A test report is the only document that separates a rebuilt pump from a repainted one. Yet many buyers accept a report containing a single pressure figure and a note saying "tested OK", which proves almost nothing about the pump's ability to hold flow under load.
This article explains what a proper bench measures, how the numbers are calculated, which acceptance criteria are meaningful, and how to run a credible check on a machine that never sees a test stand.
What a Bench Measures, and Why Each Value Matters
A hydraulic pump test bench applies load at controlled speed and temperature. Five measurements define the result.
| Measurement | Instrument | Why it matters |
|---|
| Outlet pressure | Calibrated gauge or transducer at the pump port | Establishes the load point; a flow figure without a pressure figure is meaningless |
| Flow | Turbine or gear flow meter downstream | The actual delivered volume, from which efficiency is calculated |
| Shaft speed | Tachometer or drive encoder | Flow scales with speed; without it, comparison is impossible |
| Oil temperature | Sensor in the reservoir or return line | Viscosity changes flow; tests are only comparable at the same temperature |
| Case drain flow | Metered drain return | The direct indicator of internal leakage |
A report missing any of these five is incomplete, regardless of how many pass stamps it carries. Temperature in particular is the most commonly omitted and the most consequential: a pump tested at 30 °C will show better efficiency than the same pump at 60 °C, sometimes by several percentage points.
The Test Sequence
Order matters because the first readings establish a baseline that later readings are judged against.
| Step | Condition | Purpose |
|---|
| Run-in | Low pressure, moderate speed, 20–30 minutes | Beds the new faces without thermal shock |
| Standby | Zero load, rated speed, warm oil | Baseline flow and case drain |
| Stepped load | 25%, 50%, 75%, 100% of rated pressure | Reveals leakage trend, not just a single point |
| Rated pressure hold | Rated pressure, rated speed, stabilised temperature | The headline efficiency figure |
| Peak check | Brief peak pressure, seconds only | Confirms relief and control function |
| Control response | Command displacement changes | Verifies compensator or load-sense behaviour |
The stepped-load row is the one buyers should insist on. A pump can deliver 96 percent efficiency at low pressure and 78 percent at rated pressure; a single-point test at low pressure hides the failure completely.
Calculating Volumetric Efficiency
Theoretical flow equals displacement multiplied by speed, divided by 1000 for litres per minute. Volumetric efficiency is measured flow divided by theoretical flow, expressed as a percentage.
Worked example: a pump with 71 cm³/rev displacement, driven at 2,000 rpm, has a theoretical output of 142 L/min. If the bench measures 128 L/min at rated pressure with oil at 50 °C, volumetric efficiency is 128 / 142 = 90 percent, and the 14 L/min difference is internal leakage — most of which should appear as case drain flow. If case drain flow reads only 4 L/min, the missing 10 L/min is leaking somewhere else: the test rig, the relief valve, or the plumbing.
That cross-check is the most useful habit in test interpretation. Flow deficit and case drain should broadly agree, and when they do not, the bench itself deserves suspicion.
Acceptance Criteria That Mean Something
Published limits vary by frame size and pressure class, but the structure of a sound acceptance decision does not.
| Criterion | Typical expectation | Action if outside |
|---|
| Volumetric efficiency at rated pressure | 90 percent or better for a rebuilt pump at rated temperature | Investigate rotating group before shipping |
| Case drain flow | Within manufacturer limit, and not rising steeply with pressure | Check block, plate and piston clearances |
| Flow stability | Steady reading, no oscillation at fixed load | Control instability or air ingress |
| Pressure holding | Stable at rated pressure with no drift | Internal leakage or relief passing |
| Temperature rise | Gradual and plateauing | Excess leakage converting power to heat |
| Noise and vibration | Smooth across the pressure range | Bearing, alignment or cavitation |
A single efficiency number near 90 percent with rising case drain at each pressure step is a worse pump than one at 88 percent with flat leakage. The trend carries more information than the headline.
Verifying a Pump Without a Bench
Most workshops have no test stand, which is not a reason to fit unknown parts and hope. A machine-based check produces usable numbers in under an hour:
- Warm the oil to operating temperature and confirm it is at the correct level.
- Fit a calibrated gauge at the pump outlet and a flow meter in the pressure line if one is available.
- Record standby pressure and case drain flow at neutral.
- Stall a function against relief, record peak pressure and flow, then repeat at approximately half pressure with a metered load.
- Compare the two flow readings. A fall of more than about 10 percent between half and full pressure indicates leakage.
- Sample the oil and check for metal; combine the result with case drain behaviour before deciding.
This procedure will not produce a laboratory-grade efficiency figure, but it will reliably distinguish a healthy pump from a worn one, which is the decision at hand.
Reading the Bench Itself Critically
A test report is only as good as the instrument behind it. Three questions establish whether the numbers deserve trust. First, when were the gauge, flow meter and temperature sensor last calibrated, and is the certificate current? Second, is the flow meter installed at least ten pipe diameters downstream of any bend or valve, since turbulence biases turbine meters? Third, is the relief valve on the bench known to be sealed, because a passing relief silently absorbs several litres per minute and makes a good pump look marginal?
None of these are academic. A bench with an uncalibrated gauge can pass a worn pump and, worse, reject a good one, and the supplier who cannot answer these questions is not measuring so much as estimating.
Parts Behind the Numbers
When a test shows low efficiency at pressure with high case drain, the repair is normally inside the rotating group. General-purpose high-pressure units in the 500–3,000 rpm band are represented by this high-pressure piston pump with nitrile or Viton seal options, while fixed-displacement industrial duties use the fixed-displacement equivalent rated for the same speed range. Large-frame Rexroth A20VO installations are covered by A20VO250 spare parts, where rated-pressure efficiency is the figure that matters most to the buyer.
We quote against the test data you provide. Send the pump model and nameplate data, the part numbers, the machine model, the measured flow and pressure figures if you have them, and the quantity required. Where a pump is being rebuilt rather than replaced, state the target efficiency and we will confirm which components must be renewed to reach it.
Frequently Asked Questions
What volumetric efficiency should a rebuilt pump achieve?
Around 90 percent at rated pressure and rated temperature is a reasonable expectation; many new pumps reach 93–95 percent. Anything below 85 percent at rated pressure indicates a problem.
Does a higher efficiency number always mean a better pump?
No. It must be read with pressure, temperature, speed and case drain. A high figure at low pressure and low temperature can conceal a pump that fails at working conditions.
Why does my bench report differ from the OEM figure?
Different test temperatures, oil viscosity and pressure points. Compare like with like, and ask for the test conditions to be stated on the report.
Can I test a pump on the machine after fitting?
Yes, and you should. Record standby pressure, flow at pressure, case drain flow and stabilised oil temperature in the first 50 hours, and keep the figures as the baseline for future comparison.
Uw bericht moet tussen de 20-3.000 tekens bevatten!