Abstract: The displacement of a hydraulic pump is controlled by adjusting the working volume of the piston chamber. Taking the CY-type axial piston pump as an example, this is achieved by changing the angle γ between the swashplate’s normal and the cylinder axis. When γ is at its maximum, the pump delivers full displacement; when γ is zero, no oil is pumped. If γ becomes negative, the pump reverses flow. There are various methods to adjust γ, each with unique control characteristics. This paper discusses two common control mechanisms.
The first method uses a sequence valve and a directional control valve to regulate the return spring force in the plunger cylinder (as shown in Figure 1). In this setup, the swashplate is initially set at the maximum angle due to the return spring. When system pressure exceeds the sequence valve’s set point, the valve opens, allowing pressurized oil to enter the plunger cylinder. This overcomes the spring force, reducing the swashplate angle and thus the pump’s displacement. As pressure increases further, the swashplate angle decreases until it approaches zero, causing the pump to unload and stop delivering oil. When pressure drops, the plunger returns to its original position. The directional valve improves response time during high-demand operations. This is a constant-pressure variable control system, as illustrated in Figure 2, where AB represents full displacement, BC is the variable range, and C marks the unloading point.
The second method directly applies system pressure to a multi-stage spring-loaded plunger cylinder (Figure 3). Unlike the first method, it eliminates the sequence valve and directional valve. Instead, multiple springs are used in series, with varying stiffness levels. Each spring is designed to engage sequentially as pressure increases. The first spring has the lowest stiffness, followed by the second and third. Limit blocks prevent over-compression. As pressure rises, the first spring deforms first, then the second, and finally the third. This creates a stepped change in displacement, resulting in a nearly reciprocal relationship between flow and pressure. This configuration produces a constant power curve, also known as a pressure-compensated curve, ensuring efficient energy use. It is ideal for systems like balers, where power utilization must be optimized. The design allows the pump to unload under pressure, reducing the prime mover’s required capacity while maintaining performance.
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