Selection of Bearing Types

The number of applications for rolling bearings are almost countless and the operating conditions and environments also vary greatly. In addition, the diversity of operating conditions and bearing requirements continue to grow with the rapid advancement of technology. Therefore, it’s extremely important to select the correct bearing for the duty that is expected of it. When selecting bearing types or structures, the customer should make comprehensive analysis according to the following aspects, weigh up the pros and cons, choose the bearing type suited to your application.

  • Allowable bearing space
  • Bearing load
  • Aligning capacity
  • Speed
  • Turning precision
  • Stiffness
  • Noise and vibration
  • Axial displacement
  • Frictional moment
  • Mounting and dismounting
Besides, additional factors like bearing load capacity, internal clearance, life rating, reliability, preload, lubrication, seal as well as source cost and delivery on-time should be considered.

1. Allowable bearing space

Allowable space is one of the main principles for bearing selection. During the machinery design, journal dimension is one of the first confirmed parameters through the methods of theoretical calculation or analogy. Selection of the bearing is always according to the diameter of shaft. Ball bearing or thrust ball bearing can be selected for small diameter shaft, which, usually apply for low load status. Cylindrical bearing, tapered roller bearing or spherical roller bearing can be selected for a heavy load or safety consideration. Normally, the bearing’s available space in machinery can be limited. If radial space is limited, the bearing with small radial cross section should be selected such as the kinds of bearings with a diameter series 7, 8, 9. If axial space is limited, the bearing with small axial cross section should be selected such as all kinds of bearings with width series 8, 0, 7, 9.

2. Bearing Load

2.1 Load strength

Load strength on the bearing is very important for bearing selection. Roller bearings can carry heavier loads than ball bearings when with the same boundary dimension, similarly rolling bearings without cage carry heavier loads than bearings with cages. Therefore, ball bearings are fit for low and medium loads, and roller bearings are fit for heavy loads

2.2 Load Direction.

When the bearings carry pure radial load, radial bearings should be selected, such as deep groove ball bearing, cylindrical roller bearing and needle roller bearing etc. When the bearing carries pure axial load, thrust bearings should be selected, such as thrust ball bearing, cylindrical roller thrust bearing or needle roller thrust bearing etc. When bearings carry radial and axial (combined loads), angular contact bearings are generally selected. If the radial load is larger and axial load is smaller, then the angular contact radial bearing should be selected, such as angular contact ball bearing. If the radial load is smaller than axial load in the combined load, then an angular contact thrust bearing should be selected, such as the angular contact thrust ball bearing. When the load acts eccentrically on the bearing, tilting moments will arise. If moment load is not large, the single row angular contact ball bearings in pair arranged face-to-face or back-to-back as well as single-row taper roller bearings would be more suitable. But, for some larger machinery or an application carrying a large tilting moment, such as crane, mineral machinery, antenna base for radar, rocket launcher etc., then slewing bearings would be more suitable.

3. Aligning Capacity

In normal applications, the shaft deflection would occur due to load, or mounting errors, or components deviation in production, that will make the inner ring and outer ring slant and cause uneven loads on the bearing raceway and generate excessive loads. Severe cases will cause stress concentration, and result in early failure. Therefore, aligning capacity should be considered when selecting bearings. The bearing deflection within the allowed angle error will not influence the normal working of the bearing. When selecting bearings, self-aligning ball bearing, spherical roller bearing or thrust spherical roller bearing, pillow block bearing could be selected for the larger deflection situation.

4. Speed

The speed will have little influence on the bearing selection when working under the medium and low speed (less than 0.5 times of limiting speed). However, speed must be considered in the case of the higher speed, because the inertia force from rolling elements and cages will have large effect on the load distribution, temperature, and vibration etc. under high speed, the highest speed of bearing is limited, which is the so-called “limiting speed”. “Limiting speed” depends on the working temperature of the bearing, which depends on the various factors such as structure, material, dimension, precision, clearance, load, lubrication, heat dispersion of the bearing, and cage structure, material, and guiding method etc. Friction coefficient of ball bearing is lower than that of roller bearing, so the limiting speed of ball bearings are higher than roller bearings. The speed of thrust bearing is lower than radial bearing due to the restriction of the structure. When deep groove ball bearings and cylindrical roller bearings carry pure radial load, they are suitable for high speed applications. While the single-row angular contact ball bearing carries combined load, it can operate well at high speed.

5. Running Accuracy

Running accuracy of the bearing is not only related to the parameters of structure, producing precision, clearance and stiffness of bearings, but also related to the supporting structure, material, stiffness, manufacture precision and mounting precision. Therefore, the selection of bearing precision should be considered from both aspects of bearing manufacture and application. In view of bearing manufacture, all kinds of bearings with normal precision could meet the requirements of most bearing applications. When part of the machinery requires strict running accuracy, such as machine tool spindles or high-speed shaft applications and the condition of high maintenance and with high reliability, the precision bearings must be selected to satisfy this requirement. When select precision bearings, the manufacturing and mounting precision and rigidity of matched shaft and bearing housing bore should be improved, otherwise, precision bearing would not play its function.

6. Stiffness

Bearing stiffness refers to the elastic deformation generated from the contacting position of the bearing rings and rolling elements, its deformation degree is also called stiffness. Normally, the elastic deformation of bearing is very small, therefore, it could be ignored in most situations. However, the elastic deformation is of significance in some applications, e.g. machine tool spindle bearing, the requirement of bearing stiffness should have detailed investigation. Generally, because of the contact conditions between rolling elements and raceways are linear in theory, the roller bearing such as cylindrical roller bearings and tapered roller bearings have greater stiffness than ball bearings. The stiffness can be enhanced by preloading, but, the preloading amount should be controlled. Suitable preload can improve bearing precision and fatigue life. However, if preloading amount exceeds an optimum value, then the bearing stiffness has increased little; conversely, it will have destroyed influence on the bearing operation, such as bearing friction and abrasion increasing, the temperature. The stiffness of the angular contact bearing can be realized by different bearing configuration.

7. Noise and Vibration

Bearings can make sounds during their operation. This is the result of vibration, sounds can be classified into acceptable sounds and noise. Acceptable sounds are generated by the rolling of rolling elements on the raceway and are smooth and continuous. However, noise is resulted from many abnormal conditions, such as bad component contacts or pore lubrication, foreign matter invasion, damage of bearing working surfaces, dimension error etc., these sounds have characteristics of discontinuity, or regularity, or vibration. In some applications, such as household appliances, medical appliance, office equipment etc., noise and vibration must be strictly controlled. However, the sounds and vibration of the bearings used for general application are much lower than that of the matched parts, so it is not necessary to control. Then testing for bearings noise needs to be evaluated. Noise control is usually ensured by testing the vibration value of bearings.

8. Axial Displacement

The turning shaft adopts double support in machinery. Two supports are used for confining radial displacement, while there are three methods to restrict the axial displacement, i.e. support with both ends fixed, one end fixed but the other free and both ends freely. Bearings used in the fixed end applications are called fixed end bearing. These require to be in an axial location (except where the axial displacement results from the clearance). Bearings which carry combined load or supply axial orientation together with another bearing such as deep groove ball bearing, angular contact ball bearing, self-aligning (spherical roller) bearing etc. is best suited to be a fixed end bearing. Bearings used in free end are called wandering end bearing, which, can allow the shaft to displace axially. For example, as the thermal expansion of shaft, the length of shaft will become longer, but, the bearing will not carry exceeding load or jam for this reason through a certain axial displacement. Needle roller bearing or N, NU series of cylindrical roller bearings are the best choice. The NJ series of cylindrical roller bearing can also be used as wandering end bearing. When a shaft needs a larger axial displacement, the clearance fit could be taken into consideration, but, the fit surface should be appropriately lubricated. If a deep groove ball bearing, self-aligning (spherical roller) bearing must be adopted as wandering end bearing, clearance fit could be considered, but, the fit surface should be appropriately lubricated.

9. Frictional Moment

Friction happens when the bearing running. The frictional force depends on several factors, such as the bearing type, dimension, load, running speed, characteristics of lubricants and lubrication amount. The summation of all kinds of frictions within bearing could be measured by friction moment. Generally frictional resistance of ball bearings is smaller than that of roller bearings. When bearings carry pure radial load, the friction moment of thrust ball bearings is smaller. When bearings carry pure axial load, the friction moment of thrust ball hearings is smaller also. Angular contact bearings, whose load angle is almost the same with contact angle, have the smallest friction moment under the combined load. Deep groove ball bearings and cylindrical roller bearings should be a priority selection, when customers need low friction moment bearings. If sealed bearings need to be used in low friction moment, contact sealed bearings should not be selected. All kinds of sealed bearings can be selected in the case of low frictional moment, but It is necessary to notice that bearings with rubbing seal should be avoid. The feature and volume of lubricant has a certain influence on friction moment, it is suggested to adopt low viscosity (or low consistence) lubricant hi the low friction moment application. Drop lubrication, oil jet lubrication and oil mist lubrication are recommended when using oil lubrication.

10. Mounting and Dismounting

10.1 Cylindrical Bore

Separable bearings with cylindrical bore, such as angular contact ball bearings, cylindrical roller bearings, needle roller hearings, tapered roller bearing and thrust bearings etc., can be mounted on the shaft or into the housing bore independently. If mounting and dismounting are required often hi bearing applications, the bearings mentioned above should be priority selection.

10.2 Tapered Bore

Bearings with tapered bore can be mounted directly on the journal and can be easily mounted on a cylindrical shaft with appropriate adapter or withdrawal sleeve. Self-aligning ball bearings, spherical roller bearings or double row cylindrical roller bearings with tapered bore should be selected. Bearings with tapered bore also have an effect of adjusting the radial clearance.
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