Every hydraulic cylinder relies on two main dynamic seals: one on the rod and one on the piston. On the workbench they can look alike, with similar materials and lip shapes, but they do different jobs, sit in different housings and seal on opposite diameters. Confusing the two is a common reason for leakage soon after a cylinder repair.
Rod seals
The rod seal sits in a groove in the cylinder head, also called the gland. It seals dynamically on its inside diameter against the moving rod and statically on its outside diameter in the groove.
Its job is to keep pressurised fluid inside the cylinder as the rod extends and retracts. Because it is the barrier between the hydraulic system and the outside, a failing rod seal shows up as visible external leakage: oil on the rod, on the machine and on the floor. That makes it a housekeeping, safety and environmental concern as well as a maintenance one.
A rod seal has to do something subtle. The rod carries a very thin oil film out through the seal on the extending stroke, and a well-chosen seal allows that film to be drawn back in on the return stroke rather than scraped off and left outside.
Piston seals
The piston seal sits in a groove on the piston. It seals dynamically on its outside diameter against the cylinder bore and statically on its inside diameter in the groove.
Its job is to stop fluid passing from one side of the piston to the other. In a single-acting cylinder the seal holds pressure from one direction only. In a double-acting cylinder it must seal in both directions, which calls for a bi-directional seal design or a suitable arrangement of single-acting seals.
Piston seal leakage is internal, so nothing is seen from outside. The symptoms are indirect: the cylinder drifts under load, loses force, slows down or runs warmer than usual.
Side-by-side comparison
| Aspect | Rod seal | Piston seal |
|---|---|---|
| Location | Groove in the cylinder head (gland) | Groove on the piston |
| Dynamic sealing surface | Inside diameter, against the rod | Outside diameter, against the bore |
| What it prevents | External leakage past the rod | Internal leakage across the piston |
| Pressure direction | From one side | From one side or both, depending on the cylinder |
| Typical failure symptom | Visible oil on the rod and gland | Drift, loss of force, slow or uneven movement |
| Sized by | Rod diameter and groove dimensions | Bore diameter and groove dimensions |
Why they are not interchangeable
A rod seal is designed with its dynamic sealing edge and its interference on the inside diameter; a piston seal has them on the outside. Even when the nominal dimensions happen to coincide, fitting a seal in the wrong position puts the dynamic lip against the static surface and the static lip against the moving one, which usually results in leakage and rapid wear.
Some symmetrical profiles are offered for both rod and piston use, but this should be confirmed from the supplier's data for the specific profile rather than assumed from appearance. When a seal has been removed without a record of where it came from, our guide on how to identify a hydraulic seal may help.
The rest of the sealing system
Rod and piston seals do not work alone. A typical cylinder also includes:
- A wiper: fitted at the outer end of the gland to keep dirt and moisture from being drawn in on the rod.
- Guide rings: on the piston and in the gland, to carry side loads and prevent metal-to-metal contact.
- A buffer seal: used ahead of the rod seal in heavier-duty cylinders to protect it from pressure peaks.
- Static seals: O-rings, often with back-up rings, between the fixed parts of the cylinder.
When one element is worn, the others are affected, so it is usual to replace the seals in a cylinder as a set. Pneumatic cylinders follow the same layout but use pneumatic seals designed for air service, and the two types should not be swapped.
Send us your cylinder details
We supply rod seals, piston seals, wipers and guide rings as part of our hydraulic seals range. Send the rod, bore and groove dimensions, the fluid, temperature, pressure and speed, and a drawing or the old seals as samples, to our sales engineers through the enquiry form.
