Calculating bracket and bolt stresses

[ella0509]

Hello, I can't attach a picture, so I'll try my best to give a good description.

We have an assembly block of 140kg of which the C of G is pretty much the centre of the block. The block sits on top of 4 AV mounts (one in each corner). 2 AV mounts attach the top rear face of the block to the wall - i.e. 90 degrees to the 4 blocks at the base - again, one in each top corner. The AV mounts attach through the centre with 1x M10 bolt. The AV mount attaches to the framework with 4x M6 bolts.

The block undergoes a 30g shock from below. This shock would put the 16 M6 bolts of the base mounts in tensile strain, whilst the 8 M6 bolts at the two top mounts would be in total shear.

At the moment the 1.6 Thick base plates of the AV mounts are fracturing at the corners of the M6 holes.

I'm trying to determine the stresses in these areas of the brackets so I can determine the required thickness, but I'm not sure how to determine the exact force in each corner of each bracket.

I hope you can understand my question and look forward to your responses......

Lee

 

[40818]

If i read your post correctly, your original applied force = 30 x 140kg = 4200kgf = 41188N
So if you firstly assume that the load is taken initially by bolt tension then if your load is centrally or uniformly applied, then each bolt sees 41188N/16 = 2574N
This is a somewhat simplified solution to the bolt loads as i dont have enough information. But its a starting point.
I have disocunted any effect of the sertical bolt shears, but as i said it is a basic (and could be conservative) guess. If you give more info then i might be able to give better guidance.
You could reverse engineer it a bit and if your bolts are ok, then ensure that your base plate can carry a load equal to the bolts tensile allowable.

 

[ella0509]

Thanks 40818.

During Shock and Vibration testing, the corners of the AV mounting plate shear off during the 30g Shock test. The shock test followed the vibration testing. Looking at the crack propagation, you can see some fatigue markings through about 1/3 - 1/2 thickness of the plate before the clean shear marks.

The cap head bolts were fitted WITHOUT any washers, which in itself has exaggerated the problem. We will be placing washers there on the next test run. We've also increased the size of the AV mounting bracket so that the mounting holes are not as close to the edge. I now need to run these calculations to determine the minimum required thickness of this plate, and to prove that the bolts are still sufficient if the plate does not fail.

This is where I am struggling, as I can't determine what the load in each particular corner of each particular bracket would be. I need to determine for each condition - i.e. tension / shear as the shock test takes place in all three axis.......

What other information would you require??

Regards,
Lee

 

[ella0509]

Any more thoughts guys??

Regards,
Lee

 

[Kriegen]

lee27
if no one can give you a good answer back, then maybe it is because of a lack of understanding position and placement of each of these pieces. You could give a map out of the entire problem in cartesian coordinates making a certain point on a plate the origin and going from there.

 

[zcp]

The AV mount / plate / bolt locations are unclear from your description. Also, what materials are used for each component.

To determine the forces, free body diagram for each axis of motion with the shock load as a max. Note that your load fully reverses.

To determine the stresses, forces on these mounting plates leads to failure modes and stresses (either by hand or FEA).

Then determine a fatigue strength for the component in these locations and compare to your stresses.

Not alot else we can do without seeing the physical arrangement to go into particulars.

OR you could count striations to determine cycles and back calc the forces when comparing to your fatigue strengths.

With this you may be able to show the stresses in your new plate to be less than the original. In other words, if you original FS was 0.8 and your new plate is 3X as strong, you should be good...right?

May be time to bring in the consultant for a quick FEA.

ZCP

http://www.phoenix-engineer.com

 

[desertfox]

Hi lee27

What type of AVM's are you using? The AVM's mounted
at the top ie at 90 degrees to the four at the base will
presumbly have a different stiffness than those at the base
and as your force is vertical why can you not just rely on the 4 at the bottom or am I misunderstanding something.


Regards

desertfox

 

[ella0509]

Hi desertfox.

Thanks for your reply. The 6 AV mounts are all the same specification. I did not supply enough information in the first post. The force is transmitted through all three major axis - i.e from below, and from both sides. It seems that the top brackets are fracturing when the force is from the side - i.e. pulling away from the 2 AV mounts on the wall. If there was a way to determine the way the force is split between all 6 mounts, then I can probably move forward. I'm assuming its not as simple as just dividing the total load by 6 as there are surely not all taking the same load??

Regards,
Lee

 

[ella0509]

Thanks zcp.

I was reading my emails in reverse this morning and noted desertfox reply but not yours. I am now looking into breaking the assembly down with FBD. Once I can get the load on each bracket, then we are going to run the FEA on those brackets.....

I may be back with more questions shortly, but thanks for your reply.

Regards,
Lee

 

[desertfox]

Hi lee27

Your right about dividing the load by 6 its not as simple as that.
I think also that the vertical AVM,s will see a tensile load as well as shear, as the vertical force will be offset
from the bracket consquently that force will be trying to
tear the bracket off the wall.
If bracket fastener is not preloaded sufficiently then your bracket will see tensile loads due to bending and possibly explain your fatigue cracks in the base.
We need information relating to AVM's and what pre-load your putting in the fasteners that hold tem in place.
Any info regarding the vibration might be helpful too.

Regards

desertfox