Hydraulic cylinders
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Frequently asked questions
What information do I need to replace a hydraulic cylinder?
Bore, rod diameter, stroke, mounting style at both ends (foot, flange, clevis, trunnion) and retracted pin-to-pin length. Also note working pressure and port thread size. Cylinders to ISO 6020 or ISO 6022 have standard mounting dimensions, which makes cross-referencing between makers easier.
How are cylinder push and pull forces calculated?
Push force equals pressure times piston area; pull force equals pressure times the annulus area (piston area minus rod area). For example, a 63 mm bore at 100 bar gives about 31 kN push. Because the pull area is smaller, a cylinder always pulls with less force and retracts faster for the same flow.
A cylinder leaks at the rod gland: reseal or replace?
If the rod is not scored, pitted or bent and the chrome plating is intact, replacing the seal and wiper kit is enough. If the rod is scored, new seals will tear again quickly, so the rod must be reground and replated or replaced. To order a seal kit, have the groove dimensions or the cylinder maker's kit code ready.
Why does a cylinder rod bend in service?
A long rod in compression can buckle, especially with pivot mounting at both ends and long stroke. Side loads or poor guidance also put bending moment on the rod. For long strokes, check buckling and choose a larger rod or add a stop tube.
Does a cylinder need bleeding after installation?
Yes, trapped air makes the rod move jerkily, causes knocking and can burn the seals through dieseling. Cycle the cylinder through full stroke several times at low pressure with no load to push the air back to the tank. If the cylinder has bleed screws, open the one at the highest point until oil runs out without bubbles.
Why does a cylinder drift while holding a load?
Slow drift with the valve closed is usually internal leakage past the piston seal, with oil passing from one chamber to the other. Spool leakage in the directional valve also causes drift, so check the valve too before concluding. For suspended loads that must hold firmly, fit a pilot operated check valve or counterbalance valve close to the cylinder.
How should a cylinder be mounted to avoid side loads?
A cylinder should carry only axial force, since side load wears the guide bushing, scores the rod and causes seal leaks. Clevis or trunnion mounts with a spherical rod eye let the cylinder follow the arc of the mechanism. External guides should be used where the load has a sideways component.
How is a cylinder rod protected from dirt and impact?
The wiper seal at the gland keeps dirt out, but in heavy dust a bellows boot should be added. Scratches or dents in the chrome plating cut the seals on every stroke, so avoid knocks during transport and installation. Spare cylinders stored for long periods should have the rod retracted and the ports capped.
