Lead and Pitch
The fineness or coarseness of a screw's threads are defined by two closely related quantities:
- The lead is defined as the linear distance the screw travels in one complete revolution (360°) of the shaft. The lead determines the mechanical advantage of the screw; the smaller the lead, the higher the mechanical advantage.
- The pitch is defined as the axial distance between the crests of adjacent threads.
In most screws, called "single start" screws, which have a single helical thread wrapped around them, the lead and pitch are equal. They only differ in "multiple start" screws, which have several intertwined threads. In these screws the lead is equal to the pitch multiplied by the number of starts. Multiple-start screws are used when a large linear motion for a given rotation is desired, for example in screw caps on bottles, and ball point pens.
Read more about this topic: Screw (simple Machine)
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