Short answer: A hydraulic cylinder sealing system commonly uses rod seals to retain fluid, piston seals to separate pressure, wipers to exclude contamination, wear rings to guide moving parts, O-rings for static sealing and backup rings to support seals against extrusion.
Hydraulic Seal Types and Functions
| Component | Location | Main function | Typical inspection points |
|---|---|---|---|
| Rod seal | Cylinder head or gland around the rod | Retains hydraulic fluid inside the cylinder | Lip damage, extrusion, hardening and rod condition |
| Piston seal | On the piston inside the bore | Controls pressure between cylinder chambers | Bypass wear, cuts, scoring and fit in the groove |
| Wiper/scraper | Outermost gland position | Removes dirt, water and debris from the retracting rod | Damaged lip, looseness, corrosion and contamination |
| Wear/guide ring | On piston or gland | Guides components, absorbs side load and prevents metal contact | Uneven wear, collapse, scoring and clearance |
| O-ring | Static joints or as an energizer | Seals stationary interfaces or energizes another seal | Compression set, cuts, twist and chemical change |
| Backup ring | Beside an O-ring or pressure seal | Reduces extrusion into a clearance gap | Orientation, split position, deformation and missing components |
Rod Seals
Rod seals operate on the reciprocating rod surface and prevent oil from escaping. Primary and secondary rod seals may be combined in some systems. Selection depends on pressure, speed, temperature, fluid, surface condition and housing design.
Piston Seals
Piston seals maintain separation between the pressure chambers as the piston moves through the bore. They may be single-acting or double-acting. Internal bypass can reduce holding force even when no external oil is visible.
Wipers and Scrapers
The wiper is the first barrier against mud, dust, water and abrasive particles. It is not a substitute for the rod seal. A damaged wiper allows contaminants into the gland and can shorten the life of the complete system.
Wear Rings and Guide Rings
Wear rings guide the piston and rod while absorbing transverse load. They help prevent metal-to-metal contact between piston and bore or rod and gland. Uneven wear may point to misalignment, side load or damaged hardware.
O-Rings
O-rings commonly seal static joints and can energize PTFE or other sealing elements. Size, squeeze, groove condition and material compatibility all matter. An O-ring should not be twisted or substituted merely because it appears close in diameter.
Backup Rings
Backup rings support another sealing element where pressure and clearance could cause extrusion. Their position relative to pressure direction matters. Some applications use one backup ring; reversing-pressure applications may use support on both sides.
Other Components Found in Seal Kits
Depending on the cylinder, a kit may also contain buffer seals, static gaskets, retaining elements, dust rings or specialized composite seals. Kit contents are application-specific.
Material Overview
- NBR: widely used elastomer; suitability depends on fluid and temperature.
- HNBR: selected where improved heat or ageing resistance is required.
- PU/TPU: commonly used for wear-resistant dynamic sealing profiles.
- PTFE: used where low friction or chemical and temperature performance is required, often with an energizer.
- POM/nylon/filled composites: commonly used in guide and wear elements.
Frequently Asked Questions
Does every hydraulic cylinder use all six components?
No. The exact arrangement depends on cylinder design and duty. Always reproduce the specified sealing system.
Are wipers and rod seals interchangeable?
No. The wiper excludes contamination; the rod seal retains pressure and fluid.
Can I replace only the visibly damaged seal?
Inspect the full system and follow the repair specification. Related wear rings, backup elements or hardware damage may have contributed to failure.
Use our seal-kit identification guide, browse the catalog or contact Fujis Seal.
