6D calculation of spring-forces/moments

[Helferlein]

Hi,

I'm a student in electrical engineering and I'm writing my master thesis at the moment. Ironically I'm now confronted with the deformation of springs. I'm not a physics (!) but I think and hope that you may can help me. Simple push and pull forces are not the problem.
I need to calculate all possible moments and forces and all possible combinations which can act on a spring. So I need to cover stress, torsion, shear, bending in all three spatial directions. My advisors told me to have a look at the beam theory but I didn't found an analogy for springs.

Can you help me?

Thanks in advance,
Helferlein

 

[Nidum]

Why do you need to do this ??

 

[Helferlein]

Why do you need to do this ??

The goal is to record the deformation with a camera and get the applied forces and moments out of this. It's some kind of a visual 6D force/torch sensor.

 

[Nidum]

So you have a simple helical wire spring anchored at one end and free to move in all directions at the other end ?

There is nothing intrinsically difficult about working out the deflection of a spring under compound loading . Might take you a while though .

Conceptually you have to break the spring down into elemental lengths and match forces and deflections between adjacent elements all along the spring using analytic methods .

There is also an approximate method where it is assumed that the several types of flexure do not interact and that they can be analysed independently .

Alternatively use a numerical method based on a chain of discrete elements each of which consists of one full turn of the spring . Using matrix methods and some computer time a good model of the spring response could be developed .

Personally I would use FEA or just buy a spring and test it .

 

[Nidum]

Just note :

There are lot's of other ways to design an all axis sensor .

Almost certainly there are better ways of doing what you want than monitoring the movement of the end of a single spring .

In any case a different concept for the sensor design might well make the maths much easier .

 

[Mech_Engineer]

This application sounds questionable... While it would be reasonably easy to measure length change with a vision system and correlate with the force applied to a compressing a spring, it will be very difficult to calculate/decompose some of the more complicated shear or moment loads (especially out-of-plane ones) you could "in theory" apply to a spring.

My recommendations if you're stuck to the vision system is:

1. Choose a simpler structure to analyze, like a simple cross-section fixed-end beam (Wikipedia: Beam Bending)
2. Limit the number of loads you're planning to determine, it isn't realistic to think you can decompose more than 2 or 3 of combined loads on a complex structure via analytical means.
3. Alternatively, you might consider developing a vision system which uses a photoelastic method for determining stress in parts via stress birefringence.