Announcement

Collapse
No announcement yet.

Verification needed, differential screw.

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

  • Verification needed, differential screw.

    I just want to make sure my thinking is correct on this. The way I figure it, it doesn't matter whether it's the inner or outer screw that has the larger pitch in a differential screw setup. The ratio and resulting movement will be the same. Correct?

  • #2
    "inner or outer"? Not sure I am following you here.

    Otherwise... in all the differential screw arrangements I have ever worked with where there are two different pitches threaded in to a long nut and two threaded rods, one of each pitch, threaded in to that nut.... then yes you are correct, it doesn't matter which screw end you are looking at, they move the same amount linearly-per-turn relative to the other end.

    -DU-

    Comment


    • #3
      Originally posted by Forestgnome
      The ratio and resulting movement will be the same. Correct?
      The ratio and movement distance are the same.
      However, the resulting direction will be different.
      DJ

      Comment


      • #4
        OK I get it now. DJ is correct... the direction will be different for this type of differential screw:

        depending on whether the "inner" or "outer" screw has the greater pitch.

        -DU-

        Comment


        • #5
          And in the event that you are not already aware, there is some nifty software to calculate the pitches of screw needed to obtain a particular motion that you want. Find it (and a bunch of other handy software) posted as "DIFFTHRD.ZIP" here:

          Marv Klotz's website

          Marv occasionally visits and posts here and has had all of these really handy little utilities available for free for a long time.

          So once again, Hats off to Mr Klotz for sharing his work.

          If you know this already or don't need this, feel free to ignore the above. But I figure someone might find use for this stuff that has not see it before.

          Comment


          • #6
            So based on the drawing, if the coarse thread was the outer screw, then screwing that in would result in an overall motion left due to it moving faster?

            Comment


            • #7
              http://www.google.com.au/#hl=en&gs_n...w=1920&bih=785

              Comment


              • #8
                FWIW, Thorlabs has some pretty decent prices on Differential screws, .25u per turn for $38.

                Comment


                • #9
                  They are horrible things. I can never figure out which way to turn them.

                  Best kept in the text books but if you must the answer is yes.
                  "...do you not think you have enough machines?"

                  Comment


                  • #10
                    Originally posted by Forestgnome
                    So based on the drawing, if the coarse thread was the outer screw, then screwing that in would result in an overall motion left due to it moving faster?
                    Correct. The traveling piece will move left. Not so much because the coarse thread moves "faster" but because the coarse threaded screw moves "further" to the left than the fine threaded part of the screw moves the travelling piece to the right.

                    All you have to remember (or know) is which part is coarse threaded. The coarser thread will always dominate the motion. The fine thread is really only a "divider".

                    The formula for the differential screw arrangement I posted earlier is (from Marv Klotz):

                    1/Pe = 1/Pc - 1/Pf
                    or

                    Pe = 1/(1/Pc - 1/Pf)

                    where Pe is the effective pitch of the assembly, Pc is the coarser pitch, and Pf is the finer pitch. This is for imperial threads.

                    For metric threads the formula is even simpler:

                    Pem = Pcm - Pfm

                    It is left as an exercise to the reader what happens when one of the threads is of the opposite hand. Say a right hand coarse pitch and a left hand fine pitch.

                    Macona,
                    Yeah ThorLabs has some relative bargains. Their fine thread bushings and adjuster screws are also very cheap. Especially when compared to the hassle of making them yourself. Where I work I order most of our equipment from ThorLabs. Both Edmund Optics and ThorLabs are based in NJ but ThorLabs has, IMO, better prices, quality and service.

                    -DU-

                    Comment

                    Working...
                    X