Torque Specification with lock washer 2

[icekruiser]

I have some concerns about the specified torque needed for the following assembly:

M8 bolts with flat washer and lock washer fastened through a brass flange into an aluminum housing (no inserts)
There are 6 bolts total

Forces:
2500 N (5600lbs) of tension force acting on the brass flange. Force is parallel with bolts.
AND
815 N*M of torque acting on the brass flange. Force is acting perpindicular to the bolts.


I have found numerous charts and calculations to determine the torque but when I apply the torque it seems to loose. We have had some bolts pull out.

Assuming I can not change the design (for now), are there any additional torque specs I should consider out side of a "standard" torque for a M8 bolt?

 

[jetmaker]

icekrusier,

You say that you have had bolts "pull out". What exactly do you mean??? Are they stripping the threads of the housing??

jetmaker

 

[icekruiser]

jetmaker

The bolts have pulled out of the threads in the aluminum housing.

 

[jetmaker]

icekrusier,

Then the torque is not the issue. You need to either add inserts to the housing to increase the thread shear area of the housing, or go with greater thread engagement length in the housing.

Mil spec. MIL-S-8879(c) suggests that bolt preload is calculated as 5*T/(2*rb), where T is torque, and rb is the average thread radius. In your case, the preload you are probably looking for is in the neighbourhood of 2500/6 = 416# per bolt, so say 500#. You can then calculate the approximate torque required to prevent flange separation.

Hope this is helpful,

jetmaker

 

[CoryPad]

icekruiser,

What are the material properties for the bolt?

What is the distance from the center of the flange to the center of a bolt hole?

What is the thread engagement in the Al housing? This can be calculated as bolt length minus its unengaged point, then subtract the combined thicknesses of the washers and flange.

Are there any features between the flange and housing that will resist the applied moment?

Are there any coatings within this entire system?

Regards,

Cory

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

 

[icekruiser]

Thanks for your input. I have had some machines come back with broken bolts. The broken bolts were sheared with no indication of cyclic fatigue (beach marks).

 

[jetmaker]

icekruiser,

I just want to make sure that I understand this. The failures that have been occuring are two fold. Some bolts have pulled out of the threads in the aluminum housing, and others have sheared.

Is this correct? If so, why are you concerned about the applied torque for the bolt? These failures are more an indication of improper bolt selection and thread engagement area, not bolt preload.

Please note, in my previous reply, I used the 2500N force as #. That was my mistake for not looking at the units carefully.

Thanks for the clarification,
jetmaker

 

[icekruiser]

jetmaker,

I agree with you. The parts are made overseas and assembled here in the US. They can not be changed very easily or quickly so I am trying to do as much as I can with the assembly that is in my control.

Thanks for your help

 

[jetmaker]

Thanks icekruiser,

Looking at your problem, you have a few options here. First thing, upgrade the bolt to a higher quality/strength bolt. This will help eliminate the problem of bolt shearing under the flange torque. Also, upgrading the bolts will also allow you to increase your preload by appling a higher bolt torque. If the faying surfaces of your housing and flange are reasonably rough, increasing the preload will also help to transfer more load through friction.

Next, assuming you can at least modify the parts somewhat, the best approach is to install steel inserts into the housing. If that is not possible, you might try to increase the thread length of the bolt to engage more threads in the housing. Finally, you may even be able to modify the thread profile to a fine thread which has slightly better tension capability.

Unfortunately, it sounds like the present design of the part is inadequate for the loads. There are many possible fixes, but I am not aware of what you can and can not do at your facility. I've provided some suggestions, and I hope those are of some use.

Best of luck,
jetmaker

 

[diamondjim]

I wish you would answer the questions
posed by Corypad as he seems to be on
the money. I would sample the bolts to
see if they hold up to their supposed
grade and quality. If you have sufficient
wall surface to use inserts in the aluminum,
that would be the way to go. Forget the
lockwashers. Make certain the flat washers
have enough od to reduce the bearing stresses
on the contact surfaces. Please supply
the aluminum material, its type of treatment
and the grade and quality of the bolts.

 

[icekruiser]

CoryPad and Diamondjim
What are the material properties for the bolt?
A2-70 M8X30

What is the distance from the center of the flange to the center of a bolt hole?
110mm, flange is round

What is the thread engagement in the Al housing? This can be calculated as bolt length minus its unengaged point, then subtract the combined thicknesses of the washers and flange.
12mm

Are there any features between the flange and housing that will resist the applied moment?
no (unfortunately)

Are there any coatings within this entire system?
There are no coatings.

It is cast aluminum- a gear motor housing. I don't know the type of aluminum yet. I am still waiting to here back from over seas.

If I remove the lock washer, should I replace it with some other type of lock?

Can I keep the lock washer and remove the flat washer? (The washers are contacting a brass flange.)

Thanks again for everyones help. This is the first time I used this website. I wish I would have found it years ago!

 

[unclesyd]

Normally if you can strip threads in aluminum you have enough metal to use inserts unless the Cu flange is acting as a clamp allowing the bolt to strip the Al threads. The inserts would help with the stripping and may help with the bolt shear by keeping the bolt in tension and not allowing relative movement of the two parts.
You have enough room for more smaller bolts in the long run . For the existing design you may want to use
Spirol/Drivlok pins to that the torque loads. This could be done during assembly.

Where did the bolts shear?

Is the torque load on the Cu part in the form of an impact?

How thick are the relative flanges the Cu and Al?

What is the volume of Al in the boss/flange area,

Is the Al possibly flexing under load? It appears that you are transfering the load to 1 or 2 fasteners.

There are some other possibilities besides the flat washer and lock washer.
You have some pretty good forces acting on you connection. You have a strange combination of materials of construction in your component.

 

[diamondjim]

You might want to have 2 of the 6 holes
drill almost line to line with the bolts
to reduce the shear caused by the second
load condition. These holes would be
180 degrees apart. One may be sufficient
to resist this turning torque but two
would be possible to machine and hold
the true position tolerance. If not put
in two shear pins. You must limit this
shear condition somehow to prevent the
bolts from twisting and causing extra
loads on the fasteners.
The washer could be flat but crimped where
it interferes with the flange. Lock washers
are of little value and may give you false
torque values. You can also improve the
clamped length by increasing the thickness
of the flat washers. I assume you are
using these under the bolt head as well as under
the nut.

 

[CoryPad]

icekruiser,

Your design should work with some modifications. Here are my suggestions:

1) eliminate the lock washer - they are worthless (if you want more information on this subject, perform a search of this site and you will find much information), and this will allow more thread engagement, which clearly is an issue. My calculations show that the thread engagment should be around 11.9 mm, so you are on the limit.

2) keep the flat washer - I assume the flange has clearance holes (~ 10 mm?), so you need the washer to keep surface pressure low.

3) lubricate the screws - this will allow sufficient preload to withstand the force and moment. Unlubricated stainless steel screws have large friction variation that can cause thread stripping with the aluminium component. A good target for the friction coefficient is &[ignore]mu[/ignore]; = 0.10-0.16. Contatct your fastener supplier about appropriate surface treatments.

4) use a precision preloading method - e.g. angle tightening based on lab testing. An estimate for the torque required (based on &[ignore]mu[/ignore]; = 0.10-0.16) is 15-18 N&[ignore]middot[/ignore];m.

Good luck.

Regards,

Cory

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

 

[diamondjim]

Metric Class Material Tensile Strength Yield Strength
N/mm2 psi N/mm2 psi
8.8 8.8 Steel 800 116,000 640 93,000
10.9 10.9 Steel 1040 151,000 940 136,000
12.9 12.9 Alloy Steel 1220 177,000 1100 160,000
A2-70 A2-70 302 Stainless 700 102,000 450 65,000
A4-80 A4-80 316 Stainless 800 116,000 600 87,000

 

[zepp]

An interesting thread altogether, and some good advice given, I know this comes very late in the day, but I would like to know what method is being used to torque the bolts, and are the bolts used, being tested on a bolt load meter?
I would say that if I have read the upper posts correctly, you are using stainless steel bolts into aluminium, one sugestion made above is indeed very important under such circumstances, and that is lubrication, if the threads of the bolt and the threads of the tapped hole are on the upper and lower limits respectively, what will happen is localised heating due to friction, which inturn alters the proerties of the aluminium in the thread area, It could be that threads are being pulled out because of them being britle.
Also from experiance, if the bolts are being torqued up with a hand torque wrench, this again can complicate things in respect to aluminium threads, I find from experiance that soft metals are best served with an automated method of srewing, some thing that is smooth running and none impacting, and try to eliminate side loading of the bolt, as this again will effect the threads