use 3 position open center valves for any pinch points, so that losing power causes air supply to be blocked and both cylinder ports dumped to atmosphere. please on these systems where hands may enter. a bypassed safety, and they all get bypassed for testing at least, with two position valves means missing fingers, the engineer says "I designed it to fail open" sadly, the tech had his hand pinned by the retract stroke, hit the safety, and shut off all possibility of freeing himself as he was no helplessly out of reach of the air valve and his pocket knife. it does no good to shut down and dump the air one the meat is minced, dump each dangerous cylinder that is not held from falling due to gravity if you dump it.
3 position, 4 or 5 ports, two solenoids, you lose power, the cylinder relaxes. You want it to extend, you hold on the extend coil in your PLC code and wiring. Retract? Turn off coil A and turn on coil B.
I had to deal with one that was on a swivel turret that meant the safe direction was done with single solenoids and by reversing the air flow through the whole base....That was fine until the main direction valve hung up and the safety mat forced the operator into the drum, so I redesigned it with open center valves, so that if the mat got hit, and it was switched into that side by the turret safety contacts, that relay would drop out, therefore all air got dumped, period, no special reversing manifolds only 3 position plug in Mac Valves. After it was faulted (e-stopepd) and time to reset it, the operator would be required to push it to any of its end positions, and the program would pick up the solenoids to lock it back in position as the limits were met. This was much more gentle than firing it up with cylinders at random mid stroke positions.
You can usually perform adjustments by safely putting the pieces in position, then operating the guard safety device, dump the air, the cylinder relaxes, you get your hands in there to fine tune the reed switch location, and there is no air available on this cylinder now. Nothing is bypassed, it is relaxed and compliant. Push it back and forth and set both switches dead center. You are safe, you met the need to set up equipment without bypassing anything. Some things should not be relaxed, so having the right valves at each cylinder is better than one master dump valve.
