By Josh Cosford, Contributing Editor
There are some crazy ways a hydraulic cylinder can fail, and I’ve had the opportunity to see many. There was the barrel that split open like the cut above a cage fighter’s eye, or the rod that was folded like a cotter pin when the machine ran into an immovable object. However, these exceptional cases don’t account for many of the failures I see come through the door, so let’s discuss the most common causes of hydraulic cylinder failures. (See an article highlighting pneumatic cylinders here.)
The problem may have started innocently enough, such as with a broken rod thread. But then, when the repair entailed the welding of the clevis directly to the broken rod stub, the problem went from bad to worse. The subsequent misalignment and side load lead to the creation of an ovoid bushing, a worn piston and a scored barrel. So, believe it or not, a common cause of cylinder failure is improper repair.
This is the result of a massive pressure spike in a welded-style cross-tube hydraulic cylinder when the implement impacted a wall. The wall won.
Image courtesy of Higginson Equipment
Good intentions don’t guarantee success, and, understandably, remote working locations don’t have the luxury of a local machine shop or seal supplier. Regardless, without an appropriate diagnosis and subsequent thorough repair, any jury rigging will only result in repeated failure. A cylinder rod with any scoring, pitting or bending should be replaced if a simple re-chroming won’t do the job. Seals are relatively inexpensive, so if the cylinder is already in pieces, replace every single one with the same construction and material. Many cylinders, such as NFPA-style, are nearly infinitely repairable when they’re treated well and serviced correctly.
Even in perfectly moderate operating conditions, hydraulic cylinders will experience failure after a significant service life. A cylinder that reciprocates while enjoying warm, clean oil within an industrial manufacturing environment will still experience seal, wear-strip and bearing failure over hundreds of thousands of cycles. I’ve seen plenty of cylinders dropped off at our shipping dock that the millwright exclaimed have never been off the machine since they commissioned it two decades ago. Although not an exciting failure mode to read about, still, the wheel of time grinds all.
Of course, not all machines enjoy the spa treatment operating environment that exists in semiconductors or aerospace. A hot strip mill, tunnel boring machine or drilling rig has no such luxury of operating and ambient conditions conducive to cool and clean hydraulic oil. The opportunities for contamination are high if maintenance teams aren’t thoroughly familiar with fluid cleanliness programs. Oil analysis, filtration monitoring systems and timely element replacements will prevent the buildup of damaging particles that can prematurely wear hydraulic cylinder seals.
The fourth most common cause of hydraulic cylinder failure is improperly designed or modified hydraulic systems. Attempting to use a medium-duty hydraulic cylinder with high-pressure tool pumps will lead to near-instant failure. Operating a cylinder at velocities higher than the design limitations of the standard seals will also lead to premature failure. Piston and rod seals are manufactured within the limitations of their cross-sectional profile and type of polymer, so check the catalog for the maximum rated velocity. And of course, check the catalog before you try to increase the pressure of your system, as not every seal is rated for high-pressure use.
Although the above examples are the most common causes of hydraulic cylinder failure, with appropriate training, education and maintenance, we can at least reduce the frequency of such failures. I can’t picture lightning strikes overtaking physical damage any time soon, but if we all place appropriate effort into respecting our hydraulic cylinders, old age can wear the crown as the most common cause of cylinder failures.
Filed Under: Components, Cylinders & Actuators, Engineering Basics
