Reducing the stiffness of a spring?

[some bloke]

TL;DR

I have a spring which I am using to cushion a simple device, but it has proven to be too stiff for the purpose. Is there a way to reduce the stiffness of a spring, without making it brittle or malleable?
I have access to ovens which can achieve 550°C, which can also quench in water or poly-oil.

I have a spring which I am using to cushion a simple device, but it has proven to be too stiff for the purpose. Is there a way to reduce the stiffness of a spring, without making it brittle or malleable?
I have access to ovens which can achieve 550°C, which can also quench in water or poly-oil.

summary of the device:
it's a pogo-stick, anchored to the ground with a universal joint and with a plate on top. items weighing between 10-20Kg are put on top of the plate, and then gently bounced & shaken by the user. Currently, the stiffness of the spring yields almost no bounce. It will only travel about 20mm when a 12.3Kg item is placed on top. I was hoping for significantly more, in the region of 100mm. The spring is ~300mm long, from a pogo stick.

2 options I have considered are:

1: stretch it - would this yield more travel on the spring with the same force applied? I'm not 100%, as the stiffness would remain the same.

2: Heat it and either air-cool or quench it. I suspect quenching would make it harder.

I also have ovens which can achieve 900°C, and can potentially quench from them.

Any advice gratefully received!

 

[jrmichler]

Heat treating affects the strength, but not the modulus of elasticity, so does not affect the spring rate. Stretching the spring affects the length, and would allow more travel, but does not change the spring rate. Stretching the spring is usually a bad idea because full compression afterwards can yield the spring back to its original length.

The spring rate is a function of the modulus of elasticity, number of active turns, wire diameter, and spring diameter. A good discussion of this is: https://www.engineersedge.com/spring_comp_calc_k.htm. Changing the stiffness can be done by changing any or all of the variables that control spring stiffness. One easy way to do this is to add a second identical spring in series with the existing spring. You will get twice the travel and half the stiffness.

Or just buy a different spring. A good source is McMaster-Carr: https://www.mcmaster.com/springs. They have thousands of springs to choose from, and are perfectly happy to get an order for only one spring.

 

[Baluncore]

You could wind another spring with the same length and number of turns but from wire that is thinner than the original. If I remember correctly, the spring constant is the 4'th power of wire diameter. To change by a factor of 5, the wire must be thinner by √√5 = 1.5 so wind it with wire that is 67% of the original diameter.

 

[jrmichler]

You could wind another spring with the same length and number of turns but from wire that is thinner than the original. If I remember correctly, the spring constant is the 4'th power of wire diameter. To change by a factor of 5, the wire must be thinner by √√5 = 1.5 so wind it with wire that is 67% of the original diameter.

Yes, and inversely proportional to the third power of the major diameter. Winding your own spring is a good DIY project, and you can buy spring wire from McMaster-Carr (or other sources). The photo is my first attempt at winding a replacement spring for a trash pump mechanical seal:

The largest learning experience was to release the tension before cutting the wire. The snapback distorted this spring, so I had to make another one. The second one did the job. The wire is 0.120" diameter stainless steel spring wire.