Operating with minimum energy consumption has been the subject of engineering research since time immemorial. The first step on this path was the use of logs placed under a bench on which heavy loads were moved. This created a primitive bearing that reduced frictional resistance. Further developments led to the elimination of the effect of unevenness in the ground through the use of appropriately shaped rollers, the prototype of a sliding bearing vehicle axle. Drawings of wheeled machines can be found in Egyptian paintings from the 5th Dynasty (3000 BC); remains of chariot wheels were found in Tutankhamen's tomb. Stone bearings were used as early as the Stone Age, around 4000 BC, for bearing spindles for spinning. Primitive examples of spindle bearings can also be seen in the ancient craft workshops of Europe. A fragment of deer horn was placed at the end of the spindle to act as a thrust bearing. Among the constructors of bearings there is no shortage of great names - men of science. The famous physicist Charles A. Coulomb, studying the phenomenon of dry friction in conditions of low speeds and pressures, used ball bearings of his own design, similar to some modern solutions. We cannot omit the brilliant Leonardo da Vinci, who also researched the phenomena of sliding and rolling friction. He even determined the coefficient of friction equal to 0,25, probably referring to rough, unlubricated wood-to-wood or metal-to-metal surfaces. 
[001] Typical roller bearing designs: a, b, c - ball bearings: radial, angular and axial, d, e - cylindrical and conical, f - needle roller
Today's bearing designs are characterised by great diversity. This is due to the increasing specialisation in mechanical engineering and the quest for compact, space-saving and functional solutions. In a modern machine, the bearing is one of the most responsible parts, which is affected by the complexity of operation and associated phenomena, such as temperature, contamination or vibration. In plain bearings, the shaft slides in the bearing seat. This results in high friction, although these bearings are not very sensitive to shocks, overload or contamination. In rolling bearings, on the other hand, motion is transmitted by rolling elements - balls, rollers or needles. They are characterised by low starting friction, simple lubrication, small width in relation to the shaft diameter. Another very important advantage of roller bearings is that they can be directly replaced without any adjustment or running-in. Rolling bearings are gradually replacing plain bearings in many applications. In order to simplify their manufacture and use, the overall dimensions of the bearings and their components have been standardised. Depending on the type of rolling elements, a distinction is made between ball, roller: spherical, cylindrical or tapered roller and needle roller bearings [001]. The bearing can be capable of carrying radial load (perpendicular to the axis), axial load (along the axis) or combined load. Many radial bearings, especially ball bearings, can simultaneously carry axial forces, sometimes up to 30% of the value of the radial force. As can be seen in [001], a roller bearing comprises rings 1,2, rolling elements 3 and a cage 4 separating them (not always). The inner surfaces of the rings, on which the rolling elements roll, are called raceways. 
[002] Ball bearing animation. The raceways and rolling elements of a bearing are constantly subjected during operation to sometimes huge stresses with variable amplitude, often of shock nature. The main component of the load capacity, which a bearing can carry, is its moving load capacity - in case of dynamic operation, or resting load capacity - when it is necessary to indicate, how much force can be applied to a non-rotating bearing. It turns out that bearings can carry very heavy loads. For example, if the permissible pressures for chrome steel are about 50 daN/mm2, then pressures of 500 daN/mm2 at the contact between the rolling element and the raceway are not considered critical when the bearing is in motion. The commonly used material for rolling bearings is chrome steel. The hardness of this steel after hardening is 59-64 HRC. This is a relatively high hardness, comparable to that of cutting tools. The bearing life is measured in millions of revolutions or in operating hours. It is dependent on the ratio of its operational load rating C to the size and distribution of the load P, to which the bearing is subjected during operation. This is because it is important how many rolling elements are involved in carrying the load. Figure [003] presents the load distribution in a ball bearing subjected to a lateral force P. As can be seen, the load is carried only by a part of rolling elements. The design values of the dynamic load carrying capacity of the bearing are related to this load distribution. The life of a bearing operating often under very unfavourable load conditions also depends on the selection of the lubricant. Bearings lubricated with grease do not require as precise supervision as oil-lubricated bearings, they withstand higher pressures and effectively fulfil the role of bearing seals. At higher speeds, however, oil lubrication must be used. Ensuring adequate lubrication is the basic requirement for the proper operation of the bearing. 
[003] Distribution of radial forces applied to a ball bearing.
Newly developed bearing designs require checking the assumed technical and operational parameters on test stands. Special equipment is used for this purpose, where bearings are subjected to forces and rotations selected according to the operating parameters to be checked. Usually the tests last for 250 hours, but some designs are tested for 1000 hours or are "spun" on the test stand until they are destroyed. The smallest bearings have a bore diameter of 1 mm, an outer diameter of 3 mm and a weight of 0.03 g, and reach speeds of hundreds of thousands revolutions per minute. They are used in rocket control systems, computers, medical devices, tape recorders, cameras and precision optical instruments. Giant bearings have diameters exceeding 10 m, and the balls used in them - more than 300 mm and the weight of more than 100 kg. The world's largest spherical roller bearings, used in the drum of a copper ore crusher in Warego (Australia), have dimensions of 1800x2180x375 mm and a weight of 2830 kg. To prevent damage, a temperature sensor is installed at each bearing with a switch that stops the machine if the temperature exceeds 80 ° C. The precision of movement and rigidity provided by even the largest bearings is astonishing. For example, the bearings of terrestrial antennas for receiving signals from artificial satellites must ensure such rigidity for an antenna with a diameter of more than 30 m and a mass of 325 that, when the wind blows at 40 m/s, the skew of the antenna does not exceed 0.005 degrees. The roller bearing segment has accounted for the largest share in the title industry so far and is likely to maintain this share in the future as well, registering the highest growth rate. It appears that the fastest growing demand for bearings will be in the railway and aerospace industries, and geographically in the Asia-Pacific region. 
[004] Part of the GTDE planetary gearbox. Roller bearings, function as satellites.
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