Two equivalent springs for one? 1

[Theophilus]

I've got a compression spring that gives me the function I need, but is a bit too large a diameter. Is there a way for me to figure out how to turn this single spring (21 winds over 6", .188" diameter wire, 1.0" OD) into two springs that would deliver similar force as what I've got, each with less OD? The spring is compressed just under an inch in a normal working cycle.

I'd appreciate any direction as to how to figure this out. Thanks!



Jeff Mowry

http://www.industrialdesignhaus.com

Reason trumps all. And awe transcends reason.

 

[GBor]

Does it matter if the two springs are in series or parallel?

 

[Theophilus]

I'd like them in parallel, each sharing the plates that compresses the spring, one next to the other. I'm hoping that by reducing the diameter sufficiently, I'll solve my volume issues and retain the even spring force through the cycle.



Jeff Mowry

http://www.industrialdesignhaus.com

Reason trumps all. And awe transcends reason.

 

[GBor]

Helical compression spring? If so, helical springs in parallel are additive, so your current spring constant K is equal to the sum of the two springs your are looking for, k1 and k2, or K = k1 + k2.

K = P/f = Gd^4/(8D^3N) where, G = shear modulus
D = Coil diameter
d = wire diamter
N = number of active coils

Now you can play with the parameters. Ultimately, sounds like you want each spring to have half the stiffness of your current spring. Easy way would be to half the modulus by selecting another material, or double the number of active coils per spring.

Garland E. Borowski, PE
Borowski Engineering & Analytical Services, Inc.
Lower Alabama SolidWorks Users Group

 

[Theophilus]

Thanks--yes, these are helical compression springs.

It will be interesting to see if I can find a value for each of those variables, since what I've got currently is what "works"--unknown material, but I can otherwise measure the physical dimensions. (This spring resembles a long rocker arm spring.)



Jeff Mowry

http://www.industrialdesignhaus.com

Reason trumps all. And awe transcends reason.

 

[GBor]

Probably some type of steel. G should be around 11e6 psi, but if I think about it for a minute, if you are just going to order a different spring from the same catelog, they probably give a spring constant...choose one with half the constant that you currently have.

If you are going to fabricate the spring, a quick metallurgical test will tell you what type of metal...or wave a magnet over it...if it sticks, it's a form of ferrous steel.

The other thought is that if you set K = 2*k, substitute the equation above, you get:

Gd^4/(8D^3N) = 2G1d1^4/(8D1^3N)

Assuming the same material, active coils, and coil diameter leaves you with:

d^4 = 2*d1^4

taking the forth root of both sides gives:

d = 1.19 d1

or d1 = d/1.19

That help a little more?

Garland E. Borowski, PE
Borowski Engineering & Analytical Services, Inc.
Lower Alabama SolidWorks Users Group

 

[Theophilus]

Thanks--that helps a lot. This spring came out of a product that's doing something similar to what I'm doing in a new design. It's definitely a steel spring, but I know not what particular material. Meanwhile, I can play with the number you gave me to rough things out.

Thanks!



Jeff Mowry

http://www.industrialdesignhaus.com

Reason trumps all. And awe transcends reason.

 

[israelkk]

From quick calculations the spring will give you ~160 lb at 1" deflection. If you do not compress the spring beyond the 1" then a smaller diameter spring with different wire diameter can be designed. It depends on the spring material that you need. For a music wire a 0.7" outside diameter is feasible.

However, you didn't give the work conditions of the spring such as, is it for cyclic use? Then how many cycles are required before fail. What are the environment temperatures? Does it has to be a stainless steel? etc.

http://israelkk.googlepages.com/home

 

[israelkk]

I meant that you can stay with one spring only and have the same force and smaller diameter.

 

[Theophilus]

Thanks! This will probably need to withstand several thousand (10K) cycles before failure. Conditions will normally be indoors at room temperature--nothing extreme.

A smaller diameter for the same sort of functional purpose would be great--this spring's life will probably be long-lived in the current configuration.



Jeff Mowry

http://www.industrialdesignhaus.com

Reason trumps all. And awe transcends reason.

 

[MintJulep]

Is your existing spring guided over a rod on the ID?

If not you need to make sure that your new spring won't buckle.

You could also look at nesting one spring inside the other. One RH, one LH.

 

[Theophilus]

I plan to run a guide rod through it. I need a small diameter, so spring-over-spring probably won't work out in this case.



Jeff Mowry

http://www.industrialdesignhaus.com

Reason trumps all. And awe transcends reason.

 

[GBor]

I was waiting for you to weigh in on this one, israelkk. This is your specialty, isn't it?

 

[israelkk]

GBor

Yes, design and analysis of mechanical springs is one of my specialties. But, can you explain what did you mean by "weigh in on this one". English is not my native language and especially not slang. I think I gave enough information even though Theophilus didn't give more info about the current spring material and what it is used for. Anyway, because this is a spring that is cyclical loaded or maybe even under impact loads there is no point to be more specific.

MintJulep

You can not use nested springs 6" long with those diameters the outer spring will buckle and rub on the inner spring even if the inner spring is guided.

 

[Theophilus]

israelkk, to "weigh in" on something is (roughly) to express your opinion on an issue. (I'm not sure of the origin of this expression.)

By the way, I do not need stainless steel on this. What I've got seems to be some sort of a hard steel--as you said, perhaps a "music wire"--although the wire is quite thick.

Thanks for your help.



Jeff Mowry

http://www.industrialdesignhaus.com

Reason trumps all. And awe transcends reason.

 

[israelkk]

Theophilus

Where did you get the spring? Is it a custom designed spring or a catalog spring?

 

[Theophilus]

I removed the spring from a product that used it in a four-bar link sort of arm. The spring was used to help support a load at the end of the arm.

So I don't know about the origin of the design--custom or catalog. It has ground ends to meet a flat face (each end).



Jeff Mowry

http://www.industrialdesignhaus.com

Reason trumps all. And awe transcends reason.

 

[Theophilus]

I just found a handy site with a calculator (and I was all ready to do this in Excel):

http://www.engineersedge.com/spring_menu.shtml

Thanks for your help, everyone! This should at least get me close enough to create a prototype that will function fairly closely to the spring I've got--but with two smaller-diameter springs.



Jeff Mowry

http://www.industrialdesignhaus.com

Reason trumps all. And awe transcends reason.

 

[MadMango]

If you're looking for an off-the-shelf spring, give Century Spring a look. Their web site has some useful calculators as well.

http://www.centuryspring.com/

"Art without engineering is dreaming; Engineering without art is calculating."

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[Theophilus]

Thanks! If we don't find something suitable off-the-shelf, we'll just make it, since we'll have enough volume with the production plans.



Jeff Mowry

http://www.industrialdesignhaus.com

Reason trumps all. And awe transcends reason.