We were recently asked to resolve a fault with cylinder seizures. Our client, a manufacturer of forklift trucks, was experiencing catastrophic problems with a newly designed forklift mast assembly.
The hydraulic mast assembly consisted of four equal area hydraulic displacement cylinders hydraulically connected together by a common supply line. The two outside cylinders provided free lift extending simultaneously until a mechanical stop was reached. The inner cylinders sequenced to lift the combined weight of the carriage and the free lift cylinder assembly.
Faults occurred when an outside free lift cylinder locked in a mid-stroke position. Regardless of how much external force was applied, the seizure could not easily be overcome. After eliminating possible causes such as mechanical failure and fluid line restrictions, we still could not discern the cause of the fault.
We made an informed decision to bring the cylinder back to our workshop. Once on the cylinder stripping bench, we checked the rod and barrel cylindrical surfaces but failed to detect any abnormalities. We attempted, unsuccessfully, to open the cylinder through the circlip fastened gland. Upon closer inspection we found that a high force generated from inside the fluid empty cylinder was acting against the circlip preventing removal.
In an effort to move the rod and possibly release this high internal force, we applied an external force five times the cylinder’s normal operating capacity, but failed to attain any results. It became apparent we had complex phenomenon on or hands.
We drilled a radius of small holes accurately behind the circlip, so that rods could be used to force the clip out of its retaining groove. With the circlip now removed, the seal retaining gland formerly held in place was free to move. Astonishingly, so also was the previously seized rod.
With the cylinder completely stripped and a thorough inspection performed, no visual evidence of the cause of the seizure was found. It was one outcome successfully releasing the seizure… the next challenge was finding the cause and preventing it from happening again.
Because there was clear evidence of an internal force preventing removal of the gland, the first strategy was to find the cause of this force. The force had to be hydraulically generated, as there was no other medium available. This theory was further reinforced, as by releasing the sealing gland the seizure magically disappeared. A detailed analysis of the sealing gland design uncovered the answer.
The original designers had incorporated not one but two dynamic rod seals to prevent leakage. The function of the first rod seal is to protect the second seal from high-pressure shock loads. The second seal prevents any leakage from the cylinder. Both seals were performing their job correctly, albeit a little too well! The shock seal in its design allows small quantities of fluid to pass its sealing surfaces. What it was not doing was allowing this small buildup of fluid back out into the cylinder on the return stroke. The shock seal was acting like a check valve. Over the course of numerous cylinder cycles, this buildup of fluid was slowly accumulating. As operating cycles continued and more fluid entered, the pressure compounded in this small area enclosed by both seals. This phenomenon is known as pressure intensification. The pressure kept intensifying until it was high enough to stop the movement of the rod.
Rectification was as simple as replacing the shock seal with a product that allows the free reverse flow of fluid. In this particular application, a decision was made to redesign the whole gland. All the remaining in service forklifts had the improved assembly installed. No further faults have been reported.