Structure of a Bearing
A common bearing is composed of a ball or a roller, a retainer, rings (inner ring and outer ring), lubricants and seals (selectable).
Classification of Bearings
There are several types of bearing which are classified according to the shapes of their rollers. Generally, there are ball bearing, roller bearing and a combination of the two.
Features of Bearings
A ball bearing has the characteristic of low friction. It is suitable for applications which demand high speed, high precision, low torque and low vibration. Whereas a roller bearing has a large capacity for load so it is suitable for applications which demand high load, shock load and longer bearing life.
There are various series, types and sizes of rolling bearings. In order to optimise the performance of a rolling bearing, knowing how to select the most suitable one is crucial. Generally, you can consider the following when selecting a bearing:
Application environment: including load (heavy or light, radial or axial), speed, temperature (and its change), and surroundings (corrosion, cleanness, lubrication).
Bearing functions: including installation space, rotation precision, rigidity, speed, noise, vibration, friction torque, and life.
Alignment and installation requirement: including load, installation space and structure.
Main Size
A series of international bearing standards has been stipulated in order to ensure exchanges of bearing and economic production.
Code
A series of international bearing standards has been stipulated in order to ensure exchanges of bearing and economic production.
Precision Grades
The type, size, precision and internal structure of a bearing can be symbolized by a defined code which is constituted by prepositional code, basic code and after code.
Ball and roller bearings play a very important role as mechanical elements and are used in various types of machinery. They are internationally standardized by ISO (International Standardization Organization). Besides, there are other overseas standards for these rolling bearings, such as DIN, AFBMA, JIS, etc.
Deep Groove
Ball Bearing
D>9mm ISO Normal Class ISO Class5 ISO Class5 ISO Class4
D<9mm ISO Normal Class ISO Class6 AFBMA ABEC5P AFBMA ABEC7P
Tapered
Roller Bearing
Metric Series ISO Normal Class ISO Class6 ISO Class5 ISO Class4
Thrust
Ball Bearing
Flat Seat ISO Normal Class ISO Class6 ISO Class5 ISO Class4
Equivalent Standards JIS Class0 Class6 Class5 Class4
DIN Class0 Class6 Class5 Class4
AFBMA Ball Bearing ABEC1   ABEC5 ABEC7
Roller Bearing ABEC1   ABEC5  
Basic Load Rating
The basic load rating is a constant stationary load which a group of apparently identical bearings with stationary outer rings can endure for a rating life of one million revolutions of the inner ring. The basic load rating of radial bearings is defined as a central radial load of constant direction and magnitude, while the basic load rating of thrust bearings is defined as a thrust load of constant magnitude in the same direction as the central axis. The load rating C of the SNH bearings listed in the dimension tables was calculated based on the method of ISO 281. The listed data will be adjusted subject to the development of our engineering and technology,
Bearing Life
Bearing life, in a broad sense, is the period during which bearings continue to operate and fulfil their functions. This bearing life may be defined as noise life, abrasion life, grease life, or rolling fatigue life, depending on what causes loss of bearing service.
Life of a bearing depends on its functions and applications. These functions must be performed for a prolonged period. Even if a bearing is properly mounted and correctly operated, it may eventually fail to perform satisfactorily due to increase in noise and vibration, loss of running accuracy, deterioration of grease, or fatigue flaking of the rolling surfaces.
Fatigue life is calculated by the total number of revolutions during which the bearing surface will start to flake due to stress. For seemingly identical bearings which are of the same type, size, and material and receiving the same heat treatment and processing, the rolling fatigue life of them may vary greatly even they are under identical operating conditions. This is because the flaking of materials due to fatigue is subject to many other variables. Consequently, "rating fatigue life", in which rolling fatigue life is treated as a statistical phenomenon, is used in preference to actual rolling fatigue life.
Suppose a number of bearings of the same type are operated individually under the same conditions. After a certain period of time, 10% of them fail as a result of flaking caused by rolling fatigue. In this case, the total number of revolutions is defined as the rating fatigue life or, if the speed is constant, the rating fatigue life is often expressed by the total number of operating hours completed when 10% of the bearings become inoperable due to flaking.
The speed of rolling bearings is subject to certain limits. When bearings are operating, the higher the speed, the greater the bearing temperature will be due to frictional heat.
The limiting speed is an empirically obtained value for the maximum speed at which bearings can continuously operate without failing from heat seizure or generating an abnormal amount of heat.
Consequently, the limiting speed (rpm) of a bearing varies with such factors as bearing type, size, cage form, material, load, lubricating method and heat dissipating method which involves the design of the bearing's surroundings.
The limiting speeds for bearings lubricated by grease and oil are listed in the bearing tables. The values of the limiting speeds in the tables are found according to the conditions under which bearings of standard design are operating when subjected to normal loads, i.e. C/P?12, Fa/Fr?0.20 approximately. The limiting speed figures for oil lubrication listed in the bearing tables are for conventional oil bath lubrication.
Some types of lubricants are not suitable for high speeds even though they may be markedly superior in other respects. When speeds are more than 70 percent of those listed in the tables, it is necessary to select an oil or grease lubrication that has good high-speed characteristics.
The internal clearance of rolling bearings in operation influences fatigue life, vibration, noise, heat-buildup, etc. Consequently, the selection of a proper internal clearance is very important. The internal bearing clearance is the total clearance between the rings and the rolling elements. The redial and axial clearances are defined as the total amount that one ring can be displaced relative to the other in the radial and axial directions respectively.
To obtain accurate measurements, the clearance is generally measured by applying a specified measuring load on the bearings; therefore, the measured clearance is always slightly larger than the actual internal clearance due to the elastic deformation caused by the measuring loads.
The actual internal clearance may be obtained by correcting the measured clearance by the amount of elastic deformation. In the case of roller bearings, the elastic deformation is negligible. The radial clearances in Tables7.1~7.2 are the actual clearances. The increase in internal clearance caused by the measuring load for single row deep-groove ball bearings is shown in the note in Table7.2
There are six clearance groups, namely C1,C2, Normal, C3, C4, and C5. The normal group of internal clearance of rolling bearings is suitable for most operating conditions. The clearance order is as follows: C1 represents the minimum clearance, followed by C2, Normal, C3, C4 and C5 in increasing order. Therefore, C5 has the maximum clearance.
In addition to these six basic clearance groups, there is the CM group specified for the deep groove ball bearings used in electric motors where noise must be minimized. This group, which is shown in Table 7.1, is in a narrow range just above the min. end of the normal clearance group range.
Table 7.1 Radial Internal Clearances of Bearings for Electric Motors
s (Values in µm)
Nominal Bore Diameterd(mm) Radial Internal Clearances CM Fits
Over incl. Min. Max. Shaft Housing Bore
10(incl.) 18 4 11 i 5 H 6 ~ 7
or
J 6 ~ 7
18
30
30
50
5 12 k 5
9 17
50
80
80
100
12 22
18 30
100
120
120
160
18 30 m 5
24 38
Table 7.2 Radial Internal Clearances in Single Row Deep Groove Ball Bearings under No-Load[ ISO 5753] (Values in µm)
Nominal Bore Diameter d(mm) Radial Internal Clearances
C2 Nominal C3 C4 C5
Over Incl. Min. Max. Min. Max. Min. Max. Min. Max. Min. Max.
(10mm only*) 0 7 2 13 8 23 14 29 20 37
10 18 0 9 3 18 11 25 18 33 25 45
18 24 0 10 5 20 13 28 20 36 28 48
24 30 1 11 5 20 13 28 23 41 30 53
30 40 1 11 6 20 15 33 28 46 40 64
40 50 1 11 6 23 18 36 30 51 45 73
50 65 1 15 8 28 23 43 38 61 55 90
65 80 1 15 10 30 25 51 46 71 65 105
80 100 1 18 12 36 30 58 53 84 75 120
100 120 2 20 15 41 36 66 61 97 90 140
120 140 2 23 18 48 41 81 71 114 105 160
140 160 2 23 18 53 46 91 81 130 120 180
160 180 2 25 20 61 53 102 91 147 135 200
180 200 2 30 25 71 63 117 107 163 150 230
200 225 - 32 25 80 74 134 124 189 - -
225 250 - 35 30 90 84 149 144 214 - -
250 280 - 40 35 95 89 159 154 234 - -
280 315 - 50 50 110 110 180 190 265 - -
315 355 - 55 55 125 125 200 215 295 - -
355 400 - 65 65 140 140 225 245 330 - -
Note: To convert the actual clearance into measured clearance under load, the radial clearance increase due to the measuring load, which is listed below, should be added to the actual clearance. For the C2 clearance class, the smaller value should be used for bearings with minimum clearance and the larger value for bearings near the maximum clearance range.
Values in µm
Nominal Bore Diameter d(mm) Measuring Load (Kg. f) Radial Clearance Increase
Over Incl. C2 Nominal C3 C4 C5
10(incl.) 18 2.5 3~4 4 4 4 4
18 50 5 4~5 5 6 6 6
50 280 15 6~8 8 9 9 9
Purposes of Lubrication
The main purposes of lubrication are to reduce friction and wear inside the bearings that may cause premature bearing failure. The effects of lubrication may be explained briefly as follows:
Reduction of Friction and Wear
  Direct metallic contact between the bearing rings, rolling elements and cage, which are the basic components of a bearing, is prevented by an oil film which reduces the friction and wear at the contact areas.
Extension of Fatigue Life
  The rolling fatigue life of bearings depends greatly upon the viscosity and film thickness as the rolling contact surfaces. Generally, a heavier film thickness prolongs the fatigue life.
Cooling
  Circulating lubrication may be used to carry away frictional heat or heat transferred from the outside to prevent the bearing from overheating and the oil from deteriorating.
Others
  Adequate lubrication also helps to prevent foreign material from entering the bearings and guards against corrosion or rusting.
Lubricating Methods
The various lubricating methods are divided into grease and oil lubrications. Satisfactory bearing performance can best be achieved by adopting a lubricating method that is most suitable for a particular application and operating condition. Oil lubrication is superior in lubricating efficiency, however, grease lubrication allows a simpler structure around the bearings.
Grease lubrication
  Sealed (RS, 2RS) or shield (ZZ, Z) bearings are generally factory-packed with the proper amount of good quality grease and can be used as delivered.
More than the normal amount of grease can cause heat generation or grease leakage.
Generally, SNH fills less than half of the free internal space inside bearings.
Since the brand of grease affects bearing performance, SNH usually recommends a suitable grease according to applications. For assistance when selecting grease, you may consult SNH.
Oil lubrication
  Oil lubrication is used when it is difficult to achieve satisfactory performance by using grease lubrication, for example, when an extremely low torque or a high-speed operation is required.
Particularly in the case of gyro-gimbals and synchros that are largely affected by frictional torque, low viscosity oil is used. Oil mist or oil lubrication provides low heating due to agitation and also superior cooling of the bearing.
Table 8.1 Characteristics of grease lubricants
Manufacturer Brand Thickening Type Lubricant base Operating temperature range °C Remarks
Shell Alvania R3 Lithium Mineral -20~135 General purpose
Alvania RA Lithium Mineral -40~130 General purpose
Alvania EP1 Lithium Mineral -25~110 General purpose
Alvania EP2 Lithium Mineral -25~110 General purpose
Darina 2 Poliurea Mineral -25~150 High tem. Serice
Darina EP2 Poliurea Mineral -25~150 High tem. Serice
Darina R2 Poliurea Mineral -35~150 Corrosion resistant
Aero shell grease 7 Microgel Diester -73~149 Wide range tem. Serice
Aero shell grease 15A Fluorotelomer Silicone -73~260 Wide range tem. Serice
Esso Andok B Sodium Mineral -40~120 General purpose
Andok C Sodium Mineral -30~120 General purpose
Andok 260 Sodium Mineral -40~120 General purpose
Beacon 325 Lithium Diester -54~120 Low tem.Serice
Chevron SRI 2 Urea Mineral -30~120 Water resistant
Mobil Mobilux 2 Litio Mineral -10~110 General purpose
Mobil 22 Litio Mineral -40~120 General purpose
Mobil 28 Bentonite Synthetic hydrocarbon -55~175 Wide range tem. Serice
Mobil 48     -60~170 Wide range tem. Serice
Kluber Isofelex LDS18 Lithium Diester -50~110 Low tem. Serice
Isofelex LDS18 Special A Lithium Diester -50~110 Low tem. Serice
Isofelex NBU15 Barium Diester, Mineral -30~120 General purpose
Kyodo Yushi Multemp SRL Lithium Ester -40~145 Low torque Serice
Multemp PS2 Lithium Diester, Mineral -50~110 Low tem. Serice
Multemp SCA Urea Idrocarburo
sintetico
0~160 High tem. Serice
Multemp ET150 Urea Diester -10~160  
Dupont, E.I. Krytox 240 Fluorotelomer Fluorinated -35~288 High tem. Serice
Chine Hangu Hangu 2 Lithium Mineral -10~130 Low noise Serice
Hangu HAS Lithium Mineral -40~135 Low noise serice
Hangu HTHS Lithium Mineral -40~200 High tem., High speed Serice
ZZ:
  This is non-contact shield pressed into outer ring. There is very little grease leakage and low ingress of contaminants.
2RS(contact rubber seal):
  This is rubber seal fitted into outer ring. There is light contact with inner ring. Grease is retained and ingress of external contaminants is prevented.
2RS(non-contact rubber seal):
  This is non-contact rubber seal fitted into outer ring, and it still provides effective sealing.
ZZ: pressed steel shields ZZ: removable steel shields 2RS: contact rubber seals 2RS: non-contact rubber seals
Notes: bearing with one shield(Z) or one seal (RS) is available.
The standard material for rings and balls is a vacuum degassed high carbon chromium steel allowing high efficiency, low torque, low noise level and long bearing life. For bearings requiring anti-corrosion or heat-resistance properties, martensitic stainless steel is used.
Material Symbol Chemical Composition % Equivalent
C Si Mn P S Cr Mo
High carbon chromium steel GCr15 0.95
~1.10
0.15
~0.35
0.50 0.025 0.025 1.30
~1.60
0.08 SAE 52100
Stainless steel 9Cr18 0.95
~1.20
1.00 1.00 0.04 0.03 16.00
~18.00
0.75 AISI440C
The fitting practice used for bearings is extremely important in achieving bearings' expected performance. Since ball bearings are usually used under light loads, the range between a push fit (light interference) and a slip fit (slightly loose) is generally used.
In the case of a rotating inner ring, ordinarily ball bearings are fitted to the shaft with interference, however, a slip fit is generally used for miniature bearings and instrument bearings in order to simplify their mounting, to prevent them from damaging while mounting, and to avoid changing the contact angle or preload. This is because the occurrence of creep in miniature bearings is easily prevented by tightening the side face of the inner ring against a shoulder on the shaft with a nut.
Bearing manufacturers must constantly improve the quality of their products especially the vibration and noise levels because the standards required for the performance of machines are getting higher and higher. Vibration and noise can reflect the overall quality and performance of bearings, while dimension and rotation precision can only reflect the respective characteristics.
In order to fulfill different needs of various machines, the following are the vibration levels for customers' reference.

Table 12.1 vibration of acceleration type
  unit:dB
Bore Diameter r Diameter series 0 Diameter series 2 Diameter series 3
mm Z Z1 Z2 Z3 Z Z1 Z2 Z3 Z4 Z Z1 Z2 Z3 Z4
3 35 34 32 28 36 35 32 30   37 36 33 31  
4 35 34 32 28 36 35 32 30   37 36 33 31  
5 37 36 34 30 38 37 34 32   39 37 36 33  
6 37 36 34 30 38 37 34 32   39 37 35 33  
7 39 35 35 31 40 38 36 34            
8 39 38 35 31 40 38 38 34            
9 41 40 38 32 42 40 37 45            
10 43 42 38 33 44 42 39 35 30 46 44 40 37 32
12 44 43 39 34 45 43 39 35 30 47 45 40 37 32
15 45 44 40 35 46 44 41 36 31 48 46 42 38 33
17 46 44 40 35 47 45 41 36 31 49 47 42 38 33
20 47 45 41 36 48 46 42 38 33 50 48 43 39 34
22 47 45 41 36 48 46 42 38 33 50 48 43 39 34
25 48 48 42 38 49 47 43 40 36 51 49 44 41 37
28 49 47 43 39 50 48 44 41 37 52 50 45 42 38
30 49 47 43 39 50 48 44 41 37 52 50 45 42 38
32 50 48 44 40 51 49 45 42 38 53 51 48 43 39
35 51 49 45 41 52 50 46 43 39 54 52 47 44 40
40 53 51 46 42 54 52 47 44 40 56 54 49 45 41
45 55 53 48 45 56 54 49 46 43 58 56 51 47 44
50 57 54 50 47 58 55 51 48 45 60 57 53 49 48
55 59 56 52 49 60 57 53 50 47 62 59 54 51 48
60 61 58 54 51 62 59 54 51 48 64 61 56 53 50
65 49 48 46   50 49 47 42   51 50 48 43  
70 50 49 47   51 50 48 43   52 51 49 44  
75 51 50 48   52 51 49 44   53 52 50 45  
80 52 51 49   53 52 50 45   54 53 51 46  
85 53 52 50   54 53 51 46   56 55 52 47  
90 54 53 52   56 55 53 48   58 57 54 49  
95 56 55 54   58 57 55 50   60 59 56 51  
100 58 57 56   60 59 57 52   82 81 58 53  
105 60 59 58   62 61 59 54   64 63 60 55  
110 62 61 60   64 63 61 56   66 65 62 57  
120 64 63 62   66 65 63 58   68 67 64 59  

Table 12.2 vibration of velocity type
  unit:um/s
Bore Diameter (mm) V V1 V2 V3
L M H L M H L M H L M H
5 110 72 60 90 60 50 58 36 30 35 21 18
6 110 72 60 90 60 50 58 36 30 35 21 18
7 130 96 80 110 80 65 72 48 40 44 28 24
8 130 96 80 110 80 65 72 48 40 44 28 24
9 130 96 80 110 80 85 72 48 40 44 28 24
10 160 120 100 140 100 85 90 60 50 55 35 30
12 160 120 100 140 100 85 90 60 50 55 35 30
15 210 150 120 180 130 100 110 78 60 65 46 35
17 210 150 120 180 130 100 110 78 80 85 45 35
20 260 190 150 220 160 125 130 100 75 80 60 45
22 260 190 150 220 160 125 130 100 75 80 60 45
25 260 190 150 220 160 125 130 100 75 80 60 45
28 260 190 150 220 160 125 130 100 75 80 60 45
30 300 240 190 250 200 160 150 120 100 90 75 60
32 300 240 190 250 200 160 150 120 100 90 75 80
35 300 240 190 250 200 160 150 120 100 90 75 60
40 360 300 280 300 250 220 180 150 130 110 90 80
45 360 300 260 300 250 220 180 150 130 110 90 80
50 420 320 320 350 270 270 210 160 160 125 110 100
55 420 360 360 350 300 300 210 180 180 125 110 110
60 480 360 440 400 300 370 240 180 220 145 110 130
65 300 260 420 250 220 350 160 130 180 105 80 105
70 360 310 460 300 260 390 180 150 230 110 90 135
75 360 310 480 300 260 390 180 150 230 110 90 135
80 420 360 540 350 300 450 210 180 270 130 110 160
85 420 360 540 350 300 450 210 180 270 130 110 160
90 480 420 600 400 350 500 240 210 300 145 125 180
95 480 420 600 400 350 500 240 210 300 145 125 180
100 560 490 570 470 410 565 280 245 340 170 145 204
105 560 490 670 470 410 565 280 245 340 170 145 204
110 640 570 750 540 480 630 320 290 380 190 175 225
120 640 570 750 540 480 630 320 290 380 190 175 22