Aluminium threads failure 2

[Pina87]

Hi
I need to fasten a force sensor to an aluminium plate of 20mm thickness. The manufacturer of the sensor provides that it should be fastened with two bolts M12 12.9 with a torque of 130Nm onto a plate made of stainless steel (20mm). Because of the weight I can only use an aluminium plate.

My question now is: how to I know if the threads in aluminium will withstand the torque of 130Nm.

Thnx in advance

 

[desertfox]

Hi Pina87

Well you don't give the grade of aluminium so we don't know what the yield or tensile stress is of the aluminium.
So when you have say the yield stress of the above then calculate the allowable shear stress about 58% of the yield.
Then convert the approx axial force from the M12 bolt under 130Nm of torque from this formula:-

Axial force = T/.2*d T = bolt torque

d= bolt dia

.2= friction factor

Then go to this site

http://www.roymech.co.uk/Useful_Tables/Screws/Thread_Calcs.html

and get the shear area of an M12 female thread, note the area is given per mm so multiply this area by the plate thickness.
Once you have done the above,divide the area into the bolt axial force to obtain the shear stress, this value should be below the 58% of the yield stress from earlier.

desertfox

 

[BrianE22]

If you can thread it to 1.5x - 2x the thread diameter you'll be fine. If you can't, consider using heli-coils or threaded inserts.

 

[chicopee]

Machinery handbook has a section about fastening materials of different metals. In that section you'll find the torque equations.

 

[Tmoose]

"how to I know if the threads in aluminium will withstand the torque of 130Nm"

By all means go thru the exercise to calculate "it", but For charts of what UNBRAKO felt in 1996 were reasonable tapped hole thread strength to consider for a quality socket head capscrew (similar to the fastener you described, or is it?) into brass, aluminum, cast iron and mild steel, look at page 66/68 here.

http://www.dalecompany.com/pdf/UNBRAKO.pdf

But, to really "know" how it will work, torquing some bolts just as they will be assembled in production, in test holes tapped in the actual material would be an inexpensive test.

Plenty of machine design books would offer the 1.5X diameter rule to develop full strength of the fastener in generic aluminum. If you drill a thru hole in the 20 mm plate it could be tapped to 2X diameter.
As Brian said, it Sounds pretty safe.

 

[Cockroach]

First compute the thread load as the result of torque, and apply the thread shear equations given the geometry of you threading. In this case, you must look at Box and Pin.

Once you calculate the shear areas of the nut and bolt, determine the stress by division into the load from the torque calculation. The factor of safety can then be determined noting material properties.

I hate lip service in this forum, I will post my solution when I get back to the shop.

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

 

[drawoh]

Pina87,

You can use a helical (Helicoil) thread insert. The catalogue will tell you the pullout strength. You can work out the torque.

Thread inserts are common practise with tapped holes in aluminium.

JHG

 

[brdaaw]

I second helicoils.

 

[Tmoose]

How often will this be disassembled? Helicoils also offer better wear resistance than general aluminum threads.
For a one time or occasional assembly/disassembly It may not be worth it.

I'll be interested to see if any of the calculations indicate a problem with a thread 2X diameters long.

 

[Pina87]

Guys you're great. Thnx for all the info.

Desertfox: can you please tell me, if this calculation you posted before is in any kind of mechanical book? I need to have good references and a forum just won't do it .

Thnx again

 

[TVP]

Pina87,

Yes, T = kdf is a rule-of-thumb calculation that is quite well known. Here are some links to review:

http://www.google.com/search?hl=en&...812l0l10031l29l28l1l20l0l0l235l1062l2.3.2l7l0

 

[Pina87]

Ok, than is the K=0.2 right for the metric or imperial sistem?

 

[desertfox]

Its fine for both

 

[Pina87]

Ok. according to the first post I need 35mm plate.
My aluminium is 5038 and its Rp02 is 125MPa.

Shit, isn't it?
It looks like we will need some Helicoils.

Thnx guys

 

[Walterke]

With the risk of starting a discussion about steam:
Are you sure that putting 2 bolts with a torque of 130Nm in an Alu plate will give the same strength as putting them in a steel plate?

NX 7.5
Teamcenter 8

 

[Pina87]

Hmm, I was calculating only how much force a 130Nm torque gives in a steel plate and than I calculated if such force the alu plate will sustain.
This is I realy good question, I will check it.

Thnx very much

 

[Pina87]

Ok. That's what I get : if I take the minimal friction coef. for the thread (steel bolt into alu = 0.08(lubricated)) and nut(steel on steel=0.12) I get 76kN. If the maximal friction is taken (thread=0.20(lubricated) and nut=0.18) I get 44kN. Big difference .

The problem is that I don't think the thread will be lubricated, but only the coef. for lubricated aluminium threads is known (0.08-0.20) (VDI 2230). Does anyone know why?

 

[Pina87]

So. The thread fails already under the minimal force.
I will try to calculate my own torque, that I would use for the fixation of the sensor.

 

[dinjin]

Why not use turn of the nut method? Torque is too unreliable.
You will also need a special paint or lubricant to avoid electrolysis in the threads.

 

[Walterke]

Can you perhaps make a sketch of how you're thinking of making this construction? I don't really understand why you would put thread in the aluminum AND use a nut.

NX 7.5
Teamcenter 8