Need help calculating maximum stress in a bolted assembly

[Karmann]

TL;DR

Calculating maximum stress in a bolted assembly

Trying to calculate the maximum stress that would occur in the attached assembly...

The assembly consists of a tube sandwiched between two plates that are bolted down to a larger plate with an M6 bolt. Each of the two plates have a 400N force acting on them which is perpendicular to the axis of the bolt. The M6 bolted is tightened to 9Nm, generating a clamping force of around 7500N.

I believe the location of maximum (tensile) stress would occur in the position marked by the red dot due to the bending moment created around rotation axis (green dot).

Thanks

 

[jrmichler]

Is this a homework problem? If so, we can move it to the homework forum. Also, when inserting images, it help to click on Insert, then Full Image to show the full image.

This is a linear system, so superposition holds. The procedure is as follows:
1) Calculate stresses due to bolt clamping force only.
2) Assume that all joints are perfectly bonded and the bolt clamping force is zero. Calculate stresses caused by the upper 400N force by itself.
3) Assume that all joints are perfectly bonded and the bolt clamping force is zero. Calculate stresses caused by the lower 400N force by itself.
4) Sum the results of the three calculations. If the total stress at a joint goes to zero or tensile, then the bolt clamp force is not enough to hold the joint shut.

 

[Baluncore]

Welcome to PF.

There is no nut to tighten, so it is a screw, not a bolt. You must turn the head and shank of the screw to tighten it.

The weak point is where the screw thread enters the threaded block.

There, the narrowest threaded part is subjected to clamp tension, load shear, and the torque required during assembly.

Screw head and thread, lubrication/sealant, will be important during assembly, and that choice will limit the operation.

 

[Lnewqban]

... I believe the location of maximum (tensile) stress would occur in the position marked by the red dot due to the bending moment created around rotation axis (green dot).

Welcome, @Karmann !

There is no bending moment caused by the top force if the assembly is clamped together.
The location of the red dot seems not to be correct.

The steel plates and tube should be working mainly on compression, as friction force keeps them from sliding respect to each other, or even from touching the sides of the bolt or screw.

The bottom of the thread has a smaller diameter than the body of the bolt, which makes it the weakest link under the tension load, plus the twisting force of the wrench.