You adjust a variable displacement pump by varying the stroke of its internal pistons, typically achieved by tilting the swash plate.
Here's a more detailed explanation:
Variable displacement pumps control flow output by changing the amount of fluid displaced with each stroke of their internal pistons. Unlike fixed displacement pumps which deliver a constant volume per revolution, variable displacement pumps can adjust their output to meet varying system demands, improving efficiency and reducing energy consumption.
The adjustment mechanism primarily focuses on altering the piston stroke. Here's how it works, focusing on the common swash plate design:
- Piston Arrangement: These pumps usually have multiple pistons arranged in a circular pattern around a central shaft.
- Swash Plate: The pistons are connected to a swash plate (also known as an wobble plate or cam plate) which is set at an angle. As the shaft rotates, the pistons move back and forth following the angle of the swash plate. The angle of the swash plate determines the distance the pistons travel (the stroke length).
- Adjusting Displacement:
- Tilting the Swash Plate: The key to adjusting the pump's displacement is changing the angle of the swash plate.
- Increasing the Angle: A larger angle results in a longer piston stroke. This means each piston displaces more fluid per stroke, increasing the pump's overall output flow.
- Decreasing the Angle: Conversely, a smaller angle results in a shorter stroke, reducing flow. When the swash plate is at zero degrees (perpendicular to the piston axis), the pistons don't move, and the pump delivers no flow (zero displacement).
Methods for Tilting the Swash Plate:
The mechanism for tilting the swash plate varies depending on the pump's design and application, but some common methods include:
- Manual Adjustment: Some pumps have a manual lever or screw that allows you to directly adjust the swash plate angle.
- Hydraulic Control: A common method uses a hydraulic cylinder or servo valve to position the swash plate. The position of the swash plate is controlled by hydraulic pressure directed to one or both sides of the cylinder.
- Electro-Hydraulic Control: Similar to hydraulic control, but with the servo valve controlled electronically. This allows for precise control and integration with control systems.
- Pressure Compensation: Some pumps are equipped with a pressure compensator. This automatically adjusts the swash plate angle to maintain a desired system pressure. As pressure increases, the compensator reduces the swash plate angle (decreasing flow) to prevent over-pressurization.
Example:
Imagine a hydraulic system operating a large cylinder. When the cylinder needs to move quickly, the variable displacement pump increases its swash plate angle, delivering a larger flow of fluid. When the cylinder reaches its desired position and only needs to maintain pressure, the pump reduces its swash plate angle, reducing flow and saving energy.
In summary, adjusting a variable displacement pump involves changing the angle of the swash plate, which in turn modifies the piston stroke length and the pump's flow output. The method for adjusting the swash plate angle can be manual, hydraulic, electro-hydraulic, or pressure-compensated, depending on the specific pump design and application.
