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ABOUT BEARINGS 

GENERAL DATA

 

     A bearing is a mechanical unit that provides a mobile link between two parts which rotate in relation to each other in a mechanism.

      Its function is to permit relative rotation of these parts, under load, with accuracy and minimum friction. 

      Due to their universal use, bearings must be interchangeable with respect to their dimensions, accuracy, radial internal clearance and other characteristics. The main dimensions are: d-Bore, D-Diameter, B-Width, C-Outer ring width, T-Overall bearing width, r-Corner radius, α- Contact angle. 
 

The structures of rolling bearings 

      A rolling bearing normally consists of rings (inner ring and outer ring), rolling elements and cage. The outer ring and inner ring have the rolling elements between them, which are held by the cage to ensure their smooth rolling.

      Rings (inner ring and outer ring)

      The parts for the rolling of elements on the rings are called raceways, the surfaces or these parts are called raceway surfaces. The raceways of ball bearings are also called the groove.

      Generally speaking, the inner ring and outer ring work with shaft and housing respectively.

      The inner ring and outer ring of thrust bearings are also called the shaft washer and housing washer respectively.

      Rolling elements

      The type of rolling element used allows classification of bearings into two major groups:

      -the ball bearing (where theoretically there is a point contact between the balls and the raceway)

      -the roller bearing (where theoretically there is a line contact between the rollers and the raceway). The rollers can be cylindrical rollers, needle rollers, taper rollers and spherical rollers or in other shapes for the different structures.

      Under a given load, the contact pressure between the rolling elements and the raceway is distributed along a line in the case of rollers. In the case of balls, the contact pressure is limited to a single point. This is the reason, why with the same overall dimensions, roller bearing will support heavier loads and lower speeds than ball bearings.

      Cage

      The cage is applied to embrace the rolling elements partially to ensure a distance between the two neighbor rollers in the circumferential direction.

      The cages can be pressed cages, machined cages or plastic cages.

      Compared with the cageless full complement bearings, the caged bearings have less friction and can be applied to satisfy the high-speed rotation requirement. 
 

Types of rolling bearings 

       Based on the different contact angles, rolling elements can be divided into radial bearings and thrust bearings. Or according to the structures of the rolling elements and rings, they can be classified into deep groove ball bearings, self-aligning ball bearings, angular contact ball bearing, thrust ball bearings, cylindrical roller bearings, taper roller bearings, thrust spherical roller bearings and so on.

      According to the number of rows of rolling elements, they can also be divided into single row, double row and multi-row (eg. three row, four row) bearings.  

Classification of Rolling Bearings 

A.RADIAL BEARINGS

      1.RADIAL BALL BEARINGS

            1.1.DEEP GROOVE BALL BEARINGS

                        -single row

                        -double row

            1.2.ANGULAR CONTACT BALL BEARINGS

                        -single row

                        -duplex pair

                        -double row

            1.3.FOUR POINT CONTACT BALL BEARINGS

            1.4.SELF-ALIGNING BALL BEARING

      2.RADIAL ROLLER BEARINGS

            2.1.CYLINDRICAL ROLLER BEARINGS

                        -single row

                        -double row

                        -four row

            2.2.NEEDLE ROLLER BEARINGS

                        -single row

                        -double row

            2.3.TAPER ROLLER BEARINGS

                        -single row

                        -double row

                        -four row

            2.4.SPHERICAL ROLLER BEARINGS 

B.THRUST BEARINGS

      1.THRUST BALL BEARINGS

            1.1.THRUST BALL BEARINGS (with self-aligning seat washer)

                        -single direction

                        -double direction

            1.2.ANGULAR CONTACT THRUST BALL BEARINGS

                        -single direction

                        -double direction

      2.THRUST ROLLER BEARINGS

            2.1.CYLINDRICAL ROLLER THRUST BEARINGS

            2.2.NEELE ROLLER THRUST BEARINGS

            2.3.TAPER ROLLER THRUST BEARINGS

                        -single direction

                        -double direction

            2.4.SPHERICAL ROLLER THRUST BEARINGS 

 

The structural characteristics of rolling bearings 

1.DEEP GROOVE BALL BEARINGS

      They are the most typical rolling bearings with extensive applications. The raceways in both of the rings are in arc groove form, which can carry radial load and axial load in double directions. They can be applied in situations where high-rotating speed and low noise and low vibration are required.

      Adequate lubricating grease shall be filled before hand in the sealed bearings and shielded bearings

      For bearings with filling slot, the number of the steel balls can be increased, leading to higher basic load ratings.

      Applications of these bearings can be found in automobiles, machine tools, motors, instruments, construction machines, railway vehicles, agricultural machines and varios other special machines.

      Deep groove ball bearings have the following types:

            Basic type

            Application of this type can be found in situations where there are no special requirements for mounting and sealing.

            With snap ring groove on the outer ring (-N)

            The snap ring can be used to locate the bearing in the axial direction so that the mounting in the housing is simplified.

            With one shield (-Z) on one side or two shields (-2Z) on both sides

            The shields are made of steel sheet. Adequate lubricating grease has been put in the deep groove ball bearings with two shields during manufacturing process and there is no need to heat or clean the bearing or add lubricant again. It is non-contact seal, and compared with deep groove ball bearings with seals, they have less friction and suitable for applications with high rotational speed requirement.

            With one seal (-RS) on one side or two seals (-2RS) on both sides

            The seals are made of rubber with steel sheet reinforcement. It is contact sealing. Compared with the shielded deep groove ball bearings, they are better in terms of dust-resistance, water-resistance. However, the friction shall be larger and the rotational speed is to certain extent limited.

            Cages

            Deep groove ball bearings generally use pressed cages of steel sheet or brass cages. When the outer diameter is less than (inclusive) 400mm, they use pressed cage with steel sheet and there is no suffix in the bearing code number. When the outer diameter is greater than 400mm, they use brass cages and there is no suffix in the bearing code number. 

            Deep groove ball bearings permit angular misalignment between the inner ring and outer ring. 

2.ANGULAR CONTACT BALL BEARINGS

      When the single-row angular contact ball bearing is carrying the radial load, there will be a component of force in the axial direction. Therefore it is suggested that two of these bearings be applied in back-to-back or face-to-face arrangements.

      These bearings have contact angles between the rings and balls. The standart angles are 15°, 25° and 40°. Then greater the angel, then higher is the axial load carrying capacity of the bearing. However, then smaller is the contact angle, then higher is the rotating speed.

      Single-row bearings can carry radial load and axial load in one direction.

      Double-row angular contact ball bearings have two designs of one outer ring and two inner rings and one outer ring and one inner ring. For the design with one outer ring and two inner rings, the axial clearance is easy to control during machining.

      Angular contact ball bearings are applied for high speed and high precision rotating conditions.

      These bearings are mainly applied in the machine tool spindles, high-frequency motors, gas turbines, oil pumps, air compressors, printing machines and other installations. 

3.FOUR-POINT CONTACT BALL BEARINGS

      They can carry radial load and axial load in double directions.

      One four-point contact ball bearing can replace the two angular contact ball bearings combined with DB or DF arrangements. They are used to bear the pure axial load or the combined load with greater axial load.

      A contact angle (α) is formed whenever the bearing is carrying the axial load in any direction. Therefore the rings and balls always have two contact points on any contact line.

      These bearings are mainly applied in jet plane motors and gas turbines.

      Four-point contact ball bearings are mainly used to carry axial loads in double directions. While carrying axial load, they can also carry radial load. 

      Angular contact ball bearings for paired mounting

      Based on the side face configuration, the angular contact ball bearings for paired mounting can be divided into three groups:

      a/Back-to-back arrangement;

      b/Face-to-face arrangement;

      c/Tandem arrangement.

      These bearings can carry the combination of axial load and radial load. They can also carry the pure radial load only. Except the tandem arrangement, all the other arrangements can carry axial loads in double directions. Normally, the manufacturer shall choose the arrangements for the customer, and there shall be an interface fit and both the rings and the balls are in the conditions of carrying axial pre-load, therefore improving the capacity and rigidity of the whole arrangement as one bearing unit. 

4.SELF-ALIGNING BALL BEARINGS

      The spherical form of the raceway surface of the outer ring makes these bearings self-aligned. They can automatically adjust the misalignment caused by the bending of shaft or the housing or the eccentricity.

      These bearings are mainly used in the transmission axles of wood processing machines and textile machines.

      There are cylindrical bored and taper bored self-aligning ball bearings, The tapers is 1:12. The taper bored self-aligning ball bearings can be mounted directly on the tapered shaft, or mounted on the cylindrical shaft with adapters. 

5.CYLINDRICAL ROLLER BEARINGS

      Cylindrical rollers have linear contact with the raceway, and the bearings carry heavier radial load. Applications are found in situations characterized by heavy load and shock load and also high rotation speed.

      The N type and NU type bearings can be movable along the shaft. They can adapt to the position changes between the shaft and housing caused by the thermal expansion or the mounting error and are most suitable for free end shafts.

      NJ and NF types can accept certain axial load in one direction and NH and NUP types can accept certain axial load in two directions.

      Inner ring and outer ring are separable , and they are convenient to mount and dismount.

      Applications of these bearings are mostly found in middle-sized and large sized motors, electricity generators, internal combustion engines, gas turbines, machine tool spindles and other industrial machines. 

6.SPHERICAL ROLLER BEARINGS

      Spherical rollers are put between the spherical raceway on the outer ring and the two grooves on the inner ring for these bearings. Since the center of the arc raceway on the outer ring is the same as the center of the whole bearing arrangement, these bearings are self-aligned and automatically adjust the bending of the shaft and housing and the eccentricity.

      The bearings can carry radial load and axial load in double directions. The especial radial load carrying capability makes these bearings suitable for heavy load and shock load carrying

      The tapered bore with adapter sleeve or withdrawal sleeve makes the monting and dismounting on the shaft quite convenient. 

7.TAPER ROLLER BEARINGS

      These bearings have tapered rollers which are quided by the thicker side of the cup. The design of the sliding surfaces of the cup and inner ring and rolling elements when extended converge towards the same single point on the bearing axle.

      Single-row bearings can carry radial load and axial load in one direction.

      Double-row and four-row taper roller bearings can bear the radial load and axial load in double directions.

      The capacity of carrying axial load varies with the contact angle (α). Then greater is the angle then bigger is the capacity.

      The separable cup and the cone assembly makes both mounting and dismounting easy and convenient. 

8.THRUST BALL BEARINGS

      Bearings of this type consist of the washers with raceway and the assembly of balls and cage. The washer that contacts the shaft is called shaft washer, and the one attached to the housing is called the housing washer.

      Thrust bearings in double directions have a central washer connected to the shaft.

      Single-direction bearings can carry axial load  in one direction, and double-direction bearings can carry axial load in two directions. (Neither bearing can carry radial load.)

      The bearing with housing washer which has spherical mounting surface is self-alignable, which can reduce the effects of mounting error.

      Applications oh these bearings can be found in steering mechanism of automobiles and machine tool spindles.