By Robert Sheaf, CFPAI/AJPP, CFPE, CFPS, CFPECS, CFPMT, CFPMIP, CFPMMH, CFPMIH, CFPMM, CFC Industrial Training
A forming press had a 3-inch cylinder that increased its extension speed even when no one adjusted the meter-out flow control on the cylinder’s rod side. The customer asked us to check the accumulator’s nitrogen precharge to see if it was low. They knew that lowering the precharge pressure resulted in storing more oil in the accumulator. So they thought the nitrogen gas was leaking off, and they did not have charging equipment with gauges to check it.
We told them they could check it by running the hydraulic pump until the system pressure reached 2,000 psi (138 bar), and then shut the pump down. We told them to slightly open the accumulator valve that dumps oil back to the tank and watch the gauge pressure as it slowly drops off. The pressure reading just before it suddenly drops to 0 psi would be awfully close to the gas precharge pressure. To their surprise, the pressure reading was close to the required 1,000 psi (69 bar).
The customer did not know what to do. Do you?
Increasing extension speed of a cylinder with meter-out flow controls is normally due to a leaking piston seal. Since the area on the rod side is smaller than the cap end, pressures are higher than the cap, causing it to leak past the piston increasing the cylinder’s extension speed. The cylinder needed to be repaired or replaced.