Pneumatic cylinders 2

[jadams62]

Hi all, Ive been drafted by my boss to size some pneumatic cylinders, i normally work more on the programming side so its going to require me to remember calcs i haven't done since college (30years ago), anyway, ill show you what ive come up with and hopefully you will be able to show me where I have gone wrong:

This cylinder is going to be opening a flap on a ceiling in a lab.

the cylinder will have a stroke of 750mm and need to open the flat at a rate of 75mm/s - so its gonna take 10secs to open the flap (client specified)
the air pressure at the lab is 600kPa and I have worked out the force the cylinder has to overcome is 0.25kN
so size of the cylinder is F=PxA or A=F/P = 250 / 0.6 = 150mm2

heres where i have been getting confused, because the key here is the speed at which the door opens, so i need to size my cylinder around that.

the velocity the cylinder will do is F=VxA or V=F/A = 250 / 150 = 1.6m/s which is too fast as i need 75mm/s

So A=F/V = 250/0.075= 3333mm2 - this is where im getting stuck because that seems like a very big cylinder?

Is the math correct?

Thanks in advance I am eternally grateful for any advice

John

 

[Jboggs]

Several comments... but I will limit it to these few:

GEOMETRY
All your forces, speeds, etc. will be determined by the geometry of the arrangement. I assume the door is on a hinge and pivots. Since the motion is not linear, then neither are the forces or speeds. They can both vary significantly throughout the stroke. We can't help you with the calculations if we don't know the geometry. Sketch please?

CALCULATIONS
Second, "(F)orce = (V)elocity x (A)rea"? Where did you get that? I'm confused.

FORCE
Size the bore of the cylinder based on the max load it will see divided by the air pressure you want to use. You will probably want to run it at some percentage of the available system pressure, so you have room for adjustment. That's why you will need a pressure regulator.

SPEED
Does your client specify the speed with which the door must operate, or do they specify the total time it must take? Since the actual speed is likely to vary this question makes a difference. Assuming you have adequate air supply, max speed should not be an issue. The issue will be controlling that speed. Do that with port-mounted flow control valves. Do not make the mistake of trying to control the speed with the pressure regulator. Pressure control and speed control are two different things.

VALVE
You will need a 5-port 4-way directional control valve. Size the valve for the pressure and max flow.

SUPPLY
You might want to consider a packaged Filter-Regulator-Lubricator, FRL, unit. They are compact and neat.

OTHER
Try not use the cylinder as a stop for either end of the stroke. Normally, the available cylinder stroke should slightly longer than the actual stroke required, again for field adjustment if needed. In this case use external stops to control final stopping positions.

I would strongly recommend you contact your local fluid power distributor and ask for their assistance. They do this stuff in their sleep.

My gut tells me you will end up with a relatively long and thin cylinder. If it will operate in a horizontal orientation you should also consider the moment load imposed on the internal bushings by gravity.

 

[FACS]

To add to the above comments.

Use a flow valve to control the speed, but do not control the speed of the air going into the cylinder.

Use what is called "meter out" and control the speed with the valve coming OUT of the cylinder.

The size of your cylinder is determined by the force required, and the pressure available. Don't size the cylinder using the speed.

Charlie

http://www.facsco.com

 

[jadams62]

Thanks for the tips, that helped quite a bit,

This had been put on the back burner, but im back on it again now, and i have just been informed the client already has a cylinder they want us to use.

I cant get access to the cylinder and my supervisor wants me to work out how fast the cylinder is going to move, (without a control valve) not to sure why, but there you go.

anyway the cylinder is 80mm in diameter, and pressure is 600kPa.

I would like to work out how fast it will move with and without the force of the door acting on it (0.25kN)

What calculations do I need to do, and is there any additional information i need to find out?

Much appreciated
John

 

[Walterke]

Easiest way to figure it out is to just hook it up to the source and see what it does. With the information you've given there's no way to figure it out.

I'd suggest contacting the supplier and explaining them what it is you want to do.

Alternatively, have you considered a linear actuator?

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[jadams62]

Cheers, unfortunately I cant hook it up, they are not local, I have contacted them regarding the supplier, but they don't seem to know...

Is there no way of calculating a rough number? what additional information would I require?

Cheers
John

 

[hydtools]

Ah, Walterke, a cylinder is a linear actuator.

Do you know your air system volume flow capacity? Knowing 600kPa pressure but not flow capacity is not enough. At 600kPa and insufficient flow, the piston will move as fast as the small initial volume of compressed will expand to zero pressure or the piston hits the stop. Line size? There may or may not be pressure drop at the cylinder port due to line loss.

Ted

 

[hydtools]

Look in the Bimba literature:

http://www.bimba.com/Library/#technicaltips

Sizing a cylinder.

Ted

 

[hydtools]

This may help, also:

http://www.mead-usa.com/reference/media/pneumatic_handbook.pdf

Ted

 

[dvd]

This sounds like a setup. Does your boss like you? If time is so critical that you need to make calculations based on no flow control devices, then you also need to include the complete piping arrangement including valves and assurances of constant upstream air pressure (as indicated by hydtools). Contact a pneumatics distributor and see if they have software to model your circuit (numatics has numasizing).

 

[jadams62]

HAHA, i dont think my boss does like me, but unfortunatly it is not a set up,

thanks for the replies, by volume flow capacity do you mean volumetric flow rate, i have done a little research and come up with this below, I would appreciate an opinion on it, bearing in mind I’m not an engineer, but can I work out Q (volume flow capacity) – I’m guessing I need to know the pipe supplying the air at the clients lab? And then can I calculate the speed of the cylinder

P0 = p + q

P0 = total pressure
p = static pressure
q = (density x v^2)/2
density for air = 1.403kg/m3

q = P0 – p = 600kPa = 600 000 Pa

600 000 = (1.403 x v^2)/2

v = ((2 x 600 000)/1.403)^0.5 = 925m/s

Q = A x v

Q = volumetric flow rate
v = velocity
A = area of pipe??? Or cylinder??

Thanks alot
John

 

[hydtools]

Can your client tell you what flow rate their compressor will deliver at 600kPa in lpm or m^3/min?
Since the client has selected the cylinder, what part is left for your company to play? This is where I would wipe my hands of the project and wish them well.

Ted

 

[dvd]

Air is reasonably compressible, so you can forget about the density being constant. That density you list is not the density of air at 6 bar. Also, how is the cylinder retracted? Is there piping connected to both ends of the cylinder and connected to the solenoid valve? If so, there can be some substantial backpressure on the cylinder which robs you of cylinder speed. You are better off getting someone to model this for you. Call your local Numatics, Parker, SMC, Miller, etc. rep and ask if they can help you.

 

[Walterke]

Ah, Walterke, a cylinder is a linear actuator.

I knew someone was going to give me trouble on the the moment I posted it. I meant an electronic linear actuator of course.

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[hydtools]

jadams62, your pressure assumption in your equation above is incorrect. The 600kPa would be the static pressure, not the velocity pressure. Or the total pressure = static pressure when velocity is zero. Your calculated velocity is almost Mach 3, not likely.

Ted

 

[Jboggs]

OK. IMHO, this is WAY too much discussion about a very simple project - operating a door with a cylinder.

First - I suggested the OP post a sketch. Haven't seen one yet. So ANY answer you might get from anyone (including mine below) is suspect. Geometry and gravity, like ideas, have consequences.

Second - If this is like 99% of the air systems I have ever worked with, the supply probably has much more volume flow capacity than the cylinder will consume moving the door. That means that if the cylinder is left uncontrolled it will literally slam the door in both directions. That is a very good way to break stuff, and it is why you MUST include flow control valves. They must be installed so as to restrict the flow OUT of the cylinder not in (also as previously recommended), and preferably mounted directly in the cylinder ports. You manually adjust these to achieve whatever speed you wish.

Third - You have a 80mm (~3") bore x 750mm (~30") stroke cylinder operating on 600kPa (~90 psi) system pressure with a 0.25kN (56 lb). The force the cylinder will produce is about 10 times the load, which is why I recommended previously that you are going to want to use a pressure regulator. I would cut the pressure down to maybe 50-60 psi.

If you still want someone to review your plan, contact a local fluid power distributor. They have people that do this stuff in their sleep, and can recommend specific components (directional valves, flow controls, pressure regulators, filters, lubricators, etc.) to fill your need. But even they will need a sketch and will need to understand the orientation.

 

[BillPSU]

Right on Jboggs

 

[mfgenggear]

nice

Mfgenggear
if it can be built it can be calculated.
if it can be calculated it can be built.