Pneumatic Safety.

I'm looking for some input on a problem I'm having with some long air cylinders and two hand safety.

Right now I'm using a two hand safety relay, and corresponding pushbuttons. I am energizing a two position detented air valve to shift an actuator back and forth. The power to the air solenoid coils goes through the contacts of the safety relay.

Problem is, with a detented valve, once the valve shifts into position, the clyinder will remain moving even if the safety relay is shut off (operator lets go of the pushbuttons). I could use a 3 position exhaust center valve, but then once the motion is done and it is safe for the operator to let go of the buttons, the air pressure is no longer holding the actuator in position.

Is there a solution that doesn't involve a complicated (expensive) safety relay system with bypasses and safety switches?

Basically the air actuator needs to move only when the two hand safety is pressed, but needs power to hold the actuator in position once its switch is made, like a magnetic band cylinder switch on the extended position of an air cylinder.

3-position pressure center valve will keep pressure on both sides of the cylinder when stopped in mid-stroke.

My preference for double acting cylinders + safety concerns (where gravity is not the enemy) is a 3 position open center valve. If both coils are off, the actuator can be moved back and forth by hand. The cylinder will relax, both cylinder ports being dumped out the exhaust port of the valve.

The pressure center valve may offer other safety concerns, and in practice, they tend to drift toward the rod end (no such thing as perfect piston seals).

Okay, I now see where you covered that option.

So, what about using the two position valve, plus an auxillary dump valve. The two hand safety keeps the dump valve closed, and in parallel, is held closed by one of the cylinder end limits so it doesn't dump when in position. The dump valve would be in series with the supply to the directional control valve such that when energized, air flows into the dcv(s), and when de-energized, the supply is blocked, and the supply port on the valves(s) gets dumped to atmosphere.

To use the dump valve idea or a 3 position open center in your case, I think the type of switches/relays that qualify, is going to depend on the risk assessment.

There are probably other alternatives. . .

Is it possible to use the two hand relay to reset a safety circuit, and leave it reset until some other barrier (think light curtain) drops it back out?

It's a bit hard to give ideas without "being there" in cases like these.

i would use a 3 position blocked centered valve. when you remove power, the cylinder basically stops. it will resume a smooth motion when power is reapplied.

you might also condider a double sol. spring return with the pressure port goint to the ab ports when the power is removed.

regards,
james

if the cylinder is on a vertical position, and you use an open center valve, you need to use a block valve to stop the cylinder from falling.

Another solution we use in our machines is to use a lower pressura in the cylinders (when you can of course), and that is normally accepted by the safety people

How about a Three position closed center valve and a Dual check valve. With use the Aladco 602501 Duel-Check any time safety is a concern.

sounds like possibly a holding fixture. As your aware, any pressure in the system is potentially dangerous until vented, so procedures are adapted accordingly.

Your idea of a seal in is probably the best approach, where a limit is used to seal in, or maybe a pressure sensor on the 'apply' line to cause a seal in. A 3 position, spring return valve used in conjunction with a holding circuit, would leave the operator with two handed operation to apply, but the circuit is sealed in once it reaches 'holding'.

I think I'll look into OkiePC's idea of the auxiliary dump valve being energized by the limit switch position.

Many of our systems with this problem have been solved by using a safety light curtain and a dump valve. But a few smaller machines, the cost of a safety curtain is prohibiting this setup. We try to make our air cylinders have minimal stroke (usually less than half an inch) to avoid issues with using detented valves. But there are a few that do not have this luxury.

I have had to deal with this situation for a number of years and never realy found a perfect solution. When you use a closed center valve you will get drift maybe not right away but as time goes by it will happen. When you use the open center valve you can have a problem with slaming when you energize the solinoids. Unless you shift the valve one way before the other. What we do here is use the open center valve with the aladco pilot operated check valve. Its safe but its not perfect. You might also consider a rod lock on the cylinders to stop rod travel where ever it stops. thats just my two cents.

remember the principles of pneumatics before u dump the air.
Speed is controlled by exhaust air pressure
Dump the air and you have no speed controll

If you are using a traditional air cylinder, the best method is to fit it with a brake. some manufacturers offer this option.
This is may be a possible retrofit. (add 100mm)
If you are using a rodless cylinder - then 2x 3/2 solenoid valves are better - usually used in conjunction with pilot operated check valves.
the other retrofit option is a free flowing Hydraylic cylinder in parralel - simply close the flow to stop.

some Ideas - hope they help

TOMC said:

I have had to deal with this situation for a number of years and never realy found a perfect solution. When you use a closed center valve you will get drift maybe not right away but as time goes by it will happen. When you use the open center valve you can have a problem with slaming when you energize the solinoids. Unless you shift the valve one way before the other. What we do here is use the open center valve with the aladco pilot operated check valve. Its safe but its not perfect. You might also consider a rod lock on the cylinders to stop rod travel where ever it stops. thats just my two cents.

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I like the MAC 82 and 92 series manifolds and valves for this "slam" situation. Use a dual pressure base and valve to choose the pressure delivered to the adjacent base(s) with sub base regulators, very compact. The low pressure setting was usually set a to "snap" a little bit, but not bang, because, set too low, with wear and tear and dirt and grime, the system would stall or stick and timeout before hitting the limit. The tech would bump it up five pounds, next thing you know it's on 50!

You may need to add extra regulated capacity to the low pressure port if the cylinder volume is very large to act as an accumulator if the valves on a small manifold are firing big cylinders and they need to move fast. We did it with one big piece of hose on one application. You can easily build a compact valve system that will let you choose low pressure select for starting following the e-stop to end the slams. It requires external pilot source, and their part numbers are bit difficult to sort out at times, but the valves are so configurable.

Also, lots of valve/pneumatics manufactureres offer F.L.R. (filter/regulator/lubricators) with solenoid dump valves and pressure controlled restarts all in the FLR...You just run a pair of wires to it and keep them "hot" when it's safe.

Also, lots of valve/pneumatics manufactureres offer F.L.R. (filter/regulator/lubricators) with solenoid dump valves and pressure controlled restarts all in the FLR...You just run a pair of wires to it and keep them "hot" when it's safe.

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These dumpvalves may not last long if cycled with every machine cycle. I have checked some dumpvalves and they would only last a couple months.

If a seperate valve is used to dump the air pressure, means to monitor the valve may be required. The main spool may get stuck.

AJZ said:

These dumpvalves may not last long if cycled with every machine cycle. I have checked some dumpvalves and they would only last a couple months.

Click to expand...

Good point I had not thought of.

If a seperate valve is used to dump the air pressure, means to monitor the valve may be required. The main spool may get stuck.

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In the olden days, we used regular old ASCO red hat diaphragm style 3-way valves electrically held to supply air, and dump when the pwoer shuts off. We never had any problems with them as a safety dump valve. If they leaked or didn't open the machine couldn't run. Never saw one "stick" open either.

Of course nowadays, the lawyers and code writers have to get paid too.

Looks like ASCO is into some safety stuff now too:
http://www.ascovalve.com/Applications/Products/Functional-Safety-Solutions.aspx