# Best stroke length for cylinders??



## shamrock1990 (Oct 15, 2011)

Hi all, can someone tell me what is the best length of stroke for cylinders? Im not sure what Im going to build yet, but most likely a prop that will pop up from ground level. I would guess you can extend the prop somehow, but any feedback would be appreciated! Thanks

Brendan Green


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## BioHazardCustoms (Aug 5, 2009)

It honestly depends on what you want to build and it's relative size. If you're going to build an armature that extends the prop like a lever, that also factors in. Get measurements of how high you want the prop to pop up, then do the math to figure out how long of a stroke you need on the cylinder. It would basically be: Height Desired(HD) - Height of Prop(HP) = Stroke (S). So you take the base of the prop, and the creature itself, and subtract it from the height you want to reach. That should give you the ball park number you need for the stroke of the cylinder. There is something I'm not remembering to factor in, but I'm sure someone else here will chime in and tell you what's missing. 

Also, factor in the air pressure you'll have to have for the prop. A larger cylinder will run your compressor longer than a small cylinder, and may be less terrifying than you would desire. Just food for thought.


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## shamrock1990 (Oct 15, 2011)

Thanks for the info!


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## corey872 (Jan 10, 2010)

Remember to factor in the diameter, too. It will affect how much you can lift and to some extent the speed it moves and/or how much air you use.

Diameter / 2 = radius and radius squared x 3.14 = square inches of the cylinder. With that, you can figure what force you get at what air pressure. Ie - you want to lift a 40 pound prop with a 3" diameter air cylinder:

3 in. diameter / 2 = 1.5 inches

1.5^2 x 3.14 = 7 square inches

40 pounds / 7 square inches = ~6 psi of air pressure.

Really, there are a whole list of equations out there to define a system. You can calculate things such as how much force the cylinder will have, how much air you use per stroke, how fast it will move, air pressure needed, etc. Stroke length is almost the least important. You can always limit it to less than full stroke, but it's hard to get more stroke without setting up some type of lever system.


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## niblique71 (Dec 2, 2009)

I love the explanations given so far. To simplify it a step further, If you have a single pivot point on a pneumatic prop, the closer to the fulcrum the cylinder is mounted, the bigger bore and shorter stroke it will probably need to be. The shorter stoke is needed to fit in the space, and the larger bore is required, to counteract the lack of leverage. The further away from the fulcrum the piston is mounted, the longer stroke you will need to move your prop the same distance, and then you can use a smaller bore if desired because it takes less forece to lift the same amount of weight.

That is of course assuming that you are actually lifting something. Sometimes you are just Moving something sideways, but the same rules sorta apply. It's a leverage thing.


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