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# Sizing pneumatic cylinders

02/18/2008 3:27 PM

For years I was always under the impression that you calculated the force using this calculation - Radius of the cyclinder squared x pi x psi - 20%

I was always told the 20% reduction was for air compression.

Is that really true?

Joe - jlopetrone at comcast dot net

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#1

### Re: Sizing pneumatic cylinders

02/18/2008 3:52 PM

I use the 20% reduction as a service factor - to ensure that as the cylinder ages and seals start to leak, you still have enough capacity for the application. Air compression has nothing to do with it.

Also remember that (most) cylinders "push" harder than they "pull" - the rod diameter reduces the effective piston area on the retract side.

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#2

### Re: Sizing pneumatic cylinders

02/18/2008 6:00 PM

In fact the equation which was recommended to you takes mainly into consideration the friction of seals. Old seals were not very good and so friction was high. Wear was also intensive increasing friction, the 20% are a reserve taken for the worst case. It can at least partially consider the pressure losses in not large enough pipes, a very common error in pneumatic systems.

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#3

### Re: Sizing pneumatic cylinders

02/18/2008 9:01 PM

I was told the over force figure was 25% for a cylinder that must move at a reasonable rate and that it was due to the pressure difference between the Supply and the Force to move the load. The pressure dfference is necessary to cause the air to move through the lines and still produce enough force to overcome the load resistance and move it.

I was also told the fiigure should be 50% over the load when high travel speed was required. Also, that more than 50% would gain so little extra speed it was not worth the energy waste. In that case go to a larger bore.

Check the Ross or Mac training books on their web site for more info.

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#4

### Re: Sizing pneumatic cylinders

02/19/2008 1:51 PM

The force the cylinder will produce is the area of the piston X the pressure * efficiency.

There fore F = (pi * R^2) * Pressure * %Efficiency

If you estimate there are 20% losses due to air leaks, friction, etc, then %Efficiency = 0.8. The figure for the losses is somewhat of a crap shoot, but it is better to estimate too high than too low because you can always lower the pressure to the cylinder with a regulator. You can also control the motion of the cylinder using control valves on each of the exhaust ports. Often you can order the cylinder with cushioned ends and exhaust control.

If you are calculating the force you will get on the pull stroke you must subtract the area of the rod from the area of the piston.

Travis

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#5

### Re: Sizing pneumatic cylinders

02/19/2008 3:42 PM

Joe some people will tell you anything. It is up to you to tell the difference between a good sound idea and an idea that sounds good. Trust me.

I use .7854 x Dia ^2 just because it is quick and easy. (.7854 = Pi/4).

If you have a 300 pound box and need to lift it a few inches with air pressure at 96 psig it would be futile to select a 2" bore cylinder with an area of 3.1416 square inches. 3.1416 x 96 = 301.59 pounds of force. The cylinder has internal friction from the piston seals and wear ring and rod gland seals. At least a couple of psig are often required for this friction, more without lubrication. You are not going to lift the box. If friction had not been invented and you had 1.59 pounds of force over the weight of the box it might move but so slowly that you would be late for both lunch and dinner.

Many fail to consider the difference between lifting and pushing. If your box were card board sitting on a smooth metal surface you could slide it over with a 1" Bore cylinder if the friction between the box and steel surface is less than .1. Put it this way. You could push your car with much smaller cylinder than it would take to lift it.

On a ramp is another story. Multiply by the Cos of the angle.

You also have limited selection in bore size. In the 2" bore example above you can add another cylinder or go up to the next standard bore size 2 1/2 or even 3 1/4"

You over size the cylinder by 25% for average speed (per Womak and me) to over come seal friction and have ample force to accelerate the mass. As another respondent suggested more force gives more acceleration so oversize to go faster. He stopped at 50%. I always double the force for maximum speed (subject to suitable cylinder bore selection).

It be very comforting to oversize your pneumatic cylinders and control the speed with flow controls or the force by regulating the pressure. Like toast; burn it a scrape it.

If your cylinder seals leak, replace the seals or the cylinder. Leaks are wasteful and continue while the cylinder is not being used. You would not buy a bigger engine for your car in case the tires go flat don't over size for leaks.

I could go on and I am going to. Bye

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