How does this device function (pneumatic rotary actuator)?

I have a question about how these pneumatic rotary actuators work. First off, you're only seeing about half of the unit, which is what looks like two cylinder bodies and a rotary axis in the middle. I'm fairly certain they can only travel (i.e. rotate) for a maximum number of degrees (based on the length of the stroke of the cylinders on the side), is that correct? After they reach (or partially reach) their end of stroke, I'm assuming they need to "back - up" before they can advance. When they do that, does the rotary shaft turn in reverse as well?

We're having some trouble with this thing, builder is long gone, hard to get access into it so just trying to get a better understanding of how it operates.

Thanks,
The Dude

The Dude said:

I have a question about how these pneumatic rotary actuators work. First off, you're only seeing about half of the unit, which is what looks like two cylinder bodies and a rotary axis in the middle. I'm fairly certain they can only travel (i.e. rotate) for a maximum number of degrees (based on the length of the stroke of the cylinders on the side), is that correct? After they reach (or partially reach) their end of stroke, I'm assuming they need to "back - up" before they can advance. When they do that, does the rotary shaft turn in reverse as well?


We're having some trouble with this thing, builder is long gone, hard to get access into it so just trying to get a better understanding of how it operates.

Thanks,
The Dude

View attachment 136390

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usually a gear and a gear rack connected to air cylinder. after time the air cylinder can get worn and experience blow by. that is the air leaks past piston ring orings more in certain heavily worn spots

Okay, thanks. I don't think that's the issue we're having at the moment. I think we're just simply running out of stroke. I'm hopefully going to take the things apart soon and check it out more. I still would like to understand better what happens at the end of stroke; I'm assuming it has to reverse rotation before you can "advance" again, correct?

The Dude

A common construction for these two cylinder and rotary axis devices uses a rack and pinion to drive the rotary axis. Clearly the number of teeth on the pinion with respect to the number on the rack defines the actual rotational travel. Up to, and even beyond 360° rotation is possible but the device becomes rather long. Probably units offering up to 60° or thereabouts are most common. There are several sources of off the shelf units in a range of rotation angles.

The simplest construction is clear a cylinder at each end with a rack in the middle but the device can be nearly halved in size by using two racks, one each side of the pinion, and a pair of side by side pistons. Depending on the valving and / or spring return arrangements devices can be either double or single acting so there is plenty of scope for complication. As ever with pneumatic power devices some thought needs to be given to endure safe failure mode should the air supply be lost od of insufficient pressure.

If you have intemittant faults on a device that used to work OK take a serious look at the fail safe arrangements. Even if the fault lies elsewhere understanding how the beast is supposed to behave when things go wrong gives useful insight into how its supposed to work in the first place and is generally easier to analyse.

Clive

I don't know if they have an internal pilot-check operation to prevent reverse rotation, etc, or that all up to the integrator to do on the exterior. It could be something like a nut holding the piston rod came unscrewed, or something wrong in the cushion at the end of the cylinder.

The Dude said:

Okay, thanks. I don't think that's the issue we're having at the moment. I think we're just simply running out of stroke. I'm hopefully going to take the things apart soon and check it out more. I still would like to understand better what happens at the end of stroke; I'm assuming it has to reverse rotation before you can "advance" again, correct?

The Dude

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it is usually a double side or 2 port cylinder. air goes in one end and out the other side and it is reversed to go the other way.
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it goes with out saying air goes in one side and have to be allowed to come out of other cylinder port
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blow by where air leaks internally can cause loss of power and stoppage in certain stroke locations. sometimes a rebuild kit where orings are changed with news ones solves the blowby problem

This should generally give you an idea of internals . . . Google is your friend in addition to being your overlord

With the long cylinders, it could be a spring return to center. Or since the OP is not showing a view of the entire unit, who knows what it is for certain.

Maybe you just need a bigger actuator. We can go up to almost 40K ft lbs of torque. http://www2.emersonprocess.com/site... Documents/Bettis/Brochure/DOC.4P.BRPE.US.pdf
JR

JRIowa said:

With the long cylinders, it could be a spring return to center. Or since the OP is not showing a view of the entire unit, who knows what it is for certain.

Maybe you just need a bigger actuator. We can go up to almost 40K ft lbs of torque. http://www2.emersonprocess.com/site... Documents/Bettis/Brochure/DOC.4P.BRPE.US.pdf
JR

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That would be fine if he needs a NAMUR valve actuator.
Most pneumatics folks make these actuators.
I like Festo best for longevity.

Thanks all. Motion, your basic diagram is exactly how I thought this operated but this one apparently only goes in one direction (there's a one-way clutch that doesn't allow the rotary axis to reverse direction, just the rack reverses). There's other things I'm finding out here that make me think this is just a screwed up design. It's driving a release liner for "stickers" (i.e. very slick) with a knurled shaft against two smooth steel idlers (i.e. not good grip, lots of slippage). Time for more investigation. Personally, I would have designed this with a simple air cylinder and a pawl to pick-up up an indexing hole on the tap and increment the liner forward. That might yet happen.

The Dude

Misumi urethane coated rolls work great in an S-Wrap configuration for pulling wax backed paper when peeling adhesive labels . . .

dude, i have seen them with a similar setup to yours, guts as per motion gurus pic, but the cog has a one way bearing in it, shaft had a one way bearing also. net result each stroke created x amount of motion, one i was around had a stop screw to adjust this, was also only a single acting cylinder return spring in the other cylinder.

One i worked on needed new sprag - one way bearings, the sprag rollers had got nasty flat spots after a few million cycles and would slip occasionally stuffing up the feed - losing the index, shaft they ran on was surprisingly ok though, new clutches and it was all happy again. Replacment unit was silly money from the OEM and the clutches + my time was truly peanuts in comparison.

From memory, i just undone the tie rods on the cylinder, and slid the cylinders off, removed piston one end, pulled out rack then removed the through shaft, no real magic, all pretty easy. Reassembly was just the reverse, i replaced all the orings as i rebuilt. Hardest part was restteting the stroke just right in the machine, surprising how fractions of a mm add up over a few hundred strokes.

adama said:

dude, i have seen them with a similar setup to yours, guts as per motion gurus pic, but the cog has a one way bearing in it, shaft had a one way bearing also. net result each stroke created x amount of motion, one i was around had a stop screw to adjust this, was also only a single acting cylinder return spring in the other cylinder.

One i worked on needed new sprag - one way bearings, the sprag rollers had got nasty flat spots after a few million cycles and would slip occasionally stuffing up the feed - losing the index, shaft they ran on was surprisingly ok though, new clutches and it was all happy again. Replacment unit was silly money from the OEM and the clutches + my time was truly peanuts in comparison.

From memory, i just undone the tie rods on the cylinder, and slid the cylinders off, removed piston one end, pulled out rack then removed the through shaft, no real magic, all pretty easy. Reassembly was just the reverse, i replaced all the orings as i rebuilt. Hardest part was restteting the stroke just right in the machine, surprising how fractions of a mm add up over a few hundred strokes.

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Yup, that's how this one is. Near as I can tell, the actuator is working just fine. IMO, it's just the wrong mechanism for this job. I would have either gone with some type of motor drive with urethane coated rollers (for much better grip) or (as I intend to go) like a pump shotgun. Well just have a pawl that will catch on the liner in a precisely located hole (that's already punched into the liner), driven by pneuamtic cyl and it will just push/pull all day long (the pawl won't catch on the return stroke). I did some mockups on my disk with an ink pen engaging the hole and my finger holding the tape down against the desk. PLENTY of strength on the liner to allow it to be pulled forward for the indexing even with me holding a lot more tension on it than needed.

The Dude