Pneumatic actuator system considerations

[KKerr23]

I am fairly new to pneumatic systems so bear with me here. I have designed a system to use an actuator to move different joint arms. I was considering using a single-action cylinder with a ball valve and regulator to activate the cylinder on and off. Since it is only single acting, would I need to provide a release valve in order for the cylinder to go back to its resting position? Or would it be more feasible to use a double-acting cylinder instead?

 

[zdas04]

The answer to that is really application dependent. How much precision do you need in position? The more precision, the more you need double acting. How much control do you need on the return speed? Again, the more control the more you need double acting.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

 

[KKerr23]

Since it is fairly a simple system that will be running, the precision of the motion is not that critical. The main purpose of the cylinder is to provide two positions to the arms... one position at the cylinder's resting position when not in use and one position that will move the arm upward to a general fixed position when in use. The arms are used to hold down a type of clamping mechanism so I was hoping to use some of the force of the cylinders to provide that extra push for the clamp.

 

[zdas04]

Sounds like it might be a good application for single-acting, spring return.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

 

[MintJulep]

Well, for a single-acting cylinder to return to its resting position the are needs to get out somehow.

You could use a vented ball valve it would seem.

 

[mcgyvr]

You need a 3 way valve..

See page 4

http://www.omega.com/auto/pdf/SimpValvesguide.pdf

 

[zdas04]

I always do that kind of stuff (his requirements are very similar to a pneumatic dump valve) with a 3-way valve. The cylinder is either pressurized or vented.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

 

[KKerr23]

Thanks for the options. My main idea was to just T off an exhaust valve from the main line but a 3-way valve should work even better

 

[PNachtwey]

My experience with pneumatics is that they are awfully springy. Also, the ratio of static to dynamic friction better be close to 1.
If the arms have articulated joints, good luck. It will be very difficult to move the arms without oscillations unless the mass is very low.





Peter Nachtwey
Delta Computer Systems

http://www.deltamotion.com

 

[KKerr23]

The joints are fairly simple. Basically it is set up like a clevis pin joint with a bearing between the two arms. The weight of the arms and mechanism itself is very low, only about 2-3 lbs. at best. To help it even out more, instead of a traditional regulator, I put in a hand control valve that could help with the sudden increase of pressure from our main line.

 

[Jboggs]

One of the most common mistakes I see made by pneumatic "newbies" is in the application of flow controls. The "springiness" (as you put it) of pneumatic systems is often due to the incorrect use (or disuse) of flow controls. I strongly recommend that you have flow control valves installed IN THE PORTS of the cylinder (not back at your control valve).

And understand this VERY important fact - you control the speed of a pneumatic cylinder by restricting the flow of the air OUT of the cylinder, not IN. That way you always have the whole cylinder under the higher pressure. I have seen SO MANY technicians try to control the speed of a cylinder by installing a pressure regulator on the feed. That doesn't control the speed. It controls the pressure. Size the pressure regulator to achieve the force you need when stopped. Adjust the flow controls to achieve the speed needed during travel.

 

[Jboggs]

I had some more thoughts I wanted to add a little more to my comments above. I'm assuming you are, as you said, a pneumatic newbie.

First point - my experience has taught me to use single acting cylinders only when I must, not just when I can, for several reasons. There are really no financial or space savings. Single acting cylinders have to be larger than double acting cylinders of the same stroke to accomodate the internal spring. The cost difference between a 3-way control valve and a 4-way valve is insignificant, or sometimes non-existent. I promise you, the end result of your project will be more satisfying with double acting cylinders and 4- or 5-way valves.

Second point - I mentioned installing flow control valves above. Maybe I should clarify that. Needle valves restrict flow in both directions and are not appropriate for this purpose. Flow Control valves have internal checks valves that allow full flow in one direction and restrict the flow in the other direction. That's why they are ideal for use on pneumatic cylinders. They allow the motive force powering the piston to be at full line pressure. They work best when installed directly in the ports of the cylinder.