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Threads oriented in the opposite direction are known as left-handed (LH). By common convention, right-handedness is the default handedness for screw threads. Therefore, most threaded parts and fasteners have right-handed threads. Left-handed thread applications include:
Right-hand and left-hand screw threads. The helix of a screw's thread can twist in two possible directions, which is known as handedness. Most screw threads are oriented so that when seen from above, the screw shaft moves away from the viewer (the screw is tightened) when turned in a clockwise direction.
Spiral flute screw extractors. A spiral screw extractor is itself a coarse-pitched tapered screw thread. They are generally left-handed, for use on right-handed threads, though there are right-handed extractors for removing left-handed screws. The screw is first drilled out to the proper diameter for that
Most screws are tightened by clockwise rotation, which is called a right-hand thread. [4] [5] Screws with a left-hand thread are used in exceptional cases, such as where the screw will be subject to counterclockwise torque, which would tend to loosen a right-hand screw. For this reason, the left-side pedal of a bicycle has a left-hand thread. [6]
Standardized variations include multiple-start threads, left-hand threads, and self-centering threads (which are less likely to bind under lateral forces). The original trapezoidal thread form, and still probably the one most commonly encountered worldwide, with a 29° thread angle, is the Acme thread form ( / ˈ æ k m iː / AK -mee ).
The threads of a screw are helical and therefore screws can be right- or left-handed. To properly fasten or unfasten a screw, one applies the above rules: if a screw is right-handed, pointing one's right thumb in the direction of the hole and turning in the direction of the right hand's curled fingers (i.e. clockwise) will fasten the screw ...