Timken Super Precision Bearings for Machine Tool Applications

A
ENGINEERING
Reliability Life Factor (a 1 )
The equation for the life adjustment factor for reliability is:
( )
a 1 = 4.26 x ln 100 2/3
+ 0.05
R
ln = natural logarithm (base e)
To adjust the calculated L 10 life for reliability, multiply by the a 1 factor.
If 90 (90 percent reliability) is substituted for R in the above equation,
a 1 = 1. For R = 99 (99 percent reliability), a 1 = 0.25. The following table
lists the reliability factor for commonly used reliability values.
R (percent) L n a 1
90 L 10 1.00
95 L 5 0.64
96 L 4 0.55
97 L 3 0.47
98 L 2 0.37
99 L 1 0.25
99.5 L 0.5 0.175
99.9 L 0.1 0.093
Table 9. Reliability factors.
Note that the equation for reliability adjustment assumes there is a
short minimum life below which the probability of bearing damage
is minimal (e.g., zero probability of bearing damage producing a
short life). Extensive bearing fatigue life testing has shown the
minimum life, below which the probability of bearing damage is
negligible, to be larger than shown above. For a more accurate
prediction of bearing lives at high levels of reliability, consult your
Timken representative.
Material Life Factor (a 2 )
The life adjustment factor for bearing material (a 2) for standard
Timken bearings manufactured from bearing-quality steel is 1.0.
Bearings also are manufactured from premium steels, containing
fewer and smaller inclusion impurities than standard steels
and providing the benefit of extending bearing fatigue life (e.g.,
DuraSpexx). Application of the material life factor requires that
fatigue life is limited by nonmetallic inclusions, that contact stresses
are approximately less than 2400 MPa (350 ksi), and adequate
lubrication is provided. It is important to note that improvements
in material cannot offset poor lubrication in an operating bearing
system. Consult your Timken representative for applicability of the
material factor.
Debris Life Factor (a 3d )
Debris within a lubrication system reduces the life of a rolling
bearing by creating indentations on the contacting surfaces, leading
to stress risers. The Timken life rating equations were developed
based on test data obtained with 0.040 mm (0.00157 in.) oil filtration
and measured ISO cleanness levels of approximately 15/12, which
is typical of cleanness levels found in normal industrial machinery.
When more or less debris is present within the system, the fatigue
life predictions can be adjusted according to the measured or
expected ISO lubricant cleanness level to more accurately reflect
the expected bearing performance.
As opposed to determining the debris life factor based on filtration
and ISO cleanness levels, a Debris Signature Analysis can be
performed for more accurate bearing performance predictions.
The Debris Signature Analysis is a process for determining the
effects of the actual debris present in your system on the bearing
performance. The typical way in which this occurs is through
measurements of dented/bruised surfaces on actual bearings run in
a given application. This type of analysis can be beneficial because
different types of debris cause differing levels of performance, even
when they are of the same size and amount in the lubricant. Soft,
ductile particles can cause less performance degradation than
hard, brittle particles. Hard, ductile particles are typically most
detrimental to bearing life. Brittle particles can break down, thus
not affecting performance to as large a degree as hard ductile
particles. For more information on Debris Signature Analysis or
the availability of Debris Resistant bearings for your application,
consult your Timken representative.
600
ISOMETRIC VIEW
400
μm
200
0 0
Core Structure 0-5% Bearing Area 5-10% Bearing Area Pits & Valleys
Fig. 46. Surface map of a bearing raceway with debris denting.
200
μm
400
600
800
48 TIMKEN MACHINE TOOL CATALOG