Published online by Cambridge University Press: 17 July 2025
Introduction
Screws are not only used for joining components, but also for transmission of power. Where these are used for power transmission, these are called as power screws. Their use can be seen in some of the following applications:
⢠Lifting loads such as screw jack, toggle jack.
⢠Transmission of power such as lead screw of a lathe.
⢠Converting rotary motion to linear motion such as ball screw and nut.
⢠Applying loads such as power press, vice, clamp, etc.
Types of Power Screws
Out of all the different types of threads described in Chapter 16, only square and acme threads are used for such applications due to their more strength than other screws. Square threads have square cross section of size half the pitch. Due to parallel surface of threads, it is difficult for the nut to engage and disengage. Acme threads become superior to square threads due to trapezoidal cross section with inclined surfaces forming an included angle of 29°. Owing to a bigger pitch than V threads, these threads are not cut by taps and dies and are generally cut on a lathe machine. Buttress threads are used for carpentry vice. The force is applied only in one direction while tightening. Diameters and pitch of square threads are given in Table 18.1.
Torque for Raising Load
When a screw is used to raise load, it can be considered analogues to a load being pushed on an inclined plane at helix angle Ï The Helix angle of a screw is tan–1 (Pitch / mean circumference).
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