How does torque on a screw convert to axial force?

[dobry_den]

Hi! Could anyone of you post here a link to a webpage that actually describes how a torque (of a screw) is transformed into a force that acts in the direction of the screw's axis, that makes is to screw in? Or could you briefly explain it? Any help would be highly appreciated.

 

[berkeman]

Maybe the "Mechanical Analysis" part of this page?

http://en.wikipedia.org/wiki/Screw_and_bolt

 

[dobry_den]

I mean like.. what makes the screw to go down, to screw in. After I thought about it, I came up with this idea. When I rotate the "head" of the screw, the shaft (the lower part) rotates as well. Since a part of the shaft is already in the wood (where the thread has made a groove), the groove acts against this rotation - reacting with a force F. This force acts on an inclined plane (the thread) and it is possible to decompose it into two forces, F₁ and F₂. And it is the force F₁ that makes the screw to screw in the wood (see the attached drawing).. Is this the appropriate explanation?

 

[rbj]

I mean like.. what makes the screw to go down, to screw in. After I thought about it, I came up with this idea. When I rotate the "head" of the screw, the shaft (the lower part) rotates as well. Since a part of the shaft is already in the wood (where the thread has made a groove), the groove acts against this rotation - reacting with a force F. This force acts on an inclined plane (the thread) and it is possible to decompose it into two forces, F₁ and F₂. And it is the force F₁ that makes the screw to screw in the wood (see the attached drawing).. Is this the appropriate explanation?

inclined plane: http://en.wikipedia.org/wiki/Inclined_plane

helically warped, but that is essentially what a screw is: http://en.wikipedia.org/wiki/Screw

 

[dobry_den]

Yep, I know, so do you think that my drawing is correct?

 

[mdelisio]

Yep, I know, so do you think that my drawing is correct?

I would say you're pretty close. I think you need the equivalent of a "normal" force that is perpendicular to the angled thread. It gets pretty hard to get your head around the helical screw thread, so I did a quick analysis of a much simpler screw, which consists of a bolt with a small post that acts as the thread and a hole with a helical groove cut in it.

The results of the free body diagram confirm the result you get from a "work balance" analysis:

Fvert = 2*pi*T/d

Where Fvert is the vertical force, T is the applied torque, and d is the thread pitch.

This analysis ignores friction, which is a huge simplification. Hope this helps...