Oil leaks from a hydraulic cylinder: causes and solutions

A leaking cylinder is one of the most common problems in hydraulic machines. The solution seems obvious, replace the seals, but if you do not find the cause that damaged the seal, the leak quickly returns. In this article we look at how to tell the different types of leak apart and which causes are most frequent.

External and internal leaks

External leak: oil escapes from the cylinder and is visible, usually on the rod, on the head or at the fittings. It is the easiest to detect, but also the most worrying for safety and the environment.

Internal leak (bypass): oil passes from one chamber to the other past the piston seal. It cannot be seen from outside, but it can be recognised by its symptoms: the cylinder does not hold the load, drifts down slowly, loses force or moves jerkily.

The most common causes

1. Normal wear

Every seal has a service life. After many working hours the lip wears, loses elasticity and sealing decreases. In this case replacing the seal kit solves the problem.

2. Damaged rod

Scratches, dents, rust spots or flaking chrome act like a file on the sealing lip. A new seal fitted on a damaged rod will start leaking in a short time. Check the rod carefully, also by touch: if the defects are deep, it must be reground and rechromed or replaced.

3. Contamination

Sand, dust and metal particles enter the cylinder when the wiper is worn or missing, or circulate in the system if the filters are not effective. Particles score surfaces and cut seals. Always replace the wiper together with the rod seal and check the condition of the filters.

4. Excessive temperature

Oil that runs too hot accelerates seal ageing: the material hardens, cracks and loses elasticity. Causes can include insufficient cooling, an undersized circuit or very intensive work. If the problem persists, consider materials with better heat resistance.

5. Fluid incompatibility

If the seal material is not suited to the oil used, it can swell, soften or shrink. This happens, for example, when switching to biodegradable or special fluids without checking seal compatibility.

6. Pressure peaks and extrusion

Shocks, sudden loads or badly set relief valves create pressure peaks that push the seal material into the clearance between parts. The edge of the seal looks "nibbled" or frayed. The solution may require back-up rings or profiles better suited to high pressure.

7. Worn guides and side loads

If the guide rings are worn, the rod and piston are no longer centred: the seals are squeezed on one side and wear unevenly. Asymmetric wear is often visible on the rod too.

8. Installation errors

Seals fitted the wrong way round, cut while being pushed over threads or edges, dirty grooves, lack of lubrication: these frequent mistakes cause leaks right after the repair.

How to act

1. Observe before dismantling: where is the oil coming from? Does the cylinder hold the load? Are there noises or irregular movements?
2. Dismantle and examine the old seals: cuts, frayed edges, hardening or swelling tell the story of the failure.
3. Check rod, tube and guides.
4. Replace the whole kit (seals, wiper, guides, O-rings), not only the visibly broken seal.
5. Correct the cause: filters, temperature, pressure, rod protection.

In our catalogue you will find rod and piston seals, wipers, guide rings and O-rings in a huge range of sizes. If examining the old seals does not reveal the cause, send us a few photos: our technical support can help you interpret the wear marks.

Frequently asked questions

Is slight weeping on the rod normal?

A barely perceptible film of oil can be normal, as it lubricates the rod. Dripping or a build-up of oil and dirt on the head indicates a seal that needs checking.

Can I use "stop leak" additives in the oil?

These products work by making the seals swell: they may temporarily reduce the leak, but they do not solve the cause and can alter the material. The lasting solution is replacing the seals.