Do you lift a full bucket, release the lever, and find the machine arm slowly but noticeably sinking under its own weight? Or is there constant oil weeping around the rod exit, staining the entire machine and creating dangerous puddles on the construction site? These are some of the most frustrating failures in fluid power. Experts at BRhydraulic explain why cylinders lose their seal, how to distinguish an internal leak from an external one, and why replacing only the seals is sometimes a waste of money.
1. Internal leakage: Why does the machine "lose" its load?
If an excavator or loader arm sinks, but there isn't a drop of oil visible on the outside of the cylinder, you are dealing with an internal leak. In this case, the culprit is worn-out piston seals (the component operating deep inside the cylinder tube).
In a functioning cylinder, oil pumped under the piston lifts it and is held tightly in place by a valve or control valve. When piston seals become worn, oil slowly forces its way from the bottom chamber to the top chamber under the load's weight. As a result, the cylinder retracts on its own, and the machine loses its working position.
(Note: The arm sinking can also be the fault of a worn hydraulic lock or a scratched spool in the control valve, which we described in previous articles. It is worth diagnosing this before dismantling the cylinder).
2. External leakage and the "weeping" cylinder
An entirely different fault is a situation where oil spills out at the point where the shiny piston rod exits the cylinder. Gland seals (including wipers and rod seals) are responsible for maintaining a seal at this point. Their job is to keep oil inside under immense pressure and, just as importantly, to keep dirt from entering from the outside.
Their natural wear is one thing, but sudden and intense leaks from the gland are rarely accidental. Most often, they result from physical damage to the piston rod itself.
3. Seal killers: Scratches, impacts, and dried mud
An ideally smooth, chrome-plated piston rod surface is a prerequisite for maintaining a seal. Every scratch, chrome chip, or dent from an impact by a stone or machine structural element acts like a sharp blade.
When such a scratched rod retracts into the cylinder, it literally "cuts" the lip of the rubber seal. Another enemy is dried concrete residue, ice, or hardened mud. If the outer wiper ring is old and perished, hard dirt enters the gland, destroying the main pressure seal in just a few work cycles.
4. Regeneration or a new cylinder?
Many operators try to save a leaking cylinder by buying a cheap repair kit (a set of seals) and replacing it in their garage. This is a great solution, but only under one condition: the piston rod and cylinder bore must be in perfect condition.
If you install new seals on a scratched or bent rod, the cylinder will start leaking again after a few days, or even hours, of operation. A bent rod also wears the gland on one side, which ultimately destroys the entire cylinder. In such cases, the only cost-effective solution is replacing the entire assembly.
Keeping extended piston rods clean and avoiding mechanical damage is the best insurance policy for your machine. However, if your cylinder has suffered irreversible damage, you will find heavy-duty cylinders of various diameters at the BRhydraulic.eu store (including powerful models with a 140 mm piston) that will withstand work in the harshest conditions.
