Hydraulic seals for cylinders and actuators

A rod seal keeps fluid from leaking out of the cylinder along the moving rod, so its failure shows as an external leak. A piston seal keeps fluid from passing across the piston from one chamber to the other, so its failure shows as drift, lost force or a cylinder that creeps under load. They sit in different grooves, are measured from different diameters, and are ordered separately unless they come in a kit.

At the rod end: a rod seal, a wiper outboard of it, usually a rod wear ring, and on harder duty a buffer seal between the rod seal and the cylinder. On the piston: a piston seal, or a pair of single-acting seals, with wear rings beside it. Static seals, usually O-rings with back-up rings where pressure calls for them, seal the head to the barrel and the piston to the rod.

Measure the groove, not the worn seal. For a rod seal that is the rod diameter, the groove diameter and the groove width; for a piston seal, the bore diameter, the groove diameter and the width. Note the profile (U-cup, V-packing set, T-seal or wiper), the direction of pressure and the material, and photograph the old seal's cross section; a seal kit part number or the cylinder maker's drawing replaces all of that.

For abrasive duty in mineral oil, within polyurethane's temperature range. The Parker O-Ring Handbook (ORD 5700, section 2.2.12, read 2026-09-27) describes polyurethane as hard wearing and strong, gives it a range of -40 C to 82 C, and rules it out for hot water and steam. Nitrile's range in the same handbook reaches higher, so above that temperature or in hot water the choice moves away from polyurethane; name the fluid and the temperature with the request.

The Parker O-Ring Handbook (ORD 5700, section 4.2, read 2026-09-24) says to look at back-up rings for any seal type above 1,500 psi, and below that where vibration or pressure swings make the gland breathe; where the parts close metal to metal and there is no extrusion gap, none is needed.

It sits between the piston side of the cylinder and the rod seal and takes the pressure peaks, so the rod seal sees a steadier, lower pressure and lasts longer. Buffer seals are fitted on cylinders with shock loads, high pressure or long strokes, and they are designed to let trapped pressure vent back toward the piston rather than build up behind the rod seal. They are ordered as part of a matched rod set with the rod seal and wiper.

Yes, and it is the first thing to state. The Parker O-Ring Handbook (ORD 5700, sections 2.2.1 and 2.2.12, read 2026-09-25 and 2026-09-27) lists mineral oil among the fluids nitrile and polyurethane resist, and phosphate-ester fire-resistant fluids suit EPDM: Parker's E3609-70 FDA compound data sheet (O-Ring Division report KK2223, read 2026-08-13) lists that EPDM compound as compatible with phosphate-ester hydraulic fluid and not with mineral oil products. On aerospace hardware, the handbook's section 8 specification tables (read 2026-09-24) pair the MS28775 O-ring series with MIL-H-5606 petroleum hydraulic fluid; the O-ring is made to MIL-PRF-25732, which the DLA ASSIST record (read 2026-09-24) describes as an acrylonitrile-butadiene (NBR, nitrile) rubber and lists as inactive. Fluorocarbon is specified where the fluid or the temperature demands it.