Slow approach and drift on a 100-tonne hydraulic press
A 100-tonne double-acting press has a 250 mm bore main ram fed by a variable-displacement axial piston pump rated 63 L/min at 1470 rpm, with a relief valve set at 200 bar. Faults: approach speed has fallen from 8 mm/s to 4.5 mm/s; the ram drifts 5 mm/min when the directional valve is centred and the load is held; the pump-outlet gauge reads 140 bar on the holding stroke although the load needs 175 bar. The return-line filter indicator is in the bypass position, oil temperature reaches 68 °C within an hour against 45 °C previously, and the pump is noisier with a 1× shaft-frequency component plus a new 250 Hz component. Pump case-drain flow measured 9 L/min at 175 bar; the permissible maximum is 5 L/min. The suction strainer has not been cleaned for 18 months and the reservoir breather is damaged, so wash water runs across the tank top.
1. Using the readings, separate pump wear from relief-valve and cylinder leakage, and set out a test sequence that proves which is which.
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Evidence pointing to pump wear:
- Case-drain flow 9 L/min at 175 bar is 14 % of the 63 L/min pump output, against a 5 L/min limit. Internal leakage across the piston/valve plate and slippers at pressure is the classic wear signature, and it also explains the 250 Hz noise and the fall in approach speed (effective flow to the ram is reduced).
- Low output pressure (140 bar against 175 bar required) fits a worn pump that cannot hold pressure at flow.
- Slow approach with drift and hot oil fits contamination: filter in bypass, dirty suction strainer and a broken breather letting water in.
Sequence to prove it:
- Deadhead with a certified test gauge and needle valve at low flow and watch the relief opening pressure - it must crack at 200 bar, not 140 bar. If it opens early it is the relief valve.
- Isolate the cylinder ports with a blocking valve or blind flanges and measure drift; a drifting isolated cylinder means piston-seal or valve leakage, not the pump.
- Measure case drain again at 100 bar and 175 bar - flow that rises steeply with pressure confirms pump wear.
- Check return-line flow with the actuator idle; continuous flow means leakage past the directional valve.
Only after steps 1–4 should parts be ordered.
2. The relief valve is found to open correctly at 200 bar and the isolated cylinder still drifts. What repair plan do you set, and in what order?
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The drift proves leakage in the cylinder or its holding circuit; the case-drain figure proves the pump is worn as well. Both must be corrected, because a rebuilt pump feeding a contaminated system will fail again.
Plan in this order:
- Contain the dirt source: replace the damaged breather, blank off the water path, clean the suction strainer and fit a new return filter element; correct the bypass cause rather than resetting the indicator. Flush the reservoir and lines, then change the oil and take a sample.
- Cylinder: strip, inspect the bore for scoring and the piston seal, wear rings and guide bands; fit a seal kit, check rod straightness and gland clearance. If the bore is scored, hone and fit the next seal size.
- Valves: clean or replace the directional valve and check for internal leakage on the holding port; add or restore a pilot-operated check valve for load holding if the circuit relies on the spool alone.
- Pump: replace or rebuild the piston pump - 9 L/min case drain is not repairable by adjustment.
- Thermal and acceptance: restore cooling and confirm oil stays below 55–60 °C; then run acceptance: approach speed 8 mm/s ±10 %, drift under 0.5 mm/min, holding pressure 175 bar with the relief set at 200 bar, and no case-drain reading above 5 L/min.