General understanding of bolt preload 1

[wamba1]

Is a preload applied to the threads of a bolt when the bolt is torqued into a blind hole? i.e. one threaded part and one bolt, with the bolt head bottoming out on the surface of the part. I'm having a hard time picturing how there is a preload in this case. It seems like the torque is just creating a force between the bolt head and surface of the part, but that the threads of the bolt are not actually being "stretched" like they would with a nut.

Now for the second scenario -- same bolt and threaded part, but the tip of the bolt bottoms out at the bottom of the hole. Does this have a preload on the threads?

Thank you for your help!

 

[paddingtongreen]

Preload is the force between the bolt head and the nut or other female threaded part. There is compression between the threads. If the bolt bottoms out at the bottom of a blind hole, there is still compression between the threads, but no preload, it might not even be snug tight.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[MintJulep]

Yes....but.

Preload is the tension in the bolt created by the forces applied at the thread and reacted at the head of the bolt.

In a "classical" bolted joint we tend to simplify the forces at the threads and assume that they act uniformly at a single cross-section of the bolt. Typically at mid-height of the nut.

In reality of course it's not so simple. The forces at the threads are distributed over more than one thread pitch.

So for the blind threaded hole you really need to know about the force distribution on the threads if you want to know what is happening in the bolt portion that is engaged in the threads.

The reaction forces under the head are no different than in a "classical" joint.

 

[TVP]

wamba1,

If the end of externally threaded fastener does not bottom out in the hole, then there is definitely some type of preload (pretension) in the joint when the head makes contact. How else is the force reacted?

 

[Tmoose]

http://www.hand-tools.us/images/pictures/grip-woodworkers-clamp--10in..jpg

If I place my finger in the gap near either threaded handle and twist that handle clockwise with the free hand, the captured finger will be exerting the stretching force the 2 inch long section of threaded rod must resist/exert.

The bolt torqued in a deeply threaded blind hole only has 1 or 2 threads in the stretch zone. The hole entrance would need a significant chamfer or counterbore to prevent interference with the awkward situation under the head of a "fully threaded" bolt.

http://www.allproducts.com/metal/fengyi/hex_cap_screw-l.jpg

 

[dinjin]

If there is no space between bolt head and start of thread you cannot develop preload. Preload assumes there is an intermediate part between head of bolt and start of threads.
It is in this intermediate part where the elongation, contraction takes place and a small residual into the head and into the threaded part. Maybe I too have the wrong understanding of preload.

 

[slickdeals]

This website gives you an excellent explanation on bolt pretension

http://www.boltscience.com/pages/basics2.htm