Brochure

Mtd. Ball
Bearings
G-221
®
Mounted Ball Bearing Engineering Section
Life Calculations
The L10 (rating) life for any given application and bear-
ing selection can be calculated in terms of millions of
revolutions by using the bearing Basic Dynamic Rating
and applied radial load (or, equivalent radial load in the
case of radial bearing applications having combined
radial and thrust loads). The L10 life for any given
application can be calculated in terms of hours, using
the bearing Basic Dynamic Rating, applied load (or
equivalent radial load) and suitable speed factors, by
the following equation:
L
10
= x = x
Where:
L
10
= The # of hours that 90% of
identical bearings under ideal conditions will
operate at a specic speed and condition
before fatigue is expected to occur.
C = Basic Dynamic Rating (lbs)
1,000,000 Revolutions
P = Constant Equivalent Radial Load (lbs)
n = Speed (RPM)
Additionally, the ABMA provides application factors
for all types of bearings which need to be considered
to determine an adjusted Rated Life (Lna). L10 life rat-
ing is based on laboratory conditions yet other factors
are encountered in actual bearing application that will
reduce bearing life. Lna life rating takes into account
reliability factors, material type, and operating condi-
tions.
L
na
= a
1
x a
2
x a
3
x L
10
Where:
L
na
= Adjusted Rated Life.
a
1
= Reliability Factor. Adjustment factor
applied where estimated fatigue life
is based on reliability other than 90%
(See Table 1).
a
2
= Material Factor. Life adjustment for bearing
race material. Power Transmission Solutions bearing
races are manufactured from bearing quality steel.
Therefore the a
2
factor is 1.0.
a
3
= Life Adjustment Factor for Operating Conditions.
This factor should take into account the adequacy
of lubricant, presence of foreign matter, conditions
causing changes in material properties, and unusual
loading or mounting conditions. Assuming a properly
selected and mounted bearing having adequate seals
and lubricant operating below 250°F and tight tted
to the shaft, the a
3
factor should be 1.0.
Mounted bearings are typically “slip tted” to the
shaft and rely on design features such as the inner race
length and locking device for support. ABMA recom-
mends an a
3
factor of .456 for “slip t” ball bearings.
See sample calculations on page G-224.
Vibration and shock loading can act as an additional
loading to the steady expected applied load. When
shock or vibration is present, an a
3
Life Adjustment
Factor can be applied. Shock loading has many vari-
ables which often are not easily determined. When
shock or vibration is present, the following a
3
, Life Ad-
justment Factors are recommended. The shock factor
is used in combination with the “slip t” factor.
The a
3
factor takes into account a wide range of ap-
plication and mounting conditions as well as bearing
features and design. Accurate determination of this
factor is normally achieved through testing and in-
eld experience. Power Transmission Solutions offers
a wide range of options which can maximize bearing
performance. Consult Application Engineering for
more information. See sample calculations on page
G-224.
Table 1 Life Adjustment Factor for Reliability
Reliability % L
na
a
1
90 L10 1
95 L5 0.62
96 L4 0.53
97 L3 0.44
98 L2 0.33
99 L1 0.21
50 L50 5
Table 2 Shock/Vibration Factor
Type a
3
Factor
Steady Loading 1.0
Light Shock/Vibration .5
Moderate Shock/Vibration .3
Load Ratings and Life
C
P
( )
C
P
( )
1,000,000
60 x n
16667
n
33