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The terms are derived from the Latin words for "left" (sinister) and "right" (dexter). Other disciplines use different terms (such as dextro-and laevo-rotary in chemistry, or clockwise and anticlockwise in physics) or simply use left and right (as in anatomy). Relative direction and chirality are distinct concepts.
This is known as a right-handed (RH) thread, because it follows the right-hand grip rule. 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 screw threads are also used in some other applications: Where the rotation of a shaft would cause a conventional right-handed nut to loosen rather than to tighten due to fretting induced precession. Examples include: The left hand pedal on a bicycle. [21] [23] The left-hand screw holding a circular saw blade or a bench grinder wheel on.
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]
Most hardware screw threads are right-handed helices. The alpha helix in biology as well as the A and B forms of DNA are also right-handed helices. The Z form of DNA is left-handed. In music, pitch space is often modeled with helices or double helices, most often extending out of a circle such as the circle of fifths, so as to represent octave ...
Bicycle pedals are left-threaded on the left-hand crank so that precession tightens the pedal rather than loosening it. This may seem counter-intuitive since the pedals rotate in the direction that would unscrew them from the cranks, but the torque exerted due to the precession is several orders of magnitude greater than that caused by bearing friction or even a jammed pedal bearing.
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This glossary of biology terms is a list of definitions of fundamental terms and concepts used in biology, the study of life and of living organisms.It is intended as introductory material for novices; for more specific and technical definitions from sub-disciplines and related fields, see Glossary of cell biology, Glossary of genetics, Glossary of evolutionary biology, Glossary of ecology ...