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| Illustration 1 | g01279793 |
Pump actuator (1) Swashplate (2) Outer spring (3) Control piston for the swing pump (4) Balls (5) Belleville springs (6) Pin | |
The main pump output varies from maximum output to minimum output according to the load on the machine. The angle of the swashplate determines the actual output of the main pump.
Swashplate at Maximum Angle
When the machine is not operating, the swashplate is maintained at a maximum angle by spring (2). During machine operation, the swashplate angle is varied according to the delivery pressure of the pumps and the load on the machine.
When the load on the machine is light, the delivery pressure from the right pump and the left pump is low and spring (2) is not compressed by the force of pump's pistons. Therefore, the swashplate maintains the maximum angle, and the pump output is maintained at the maximum flow.
As the load on the machine increases, discharge pressure from the main pump will resist the movement of the pistons. If the pistons are not able to move up the incline of the swashplate toward the port plate, the angle of swashplate (1) will be reduced by compressing spring (2). Swashplate (1) pivots on two balls (4). When the total force of the delivery pressure against the pistons exceed the force of the spring, the angle of swashplate (1) changes in order to destroke the pump.
Swashplate at Minimum Angle
When the swing pump is used, the swing pump's discharge pressure will act on control piston (3). The control piston will act on swashplate (1). The force of control piston (3) adds to the force of the main pump in order to destroke the pump.
As the load on the machine increases, the discharge pressure increases. The force that tries to reduce the swashplate angle also increases, and spring (2) begins to be compressed. If the discharge pressure increases further, belleville springs (5) will also begin to be compressed. When spring (2) and spring (5) are completely compressed, the swashplate will be at the minimum angle, and the pump will output the minimum flow.
The swashplate is held at a position that balances the output of the right pump, the left pump, and the swing pump. Therefore, the hydraulic horsepower is constant regardless of the load on the machine.
Constant Horsepower
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| Illustration 2 | g00833197 |
After the main pump reaches the destroke point, pump output and hydraulic horsepower are nearly constant. (1) Destroke point (2) P-Q characteristic curve | |
The curve in Illustration 2 indicates the relationship between pump flow and delivery pressure. The p-q characteristic curve is established under the following conditions: The right pump delivery pressure (P1) is added to the left pump delivery pressure (P2) and to the swing pump's delivery pressure (P3).
The swashplate angle in the main pump is changed by the combined delivery pressure of the right pump, the left pump, and the swing pump. As the total delivery pressure increases, output flow (Q) begins to decrease from point (1). Below point (1), pump output is nearly constant at any given point in order to maintain a constant hydraulic horsepower.
The break in the curve at point (2) occurs when the swashplate contacts pin (6). Additional discharge pressure will begin to compress belleville springs (5) and spring (2). Belleville springs add additional spring force. The additional force causes the p-q curve to flatten.