Best way to stop air cylinder with a light curtain application?

[brengine]

Hello,

What are your thoughts about the best way to stop air cylinder with a light curtain application? Lets say it's mounted vertically and is connected to a couple hundred pound load (i.e. enough to fall and hurt somebody or something).

Pneumatic valve choices and issues that I have heard of but I really don't fully understand the impact of them all:

1) 3-position-center-open.
---->The load will fall when the light curtain is broken.
---->Tooling might slam at the top position once system is re-energized.

2) 3-position-center-open with pilot-operated-check-valve on the bottom port of the cylinder
---->Response time to stop load might not be fast enough for application.
---->Load could drift up.
---->Tooling might slam at the top position once system is re-energized.

3) 3-position-center-blocked or pilot-operated-check-valves at both ends of the cylinder.
---->Response time to stop load might not be fast enough for application.
---->Cylinder might drift
---->Cylinder is locked in position. It will take a crowbar to move it, or you'd have to remove the pilot-operated-check-valves.

4) 3-position-center-pressurized.
---->Load will drift towards the rod end of the cylinder because the rod side of the piston has a smaller surface area, therefore will have less force than the force on the non-rod side of the piston.

5) 3-position-center-pressurized where each side of the cylinder is pressurized with a different pressure. When setup correctly it will balance the load by compensating for the different surfaces areas on the piston (i.e. rod side will need a higher pressure since it has a smaller surface area).
---->Response time very good
---->Complex and could be difficult to plumb.

Just curious what others have to say about this issue.
Thanks,
Ken

 

[skills]

As an addition, you might consider using a double rodded cylinder so the annulus difference is zero and use the pressure centered valve so that the locked state has a net zero force; however, the ONLY real fail-safe method is to deenergise, kinetically and potentially the load by lowering it to the bottom. If you seek to merely stop it in a screen violation, then above will work, assuming the kinetic force doesn't exceed the pressure ratings of fittings, lines, cylinder, and the like. The pressure produced to decelerate the load is applied to system and may easily exceed the system ratings by multiples. This is a tricky situation as the safe response instantaneously and then for a duration are two different outcomes.

Hope this helps.

Randall

http://www.principaldynamics.com

 

[tygerdawg]

I hate to say it, but you actually may consider floating this question in front of a (gasp!) salesman for pneumatic cylinders. A GOOD, OLDER salesman, mind you, not some younger guy just learning the ropes.

I had a sticky design issue some time back, and mentioned it to my local rep for SMC Pneumatics. Because he IS a salesman, he has actually seen hundreds of applications (and then discussed with his Tech Support contacts). He made a wonderful, innovative, elegant recommendation, advised all of the problems I would see and how to overcome them, etc. Problem solved, happy customer.

TygerDawg

 

[automatic2]

I would tend towards hydraulics with an anti fall valve, for real protection. Pnuematics have poor positioning quality, they are best applied to exert a force over a defined range, from point A to point B. Alternatively, your logic could be full retraction or extension on curtain signal. Don't expect to have success anywhere in between using pnuematics.

 

[Barry1961]

The standard way to do it is with a rod lock. It is a spring loaded gripper that is mounted to the front of the air cylinder. You have to supply air pressure to it to move the cylinder.
This is SOP when designing machines for automotive industry.
Whoever you are buying cylinders from should have this as an option, if not you can buy them seperately.

Do a google on cylinder rod lock

Barry1961

 

[brengine]

Barry1961,

As I understand them, rodlocks are used to hold the cylinder in position not to act as a stopping device for the cylinder. Do you know what method is usually used to first stop the cylinder before the rodlock is engaged?

Our current method we use is #5 from my original post above (i.e. 3-position-center-pressurized where each side of the cylinder is pressurized with a different pressure) and a check valve at the bottom of the cylinder.

Thanks all for the input, I will be passing this along to see what others here think of the method's you've recommended.

Ken

 

[Barry1961]

How you need to stop your air cylinder when the light curtian is tripped will depend on a few things. If I remember the OSHA spec is based on a person moving at 60" a second when they come through the curtian. All motion needs to be stopped before they reach the moving part.

Check the OSHA site on that speed and calculate how fast you need to stop.

Some variables it would be nice to know:

How often will this happen per shift/life?
How fast is the load moving?
What is bore and stroke of cylinder? Volume=spongey
What size air lines and distance to valve?

If this is going to be a common occurance you may need to put real brake on it.

Barry1961