Screw thread
A screw thread is a helical or tapered structure used to convert between rotational and linear movement or force.
Screw threads have several applications:
- Fastening
- Fasteners such as wood screws, machine screws, nuts and bolts.
- Connecting threaded pipes and hoses to each other and to caps and fixtures.
- Gear reduction via worm drives
- Moving objects linearly by converting rotary motion to linear motion, as in a screw jack.
- Measuring by correlating linear motion to rotary motion (and simultaneously amplifying it), as in a micrometer.
- Both moving objects linearly and simultaneously measuring the movement, combining the two aforementioned functions, as in a leadscrew.
In all of these applications, the screw thread has two main functions:
- It converts rotary motion into linear.
- It prevents linear motion without the corresponding rotation.
In most applications, the thread pitch of a screw is chosen so that friction is sufficient to prevent linear motion being converted to rotary, that is so the screw does not slip even when linear force is applied so long as no external rotational force is present. This characteristic is essential to the vast majority of its uses.
A screw thread may be thought of as an inclined plane wrapped around a cylinder or cone. The tightening of a fastener's screw thread is comparable to driving a wedge into a gap until it sticks fast through friction and slight plastic deformation.
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