# Pneumatic Actuation Question

We are planning to use pneumatics to retract and extend our intake mechanism. I realize this has been done thousands of times, but we cannot wrap our heads around how to move in an arc by extending and retracting horizontally. Is there a guide or formula that you use to help you determine the best mounting point for the cylinder and the rod? Any tips?

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Send a pic of what you are trying to do and Iâ€™ll write a detailed explanation

try to make the cylinder close to perpendicular to the arm, at the end of travel that requires the most forceâ€¦which is usually in the â€śdownâ€ť position.

Trial and error is OKâ€¦

and adding some help in the form or gas springs or constant force springs is also OK.

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Spectrum did a good video. Around half way he talks about pneumatics and intakes and some of the math behind it.

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Ok, I watched the first video and set-up a sketch like he did, but can you tell me why he doubles the extended cylinder length? In the video he has a series of center lines, one with 2 and one with 3. He sets one of the 3 to his cylinder length, and 1 of the 2 to his stroke. But then he sets the other 1 of 2 equal to the cylinder and then the other 2 of 3 equal to stroke so it seems like it is cylinder plus stroke plus stroke again. Any idea why?

For most cylinders 4099 has used (everything thatâ€™s a â€śround body air cylinderâ€ť on McMaster), at least, for a given bore size, the retracted length can be expressed as a sum of some fixed part and the stroke length. Doing it this way lets you easily iterate on stroke length (you just have to change one number instead of having to go into the McMaster catalog).

For example, look at https://www.mcmaster.com/6498K636/ and https://www.mcmaster.com/6498K637/. These are the same cylinder, with 1" difference in stroke length, so the difference in retracted length is 1". In this case the â€śfixed partâ€ť would be 4.25" + whatever the rod end adds. Make sure you are using the length between your pivots, by the way, not the total length of the cylinder, when you do this.

So you can express the retracted length as rod end/clevis length + fixed part + stroke length, and then the extended length is rod end/clevis length + fixed part + stroke length + stroke length.

I get the clevis plus stroke plus length, but why does the stroke double on the extended length?

The piston has to be at least as long as the stroke length, and it has to stow somewhere when the cylinder retracts. So the retracted length of the cylinder has to be at least as much as the stroke length. There is also other stuff in front of and behind the piston that makes up that retracted length which is the â€śfixed partâ€ť. This is why the stroke length shows up at all in the retracted length.

Then, pretty much by definition, the extended length has to be the retracted length plus the stroke length â€“ the stroke length literally just means how much the cylinderâ€™s length changes between extended and retracted. So a 4" stroke cylinder would mean that if the retracted length was 11.5" (made up number), the extended length would have to be 15.5".

Am I understanding your question right? Feel free to ask again if I didnâ€™t grasp where the misunderstanding was.

Do you have a pneumatic cylinder at your shop? If you do, go measure the length with the piston retracted and again with it extended.

I have a cylinder that measured 8.25 retracted (Counting clevis) and 12.25 extended, so a 4" stroke. If I drew that per the video I would make one measurement 8.25 and then I would make the 2nd measurement 4, but in the video he then sets the 2 remaining lines on the extended cylinder equal to the extension (which would be 8", not 4). Iâ€™m just not following why you would double the extension. Seems that would make your measurements incorrect.

I have a cylinder and have measured it. 8.25" retracted + 4" extension. So it makes sense to me that Iâ€™d use those numbers in my drawing, but in the video the guy doubles the extension on one of the lines and Iâ€™m not following why.

Having not watched the video Iâ€™m assuming the fixed length on your in hand cylinder is 4.25 with 4 in stroke. Hence the 8.25 and 12.25. Generally if you stay in the same bore size and brand you will find that they all have the same fixed length.

So one measure is the fixed length. Then stroke for retracted.
The other would be fixed length + stroke + stroke to get your extended length.

Doing it this way you can do it as a formula and make the adjustments much quicker by changing just the stoke length.

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044-DP.pdf (686.1 KB)
This is the cylinder I have. Based on the drawing what would be my fixed length? I am assuming it would be 8.25, but I am not sure.

If the cylinder measures 8.25" when retracted, and 12.25" when extended, then you know your stroke length is 4". Therefore your fixed length is 4.25" (8.25" - 4"). Therefore the extended length is 4.25" + 4" + 4", or fixed length + 2Ă— stroke length

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The reason you model it this way is if you are using a 4in stroke cylinder but decide you need 5in, as long as you stay in the same cylinder family/bore size you can just change you 4in/8in dimensions to 5/10 and the 4.25 will stay constant.

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Is the fixed length always the retracted length minus the stroke? Iâ€™m just not following why you wouldnâ€™t just use 8.25 and 12.25

I think youâ€™re overthinking it. You do use 8.25 and 12.25. Weâ€™re explaining why the drawing used 2Ă— stroke length

Ok, so in my drawing I need to use the 8.25 as retracted and 12.25 as my extended lengths, right? (Or those numbers plus the distance to the center of my rod end/clevis) Using those numbers makes sense to me, but I didnâ€™t understand the video using the 2x stroke length etc.

I havenâ€™t watched the video and havenâ€™t seen your drawing, so I canâ€™t speak specifically to those. Having said that, my understanding of what the video was doing was simply a shorthand to make editing easier.

A cylinder has 2 positions, retracted and extended. The measurements for these 2 positions include 2 dimensions, cylinder length and stroke length. You can then logically split cylinder length into 2 components, fixed length (A) and stroke length (B) as illustrated by my crude ASCII art below

``````+----------------+
| A :     B      |=
+----------------+

+----------------+
| A :     B      |====B====
+----------------+
``````
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