Bearing (mechanical)
A bearing is a sliding or rolling component used to reduce friction in a machine. The first bearings were wood, but ceramic or glass can be used, and steel is quite common. The path that a bearing travels is called a "race". Bearings need to be as hard as the race that they are rolling against, or either the bearing or race will wear out prematurely. Different types exist for both linear and rotary motion.
Linear bearings
An early type of linear bearing was an arrangement of tree trunks laid down under sleds. This technology is known to date at least as far back as the construction of the Pyramids of Giza. Modern linear bearings use a similar principle, sometimes with balls in place of rollers.
Rotary bearings
Rotary bearings are required for many applications, from heavy-duty use in vehicle axles and machine shafts, to precision clock parts. The simplest rotary bearing is the sleeve bearing, which is just a cylinder inserted between the wheel and its axle. This was followed by the roller bearing, in which the sleeve was replaced by a number of cylindrical rollers. Each roller behaves as an individual wheel. The first practical caged-roller bearing was invented by horologist John Harrison in his H3 chronometer of 1760.The ball bearing was an improvement on the roller bearing. Roller bearings are still used, however, when the sideways forces are too great for ball bearings.
An early example of a wooden ball bearing, supporting a rotating table, was retrieved from the remains of a Roman ship in Lake Nemi[?], Italy. The wreck was dated to 40 BC. Leonardo da Vinci is said to have described a type of ball bearing around the year 1500. One of the issues with ball bearings is that they can rub against each other, causing additional friction, but this can be prevented by enclosing the balls in a cage. The captured, or caged, ball bearing was originally described by Galileo in the 1600s. The mounting of bearings into a set was not accomplished for many years after that. The first patent for a ball race was by Philip Vaughan of Carmarthen[?] in 1794. The modern, self-aligning design of ball bearing is attributed to Sven Wingquist of the SKF ball-bearing manufacturer in 1907.
Some modern bearing assemblies require routine addition of lubricants, while others are factory sealed, requiring no further maintenance for the life of the mechanical assembly. The lubricant is intended to reduce friction. However, if the lubricant becomes contaminated by hard particles, such as steel chips from the race or bearing, sand, or grit, the lubricant quickly begins to act as a grinding compound. This greatly reduces the operating life of the bearing assembly.
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