Material choice for 1.25" threaded fastening 1

[mhf23]

Hiya,
I'm designing a fitting which will have the male half of a 1.25" screw thread fastening on one end. The female half is being made by another company, and is anodised aluminium. The whole fastening is going to be regularly done and undone by hand - the part I'm designing is kind of an optional add-on.

I know aluminium-aluminium threads have a serious problem with seizing. It's my understanding that anodized aluminium threads also tend to seize, although I'm assuming this is for different reasons.

My material choices are:
Stainless steel (303, probably)
Brass
Anodized aluminium.

Does anyone have any experience of this sort of situation? Anyone recommemnd any particular material? Weight is also a consideration, so the lighter the better.

Ta

Matt Freeman,
Design Engineer,
UK

 

[mechengdude]

You say that the parts will be put on by hand. I've never had any trouble with two anodized pieces of aluminum being assembled hand tight.

I've had good luck with using a hard anodize per MIL-A-8625 Type III. Might want to give that a look.

 

[MintJulep]

Loads?

 

[CorBlimeyLimey]

Does it have to be a metal?

If it does, could it have a threaded non-metal liner?

 

[mhf23]

[MintJulep] and [CorblimeyLimey]: by way of answering more than one question at a time, although it is supposed to be only hand-tight, it's going out to customers as part of a measuring system and I wouldn't put it past customers to 'hand tighten' it with a pipe wrench. On that basis, and because it's likely to be screwed and unscrewed maybe twice a day for its entire design life, I've so far ruled out polymers... unless you guys know better...

Matt Freeman,
Design Engineer,
UK

 

[CoryPad]

You can keep the metal-to-metal joint, you just need to apply a lubricant to the threads. Your choices include PTFE masking (also known as Teflon tape), anti-seize (Loctite has a range of products), or a molybdenum disulfide lubricant (Dow Corning offers a few).

Regards,

Cory

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

 

[CorBlimeyLimey]

OK, but you didn't answer MJs question of the loads involved.

Is the part being used to lock/clamp something in place or for positioning?

Is the thread fine or coarse?

Can a lubricant be used?

 

[mhf23]

Sorry, I'll try again:

I don't think lubricant can really be used - it's an often handled part, so we can't guarantee the state the customers will keep it in, so it's best to assume it's unlubricated. Sure, you could use something pre-impregnated, but I think that's overkill.

The drawing specifies a 1.25" -20 UN thread. I'm a metric boy myself, but I think that's a coarse thread.

As to the loads - to be honest, I don't have any figures I can give you right now, it's all a bit back-of-fag-packet. I'll try and post some figures before the end of the day.

Matt Freeman,
Design Engineer,
UK

 

[KENAT]

Stainless or Corosion resistant steel can be problematic with Aluminium from a corrosion point of view if you have anything aproching a harsh environment.

As suggested above hard annodized will probably be better than standard for this application.



KENAT, probably the least qualified checker you'll ever meet...

 

[NickE]

I'd go with Brass, cause it wont sieze/gall and I dont (offhand) think that it will make a bad glavanic couple with the Al. (The anodizing is likely to get broken through/worn away.)

Also the brass is going to be easy to produce, will thread easy, and be a completely different color, that way when the guy in the field drops it he can find it again.

Nick
I love materials science!

 

[scastillo]

If feasable, I would suggest you look into using a threaded insert for the female piece. This is based on my own experience, where mating aluminum parts were stripped because of reasons you've already stated (over torque or lack of lubricant).

 

[dcstorm]

We use a dry film lube of Al-Al threads per MIL-PRF-46010. It is cured on the threads, and does not come off from handling or installation of threads. Might do the trick for you.

Sample callout:
APPLY EVERLUBE 9002 IAW MIL-PRF-46010 TO THE THREADS, .0003 TO .0005 THICK. CURE AT 400 DEG F +/- 15 DEG F FOR 120-140 MINUTES.

 

[mhf23]

That's great, thanks all for your help!

Matt Freeman,
Design Engineer,
UK

 

[kenvlach]

dcstorm, are you applying the MIL-PRF-46010F SOLID FILM LUBRICANT over anodize?

Combine PTFE or similar dry film lubricant with anodize to get wear resistance + lubricity. Have professionals do it for you:
Some suppliers:
In the US:

http://www.magnaplate.com/coatings/tufram.html

In the UK:

http://www.poeton.co.uk/w1/a350.htm