Spring design guideline!

[krisreliable]

I am currently looking at a spring failure which is designed to be pre-loaded during assembly @54% deflection (400 lbs of load) and could see deflection upto 75% during service.

What are some of the design factors that I should consider to verify if this spring is rated for this application? Is pre-loading at this deflection for a long period of time a concern? I would appreciate any insight on this.

 

[israelkk]

Did the spring failed or you are looking to see if it may fail?

If it did fail then more detail info may help.

 

[desertfox]

Hi krisreliable

How long was the spring in service, it might be a fatigue failure but could also be one of several other failure modes.
Can you load a picture up of the failed spring and physical details of the spring itself I presume its a compression spring but you haven't actually confirmed that.

You might like to look at this site about springs and in addition it as links to other sites regarding springs.

http://www.roymech.co.uk/Useful_Tables/Springs/Springs_helical.html

desertfox

 

[krisreliable]

Thanks for the response guys. The spring did fail and yes it is a compression spring.

The tool was pre-loaded (55% deflection) during assembly and kept in the shop for about a month. Was put in service for about a few hours (75% deflection) and was again put back in the shop for a couple of months and found broken during dis-assembly. So it is really unclear when the spring could have broken.

I doubt fatigue condition existing here as the loading conditions on the spring do not change. free height of the spring is 6" with wire dia of 0.28".

krisreliable

 

[israelkk]

Without giving the full details of the spring and assuming the spring is not over stressed, I would suspect hydrogen embrittlement if the spring has gone chemical cleaning or plating.

If you want us to give more intelligent answers please provide:

Spring material
Spring outside diameter.
Spring total and active number of coils
Spring solid height
spring free height
Spring working environment and temperature
Is the spring plated and which plate?

 

[krisreliable]

sorry about that!

Spring material - Chrome vanadium silicon
OD - 3.275 in
Total & active coils - 5.559
max solid height - 1.5"
free height - 5.595"
no plating
I wouldnt expect to see any temperature. its drilling tool. but it is isolated from drilling mud by seals.

krisreliable

 

[israelkk]

Something doesn't add up with your spring data and the 400 lbf load at 54% deflection. According to my calculations spring with your dimensions can give no more than 254 lbf at solid height. Is the free height - 5.595 inch was measured after the spring was loaded or on a new spring?

 

[desertfox]

Hi krisreliable

What are the ends of the spring like are they squared and ground ? the reason I ask is you have active and total number of coils as 5.559, if the spring ends are closed, closed and ground then your active coils will be less than what you posted earlier and the knock on of that is you will have a much stronger spring but a highly stressed one just glancing at the data.
If possible can you upload a file of the broken spring ends.

desertfox

 

[krisreliable]

Yes it is closed & ground! I do think it is highly stressed as well looking at the shear stress values at the max deflection. Here is a picture; let me know what you think! Thanks.

krisreliable.

 

[GregLocock]

Looks like an inclusion in the steel to me. How close to the end is this?

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[desertfox]

Hi krisreliable

Looking at the failure I would say that the spring failed under tensile principle stress and I can see what I think are river lines running back on the surface to the centre of the section.
If the ends are squared and ground your active coils are not the total number of coils but less by about 2 I think off the top of my head.
Are we certain the spring is only statically loaded.

desertfox

 

[krisreliable]

It is under static load once assembled. It is put in service only for a few hours and it might see minor variations in the load during that time.

BTW I did study some inclusion rating on this and nothing jumped out!

krisreliable.

 

[desertfox]

Hi krisreliable

According to my approximate calculations your shear stress is running at 83% of the torsional yield stress value at 54% compression of spring and going over 100% just, at 75% compression of spring. I based the torsional yield stress value at 50% of uts using the figures off the RoyMech site
(link given in my first post)so you should do a check with your spring supplier for more accurate data.
So it does look to me that your spring is highly stressed and normally for purely static duty I would say 70% of uts is probably the maximum you could go in relation to a torsional yield limit which your spring appears to exceed anyway.
Have you talked to your spring supplier? he may be able to shed some light on your failure.
Thanks for the picture, any chance you can post one of it at 90 degrees to the fracture surface?

desertfox