When one of your hydraulic machines loses actuator speed as the oil warms, you need enough evidence to decide whether or not removing the hydraulic gear pump is justified. However, a gauge reaching its relief pressure simply describes the resistance being overcome; it does not quantify the flow available at that pressure. For the full picture, add flow and temperature to the pressure reading, then track delivery across controlled load points. For a fixed-displacement pump, the following six checks will show you whether the loss follows the pattern of internal wear or whether your diagnostic efforts should be directed elsewhere.
Check 1: Confirm what the pump should deliver at the test speed
The theoretical flow is:
Qth = (Vg × n) / 1000
Here, Vg is displacement in cm³/rev and n is speed in rev/min, so a 25 cm³/rev pump at 1,500 rev/min should theoretically deliver 37.5 l/min. If its drive falls to 1,350 rev/min under load, the expected figure becomes 33.75 l/min before leakage is considered. Record speed at each load point so that any ‘drive droop’ is accounted for. For a variable-displacement unit, you’ll also need to confirm its commanded displacement and compensator state.
Check 2: Test at the temperature where the weakness appears
Oil viscosity naturally falls as temperature rises, allowing more leakage through worn internal hydraulic parts. If your machine loses output after 40 minutes, reproduce that thermal condition before testing; bring the circuit to its normal fault temperature, allow the reading to stabilise and record temperature alongside every pressure-and-flow result. You can only compare readings meaningfully when the fluid grade and temperature are consistent.
Check 3: Use low-load flow to establish your baseline
Install a suitably ranged inline tester in accordance with the machine and tester instructions, then record delivery at a fixed speed with the loading valve fully open. At low pressure, the differential driving oil across internal leakage paths will be small, including where wear has increased the clearances. Use this flow as the baseline from which to assess the loaded readings.
Check 4: See how much flow remains as pressure rises
Increase the load progressively, staying within the ratings of the complete test circuit, and record flow at both an intermediate pressure and at the machine’s normal working pressure. Suppose our example hydraulic gear pump delivers 35.8 l/min at 50 bar and 30.6 l/min at 200 bar, with speed and temperature unchanged. The 5.2 l/min fall shows you how much additional delivery is being lost as the internal leakage paths are exposed to higher differential pressure.
Check 5: Rule out conditions that can imitate pump wear
To rule out other hydraulic parts faults that can imitate pump failure, measure the inlet vacuum or charge pressure at the pump and then check the pressure downstream of the tester. A collapsed suction hose, blocked reservoir breather or inadequate charge supply can prevent the pumping chambers from filling properly, and undersized test hoses and couplings can also alter the operating conditions you are trying to reproduce. These influences need correcting before the flow loss is assigned to the rotating group.
Check 6: Make the decision against the correct benchmark
At 200 bar, the example pump’s volumetric efficiency against theoretical delivery is:
ηv = (Qactual / Qtheoretical) × 100 = (30.6 / 37.5) × 100 = 81.6%
There is no universal rejection percentage for every pump design, so compare the result with the manufacturer’s performance curve or a known-good unit tested at the same speed, pressure, fluid and temperature, allowing for instrument uncertainty. A result within that benchmark will support leaving the hydraulic gear pump in place and continuing the investigation elsewhere in the circuit.
What next?
Hydrastar supplies a range of Webtec portable hydraulic testers and diagnostic tools to improve your fault-finding processes. Speak to one of our engineers today to find out more.
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