Scroll thread transmission 2

[PrintScaffold]

Greetings!
I am very interested in the theory behind the scroll thread transmission, which is used in spiral ring chucks of lathes. Is there a good description, and ultimately, design rules available sowewhere?

 

[Tmoose]

Is this the mechanism you are interested in?

http://www.belarus.net/plant/beltapaz/ki_tap7.jpg

 

[PrintScaffold]

Yes, exactly. But not the whole mechanism (which is pretty obvious), but only the spiral transmission.

 

[PrintScaffold]

I did not find any background info about the spiral transmission in the books. All books basically only write about the existence of this concept - no additional description. No info elsewhere?

 

[MintJulep]

So that's how those work.

It's just a screw thread (which is just a wedge) wrapped in a different way.

You'll find the fundamentals of screw threads in any basic mechanical design text. Just twist everything around by 90 degrees.

 

[PrintScaffold]

It doesn't seem correct at a glance. Screw thread is a helix, while what I am interested in is a spiral. There's a difference. And profile of a spiral transmission is not a wedge, it's rectangle.

 

[MikeHalloran]

The spiral and the helix are the result of different coordinate transformations, both starting from a wedge. The kinematics are the same at the point of contact.

Note that the teeth on the back of the chuck jaws have to be relieved on both faces, i.e. made more convex than is ideal, because they have to mesh with faces of decreasing local radius as the scroll is tightened.





Mike Halloran
Pembroke Pines, FL, USA

 

[PrintScaffold]

Note that the teeth on the back of the chuck jaws have to be relieved on both faces, i.e. made more convex than is ideal, because they have to mesh with faces of decreasing local radius as the scroll is tightened.

Exactly what I meant saying it's not the same as the thread helix. And that's what I particlarily interested in - how exactly do the chuck teeth have to be relieved. Is there any universal solution which applies to the majority of the designs? Or is it more down to the designer?

And do the teeth on the juck jaws also follow the spiral? Or do they derive from the concentric circles?

 

[MikeHalloran]

I know of no standards, and no reason for them to exist.

Of the chucks I have disassembled:
The teeth follow the spiral.
The teeth within a given chuck differ from one another; the teeth are in different radial positions.
The teeth are always numbered or otherwise marked to fit in a particular location on a particular chuck.


Mike Halloran
Pembroke Pines, FL, USA

 

[PrintScaffold]

Could you produce the basic sketch of jaw's teeth?

 

[MikeHalloran]

Sure, I could.





Mike Halloran
Pembroke Pines, FL, USA

 

[Tmoose]

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

http://www.cartertools.com/3jawdis2.jpg

http://www.lathemaster.com/images/4_inch_chuck.JPG

 

[PrintScaffold]

I kindly ask you to send a sketch here.

I really can't find any detailed info on spiral transmissions. Although the books that I have explain a lot about all other stuff...

 

[PrintScaffold]

Thanks Tmoose! That clears quite a number of questions.

 

[Compositepro]

The teeth on the jaws could be cylindrical pins and they would still work.

 

[MikeHalloran]

Tmoose's second link clearly shows that the teeth are displaced from one jaw to the next, so the jaws within a chuck are not identical.

It also shows teeth having one convex face and one concave face, which is representative of a chuck with non-reversible jaws.

Chucks with reversible jaws typically have two convex faces.

Now you should have enough information to make a decent drawing, never mind a sketch.



Mike Halloran
Pembroke Pines, FL, USA

 

[PrintScaffold]

Yes, I believe so. Thank you all!