Wiper Seals

What are wiper seals?

Wiper seals, also called scrapers or dust seals, are fitted at the very outside of a hydraulic or pneumatic cylinder, where the piston rod enters the cylinder head. As the rod retracts, the wiper scrapes off dirt, dust, mud, water, and grit, so they cannot be dragged into the cylinder. They do not hold the fluid pressure themselves, but they protect the rod seal and the hydraulic system from the contamination that causes most seal failures.

A standard wiper seal has these main features:

• A wiping lip, which presses against the rod and scrapes it clean as it retracts
• A body that locates the wiper in the cylinder head, either by snapping into a groove or by being pressed into a bore
• Often a metal case or reinforcement, for a secure fit in the housing
• On some designs, a second inner lip to help keep the oil film in.

The wiping lip faces outwards.

• It points away from the cylinder, so it scrapes contamination off the rod rather than pushing it in.
• It must keep close contact with the rod to remove fine dust and grit, while letting the thin oil film on the rod return into the cylinder.

Wiper seals are sized by the rod diameter, the housing diameter, and the height of the wiper.

Why are wiper seals essential?

Contamination is one of the biggest causes of hydraulic failure. Every time a cylinder rod extends, it is exposed to dirt, dust, mud, and water. Without a good wiper, this contamination is pulled back into the cylinder when the rod retracts, where it scores the rod, damages the rod seal, and pollutes the hydraulic oil, leading to leaks and wear throughout the system.

A wiper seal is a small, low-cost part, but it protects much more expensive components. Fitting the right wiper, and replacing it when it wears, extends the life of the rod seal, the rod, and the whole hydraulic system.

Typical applications include:

• Excavators, loaders, and dumpers
• Tractors and agricultural machinery
• Forestry and waste handling equipment
• Cranes and lifting equipment
• Tipper trucks and mobile hydraulics
• Industrial presses and machine tools
• Pneumatic cylinders in dusty environments
• Mining and quarrying machinery

The main types of wiper seals

• Snap-in wiper seals - Rubber or polyurethane wipers that snap into a groove in the cylinder head. They are easy to fit and replace, and are widely used in mobile and industrial hydraulics.
• Press-in metal-cased wiper seals - Wipers with a metal outer case that are pressed into a bore in the cylinder head, like an oil seal. The metal case gives a firm, secure fit, ideal for harsh conditions and heavy-duty machinery.
• Double-lip wiper seals - Wipers with an outer lip to remove dirt and an inner lip to help keep the oil film inside the cylinder. They give extra protection and are often used in tandem with the rod seal.
• PTFE wiper seals - A PTFE wiping ring with a rubber O-ring energiser, giving low friction and good scraping performance, often used with PTFE rod seals.
• Pneumatic wiper seals - Lighter wipers designed for air cylinders, sometimes combined with the rod seal in one part.

How to choose the right wiper seals?

• Housing design - Choose a snap-in wiper if the cylinder head has a groove, or a press-in metal-cased wiper if it has a plain bore.
• Conditions - For very dirty, muddy, or abrasive environments, choose a tough polyurethane or metal-cased wiper. For extra protection, choose a double-lip design.
• Matching the rod seal - Choose a wiper that suits the rod seal it protects, such as a PTFE wiper with a PTFE rod seal.
• Material - Choose polyurethane for wear and abrasion resistance, nitrile for general use, and FKM for higher temperatures.
• Size - Match the rod diameter, the housing or groove diameter, and the height of the wiper exactly.

Essential references and specialised ranges

How the numbering works:

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

Rod diameter x housing diameter x height

For example, a 40x48x7 wiper seal fits a 40mm rod, sits in a 48mm diameter housing, and is 7mm high.

Some wipers show two height figures, one for the part that sits in the housing and one for the overall height including the lip.

The profile code identifies the wiper design, such as snap-in, metal-cased, or double-lip. Each manufacturer uses its own codes.

Suffixes:

Manufacturers often add codes for the material, such as:

PU or AU = polyurethane

NBR = nitrile rubber

PTFE = polytetrafluoroethylene

FKM = fluoroelastomer

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

FAQ - All you need to know about wiper seals

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

A rod seal holds the fluid pressure inside the cylinder. A wiper seal sits outside it and does not hold pressure. Its job is to scrape contamination off the rod so it cannot reach the rod seal.

Do I need a wiper seal on every cylinder?

Almost always. Any cylinder with a rod that extends into a dirty, dusty, or wet environment needs a wiper to protect the rod seal and the hydraulic oil.

Should I replace the wiper and rod seal together?

Yes, it is good practice. A worn wiper lets dirt reach the rod seal, so by the time a wiper is worn, the rod seal has often been affected too.

Which way round does a wiper seal go?

The wiping lip must face outwards, away from the cylinder, so it scrapes dirt off the rod as it retracts. Fitting it the wrong way round will push dirt into the cylinder.

How do I fit a wiper seal?

Clean the groove or housing bore and check the rod for damage. For snap-in wipers, ease the wiper into the groove by hand, with the lip facing outwards. For metal-cased wipers, press the wiper squarely into the bore with a suitable fitting tool. Protect the lip from threads and sharp edges when fitting the rod.

How do I identify the wiper I need?

Measure the rod diameter, the groove or housing diameter, and the groove depth or housing height. Note whether the old wiper snaps into a groove or is pressed into a bore, and whether it has a metal case.

What are the signs that a wiper seal needs replacing?

Warning signs include:

• Dirt, dust, or grit building up on the rod and around the cylinder head
• A torn, worn, or missing wiping lip
• Scoring or scratches appearing on the piston rod
• Oil leaking from the rod seal behind it
• Dirty or contaminated hydraulic oil
• A wiper that has worked loose from its groove or housing