Need to know how much force in psi

I'm wondering if anyone can help me out with calculating how much air pressure is needed to move a piston fast enough given the surface area, distance needed to move, and total mass that is being moved. Sorry if this is in the wrong forum.

Surface area - 305mm²
Mass - 15.6 grams
Movement - 25.9mm

Given that information and that I need to be able to move the object forward and back 30+ times a second but preferably 40+ times a second. Is there a way to calculate how much air pressure is needed to get my desired results?

Thanks. And sorry again if this is the wrong forum. If anyone can point me in the right direction it would be greatly appreciated.

Implicit in what you describe is the need for a valve, either an electrically operated solenoid valve, an air operated valve or a mechanical that can be operated 40 times a second.

I think this operating frequency is really pushing it for a solenoid valve and is totally impossible for an air operated valve. You may be able to drive a mechanical valve fast enough but then I think you would come up against the problem that the compressed air simplay cannot move fast enough to give the rate you want.

Maybe someone will come on with an example that shoots me down but I think what you are proposing is simply impossible.

An electronic solenoid valve would be used in order to drive the piston. Humphrey makes noids that can cycle 40+ times a second. What I need to know is how much air pressure is it going to take for the piston to fully cycle that many times in a second. 40 is really pushing the limits of whats really needed.

It's all extremely simple calculations and I can do them for you, but I think you would be better served by an oscillating mechanism.

Air is springy. Getting it to efficiently change direction quickly is tricky. Tubing size and length would be critical. The cylinder in your arrangement is not going to like the vibration, the impact or the internal seal friction. I doubt the cylinder would last very long. Same with the valve. I just can't see it happening.

With that being said, I will dig into the numbers shortly (at work at the moment...).

Scott

On second thought - look at a pneumatic hammer (air chisel). Take one apart and look the the valve. It is different type of control valve and it operates at some pretty high frequencies for an air operated device. This arrangement could be altered or used as is to run your application on 100PSI.

Scott

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Originally Posted by mxtras

....Air is springy....Scott

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Take the temperature down low enough and it isn't :).

The air hammer suggestion is good but I doubt whether even that can go at 40 hertz. It would be the best approach, however, have the piston trip its own valve. This reduces the mass of air in supply lines that has to change direction.

Another thought is to include springs. Balance the piston between opposed springs and tune the spring rate and moving mass to get a resonant frequency at 40 hertz.

Dear mxtras and Geof,

Over here (UK) Burkert do a wide range of solenoid valves. Open and close times are around 10ms each way. I do not know how much coil saturation will slow this down as regards maximum frequency of continuous operation, but 50 Hz might be the upper limit for (say) 5 watt coils. It might be higher if you use more wattage for the coil.

Some of their range are good for 20 bar and over (300 PSI).

Yep, air is springy, and it is going to be hot at high pressures, but I think it might just be able to be done.

Modifying a road breaker might be easier...

Best wishes,

Martin

Take two valves and half the time one has to open and close.... Works like a charm.
Lemo

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Originally Posted by Lemo

Take two valves and half the time one has to open and close.... Works like a charm.
Lemo

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Dear Lemo,

Good concept. No need to stop at two valves either...

Best wishes,

Martin

I'm sorry if this post might come across in a negative way. But I don't think any of you understand what I'm asking. I already have a solenoid valve that can handle the number of cycles I'm looking for. What I'm asking is how much force is required to move object "A" a distance of 25.9mm if it has a mass of 15grams. Now the surface area the air can contact is roughly 305mm². That will help determine the air pressure needed. What I want to know is what is the air pressure required. I'm talking about Kinematics here. But going over equations that I don't fully understand isn't helping me. So if anyone who does understand Kinematics, I'd greatly appreciate some input. I've given all the required information to get the answer I need. A formula would be awesome if anyone has it.

You need to measure that. There is much unknown in the setup. Flex of the lines, line diameter, amount of flow, of course pressure, exhaust speed, possibly a vacuum drawing exhaust for higher speed, and so on. Why not hydraulic? Easy to calculate by comparison. I know.... you have all the parts... let me be a steam boat... but but but but...
Lemo

it isn't really kinematics, you need to solve a system of non-linear dynamic differential equations. My guess is the main problem will be estimating friction. It isn't likely that back of the envelope calcs will be good enough, because you have to deal with air flowing in and out to determine the net pressure. You know you need at least Pressure = mass x acceleration/Area. Estimating the acceleration will depend on how the thing will move.

Since the piston has to reverse direction obviously, it must have accel and decel. What will decel/stop the piston at the end of each stroke in each direction? Will it slam into a hard stop? How will the air on the low pressure side of the piston escape?

There's a ton of variables here. In a severely oversimplified model, assume the piston is sitting in a pipe with no friction and some kind of physical stop at the limits of travel 25.9mm apart. Air pressure changes are instant.

- To go forward and back 40 times in one second means .025 seconds to go from rest, travel 25.9mm, assume instant decel into the physical stop, zero dwell while the solenoid valve reverses the air pressure (we'll naively assume instantaneous) then from rest again, travel 25.9mm in the reverse direction with instant decel against the original stop.
- If accel is linear (it won't be), then the piston must cover the first move of 25.9mm in .0125 seconds for an average speed of 2 meters/second or a peak of 4m/s when it hits the physical stop.
- So to accelerate from 0 to 4m/s in .0125 seconds will take about 320m/s*s acceleration.
- Force=mass*accel. Thus F = (.0156 Kg) * (320m/s*s) = 5 Kg*m/s*s
- Pressure = (5Kg*m/s*s) / (305 sq mm) = 16400 N/m*m or Pascals = 2.4lb/in*in.

Someone check my math since I'm feeble with this stuff these days.

That's the way too theoretical idealized calculation. Zero friction, linear accel in each direction, instant pressure changes, instant decel. All of which is hogwash in the real world.

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Originally Posted by Caprirs

.......- Force=mass*accel. Thus F = (.0156 Kg) * (320m/s*s) = 5 Kg*m/s*s
- Pressure = (5Kg*m/s*s) / (305 sq mm) = 16400 N/m*m or Pascals = 2.4lb/in*in.

Someone check my math since I'm feeble with this stuff these days.

That's the way too theoretical idealized calculation. Zero friction, linear accel in each direction, instant pressure changes, instant decel. All of which is hogwash in the real world.

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I did some number crunching and came up with 8.16 x 10E4 Pascals so I must have missed a factor of two and maybe thrown in an extra power of ten somewhere. But I agree totally with the hogwash conclusion.

My gut, which is often a reliable source, tells me it is not-doable, and until someone builds something that buzzes along at 40 hz that is what I am sticking with.:)

Hand held air hammers/chisels are often rated around 2000 blows/minute as are some impact wrenches. That's about 35 Hz. So it's obviously do-able to make something using air power. However, in an pnuematic impact wrench, there is no electrical solenoid controlling a valve to reverse a piston. It's all mechanical rotation with the "flywheel" always rotating in the same direction, no reversing. Haven't taken apart an air hammer/chisel to know how they work.

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Originally Posted by Caprirs

... Haven't taken apart an air hammer/chisel to know how they work.

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One method is that there are valves at each end and the piston is held against one end by a spring; this holds that inlet valve open meanwhile the opposite end exhaust is open. The air comes in and as soon as the piston moves the inlet closes, the exhaust is closed and stays closed until the piston nears the end of its travel and then an actuator rod is moved by the piston to open the exhaust. The piston coasts into the other end closing that exhaust and opening that inlet and the sequence reverses. Yes they can go fast because the piston operates the valves and there are no long and maybe constricted passages for the air to pass through.

RedDog,
I find it difficult to answer this type of question without having any knowledge of the use and the purpose itself. I also know for a fact that we often try to make something that already exists as we (I am the first to do this) think that what we need does not exist for our specific application.
Please take a look at the Festo automation Website and search for "Fluidic Muscle", this might do the trick for your specific application. I hope you will be impressed as I was a few years ago when I saw them at Festo's booth as this thing can really vibrate.

Best regards

Bruno

Sorry guys. I know I was being really vague in all the information I was giving. I do know this all entirely possible as its done in paintball markers all the time. And the solenoids they use can handle the types of cycles I'm looking for. This is a paintball marker design I'm working on. I was trying to get a rough estimate of the pressure I'd be looking for to know what what springs to look for in a low pressure regulator.