Hydraulics & Fluid Power Reference Library · Technical Reference

Cylinder Seals and Seal Kits

A hydraulic cylinder uses several seal types, each with its own job: rod seals and piston seals hold pressure, wipers keep dirt out and wear rings guide the moving parts. A seal kit replaces all of them for one cylinder model.

Ref HY-0028ISO 5597ISO 7425

Draft, open for review. Check every value against the governing standard and the manufacturer’s data before use.

What it is

Cylinder seals keep oil in, keep dirt out and let the rod and piston slide. Each sits in its own groove (gland). The housing dimensions for many seal types are standardized, so seals from different makers can fit the same groove when the profile and size match.

How to identify

Remove seals carefully and lay them out in order. Note which groove each came from and which way any lip faced. Measure the groove, not the worn seal, and record bore, rod diameter and groove width. Many seal kits are sold by cylinder model, so the cylinder tag or part number is the fastest route.

Seal types and jobs

Seal	Location	Job	Common materials
Rod seal	Head gland, around the rod	Holds pressure at the rod	Polyurethane, NBR, PTFE with energizer
Buffer seal	In front of the rod seal	Takes pressure peaks and protects the rod seal	Polyurethane, PTFE
Wiper (scraper)	Outer end of the head	Keeps dirt and water out	Polyurethane, NBR
Piston seal	On the piston	Holds pressure across the piston	PTFE with energizer, polyurethane, NBR
Wear ring (guide ring)	Piston and head	Guides parts, stops metal contact	Filled PTFE, polyester or phenolic resin
Static seal	Head to barrel, piston to rod	Seals non-moving joints	O-ring with backup ring

Key points

Lip seals are directional. The lip faces the pressure. Installing one backward causes an instant leak.
Wipers come as snap-in or press-in types. Double-acting wipers also hold back an oil film.
PTFE-based seals with an elastomer energizer are low friction and good for high speed, but need proper sizing tools to install.
Material must suit the fluid and temperature. NBR suits petroleum oil; FKM handles higher temperature; some fire-resistant fluids need other compounds.
O-rings in static grooves often need backup rings at higher pressures. See the AS568 O-ring chart.
ISO 5597 covers piston and rod seal housings. ISO 7425 covers housings for elastomer-energized PTFE seals.

Common mistakes

Reusing the old wiper or wear rings with new seals.
Cutting a seal on rod threads or sharp port edges during assembly. Use a sleeve or tape the threads.
Installing a rod seal or piston seal backward.
Missing the scored rod or barrel that destroyed the old seals.
Choosing a seal compound that is not compatible with the fluid.

Governing standards: ISO 5597 (piston and rod seal housings); ISO 7425 (housings for elastomer-energized PTFE seals); ISO 6195 (wiper housings); seal maker's catalog.

Reference only. Verify against the current governing code or standard, the manufacturer's data and the project documents before use. Apps for Distributors disclaims liability arising from reliance on this reference.

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