By Sastry Ganti
Edited by Kenneth Korane
Hydraulic cylinders are a common sight on industrial machinery generating large forces. These workhorses routinely run day-in and day-out without notice. But even the best designs eventually succumb to seal wear and cylinders start to leak.
The loss of performance and resulting mess cannot be tolerated for long. Normal procedure is to disassemble the cylinder, replace all the seals and worn elements, and return it to production. Often, a spare can be installed while the original cylinder is rebuilt.
This is fine when cylinders are reasonably sized and can be easily removed for repair, but it is not practical for extremely large cylinders. For example, metalforming machine tools occasionally use such large main cylinders that the cylinder itself forms half the machine. Consider one typical cylinder with a 44-in. bore, 18-in. rod diameter, and 50-in. stroke. Rated at 4,900 psi, it delivers a 3,725-ton push and pulls 3,100 tons. Standing about 12 ft tall (retracted) and forming the base of the machine, it mounts in a deep pit. This hampers external leak detection and maintenance, so the cylinder must be designed for years of service-free operation.
Let's look at potential failure points. A typical cylinder has static seals where the tubular body joins the end plates. These seals may see some roll due to pressure fluctuations and the resulting deformation and relaxation of cylinder components. If well designed and properly installed, they easily outlast other seals.
The cylinder shown in the accompanying graphic has several low-friction piston rings that ride against the cylinder tube ID. Though dynamic seals, they are well lubricated from both directions. This multiple-seal system generally provides long cylinder and seal life.
This leaves the dynamic seal between the piston rod and bushing as the most likely point of premature failure. This seal receives adequate lubrication only on the internal side. Also, if not properly protected on the external side, the seal is vulnerable to damaging dirt and debris. Both factors increase the likelihood of premature failure.
Extremely large cylinders are expensive. For a modest increase in cost, they can be equipped with an additional seal in the pistonrod bushing that acts as a backup and extends the period between rebuilds. This design includes an additional O-ring, a common lube connection between gland chambers and the lube reservoir, and an overflow tank.
When the cylinder operates, the lower seal provides dynamic sealing. The lubricant (which is probably the same type of fluid actuating the cylinder) from the reservoir lubricates the top of the lower seal, both sides of the upper seal, and the O-ring.
The O-ring protects the main seals from external contamination and retains the lubricant. The housing for the O-ring can be designed for speedy maintenance and replacement without the need to disassemble the cylinder. The accompanying graphic shows one possible design for this housing. Maintenance personnel can remove and replace the split O-ring and split retainer plates as necessary.
Also note that oil in the reservoir is at about the same level as the O-ring. In normal operation reservoir oil level should not significantly change, only lowering slowly due to adhesion to the piston rod and eventual escape to other areas.
Eventually, the lower seal will wear out and fail. Fluid from the cylinder will escape past the lower seal into chamber A, and from there, flow into the overflow tank, indicating a major leak. When this occurs, plug the circuit at point P. The upper seal now becomes the main dynamic seal. Thus, the cylinder continues to operate normally, and users can postpone rebuilding the cylinder for a lengthy period.