Fixed Displacement Pump
A fixed displacement pump sweeps the same volume every revolution, and its flow rate changes only with drive speed. Because it keeps producing flow even when no work is being done, it is used together with open center valves. Gear pumps are the most common example of this class.
This pump's behavior is as predictable as it is wasteful. Since flow cannot be cut off, the unused oil has to go somewhere: in an open center valve it returns to tank, while in a closed center system it opens the relief valve and converts all that energy into heat. The second case raises oil temperature quickly and stresses the system.
- Flow calculation: displacement (cm³/rev) × speed (rpm) ÷ 1000 = theoretical l/min.
- Idle effect: when engine speed drops, flow drops proportionally and motion slows. This is not a fault.
- Speed control: the only way is to throttle the orifice at the valve; the throttled energy turns into heat.
When you convert a manual lever to electronic control, this relationship does not change. The command you send remotely sets the lever opening, the opening sets the orifice, and the orifice sets the speed. If you have a setup that also affects engine speed at the same time (a throttle/rpm kit, for example), account for the fact that speed is influenced by two variables at once.
Where this term is used: What Is a Hydraulic Lever Retrofit Kit? Definition, Components, Operating Principle