A worn seal comes out of a cylinder with no part number, and the machine is standing. It is a familiar situation in maintenance, and a few careful minutes at this point usually save a wrong order later. This guide sets out a practical method for recording what is needed to identify a replacement.
Note where the seal came from
Before cleaning or discarding anything, record the position and orientation of every sealing element. A photograph of the stripped gland and piston, taken before the seals are removed, is often the single most useful piece of information.
- Was the seal in the gland (rod side), on the piston, or at a static joint?
- Which way did the lips face in relation to the pressure?
- If there were several elements, in what order were they fitted?
- Is the cylinder single-acting or double-acting?
The position matters because rod and piston seals are designed differently, as explained in our article on the hydraulic rod seal vs piston seal.
Record the profile
The cross-section defines the seal type. If the seal is scrap, cut through it cleanly with a sharp blade, then photograph or sketch the cut face. Common hydraulic profiles include:
- U-shaped seals with two lips, either symmetrical or with one lip shorter than the other;
- compact seals that combine a wear-resistant sealing ring with an elastomer energizer;
- multi-piece vee packing sets with support rings at each end;
- PTFE-based rings energized by an O-ring;
- wipers, with or without a metal case;
- guide rings, which are flat-section bearing elements rather than seals but are normally replaced at the same time.
Count the pieces, including any back-up rings, and note which were found together in one groove.
Measure the hardware, then the seal
The groove tells you more than the worn seal does. Measure the metal parts first:
- the rod diameter (for gland seals) or the bore diameter (for piston seals);
- the groove diameter;
- the groove width.
Then measure the seal itself: inside diameter, outside diameter and height, plus the cross-section if it can be measured. Treat these figures as a guide only. A used seal may be worn, swollen by the fluid, shrunk, or permanently flattened, and soft seals are easily distorted by the measuring instrument.
Note whether the equipment is built to metric or inch dimensions. A near-equivalent in the other system may appear to fit and still not seal properly, so record the measurements as accurately as you can and say which unit you used.
Look at the material and any markings
Colour is not a reliable guide to material, because conventions differ between suppliers. A few observations still help:
- Polyurethane feels tough and relatively stiff, and returns to shape after being flexed.
- Rubber such as NBR is softer and more elastic.
- Fabric-reinforced seals show a visible weave in the surface.
- PTFE-based parts are hard and slippery, and do not stretch back like rubber.
Copy any moulded numbers, letters or symbols exactly as they appear, even if they are incomplete. Also note the condition of the old seal. Hardening, swelling, nibbled edges or scoring all say something about the duty, and may mean a like-for-like replacement is not the right answer. Our article on the common causes of seal failure describes what to look for.
Record the operating conditions
Dimensions identify a seal that fits; operating conditions identify one that suits. Note the hydraulic fluid, the working pressure and any shock loading, the temperature range, the speed and stroke, and the environment the cylinder works in. The make and model of the machine and the function of the cylinder are useful as well.
Send us what you have
Our sales engineers regularly help customers identify hydraulic seals from samples and partial information. Use the enquiry form to send:
- the rod, bore and groove dimensions, and the seal dimensions as measured;
- photographs of the seal, its cross-section and its position in the cylinder;
- the fluid, temperature, pressure and speed;
- any markings, a drawing if one exists, or the old seal as a sample.
The more complete the information, the quicker a suitable replacement can be confirmed.
