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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.
Chirality, however, is observer-independent: no matter how one looks at a right-hand screw thread, it remains different from a left-hand screw thread. Therefore, a symmetric object has sinistral and dextral directions arbitrarily defined by the position of the observer, while an asymmetric object that shows chirality may have sinistral and ...
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]
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.
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It normally consists of two threaded eye bolts, one screwed into each end of a small metal frame, one with a conventional right-hand thread and the other with a left-hand thread. The tension can be adjusted by rotating the frame, which causes both eye bolts to be screwed in or out simultaneously, without twisting the eye bolts or attached cables.
Similarly, the flyer whorl of a spinning wheel uses a left-hand thread to keep it from loosening. A turnbuckle has right-handed threads on one end and left-handed threads on the other. Some gas fittings are left-handed to prevent disastrous misconnections: oxygen fittings are right-handed, but acetylene , propane , and other flammable gases are ...
This can cause threaded parts to either tighten or loosen under a load, depending on the direction of rotation, typically with a force that can far exceed the typical torque of a wrench. For example, this is a common problem in bicycle pedals, thus on nearly all bikes built after the 1930s, the left-side pedal is equipped with left-hand ...