Help! Saftey question.

[Guest fcflores]

We currently have a double sided press that is controlled with one SLC 5/04. Each side of the machine has its own light curtain, E-stop and start button. The left and right press is controlled with one Hydraulic pump. The problem we are having is that we cannot extend and retract both sides at once due to Pump sizing. So today I received an email stating that they would like the operation to run as follows. Left press: 1. load part 2. Operator hits cycle start. 3. left press in motion 4. cycle complete Right press: 1. Load part 2. While the left press in motion the operator will hit the start button and walk away to do another job function. When the start is hit the plc will store the command in the logic until the left side is complete, once the left side is complete the right side will automatically auto start. 3. right press in motion 4. cycle complete My question is, is this allowable programming to store start commands? In addition is it allowable to auto start a machine unattended? Note: I am going to reset start storage bit whenever the light curtain is broke; changed from auto to manual and whenever the E-stop is pressed. Thanks......

[OkiePC 14]

I am NOT a press safety expert, but have been around machines that do what you describe. In addition to the things you have already considered, you may be required to post a sign on the machine stating "WARNING, THIS MACHINE MAY START AUTOMATICALLY" In the logic, you may also want to drop out the auto_start logic after a timeout period in addition to the other things you have... It would be advisable to get an expert safety analysis of the situation done by a professional in that field. JMHO Paul

[paulengr 44]

It's a press. ANSI & OSHA has a lot to say about presses and preventing operators from doing stupid things that cause their appendages to get pressed. Including the fact that an E-Stop at least at the current time HAS to be a hard wired function...ie, it kills power or outputs on the PLC. Just recently, NFPA 86 (the fire codes for burners) has been updated to allow "safety PLC's" so they are the first as far as I know to allow AB's GuardPLC's and such to be used. No "soft" E-Stops...where the PLC releases everything. Unlike all other circuits, you wire up the E-Stop circuit with normally closed contacts wired in series and you are supposed to use "force guided relays" although there is a post on this site that discusses improvements over the force guided relay system using non-forced guided relays. In practice, the PLC should work the same as the hard wired stuff. There are a couple ways to do this. One is that on reset/power-on, you clear any latches (like storing a "START" command). This is standard PLC practice anyways. Then you kill power to the PLC with the E-Stop circuit. The second approach is that you code the E-Stop signal as an input and do the same thing in the PLC code. Here is where the divergence of opinion happens. Personally, there is NO difference. The PLC is picking up an external input state ("E-Stop or Power Fail") and interpreting it in exactly the same way. However, some plants may require one extreme or the other. Finally, I have a suggestion. Put all your "fail safe" type logic in a SINGLE ladder. This one ladder should be fairly short and quite often coded as the ladder that calls all other ladders. This makes it darned clear to everyone involved where the safety code is at and what it does. If you feel the need, password protect this single ladder different from the others (modification privileges on that section are more restricted than the rest). DO NOT do the crap that I found in the plant I'm in now. They just put MCR zones around the whole program which were triggered off an input bit from the E-Stop circuit and didn't hardwire ANYTHING. Not only is this outright illegal by OSHA standards for presses (the application was not a press), but it is really piss poor practice. When the E-Stop is reset, what happens? Yep...everything comes back live immediately as if nothing ever happened...the machine takes off trying to complete it's cycle although you have a huge hardware state ambiguity problem because everything is going from full dead stop to full speed ahead instantly from a possibly unknown state!

[BobLfoot 302]

Paul lists a mouthful so I'll tackle it in sections. Could not agree more. Sounds Like you have a mess and a Big OSHA fine headed your way unless you've begun to correct it Paul. Not to mention you could be personally liable. Good luck and I hope your management takes the problems you've identified as seriously as most of us here would. Finally my $0.02 it is not the start that will be your biggest problem. You need to design your machine in such a way that is has 2 independnet control reliable and ANSI/OSHA compliant safety systems one on each half of the machine. I think this will probably mean redundant pressure relief valves so that when one side is down it cannot develop pressure to move even though the pump is running to move the other side. ROSS makes a nice family of press controls that might be the ticket. Edited 18 Oct 2006 by BobLfoot