E-Stop

Hi,

There is this E-stop issue, which I'm trying to understand thoroughly. There is a pneumatic cylinder, and when the e-stop is pressed the machine stops but the air was still trapped inside the cylinder because of which it was difficult to move the piston back to the originally position.

I figured out that it's very important to determine the nature of the valve and its actions. For this cylinder, the power stroke is controlled by the single solenoid spring return and the approach stroke is controlled by the double solenoid.

My biggest concern is, when the e-stop was pressed why was the air trapped inside the cylinder.

I want the power to cut off and the piston to return to neutral position. How do I achieve this ?

Edit: removed wrong information

The emergency stop should actually place the machine in a safe state according to the Machine Directive in EU (from what I recall some years ago when a similar issue was handled). Moving parts / closing / open valve after pressing em stop is allowed. Side note...

For emergency stop circuits in the OP case for his info they commonly use a second time relay meaning 2 power feeds one cut off directly and one after a couple of seconds.

Ok, my mistake. Thanks Spaderkung, I'll edit my post to remove that.

To answer the original question then, you may have a couple of options depending on what type of solenoid bank you're using. Does each individual solenoid have its own power connection supplying it, or is the entire bank powered by a single run?

Let's take this one step at a time.

Since we cannot see your pneumatic diagram, we will only be guessing.

the power stroke valve. - Probably an air valve that will dump air pressure to the atmosphere. when you hit the e-stop, can you move any other air cylinder? be sure and put the air cylinder(s) back where you found them.

the approach cylinder. - Sounds like a double solenoid, blocked centered valve. when power is removed, all ports to the valve are blocked and the cylinder will not move.

IMPORTANT !
what does this air cylinder hold in place?
what is the purpose of the air cylinder in general?

the purpose of a valve of this type is to hold the cylinder in place, hold something, prevent movement, AND keep from damaging / destroying something or hurt someone when the air is returned and the valve is told to move.

let's say you replace this valve with an open centered valve and you hit the e-stop. The air cylinder will move freely in both directions with little effort.
let's also say that the cylinder has a 24" stroke and in at midstroke when the e-stop is pressed.

when power is restored and the air is reapplied, the cylinder will still stay in place ( you have not applied power to either side of the valve and the plc outputs should all be off at this point).

WHEN power is applied to either side of this valve, it will take off like a rocket until it hits something, someone, or hits the end of the cylinder stroke. parts can fly in any direction, machine parts can and do break and create flying metal, hands can and DO get smashed, and air cylinder rods bend?

The entire system must be looked at, not just one cylinder!
you MUST walk yourself through the entire machine operation process and figure out what it does in a step by step sequence.
When you get this done, repeat the process and hit the e-stop at each step and re-apply power and air. What can go flying, what can move like a rocket, can someone get their hand in their and what is the result?

this is your risk analysis. design the system and go through a risk analysis with engineering, safety, maintenance, and operators to see if your design is safe to everyone.
if you can ask "WHAT IF" in any situation, you MUST have an answer ! these answers help you design a safer pneumatic system.

I was fortunate enough to jerk my arm out from under a 300 lb. weight when the plant lost power. the weight fell 30" and crushed a car part. someone put an open centered valve on instead of a blocked centered valve. This happened at my customer's site in front of the plant manager. all work stopped and we had to inspect every machine we put in. this was the only valve that was incorrect. A new hire grabbed parts designed for another machine.

I hope this helps.
james

It's a very simple machine. Crimping press machine. It has only one cylinder. When I hit the E-stop, the machine stopped. Since the cylinder was in power stroke, we were trying to move to the original position but we couldn't since the air was trapped inside the cylinder. I don't know why.

I'm not sure if we would need another valve to remove air when the E-stop is pressed. We want the cylinder to release when E-stop is pressed.

Could you post the part number of the cylinder?

ashika said:

It's a very simple machine. Crimping press machine. It has only one cylinder. When I hit the E-stop, the machine stopped. Since the cylinder was in power stroke, we were trying to move to the original position but we couldn't since the air was trapped inside the cylinder. I don't know why.

I'm not sure if we would need another valve to remove air when the E-stop is pressed. We want the cylinder to release when E-stop is pressed.

Click to expand...

An emergency stop is meant to protect life & limb, not to help Fat Frank from maintenance get a stuck part out.

What if someones hand is underneath the press and someone hits the e-stop, sounds like your design would make it fall on their hand.

What you WANT the cylinder to do when you hit the e-stop and what it actually does, may not be in agreement. I know what I would want it to do though.

Now imagine your in the witness stand staring at the man with a missing hand? Sounds dramatic? It does happen. Get help from someone used to designing safety functions.

We use similar valving on our covers which can range from a few hundred pounds to a few thousand.

Basically the power moving the cylinder ( be it air or hydraulics ) has to be supplied to the inlet of the cylinder in order for the powered side of the valve to be open ( I guess a pilot if you will). If the air pressure ( or hydraulic ) is removed, the inlet valve on either side clamps shut. It locks the cylinder in place. This will happen during an "E-Stop" or loss of power.

All our larger machine have a safe start procedure where by all the motioning valves are checked for position before the piloting supply is returned to the inlet valves on the cylinders. The machine will start in a static mode, and any movement has to be made by the operator.

janner_10 said:

An emergency stop is meant to protect life & limb, not to help Fat Frank from maintenance get a stuck part out.

What if someones hand is underneath the press and someone hits the e-stop, sounds like your design would make it fall on their hand.

What you WANT the cylinder to do when you hit the e-stop and what it actually does, may not be in agreement. I know what I would want it to do though.

Now imagine your in the witness stand staring at the man with a missing hand? Sounds dramatic? It does happen. Get help from someone used to designing safety functions.

Click to expand...

To his point, there is a distinct difference between a Stop, and an EMERGENCY Stop. Emergency stopping is intended to place a machine into a SAFE condition, convenience has zero consideration in that process. If your operators are using the E-Stop as a normal Stop command simply because the button is easier to hit, you have a training problem.

What you might consider with regard to your cylinder position issue however is a keyed switch that retracts the cycling at AFTER an E-Stop and only AFTER a full operational safety check can take place. Give that key to select TRAINED safety officers.

you said the magic words "CRIMPING PRESS" !!!!

since I do not know the machine design, or anything else related to the machine, I strongly urge that you get with everyone and do a risk assessment.


May I suggest the following.
install manual controls that will allow you to manually retract the cylinder to the home position.

regards,
james

jraef said:

To his point, there is a distinct difference between a Stop, and an EMERGENCY Stop. Emergency stopping is intended to place a machine into a SAFE condition, convenience has zero consideration in that process. If your operators are using the E-Stop as a normal Stop command simply because the button is easier to hit, you have a training problem.

What you might consider with regard to your cylinder position issue however is a keyed switch that retracts the cycling at AFTER an E-Stop and only AFTER a full operational safety check can take place. Give that key to select TRAINED safety officers.

Click to expand...

It would be difficult to improve on this answer.

Props to the original poster for reaching out to figure out the correct solution. He needs to follow thru with getting this fixed correctly by people who are knowledgeable.

To stop motion and relieve pressure, you need a 3 Position valve. When power to the valve is dropped, the valve will spring center to the middle position which will vent cylinder lines to atmosphere to relieve any trapped air.

This is really a design question. If it's a purchased machine, I'd say reach out to OEM and verify this is correct operation. If it is, then what is the process to "reset" after an e-stop. It honestly sounds like this is what you want it to do in the event of an e-stop, and there's probably a bypass valve or dump procedure to make it easier to put back into service.