A company that makes carbon fiber leaf springs for the GM Corvette had to replace the main pilot operated directional valve that was scarred badly by a broken spring from the load check that failed. They replaced both the check valve and directional valve from their stores room and after cycling the press several times to remove the air, maintenance turned it over to production.
The production group reported back that all was working fine other than the press would slowly drift down when they would break for lunch or when they would do a die change. Maintenance replaced the pilot operated check and the directly operated relief. But the press would still drift down. They then blocked the rod side outlet of the cylinder and determined the cylinder piston was not leaking since the drift stopped.
Any idea what could be causing the ram to drift down?
The drifting ram problem was solved when they examined the new valves model number and discovered that one letter was different. It indicated the main spool center position was all ports blocked. When the hydraulic system was idling, both valve ports “A and B” would see normal leakage from the pressure port forcing the pilot operated check to open. They purchased a spool with the original setup, and it stopped the drift.
If you have an all ports blocked center position valve and block the cylinder ports with pressure gages, the gages will build up to half (1/2) of system pressure seen on the “P” port. This pressure will open the pilot operated check allowing the ram to drift.
Robert Sheaf has more than 45 years troubleshooting, training, and consulting in the fluid power field. Email email@example.com or visit his website at www.cfcindustrialtraining.com. Visit fluidpowerjournal.com/figure-it-out to view previous problems.