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"Differential Thread"... What is the benefit?

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  • "Differential Thread"... What is the benefit?

    Someone mentioned the differential thread on another post and I'd like to explore it more as I've heard the term, made one but don't see the advantage?

    Seems to me that there will always be some slop between thread and mate. When you double the threads and mates in a mechanical application I would think you would double the slop but these are used for refined applications if I'm not mistaken?

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    Thank you to our families of soldiers, many of whom have given so much more then the rest of us for the Freedom we enjoy.

    It is true, there is nothing free about freedom, don't be so quick to give it away.

  • #2
    Slop does not destroy the usefulness of a lead screw, and it won't destroy the usefulness of a differential thread either.

    The advantage is a fine-thread movement without needing an actual fine thread. All slop-eliminating methods would be used to mitigate the effect of slop, taking it up etc.

    But it won't be doubled..... it will have the proportional slop of each size thread, in that thread..... slop larger than the pitch is obviously not possible, for example, so a finer thread has to have less possible slop..
    1601

    Keep eye on ball.
    Hashim Khan

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    • #3
      YOD,
      I needed some fine threads many years ago for steering laser mirrors.
      The originals had 1/2" x 40 tpi on them but that was still a bit course, the main problem was these being in constant use soon wore out.

      I made new threads using the differential method [ which at the time I thought I had invented ] using 1/2" x 20 running with 1/2" x 26 which are both common British threads in the Cycle series.

      This gave me a fine action but with deep threads to prevent wear and after trialling these on one machine they were all fitted with them and none were replaced up until they were scrapped out.

      .
      .

      Sir John , Earl of Bligeport & Sudspumpwater. MBE [ Motor Bike Engineer ] Nottingham England.



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      • #4
        It's used on tap handles to increase the clamping force.

        Here are examples used for metrology including micrometer screws and laser mirror adjusters.

        http://www.photonlines.com/optosigma/screw.html
        Last edited by Evan; 12-31-2009, 10:26 AM.
        Free software for calculating bolt circles and similar: Click Here

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        • #5
          Hi all,

          If the goal is to advance less, than the second thread has to be left-handed, is it not?
          Say we start with a 2 mm RH thread that acts on a 1.5 mm LH thread, the outcome would be an .5 mm advancement after one turn. Right?

          Regards,

          Danny
          Last edited by DannyW; 12-31-2009, 10:42 AM.
          ---------------------------
          Wer anderen etwas vorgedacht, ....... When you propose a solution for someone's problem,
          wird jahrelang nur ausgelacht. ....... you will be ridiculed for years.
          Begreift man die Entdeckung endlich, ....... When the discovery is finally understood,
          so nennt sie jeder selbstverstÙ†ndlich. ....... everyone will say it is obvious.
          -- Wilhelm Busch --

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          • #6
            Here is a demonstration model. It should make it clear.

            Free software for calculating bolt circles and similar: Click Here

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            • #7
              Originally posted by DannyW
              Hi all,

              If the goal is to advance less, than the second tread has to be left-handed, is it not?
              Say we start with a 2 mm RH tread that acts on a 1.5 mm LH tread, the outcome would be an .5 mm advancement after one turn. Right?

              Regards,

              Danny
              The threads are given at "16 pitch" and "18 pitch" which is most likely in error. The poster probably meant 16 threads per inch (tpi) and 18 tpi.
              If both threads are right handed and the screw is turned clockwise then the screw will move to the left. However the traveling piece (which I'm assuming cannot rotate but can slide left or right) will move to the right.
              The screw will move 1/16" for each rotation (left) and the traveling piece 1/18" (right). Subtract the two and get the net movement of the traveling piece to the left.
              Now if the 18 tpi were left handed then the traveling piece will also move left. The net distance travelled by the traveling piece will be 1/18 plus 1/16 (left).

              Ed P

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              • #8
                Thanks guys. I understand how the concept works but it was the slop I had trouble dealing with and JTiers pointed out something I had overlooked. And that is the slop is taken up when the threads all mate up. I suppose in actual usage, this differential screw might adjust one item against another but a lock down screw would be necessary to hold all in place as the slop inherent in both sets of threads would come into play.

                The durability issue that John points out was another point that had escaped me. The example Evan post was one I had not seen. I can understand how it might be used where the more conventional one (such as the optical) might as you could apply more force to it to move larger objects.

                Has anybody made a triple differential thread?
                Last edited by Your Old Dog; 12-31-2009, 11:28 AM.
                - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
                Thank you to our families of soldiers, many of whom have given so much more then the rest of us for the Freedom we enjoy.

                It is true, there is nothing free about freedom, don't be so quick to give it away.

                Comment


                • #9
                  I did post this once before... nice idea
                  I was going to use it as a feed for a line boring bar, but went a different direction thanks to a UK buddy

                  http://www.youtube.com/watch?v=iPkoGug-oC0
                  please visit my webpage:
                  http://motorworks88.webs.com/

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                  • #10
                    My fellow designer pulled the "differential screw thread trick" out
                    of his pocket, and used it on a inspection fixture.

                    The large gear product weighing around 5 tons was needed
                    to be finely (maybe 30 - 40 pitch) adjusted for leveling or elevation
                    using a screw on a c.m.m. fixture.

                    The screws were around 2" in diameter (also support the
                    "v" blocks in alignment) but had a very fine adjustment.

                    Which the inspector (c.m.m. operator) appreciates daily

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                    • #11
                      Got it Ed!

                      And Evan's model does what? Given both RH threads and turning clockwise?

                      The middle piece goes to the right?

                      Right?


                      Danny
                      Last edited by DannyW; 12-31-2009, 11:55 AM.
                      ---------------------------
                      Wer anderen etwas vorgedacht, ....... When you propose a solution for someone's problem,
                      wird jahrelang nur ausgelacht. ....... you will be ridiculed for years.
                      Begreift man die Entdeckung endlich, ....... When the discovery is finally understood,
                      so nennt sie jeder selbstverstÙ†ndlich. ....... everyone will say it is obvious.
                      -- Wilhelm Busch --

                      Comment


                      • #12
                        Originally posted by DannyW
                        Got it Ed!

                        And Evan's model does what? Given both RH threads and turning clockwise?

                        The middle piece goes to the right?

                        Right?


                        Danny
                        Yes - and the handle winds in at the pitch of the left most thread.

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                        • #13
                          I used this trick on a steam engine valve linkage coupling nut once; fine thread rod eye on one side, coarse thread rod from the eccentric on the other and both w/ jam nuts. Works very well, and is really simple; you cab adjust the valve timing while the engine runs (slowly).

                          - Bart
                          Bart Smaalders
                          http://smaalders.net/barts

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                          • #14
                            Hmm, I could sware my tap/die handles have diffrential thread, but its 'reversed' (Left hand/right hand thread) in that it gives me extra travel for less turns, Not more torque for more turns.

                            Its definately a cool trick.
                            as far as the 'slop' is conserned, its not a big problem if you can insure its allways loaded from one direction (One way movement stops, lifting parts, clamping parts, etc etc etc) Or just have an application where a little slop isent a consern, or if you used slop reduction methods like nylok nuts, etc.
                            Play Brutal Nature, Black Moons free to play highly realistic voxel sandbox game.

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                            • #15
                              .....and on a different but somewhat related (and I trust interesting) topic I recall reading once that Maudsely et al would parallel two lead screws to cut a new lead screw which would have only the the average error of the pair.

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