A local chip maker called because their tortilla press would run fine for awhile, but then during the middle of a cycle the press would come down and not return to the up position. Cycling power to the machine it would resume operation sometimes for hours sometimes minutes.
The lead CSI inspected the PLC code, which confirmed that the UP solenoid was energized. Then, he ruled out hydraulic failure. Upon further investigation the CSI found a third solenoid labeled BYPASS. Neither the client nor the factory were able to provide hydraulic system prints, so the CSI performed a detailed inspection of the PLC code and discovered that BYPASS must be energized during UP or DOWN movement. During the stall the PLC output light was on — but there was no power to the BYPASS solenoid.
The PLC relay output had failed. The relay contacts inside the PLC had failed. Further investigation revealed this was not the first time.
The CSI revised the machine design, installed a PLC triac output card and remote-mounted relays to handle the load of the hydraulic solenoids. Now, if a failure does occur the client replaces a $10 relay instead of a $500 output card.