DIFFERENCE BETWEEN COLD COIL COMPRESSION SPRING & HOT COIL COMPRESSION SPRINGS 6

[mvalmiki]

HI

I REQUEST ALL TO PLEASE LET US KNOW THE DIFFERENCE BETWEEN COLD COIL COMPRESSION SPRING & HOT COIL COMPRESSION SPRINGS IN TERMS OF ITS MECHANICAL PROPERTIES,WHICH SPRING LIFE IS BEST. WE DID NOT UNDERSTAND WHY SPRINGS WERE BREAKING DURING THERE SERVICE LIFE EVEN WHEN BOTH THE SPRING WERE LOADED WITHIN THERE STRESS LIMITS. & WHAT BEST PRACTICE NEED TO CONSIDERED TO AVOIDE THE CREEP OR SAGGING AFTER CERTAIN TIME OF SERVICE.

THANKING ALL IN ADVANCE FOR YOUR VALUABLE FEED BACK IN ADVANCE.

MAHESH

 

[israelkk]

Can you please give the specific of the spring? A drawing with mechanical requirements will be best.

 

[swall]

You need to have a failure analysis performed by a qualified metallurgical engineer before you can proceed. I would also point out that the usage of hot coil vs cold wound springs is primarily a matter of wire or bar size. Small springs are cold wound; large springs are hot coiled.

 

[Tmoose]

" WE DID NOT UNDERSTAND WHY SPRINGS WERE BREAKING DURING THERE SERVICE LIFE EVEN WHEN BOTH THE SPRING WERE LOADED WITHIN THERE STRESS LIMITS. "

Which "stress" limits/ Yield, endurance, ???

It does not take much of geometry aberration to increase local stress.

The legend is The nearly magical fatigue improvement of Shot peening was discovered by accident when some rusty or dirty valve springs were reclaimed by grit blasting.

As suggested by others, an important step is careful inspection of the fracture surfaces etc for initiation at a surface defect or material flaw or something else altogether.

 

[TheTick]

Two things come to mind that have been issues for me in the past...

1. One version of the popular SMI spring calculator software (version 5 or 6?) had an bug that was miscalculating spring life. This resulted in many spring failures.

2. Shot-peening springs can radically improve fatigue life.

 

[MintJulep]

Very often you will find that no one told the actual loads what your design assumptions were.

And the actual loads have decided on their own to be much higher.

 

[tbuelna]

As swall noted, typically the only reason helical compression springs are hot wound is because the wire diameter (>1/2") and/or coil configuration won't allow cold winding. Cold winding of alloy steel would normally be preferred if practical, as it allows the use of high quality cold-drawn and pre-tempered wire. Hot winding of alloy steel requires the spring to be heat treated after winding.

A helical compression spring is similar to other high performance metal components like gears or shafts when it comes to fatigue. And they will benefit from the same approaches used to optimize fatigue performance such as careful attention to raw material cleanliness/metallurgy/microstructure, heat treatment, surface texture, and mechanical working such as shot peen.

To minimize the degree of "creep or sag" a helical compression spring might experience during its service life, sometimes the spring can be pre-set by compressing the spring to coil-bind before installation. But this is only applicable when the torsional stress at coil-bind is somewhere between about 40-60% of the wire's min tensile strength.

As others noted, there are a couple things you should do before taking any corrective action. Perform a thorough examination of your damaged springs to establish how they failed, collect as much data on the failed springs as possible including raw material pedigree, manufacturing processing and QA records, and any service/operational history, and consider doing some stress and fatigue analysis to determine if the basic design of the springs is up to the task.

 

[desertfox]

Hi

If you can provide details of the spring we may be able to help you further, it might be your spring is failing under fatigue which would explain why the spring is not stressed beyond its safe limits.

 

[mvalmiki]

hi guys..

thank you very much for your valuable feed back..

we observed that the springs were failing due to unethical manufacturing process (heat treatment was not properly done) followed by manufacturer..

 

[mvalmiki]

hi all,

we have further question to all..

as per DIN and IS standards springs above 8 or 12mm dia wire/rod should be hot coiled due to some manufacturing difficulties..

now we have approached some manufacturers they say that they can coil the springs up to 30 mm dia wire/rod with cold coiling process.

please suggest whether the spring made from wire dia 12mm to 30mm with cold coiling process would be equally good as hot coiled springs??

awaiting your valuable suggestion..

 

[mp87]

now we have approached some manufacturers they say that they can coil the springs up to 30 mm dia wire/rod with cold coiling process.

Spring manufacturers run faster than standards. 30 mm still sounds quite high to me, but I remember a European manufacturer that was able to reach 15 mm some two years ago.

Usually cold coiled springs are better as they do not have some limitations on alloying elements which are instead present in hot coiled ones.

EDIT: Dimensional tolerances are tighter for cold coiled springs, too.

 

[btrueblood]

What mp87 said, plus, cold coiling work-hardens the material resulting in higher yield strength, which benefits the fatigue life of the spring.

 

[TVP]

I concur with the others, cold coiling tends to be better than hot coiling. I am not familiar with any equipment that can crop and coil wire as large as 30 mm, more like 19-20 mm.