How do pneumatic cushioning seals work?

As the piston approaches the end position, a cushioning sleeve on the piston rod enters the cushioning seal in the cylinder head. At that moment, the normal exhaust path is closed. The air trapped between the piston and cylinder head can no longer escape freely and is compressed. This back pressure decelerates the piston over the final centimetres of the stroke. A small adjustable passage in the cylinder head allows the piston to reach its final position.

An adjustment screw controls how much air is allowed to pass through and therefore how strongly the cylinder is decelerated. Never close this screw completely, as the piston will then be unable to reach its end position. Pneumatic cushioning seals only work correctly when this passage is properly adjusted.

During the return stroke, the opposite happens. The compressed air pushes the lip of the cushioning seal aside, allowing air to flow immediately past the seal to the full piston area. As a result, the cylinder can start immediately at full speed. Pneumatic cushioning seals therefore work like a non-return valve: they seal in one direction and allow flow in the other.

This characteristic is lost when the seal becomes hardened or cracked. The cylinder will then no longer decelerate correctly and may also start more slowly because the air can no longer find a free passage. Both symptoms can therefore originate from the same component. This explains why a cylinder can sometimes hit hard at the end of its stroke while also starting slowly.

Which pneumatic cushioning seals do you need?

Four important factors should be considered when selecting pneumatic cushioning seals.

1. The cushioning system of your cylinder

There are three ways to decelerate a pneumatic cylinder. With elastic cushioning, a rubber buffer ring absorbs the impact. This is simple and maintenance-free, but it is insufficient for greater dynamic forces. With adjustable pneumatic cushioning, the trapped air acts as a buffer and the exhaust flow is adjusted using a screw. This works in almost every application, including heavy loads and long strokes, although the adjustment requires periodic checking.

With self-adjusting cushioning, the cylinder regulates the cushioning automatically. Pneumatic cushioning seals are used with the second and third methods. If you are unsure which system your cylinder uses, check whether there is an adjustment screw in the cylinder head. If there is, the cylinder uses adjustable pneumatic cushioning.

2. Material

The seal often consists of an elastic sealing section and a harder guiding section. NBR is the standard material for the sealing section and is suitable for the vast majority of applications. Polyurethane offers greater wear resistance and a longer service life at high cycle rates. FKM (Viton) is suitable for high temperatures or aggressive cleaning agents, while EPDM can be used with water and steam. The guiding section is generally made of POM or nylon.

Select pneumatic cushioning seals according to the operating environment and the number of strokes per minute, rather than price alone. A material that is too soft can lose its elasticity, causing the cushioning function to deteriorate. If in doubt, ask us for the relevant resistance data.

3. Mass, speed and cycle time

The heavier the moving mass and the higher the speed, the more energy the cushioning system must absorb. A cylinder moving a lightweight gripper has very different requirements from a cylinder moving a fully loaded pallet. If the machine also performs thousands of strokes per day, service life becomes more important than the initial purchase price.

Provide us with the cylinder dimensions, moving mass and number of strokes per minute. With this information, we can select suitable pneumatic cushioning seals and advise you on the appropriate material. Without this information, the selection becomes guesswork, and the consequences may only become apparent after a few weeks of operation.

4. Recognising wear

Worn pneumatic cushioning seals produce recognisable symptoms. The cylinder may begin to make a noticeable tapping or banging sound at the end of the stroke because the cushioning no longer works. The cylinder may also start slowly because the lip no longer moves aside correctly and the air cannot flow freely.

If the cylinder operates irregularly, first check whether the adjustment screw is still set correctly before replacing the seal. An incorrectly adjusted screw can produce the same symptoms as a worn cushioning seal.

In practice, pneumatic cushioning seals are usually replaced during every cylinder overhaul together with the piston and rod seals. It is also advisable to replace the pneumatic wiper, as this is a relatively inexpensive component that prevents contamination from remaining on the piston rod.

Materials for pneumatic cushioning seals

Pneumatic cushioning seals are mainly available in NBR, PU (polyurethane), FKM (Viton) and EPDM, usually combined with a guiding section made of POM or nylon. Other materials are available on request. When selecting a material, consider the temperature, cleaning agents that come into contact with the machine and the lubricant. If your installation uses lubricated compressed air or lifetime grease lubrication, please provide this information.

Not every elastomer is compatible with every lubricant, and a swollen seal can no longer seal correctly. We supply all of the above versions in common sizes, with non-standard types available at short notice. We are happy to help determine the most suitable combination for your machine.

Determining the dimensions of pneumatic cushioning seals

To determine the correct type, you need three dimensions:

d – the inside diameter, i.e. the diameter of the cushioning sleeve on the piston.

D – the outside diameter, i.e. the diameter of the groove in the cylinder head.

L – the groove width, measured in the longitudinal direction of the cylinder.

If the sealing lip or guiding section extends beyond the groove, also measure the overall height. In addition, note the operating direction because pneumatic cushioning seals only seal on one side and must therefore be installed with the lip facing the correct direction. Take a photograph of the orientation during disassembly so that you can verify the correct position during installation. Measure using a vernier caliper and do not round the dimensions.

Pneumatic cushioning seals are often hardened or deformed after use. If in doubt, measure the groove in the cylinder head rather than the seal itself. If you are working with a standardised cylinder, provide the bore and cylinder type so that we can determine the dimensions from the standard. If you are unsure, send us a photograph together with the measured dimensions and we can help identify the correct type.

Custom-made pneumatic cushioning seals from Wezep

Pneumatic cushioning seals are often cylinder-specific and can therefore disappear from the market more quickly than standard seals. This can be particularly challenging with machines from manufacturers that are no longer in business. In such cases, we can identify an equivalent for you or manufacture custom pneumatic cushioning seals from our range of profiles. Even a single piece is possible. From our location in Wezep, near Zwolle, we supply customers throughout the Netherlands and Belgium.

Are you looking for unusual dimensions or do you have questions about our products? Please feel free to contact us. We are available on weekdays between 08:30 and 17:00.