Spring theory

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zebedeesknees
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Spring theory

Postby zebedeesknees » Tue Feb 01, 2022 7:59 pm

I had not intend to mention this, but must, so new topic so as not to annoy anyone..

Shortening a coil spring makes it shorter - self evident.

Shortening a coil spring makes it stronger - counter intuitive.

Think of a coil spring as a torsion bar/spring wrapped into a helix. Shortening it makes it stiffer.

Reducing the wire diameter or the diameter of the coils makes it softer.

Ted.
(A purists' purist)

davebradwell
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Re: Spring theory

Postby davebradwell » Tue Feb 01, 2022 8:39 pm

..er diameter of coils....?

You know really!

DaveB

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zebedeesknees
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Re: Spring theory

Postby zebedeesknees » Tue Feb 01, 2022 9:39 pm

davebradwell wrote:..er diameter of coils....?

You know really!

DaveB


Yes Dave, and I was expecting you to beat me to it over shortening. For this ol' brain, twisting a greater radius seems clearly to be easier.

Take a conical helical spring such as used to return solenoid cores, and flatten it slowly against a flat surface. You'll find that the outer coils touch the surface before the inner ones.

Or, take two identical lengths of wire and coil one into a large diameter helix, and the other into one of a smaller diameter and try compressing them.

Ted.
(A purists' purist)

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Will L
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Re: Spring theory

Postby Will L » Tue Feb 01, 2022 10:17 pm

zebedeesknees wrote:...
Shortening a coil spring makes it stronger - counter intuitive...
This I worked out for myself some time back. I agree that lots of people have made the mistake of believing the opposite.
...Reducing the wire diameter or the diameter of the coils makes it softer.

Obviously that's true for reducing the wire diameter, but how physically do we explain it for reducing the diameter of the coils? I would have expected the contrary.

Crepello
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Re: Spring theory

Postby Crepello » Wed Feb 02, 2022 10:21 am

Will L wrote:
zebedeesknees wrote:...
Obviously that's true for reducing the wire diameter, but how physically do we explain it for reducing the diameter of the coils? I would have expected the contrary.

Regarding the effect of coil diameter, if the load is assumed to align with the central axis of the cylinder
bounded by the coils, then the torsion in the rod forming the coils is generated by the load force multiplied
by its offset from the rod, which is half the diameter of the coil. A given force applied to a larger diameter
coil will generate a greater torque, and thus greater deflection, than when applied to a smaller diameter
coil.

davebradwell
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Re: Spring theory

Postby davebradwell » Wed Feb 02, 2022 6:32 pm

It seems very simple - stiffness of coils spring = G.d**4/8.N D**3. It's available anywhere.

The bigger you make D then the lower the stiffness. Don't blame me!

DaveB

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zebedeesknees
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Re: Spring theory

Postby zebedeesknees » Wed Feb 02, 2022 6:44 pm

Will L wrote:
zebedeesknees wrote:...
Shortening a coil spring makes it stronger - counter intuitive...
This I worked out for myself some time back. I agree that lots of people have made the mistake of believing the opposite.


You may also remember Will, there was a 'discussion' on E4um years ago on the subject. That was when we were allowed to freely express our opinions...

...Reducing the wire diameter or the diameter of the coils makes it softer.

Obviously that's true for reducing the wire diameter, but how physically do we explain it for reducing the diameter of the coils? I would have expected the contrary.


Lots of spring stuff is far from obvious, probably why there is still some resistance to progress in that area.

It's the demography you know, the older one gets, the more conservative, upper and lower case...

Ted. (74 next week but resisting...)
(A purists' purist)

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zebedeesknees
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Re: Spring theory

Postby zebedeesknees » Wed Feb 02, 2022 6:56 pm

What I really want is working self-contained sprung buffers.

Chris Gibbon produced some fabulous castings for the Husky, but the beams are so high relative to the running plate that the conventional way of springing through the beams is impossible. The same applies to the Southern A1 and A1X, and probably more.
I suspect that the real thing used volute springs, though winding those in 4mm scale is too much of a challenge, conical helixes would work, but then there is the issue of retaining the buffer head. On four... Nah..
Anyone?

Ted.
(A purists' purist)

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Will L
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Re: Spring theory

Postby Will L » Wed Feb 02, 2022 7:09 pm

zebedeesknees wrote:...there was a 'discussion' on E4um years ago on the subject. That was when we were allowed to freely express our opinions...
but I always tried not to be rude
...It's the demography you know, the older one gets, the more conservative, upper and lower case...

Ted. (74 next week but resisting...)

Possibly, but 16 years working with Citizens Advice has had the opposite effect, upper case anyway.

If found being 74 isn't too bad, its the prospect of getting to 75 thats bothering me.

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zebedeesknees
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Re: Spring theory

Postby zebedeesknees » Wed Feb 02, 2022 7:33 pm

Will L wrote:but I always tried not to be rude

I didn't, but I do on here. I thought that being accused of being a 'smatarse' warranted a response or deletion, but apparently there are different rules for different members, or the moderator has blind spots...

If found being 74 isn't too bad, its the prospect of getting to 75 thats bothering me.

It's only one more Will, tiny percentage now!

Ted.
(A purists' purist)

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johndarch
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Re: Spring theory

Postby johndarch » Wed Feb 02, 2022 7:38 pm

Will L wrote:
If found being 74 isn't too bad, its the prospect of getting to 75 thats bothering me.


75 doesn't seem to me to be much different to 74. We were going to celebrate the event in style but Covid put paid to that. Saved us a lot of money though!!

davebradwell
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Re: Spring theory

Postby davebradwell » Wed Feb 02, 2022 8:06 pm

It must be better than not getting to 75, Will.

DaveB

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Julian Roberts
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Re: Spring theory

Postby Julian Roberts » Thu Feb 03, 2022 2:40 am

davebradwell wrote:It seems very simple - stiffness of coils spring = G.d**4/8.N D**3.


Well of course

s holmes wrote: elementary my dear Watson


Defo an insomniacs' thread...the very first post loses me

zebedeesknees wrote:
Shortening a coil spring makes it shorter - self evident.

Ted.


My ol' brain asks, do you mean shortening it by cutting a bit off of it? Or by compressing it? If the latter would you twist it some more times? Could you or would that change the diameter, hence ....errr.... the strength......... :?:

Well ... anytime I think that even a simple (bendy bit of straight wire) spring Is what I need I get into a load of incomprehensible effects and I just don't understand what's going on...other than, bend it and see what happens, try to solder it on without it moving when it's doing what you want it to do....start again.....try a different gauge of wire...different material.....

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John Bateson
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Re: Spring theory

Postby John Bateson » Thu Feb 03, 2022 8:36 am

Will, if found being 74 isn't too bad, its the prospect of getting to 75 thats bothering me.

Didn't bother me at all - don't remember it.

And after reading through this lot its not just various body parts that hurt, now my brain has joined the set of painful areas.

John
Slaving away still on GCR stuff ...

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LesGros
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Re: Spring theory

Postby LesGros » Thu Feb 03, 2022 9:04 am

John Bateson wrote:
Will, if found being 74 isn't too bad, its the prospect of getting to 75 thats bothering me.
Didn't bother me at all - don't remember it.
And after reading through this lot its not just various body parts that hurt, now my brain has joined the set of painful areas.
John


A beneficial(?) effect of COVID-lockdown brain fog is that one does not remember the headache . . .

Commiserations to all who feel they need them. :D
LesG

The man who never made a mistake
never made anything useful

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Will L
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Re: Spring theory

Postby Will L » Thu Feb 03, 2022 11:09 am

Crepello wrote:Regarding the effect of coil diameter, if the load is assumed to align with the central axis of the cylinder bounded by the coils, then the torsion in the rod forming the coils is generated by the load force multiplied by its offset from the rod, which is half the diameter of the coil. A given force applied to a larger diameter coil will generate a greater torque, and thus greater deflection, than when applied to a smaller diameter coil.

I'm just beginning to get my head round this. I think the confusing thing here is that we are considering the length of the coil not the length of the spring material it takes to make it. Is it not correct to say, to get a greater diameter of coil over the same effective length of the coil, you need a greater length of the spring material and the longer the length of a spring material the softer the spring it makes its going to be? Because that makes logical sense and fits with the fact that shortening a spiral spring makes it stiffer .
John Bateson wrote:
Will, I found being 74 isn't too bad, its the prospect of getting to 75 thats bothering me.

Didn't bother me at all - don't remember it.

Think I'd worry about not remembering it John, however next year me also I suspect (hope). Its just that the maternal side of my family had a nasty habit of not getting past 75

And after reading through this lot its not just various body parts that hurt, now my brain has joined the set of painful areas.


I find this sort of pain between the ears helps deaden those which increasingly are occurring else where.

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Winander
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Re: Spring theory

Postby Winander » Thu Feb 03, 2022 12:02 pm

Will L wrote:the longer the length of a spring material the softer the spring it makes its going to be


That is the part that is counter-intuitive to me. I think longer wire, more resistance simply because there is more of it. I only resolved it when I imagined the wire straight - longer = more bendy. Then I thought about tighter coils (smaller diameter) in a spring making it stiffer and decided to lie down for a while.
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davebradwell
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Re: Spring theory

Postby davebradwell » Thu Feb 03, 2022 1:04 pm

Think of each coil as a lever acting on a piece of the wire in torsion - the longer the lever (ie diameterD) the less force needs to be applied. Anyway, the formula leaves no room for doubt.

DaveB

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Winander
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Re: Spring theory

Postby Winander » Thu Feb 03, 2022 1:22 pm

A very clear explanation thanks Dave. A spring, something that appears simple but requires so much explanation.
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zebedeesknees
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Re: Spring theory

Postby zebedeesknees » Thu Feb 03, 2022 5:27 pm

Winander wrote:A very clear explanation thanks Dave. A spring, something that appears simple but requires so much explanation.

Relaying this topic to a good friend (M.Sci theoretical mathmatics) this morning, his reaction was "Too many variables!"

Ted.
(A purists' purist)

RAO
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Re: Spring theory

Postby RAO » Thu Feb 03, 2022 7:22 pm

My conclusion is:-
The longer the wire the less is the force it produces.
The heavier the wire the more force it produces.
Taking the same length of wire!
The tighter you coil it the more force it will produce [as it is less flexible].
I'd love to be proved wrong, but this is what i've found making pick-ups for various locomotives.

davebradwell
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Re: Spring theory

Postby davebradwell » Thu Feb 03, 2022 9:19 pm

The length of wire doesn't really come into it - if you halved the diameter of the spring it would be 8 times stiffer. To use the same length of wire you'd then wind twice as many coils so that would make it half as stiff. Combine the two and same length of wire gives spring that is 4 times stiffer.

It's a simple fraction with the 4th power of wire diameter on top and on bottom 3rd power of spring diameter X number of coils. A tiny increase in wire gauge gives a large increase in stiffness whereas increasing coil diameter reduces stiffness slightly less quickly.

DaveB

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Re: Spring theory

Postby davebradwell » Thu Feb 03, 2022 10:16 pm

Late extra.
If you are talking about pick-ups the discussion above only relates to stiffness of coil springs in compression. If you are winding/unwinding a coil spring it is a completely different situation and as a first guess is completely independent of D - just length and wire diameter. Cube of length, 4th power of dimeter.

DaveB

Phil O
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Re: Spring theory

Postby Phil O » Fri Feb 04, 2022 8:14 pm

Having been involved in the making of safety valve springs and one or two lesser ones during my apprenticeship nearly 50 years ago, the most important thing after the actual grade of steel, was the heat treatment of the finished article. After we had made them, we passed them through a hatch, to the test and measurement centre, where they were tested to see if they matched the spec, failures were re- heat treated, a second failure was chucked in the bin. Producing the sort of springs to match the prototype in 4mm would be nigh on impossible. Stretching a spring weakens it. I did have all the notes on the do's and don'ts of spring manufacture, but I probably chucked it out when I took redundancy.

Cheers

Phil.


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