Fluid Power Journal

Blocked Ports and Failing Piston Seals

By Ernie Parker, CFPAI, CFPSD, CFPS, CFPMM, CFPMT, CFPMIP, CFPMMH, CFPMIH, CFPE

You may have seen the brain teaser in your lifetime in which a hydraulic cylinder is halfway extended with a load on it.  Both ports are blocked and the piston seal fails. The question is, What will the rod do with a load on it?  The answer is that, other than a slight compressibility of the oil, it will stay in place. But now the load is supported only by the area of the rod, and the pressure may skyrocket.

This problem is going to be the same scenario, but with air.

Given:   

2” x 8” x 1” cylinder       or          50 mm x 200 mm x 25 mm

Load is 314 lbs.               or          1374 N

Rod is extended upward at 4 inches or 6.35 mm

Both ports are blocked with no leakage

Questions:

What will the cylinder rod do if the piston seal starts to leak internally?

What is the resting pressure inside the cylinder when the piston stops moving?

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