Bearing, Rolling-Contact

Bearing, Rolling-Contact

 

(or rolling-element bearing), a support for a rotating part of a mechanism or machine that functions predominantly under conditions of rolling friction and usually consists of inner and outer rings (races), rolling elements, and a separator to keep the rolling elements apart and to guide their motion.

A distinction is made between ball bearings and roller bearings, depending on the shape of the rolling elements; the latter type may have various roller shapes. The outer surface of the inner ring and the inner surface of the outer ring have roller tracks whose shape conforms to the rolling elements used in the particular type of bearing. Sometimes, to reduce the overall radial dimensions, rolling-contact bearings have only one ring; in this case the roller track is formed directly on the shaft or on the surface of the housing member. Some rolling-contact bearings—for example, needle bearings—may be made without a separator. They are characterized by a large number of rolling elements and, as a result, have a high load-carrying capacity. The limiting speed of rotation of bearings without separators is lower because of the higher frictional torque.

Rolling-contact bearings are divided into four groups according to the direction of action of the load: (1) radial bearings, which take only radial loads (for example, roller bearings with needle rollers) or radial and limited axial loads (single-row radial ball bearings); (2) radial-thrust bearings, which take combined —that is, radial and axial—loads (bearings with tapered rollers); (3) thrust-radial bearings, which take mainly axial loads, but also minor radial loads (they have limited application); (4) thrust bearings, which take only axial loads.

Rolling-contact bearings may have one or more rows of rolling elements and a variety of designs. They are divided into types according to a set of characteristics. In addition to the main types of such bearings, there are also design variations. Radial-thrust ball bearings are made with various rated angles of contact, usually 12°, 26°, and 36°. As the angle of contact increases, the axial stiffness and the ability to take axial loads are increased, but the radial stiffness and power-speed coefficient are decreased. The load-carrying capacity and stiffness of the support, as well as the precision of shaft rotation, are improved by the installation of duplex radial-thrust rolling-contact bearings. Ball bearings with a split inner or outer ring take axial loads in either direction and accurately secure the axial position of the shaft.

The design of bearings may vary depending on the means of mounting (mounting on the shaft or attachment to the housing). Bearings designed to be attached to the tapered necks of shafts have a tapered bore. Spherical rolling-contact bearings on adapter sleeves are mounted on the smooth parts of shafts (that is, parts without beads). The outer rings of radial ball bearings are sometimes made with a slot for an adjusting washer, which simplifies the axial attachment in the housing. The rings and the rolling elements are fabricated from high-carbon steels tempered to great hardness or sometimes from casehardened mild steels. Type ShKh15 chrome steels are the most common. Stainless or heat-resistant steels are used in some cases. The separators of rolling-contact bearings in mass production are made of mild steel or, less frequently, of stainless steel and brass by punching from strip or sheets. The huge separators of bearings designed for high-speed operation are made of brass, ductile cast iron, bronze, Duralumin, or graphitic steel, as well as textolite and other plastics.

The accuracy of manufacture of rolling-contact bearings is regulated by grades: 0 (normal) and 6, 5, 4, and 2, in order of increasing accuracy. There is a common standard for the overall dimensions of rolling-contact bearings in all countries. Numerical notations are used to mark the bearings. For bearings having internal diameters from 20 to 495 mm, the first and second digits from the right correspond to the diameter divided by 5. For diameters greater than 9 mm, the third and seventh digits designate the series of external diameters and widths. The series of bearing diameters provided for in the standards are superlight, extralight, light, medium, and heavy; the series of widths are narrow, normal, wide, and extra-wide. The most common bearings are those of the light narrow series, which are designated by the digit 2 in the third place and 0 in the seventh place, and the medium narrow series, which have a 3 in the third place and 0 in the seventh place.

The fourth digit indicates the type of bearing: 0 is a radial ball bearing, 1 is a radial two-row spherical ball bearing, 2 is a radial bearing with short cylindrical rollers, 3 is a radial two-row spherical roller bearing, 4 is a radial roller bearing with long cylindrical rollers or a needle bearing, 5 is a radial roller bearing with spiral-wound rollers, 6 is a radial-thrust ball bearing, 7 is a tapered roller bearing, 8 is a thrust ball bearing, and 9 is a thrust roller bearing.

The fifth and sixth digits indicate the specific design features of a bearing. In the conventional designation of a rolling-contact bearing, zeros are not shown to the left of the last significant digit. The grade of fit appears to the left of the conventional designation, after a dash. Bearings that are nonstandard with respect to design, materials, technology, and heat treatment are marked with additional symbols.

The factory manufacture of rolling-contact bearings was begun in 1883 in Germany. Bearings are now produced in the USSR with inner diameters from fractions of a millimeter up to 1,345 mm and with weights from fractions of a gram up to 4 tons. They are used in various machines and instruments, in which they operate over a wide range of rotational speeds (up to 200,000 rpm) at temperatures of up to 1000°C; ball bearings have been made that can operate in a high vacuum.

The widespread use of rolling-contact bearings stems from a number of advantages over plain bearings, including lower drag torque, especially at the start of motion but also at low and medium speeds of rotation; greater load-carrying capacity per unit of bearing width; complete interchangeability; simplicity of maintenance; lower consumption of lubricating materials and nonferrous metals; and less stringent requirements for the materials and heat treatment of the shafts. The shortcomings of rolling-contact bearings include limited service life, particularly at high speeds; the great range of service life; the high cost of short-run and individual production; larger radial dimensions; and lower capacity for vibration and impact damping compared to plain bearings.

The energy losses in rolling-contact bearings result from a complicated physical process. The drag torque depends on the simultaneous effect of a number of phenomena: the slipping of the rolling elements along the contact surfaces and recesses of the separators, the internal friction losses in the material of the contacting elements (elastic hysteresis), the sliding of the massive separator along the centering flanges of the rings, and the resistance of the lubricant and the surrounding medium. The drag torque can be found approximately by using an arbitrary concept of the normalized dimensionless coefficient of friction fn: T = 0.5P·fn · d, where P is the load on the bearing and d is the diameter of the bore in the bearing.

The value of fn ranges from 0.0015 to 0.02 (the smaller values apply to ball bearings operating under radial loads with fluid lubrication). Various materials are used to lubricate rolling-contact bearings: light oils, plastic lubricants, and in special cases, solid materials. The most favorable operating conditions for rolling-contact bearings are provided by light oils characterized by such attributes as stability during operation, relatively low rotational drag, torque, and good ability to carry off heat and to cleanse the bearings of wear products. Plastic lubricants are better than light oils in that they protect the surfaces from corrosion, and they require no complicated gaskets to be retained in the assembly.

Rolling-contact bearings are designed for longevity (service life) according to dynamic and static load-carrying capacities. The design methods are standardized in the USSR and conform to the recommendations of the Council for Mutual Economic Assistance (COMECON) and the International Organization for Standardization (ISO). The longevity of the bearings is understood to mean the rated service period expressed in the number of revolutions or the number of hours of operation for which not less than 90 percent of a given group of bearings must operate under identical conditions without the appearance of any signs of metal fatigue (pitting). The relationship between the rated service life in millions of revolutions (L) or in hours (Lh) and the equivalent dynamic load (P) is given by the empirical functions L = (C/P)α millions of revolutions and Lh = 106L/60n hours. In these expressions, C is the dynamic load-carrying capacity of a bearing, the constant radial or axial load (for thrust and thrust-radial bearings) that one of a group of identical bearings with fixed outer rings can withstand during the rated service life of 1 million revolutions for the inner rotating ring; P is the equivalent dynamic load, a constant radial or axial load (for thrust and thrust-radial bearings) that, when applied to a bearing with a rotating inner and fixed outer ring, gives the same rated service life as under actual load and rotation conditions (the value of P is found from formulas in which a composite load is reduced to a radial or axial load having an equivalent destructive action); α is an exponent, which is equal to 3 for ball bearings and 3.33 for roller bearings; and n is the rate of rotation in rpm. Bearings taking an external load when immobile or rotating at not more than 1 rpm are selected or tested for static load.

The static load-carrying capacity (C0) is taken as the load acting on a bearing such that in the most heavily loaded area of contact the total residual strain developed in the rolling elements and races does not exceed 0.0001 of the diameter of the rolling element. The values of dynamic and static load-carrying capacities in kilograms-force, or newtons, are indicated in the catalogs for each standard bearing size. As the quality of rolling-contact bearings is improved, these values will increase. A substantially longer service life is possible, for example, through the advancement of technology and the use of electroslag, vacuum-arc, and double (electroslag and vacuum-arc) remelting of the steels.

REFERENCES

Podshipniki kacheniia: Spravochnoe posobie. Moscow, 1961.
Detail mashin: Atlas konstruktsii, 3rd ed. Edited by D. N. Reshetov. Moscow, 1968.
Sprishevskii, A. I. Podshipniki kacheniia. Moscow, 1969.
Detali mashin. Raschet i konstuirovanie: Spravochnik, 3rd ed., vol. 1. Edited by N. S. Acherkan. Moscow, 1968.
Podshipniki kacheniia: Katalog-spravochnik. Moscow, 1972.
GOST 18854-73.
GOST 18855-73.

V. N. IVANOV

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