NAF-PH4 strain relief

[brin]

Does anyone have information or a copy of NAF-PH4? Many older drawings (1950's era) refer to this specification for strain relief of springs for aircraft use. As far as I can tell this is from the Naval Aircraft Factory (NAF).

thanks

 

[sbozy25]

I do not have this spec. in my military spec files. More than likely this is an obsolete spec. replaced by a mil spec. What material are you talking about when that is being referenced, I might be able to point you at the mil spec you need. Or you can go to

http://assist.daps.dla.mil/online/start/

and see if they can get you a copy. That is the website that we get all our DOD specs from.

 

[TVP]

I have never seen this designation before. I even checked the 1950 edition of MIL-S-6715 Springs; Helical, Aircraft which only references AN-H-1 for heat treating.

 

[brin]

most of the springs are A228 music wire. The "strain relief per NAF-PH4" is a general note even where the spring material is not specified. I looked at MIL-S-6715 v1950 also. From what I understand, strain relief is used to remove some of the residual stress due to forming, and improve the life of these parts. Would that be correct?

 

[israelkk]

Not exactly, sometines the residual stresses are intended to stay such as in the case of a preset (set-remove, scragging).

 

[israelkk]

I want to suggest that you calculate the stresses in the spring and see if it needs presetting or not, if it needs, do not remove the residual stresses after preset. You can remove them after coiling and then do the preset with no further processing. The preset operation should be the last operation on the spring i.e. after coating too.

 

[TVP]

I agree with israelkk that the low-temperature heat treatment (my description that I will explain later) should be performed after coiling and that pre-setting should be the last step. If shot peening is to be performed, then there should be two low temperature heat treatments performed. A typical sequence would look like this:

coiling --> LTHT --> grinding --> shot peening --> LTHT --> coating --> pre-setting

The first LTHT is mostly for relieving the tensile residual stresses that are introduced during the coiling process. The second LTHT is not really a stress-relief treatment, but instead increases the yield point due to strain aging. People can become confused if the term stress relieving is used to describe both treatments, because they really are two different steps with two different goals.

 

[brin]

Thanks Israelkk, TVP, sbozy25- this adds to my limited knowledge about springs. so to confirm, the spring is typically formed (coiled), then stress relieved fully (time and temperature based on manufacturer preference?), and then pre-set to induce favourable residual compressive stress. does that sound about right?

Also, MIL-S-6715 para 3.12 provides strain relief temperatures of 350 deg F (1 hr) to 600 deg F (10 min). Is this typical for strain relief?

 

[sbozy25]

Depending on the material, yes that is the norm for stress relieving.

Here is a typical manufacturing routing at my facility.

Coil
Stress Relieve
Grind (if ends need it)
Shot Peen (if calculated stresses are high enough to warrant it)
Stress relieve again (only if peened)
press (to remove initial set, but only if calculations warrant it)
Rust preventative coating
Inspect
Ship

I will tell you this, never, and I mean never try to tell the manufacturer how to make their parts. They are the experts, just tell them what you want, and let them decide. Nothign irritates me more than a customer that trys to tell me how to run the parts, especially when half the time they have un-needed steps that are costly and do nothing. If I come across that, I usually quote the job much higher than I normaly would because of the time wasted...

 

[brin]

sbozy25- point taken, thanks. My problem is that as OEM we have to be able to explain everything about the part to the regulatory agency.

 

[sbozy25]

Oh yes I understand that... We get requests all the time from customers asking us to explain our processing. We can even provide the documentation showing the operators training and skill level if needed.

 

[israelkk]

I would like to add that if the Rust preventative coating is a electroplating coating such as zinc, cadmium etc. A hydrogen embrittlement relief is needed by heating it to 190 Celsius degrees for up to 24 hours. Therefore, the preset operation has to be the absolutely the last operation i.e. after the hydrogen embrittlement relief operation.

 

[sbozy25]

Yup, you are right... This actually makes me think of an interesting situation I come across often. There are many big name companies that use my company for their spring needs. Quite a few of them want us to directly ship from the coater to them. Which means the last press step will not be performed. Now in some cases it is ok, but many of these springs are high in the stress percentages and should be pressed and never are! Drives me up the wall!!!! Especially when I get a mad phone call about parts taking set the first time they are cycled....

 

[israelkk]

sbozy25

Springs taking set the first time they are cycled are unknowledgeable designer fault. Most mechanical engineers do not even know what it means, what it gives, when to use it, how to calculate it, etc. Knowing the formulas for spring deflection and spring stresses due to deflection or load is definitly not enough.

http://israelkk.googlepages.com/home

 

[sbozy25]

Yeah I agree with you 100%

I just wish they would listen to those of us that deal with it every day and believe what we tell them... oh well, more $ out of their pocket when they have to have us re-process the parts for them.