A new hydraulic system was installed on an old “Clamp and Drill” machine. A PLC would shift a directional valve clamping the part and a proximity switch would then signal the PLC to advance the air operated drill. At the same time, the clamp directional valve would center, trapping the clamp pressure. When talking to the maintenance man and operator, they said that when they started up the new installation, the electrical controls engineer had the cylinder retracting when it should be extending. When they contacted the engineer, he told them to switch the cylinder hoses and everything should be OK.
After switching the hoses, they found that the pressure reducing valve would only work on the rod side and allow the cap end to build to system pressure, causing damage to the clamped part.
The engineer was told of the new problem, so he switched the wiring to the solenoids and also returned the hoses to their original positions. This now allowed reduced pressure of the cap end and allowed the cylinder to extend when it was signaled to extend.
A different problem developed. The reduced pressure on the cap end of the cylinder, after about 15 seconds, would start to slowly creep up to system pressure.
By Robert Sheaf, CFPAI/AJPP, CFPE, CFPS, CFPECS, CFPMT, CFPMIP, CFPMMH, CFPMIH, CFPMM, CFC Industrial Training