and higher drive pressure will destroke the swashplate. Higher signal pressure at the actuator piston (2) will upstroke the swashplate.
The position of the swashplate (10) determines the direction of flow in the two passages of the main loop (8) and (13).
The swashplate (10) is at the maximum position. The slipper plate (14) matches the angle of the swashplate. The pistons (11) match the angle of the slipper plate. The pistons move in and out of the barrel (3). As the pistons move out of the barrel, oil flows behind the pistons. This oil is supplied under pressure from the charge circuit through passage (9) .
Oil that is pushed ahead of pistons (11) goes through the outlet passages of the port plate (5). The oil leaves the pump through the passage for the main loop line (13). The oil flows to the piston motors (hydrostatic). The oil then flows back to the passage for the main loop (8) in the piston pump (hydrostatic).
Charge Relief Valve
The charge relief valve limits the pressure of the charge oil that is not sent 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 is located on the side of the piston pump (hydrostatic) toward the front. The charge relief valve is below the speed sensing valve.
Speed Sensing Valve
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| Illustration 2 | g00665887 |
The speed sensing valve (14) 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.
The Crossover Relief Valves
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| Illustration 3 | g00665890 |
There are four crossover relief valves (15) on the piston pump (hydrostatic). There are two crossover relief valves in each loop of the hydrostatic circuit. The loops of the hydrostatic circuit are closed circuits.
Relief valves limit spikes of high pressure. The maximum pressure of each loop line of the closed circuit is limited to 38200 kPa (5541 psi) by the crossover relief valves. Relief valves are not capable of passing a large flow of oil. The oil from each valve flows into the low pressure side of the hydrostatic system.