ABSTRACT
Screw Mechanism Basics Screw mechanisms consist of two main parts, a rod formed or cut with one or more helical threads, and a nut that engages those threads. Screw mechanisms can translate rotary motion into linear motion or, more rarely, linear into rotary. The four different setups shown in Figure 20.1 result from driving either the screw or the nut, and whether the drive is rotational or linear. In addition to the screw and nut, mounting blocks which are designed to support the screw and allow it to spin freely are available. Screw mechanisms are available either as individual parts or as full assemblies which usually consist of one screw, one nut and two mounting blocks. There are two main categories of screw mechanisms, distinguished by the type of movement between nut and screw: sliding and rolling. There are some common terms which apply to both categories:
Efficiency - The ratio of power applied to the screw mechanism to power actually transmitted. Sliding contact screws have efficiencies ranging from 10% to 80%, while ball screw efficiency is generally 90% or greater. Any screw less than 50% efficient will self-lock, meaning no amount of force axially along the threaded rod will spin the nut, nor will any force on the nut, aligned in the direction of the long axis of the rod, spin the rod. Conversely, if efficiency is 50% or higher, forces on the nut will spin the rod, and vice-versa. A self-locking lift would use a screw with an efficiency well below 50%, while the classic Yankee screwdriver uses a high efficiency screw to convert linear motion into rotation.
