Symmetrical Seals

What are symmetrical seals?

Symmetrical seals are U-shaped lip seals with two sealing lips of equal height, one on the inside and one on the outside. Because both lips are the same, the seal can be used either as a rod seal, sealing on its inner lip, or as a piston seal, sealing on its outer lip. This makes symmetrical seals one of the most versatile and widely used seals in hydraulic and pneumatic cylinders.

A standard symmetrical seal has these main features:

• A U-shaped profile, open on the pressure side
• An inner lip and an outer lip of equal height, one sealing against the rod or bore and the other against the groove
• A solid heel on the closed side, which supports the seal and resists being pushed into the gap
• A one-piece design, usually in polyurethane or nitrile rubber.

The matching lips are what make the seal so flexible.

• Pressure enters the open side of the U and pushes both lips outwards, so the higher the pressure, the tighter the seal.
• Because the inner and outer lips are the same, one seal design can be fitted in either a rod groove or a piston groove.

Symmetrical seals are sized by their inside diameter, outside diameter, and height.

Why are symmetrical seals essential?

Hydraulic and pneumatic cylinders need reliable seals on both the rod and the piston. Symmetrical seals cover both jobs with one simple, cost-effective design, which makes them easy to stock and quick to replace. Their pressure-energised lips give a tight seal under load, while the solid heel helps them resist wear and extrusion.

They are a popular choice for repairs, maintenance, and general-purpose cylinders, where a straightforward, dependable seal is needed without the complexity of multi-part sealing systems.

Typical applications include:

• Hydraulic cylinders on agricultural and construction machinery
• Pneumatic cylinders and actuators
• Hydraulic jacks, rams, and presses
• Lifting equipment and tail lifts
• Mobile hydraulics on trucks and trailers
• Material handling equipment
• General cylinder repairs and maintenance

The main types of symmetrical seals

• Polyurethane symmetrical seals - The most common type, with excellent wear, tear, and extrusion resistance. They suit high-pressure hydraulic cylinders and demanding use.
• Nitrile symmetrical seals - Softer seals with good sealing at lower pressures, often used in pneumatic cylinders and lighter hydraulic applications.
• Fabric-reinforced symmetrical seals - Seals with a fabric layer built in for extra strength and extrusion resistance, used in heavy-duty and high-pressure applications.
• Symmetrical seals with back-up rings - A symmetrical seal fitted with a separate back-up ring behind the heel, to stop the seal being pushed into the gap at very high pressures.

How to choose the right symmetrical seals?

• Rod or piston use - Check the seal suits the position. For rod use, the inside diameter must match the rod. For piston use, the outside diameter must match the cylinder bore.
• Pressure - Choose polyurethane or fabric-reinforced seals for high pressure, and add a back-up ring where pressures are very high or the gap is large.
• Material - Choose polyurethane for toughness and wear resistance, nitrile for softer sealing and lower pressures, and FKM where high temperatures are involved.
• Single-acting - A symmetrical seal holds pressure from one side only. For a double-acting piston, fit two seals back-to-back or choose a double-acting seal.
• Size - Match the inside diameter, outside diameter, and height to the groove exactly.

Essential references and specialised ranges

How the numbering works:

Symmetrical seals are referenced by their dimensions, in mm, often with a profile code:

Inside diameter x outside diameter x height

For example, a 40x50x10 symmetrical seal fits a 40mm rod, or a piston groove with a 40mm groove diameter, in a 50mm bore, and is 10mm high.

The profile code identifies the seal design and is usually shown before or after the dimensions. Each manufacturer uses its own codes.

Suffixes:

Manufacturers often add codes for the material, such as:

PU or AU = polyurethane

NBR = nitrile rubber

FKM = fluoroelastomer

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

FAQ - All you need to know about symmetrical seals

What does symmetrical mean for a seal?

It means the inner and outer sealing lips are the same height. This lets the seal be used as either a rod seal or a piston seal, as both lips seal equally well.

What is the difference between symmetrical and asymmetrical seals?

An asymmetrical seal has one lip shorter than the other and is designed for one position only, either rod or piston, which can give better sealing in that position. A symmetrical seal can be used in either position.

Which way round does a symmetrical seal go?

The open side of the U, between the lips, must face the pressure. Fitting it the wrong way round will cause it to leak.

Can a symmetrical seal work in both directions?

No. It holds pressure from the open side only. For a piston that is powered both ways, fit two seals back-to-back or use a double-acting piston seal.

How do I fit a symmetrical seal?

Clean the groove and remove any burrs or sharp edges. Lightly lubricate the seal, then ease it into the groove without twisting or overstretching it, with the open side facing the pressure. Use a fitting sleeve or lead-in chamfer to protect the lips when assembling the rod or piston.

How do I identify the size I need?

Measure the rod diameter or cylinder bore, the groove diameter, and the groove depth and width. If you have the old seal, measure its inside diameter, outside diameter, and height.

What are the signs that a symmetrical seal has failed?

Warning signs include:

• Oil leaking along the rod, for rod seals
• The cylinder drifting or losing power, for piston seals
• Worn, torn, or flattened lips
• The heel nibbled or squeezed into the gap
• Hardened or cracked material from heat or age