Rod Seals

What are rod seals?

Rod seals are the seals fitted in the end of a hydraulic or pneumatic cylinder, where the piston rod slides in and out. They stop the pressurised fluid inside the cylinder from leaking out along the rod, while letting the rod move freely. Because they are the last line of defence between the fluid and the outside world, rod seals are critical for preventing leaks, keeping oil off the machine, and protecting the environment.

A typical rod sealing system includes:

• A rod seal, which seals against the moving piston rod and holds back the fluid pressure
• In many designs, an energiser, such as a rubber O-ring, which keeps the sealing element pressed against the rod
• A wiper or scraper, fitted on the outside, which cleans dirt off the rod as it retracts
• Guide rings, which support the rod and keep it centred in the cylinder head.

Rod seals work in a demanding position.

• They seal against a rod that is constantly moving in and out, often at high pressure.
• The rod is exposed to dirt, water, and weather each time it extends, so the rod seal relies on the wiper to keep contamination away.

Rod seals are sized by the piston rod diameter, the groove diameter in the cylinder head, and the groove width.

Why are rod seals essential?

A leaking rod seal is usually the first sign of a worn cylinder, and one of the most visible. Oil escaping along the rod wastes fluid, makes the machine dirty and unsafe, can contaminate the ground, and lets dirt get into the hydraulic system. A good rod seal keeps the fluid in and the system clean, which protects the pump, valves, and the rest of the cylinder.

Rod seals also need to leave a very thin film of oil on the rod as it extends, then draw it back in as it retracts, so the rod is lubricated without oil escaping. Choosing the right seal design and material gives a dry, long-lasting seal.

Typical applications include:

• Excavators, loaders, and other construction machinery
• Tractors and agricultural equipment
• Cranes, tail lifts, and lifting equipment
• Industrial presses and machine tools
• Tipper trucks and mobile hydraulics
• Pneumatic cylinders and actuators
• Forestry and waste handling machinery
• Hydraulic jacks and rams

The main types of rod seals

• U-cup rod seals - Single-acting lip seals with a U-shaped profile. The lips are pushed against the rod by the fluid pressure, giving a tight seal. They are the most widely used rod seal and are often made from polyurethane.
• PTFE rod seals with energiser - A PTFE sealing ring backed by a rubber O-ring. They give very low friction and smooth movement, with no stick-slip, and suit high speeds and wide temperature ranges.
• Buffer seals - Fitted in front of the main rod seal, on the pressure side, to absorb pressure spikes and protect the rod seal. Used in heavy-duty cylinders as part of a tandem sealing system.
• Pneumatic rod seals - Low-friction seals for compressed air cylinders, often combining a sealing lip and wiper in one part.
• Wipers and scrapers - Fitted outside the rod seal to remove dirt, dust, and water from the rod. They are essential for protecting the rod seal and are often replaced at the same time.
• Rod guide rings - Bearing strips fitted in the cylinder head to support the rod and stop it touching the metal, which protects the seals from side loads.

How to choose the right rod seals?

• Hydraulic or pneumatic - Choose hydraulic rod seals for high pressure and oil, and pneumatic rod seals for compressed air and low friction.
• Pressure - Check the seal is rated for the cylinder’s maximum pressure. For heavy-duty cylinders with pressure peaks, consider a buffer seal in front of the rod seal.
• Speed and friction - For high speeds or smooth, precise movement, choose a low-friction PTFE seal. For general use, a polyurethane U-cup is tough and reliable.
• Material - Choose polyurethane for wear resistance and toughness, nitrile for general sealing, PTFE for low friction and wide temperature range, and FKM for high temperatures.
• Environment - In dirty or outdoor conditions, make sure a good wiper is fitted with the rod seal, as contamination is a common cause of rod seal failure.
• Size - Match the rod diameter, the groove diameter in the cylinder head, and the groove width exactly.

Essential references and specialised ranges

How the numbering works:

Rod seals are usually referenced by a profile or type code and their dimensions, in mm:

Rod diameter x groove diameter x groove width

For example, a 40x50x6 rod seal fits a 40mm piston rod, sits in a 50mm diameter groove in the cylinder head, and fits a groove 6mm wide.

Each manufacturer uses its own profile codes to identify the seal design, such as a U-cup, PTFE rod seal, or buffer seal. The profile code is usually shown before or after the dimensions.

Suffixes:

Manufacturers often add codes for the material, such as:

PU or AU = polyurethane

NBR = nitrile rubber

PTFE = polytetrafluoroethylene, often filled with bronze or glass for strength

FKM = fluoroelastomer

Suffix codes vary between manufacturers, so check the supplier’s catalogue for the exact meaning.

FAQ - All you need to know about rod seals

What is the difference between a rod seal and a wiper?

The rod seal holds the fluid pressure inside the cylinder. The wiper sits outside the rod seal and does not hold pressure; its job is to scrape dirt off the rod as it retracts, so contamination does not reach the rod seal.

Why is my cylinder leaking oil along the rod?

The most common causes are a worn or damaged rod seal, a scored or pitted rod, a worn wiper letting dirt in, or worn guide rings letting the rod move off-centre. Check the rod surface as well as the seals when repairing a leaking cylinder.

Should I replace the rod seal and wiper together?

Yes. The wiper protects the rod seal, so if one is worn, the other has usually been affected too. Replacing both, along with any guide rings, gives the best result.

Do rod seals need lubrication?

Hydraulic rod seals are lubricated by the hydraulic oil. Pneumatic rod seals rely on grease applied when fitted, or on lubricated air. Always lightly lubricate a new seal with a compatible fluid or grease before fitting.

How do I fit a rod seal?

Clean the cylinder head groove and check the rod for scratches, pitting, or damage. Lightly lubricate the seal, then fold or ease it into the groove without kinking it, making sure the lips face the pressure side. Protect the seal from threads and sharp edges with a fitting sleeve when sliding the rod through.

How do I identify the seal I need?

Measure the rod diameter, the groove diameter in the cylinder head, and the groove width. Note whether the cylinder is hydraulic or pneumatic, and the type and material of the old seal.

What are the signs that a rod seal has failed?

Warning signs include:

• Oil leaking or dripping along the piston rod
• An oily film or ring left on the rod after it retracts
• Dirt or grit building up around the cylinder head
• Air getting into the hydraulic system, causing spongy movement
• Torn, worn, or hardened seal lips when the cylinder is opened
• Scoring or pitting on the piston rod