We have a 1986 Morton Mill with fanuc servos and drives. (30M type). The Z axis is now sinking every time the power is turned off or the E-Stop button is pressed. (Used to happen occasionally, now every time.) This happens even if the machine has been sitting at the same Z height for a long time. The machine has a tracing head with a dead weight counterbalance, a hydraulic counterbalance, and it would seem a Z brake on the servo motor. (At least the controller has a Z brake relay and the relay is powered and working.) I can still see the dead weight so I believe I can rule that out but I don't know whether the Z brake is never engaging (stuck open?) or whether the hydraulic CB leaks? Does it seem like the Z brake should be able to hold the head in position whether the hydraulic CB is there or not? I recognize any sort of valve that would hold the pressure on the CB w/o the pump running... Actually when the unit is sinking I can see MORE pressure on the CB than when the machine was running. It runs at about 400psi when running and goes up to about 550psi when it's sinking. Until it hits something, then the pressure bleeds off...
I really would like any input that people could provide on how it would be best to proceed in diagnosing this problem. Thanks.