Spring Creep? 1

[hamsuplo]

I have an extension spring that is used (theoretically well below its yielding point) and used at room temperature.

It is under constant loading (yield load is around 65kg~, constant load is no more than 10-20kg) and within one day it stretched out and didn't return to its original shape.

Obviously there is some problem with the spring. The material is steel. Assuming that the material is perfectly fine, what could cause this "creep" to happen? Heat treatment time to little?

Thanks in advance

 

[TVP]

Relaxation or load loss is one of the most common failure modes for tension springs (extension springs). Here are some links for more information:

http://www.mw-ind.com/pdfs/ResidualStressProcessing.pdf

http://www.lambdatechs.com/documents/275.pdf

http://www.navsea.navy.mil/nswc/carderock/src/mechrel/products/handbook/CHAPTER4RevE.pdf

http://www.leespring.com/downloads/uk/leespring_engguide.pdf

 

[israelkk]

Without knowing the exact data of the spring it is impossible to give an intelligent answer.
A full manufacturing drawing or a catalog number (if it is a standard off the shelf spring) is best.
Otherwise, the following data is a start:
Number of coils
Wire diameter
Outside diameter
Shape of ends (drawing)
Wire material
Spring body length between the ends

 

[hamsuplo]

the spring is 28coils, 3.2mm diameter, and standard steel

i've looked up some information on relaxation/creep and it depends on the degree of stress, temperature and time, which is why it confuses me since the stress is low, the temperature is room temperature and the time is only 1 day.

what are the possible causes of failure for this stretching of the spring?

 

[hamsuplo]

Also another question, what happens to a spring if you don't heat treat it? Does this make the spring more likely to suffer creep?

 

[TVP]

You need to know what the stress in the spring is prior to use. If the spring has not been heated after coiling, then it will have residual stresses that can cause relaxation, even if the theoretical in-service stresses are low.