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Discussion Starter · #1 ·
Hi guys,

I am going to put my Spider Prop back up this Year. Anyway, the last time I used this Prop, the Cylinder could just barely lift it, so I am looking for a new Cylinder. Does anyone know how to spec Cylinders? I have a 17.8 cfm, 120 psi Compressor (60 Gallon)-

Looks like the biggest Cylinder that's priced within reason is a 6" Stroke, but I wonder how well I can power a 1 1/2" Cylinder?

Thanks for your responses-
 

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Size of cylinder.

Here is what you need to consider.

Weight of the prop. Any leverage advantage or for that matter disadvantage. Most pneumatic props are working with a disadvantage. IE the end movement of the prop is greater than the throw of the cylinder.

So lets say your prop weighs 5 lbs and the mechanical movement is 4 to 1. So if your cylinder moves 4 inches, your prop moves 16 inches. This is a basic calculation and likely the more experienced will correct me, but I would think that with pressure of 100 PSI, to lift this prop you would need a cylinder with a piston diameter of approx 1/2 inch in diameter. A cylinder with a 1/2 inch piston has on the non-rod side an area of rounded off .19 cubic inches. So if you apply 100 PSI to this area, you will provide 19 lbs of pressure. Likely this would not lift the prop though, because of friction and loss.

Hopefully this will give you an idea on how to compute your needs. It is not the distance of travel that you need to look at, other than how it relates to the distance you want your prop to move. Again this factors in. If you change the lever point to increase the movement per the distance of throw, then you will need to increase the size of the piston.

Hope this helps.
 

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Discussion Starter · #3 ·
Thank you for your response,

I am a little dense on this, but if I have, say a 4:1 ratio, and my Prop and Linkage weigh, say, 40#-will a 1 1/2" Piston lift it or am I still dense? (well, I know I'm still dense, but-:D)

naother question is how to figure my linkage-it is a large Spider (with about a 6-8' span including legs), and it will be suspended from the ceiling-I had it set up this way about 4 Years ago, and I used air to hold it up, and then dropped the air so it would fall-how would you firgure the ,m say, 4:1?

Thanks for helping the intelligence challenged! :eek:
 

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Pi X .75 X .75=1.76 square inches for the piston diameter.
You mechanical linkage is 4:1 so the 40 lbs of the spider and the mechanism makes the force required to lift it 160 lbs.
If you take 160 and divide it by 1.76 square inches you get 90 psi that your your compressor must provide to lift the spider and assy.
I would use a 2" so you can drop your pressure a bit. It's easier to avoid leaks with less pressure. You'll also get smoother motion with a bigger cylinder.

I hope this helps....
 
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