Required Wall thickness for Aluminium Pneumatic Cylinder

[cerosuk]

Hi,

I am in need of some help. I am currently working on a robot design for the uk series of robot wars. We have previously entered with a full pressure pneumatic robot called ceros which can be viewed at

http://www.ceros.org.uk.

The previous incarnation used a hydraulic cylinder modified for use at 750psi as standard pneumatics would not take it however this weighed in at 11 kgs and almost put us over the weight limit requirements. This time round we are looking to manufacture our own ram using aluminium tube with end plates and tie rods to pull the plates together. What I was wondering is this. We are trying to work out how to calculate a safe wall thickness for a seamless tube of aluminium 6082 to have the cylinder body made from it would need to be able to hold 900 psi safe working pressure and be tested to 110% of its safe working pressure so a maxiumum of 1000psi. The tube inside diameter would be 120mm and the length of the tube would be 185mm. We have some calculations down on paper but at the moment we think we are heading in the wrong direction. Can anyone help out ?? We really need to work this out before we proceed any further.

Any help will be greatly appreciated.

Kind regards

Francis Smith

http://www.ceros.org.uk

teamnd@hotmail.com

 

[prex]

This is a very high pressure for an air operated device.
You should be aware of the fact that compressed air is risky, whereas hydraulic oil is not (the first becomes a bomb in case of failure, the latter would just make puf!).
Also I wonder how you intend to obtain such a high pressure with air: it is far from being impossible, of course, but it is quite uncommon if you intend to use compressors of wide availability.
Last but not least, aluminum is a material whose strength characteristics may widely change if you don't intend to weld it, as I suppose: you should ask a supplier for certified strengths and also for availability of diameter and thickness of interest (I guess more than 1in. thickness!).
Yet another point: personally I don't know, but possibly an air cylinder in aluminum requires a check for stiffness against leakage through the joint, you should consult a supplier of similar cylinders.
In conclusion I would prefer to see you having clearer ideas about your intentions, before giving advice on such a delicate matter.

prex

http://www.xcalcs.com

Online tools for structural design

 

[cerosuk]

Hi Prex

Thanks for the reply. The power source is carbon dioxde which is stored at between 750 and 900 psi. I know of the dangers of the high pressure gas, I was in a situation where someone created compression ignition within a cylinder and would never ever want to be in that situation again which is the reasoning behind my question. Saftey is my top priority I show huge respect for these pieces of machinery and always over engineer what I build with regards to high pressure gas. The plan of the design of the ram was to have the aluminium tube as the cylinder wall, use steel plates 10mm thick top and bottom for the plates and titanium tie rods to hold the construction together. The tie rods were to be 12mm thick with spcers to prevent deformation of the top and bottom plates. Gas would be fed into the lower portion of the ram via 3 1/2" 2/2 vavles which feed from a high pressure buffer tank direct from 2 fire extinguishers all piped up using hydraulic hose. This is very similar to the setup last year except the ram is different. Last year we had no problems with the saftey aspect of the constrauction and hope to have the same this year. If we were to go for a steel tube as the wall what would you suggest then with reagrd to thickness I mean would it be better for us to weld the plates in rather than using tie rods to hold plates together or a combination of both ??

Any help you can give would be appreciated

Kind regards

Francis

http://www.ceros.org.uk

teamnd@hotmail.com

 

[kenvlach]

I think a safer, simpler design would be to modify an aluminum compressed gas cylinder of the right size. (It's been a while, but) I believe that aluminum compressed gas cylinders are seamless, made by a deep drawing process, with the top end spun down to a greater thickness & then threaded.
Check out
"Standards for Visual Inspection of High Pressure Aluminum Compressed Gas Cylinders
Document Number: CGA C-6.1
Compressed Gas Association
01-Jan-2002
Description: A guide for visual inspection and hydrostatic testing of aluminum cylinders having a service pressure of 1800 psig [my emphasis] and over. This publication covers preparation for inspection and hydrostatic testing as well as inspection criteria for defects."

http://www.cssinfo.com/info/cga.html

Also:
"Pressure Aluminum Compressed Gas Cylinders, 1995
CGA Pamphlet C-6.2, Guidelines for Visual Inspection and 173.34

Requalification of Fiber Reinforced High Pressure Cylinders, 1988
CGA Pamphlet C-6.3, Guidelines for Visual Inspection and 173.34

Requalification of Low Pressure Aluminum Compressed Gas Cylinders,1991
CGA Pamphlet C-7, A Guide for the Preparation of Precautionary 172.400a"

http://www.c-f-c.com/gaslink/defense_mil.htm

I believe C-6.3 covers fiberglass-reinforced aluminum cylinders. These are inspection rather than manufacturing standards, but should guide you in the right direction.

Hope this helps,
Ken

 

[kenvlach]

Federal specification RR-C-901D CYLINDERS, COMPRESSED GAS: SEAMLESS SHATTERPROOF, HIGH PRESSURE DOT 3AA STEEL, AND 3AL ALUMINUM, TABLE III. Cylinder sizes: high pressure industrial and medical gases, aluminum (DOT specification 3AL) includes aluminum cylinders of 5.3” diameter (I presume OD) in 14.0, 14.2 and 17.1” heights. Doesn’t give wall thicknesses. I found it interesting that to verify shatterproof, a cylinder filled to the rated pressure must pass a gunfire test. “An armor-piercing projectile 0.50 caliber in size shall be fired at the cylinder. The 0.50 caliber projectile shall strike the cylinders at a velocity of 2800 feet per second, ±100 feet per second.” A free download from ASSIST:

http://assist.daps.dla.mil/docimages/0004/03/71/901D.PD5

All necessary mechanical specifications (e.g., wall thickness formula, material Al 6061-T6) are given in Code of Federal Regulations (CFR)
49 CFR 178.46 - Specification 3AL Seamless Aluminum Cylinders.

http://frwebgate5.access.gpo.gov/cg...SdocID=992729221627+1+0+0&WAISaction=retrieve

 

[unclesyd]

Why don't you look at Aluminum hydraulic cylinder.

There are aluminum bodied jacks in you size range.

 

[cerosuk]

Hi Guys,

Thanks for the information, all very useful and helpful in this case. I Think I have found an alternative soultion. I have managed to loacte some spare weight and am going to modify a steel hydraulic clyinder again, at leat i know this can handle the pressure (one in question is rated to 10000psi) the wall thickness is 10mm which is going to be milled down to 6 leaving 10mm ring at the top to bolt a flange plate on, it will have a welded bottom to it and use the original piston and rod. Thanks for all your help, with regard aluminium pressure cylinders I have been looking at modifying a co2 fire extinguisher which is rated to 3 times the pressure we were looking at but I keep getting told this is not safe by so many people I just gave in. The wall of these cylinders is 8mm 6061 t6 aluminim and this holds 212 bar so i dont know but i think I know where I am heading now. Thanks once again

Francis

 

[kenvlach]

212 bar = 3075 psi, and this presumably includes a safety factor. By the way, the aluminum gas cylinders I mentioned above were rated 1800 psig or higher.
However, I like unclesyd’s suggestion, since it eliminates machining. And aluminum hydraulic cylinders “are 60% lighter than comparable tonnage steel jacks.” Hard anodized piston rods and cylinder bores, too. See part number 29035T73 at

http://www.mcmaster.com/.

30 ton capacity, 6” lift, rated 10,000 psi. With some searching, you could find something lighter that still has a decent safety factor.

 

[cerosuk]

Basically on the UK fire extinghuishers the burst disk is set to 212 so the point at which the cylinder goes will be a bit more than this. I like the idea of these aluminim rams you mention I had a look on mcmaster and they dont have an eye connection at the top which is great as we are using a rolling piviot on the arm I am going to try and find a similar thing in uk if I can.

Thanks again

Francis

 

[unclesyd]

You can get a double acting type jack where you can take the pad off the ram and screw an in an eye.
We use them for push-pull all the time just by changing to the eye and swaping the hose.

 

[laidback50]

My company manufactures AA6061 aluminium alloy cylinders designed to EC Directive 84/526/EEC. All are pressure tested to 250 bar and have a typical failure pressure of 440 bar. They are used for containing liquid CO2 for a domestic appliance (drinks machine).
A 90mm diameter cylinder weighs around 1.6kg, has a water capacity of 1.34 litre and takes a charge of 1kg of liquid CO2 when using a fill ratio of 0.75kg CO2/litre water capacity.
Similar 60mm diameter cylinder weighs 0.7kg, 0.605 litre w.c. and is charged with 450g of liquid CO2.

Bursting discs are always fitted to CO2 cylinders to safeguard against overpressurisation but for a robot I think I would sleeve it with a thin steel tube to give better penetration resistance.

I was a little nervous when I switched from steel cylinders to aluminium but they are very strong, considerably lighter and you don't typically have a problem with corrosion.