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Hydraulic Rod Seals: Types, Materials, and Selection Guide

Quick Answer: A rod seal is a dynamic hydraulic seal fitted in the cylinder head. It seals the reciprocating piston rod and prevents hydraulic fluid from leaking to the outside. CORASEALS supplies rod seals in NBR, FKM, HNBR, PU, and filled PTFE for mobile, industrial, and heavy duty hydraulic cylinders.

What Is a Rod Seal?

A rod seal is located in the rod gland or cylinder head. The seal lip contacts the moving piston rod. During cylinder extension and retraction, the seal must hold system pressure on the rod side and prevent external leakage. The rod seal also controls the lubrication film on the rod surface. Too much lubrication film causes visible leakage. Too little film causes dry running, high friction, and premature seal wear.

Rod Seal vs Piston Seal

A rod seal seals on the piston rod and the cylinder head. A piston seal seals on the piston body and the cylinder bore. Rod seals are exposed to the external environment and must work with a wiper seal. Piston seals are internal and do not need to block outside contamination. Both seals are dynamic and must handle pressure, speed, and temperature changes.

Common Rod Seal Profiles

Rod seal profiles include U-cups, lip seals, compact seals, V-pack sets, and filled PTFE step seals. Some customers refer to profiles by codes such as UN, UHS, HBY, or KDAS. These codes are not always the same across catalogs. Always confirm groove dimensions, cross section, and material before replacing a rod seal with a different code.

A complete cylinder sealing system often includes a rod seal, a buffer seal, and a wiper seal. The buffer seal protects the rod seal from pressure spikes. The wiper seal blocks dust, dirt, and moisture from reaching the rod seal.

Rod Seal Material Selection

The right rod seal material depends on hydraulic fluid, temperature, speed, and pressure. Common material options are listed below.

• NBR rod seals. Suitable for mineral oil based hydraulic fluids and water glycol. Not suitable for all high temperature applications.
• FKM rod seals. Selected for higher temperature and synthetic fluids. FKM offers better chemical resistance than NBR in many hydraulic systems.
• HNBR rod seals. Used when wear resistance and thermal stability are required.
• PU rod seals. High abrasion resistance for construction equipment, agricultural machinery, and heavy duty cylinders.
• Filled PTFE rod seals. Low friction and high pressure capability. PTFE rod seals are often used with an elastomer energizer.

Material selection must be confirmed with the seal supplier. A compound that works in one hydraulic fluid may fail in another.

How to Measure a Rod Seal

Measure the piston rod diameter first. Then measure the groove diameter and groove width in the cylinder head housing. For an installed seal, remove the seal carefully and measure rod diameter, housing bore, and seal height. Use the same unit system throughout, either mm or inch. Do not convert rounded values.

Common dimensions needed for a rod seal are ID, OD, and height or cross section. For a rod seal, the ID is normally the rod diameter. The OD is the groove or housing bore diameter. The height is the axial width of the seal.

What Causes Rod Seal Leakage?

Rod seal leakage can be caused by a worn rod surface, a damaged chrome layer, incorrect surface finish, contaminated hydraulic oil, high oil temperature, pressure spikes, or incorrect groove dimensions. Installation damage is also common. A new rod seal can leak if the rod has scratches or if the seal lip is cut during assembly.

If a cylinder leaks after replacing the rod seal, inspect the rod surface, the wiper seal, the buffer seal, and the cylinder head groove. Check for contamination and verify the new seal ma