Reducing length of conical spring but keeping stiffness and number of coils the same

[Pand3mic]

I need to reduce the length of a conical helical compression spring without changing the spring stiffness or the number of coils in the spring. To do this, will I need to change the spring wire diameter?

Spring length is changing from 7mm to 5mm.
Number of active springs = 3
Large outer diameter = 5.8mm
Small outer diameter = 4mm
Diameter of spring wire = 0.4mm
Modulus of Rigidity = 78.5 GPa or 78500N/mm^2
Spring stiffness k = 1N/mm


I am finding it difficult searching for the correct equation to use.

I am also inexperienced in spring design and would appreciate anybody's help on this.

Thanks in advance!

 

[desertfox]

Hi

This site should help with the formula

http://www.roymech.co.uk/Useful_Tables/Springs/Springs_helical.html#Compression

If you change the wire diameter you automatically alter the spring stiffness

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[Pand3mic]

Thanks for the reply. I was looking at that site trying to figure it out.

What I mean is; is it possible to keep the same spring stiffness, the same number of coils (turns) and same wire diameter if I was was to reduce the free length of the spring from 7mm to 5mm?

My instincts suggest that if I was to reduce the free length of the spring while keeping the same stiffnes and same number of coils, then something else would have to change?

 

[desertfox]

Hi you could reduce the spring height by just rewinding the spring with a smaller helix angle but only if you have enough clearence between the coils on the existing spring.
In addition if you do manage to rewind the spring with a shorter length then the springs working length might be compromised and also the load at length for your application.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[israelkk]

What do you mean by "Spring length is changing from 7mm to 5mm"?
What is the force/s needed from the spring?
What is the spring heights at the desired force/s?

 

[Pand3mic]

By spring length, I mean the free, unloaded length of the spring.

I do not know the force exerted on the spring as I cannot measure it. The height of the fully compressed spring is about 0.8mm.

I realise that I am probably missing too many variables here but theoretically will the actual diameter of the spring wire in a conical spring change if the free length was reduced while keeping the (a) the stiffness, (b) the number of coils the same?

Thanks in advance!

 

[MikeHalloran]

Your first problem is that a conical helical spring does not have a single valued rate or stiffness; the spring gets stiffer as more coils reach the seat and become inactive.

You really need to measure what you have before you start changing it, so you will know in which direction to go, and how far to go.

In the case of a conical spring, computing a rate or a stiffness is largely a waste of time. It is better to measure the force necessary to compress the spring to a series of lengths. A force gauge is the most expensive part of the necessary instrumentation. The rest is just platens, some forcing means like a large C-clamp, and some calipers.

You can make a graph of force/length, then overlay a sketch of how you'd like the curve to move, and send that to spring suppliers for estimation and suggestions.



Mike Halloran
Pembroke Pines, FL, USA

 

[israelkk]

MikeHalloran

You are correct when the coils pitch is constant then, the spring rate is progressive because the largest coils touches the next coil first and so on. However, to make a constant rate conical spring the pitch is made variable such that all coils will bottom at same time.

 

[Pand3mic]

Hi Mike,

Thanks for your breakdown and explanation. It has made things much more clear.