Rod seals: how to choose the right one
The rod seal is the first barrier against oil leaking out of the cylinder. Here is how to choose it according to pressure, speed, temperature and groove dimensions.
In a hydraulic cylinder the rod seal has a delicate job: it must keep pressurised oil inside the cylinder while the rod moves back and forth thousands of times a day. If it does not work properly, the leak is immediately visible: oil on the rod, drops under the machine, loss of performance. Choosing the right profile is therefore essential for the reliability of the whole system.
What a rod seal does
The rod seal works dynamically: its sealing lip rests on the chromed surface of the rod and must let only a minimal film of oil through, enough to lubricate, then carry it back inside on the return stroke. At the same time it must withstand system pressure, pressure peaks and temperature changes. In many cylinders it works together with other components: a wiper on the outside, which blocks dirt, and one or more guide rings, which keep the rod centred.
The main profile types
- U-rings (single or asymmetric lip): the most common profile, easy to fit and suitable for most standard applications.
- Compact seals or seals with anti-extrusion ring: designed for higher pressures, where the material must not be pushed into the gap between rod and head.
- PTFE seals with an elastomer energiser: low friction, no stick-slip, ideal for slow and precise movements or for high speeds.
- Tandem sealing systems: a primary and a secondary seal, for heavy-duty applications or where leakage must be practically zero.
In our catalogue you will find these families divided by profile, each with its available sizes and material.
Parameters to consider
Working pressure and peaks
Knowing the nominal pressure is not enough: peaks caused by shocks and sudden loads also count. The higher the pressure, the more important the material (polyurethane or PTFE compounds) and an anti-extrusion element become.
Rod speed
High speeds generate heat through friction. In these cases low-friction PTFE profiles perform better than traditional elastomers.
Temperature and fluid
The material must be compatible with the oil used and with the working temperature. NBR is a common choice for mineral oils at medium temperatures; for higher temperatures or special fluids, FKM (Viton) or other compounds are considered. The temperature indications for each product are shown on its page on the website.
External environment
Dust, mud, sand or metal chips drastically shorten seal life if the wiper is not adequate. The rod seal and the wiper should always be chosen together.
How to measure
The three key dimensions are:
- d – rod diameter;
- D – groove bottom diameter in the cylinder head;
- H (or L) – groove height.
If you only have the old seal, remember that a worn or deformed seal can give misleading measurements: whenever possible, measure the groove and the rod directly with a caliper. On our website you can then use the search by dimensions with these values: you will see only compatible profiles.
Mistakes to avoid
- Fitting the seal the wrong way round: the sealing lip must face the pressure.
- Passing the lip over threads or sharp edges without protection: a tiny cut is enough to cause a leak.
- Reusing a worn wiper with a new seal.
- Ignoring the condition of the rod: scratches, dents or damaged chrome will destroy even the best seal.
Not sure about the profile?
If you are not sure which seal to choose, contact us with the dimensions, pressure, temperature, type of oil and, if possible, a photo of the part. Our technical support will help you find the most suitable solution among the available profiles.
