Differential screw
![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.](/Images/godic/202501/09/DifferentialScrew4224.gif")
![Micrometer adjuster with differential threads. The threads 11 on rod 4 have one pitch (e.g. 25 tpi, blue), while the threads 10 in the barrel 1 have another (e.g. 20 tpi, orange). A full turn of the thimble 12 rotates the nut sleeve 13 and 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.](/Images/godic/202501/09/Differential_adjuster_from_patent_585184_fig_1,_24224.png")
![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]](/Images/godic/202501/09/Lanz_&_Betancourt-_Analytical_essays_on_the_construction_of_machines,_pg_181_-_Plate_1_-_Fig_D34224.png")
,_by_Joshua_Rose,_Figure_417,_1887_-_illo0135e4224.png")
A differential screw is a mechanism used for making small, precise adjustments to the spacing between two objects (such as in focusing a microscope, moving the anvils of a micrometer, or positioning optics). A differential screw uses a spindle with two screw threads of differing leads (aka 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.