A hydraulic system at a steel mill’s exit end pushes and pulls a sled that moves finished steel coils to the side for unloading. The double-acting cylinder that drives the sled has a 5-in. bore, 120-in. stroke, and 3½-in rod diameter. It has its own hydraulic power unit (HPU): a gear pump driven by a variable-frequency drive and mounted on top of a 30-gal reservoir. The simple circuit also contains a relief valve, directional-control valve, and return-line filter.
The operator of the line complained to maintenance about a strange noise coming from the HPU. Maintenance people found the noise did not occur when the 10-ft.-long cylinder was retracted. However, a loud noise could be heard when cylinder’s piston rod about 7 ft. or more.
The HPU’s oil-level sight gauge was checked, but it showed the level to be comfortably above the low-level mark. Feeling other components wouldn’t cause such a noise, they decided to change the pump—the old “change the pump if you don’t know what’s causing the problem” scenario. Unfortunately, it did not solve the problem. What should they look at next?
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