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5. SELECTION OF BEARING SIZE

5.1 Bearing Life of grease-prelubricated bearings (refer to Section 12, Table 5. 1 Fatigue Life Factor fh for Various Bearing Applications
Lubrication, Page A107) can be estimated. Since noise
The various functions required of rolling bearings vary life and abrasion life are judged according to individual Operating Periods ~3 2~4 Fatigue Life Factor fh 4~7 6~
according to the bearing application. These functions standards for different applications, specific values for • Agricultural
must be performed for a prolonged period. Even if noise or abrasion life must be determined empirically. Infrequently used or only • Small motors for equipment 3~5 • Paper making
bearings are properly mounted and correctly operated, for short periods home appliances machines
they will eventually fail to perform satisfactorily due 5.2 Basic Load Rating and Fatigue Life like vacuum • Motors for home
to an increase in noise and vibration, loss of running cleaners and heaters and air • Waterworks pumps
accuracy, deterioration of grease, or fatigue flaking of 5.2.1 Basic Load Rating washing machines conditioners • Electric power
the rolling surfaces. The basic load rating is defined as the constant load • Hand power tools • Construction stations
Bearing life, in the broad sense of the term, is the applied on bearings with stationary outer rings that the equipment • Mine draining
period during which bearings continue to operate and inner rings can endure for a rating life of one million Used only occasionally • Small motors • Conveyors pumps
to satisfy their required functions. This bearing life revolutions (106 rev). The basic load rating of radial but reliability is impor- • Deck cranes • Elevator cable
may be defined as noise life, abrasion life, grease life, bearings is defined as a central radial load of constant tant • General cargo sheaves
or rolling fatigue life, depending on which one causes direction and magnitude, while the basic load rating of cranes
loss of bearing service. thrust bearings is defined as an axial load of constant • Rolling mill roll • Pinion stands • Factory motors • Crane sheaves
magnitude in the same direction as the central axis. necks • Passenger cars • Machine tools • Compressors
Aside from the failure of bearings to function due The load ratings are listed under C r for radial bearings Used intermittently for • Escalators • Transmissions • Specialized
to natural deterioration, bearings may fail when and C a for thrust bearings in the dimension tables. relatively long periods • Vibrating screens transmissions
conditions such as heat-seizure, fracture, scoring of 5.2.2 Machinery in which Bearings are Used and • Crushers
the rings, damage of the seals or the cage, or other
damage occurs. Projected Life Used intermittently for • Centrifugal • Mine hoists
Conditions such as these should not be interpreted It is not advisable to select bearings with unnecessarily more than eight hours separators • Press flywheels
as normal bearing failure since they often occur as a high load ratings, for such bearings may be too large daily • Air conditioning • Railway traction
result of errors in bearing selection, improper design and uneconomical. In addition, the bearing life alone equipment motors
or manufacture of the bearing surroundings, incorrect should not be the deciding factor in the selection of • Blowers • Locomotive axle
mounting, or insufficient maintenance. bearings. The strength, rigidity, and design of the shaft • Woodworking boxes
machines
5.1.1 Rolling Fatigue Life and Basic Rating Life Fig. 5.1 Example of Flaking • Large motors
• Axle boxes on
When rolling bearings are operated under load, the Life railway rolling stock
raceways of their inner and outer rings and rolling Fig. 5.2 Failure Probability and Bearing Life
elements are subjected to repeated cyclic stress. Used continuously and
Because of metal fatigue of the rolling contact surfaces high reliability is impor-
of the raceways and rolling elements, scaly particles tant
may separate from the bearing material (Fig. 5.1).
This phenomenon is called "flaking". Rolling fatigue on which the bearings are to be mounted should also By designating the basic rating life as Lh (h), bearing
life is represented by the total number of revolutions be considered. Bearings are used in a wide range of
at which time the bearing surface will start flaking due applications and the design life varies with specific speed as n (min–1), fatigue life factor as fh, and speed
to stress. This is called fatigue life. As shown in Fig. applications and operating conditions. Table 5.1 gives factor as f n, the relations shown in Table 5.2 are
5.2, even for seemingly identical bearings, which are an empirical fatigue life factor derived from customary
of the same type, size, and material and receive the operating experience for various machines. Also refer obtained:
same heat treatment and other processing, the rolling to Table 5.2.
fatigue life varies greatly even under identical operating Table 5. 2 Basic Rating Life, Fatigue Life
conditions. This is because the flaking of materials 5.2.3 Selection of Bearing Size Based on Basic Load Factor and Speed Factor
due to fatigue is subject to many other variables. Rating
Consequently, "basic rating life", in which rolling fatigue Life Ball Bearings Roller Bearings
life is treated as a statistical phenomenon, is used in The following relation exists between bearing load and Parameters
preference to actual rolling fatigue life.
basic rating life: 3 Basic
Suppose a number of bearings of the same type are ( )For ball bearings Rating 3 10 10
operated individually under the same conditions. After C . . . . . . . . . . . . . . . . (5.1) Life
a certain period of time, 10 % of them fail as a result of P =500 fh3 3 =500 fh 3
flaking caused by rolling fatigue. The total number of ( ) ( )Lh=6100n6C
revolutions at this point is defined as the basic rating L= P L h=6100n6 C
life or, if the speed is constant, the basic rating life P
is often expressed by the total number of operating 10
hours completed when 10 % of the bearings become ( )For roller bearings C
inoperable due to flaking. L= P 3 . . . . . . . . . . . . . . (5.2) Fatigue C
Life P
In determining bearing life, basic rating life is often the Failure Probability where L : Basic rating life (106 rev) Factor fh = fn C fh = fn
only factor considered. However, other factors must Rating Life P
also be taken into account. For example, the grease life P : Bearing load (equivalent load) (N), {kgf}
Average Life ..........(Refer to Page A30) ( )fn= 1 ( )fn= 3
A 24 Speed 106 3 106 10
C : Basic load rating (N), {kgf} Factor
For radial bearings, C is written Cr 500×60n 500×60n
For thrust bearings, C is written Ca
= (0.03n)- 1 = (0.03n)- 3
3 10

In the case of bearings that run at a constant speed, n, fn......Fig. 5.3 (See Page A26), Appendix Table 12
it is convenient to express the fatigue life in terms of
hours. In general, the fatigue life of bearings used in (See Page C24)
automobiles and other vehicles is given in terms of
mileage. Lh, fh....Fig. 5.4 (See Page A26), Appendix Table 13

(See Page C25)

A 25