Variable Displacement Pump
A variable displacement pump can change the volume it sweeps per revolution through a regulator, according to demand. When no work is being done it pulls its displacement to zero and delivers only enough oil to cover leakage. It is the core component of closed center and load sensing circuits.
The regulator tracks two quantities: system pressure and the signal coming from the load sensing line. When pressure reaches the set limit the pump destrokes itself (pressure compensation); in the load sensing type it holds the difference between pump outlet and the pressure of the heaviest working section typically constant at 15-25 bar. As a result, the flow delivered matches how far the lever is opened, and no more is produced.
- Gain: almost no heat buildup at idle; fuel consumption and oil temperature drop noticeably.
- Cost: more complex, more expensive, and more sensitive to oil cleanliness.
- Symptom: pump noise not changing at idle and the tank not heating up point to this type.
In these systems the relationship between lever travel and speed is more linear than in a fixed displacement system, because flow does not stay constant while an orifice closes down; demand is met directly. This is why teaching the endpoints of the calibration properly makes a more noticeable difference in electronic lever control.
Where this term is used: What Is a Hydraulic Lever Retrofit Kit? Definition, Components, Operating Principle