Installing, Running and Maintaining Hydraulic Cylinders in Press Work

Installing and Aligning Cylinders in a Press Frame

Installation decides how long a cylinder survives, and mistakes made during fitting are expensive to undo. The cylinder must run on its own axis: if the platen tilts even slightly across the stroke, the rod sees a bending load on every cycle, and the symptom appears months later as scoring on one side of the rod chrome.

Check the frame before the cylinder is delivered. Confirm that the crown and platen are parallel, that the tie-rod holes and port fittings are clean, and that the space reserved for the cylinder matches the drawing, including retracted length and port clearance. Alignment is checked with a straight edge and a feeler gauge across the mounting faces, and with a dial indicator across the platen on a multi-cylinder press. A run-out above 0.10 mm is worth correcting before the cylinder is bolted down.

Cylinder Mounting, Tie-Rod Torque and End Stops

Mount the cylinder on clean, unpainted, flat seats with the ports plugged. Paint or packing trapped under a foot tilts the body by a fraction of a millimetre and preloads the rod unevenly, a leading cause of early seal and rod failure.

Tie rods are tightened diagonally in two or three passes to the manufacturer's figure, with clean, lightly lubricated threads. Over-tightening stretches the rods and thins the barrel wall, while under-tightening lets the caps move under load. If no torque is published, target the desired preload divided by the number of rods, at roughly 30 to 50 percent of the rod yield strength. Set the cylinder fully retracted but not hard against the internal stop, leaving 2 to 5 mm of headroom, and adjust external platen stops so the cylinder does not carry the shock of the stop.

Pipe Routing, Supports and Hoses

Pipe routing matters as much as the cylinder itself. Lines should be short, straight and of generous bore, because flow through a cylinder port at working speed is high and pressure drop along a long thin line shows up as lost speed and heat. Rigid steel pipe is preferred near the cylinder, with flexible hose only for the short connection that must accommodate movement. Support every pipe run independently of the cylinder body: a pipe hanging on its fitting will fatigue it, and a port carrying the weight of a run will leak at the thread. Use split flange blocks or pipe clamps at the fitting, and keep supports clear of the swept envelope.

Hoses must be rated for working pressure, minimum bend radius and pressure pulses, and replaced on a schedule rather than on failure. Abrasive swarf from stamping settles in hose loops and cuts the inner liner from the inside, so hoses in a stamping press need more frequent inspection.

Air Bleeding and First Commissioning

Air in the system causes most commissioning problems. Compressed air in the oil makes a cylinder move erratically, makes the pump noisy and accelerates seal damage. Fill the tank with the specified oil, run the pump against a closed valve to purge the high-pressure lines, then bleed each cylinder from the highest point down.

Bleed at the ports and the rod end with the cylinder positioned so air rises to the bleed screw. Open it a fraction of a turn and let oil run until it flows as a solid stream with no bubbles, then close and repeat at every high point. On a blind rod end the air collects in the blind end, so drive to full extension before the final bleed, then commission with the press unloaded where possible.

Setting Cylinder Speed and Working Pressure

Two settings define how a press cylinder behaves: the working pressure and the speed of each direction. Set the relief valve to the working pressure, not to the highest the pump can deliver, and confirm that no component in the circuit has a lower rating. A limit set too low thins the oil and increases pump wear; one set too high removes the protection the frame relies on.

Speed is set with the flow control valve at the cylinder port for the advance stroke, and by the directional valve for the return. Change speed in small steps and watch the gauge. If pressure rises as speed falls, the cylinder is fighting the load rather than the valve; if pressure falls as speed falls, the throttle is doing the work. Record the settings, because a cylinder commissioned at a known pressure and speed should be returned to the same values.

Cylinder Drift and Speed Control Faults

Cylinder drift is the most common complaint on a press with several cylinders. The symptom is a platen that sits unevenly, a tool that binds on one side, or a position error reported at the top of the stroke. The cause is usually hydraulic imbalance rather than a mechanical defect.

Start with the cheap checks: oil temperature and level, filter condition, and air in the system. Air compresses under load, so the cylinder reaching the stop first appears to drift while the lagging one appears cushioned. A cylinder that drifts only while the oil is cold points to air or a suction restriction on that branch. If the drift follows the cylinder, suspect a binding guide, a rod burr or a hardened piston seal; if it follows the load, suspect misaligned tooling or a platen not parallel to the table.

Seal Wear, Rod Scoring and Piston Surface Damage

Rod scoring ends the life of most cylinders, and it almost always has an upstream cause. Fine axial scratches across the seal land suggest contamination that entered past a damaged wiper; a single deep groove at one height suggests trapped swarf in the gland; a bend or crease in the rod points to side loading or buckling.

Seal wear appears first as an increased film of oil on the rod, then as a rising oil level in the tank, then as oil weeping from the gland. A rod seal leaking past its wiper on an open-air machine carries abrasive dust into the seal area on every stroke, so inspect the wiper rather than only the seal. Piston damage typically follows ignored external leakage: with a worn piston seal, oil crosses into the rod side on each stroke and the pressure difference across it rises, producing heat and eventually scoring of the bore.

Measure bore and rod before ordering parts: check the bore at the piston seal land and the wear band, and the rod diameter at three points along its exposed length.

Diagnosing External and Internal Leakage

Leakage diagnosis starts by deciding where the oil comes from. External leakage is visible at the gland, end cap seals, pipe fittings and the rod end connection. Internal leakage is invisible: it shows as a cylinder that will not hold position, a pump running continuously with the valve closed, or a platen creeping down under gravity.

Clean the gland and rod area, run several strokes and look for fresh oil. If the leak is at a fitting, tighten it and check the flatness of the sealing face, since a scarred face leaks again. If it is at the gland, the rod seal is worn or the rod scored, and tightening the gland nut further is not a repair. Measure internal leakage with a flow meter on the cylinder port while the valve is closed. Small internal leakage is acceptable on a down-moving load, but on an overhung load it is a safety problem.

Inspection Intervals, Rebuild or Replace, and Safe Depressurisation

Inspection works when it is scheduled and written down. Daily the operator should look for oil on the rod and at the joints, listen for unusual noise, and note any change in stroke time. A slowly lengthening return time is often the first sign of a developing seal problem.

Weekly and monthly the technician checks oil level, filter differential pressure, hose condition, oil temperature and the rod chrome. Rebuild or replace is decided by measurement, not appearance: a cylinder that has lost a rod seal but has a clean rod and bore can be rebuilt with a seal kit, while one with a scored rod, damaged bore, bent rod or compromised tie rod is not a rebuild candidate.

Before any work begins make the system safe: stop the press, isolate the electrical supply, close and tag the isolation valves, and bleed residual pressure to zero at the cylinder and any accumulator. A cylinder supported only by the hydraulic circuit must be mechanically blocked before an end cap is removed, because a hose failure can drop the load without warning. Support the rod from a sling, and cover the bore to keep swarf out.

Frequently Asked Questions on Cylinder Installation and Maintenance

How do I bleed air from a hydraulic cylinder?

Position the cylinder so that trapped air collects at the highest point, open the bleed screw a fraction of a turn and let the oil run until it flows without bubbles, then close it. Repeat at every high point and at the blind end of the cylinder. Bleeding is only complete when the cylinder has been driven to full extension with the oil warm, because air migrates as the oil thins.

What causes a cylinder to drift when the directional valve is closed?

The most common cause is internal leakage across the piston seal combined with a counterbalance valve that is missing or unseated, so the load is not held hydraulically. Check the counterbalance and pilot-operated check valves first, then measure leakage flow with a flow meter. Air trapped in the cylinder also causes drift that looks worse when the oil is cold.

How often should cylinder seals be replaced?

Plan a seal change whenever the cylinder is rebuilt, and rebuild after roughly three to five million cycles on typical press duty, or sooner if the rod is exposed to abrasive contamination or the oil runs hot. Replace the seals at the first sign of an increased oil film on the rod, because contamination drawn past a damaged wiper shortens the life of the replacement seal and of the rod itself.

Is it safe to remove a cylinder end cap without releasing the pressure?

No. A cylinder can hold a press load in the hydraulic circuit alone, and a hose failure or a valve that does not seat can release that energy without warning. Before removing any end cap, isolate the electrical supply, close and tag the isolation valves, bleed the pressure to zero at the cylinder and at any accumulator, and block the platen mechanically so the load cannot fall.