Press ram drifting and cycle slowing after a return-hose change
A 100-tonne hydraulic press used for deep drawing has a 200 mm bore main cylinder, a 300 mm working stroke and a pump delivering 60 L/min at 160 bar. Power-pack oil temperature used to settle at 48 °C but now runs at 68 °C. Over three weeks three symptoms have appeared: the ram drifts down about 4 mm/min when the directional valve is in neutral with the pump running; the draw stroke has gone from 9 s to 13 s; and the return-filter indicator sits in the red bypass zone. Oil was last changed 5,000 running hours ago against a 2,000-hour schedule, and the last oil analysis is 14 months old. Operators say the drift began after a weekend when a fitter replaced a hose on the return line. No gauge is fitted on the main line; the only pressure gauge is on the accumulator charging panel. The press runs two shifts.
1. Set out the safety steps and the diagnostic sequence you would follow before condemning any component.
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Safety first - a drifting ram is a stored-energy hazard.
- Lock out and isolate the pump motor; do not trust the directional valve to hold the load.
- Bleed the accumulator down through its unloading valve and confirm zero on the charging-panel gauge, then mechanically block or prop the ram before anyone reaches underneath.
- Let oil cool below 50 °C before opening anything; 68 °C oil at pressure is a serious burn risk. Have a spill kit ready.
Instrument before diagnosing. With no main-line gauge you cannot separate a passing relief valve from a worn pump, so fit a calibrated 0–250 bar test gauge at the pump outlet and at the cylinder inlet.
Sequence.
- Oil sample from mid-reservoir: ISO cleanliness code, water content, viscosity at 40 °C. A filter indicator in bypass after 5,000 h on a 2,000-hour schedule means a clogged element passing dirty oil - replace it and resample.
- Check the cooler (water flow, fouling, ambient) and confirm oil temperature with a second thermometer.
- Drift test: isolate the cylinder by blanking its ports with line stops, then repeat the 4 mm/min test. Drift with the cylinder isolated means piston-seal or barrel leakage; no drift means valve spool or check-valve leakage.
- Verify the return hose fitted that weekend: correct part number, internal diameter, no kink, no collapsing liner. A throttled or delaminating return hose raises back pressure, heats the oil and can push the filter into bypass - one fault explaining three symptoms.
2. Using the flow and area figures, rank the likely root causes and state the corrective and preventive actions.
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Use the numbers to rank the causes. Cylinder area = π/4 × 20² ≈ 314 cm². Over a 300 mm stroke the draw volume is 314 × 30 ≈ 9.4 L, so at 60 L/min the ideal draw time is 9.4/60 × 60 ≈ 9.4 s - matching the old 9 s. At 13 s the cylinder is effectively receiving 9.4/13 × 60 ≈ 43 L/min, about 17 L/min short. The drift of 4 mm/min equals 314 × 0.4 ≈ 126 cm³/min, i.e. about 0.13 L/min.
So 0.13 L/min of leakage cannot explain a 17 L/min loss. Ranked:
- Relief valve passing or pump worn - accounts for the 17 L/min deficit and the 20 K temperature rise, since the shortfall is dumped as heat. Confirm by blanking the pump outlet and reading pressure against the relief setting at flow.
- Restricted or collapsing return hose fitted at the weekend - cheapest to check and explains back pressure, heat and filter bypass together.
- Contamination from 5,000-hour oil through a bypassed filter - the likely origin of spool scoring and seal wear.
- Cylinder piston seal - explains the slow drift only; check after pump and valve are cleared.
Corrective: change oil and element, clean the suction strainer, restore the correct return hose, re-set the relief against a calibrated gauge, reseal the cylinder if drift remains.
Preventive: oil change at 2,000 h, six-monthly oil analysis, differential-pressure indicator on the filter, a permanent main-line gauge and test point, and a hose specification check at stores issue.