Compression Spring Design

[matthewcsmith]

Hi everyone,

I'm trying to learn about compression springs. I've got 2 designs which work very well in their environment and have been evaluating the designs to try to replicate similar life & performance just in a more compact design at lower force levels.

After looking at the calculations for flat & ground end compression springs I see that the number of active coils is N-2 where N is the total number of coils. So, I go about calculating the solid height. After some measurement and physical compression testing, there's no way this calculation is correct. The spring could be compressed MUCH further before it coil binds.

What am I missing here?

 

[israelkk]

Can you give real physical data?

1. Free height
2. Outside diameter
3. Wire diameter
4. Number of total coils
5. Wire material
6. Forceqs at the measured deflection/s
7. Is it for static use or cyclic (fatigue) use?
etc.

Without this info there is no way to understand your question and answer it.

 

[matthewcsmith]

Sure, here are the actual dimensions.

Free Height: 81mm (3.19")
OD: 63mm (2.48")
Wire: 13mm (0.512")
Total # Coils: 5.25
Mat'l: nickel alloy
Working Force: ~780kgf @ 69.5mm (1720# @ 2.736")
Static loading

By my calculations, this yields a solid height of 68.3mm (2.687"). In actuality, the springs are able to compress well past this amount. At 68.3mm height, the spring is able to compress about 10mm more!

Any thoughts?

Matt Smith
Mechanical Engineer
'04 RHIT Grad

 

[israelkk]

What kind of nickel alloy? Inconel X-750? What kind of heat treatment? or what is the module young (E) and shear modulus (G) and what is the Ultimate tensile strength?

 

[btrueblood]

Total # coils = 5.25 - I get your calculated solid height by multiplying wire diameter by this number. But, depending on what you mean by "Total # coils", the difference in solid height may be:

(for closed & ground springs)
Each end of the spring is ground by (approximately) 1/2 of the coil diameter, so the solid height is given (approximately) by :

solid ht. = wire dia. * ((total coils) -1)

Thus, the effective solid height is shorter than your calculation by about 12 mm.

 

[israelkk]

matthewcsmith

I think there is a confusion what it means closed and ground. Usually such spring are Squared (closed) and ground where the ground is at least 270 degrees of the end coils. In this case the solid height is = total number of coils * wire diameter (68.3mm).

It is best if you can post a picture of the spring.

If the actual spring you have has smaller solid height maybe it has less coils.

 

[matthewcsmith]

After considering some of your comments and visual inspection of the spring, I think I have some idea of what's going on here.

The ends are closed & ground. The grinding process looks to have removed maybe half of the wire diameter. BUT I think the big issue here is the wire diameter compared to the spring pitch. The wire is pretty large (at least it looks it to me) and the pitch is not so large. The last bit of the closed end runs parallel to the coil below it.

Depending on how much you grind the end of the spring, will determine what the solid height of the spring it.

Does this make sense? If I wanted a spring with additional compression travel than what the standard calculations say is possible, how do I specify how much to remove from the closed ends?


I have attached a picture.


Matt Smith
Mechanical Engineer
'04 RHIT Grad

 

[matthewcsmith]

Another picture

Matt Smith
Mechanical Engineer
'04 RHIT Grad

 

[israelkk]

This spring in the picture has a little less than 5 coils!!.

Can you specify what do you want from the spring in terms of forces and travel. You still didn't supply information regarding the exact material specification (or alternatively, the E - module young, G - shear modulus and ultimate tensile strength of the spring wire after heat treatment).

 

[TVP]

Matt,

I agree with israelkk, this spring has less than 5 coils, not 5.25. And yes, the fact that this spring has a small coil diameter, but a large wire diameter, has a significant effect on the solid length. Your specification for squareness and end parallelism will also have an effect on the solid length, as these will influence the amount of grinding that is required.

 

[matthewcsmith]

Thanks for the information. The springs are performing well and meet the required force at the required height. So, it's not a big issue.

After futher discussions with the manufacturer, some things are varied a bit to keep the required specifications. Looks like pitch, # of turns, and grind are just a few of them.

Matt Smith
Mechanical Engineer
'04 RHIT Grad