Piston seals: single or double acting?

While the rod seal prevents oil from escaping, the piston seal has a different but equally important role: it must separate the two chambers of the cylinder, so that pressure on one side of the piston becomes force and motion. When the piston seal wears, oil passes from one chamber to the other (so-called internal leakage): the cylinder loses force, does not hold the load in position, and movements become slow or irregular, often with no visible external leak.

Single acting and double acting

The first selection criterion depends on the type of cylinder.

• Single acting cylinders: pressure acts on one side of the piston only, while the return stroke is by gravity, load or a spring. Seals that hold pressure in one direction are used, often lip profiles similar to rod seals.
• Double acting cylinders: oil pushes alternately on both sides of the piston. A seal working in both directions is needed: a bidirectional seal or two unidirectional seals fitted back to back.

The most common profiles

Compact seals

They consist of an elastomer sealing element, often with anti-extrusion rings and, in some models, integrated guide elements. They fit in a single groove, are robust and tolerate dirt well, which is why they are widespread in mobile machinery such as tractors, loaders and excavators.

PTFE seals with energiser

A filled PTFE ring (bronze, carbon, glass) works against the bore, while an elastomer O-ring or square ring pushes it and ensures sealing. They offer very low friction, no stick-slip and good temperature resistance: they suit precise movements, high speeds or applications with frequent reversals.

Lip seals (U-rings)

Used mainly in single acting cylinders, or fitted in pairs in double acting ones. They seal well even at low pressure.

Polyurethane seals

Polyurethane offers excellent resistance to abrasion and extrusion, so it is often chosen for heavy-duty applications. Compatibility with temperature and fluid must be checked.

The role of guide rings

The piston seal must never carry side loads. That is the job of guide rings, made of phenolic resin, polyester or filled PTFE, which centre the piston in the bore and prevent metal-to-metal contact. When you replace the piston seal, always check the guides too: if they are worn, the piston tilts, the seal is squeezed on one side and quickly fails.

How to choose and measure

To identify the correct seal you need:

1. D – bore diameter of the cylinder tube;
2. d – groove bottom diameter on the piston;
3. L – groove width.

Add the operating conditions: maximum pressure, speed, temperature, type of oil and contamination. With this information you can use the search by dimensions on our website or ask our technical support for advice.

Signs that the piston seal needs replacing

• The cylinder does not hold the load and slowly drifts down;
• Available force has dropped at the same pressure;
• Movement is irregular or jerky;
• The oil runs hotter than usual.

In all these cases it is worth dismantling the cylinder, checking tube, piston and guides, and replacing the whole seal kit. Changing only one seal while leaving the worn ones in place often leads to another repair shortly afterwards.

Frequently asked questions

Can I use a rod seal on the piston?

Generally not: profiles are designed to work on a specific side (inner diameter for the rod, outer diameter for the piston). Even when the dimensions look similar, the lip geometry and preload are different.

One compact seal or two lip seals?

Compact seals are easier to fit and tolerate dirt well; two opposed lip seals often give excellent sealing at low pressure. The choice also depends on the shape of the existing groove.

How often should the piston seal be changed?

There is no fixed interval: it depends on working hours, pressure, oil cleanliness and side loads. The most reliable signal is the cylinder no longer holding the load.