How To Repair Pneumatic Cylinders

Post By: Tom Rowse On: 25-05-2023 Read Time: 6 minutes - Guides - Pneumatics

Pneumatic systems are becoming more common as their applications become more diverse. New technology has allowed the use of compressed air in many more processes, and many different types of air cylinder have been developed.

Along with the original pancake compact cylinder and lighter-duty roundline cylinders, you’ll also find ISO-compliant designs and American NFPA cylinders for fire protection. With so many different cylinder styles and applications, it’s easy to make mistakes in configuration or suffer component failures. That’s when you need to know the basics of identifying failures identifying failures and how to repair pneumatic cylinders.

What Can Go Wrong?

The components with the greatest likelihood of failure in pneumatic cylinders are the seals. To be more specific, this usually means the rod and piston seals. Rod and piston seals are by their nature dynamic. In other words, their normal operation causes them to undergo friction, and friction inevitably causes wear. You might be scrupulous about keeping your seals well-lubricated, but they will wear over time, no matter how careful you are. They won’t disappear altogether, but they’ll begin to show signs of wear which could lead to more significant problems.

The most common problem with pneumatic cylinders is leaking seals leaking seals, which need to be replaced. Sometimes other repairs or modifications may be required, such as replacing other components which are causing the wear. For example, uneven wear across the bushing – if there is one – can be a result of side load forces. These may also cause uneven wear on the piston, which means that you’ll need to replace those items in either case. If the cylinder is misaligned, this may also cause the piston rod to wear unevenly. If you see patches of chrome missing, or perhaps some scratches on the metal, this is a sign that extreme wear of the cylinder bushing has allowed it to come into direct contact with the head.

Diagnosis

Once you’ve determined that a cylinder is malfunctioning, you must inspect it carefully to discover what’s wrong with it. You may know that already, but even when you think you recognise the most obvious symptoms, these may not always identify the only component failures. In most cases, the cylinder will probably look OK at first glance, and the chances are it only needs the seals replacing. You should be able to identify problems like a bypass across the piston seals, or leakage from the rod seal, by means of a quick pressure test.

If you’re going to open the cylinder up anyway, you might as well replace all the seals. Seals for pneumatic cylinders are usually inexpensive, and may well just require a series of O-rings. If you keep O-ring kits in stock and have the necessary tools, it’s a fairly simple matter to replace the seals yourself, rather than paying an outside contractor.

Pneumatic Cylinders

Once you’ve determined that a cylinder is malfunctioning, you must inspect it carefully to discover what’s wrong with it. You may know that already, but even when you think you recognise the most obvious symptoms, these may not always identify the only component failures. In most cases, the cylinder will probably look OK at first glance, and the chances are it only needs the seals replacing. You should be able to identify problems like a bypass across the piston seals, or leakage from the rod seal, by means of a quick pressure test.

If you’re going to open the cylinder up anyway, you might as well replace all the seals. Seals for pneumatic cylinders are usually inexpensive, and may well just require a series of O-rings. If you keep O-ring kits in stock and have the necessary tools, it’s a fairly simple matter to replace the seals yourself, rather than paying an outside contractor.

Once you’ve broken down the cylinder, you can inspect the remaining hard components for wear, looking out for cracks or other physical damage. The dynamic operation of the piston, its barrel, rod and bushing are the most likely areas where you’ll find scoring or damage. You should have wear or guide strips inside the piston, made of a low-friction buffer material intended to protect the moving parts. These do offer a considerable improvement to the life of the cylinder, but of course they, too, will eventually wear out. If that worn-out wear strip no longer separates the barrel from the inside of the piston, these components may wear unevenly.

Bigger Problems

Sometimes it happens that small or cheap cylinders simply aren’t worth taking the time and trouble to repair. For example, if you find that the piston is sliding when it comes in contact with the barrel, then the barrel is likely to need servicing. With luck, you may be able to grind the barrel, so that you can reinstate an ideal, smooth, round finish to its internal diameter (ID).

If the barrel ID is already too worn for grinding to achieve a clean finish, you can chrome it first. This will add a layer of material to the inner surface, which you can grind more finely to return the barrel ID to its original dimensions. Too much grinding without an additional layer will increase the clearance between the barrel ID and the piston, making the seal ineffective.

Another area of wear in replaceable components is the bushing, which supports the rod as well as providing a sacrificial surface. This means that it’s the bushing that wears out rather than the cylinder head, which is far harder to replace. Some manufacturers use a wear strip for this function as well, but you’ll also find bushings made of oil-impregnated bronze. This is designed to lubricate the area as it wears, thus reducing the amount of abrasion.

Finishing Up

You might find some pneumatic cylinders without any form of wear strip or bushing, in which case they’re hardly worth repairing. Most of the wear will be to the piston rod, as it’s exposed to all the environmental damage. This can be from humidity as well as dust and dirt, which will cause physical damage such as corrosion, scratches and excessive wear. The rod is the most frequently replaced component in a cylinder, after its seals.

Once you’ve repaired or replaced any major parts, you can reassemble the cylinder with new seals. You should then test the seals with soapy water, especially around the rod seal, cushion pockets, and end seals if necessary. If you see any bubbles, then you’ve still got a leak, and you’ll have to reinspect the cylinder. Then check the piston seals by applying pressure to one port to see if any air leaks out from the opposing port.

Proactive Maintenance and Lifespan

Some cylinders weren’t ever meant to be repaired, but if you’re constrained by time or costs, you might be able to do something yourself. If you have the ability and can’t wait for – or afford – a replacement, the above guidelines should help. With all pneumatic cylinders, especially heavy-duty cylinders, regular proactive maintenance should provide an almost indefinite life span.