Re: rolled ball screw - STANDARD SCREW INSTEAD?

Very nice explanation thanks, I sort of known but
I never had it explained so well.
But how do Ballscrews fall into this they have
a less then favorable contact angle.
I guess the hardness of the components and the fact
that there would not be any friction makes them
less vulnerable to the spreading effect.


--- In CAD_CAM_EDM_DRO@yahoogroups.com, "jmkasunich
<jmkasunich@y...>" <jmkasunich@y...> wrote:

> --- In CAD_CAM_EDM_DRO@yahoogroups.com, "Lloyd Leung" <lloyd@l...>
> wrote:
> > Here is my understanding of the different types of threads:
> >
> > ACME thread (rectangular) is used in machinery, because the
> > sides of the wall are used to transfer power, and not grip.
> > _|~|_|~|_|~|_|~|_
> >
> > Triangular threads are meant to grip and bite, and not used
> > to transfer rotary motion into linear motion.
> > _/\_/\_/\_/\_/\_/\_
> >
> >
> > Buttress threads are meant to have staying power, so it won't
> > budge and hopefully won't vibrate out.
> > _/|_/|_/|_/|_/|_/|_
> >
> >
> > Hope that helps. (if my ASCII drawings don't come out, oh well)
> >
> They came out fine - good explanation. I'll throw in a little
> more detail.
>
> The difference between the thread types is friction. The thread
> flanks on the triangular thread are angled. That means that the
> axial load on the screw (horizontal in the pictures) results in
> a radial force too. This wedging action increases the total
> force which increases friction. That's a good thing for a bolt,
> bad for a leadscrew. Imagine for a minute that you take the
> triangle thread, and make the slope of the sides even shallower.
> (Lower the height, increase the width of each thread.) The
> wedging action gets more pronounced as the angle gets
> shallower, and eventually you get a morse taper, in which
> the wedging action is so strong that it locks up.
>
> On the other hand, for a leadscrew, we want to minimize friction.
> The lowest friction occurs when the sides of the thread are
> vertical, since there is no wedging action if there is no angle.
> Square threads are used on some screws, but they are hard to
> make. The Acme thread is a compromise - it a very small amount
> of wedging action, but is much easier to cut.
>
> The buttress thread is a special case, used on screws that will
> always be loaded from one side - like a knee elevating screw.
> They are designed so that the loaded face is vertical, so it
> has the low friction of a square thread. By putting an angle
> on the unloaded face, the thread is easier to cut, and much
> stronger than the square thread. Because of the angled side,
> a buttress thread loaded in the wrong direction will have
> lots of friction, and may bind up completely.
>
> John Kasunich
>
>
>
>
>
> The square thread has vertical sides, so there is