Shear Limit of M5 Bolt 2

[wrhig]

I am trying to calculate the amount of weight that a 4 bolt hole pattern (4" x 2&quot of M5 bolts can support without damage to the bolts and/or tapping pad they are mounted to.

The bolts are 10.9 class (grade 8, medium carbon steel) bolts with the following specs

Proof Load (MPa) = 830
Yield Strenth (MPa) = 940
Tensile Strength (Mpa) = 1040

If you assume that the bolts are 20 mm long and 10 mm is threaded into the tap pad, how much weight could the above bolt hole pattern support.

I would greatly appreciate some help on this matter.
Wes

 

[CoryPad]

Are the bolts pretensioned? Are the holes oversized so that the shear force is resisted by the friction force at the joint interface? If so, let us know what is the pretension level (or torque level), the bolt coating (friction coefficient if known), and the materials for the pad and whatever it is connected to.

If you don't consider pretension, and the force is shearing the bolts through the threaded region, then the mass that would cause bolt fracture is approximately:

m = F/A

where

m = mass
F = force due to mass
a = gravitational acceleration (assumed to be 9.8 m/s²)

F = 5 &[ignore]middot[/ignore]; &[ignore]tau[/ignore]; &[ignore]middot[/ignore]; A

where

&[ignore]tau[/ignore]; = bolt shear strength (assume to be 0.6 * 1040 MPa)
A = screw root diameter area = 12.69 mm²

m ~ 4000 kg





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[wrhig]

The tapped pad is a medium carbon steel. The bracket is made of B211 Aluminum Alloy. The pads and bracket are both painted. The holes are oversized. The bolts are torqued to around 4 Nm.

The bracket is mounted to 4 tap pads on a ballistic steel side wall that is angled at around 60 degrees to the horizontal.

If we disregard the pretension and tap pad, your calculations show that one M5 can hold 4000Kg before it will break!!! Does that mean the 4 bolt pattern could support 16000kg!!!

What is screw root diameter area?
What pretenstion do you recommend?
Does the amount of thread engagment change the max load?
What does Proof Load, Yeild Strength and Tensile Strength mean?

Hopefully you'll bare with me...I'm an electrical engineer!
Thanks alot,
Wes

 

[CoryPad]

My calculations were for 5 bolts, so the total mass is 4000 kg, not 16 000 kg.

Screw root diameter area is the area associated with the smallest diameter of the screw thread. For an M5 (which has a major diameter of 5 mm), the root diameter is ~ 4 mm.

Recommended pretension for general purpose joints is ~ 0.75 &[ignore]middot[/ignore]; yield strength.

Thread engagement can change max axial load, but won't affect allowable shear load much.

Proof load is defined as 0.9 &[ignore]middot[/ignore]; yield load. Yield load is equal to yield strength &[ignore]middot[/ignore]; stress area. Yield strength is the stress that produces 0.2% offset strain. Stress area is a little larger than the root diameter area. Tensile strength is the maximum stress that a material can resist.

4 N m is a low torque for these bolts. You probably are achieving only 0.5 &[ignore]middot[/ignore]; F_y, which is ~ 5 kN. Five bolts with 5 kN force gives 25 kN joint preload. If the friction coefficient between the coated steel and aluminium pieces is ~ 0.1 (conservative estimate), then the joint can resist 2500 N before it slips. This is ~ 255 kg.

If this is a critical structure, you should use the services of a qualified engineer. Good luck.






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