Chapter 11
Chapter 11

Load-Life Relationship
Using a regression
equation to represent
the line,
◦ a = 3 for ball
bearings
◦ a = 10/3 for roller
bearings
(cylindrical and
tapered roller)

The desired design load FD and life LD come from the problem statement.

The rated life LR will be stated by the specific bearing manufacturer. Many catalogs rate at LR =
106 revolutions.

The catalog load rating C10 is used to find a suitable bearing in the catalog.

It is often convenient to define a dimensionless multiple of rating life
Reliability vs. Life

Defining the life measure in dimensionless form as x = L/L10, the reliability is expressed with a
Weibull distribution as
1|Page
Chapter 11
Reliability vs. Life
 Solving Eq. (11–4) for x yields
Relating Load, Life, and Reliability
Relating Load, Life, and Reliability
 Along a constant load line ( AB), Eq. (11–4) applies:
 Along a constant reliability contour ( BD), Eq. (11–2) applies:
 Solving for xB,
2|Page
Chapter 11
Relating Load, Life, and Reliability
 Catalog information is at point A, at coordinates C10 and
x10 =L10 /L10 =1, on the 0.90 reliability contour.
 The design information is at point D, at coordinates FD and xD,
on the R=R D reliability contour.
 The designer must move from point
D to point A via point B.
Fig. 11–5
24
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

For two bearings on a shaft, R = RARB
If the bearings are to be identical, each bearing should have a reliability equal to the square root
of the total desired reliability RA=RB = √R.
3|Page
Chapter 11
Combined Radial and Thrust Loading
 The data can be approximated by
Combined Radial and Thr
two straight lines
 It is common to express the two
equations as a single equation
were
i = 1 when Fa /(VFr) ≤ e
i = 2 when Fa /(VFr) > e
 X is the ordinate intercept and Y is
the slope
 Basically, indicates that Fe equals Fr
for smaller ratios of Fa/Fr, then
begins to rise when Fa/Fr exceeds
some amount e
Fig. 11–6
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◦
V = 1 for inner ring rotation
◦
V = 1.2 for outer ring rotation
 X and Y factors depend on geometry
36
and construction of the specific
bearing.
4|Page
Chapter 11
Table 11-1

X and Y are functions of e, which is a function of Fa/C0.

C0 is the basic static load rating, which is tabulated in the catalog
Variable Loading with Piecewise Constant Loading
45
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5|Page
Chapter 11
Variable Loading with Periodic Variation
Variable Loading with Piecewise Constant Loading
Fig. 11–11
50
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6|Page