Piston Seals

What are piston seals?

Piston seals are the seals fitted around the piston inside a hydraulic or pneumatic cylinder. They seal the gap between the piston and the cylinder bore, so fluid or air pressure on one side of the piston cannot leak across to the other. This is what lets the cylinder push, pull, and hold a load. Without effective piston seals, a cylinder loses power, creeps under load, or fails to move at all.

A typical piston seal arrangement includes:

• A sealing element, which presses against the cylinder bore to stop fluid passing the piston
• In many designs, an energiser, such as a rubber O-ring, which pushes the sealing element outwards
• Wear rings or guide rings, which keep the piston centred in the bore and stop metal-to-metal contact
• A groove in the piston that holds the seal in place.

Piston seals can be single-acting or double-acting.

• Single-acting seals hold pressure from one side only, and are used in cylinders that are powered in one direction.
• Double-acting seals hold pressure from both sides, for cylinders that are powered in both directions.

Piston seals are sized by the cylinder bore diameter, the groove diameter in the piston, and the groove width.

Why are piston seals essential?

The piston seal is what turns fluid or air pressure into force in a cylinder. A good seal keeps the full pressure on the piston, so the cylinder delivers its rated force and holds its position. A worn or damaged seal lets fluid bypass the piston, so the cylinder becomes slow, weak, or drifts under load, which can be a safety risk in lifting equipment.

Piston seals also have to cope with constant movement, pressure changes, and sometimes dirt and heat. Choosing the right design and material gives a longer-lasting seal, less downtime, and more reliable machinery.

Typical applications include:

• Hydraulic cylinders on construction and agricultural machinery
• Lifting equipment, such as loaders, cranes, and tail lifts
• Industrial presses and machine tools
• Pneumatic cylinders and actuators
• Mobile hydraulics on trucks and trailers
• Injection moulding machines
• Hydraulic jacks and rams
• Material handling equipment

The main types of piston seals

• Single-acting lip seals (U-seals) - Seals with a lip that is pushed against the bore by pressure from one side. They are simple, effective, and widely used in single-acting cylinders, or in pairs back-to-back for double-acting cylinders.
• Double-acting compact seals - Seals that combine a sealing element, energiser, and sometimes guide rings in one assembly, to seal in both directions in a short space.
• PTFE piston seals with energiser - A PTFE sealing ring with a rubber O-ring behind it. PTFE gives low friction and smooth movement with no stick-slip, while the O-ring keeps it pressed against the bore. They suit both directions and higher speeds.
• Polyurethane piston seals - Tough seals with excellent wear and extrusion resistance, widely used in hydraulic cylinders working under high pressure.
• Pneumatic piston seals - Lighter, low-friction seals designed for compressed air cylinders, often with built-in lubrication grooves.
• Wear rings and guide rings - Not seals in themselves, but fitted alongside piston seals to guide the piston and protect the seal from side loads.

How to choose the right piston seals?

• Single or double-acting - Choose a single-acting seal for cylinders powered in one direction, or a double-acting seal for cylinders powered both ways.
• Hydraulic or pneumatic - Hydraulic seals are designed for high pressure and oil, while pneumatic seals are designed for lower pressure, air, and low friction.
• Pressure and speed - Check the seal is rated for the cylinder’s maximum pressure and piston speed. For high pressure, choose a tough material such as polyurethane, or add back-up rings.
• Material - Choose polyurethane for toughness and wear resistance, nitrile for general sealing, PTFE for low friction and wide temperature range, and FKM for high temperatures.
• Size - Match the cylinder bore, the piston groove diameter, and the groove width exactly. The seal must suit the groove dimensions as well as the bore.

Essential references and specialised ranges

How the numbering works:

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

Cylinder bore diameter x groove diameter x groove width

For example, a 50x39x4.2 piston seal fits a 50mm cylinder bore, sits in a 39mm diameter groove in the piston, and fits a groove 4.2mm wide.

Each manufacturer uses its own profile codes to identify the seal design, such as a single-acting lip seal or a double-acting PTFE 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 piston seals

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

A piston seal seals between the piston and the cylinder bore, inside the cylinder. A rod seal seals around the piston rod where it leaves the cylinder, keeping fluid in and dirt out. Both are needed in most hydraulic cylinders.

How do I know if my piston seals are worn?

Common signs are the cylinder drifting or creeping under load, losing speed or power, or moving unevenly. Because piston seals leak internally, there may be no visible leak, so a drop in performance is often the first sign.

Should I replace the wear rings at the same time?

Yes, it is good practice. Worn wear rings let the piston touch the bore, which damages new seals quickly. Replacing them together protects the new seals and the cylinder.

Do piston seals need lubrication?

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

How do I fit piston seals?

Clean the piston groove and the cylinder bore, and remove any burrs or sharp edges. Lightly lubricate the seal and ease it into the groove without overstretching it, using a fitting tool where possible. PTFE seals may need to be warmed or sized after fitting. Guide the piston into the bore carefully, using a lead-in chamfer or fitting sleeve to protect the seal.

How do I identify the seal I need?

Measure the cylinder bore diameter, the groove diameter in the piston, and the groove width. Note whether the cylinder is single or double-acting, hydraulic or pneumatic, and the type and material of the old seal.

What are the signs that a piston seal has failed?

Warning signs include:

• The cylinder drifting or creeping under load
• Loss of speed, power, or force
• Uneven, jerky, or juddering movement
• Hydraulic oil overheating
• Torn, worn, or flattened seals when the cylinder is opened
• Scoring on the cylinder bore