Abstract: The displacement of a hydraulic pump is controlled by adjusting the working volume of the pump chamber. Taking the CY-type piston pump as an example, this is achieved by changing the angle γ between the swash plate's normal and the cylinder's rotational axis. When γ is at its maximum, the plunger chamber has the largest working volume, resulting in full flow output. Conversely, when γ is zero, the working volume becomes zero, and the pump stops delivering oil. If γ is negative, the pump operates in reverse. There are various methods to control the γ angle, each with unique characteristics. This paper explores two common approaches.
The first method uses a sequence valve and a directional control valve to regulate the return spring force on the variable plunger (see Figure 1). In the initial state, the swash plate is set at its maximum angle γ due to the return spring’s force, ensuring full displacement. When the system pressure exceeds the sequence valve’s set point, the valve opens, and the directional control valve shifts, allowing pressurized oil to enter the plunger cylinder. This forces the variable plunger to move against the spring, reducing the swash plate’s angle and decreasing the pump’s displacement. As pressure continues to rise, the swash plate angle decreases further, leading to near-zero flow—essentially unloading the pump under pressure (since power N = p × Q, and N approaches zero when Q does). Once the system pressure drops, the plunger returns to its original position. The directional valve enhances response speed during high-demand conditions. This control method is known as constant pressure variable control, with characteristic curves shown in Figure 2. The curve shows a full-displacement region (AB), a variable region (BC), and an unloading point (C).
The second method involves direct pressure oil acting on a multi-stage spring-loaded piston cylinder (see Figure 3). Instead of using a sequence valve, the system pressure directly acts on the variable plunger. The spring is replaced with multiple springs in series, each with different stiffness levels. This design allows for a non-linear relationship between displacement and pressure, approximating the function Q = 1/p × N. By carefully selecting spring stiffness and limit blocks, the displacement-pressure curve can closely resemble a constant power curve, also known as a pressure compensation curve. This ensures that when pressure increases, displacement decreases proportionally, maintaining efficient operation. The system can also unload under pressure, making it ideal for applications like baling machines, where energy efficiency is critical. This control mechanism improves overall system performance and reduces the required motor capacity.
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