Hydraulic Cylinders for Press Work: Types, Sizing and Selection

What a Hydraulic Cylinder Does in a Press

A hydraulic cylinder is the only component in a press that converts fluid pressure into mechanical work. The pump, motor, tank and relief valve only store and deliver energy, while the frame, platen and bolster only react against it. The cylinder is therefore the boundary between the hydraulic circuit and the machine structure, and most press failures originate there.

It must do five things at once: develop axial thrust over the working stroke, carry the forming load without buckling, resist the bending moment from off-centre tooling, hold the load safely when the control valve is closed, and return to the open position for the next cycle.

The duty cycle is unusual too. A production press runs a few hundred strokes per hour with a fast approach, a slow working speed under load and a quick return, so working speed and cycle frequency rather than peak force tend to set the fatigue life of the rod and seals.

Single-Acting and Double-Acting Cylinders

The choice is decided by how the ram returns. A single-acting cylinder extends under pressure and retracts only when that pressure is released, so a spring, the weight of the platen or a separate return ram must drive the return. This suits bench and tabletop presses where gravity does the work.

A double-acting cylinder extends and retracts hydraulically, removing the spring and giving control of both directions. In most production presses this is better, because the return can run two to three times faster than the advance and cycle time falls accordingly.

Load holding still needs attention: a closed-centre valve holds the cylinder, but its internal check is not a positive lock, so an overhung ram needs a counterbalance or pilot-operated check valve.

Tie-Rod, Welded and Telescopic Bodies

Cylinder bodies come in three constructions, and the choice is mostly about stroke, side-load capacity and repairability.

The tie-rod cylinder is a barrel, front and rear caps and tie rods clamping the stack under tension. The rods carry load across the joint, so it accepts higher working pressure than a welded body and can be opened for inspection, but tie rods add 15 to 25 percent to the closed length.

The welded body joins barrel and caps with a continuous weld ring: shorter and cheaper, but not openable without cutting. A telescopic cylinder nests stages, so a 100 mm bore two-stage unit gives 1500 to 2500 mm stroke in about 700 mm closed length.

Bore, Rod Diameter, Stroke and Stroke-to-Bore Ratio

Bore sets the force, rod diameter sets the bending strength and stroke sets the space the cylinder occupies. Buyers routinely pick the bore from the force requirement and leave the rest to the catalogue, which is where press cylinders begin to fail.

The stroke-to-bore ratio is the first constraint. For a single-stage cylinder a ratio of about 1.5 is comfortable and 2.0 is close to the practical limit. Telescopic cylinders break this rule and reach ratios of 4 to 6, but they need an anti-rotation guide and a well designed flow divider.

Rod diameter is typically 50 to 70 percent of bore on tie-rod cylinders and 60 to 80 percent on welded bodies, with heavy-duty designs pushing higher to resist buckling. Retracted length must include barrel, caps, rod end and flanges, and it is this figure that must fit between crown and platen. Leave 2 to 5 mm of headroom at full retraction so the rod never takes the shock of a mistimed command.

Thrust Calculation and the Pressure-Area Relationship

Thrust follows one rule: force equals pressure multiplied by piston area. Because area rises with the square of the bore, a cylinder 25 percent larger in bore delivers 56 percent more force at the same pressure, so oversizing wastes flow, generates heat and buys capacity that is never used.

Extend thrust uses the full bore area; retract thrust uses the annular area between bore and rod. A 160 mm bore cylinder with a 100 mm rod has a piston area of 20 106 cm2 but an annular area of only 12 252 cm2, so it retracts with about 61 percent of its extend force.

Confirm that the relief setting equals the working pressure and that the cylinder's maximum allowable pressure exceeds it with margin. Pressure and flow are not interchangeable: halving the working speed means halving the flow, which changes the heat load in the tank.

Retraction Speed, Flow Requirements and Speed Control

Cylinder speed is set by flow: litres per minute equals piston area in square centimetres times speed in centimetres per second times 0.06. A 100 mm bore cylinder has a piston area of 7 854 cm2, so at 64 mm per second it needs about 30 L per minute, and at 20 mm per second it needs 9.4 L per minute.

Most presses therefore run a pump considerably larger than the working stroke needs, and the difference is taken out by a flow control valve. A meter-out throttle at the cylinder port gives a slowed approach and holds the slow working speed under load. Return speed is set by the flow the directional valve passes to the rod port, and the smaller annular area makes it faster.

Speed control valves are cheap but waste energy as heat. The alternative is to size the pump close to the average working demand or run a variable-speed drive, which moves the heat into the drive. Tank size, oil grade and cooler capacity follow the average power, which equals force times speed.

Side Loads, Buckling Limits and Rod Alignment

A press cylinder is a slender column, and slender columns buckle. The rod is loaded in compression on every extension and also carries the bending moment from any offset load. The practical rule used by manufacturers is that the rod diameter should be at least about one twentieth of the stroke length, and larger where the load is off-centre.

Buckling is not the only limit. Manufacturers publish allowable side load and bending moment for each bore and stroke, and those figures fall sharply as the stroke grows. Where tooling is offset, or a side-mounted cylinder works through a beam, the mounting must be checked against the published moment rather than assumed.

Spherical rod eyes absorb small angular misalignment without loading the rod sideways, and anti-rotation guide rails keep the cylinder from taking any side force at all. The cylinder must also be square to the platen: a misalignment of 1 mm over a 500 mm stroke creates a measurable bending load.

Choosing a Cylinder for a Press Frame and Synchronising Multiple Cylinders

A cylinder is chosen for a press frame by working backwards from the frame, not from the catalogue. Measure the space between crown and platen with the ram retracted, then confirm the mounting interface: bolt circle and thread, flange thickness, port position and the direction the ports point. Port position often decides the matter, because ports facing the wrong way must be piped around the frame.

The mounting is a flange or foot mount to the crown, with the rod end attached to the platen through a flange, a clevis or a spherical rod eye. Choose the connection that lets the rod eye align itself, and keep flange bolts out of the bending zone.

On multi-cylinder presses, synchronisation is not automatic. Two cylinders fed from the same pump at the same pressure will not necessarily move together, because seal friction, oil volume and load differences create imbalance. A flow divider guarantees equal volume, while an electro-hydraulic synchronisation valve meters the faster cylinder back to the slower one. Fit make-up check valves so the cylinders can be matched.

Sealing Options and the Cost of Sizing Mistakes

Seals decide both the life and the behaviour of a cylinder. The set normally comprises a piston seal, a rod seal, a wiper and a buffer seal, and each fails differently. A rod seal damaged by a scored rod leaks oil down the outside of the cylinder, and a hardened wiper lets abrasive swarf into the seal area, turning a small leak into a rod failure.

Elastomer choice is a compromise of temperature, media and speed. Nitrile rubber is the default: cheap, tolerant of mineral oil and water, good to roughly 80 degrees Celsius. Polyurethane is the wear-resistant choice for high-cycle presses, good to about 100 degrees Celsius, but poor in hot water. FKM handles high temperature and aggressive media but is costly.

Cost of ownership is where sizing mistakes finally show. An oversized bore wastes pump capacity, an undersized rod bends, and a cylinder rated below the relief setting will be damaged by the first operator who opens the valve too far.

Frequently Asked Questions About Hydraulic Cylinders

How do I calculate the thrust required from a hydraulic cylinder?

Calculate the required force first: the forming force for the material, plus the weight of the moving platen and tooling, plus a margin of 15 to 25 percent. Divide that force by the maximum working pressure to obtain the piston area, then convert area to bore using bore equals the square root of four times area divided by pi.

What is the difference between a tie-rod cylinder and a welded cylinder?

A tie-rod cylinder is a bolted stack clamped by external tie rods. It can be dismantled for inspection and normally tolerates a higher working pressure, but it is longer for a given bore and stroke. A welded cylinder has barrel and caps joined by a continuous weld, which is shorter and cheaper, but cannot be opened without cutting.

How much of the cylinder force is available during retraction?

Retraction uses the annular area between bore and rod rather than the full piston area. For a 160 mm bore with a 100 mm rod the extend area is 20 106 cm2 and the annular area is 12 252 cm2, so retraction delivers about 61 percent of the extend thrust. Confirm that the retracted force still exceeds the weight of the platen and tooling.

How do I know if a cylinder is undersized for side loads?

Compare the published allowable side load and bending moment for the bore and stroke with the worst tooling offset. As a quick check, the rod diameter should be at least about one twentieth of the stroke length and the cylinder should sit square to the platen within 0.1 mm per 100 mm of stroke. Rod scoring at the same height on every return stroke signals buckling or side loading.