Hydraulic & pneumatic seals

A piston seal seals between the piston and the cylinder bore, stopping oil passing from one chamber to the other. A rod seal seals around the rod to keep oil in, and always works with a wiper on the outside to stop dirt being drawn in on the rod. Their sections and lip directions differ, so they are not interchangeable.

They are manufacturers' profile codes: UN is a symmetrical U-cup, DH is typically a single-lip wiper, KDAS is a multi-part piston seal. Different makers use different codes for the same profile, so when substituting compare the cross-section drawing and the three groove dimensions, not the code name alone.

For a rod seal: rod diameter, groove root diameter and groove length. For a piston seal: cylinder bore, groove root diameter on the piston and groove length. Measuring the metal parts is more accurate than measuring a worn seal.

PU (polyurethane) resists wear and extrusion well and suits high pressure and mineral oil up to about 80 to 100 °C. NBR is softer and fits low to medium pressure. For oil above 100 °C or fire-resistant fluids, look at FKM or other materials, because PU hydrolyses with water and heat.

Not advisable. Pneumatic seals are designed for around 10 bar and low friction with thin lips; in high-pressure hydraulics they extrude and tear. Conversely, hydraulic seals in an air cylinder raise friction and cause stick-slip at low speed.

A single-lip wiper has only an outward lip that scrapes dirt off the rod as it retracts. A double-lip wiper adds an inner lip that holds back the oil film passing the rod seal, and is typically used with tandem rod seal systems. On muddy construction sites, a metal-cased wiper pressed into its bore is the better choice.

A wet rod with an oil ring collecting at the wiper means the rod seal is weakening. A cylinder that drifts with the valve closed, or slows under the same load, usually has a piston seal passing oil between chambers. Squealing at low speed may come from dry seals or seals hardened by heat.

PTFE has very low friction and no breakaway stiction, so the cylinder moves smoothly at low speed without stick-slip. Because PTFE is not elastic, it is usually paired with a rubber O-ring energiser, as in stepped rod seals or cap type piston seals. PTFE also tolerates many synthetic fluids and higher temperatures than PU.