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Say you have a lead screw rotating at a constant angular velocity. Without being able to adjust the lead screw pitch or angular velocity. Is it possible to use a nut which will only travel half the pitch upon each full rotation of the screw.

I found a source saying using a half nut, commonly used in lathes and only engaging one half (eg it covers only half the circumference of the screw), allows for this to occur. But upon a full rotation I can't picture it being half the pitch rather than the full pitch.

I'm also aware that using a multi-start nut would allow for going twice the pitch but can you reduce it in any way?

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2 Answers 2

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You may be able to accomplish your objective by using what is described as a differential screw. Wikipedia provides a rather complex description with an image or three.

Thingiverse provides a 3D printable model, one of which I have printed. In the linked model, the screw pitch difference is 0.25 (metric) which means that one turn of the barrel creates that much movement, despite having M2 and M2.25 pitch respectively.

Differential screw model Image from linked site.

Based on your description, it seems likely that you could accomplish your goal. The linked site also includes OpenSCAD files for one's own customization and includes the screw libraries necessary to make the entire construction operational.

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  • $\begingroup$ that seems to just change the pitch of the screw on one side $\endgroup$
    – Tiger Guy
    Commented Sep 28, 2023 at 17:14
  • $\begingroup$ When turning the hex end (upper left corner of above image) in a given direction, the travel is going to be (in this example) 0.25 mm travel (pitch) even though the internal threads are respectively 2.0 mm and 2.25 mm pitch. The model presented is an example of a suggested starting point. It may be necessary to incorporate additional elements or create a bore in order to create the described circumstances. I don't see how it can be accomplished without a differential thread. $\endgroup$
    – fred_dot_u
    Commented Sep 28, 2023 at 17:27
  • $\begingroup$ the example moves the two nuts at different speeds, so the distance between them changes at a low rate. both nuts have fixed threads and pitches $\endgroup$
    – Tiger Guy
    Commented Sep 28, 2023 at 18:00
  • $\begingroup$ My issue with a differential screw is they need to travel the full length of the screw and a differential seems like it would block that $\endgroup$
    – Stan
    Commented Sep 29, 2023 at 20:04
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With a single lead screw -- no.

The reason is because a lead screw has spiral symmetry -- i.e., if you had a lead screw that was both absolutely perfect and had zero friction, then there would be no way to tell the difference between that screw rotating, and that screw moving along its axis.

Note that the two cases you mention won't work:

A half not is not a nut that magically halves the speed it'll travel along a lead screw. Rather, a half nut is a nut that's been split into two, that can be clamped around a lead screw or released. When it's engaged, it acts like a regular nut; when it's disengaged, it's just not connected to the lead screw.

A two-lead nut would not engage with a single-lead screw, even if everything else is the same size. While it may look superficially similar, a two-lead nut has a double helix on its inner side, each of which is on a different slope from an otherwise similar single-lead nut. Try to screw a two-lead nut onto a one-lead screw, or visa versa, and at best it'll just be balked, while at worst it'll jam.

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  • $\begingroup$ Sweet that makes sense, I did think the half nut wouldn't actually do anything but that clears up some other questions I had aswell. Thanks $\endgroup$
    – Stan
    Commented Sep 29, 2023 at 17:58

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