Re: Spring Design 1

[mikemkc]

Does anyone know of a good share/free ware program for spring design?

Thanks,

mikemkc

 

[GregLocock]

Don't know if it's any good, it does seem OK

http://www.rentoncoilspring.com/design.html

Note that it does not include rate softening, or fatigue life.



Cheers

Greg Locock

 

[ivymike]

You can also make a spreadsheet for the purpose fairly simply. Shigley's _Mechanical Engineering Design_ has all the necessary equations, if I remember right. The only catch will be coming up with good size sensitivity and fatigue strength properties for your materials. I've made such a spreadsheet myself, but unfortunately I can't share it (made on company time with a small amount of proprietary information).

As far as general guidelines go, I constructed my spreadsheet as follows (Shigley can help with this). My goal was finding the highest frequency spring that would fit in a particular package and still survive both static and fatigue loads:

inputs -
wire material, UTS, fatigue strength vs size info, density, shear modulus, max displacement, wire diameter, minimum fitted force, number of dead turns, min and max limits on force @ max displacement, package ID, package OD, minimum solid clearance

calculations -

for each of a series of values of force @ max displacement, I calculate the following:
spring index, mean diam, outer diam, inner diam, concentration factors, max working stress, max solid force

then I calculate max stiffness based on allowable solid force, max stiffness based on min fitted force, max stiffness based on allowable stress range

then I take the max of those three, which I call max allowable stiffness, and I output an indicator to show which limit was the most restrictive (fitted force, stress range, min force)

then based on my max allowable stiffness I calculate fitted force, fitted stress, stress range, fatigue cover factor, number of active turns, solid length, fitted length, solid force, and natural frequency.

the end.

Sorry, I can't share all of the equations.

 

[GregLocock]

Sounds good. I have a mathcad worksheet that has no proprietary info in it, if you need it. To be honest, as mike suggests, it is better to code it up yourself, then you know what assumptions you have made.

The Bosch blue book and MIL handbook 5 are invaluable sources of fatigue properties for materials for springs, but you should also chat to your spring manufacturer.

One persistent issue I came up against was the rate softening under load - our spring supplier just didn't 'get' the idea that the actual rate at nominal design was my important parameter.

Cheers

Greg Locock