A nut is a component with an internal screw thread that engages a matching external thread. In a lead screw mechanism, it transfers load through thread contact and can provide the translating output. Either the screw or the nut can be the rotating member, provided the other required motions are constrained.[ref]
Selecting the Optimal Screw Technology by Tolomatic. Introduction, printed p4; Acme Screws, printed p7, load transfer through sliding thread surfaces
The useful motion depends on relative rotation. Consider an ideal assembly with screw lead per relative revolution. Restraining the nut against rotation while turning an axially restrained screw through a quarter-turn produces of nut travel. If both rotate together through the same angle without relative rotation, that common rotation produces no axial advance through the thread geometry.
An ideal work calculation connects the motion conversion to load transfer. For steady slow motion without friction or changes in stored energy,With and , the ideal axial force is approximately . Real contact friction and other loads enter the drive-torque balance; the ideal calculation is not a torque specification for a manufactured assembly.
A matching thread must establish a suitable contact path, rather than simply fit through the nut's opening. Clearance can permit backlash when the loaded contact surfaces change on reversal. After contact is established, further displacement under changing force includes mechanical compliance. Counting rotations alone therefore does not measure the load-dependent position of the nut.
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