Rotating a bolt through two nuts with slightly different thread pitches for each changes the separation of the nuts by the difference between the threads.Micrometer adjuster with differential threads. The threads11on rod4have one pitch (e.g. 25 tpi, blue), while the threads10in the barrel1have another (e.g. 20 tpi, orange). A full turn of the thimble12rotates the nut sleeve13and its two threads (20 tpi outside and 25 tpi inside) to move the rod 1/20 - 1/25 = 0.01 in (0.25 mm) relative to the barrel.Differential screw illustration from an 1817 machine design handbook. ab & ef have one thread pitch while cd has a different one. One turn of AB moves the whole spindle one ab thread distance; simultaneously, M moves one cd thread distance and the amount that AB moved. M's overall movement is thus the difference between ab and cd.[1]Differential screw leveling feet, drawing by Joshua Rose, 1887.
A differential screw is a mechanism used for making small, precise adjustments to the spacing between two objects (such as in focusing a microscope,[2] moving the anvils of a micrometer,[3][4] or positioning optics[5]). A differential screw uses a spindle with two screw threads of differing leads (in case of a single lead equal to the thread pitch), and possibly opposite handedness, on which two nuts move. As the spindle rotates, the space between the nuts changes based on the difference between the threads. These mechanisms allow extremely small adjustments using commonly available screws. A differential screw mechanism using two nuts incurs higher friction and therefore requires more torque to turn than a simple, single lead screw with an equivalent pitch.[6][7][8]
^Cite error: The named reference Lanz was invoked but never defined (see the help page).
^"Micrometer Heads Series 110-Differential Screw Translator (extra-Fine Feeding) Type". Product Catalog. Mitutoyo, U.S.A. Archived from the original on November 9, 2011. Retrieved December 11, 2012.
^"Tutorials: Opto-Mechanical Components, Translation Stages, Drive Options". Archived from the original on 2010-01-15. Retrieved 2012-12-14.
^"Screw Threads - The Theory Of Machines - Engineering Reference with Worked Examples". Retrieved 2012-12-24.
^Kostelnicek, Dick (March 2010). "That Ol' Differential Screw Clamp" (PDF). Home Metal Shop Club Newsletter. 5 (3): 5–6. Retrieved 2012-12-24.
^Kuhrt, O.C. (1916). "A Toolmaker's Novel Clamp". American Machinist. 44: 32.
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