safety question regarding a pneumatic dump valve

[gravitar 3]

I just finished wiring a small assembly machine for a shop, and they've taken me to task for how I've wired the pneumatic safety dump valve they've provided. The way I have it now, the light curtain, E-stop, and back door interlock switch all interrupt the output power when opened/interrupted, and the dump valve is wired to release all pressure when any of these safeties are violated. They tell me they don't want it that way, but rather to only open the dump valve if the auto cycle were interrupted. They want to leave the lines pressurized otherwise. I told them that's now how I'm accustomed to wiring machines, and that I thought it was a violation of OSHA rules to do it that way. They think I'm nuts. which one of us is right?

[jstolaruk 9]

OSHA requires that the operator never be placed in harms way, so in your case, if any of the pneumatic devices could harm the operator when he is present then the energy needs to be removed. That could include rod-locks for vertical movements to freeze their position so that they can't float down as air exhausts and (slowly) create a crush condition. Is the light curtain there to allow the operator to enter each cycle? OSHA does allow the use of a muting circuit to override the light curtain's ability to drop energy when all of the movements are in the home position. Home position would have to be monitored with safety grade PSD (prox or limit) in order to be control reliable. We do this 98% of the time on our assembly presses so the the hydraulic system's motor isn't constantly cycled and we can meet cycle time. So, I would say that what they are requesting is not unusual and can be accomplished if the home position monitoring safety circuit is added. edit: Home position should be replaced with safe home position. Edited 27 Mar 2007 by jstolaruk

[paulengr 44]

Two different problems. If power is cut off to the pneumatic cylinder actuators, will they still move? Most of the time if you have a double-acting system (where you have to energize an output for each direction instead of a spring-loaded system that requires only a single output), it's not going to move. At that point, anything that would have the potential to harm somebody if it's moving should be disabled (de-energize the output contacts, usually by disconnecting power to the output card's output terminals via a common power wire). If there's a lot of stored potential energy such as a 10 ton machine lumbering down a track or a hoist which could freewheel and drop something, then you need to install a mechanical brake that engages whenever you drop power to the brake to stop the machine from moving. If it is an E-Stop situation, then obviously you cut power to everything ASAP. If the machine is controlling a hydraulic pump or air compressor, that's included (the E-Stop situation may mean somebody removed a guard and somebody got their shirt/hand/whatever caught in the motor shaft). However, as mentioned previously, if something has the potential to be in motion and you'll increase the safety risk, you need either rod locks as previously mentioned, or a pneumatic "hold" circuit which maintains position of the cylinder when the input pressure is removed. The point of an E-Stop is not to remove power. That's a STOP circuit which is entirely different. The point is to stop all machine motion as quickly as possible. The THIRD situation is a lockout/tagout situation. In this case, obviously pneumatic pressure should be released everywhere. This is where those "hold" circuits that I just mentioned have to have a dump valve in order to remove all stored energy. At this point the proper procedure is to remove all energy sources so the act of disconnecting power (not just triggering E-Stops, which are not valid lockout devices) will also release the pneumatic pressure. This may again not be quite the same as a STOP. Simply cutting off power to everything and sending everything to a "zero energy" state is not automatically a safe system. All machines are required to have a STOP by Code. If you read OSHA's regulations (I have), you will find that E-Stops are mentioned in only one small section of a specific piece of equipment and only in passing. OSHA does require lockout/tagout devices and implies that you do everything else safely. The easiest way to demonstrate this is by complying voluntarily with somebody's written code. NFPA's machine safety code explains all this E-Stops vs. Stop stuff and lays out how to do this stuff properly in a fairly clear manner. Plus, it's a heck of a lot shorter to read than some other codes (NEC).