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Aluminium threads failure
2

Aluminium threads failure

Aluminium threads failure

(OP)
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 :)  

RE: Aluminium threads failure

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  

RE: Aluminium threads failure

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.

RE: Aluminium threads failure

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

RE: Aluminium threads failure

"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.

RE: Aluminium threads failure

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

RE: Aluminium threads failure

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

RE: Aluminium threads failure

I second helicoils.

RE: Aluminium threads failure

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.

RE: Aluminium threads failure

(OP)
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 :)   

RE: Aluminium threads failure

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

RE: Aluminium threads failure

Its fine for both

RE: Aluminium threads failure

(OP)
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 :)

RE: Aluminium threads failure

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

RE: Aluminium threads failure

(OP)
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 :)

RE: Aluminium threads failure

(OP)
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?

RE: Aluminium threads failure

(OP)
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.  

RE: Aluminium threads failure

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.

RE: Aluminium threads failure

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

RE: Aluminium threads failure

(OP)
Sorry I meant the friction coef. betwen the head of the bolt and the sensor. There is no nut involved.

Sorry :(

RE: Aluminium threads failure

Speaking of nut.  Can you just forego threading the aluminum plate and put a nut on the back side of it with a washer?   

RE: Aluminium threads failure

(OP)
Jap, this is what we're going to do now, because it looks like we don't have any other choice.

Thanx for all the information, they were realy helpful.

RE: Aluminium threads failure

If so, use a special larger O.D. washer to ensure that you do not have too much pressure under the washers.

RE: Aluminium threads failure

(OP)
We are thinking about using a flat head bolt (DIN 7991), because we don't want any unevenness on the upper side of the plate.  So I need to figure out if the aluminium plate will sustain the pressure under the head of the srew.
Are there any special formulas for calculating the pressure for this kind of bolts or can I just take the same one like for a normal bolt with a washer.

Thnx

RE: Aluminium threads failure

Hi Pina87

I would do a bearing stress calculation as you describe.

desertfox

RE: Aluminium threads failure

VDI2230 defines this property as the "limiting surface pressure", with the symbol pG.  Wrought aluminum alloy AlMgSi1 F31 (3.2315), similar to 6061-T6 in North America (UTS 290 MPa, YS 250 MPa), has a value of pG = 260 MPa.  The same alloy at somewhat lower strength (UTS 260 MPa, YS 200 MPa) has pG = 230 MPa.  Wrought alloy AlMg4,5Mn F27 (3.3547), similar to 5052 (UTS 260, YS 110), has pG = 230 MPa.
 

RE: Aluminium threads failure

Will you be tightening the nut or the screw?
 

RE: Aluminium threads failure

(OP)
The nut.  

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