Re: rolled ball screw - STANDARD SCREW INSTEAD?

--- 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