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I have a steel coil spring that is used by compression. It is too strong, and I would like to reduce its strength by some fraction, ideally keeping its length.

Reducing it to roughly half would be nice. Uncontrolled heating and cooling in air would make it very unelastic, I think, and shorter after the first compression.

It can be manually handled. (The dimensions are: cylindrical with diameter 3.5 mm, length 60 mm, and round wire wire diameter 0.5 mm.)

I would be interested in the theory, even if it's something that I can not do, like precisely controlled heat treatment. I have only two of them, it can be very slow.

Volker Siegel
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    The Young's modulus (elastic modulus) of steel in remarkably consistent over various alloys and carbon contents. The difference that heat treatment makes is to raise or lower the bending stress that the steel can withstand. So anything you do with heat will just make it deform and stay deformed. – TimWescott Dec 06 '19 at 21:07
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    Have you thought about of, instead of manipulating the compression springs, adding some tension springs as “negative strength” springs? You could possibly run them inside the compression springs, to save space. – leftaroundabout Dec 07 '19 at 14:38
  • Short answer: Nope. – Hot Licks Dec 08 '19 at 23:46
  • @HotLicks That is a good summary, I'd give it a +1 as an answer! – Volker Siegel Dec 09 '19 at 02:54
  • Do note that if you have two springs, and there is no requirement that the length be maintained, you can just stack them into a single assembly twice as long. – Hot Licks Dec 09 '19 at 03:25
  • @leftaroundabout That's a really brilliant idea! It does not work for my example case, because of it's geometry, but as a general answer, relevant for later reader, I think it may be the best answer, after 7 already exist! (I see SE an an important archive, so I think answers are important even when they do not solve the practical problem at hand). Again, it's just brilliant! Please make it an answer! – Volker Siegel Dec 14 '19 at 07:34
  • When you said "I have a steel coil spring..." and "...keeping its length" it makes me (and it looks like others) think this is a one off which is why the hacks. If you are designing something new, different answers are appropriate. – user1683793 Dec 04 '24 at 20:21
  • In Machine Design class, I learned (as did most of the other people responding) that coil springs are effectively torsion bars that have been wrapped to make the coil. Reducing the diameter or increasing the number of coils (ie, making the "torsion bar" longer) will make the spring weaker, as will reducing the Young's modulus of the material they are made out of. Removing material by acid or electrolysis will make the coils thinner. The idea of reshaping the coil, I'm not sure how that would work. – user1683793 Dec 04 '24 at 21:17

11 Answers11

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You could try a bath of concentrated acid. As long as you could maintain circulation so the concentration was fairly constant, a spring presents a very uniform cross-section and should be dissolved at a consistent rate.

I would recommend keeping one of the springs as a reference so that you can measure the result after a short period and adjust your time correspondingly to obtain the desired result.

Gonzonator
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By "strength" do you mean stiffness/modulus? The modulus does not change with tensile or yield strengths. Make it with wire of 1/2 the diameter for 1/2 the modulus. Acid will hydrogen stress crack hardened steel ( This condition has many other names like "embrittlement".). Make it out of aluminum and get 1/3 the modulus or titanium and get 2/3 the modulus ( approximately). Doing your own heat-treatment could work ; simple enough to try : Heat to orange in a gas flame ( 1600 F +/-) hold time - a couple seconds , IMMEDIATELY quench into cold salt water. Temper at 300 to 400 F ,somewhere around an hour is traditional. Good for carbon and most low alloy steels. High alloy or tool steel -another story. The heat-treatment risk is the exceptionally small thickness which could partially decarburize in a few seconds at 1600 F. Commercially it would be heat-treated in a protective atmosphere or vacuum , possibly with induction heating.

blacksmith37
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Probably just another hypotetical idea: If you squeeze the wire from circular into a square shape, it will have about 54% of the original rate. If you squeeze it slightly more into a rectangle, it will at some point become 50%. And 1/2 the diameter of the wire does not gie you 1/2 the rate. It would give you 1/16 of the rate. 85% of the diameter gives you 1/2 the rate.

Ingo
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Another hypothetical way to reduce the amount of material in the spring would be to fix it in a jig stretched to double its normal length.

Then, heat it sufficiently to anneal it and remove the tension in it.

Then, re-temper it and cut it in half, back to its original length.

But just buying a new spring would be a much simpler method.

alephzero
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While I think the concentrated acid solution is brilliant, I'd like to suggest a refinement.

Rather than use just any old acid, you should specifically use a chemical etchant.

Typically, a chemical etchant's nucleophile strength does not come from Hydrogen, so it should have the benefit of much lower -if not entirely eliminated- hydrogen enbrittlement. (At least theoretically, anyway.)

A good etchant for this might be Ferric Chloride, [FeCl3]. Copper(II) Chloride [CuCl2] is another possibility.

Getting an even and precise etch will also be up to idiosyncrasy's of the exact setup you try and run. Chemical concentration, bath agitation, cleanliness of the work, temperature, ...

Another, related, option is to use electrochemistry instead of direct chemistry.

Specifically, use the reverse of electrowinning, electroetching.

One of the pros of this method is that, with controlled current + time, you could fairly easily calculate and control the exact mass of the material removed by the relationship between the unit of electric charge (coulombs), and the unit of molar mass (moles.) This extrapolates well to controlling the exact change in spring's mechanical property.

With direct chemistry, concentration + temperature + time = mass of material removed. Even if you precisely dial in the last two variables, the first one is going to be a ever changing target, as the work will dilute/poison the bath.

Probably the biggest con of this (aside from requiring a semi-sophisticated electronic power supply) is that surface irregularities will become much more of a problem than with chemical methods. In particular, any insulator will be practically inert to etching with the electrochemical method.

Conversely, an insulator might still be attacked by a pure chemical method. e.g. Any oil on the surface might become esterfied to something soluble, be freed from the surface, thus allowing the etchant to get at the actual work piece.

Charlie
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I would consider perhaps the easiest way would be to change the leverage...

Failing that, I would get a new spring made with the details necessary and spare one 10% above and below your "new" value.

Solar Mike
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Another way to decrese the stiffness without making the spring longer:

Making it longer would obviously decrese the stiffness. And adding other springs in series would do also. You could add springs in series and put them one in another. The inner springs should be shorted and connected end-to-end. You could have three springs in total. The second connected to then end of the first, going back inside. At some point (say half of the outer springs lengt) you append another spring, which is again inside the second. It should be long enough to reach out of the first one. Well, you may have to cut the first one a little bit.

I don't expect this will be useful, but it really depends on factors as your packaging, available of springs, what you really want to achieve...

Ingo
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A possible way you could fix this, if you have access to something like a blowtorch, or better yet, a blacksmithing forge: you can heat the steel up until it is glowing orange turning yellow, and then find a place to set the spring down while it cools without anything touching it. If you allow it to cool slowly enough, the spring will become annealed, softer and more malleable and flexible.

TheHashtagKing
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Run the coils back and forth over sandpaper. Try to do this evenly to maintain the wire's diameter consistency.

Rusty
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What if you just heat it in its long relaxed state? Or heat just a portion of it's length?

John
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Slide the spring over a rod. Walk over to your grinding wheel. Grind off the outer 15% of the wire's diameter along the entire length of the spring. Check the rate. Rinse and repeat.

Greg Locock
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