Physic's Formula Question

[Deviant]

Can anyone tell me the formula's that I need to calculate the output air pressure from a cylinder/piston/spring setup.

IE, If I have a cylinder that has a diameter of .5 inch, with a length of 1.5 inches. How strong will the spring need to be to output X pressure from an nozzle with a diameter of 3mm.

((The numbers can and will need to vary, but I'd like to know the formula's so that I can calculate the desired air pressure.))

Also, I'd like to know how to calculate the pressure needed to move an object at a specific weight to a specific velocity within a certain distance.

I hope someone can help out. I vagulely remember doing similar calculations long ago in school.

Thanks.

[Al_The_Man]

Here is what you might need http://www.team116.org/2003/lessons/...Pneumatics.pdf
Al.

[Deviant]

Very close to what I need.

I actually need to calculate the air pressure generated by a spring powered piston. Not the force generated by an air driven piston.

It could be that the formula's on the page will do what I need. However, it's making my head hurt thinking of how convert them for use.

[2muchstuff]

It has been a while for me too, but here goes:

.500" piston bore
area of a circle: pi(r*r)

A=3.1415(.250*.250)
A=3.1415(.0625)
A=.1963 square inches

100 pounds of applied force to this piston will give you 19.63 pounds of force. The amount of force will be the same whether it is mechanical or pneumatic. As for the amount of pressure at the nozzle, it will be the same no matter what size it is. All a nozzle does is regulate/restrict the amount of flow thru it.

In moving an object, a few other factors are involved. One is the amount of friction between the object and what it sits on. Otherwise Force = Mass times Acceleration.

Sorry, that is what comes to mind as of right now, hope this helps you out some.

[2muchstuff]

To figure the amount of force of a spring driven piston, you need to know the spring's constant. It is figure that tells you how much force to compress the spring a specified distance. Take that figure and multiply it by the distance of the spring's compression, that will give you the amount of force. Now multiply that by the square inches of the piston face to get the amount of air pressure.

[Deviant]

I don't think that the nozzle has zero impact on the the output force of the air.

Example, if you have a syringe with the needle on, and you squeeze the piston, you will spray water across the room. Again the same syringe without a cap on the end would cause the water to splash on the floor.

I'm sure it has something to do with the compression factor and the ratio of the piston diameter to the area the the energy is actually applied to.

Ah well, maybe some MIT student that breaths physics will jump in on this thread and set is all right.

There are similar products that use both spring and compressed air for their power supply. Both have similar results. The compressed air one use a supply of 80-150psi or so, while the spring gun isn't using anything close to an 80 pound spring.

[pmurray]

Deviant,

What you need is to use Bernoulli’s Equation. Do a google search on it – there will be lots of hits and info on how to use it. Here is one link:

http://www.grc.nasa.gov/WWW/K-12/airplane/bern.html

With out going through numbers in your example, if your nozzle is releasing to the atmosphere, then you will only have dynamic pressure at this point (rV*V/2, r = density of fluid, V = Velocity). This will be equal to the static pressure in your system, which was explained how to get in the above posts.

Try working through it, if you are still having trouble, give a shout and I’ll show you some more.

Hope this helps. . .

plm

[Deviant]

Definately something close to what I need.

According to what I've read so far. Bernoulli's equation applies to a steady flow of incompressible fluid.

I'm not sure how that relates to a short burst of air, which can be and probably is compressed, at least some what.

I'll keep reading.

[2muchstuff]

If I may so bold as to ask, what are you trying to build.

[pmurray]

Deviant,

They are referring to the compressibility property of the fluid. You can assume the air is an incompressible fluid and also disregard pressure losses due to friction etc. – you will be close enough. Also, the terms in Bernoulli’s Equation are in units of pressure heads – so you will convert your psi to a head of pressure.

Keep in mind; I’m recalling some of this stuff from fluid dynamics and a thermo course from over 30+ years ago – I’m a little rusty.

plm

[trubleshtr]

Quick reference is


F
-----
P | A

Where :
F = force
P = pressure
A = Area
You will need to determine spring rates for the spring itself.

Question, Are you looking to find the force generated by the rod, or the amount of air pressure coming out the port on the cylinder when the spring is pushing on the piston?
just trying to clarify.

[lerman]

Originally Posted by Deviant I don't think that the nozzle has zero impact on the the output force of the air. Example, if you have a syringe with the needle on, and you squeeze the piston, you will spray water across the room. Again the same syringe without a cap on the end would cause the water to splash on the floor. I'm sure it has something to do with the compression factor and the ratio of the piston diameter to the area the the energy is actually applied to. Ah well, maybe some MIT student that breaths physics will jump in on this thread and set is all right. There are similar products that use both spring and compressed air for their power supply. Both have similar results. The compressed air one use a supply of 80-150psi or so, while the spring gun isn't using anything close to an 80 pound spring.

When you take the needle off the syringe, you cannot apply the same force because you can't push fast enough. The volume flow of a gas through an orifice is determined by the properties of the gas, the size and share of the orifice, an by the upstream pressure (assuming that it is several times the downstream pressure).

Ken