Usage
Typical nuts, screws, bolts, and bottle caps are tightened (moved away from the observer) clockwise and loosened (moved towards the observer) counterclockwise, in accordance with the right-hand rule.
A way of remembering this that is based on the right-hand rule is to place one's loosely-clenched right hand above the object with the thumb pointing in the direction one wants the screw, nut, bolt, or cap ultimately to move, and the curl of the fingers, from the palm to the tips, will indicate in which way one needs to turn the screw, nut, bolt or cap to achieve the desired result. Most threaded objects are susceptible to application of the above; for a countably small number of exceptions (read: "left-handed" threads, or "reverse threads"), one substitutes the left-hand rule instead.
The reason for the clockwise orientation of most screws and bolts is that supination of the arm, which is used by a right-handed person to turn a screw clockwise, is generally stronger than pronation. Also, it was wise to adopt a single standard version for most screws and bolts – in order to eliminate endless confusion.
Sometimes the opposite sense of threading is used for a special reason. A thread might need to be left-handed to prevent the prevalent stresses that are present from loosening it. For example, some older automobiles and trucks had right-handed lug nuts on the right side of the vehicle and left-handed lug nuts on the left side of the vehicle. As the vehicle moved forward, the lug nuts tend to tighten. For bicycle pedals, the one on the right must be reverse-threaded, or the pedal will unscrew during use; similarly, the flyer whorl of a spinning wheel uses a left-hand thread to keep from loosening in normal use. A turnbuckle has right-handed threads on one end and left-handed threads on the other end. Some gas fittings are left-handed to prevent disastrous misconnections. For example, oxygen fittings are right-handed, but acetylene and other flammable gases use left-handed fittings.
In trigonometry, and in mathematics in general, plane angles are conventionally measured counterclockwise, starting with 0° or 0 radians pointed directly to the right (or east). In navigation, compass headings increase in a clockwise direction around the compass face, starting with 0° at the top of the compass (the northerly direction).
Read more about this topic: Clockwise
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