Does a bolt in a threaded hole have a preload or clamping force? If not, what is keeping the joint together.

[EngTipsAccount1234]

Say you have a bolt going through an unthreaded hole in a plate, then into a second plate with a hole that is threaded. You are trying to hold the two plates together with this connection. Is the bolt being stretched at all to create a preload or clamping force? If not then what is keeping these two pieces together when an external load is applied?

For a connection with a bolt and nut I understand that the nut stretches the bolt to create a clamping force. But I don't understand how that clamping force would arise in a connection without a nut like I described. Yet this connection is used all the time to hole parts together.

 

[SwinnyGG]

A bolt is not intelligent - it does not 'know' whether it is threaded into a nut, or into a threaded hole in a casting or plate or whatever.

It stretches and applies preload to the joint just the same.

 

[Stick.]

Take your two plates clamped with a bolt and nut, and while tightening the connection, imagine holding the nut steady with a wrench and spinning the bolt. I'm assuming that's basically the connection you say that you understand. Now instead of holding the nut with a wrench, before tightening, weld it to the plate and then tighten. Nothing has really changed in terms of bolt stretch, clamping force, etc. It's still effectively the same as far as bolt stretch and clamping is concerned. Now is having a nut welded to the plate really any different than a plate with a tapped hole?

Also, drawing a free body diagram of the joint never hurts to help you wrap your head around the forces involved.

 

[HTURKAK]

Is the bolt being stretched at all to create a preload or clamping force?

If there is no external tension force to separate the two plates , preload = clamping force.

 

[dgeesaman]

Say you have a bolt going through an unthreaded hole in a plate, then into a second plate with a hole that is threaded. You are trying to hold the two plates together with this connection. Is the bolt being stretched at all to create a preload or clamping force? If not then what is keeping these two pieces together when an external load is applied?

For a connection with a bolt and nut I understand that the nut stretches the bolt to create a clamping force. But I don't understand how that clamping force would arise in a connection without a nut like I described. Yet this connection is used all the time to hole parts together.

The bolt is definitely under preload, otherwise there would not be any torque when it's tightened and it wouldn't hold anything securely.

Think of the threaded hole in the mating plate as being a nut. That's how it behaves. It doesn't care if it's clamping one, two, or more members in between.

Within the threaded length of the nut/plate, the thread that's nearest the screw head has full preload. The preload taken by each subsequent thread going away from the head decreases with the preload being zero at the last/deepest thread.

There are a few downsides to threading into one of the mating parts:
- The portion of screw that is preloaded can be very short, and its ability to flex and absorb vibration is reduced.
- The area between the plates that is held in direct compression is narrower. If you're trying to keep the mated parts very flat or compress a sealing element this can be troublesome. Putting a heavy/wide washer under the screw head can help offset this.
- The tapped hole is not as easily replaced as a separate nut, and its strength and thread length is often limited to the second plate material.

 

[rb1957]

sigh, yes, there is preload (if there is enough torque). why wouldn't there be ??

another example is a barrel nut ... the 2nd plate isn't threaded but has a transverse hole and a threaded nut fits inside the hole.

 

[r6155]

@ EngTipsAccount1234
It is best to have a sketch to avoid mistakes.

 

[mfgenggear]

Op
Is making jest question.