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aluminum spring

aluminum spring

(OP)
Hello, I am trying to find some aluminum spring material. The shape of the spring needs to be rectangular (1 ½”L x 3/8” W x 1/16” D). I don’t know the spring constant at this point but it has to be higher than what you get from typical material machined out of stock from the hardware store.

The spring will be held at one end and the other end will be moving in an arc. I don’t know the spring constant but I know what displacement I need from the high tension position to the low tension position and that’s about 5/8”.

My question is: does anyone know where I could purchase such a material even if it was not that exact profile that I need I could machine it.
Thanks,

Jacques

RE: aluminum spring

There is no form of aluminum that can do that.

RE: aluminum spring

(OP)
This is what I originally thought then I googled "aluminum spring" and there are some manufacturers that make aluminum helical springs. What are they doing that's different?

Also in my case I am not worried about a fatigue limit, the spring only needs to work once.

Cheers,

Jacques

RE: aluminum spring

Little things like modulus (AL springs exert little force) and yield strength) come into play long before fatigue, butt hat is an issue with Al also.
You start with the force and displacement, and then design the spring, not the other way around.

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P.E. Metallurgy, Plymouth Tube

RE: aluminum spring

(OP)
Yes, I have no problem designing the spring from 1st principles. My main issue is finding the right aluminum alloy. Aluminum 7075 seems to be a good candidate, it has a 61,00 psi yield strength.

RE: aluminum spring

So you meant helical spring when you said it was a rectangular bar? It's hard to get good answers from really bad questions.

RE: aluminum spring

(OP)
Don't know where you get helical spring from. it's a rectangular cross-section bar. It's also hard to get good comments without getting side-swiped like that.

RE: aluminum spring

some manufacturers that make aluminum helical springs

Want less side-swiping? Put up a picture of the item you are designing and the calculations that led you to believe that it can work.

RE: aluminum spring

(OP)
Thanks. I have found a source McMaster Carr, unfortunately they don't deliver in Canada and I'll have to get around that.

RE: aluminum spring

1. The spring constant is based on geometry and material, any Al alloys of the same size and shape will have the same K. It is just that stronger alloys will flex further before taking a set.
2. Have you bothered to calculate the stress involved for flexing this piece 5/8"? Just as a guess it sounds very high.
3. 61ksi is very weak for spring material, since you need to work at about 20% of yield to get good life. Think spring steels with 200ksi strengths.
4. All Al alloys have poor fatigue resistance. If you are looking at more than a few thousand cycles you need to start looking at other alloys.
5. Why not steel (or a stainless)? You could make it a lot thinner and lighter, and get more force and longer life.

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P.E. Metallurgy, Plymouth Tube

RE: aluminum spring

(OP)
Thanks, all very good points
2. Not yet, will do shortly. I am going to start with a much thinner plate 0.030", stress will be lower;
3. Good point I am going to look for a stronger aluminum alloy;
4. I only require 1 cycle;
5. It needs to be aluminum, the higher concentration of aluminum the better, it is part of something that needs to be 100% recyclable.

RE: aluminum spring

JJJ, so the entire thing is Al, no other materials at all?
One cycle you might get.
If the stress is too high consider stacking multiple thin leaves, they would need a film or lubricant between them but that is minor.

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P.E. Metallurgy, Plymouth Tube

RE: aluminum spring

You've made 6 posts, and not once did you say what load is applied to get your 5/8" deflection.

So, what spring constant do you need? Once you've figured that out, you can then use existing formulas to figure out what material properties are required.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm

RE: aluminum spring

Since he didn't post a load I figured that he would take whatever he got, since he has fixed all of the other variables.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

RE: aluminum spring

I don't have my aluminum yardstick handy, but I do have some 6061 (probably) Al name plates that are 20 about mils thick; they'll deflect about 5/8", but only over about a 3-inch span, which means that for 1 1/2" span, something on the order of 5/16" deflection is more realistically possible. Steel at roughly the same thickness can easily do the required deflection over the required span.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm

RE: aluminum spring

IRStuff, He was looking at material over 3 times as thick and half the length of the test. The stiffness is around 30 times higher from thickness alone ((.063/.020)^3), and supporting the shorter material has to take up some of that length, so maybe 2.5 times stiffer due to shortened length; overall around 80 times the sample yield strength. Now he's shifted to .030 for some reason which is still over 3 times as stiff; pushing a required yield to 8 times that of your sample.

A first principles calculation would have calculated the strain required to get the desired deflection. Since that is geometry driven and no geometry is given, we are left in the dark as to what the OP had in mind. I would have started with the desired deflection and the allowable strain to determine the material thickness and then look at the width limit to set the required modulus of elasticity to get the desired force; possibly with the use of multiple leaves if the combinations are unsuitable.

RE: aluminum spring

What is driving you to aluminum? Is it just recyclability? Steel can be recycled too!

RE: aluminum spring

(OP)
You all make very good points. The 7075 material is not expensive, and I can get a variety of thickness'. I'm working this experimentally as I have to do tests that will prove the principle, that is the desired deflection for the span of the bar. Multiple leaves may be required to get the desired deflection however not sure what bonding material I could use as contamination is a problem, the parts must be food grade compatible.

RE: aluminum spring

If you use multiple leaves you do not want to bond them- multiple leaves work to increase deflection by reducing strain, as the leaves are allowed to slide against each other. You need lubrication between the leaves, not adhesion. If you bond them together you get the same stresses and strains as you would with a full section.

RE: aluminum spring

(OP)
Yes of course, good point.

RE: aluminum spring

If the parts must be food grade compatible and recyclable, I don't see why stainless steel would not be acceptable. It would make a MUCH better spring than aluminum.

RE: aluminum spring

This sounds like an application for 'Belleville-style' stacked spring-disks... or perhaps bellows-style/stacked/welded spring disks.

These could be from punched/formed from thin aluminum high strength aluminum sheet blanks assuming a suitable corrosion protective mechanism.

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

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