Pneumatic System Troubles

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My first thought is to adjust the cylinders position so that the end of travel of the piston is the stopping point of the process.

Pneumatic cylinders use air, which compresses. It's like a spring. You need a mechanical stop. Maybe not end of travel, but something to positively stop travel at the proper point.

Good luck!

I wish a positive stop was an option. I need it to stop short of the full stroke on the down stroke and up stroke both.

Talk to these guys: basic options - Bimba

It would appear to be residual pressure in the lines that is causing this.

Could you set it up so that the sensor not only closes the air valve, but also opens the exhaust valve? That should remove all pressure from the line and stop the ram immediately.

That is my thoughts as well but I am using a 5port 3 way valve with the center normally closed. I thought when one side opens it automatically exhausts the other.

That may be your problem. I am assuming your valve is a 5 port 3 position valve (not 3 way). When the valve shifts to the center (both ports closed) position, the pressure on one side of the piston is greater than the other so the piston will continue to travel until the pressures equalize. To achieve a more immediate stop you should use a valve with both ports open to exhaust in the center position. However this may lead to other problems as the cylinder is now free floating.

I general, there is no easy inexpensive way to achieve accurate positioning with a pneumatic cylinder without using positive mechanical stops.

One option to consider, that will work well with a PLC, and provide accurate stopping, is to replace the pneumatic cylinder with an electro-mechanical actuator. The price of these devices is becoming very competitive with pneumatic actuators.

Then maybe an adjustable stroke kit is what you need

First good post from the top down!!GA

there is no way to stop the pneumatic in the position rather then fully closed or open ,you have to use a mechanical stopper or the other solution is little expensive to stop the cylinder on any place in between ,that's the pneumatic cylinder with hydraulic brake which can be stopped on any position .

First a remark if you want to control a pneumatic cylinder it is better to use two 3/3 valves which are controled independently one of the other so that you can adapt the fucntion as over-lap or underlap as needed. This is NOT possible with usual 5 positions controls which have either an over or an under lap by design.

As for the problem you mention I would appreciate a better detailed description, step by step, to be able to give you a good counsel. May be you can take pictures and send them.

The sensors send a signal to do something. It might stop the air supply to the cylinder, but it will not stop the piston, the air inside continues to expand. You need some sort of mechanical stop at the end of the desired stroke - as others have said.

There are several ways to achieve this. You need position feedback. The way we have done it is with 4 bang bang valves, a position sensor and a PLC with some simple software.

Any pneumatics company will advise you - there is nothing unusual about it. You can make a 6" stroke cylinder stop in any position with only 1 mm of bounce. and do it 10 times per second if required.

Ask your supplier. If they can't help I can give you the name of one who can.

Precise control is almost if not impossible using a pneumatic cylinder, Hydraulic will do the job.

The position of a pneumatic cylinder can be accurately controlled with a portioning valve and linear feedback. Maybe over kill for this application.

How about an open center valve with a pilot check? When the valve centers the po check should be able to hold position.

Pneumatic cylinders are ordered and sold in stroke lenth and one should actualy buy the correct stroke to make sure that max stroke is your pressure position. The sensor is not to tel your plc to stop the cylinder, it is a sensor to tel the plc that the step is comlete and the program can advance to the next step. The easiest way to adress your problem is to make the opposite (fixed) mounting side adjustable and set it to ensure that maximum stroke will be the position where the pressure in the cilinder will be maintained to complete the next step. Your process sounds as if the presure must be maintained until the crimping process is completed. Good luck, sure you will have somebody to help you with a simple adjustable mounting bracket.

These cylinders are moving some devices to bend the 90 and crimp. Place adjustable stops on those devices.

You will not make a consistent product unless you bottom your pneumatic cylinders against (adjustable) stops. Set your sensors to trigger slightly early and add a delay timer into the PLC to ensure that the cylinder fully completes it's stroke. That way you never get a hang up in the sequence when dirt builds up on the stop and the sensor target fails to reach the sensor. It is easier to adjust a software timer precisely than to move a proximity switch about. Once you have the system running as required optimize it by shaving 0.1s of a second off the timers. That may seem petty but on a 3 second cycle time every 0.1 second saved is equivalent to 1600 extra cycles every week on 8 hour single shift working.

I'm going to throw my 2 cents worth in since this is somewhat my area of expertise. There are a couple of GA's here, and I'm going to confirm one and recommend another solution.

For the sake of simplicity, a pilot operated check valve (some call this a blocking fitting) in each cylinder port is a good idea as it will best minimize the over travel when the valve is de-energized and the cylinder pressures equalize. Used with port flow controls it will give good control of the cylinder. When using this setup, a 3 position valve with ports to exhaust (AB-R) will give good control if using the cylinder lines to also pilot the POCV's.

The other thought that comes to mind is to use a double rod cylinder which will give equal area on each side of the piston, along with a 3 position valve that has the air inlet connected to the ports in the center position, (P-AB) and of course port mounted flow controls.

Hopefully that will make sense. I can expound or diagram if needed.

Your comments are welcome.

My problem with any pneumatic cylinder that isn't operated to a positive stop is that it will never offer repeatability. You still have springs on both sides of the piston, even with center piston, double rod cylinders.

That's just my opinion, based on limited experience.

I agree with your evaluation!

Thank you Lyn. You, as well as several others who mentioned it, are absolutely correct regarding hard stops, there's nothing better. And yes, as others have stated, hydraulic, electric, servo and proportional solutions controlled with a PLC or the like would all be more accurate. But depending on the level of accuracy that he needs, along with his budget, a properly designed pneumatic solution may well get the job done for a few hundred dollars. Granted, we are still short of some details but from what we have learned I am comfortable with it.

Another solution would be to use a multi-position cylinder set-up. It sounds like he must need an intermediate stop between stroke extremes. Again, depends on the demands of the application.

Ideally he should get a good fluid power vendor in to help him. The OP hasn't stated where he is, I would go in myself if he was nearby.

Have a good day Lyn.

Maybe the OP doesn't require the accuracy I am assuming for a bending and crimping operation.

That's anybody's guess.

Thanks for lending your expertise to the discussion.

In my experience Pneumatic positioning is not appropriate unless hard stops are used. They can be external or internal to the cylinder.

Hydraulic can position fairly reliably and hold position.

Electric "cylinders" or actuators can be very effective.

What is the require stroke length and force?

Google 'positioning pneumatic cylinders'. It is commonplace, quick, accurate and we do in in several applications thousands of times per day.

A drawing would be helpful but here goes.

Are you using the forward stroke for one operation and the reverse stroke for the other operation? If so you can try placing rubber stops inside the cylinder on the shaft to control the forward stroke and rubber stops on the exposed shaft for the retraction stroke.

If this is a no go how about sending a sketch?

Link your air piston to a double acting piston filled with oil with both ports connected via a valve in the oil line to open and close from a signal from a sensor at the desired stop. Oil will flow from one end of the piston to the other end and will lock in position when the valve closes. A semi hydraulic solution from an idea by Bongo #12.