How to build an Adjustable Tension Spring?

Question

Is there such a thing as an off-the-shelf adjustable tension spring? I've searched extensively without success.

The problem I'm trying to solve is as follows:

I have a tension spring hanging from a fixed point and I want to:

1. Achieve the same deflection with different masses hanging from the spring, and
2. Be able to set the different masses into oscillation and have them oscillate through similar (doesn't have to be exact) displacements

I've thought of two ways to solve this problem and am hoping to ask opinion on whether both will work and, if yes, which will work best?

Solution 1:

Engineer a simple screw mechanism for one end of the spring. For heavier weights it is "screwed in" such that it collect coils - effectively making the spring shorter and increasing the spring constant. For lighter weights it can be screwed out to achieve the opposite effect. This should allow me to achieve the same deflection with different weights, though I'm unsure of the affect it will have on the oscillation of them? (as the spring constant will be different)

A good example of similar screw mechanism can be seen here - https://youtu.be/YNI-U5IiLeI?t=174.

Solution 2: Use a compression spring in a coilover arrangement such that it's possible to pre-compress the spring for heavier weights using a nut that screws up a pipe (red arrow below). This should allow me to achieve the same deflection for different stationary masses and I am pretty sure will also provide similar displacements when they are in oscillation (provided a constant rate spring is used), but the downside would be that there is more friction in this system as the rod will need to pass through a guide/bearing. It's also quite a bit more of complicated build.

I've tried to sketch this setup below

Very interested to know if either of these will work? Or if there's something all the shelf that can do this?

Answer 1

I don't think either of the ways you are proposing with change the stiffness of the spring. What they will change is the starting value at compression (so you will immediately start from another point in the curve).

IMHO, the simplest way to achieve what you want is to reduce/increase the length of the spring.. The spring constant is inversely proportional to the length of the spring.

• If you double the length of the spring the spring constant will be half.
• if the length of the spring halves, the spring constant will double.

So if you double the mass, but the same displacement and behavior is expected then you can change the location of the support. I.e. support in the middle. This will have the effect that the spring constant will double, and overall the $\frac{K}{m}$ will remain the same (where K is the stiffness constant and m is the mass of the system).

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A word of caution: if you want to investigate the damping behavior (or aspects of it) or if the damping is not zero, then the above method will not produce the exact same results. The reason is that the $\frac{c}{m}$ coefficient will change, and you are not certain that halving the distance will have the same effect as the spring.