Tightening Of Bolts?

[evmundo]

What is the required ratio of proof load to tightening torque of a Screw/Bolt, how much of this torque is used to overcome the under head friction and how much is used to put the fastener under tensile load. What is tighteneing to yield?

 

[CoryPad]

Tightening to yield means that the externally threaded member is intentionally loaded to or just past its yield strenght. This insures the largest possible joint preload, while avoiding excessive plastic damage to the fastener.

For the other information, see the following posts:

thread288-20937, thread725-22078, thread727-22951, and thread288-21503

 

[Kenneth]

You might also take a look at

http://astimage.daps.dla.mil/docimages/0000/22/90/70455.PD9...

 

[thorangle]

Shigley and Mischke, Mechanical Engineering Design, 5th Ed., Chapter - 8, provides a pretty good starting point for understanding how to calculating fastener preload as well as discussing friction issues.

There are many factors to consider in determining fastener preload:

Friction between threads.
Friction under fastener head (as you have stated).
How many times the fastener/joint will be assembled/dis-assembled.
Cyclical loads.
Static loads.
Grade/Class of fastener used.
Corrosion concerns.
Design life of the fastened joint.
How the preload is applied to the fastener.
Joint stiffness.

Some or all of these issues need to be addressed in the design and analysis of a bolted joint.

Regards,