The English word corresponding to screw is Screw. The meaning of this word has changed significantly in recent hundreds of years. At least in 1725, it meant "mating".
In addition to the knowledge in the name, it took thousands of years for small screws to be tightened clockwise and loosened counterclockwise from their invention.
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Why do screws have to be tightened clockwise?
Plato's friend invented the screw
The six simplest mechanical tools are: screws, inclined planes, levers, pulleys, wedges, wheels, and axles.
The screw is among the six simple machines, but to put it bluntly, it is nothing more than an axis and an inclined plane winding around it. Today, screws have developed standard sizes. The typical way to use a screw is to turn it clockwise to tighten it (as opposed to turning it counterclockwise to loosen it).
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Clockwise tightening is mainly determined by right-handers
However, since the screws at the time of their invention were all made manually, the fineness of the screws was not consistent and was often determined by the craftsman's personal preference.
By the mid-16th century, French court engineer Jaques Besson invented a lathe that could cut screws. It later took 100 years for this technology to be promoted. The Englishman Henry Maudsley invented the modern lathe in 1797. With it, the fineness of threads was significantly improved. Despite this, there is still no unified standard for the size and fineness of screws.
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This situation changed in 1841. Maudsley's apprentice Joseph Whitworth submitted an article to the Institution of Municipal Engineers calling for the unification of screw types. He made two suggestions:
1. The inclination angle of the screw thread should be 55° as the standard;
2. Regardless of the diameter of the screw, the number of wires per foot should adopt a certain standard.
Although screws are small, they require n types of machine tools and n+1 types of cutting tools to make them in the early days.
Early screws were not easy to make because their production process "required three cutting tools and two machine tools."
In order to solve the production and manufacturing problems of British standards, American William Sellers invented a flat-top and flat-heeled thread in 1864. This small change allowed screws to be manufactured with only one tool and machine tool. Faster, easier and cheaper.
The thread of Sellers screws became popular in the United States and soon became the standard for Amtrak applications.
Characteristics of bolted joints
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Figure B: Bolted joint properties
Main variables of the tightening process:
(1) Torque (T): the applied tightening torque, unit Nm (Nm);
(2) Clamping force (F): the actual axial clamping (pressing) size between connecting bodies, unit Newton (N);
(3) Friction coefficient (U): the torque coefficient consumed by bolt heads, thread pairs, etc.;
(4) Rotation angle (A): Based on a certain torque, the bolt will have a certain axial elongation or the connector will be compressed and the thread angle needs to be turned.
