Safety wiring on manually loaded press station

L

Thread Starter

Lanco Guy

Hey Folks,
We have a light curtain that an operator reaches through to load parts into an assembly fixture. There is a ross safety valve supplying air to the cylinders on the fixture. We want to avoid dumping the safety air valve every time the curtain gets broken. We need to override the curtain by a safe means.

If there was one cylinder, we could simply monitor the cylinder home position with a non-contact safety senor. That sensor would allow air to remain while the curtain is broken. If the curtain is broken and there is a fault that caused the cylinder to move, then the non-contact switch would be opened and the air would immediately dump. Now comes my point where we have many harmful cylinders behind the curtain. In order for air to remain while loading parts, would we really need to safe sense the home position of all those cylinders? Or is there another way?
 
Before you attempt to modify \ override equipment safety features, are you sure that your local Health and Safety regulations permit you to modify equipment that has been certified safe in its original setup?

I believe that any modifications you attempt, have to be certified by your Health and Safety Authority before you get your operators to work on the modified machine.

I have stated all the above, as it seems that you want to kill a safety feature on your machine, and insert some other safety system. Have you evaluated what will be your gains against the risks you are taking?
 
Actually the machine does not exist yet. We manufacture machines. Answers geared towards the question posted would be appreciated.
 
Have you considered single acting spring return cylinders whose fail safe position is the lifted position? That way you can get your protection curtain to simply interrupt the control supply to all your cylinders and they will automatically go to the lifted position. Additional dumping of air is not required.

How is that for an answer geared directly to your question?
 
C

Control_Lurker

> Actually the machine does not exist yet. We manufacture machines.

What you really need is a Failure Mode, Effects, Criticality Analysis (FMECA). See http://en.wikipedia.org/wiki/Failure_mode,_effects,_and_criticality_analysis and many others from your favorite search engine. (Remove any spaces as necessary)

Anyone who gives you a "try this or try that answer" will be thinking of only one or maybe two cases. They will not know your complete problem nor other unsafe conditions it introduces. It is a lot of work to analyze the complete problem in all on, off, failed on, and failed off for every component and operational state. It's called due-diligence. But if this is done correctly AND before you start, it may save your butt in court. If someone gets hurt you can bring out your analysis, explain why it shouldn't have happened, how you thought through that case ahead of time, and what you did to prevent it from happening. It may also save time and help you design the correct product the first time rather than throwing on patches to handle disparate safety cases late in production.

By the way, what does the specification say about safety or air dumping requirements? Let me guess you don't have one, it's not specified, or it conflicts with other requirements.

Hope this helps.
 
Thanks JoJo,

Our engineers are reluctant to go with that type of cylinder in this particular case. They tend to not have the life expectancy we need.
 
B

bob peterson

I don't see a way around what you are currently doing if movement of the cylinders might harm someone.

--
Bob
 
J

James Ingraham

You don't want to go in to court and say, "Well, a random guy on Control.com told us it was okay." So I agree with some of the other posters who are saying you need to really sit down and think about what it is you're trying to accomplish. I will suggest a few things, but I'm not actually advocating a specific plan of action.

1) In no circumstance is one sensor enough. If the cylinder starts to move, and the the sensor is stuck, you have a serious problem. Two sensors (or one safety-rated sensor) would be an absolute minimum.

2) I assume the light curtain is there to make sure someone doesn't get hurt. You apparently have air-powered actuators that are capable of hurting people. I'm in agreement with Bob Peterson on this one; how else are you possibly going to ensure the safety of the operator if you don't cut the air? Even your "detect movement" idea is no good if a stuck cylinder builds up pressure and suddenly breaks free, slamming forward (or backward.)

3) Figure out why cutting the air is a problem. Don't want to waste the cfm? Add a re-circulating tank. You don't want other parts of the machine to move? Add brakes. Your program doesn't recover well? Fix the software.

-James Ingraham
Sage Automation, Inc.
 
Thanks for the chat guys. We will be dumping air via a cat 4 safety valve every time the curtain is broken.
 
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