Stress Relieving- torsion springs

[eng1234]

I'm looking to find more information about strength/life cycle increases for a torsion spring that is heat treated after manufacturing. Our manufacture claims a 12-15% increase in the cycle life of the spring if we "bake" the spring after forming with the following process 20min. @450 deg F. Can someone direct me to a source to validate this information?

 

[metengr]

Can you provide more information? What is the spring material, specified heat treatment and dimensions?

 

[Goahead]

It is hard to discuss the validity of a treatment without knowing to which material it has to be applied.
A different cold surface treatment applicable to various spring materials for improving fatigue life would be shot peening.

http://www.welding-advisers.com/

 

[swall]

The coiling process will leave the spring I.D.in residual tension. Since torsion springs are loading in bending, these residual stresses can be a problem if the spring operation tends to increase the body diameter. In this situation, the spring should definitely be stress relieved. If the loading on the spring tends to decrease the body diameter, then the only benefit you get from stress relief will be dimensional stability or stress relief in the end hooks or end bends. Can't comment on the stress relief temperature without knowing the material.

 

[eng1234]

Metengr

Here is some additional information regarding the initial question posted:

Material: A228 Music wire
Wire diam: 0.135"
spring OD: 1.0"
active coils: 241
Max torque: 62 in-lbs.

At this state of torque it puts the stress on the spring at 104% (according to our SMI spring program). Based on wanted a longer cycle life, we are investigating a post mfg process to improve the claim of 12-15% more life in the spring if we relieve the internal residual stresses? We have having samples run to cycle test, but wanted to read more about it in the meantime.

 

[swall]

Typical stress relief for music wire is 375-450F, so the proposed 20 minutes at 450F is right in there. Cycle testing is a good way to evalute the effectiveness, unless you want to spend the time and money for X-ray diffraction. Most stress relief cycles used by commerical spring manufacturers do not give anywhere near 100% stress relief, by the way.

 

[eng1234]

swall

Thanks for the reply and confirming that the stress relief process our manufacturer is recommending is in the ball park. Of course this additional process is going to cost us money on an already expensive spring. Do you know if the 12-15% claim holds water?

Thanks!

 

[swall]

The claim does not strike me as being totally out of line. But basically, you're in the "trust but verify mode" here. Take their recommendation for a trial batch of springs and conduct cycle life testing against a control group. Make sure the stress relief performed on the trial batch is representitive of what the production process will be, i.e. if the production process is to be on a belt furnace, make sure the trial lot is run that way (rather than in a batch furnace).

 

[EdStainless]

The other advantage to baking springs after forming is that you will get more consistant performance.
Normally the improvement in median life does not change much, but the minimums improve a lot.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.

http://www.trenttube.com/Trent/tech_form.htm

 

[israelkk]

eng1234

According to your data the spring free length is 33.9 inch am I correct?
Is the spring designed to deflect 14 revolutions?
The stress at that moment is 291ksi where the minimum ultimate tensile strength of 0.135" Music wire diameter is 258ksi.

A quick calculation gives that this spring can not be loaded beyond 40 lb-in without permanent set.

http://www.webspawner.com/users/israelkk/

 

[israelkk]

eng1234

Just wanted to add that swall is correct saying that in general these residual stresses can be a problem if the spring operation tends to increase the body diameter, then the spring should definitely be stress relieved.

More then that the concentration factor (Wahl factor) in this case is much higher than the case where the loading on the spring tends to decrease the body diameter.

When the loading on the spring tends to decrease the body diameter then these residual stresses are favorable because the outside diamter is in compressive stress condition due to the forming process. Therefore, stress relieving will make more damage then help.

 

[NickE]

>>"At this state of torque it puts the stress on the spring at 104%"<<-eng1234

104% of what? Yield? Tensile? Endurance Limit?

 

[eng1234]

Thanks for all the great points to consider. Yes Israelkk, you are correct with the spring length it's aboue 35 in (but has inactive coils on the ends).

In our application the spring has 7 initial turns then alternates up to 15 turns and back down to 7. My current cycle testing being run has shown no set or drop in torque after 5,000+ cycles. However I question the validity of the test if we are not holding the spring at max torque for any length of time between cycles.

Our spring program shows the following data:
wire diam: 0.135"
Mean diam: .865"
Initial torque (@ 7 turns): 30.98 in-lbs
Max torque (@ 15 turns): 66.388 in-lbs
State of stress: 104% of Tensile strength
Body length: 34.695"
(no corrosion factors have been accounted for)

Interesting that releiving residual stress may infact be harmful since the spring decreases in diameter when in use, and any grain realignment would make it easier to reduce the diameter instead of resist it.

Besides adding additional wire (weight and cost) can we shot peen or increase cycle life without changing wire diameter using other methods

Thanks to all for your comments!!

 

[israelkk]

First, I assume the spring is guided on a shaft or a hole and if the inside or outside diameter of the spring touches the shaft or hole there may be a friction between the inner/outer side of the spring and the shaft/hole and the 66.388 in-lbs is composed of the spring resistance and the friction with the shaft/hole combined. If this is the case then the spring may see much less torque than 66.388 in-lbs.

The minimum ultimate tensile strength of 0.135" Music wire diameter is 258ksi however, the maximum is 285ksi. Therefore, it may be that the actual wire you are currently using is on the upper level of the spec and therefore you do not see any detectable set. However, you have to design for the minimum spec values. There is also a chance that the actual wire diameter is more than 0.135" (in the upper level of the wire diameter tolerance).

According to the Associated springs Company (SPEC) for static applications you can design for up to 100% of the ultimate tensile strength “for springs without stress relieve” and 80% for stress relieved springs. However, your case is definitely cyclic. The SMI Handbook of Spring Design does not discuss the stress levels issue.

In each cycle your spring approaches UTS therefore, from fatigue design point of view (on the SN curve) the spring life should be less than 1000 (out of the graph). The reasons I mentioned earlier may be the cause that your spring in the tests show higher life cycles.

You should notice that the spring inside diameter may decrease to 0.952 inch when maximum loaded.

Can you provide the guide shaft (hole) diameter?
Can you describe how the spring is loaded? Is it loaded with pure torque or it bends due to the force on the arms?
How many of the 241 coils are truely active inactive?
How long are the arms?

http://www.webspawner.com/users/israelkk/

 

[eng1234]

Israelkk,

Once again thanks for your comments. Here's another point we had to consider:
- Upon assembly of this spring we stretch the spring 2.5" in length over a 1/4" OD rod so we impose torsion stress in the spring and buckling in the rod. However as we roll up the torque this torsion strength should go to zero as the spring grows in length and increase the bending stress in the coils.
- Can you advise where one could fine Ultimate Tensile Strength tables for various wire sizes?
- We have design software that advised us the reduced diameter at max torque so I think we have that covered.
- The springs has an inward hook end that comes in contact with a zinc cast part.
- Yes 241 total coils and they are all active.

Thanks!

 

[sreid]

How about a modern ultra high strength steel like Carpenter AerMet 310.

http://www.cartech.com/products/wr_...?leftn=our_products&lefto=empty&lefti=our_sa0

 

[israelkk]

eng1234

As I mentioned in my previous post from a stand point of fatigue this spring is not intended for cyclic application. Just be aware that in future batches of manufactured spring the spring may not last even 1000 cycles. It is very difficult in such springs to avoid nicks, scratches and tooling marks on the inward hook bend radius which will promote stress concentration points and as a result a premature failure due to cyclic application.

For music wire UTS for various wire diameters look at

http://assist.daps.dla.mil/quicksearch/

and do a search for QQ-W-470.