Cylinder Autopsy Reveals Failure Cause

Inside a Hydraulic Cylinder Failure

A hydraulic repair facility in Australia recently shared an experience about a double-acting hydraulic cylinder that their customer said would extend, but not retract.

When the cylinder came into the shop, the piston rod was almost completely retracted into the cylinder tube. Further inspection revealed the rod was stuck in place, and it wouldn’t extend or retract. Reasonable attempts were made to extend the rod using shop equipment, but it wouldn’t budge. The cylinder’s rod-end cap was also immovable and seemed to be seized to in place. When they pumped hydraulic fluid at low pressure into the cap-end port, fluid came out the rod-end port. They suspected the piston rod was bent inside the cylinder, which could explain why it was stuck in place.

Technicians quickly realized the cylinder suffered a serious failure, so they ordered a replacement. The cylinder would become scrap, but the technicians still wanted to find out what happened. They decided it would be interesting to dissect the cylinder to see if they could determine what happened.

After draining fluid from the cylinder and determining that no residual pressure remained, a technician used a grinder to cut a rectangular window into the cylinder tube. This revealed the problem right away. The two-piece piston had come apart, causing it to be wedged against the inside diameter of the cylinder tube. Also, the piston nut was wedged between the piston and end cap.

The customer mentioned that the cylinder had been “rebuilt” by a different repair shop previously. Because piston nut showed no cracks or other failure modes, it likely was not tightened to specifications and simply worked loose over time. It’s also possible that a standard nut was used, rather than one designed as a piston nut. Whatever the cause, a lack of proper training or careless work caused unnecessary expense and downtime to a mining contractor.

By: Alan Hitchcox
After serving as a vehicle mechanic in the US Army, Alan attended college full time to earn a Bachelor of Science while also working full time for a power transmission and fluid power distributor. He then became a technical editor and wrote or edited hundreds of technical articles for 38 years, with the last 32 on Hydraulics & Pneumatics magazine before becoming semi-retired in 2020.