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 |
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| 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). |
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| 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. |
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| 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. |
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 |
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| 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: |
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Application environment:
including load (heavy or light, radial or axial), speed, temperature (and
its change), and surroundings (corrosion, cleanness, lubrication). |
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Bearing functions:
including installation space, rotation precision, rigidity, speed, noise,
vibration, friction torque, and life. |
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Alignment and installation
requirement: including load, installation space and structure. |
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| Main
Size |
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| A series
of international bearing standards has been stipulated in order to ensure
exchanges of bearing and economic production. |
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| Code |
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| A series
of international bearing standards has been stipulated in order to ensure
exchanges of bearing and economic production. |
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| Precision
Grades |
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| 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. |
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 |
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| 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. |
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|
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 |
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 |
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| Basic
Load Rating |
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| 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, |
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| Bearing
Life |
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| 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. |
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| 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. |
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| 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. |
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| 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.
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 |
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| 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. |
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| 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. |
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| 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. |
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| 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. |
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| 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. |
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| 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.
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|
| 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. |
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| 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 |
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| 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. |
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| 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 |
|
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| 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 |
- |
- |
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| 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 |
|
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| 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: |
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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. |
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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. |
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|
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. |
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|
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. |
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|
|
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. |
|
|