NTN - Precision Rolling Bearings

Technical Data
NTNs new bearing life formula
The correlation between the NTNs life correction
factor a NTN and corrected rating life Lnm is defined by the
formula (3.14) below.
C
p
Lnmaa NTN
3.14
P
where,
Z
Pmax
Fig. 3.2 Calculation model
Lnm Corrected rating life
a1 Reliability coefficient
a NTN Life correction factor that reflects material
properties, fatigue limit stress, contamination
with foreign matter and oil film parameter
() (0.1 a NTN 50)
C Basic dynamic load rating
P Dynamic equivalent load
p Index 3 (ball bearing) 10/3 (roller bearing)
(1) Effect of fatigue limit
NTNs new bearing life formula introduces a concept
of fatigue life according to which the bearing life is
infinitely long at a particular contact surface pressure as
illustrated in Fig. 3.3 assuming no foreign matter is
trapped in the bearing and the bearing is reliably
lubricated.
y
(2) Effect of foreign matter
The effect of foreign matter is treated as surfaceinitiated
flaking that starts from a dent resulting from
trapped foreign matter. NTN performs a bearing life
calculation, assuming that the size of foreign matter and
the stress concentration area in the middle portion (the
size of this area corresponds with that of the foreign
matter) in the surface layer as well as the amount of
foreign matter significantly affect the bearing life.
Dented raceway
Normal raceway
Fig. 3.4 Contact surface pressure distribution resulting
from dent
(3) Effect of oil film parameter ()
The oil film parameter can be used to calculate
bearing life. The oil film parameter, designated by, is
the ratio of the oil film thickness to the roughness of the
surface. It can be used to calculate the average stress
across the surface layer of two contacting surfaces,
such as a rolling element and raceway. From this
surface layer stress, the contact surface pressure can
be determined. Bearing life is then calculated from the
contact surface pressure.
Conditions of two objects on surface layer
Calculation model
Object having
composite roughness
of two objects
Smooth surface
Contact surface pressure
equivalent to fatigue limit
Contact stress
Rating
life
Life reflecting fatigue limit
Life
Fig. 3.3 Basic concept of fatigue limit
Oil film
Fig. 3.5 Model of stress load onto the surface layer
New life calculation formula chart
Various statuses of contamination with foreign matter
are defined in Table 3.2. The values of ISO codes and
NAS classes are those for ball bearings that are
subjected to more severe operating conditions.
Table 3.2 Status of contamination
Condition of contamination Extremely clean Clean Normal
Lightly Moderately Highly Severely
contaminated contaminated contaminated contaminated
Contamination coefficient 1 0.8 0.5 0.4 0.3 0.2 0.1
Guideline for application
ISO cleanliness code
(ISO 4406)
NAS class
Filtered
Less than 10 μm 1030m 3050m 5070m 70100m
100 μm or
more
No filter
Ingress of
much dust
13/10 15/12 17/14 19/16 21/18 23/20 25/22
0 3 6 8 9 10 12
19