In brief
A hydraulic cylinder is an actuator that converts hydraulic pressure into linear motion with high force, for example in presses or rolling mills.
Contents
What is Hydraulic cylinder?
Inside the cylinder tube, a piston with piston rod moves, displaced by oil pressure on one or both sides. The force is the product of pressure and effective piston area. Hydraulic cylinders generate considerably higher forces than pneumatic cylinders in a compact design and can be positioned sensitively.
Single-acting cylinders work in one direction only and retract by load or spring, while double-acting cylinders are pressurised in both directions. There are also double-rod cylinders with a through rod, telescopic and plunger cylinders. In terms of construction, a distinction is made between tie-rod, round-body and welded cylinders. Key ratings are piston and rod diameter, stroke, nominal pressure, mounting style, end-position cushioning and seal system. For certain series, mounting dimensions are standardised in ISO 6020 and ISO 6022.
Common causes of failure are worn seals, scoring on the piston rod, contaminated oil, side loads due to misalignment and corrosion. In steelworks or on injection moulding machines, heat and scale are added. A seal kit is often sufficient; if the rod is damaged, a repair is necessary. For cylinders without documentation, a replacement can be reproduced on the basis of measurements, installation situation and operating data.
In practice: maintenance and procurement
For replacement or repair, nameplate data, piston and rod diameter, stroke, operating pressure, mounting style, port thread and medium are needed. For special cylinders without documentation, a dimensioned drawing helps; seal kits should be stocked for critical cylinders.
Synonyms and abbreviations: hydraulic ram · hydraulic actuator · differential cylinder · plunger cylinder · double-acting cylinder
Frequently asked questions
What is the difference between a single-acting and a double-acting hydraulic cylinder?
A single-acting cylinder is pressurised with oil on one side of the piston only; it retracts by the weight of the load or by a spring. A double-acting cylinder has two ports and is moved hydraulically in both directions. Double-acting cylinders are common in presses, hoists and conveying equipment because they deliver force in both directions.
How is the force of a hydraulic cylinder calculated?
The force is the product of pressure and effective area. When extending, the full piston area acts; when retracting, only the annular area, i.e. piston area minus rod cross-section. A cylinder therefore produces less force when retracting. In practice, friction and pressure losses reduce the theoretical value.
What information is needed to replace a hydraulic cylinder?
Piston diameter, rod diameter, stroke, maximum operating pressure, mounting style at base and rod, port type and size, installation length as well as medium and temperature are needed. Details of end-position cushioning and any position measuring system are also important. The nameplate and manufacturer part number shorten clarification considerably.
Why does a hydraulic cylinder leak oil or drift down?
External leakage at the rod usually indicates a worn rod seal or a damaged rod surface. If a cylinder drifts down under load, a defective piston seal or a leaking valve in the circuit may be the cause. A test should distinguish between the two before parts are replaced.