Spring Design û Change in Diameter with Compression 5

[Jalipa]

Hi Guys,

A handbook I’ve got states that a spring has a 3-4% change in diameter when compressed.

I want to fit this spring in a fairly tight hole.

Does anyone know how the change in diameter is calculated?

I seem to recall that there was nifty little formula but I cannot find it.

Can any one help?

Thank you,

J

 

[BlueBeam]

Hi,
I´ve also learned a lot of formulas about such stuff, but in the end its only important that it works. For ´springs´ you probably want to use, you just multiply the nominal OD with 1,166667 and round up the result on one place after the comma.

regards,

MT

 

[Chapstick]

I would give Dayton a call. I worked a couple years designing progressive metal stamping dies and I remember doing this calc but have long forgotten. They will only be able to help you tho with die springs tho. If it's a wire spring you are dealing with im not much help.

http://www.daytonprogress.com

 

[unclesyd]

Go to the following:

http://www.efunda.com/designstandards/springs/spring_introduction.cfm

I don't know if they have the information you need but they have a lot on spring calculations.

 

[desertfox]

Hi Jalipa

For a compression spring which has ends that are prevented from unwinding during compression the formula for expansion on diameter is as follows:-

change in dia= 0.05 * (p^2-d^2)/D

where d = wire dia

D = mean dia of coils

p = pitch of the coils when spring is in its
free state
formula represents spring going from free height to solid
length.

regards desertfox

 

[Jalipa]

Thanks Desert Fox!!

J

 

[israelkk]

Please, when you give formulas include the units too, especiallly where numerical factors such as 0.05 is included.

 

[TheTick]

There is software available from SMI, the Spring Manufacturers Institute. Check Google to find the website. Their software is the de facto standard among spring makers.

We use the old DOS version of the SMI software. A bit clunky, but the math is solid. The v5 software had some programming errors that made it worse than useless, but v6 is supposed to have fixed that.

All this machinery making modern music can still be open-hearted.

 

[Jalipa]

I now have several formulas:-

Alternative version of the formula above (from an Italian Spec....don't ask):-

change in dia = p^2-d^2/20D

and

change in dia = p^2-0.8d^2pd – 0.2 d^2/10D

Units would be s.i. (metric)

I have a demo copy of Institute of Spring Technology (IST) Spring Software....it is better than say the formulas/calculators on Efunda, the Engineerstoolbox and anything else I've found on the web by far...it is also cheap (not that i've bought it yet) but it is modular...and generally whatever i seem to want isn't on the modules on the demo.

Their website is

http://www.ist.org.uk

I don't know if this is the same as the the SMI software.

J

 

[israelkk]

Can you please explain why you want the spring in a tight hole?

Take into consideration the following:

1. A spring is not a precision product. The coils may have a slight lateral shift (especially the end coils).

2. There is a commom tolerance on the outside diameter if you want to keep the price reasoable.

3. Even in a squared and ground spring the axis of the spring may have a +/- 3 degrees of tilt.

4. The spring axis is rarely a straight line it usually has a curve shape which increase with the increase of the load.

 

[Jalipa]

I say ‘tight’ in that I don’t have a lot of control over its size.

Cost reduction is my aim and I am hoping to reduce the number of parts in an existing assembly. Currently, I have a two springs in a blind hole, which apply a pre-load. There is also a rubber ‘o’ ring; elsewhere; which also provides some pre-load. (The ‘o’ ring was a latter fix).

What I want is one spring to replace all three parts.

Ideally without changing the part where it fits.

Consequently, I need a spring of sufficient rate to provide the required load. However, this could (and did) mean increasing the diameter of the spring slightly.

I was concerned that a larger spring, when compressed, might jam in the hole. Thus, wanted to establish how much the diameter will increase.

Btw – I’ve got a spring that I want now ;-)

 

[sprintcar]

Why not look at Wave Springs as an option? I went from a group of compression springs to a single wave and saved $$.. plus NOT as diameter sensitive.
I used

http://www.smalley.com

but there are a bunch of others

Keep the wheels on the ground
Bob
showshine@aol.com

 

[earnest]

I too, would love a simple formula for spring expansion (and contraction). I checked SMI but the software is $600.

So, that brings me back to this thing:

change in D= 0.05 * (p^2-d^2)/D

If pitch is units of 1/L, then this isn't math. If its in L per turn what are the units?

Also, the expansion is surely a function of compression from free length.

Any clarification is appreciated.

 

[israelkk]

Associated Springs co. design handbook 1987 edition at page 31 gives:

O.D.(at solid)=SQRT(D*D+(P*P-d*d)/(Pi*Pi))+d

D=spring mean diameter with no load
d=wire diameter
p=coil pitch
Pi=3.1416...

The spring outside diameter at no load=D+d
The difference O.D.-(D+d) is what you are looking for.

 

[Jalipa]

Spring Design Software:-

http://www.ist.org.uk

sell their "base module" for $167 and $335 for the Design module. There is also a Graphics module for pretty pictures(also $167)

J.

 

[israelkk]

For the last 30 years I designed hundreds of spring from many types, Helical compression, extension, Conical, Torsion (round and rectangular wires) Belleville (Snap action), etc.

But never has a problem with tight hole problem. IF you are trying to use the spring for other then producing force I think that approach is risky, If you design the surrounding of the spring too then I am sure you can make adjustments (enlarge the hole).

If you are trying to fit a spring into a hole and you can not increase the hole, I can try to design a spring for you to meet your demands. There are many ways you can try. You can switch to a stronger wire or you can design a spring with preset, etc.

If you can define your problem more accurately I may help you.

israelkk2001@yahoo.com

http://www.webspawner.com/users/israelkk/

 

[unclesyd]

All the spring design software you will ever need.

http://www.hexagon.de

 

[philalger]

why dont you try uratane as a spring it has a high spring ratio over a short distance try danly uk on 0121 585 7171 uk

phil

 

[Jalipa]

A high spring ratio means a large increase in force over the working length. Often that might not matter – however I require the spring to apply a ‘constant’ load. So I need to increase the level spring force but keep a narrow variation band. To do that I need to look at free length, wire diameter and number of coils. I then have problems of solid height versus working length and stress. Other constraints are the bore depth and diameter which I cannot change (increasing spring diameter improves stress). I looked at wave springs (thanx 4 the ID Sprintcar!!) but ruled them out because of the v high force variation.

I've got a compression spring now.

But I've never heard of uratane.

Jalipa

 

[TVP]

I believe that was meant to be urethane as in an elastomeric polyurethane.