In such situations, therefore, it may be advantageous to use a variable-displacement pump, which will only create the amount of flow actually needed to move the actuator at the required speed. Although the pump itself will be more complicated, and therefore more expensive, in most cases it will be a cost-effective solution by virtue of the lower energy consumption and the reduced heat generation.
the right-hand side is fed with pressure from the pump outlet port and tends to push the ring towards the zero-flow position. This, however, is resisted by an adjustable spring on the left-hand side. Therefore, the pump is held at maximum flow until the outlet pressure is high enough to compress the spring, allowing the control piston to start reducing the pump flow rate. The flow rate will then reduce to a level that is sufficient to maintain the outlet pressure at the setting of the spring.
The use of a circular cam ring means that, unlike the fixed-displacement vane pump, the variable-displacement version is no longer balanced from the point of view of internal pressure forces. In practice this limits the maximum pressure capability of the pump, typically up to approximately 210 bar (3000 psi). However, the design does provide a relatively low-cost, low-noise variable-displacement pump that is ideally suited to applications such as machine tools and food machinery.
Variable-displacement piston pumps
A typical variable-displacement axial piston pump is illustrated in Fig. 2.37. The pump works on the same principle as the fixed-displacement version except that the swashplate can now be tilted to different angles, which varies the stroke length of each piston as it rotates, and hence the output flow of the pump. When the swashplate is perpendicular to the drive shaft the flow will be zero, but when tilted to its maximum angle (typically 18–20°) the flow will be at a maximum.
CONTROL PISTON
COMPENSATOR VALVE SPOOL
Fig. 2.37 Variable-displacement axial piston pump (Image courtesy of Eaton Corp.)
Normally a control piston at the top of the pump case is used to tilt the swashplate to the required angle, although in some cases the swashplate can be moved mechanically or electrically. A spring, together with the internal forces acting on the swashplate, tends to move the swashplate to the maximum angle (maximum flow) position so that, when pressurised, the control piston will extend and act to reduce the flow of the pump. The fluid pressure inside the control piston is, in turn, DEFINITION
A variable-displacement pump that reduces its flow at a predetermined pressure is known as a pressure- compensated pump.
regulated by an adjustable pressure-compensator valve mounted on the rear of the pump.
When the pump outlet pressure is less than the setting of the spring in the compensator valve, the spool remains pushed upwards to connect the fluid in the control piston to tank via the pump case. The swashplate then moves to the maximum angle and the pump provides maximum flow. When the outlet pressure rises to the spring setting, however, the spool is pushed down, and fluid under high pressure enters the control piston to start to de-stroke the pump.
As with the variable-displacement vane pump, the pump flow will reduce to a level where the outlet pressure is maintained at the setting of the spring in the compensator valve. If the pump outlet is blocked (e.g. by a closed port directional valve), the pump flow reduces to virtually zero – just sufficient to maintain leakage in the system. If the pump outlet is restricted (but not blocked completely), the pump produces whatever flow the restriction can pass at the pressure setting of the compensator valve.
The same general principle applies to variable-displacement bent-axis piston pumps (Fig 2.38). In this case the angle of the cylinder block can again be varied to change the piston stroke and hence the pump flow. However, the control piston this time is mounted on the rear of the pump and tilts the cylinder block by means of a link rod.
MAXIMUM-DISPLACEMENT ADJUSTMENT
Fig. 2.38 Variable-displacement bent-axis piston pump
With variable-displacement pumps there is normally an option to limit the maximum flow of the pump by means of fixed or adjustable stops, which prevent the cam ring, swashplate or cylinder block moving to its maximum flow position. So, by using a variable-displacement pump in conjunction with a flow-control valve, the system is now more efficient in that the pump will only produce the required flow rate as set by the flow control, and no flow will be wasted over the relief valve (Fig. 2.39).
WARNING A pump relief valve is often required in conjunction with a pressure-compensated variable-displacement pump in order to relieve peak pressures and also to provide protection should the control piston or its control valve jam. However, the pressure-relief valve should normally be set at least 20 bar (300psi) higher than the compensator setting.