21-12-2012, 02:06 PM
Bearing
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ABSTRACT
The terms rolling-contact bearing, antifriction bearing, and rolling bearing are all used to
describe that class of bearing in which the main load is transferred through elements in
rolling contact rather than in sliding contact. In a rolling bearing the starting friction is
about twice the running friction, but still it is negligible in comparison with the starting
friction of a sleeve bearing. Load, speed, and the operating viscosity of the lubricant do
affect the frictional characteristics of a rolling bearing. It is probably a mistake to describe
a rolling bearing as “antifriction,” but the term is used generally throughout the industry.
From the mechanical designer’s standpoint, the study of antifriction bearings
differs in several respects when compared with the study of other topics because the
bearings they specify have already been designed. The specialist in antifriction-bearing
design is confronted with the problem of designing a group of elements that compose
a rolling bearing: these elements must be designed to fit into a space whose dimensions
are specified; they must be designed to receive a load having certain characteristics;
and finally, these elements must be designed to have a satisfactory life when
operated under the specified conditions. Bearing specialists must therefore consider
such matters as fatigue loading, friction, heat, corrosion resistance, kinematic problems,
material properties, lubrication, machining tolerances, assembly, use, and cost.
From a consideration of all these factors, bearing specialists arrive at a compromise
that, in their judgment, is a good solution to the problem as stated.
We begin with an overview of bearing types; then we note that bearing life cannot
be described in deterministic form. We introduce the invariant, the statistical distribution
of life, which is strongly Weibullian.1 There are some useful deterministic equations
addressing load versus life at constant reliability, and we introduce the catalog rating at
rating life.
Bearing Types
Bearings are manufactured to take pure radial loads, pure thrust loads, or a combination
of the two kinds of loads. The nomenclature of a ball bearing is illustrated in Fig. 11–1,
which also shows the four essential parts of a bearing. These are the outer ring, the
inner ring, the balls or rolling elements, and the separator. In low-priced bearings, the
separator is sometimes omitted, but it has the important function of separating the
elements so that rubbing contact will not occur.
In this section we include a selection from the many types of standardized bearings
that are manufactured. Most bearing manufacturers provide engineering manuals
and brochures containing lavish descriptions of the various types available. In the small
space available here, only a meager outline of some of the most common types can be
given. So you should include a survey of bearing manufacturers’ literature in your studies
of this section.
Some of the various types of standardized bearings that are manufactured are
shown in Fig. The single-row deep-groove bearing will take radial load as well
as some thrust load. The balls are inserted into the grooves by moving the inner ring
to an eccentric position. The balls are separated after loading, and the separator is
then inserted. The use of a filling notch (Fig. 11–2b) in the inner and outer rings
enables a greater number of balls to be inserted, thus increasing the load capacity.
The thrust capacity is decreased, however, because of the bumping of the balls against
the edge of the notch when thrust loads are present. The angular-contact bearing (Fig.
11–2c) provides a greater thrust capacity.
Bearing Life
When the ball or roller of rolling-contact bearings rolls, contact stresses occur on the
inner ring, the rolling element, and on the outer ring. Because the curvature of the
contacting elements in the axial direction is different from that in the radial direction,
the equations for these stresses are more involved than in the Hertz equations presented
in Chapter 3. If a bearing is clean and properly lubricated, is mounted and
sealed against the entrance of dust and dirt, is maintained in this condition, and is
operated at reasonable temperatures, then metal fatigue will be the only cause of failure.
Inasmuch as metal fatigue implies many millions of stress applications successfully
endured, we need a quantitative life measure.
Relating Load, Life, and Reliability
This is the designer’s problem. The desired load is not the manufacturer’s test load
orcatalog entry. The desired speed is different from the vendor’s test speed, and the
reliability expectation is typically much higher than the 0.90 accompanying the catalog
entry. Figure 11–5 shows the situation. The catalog information is plotted as point A,
whose coordinates are (the logs of) C10 and x10 = L10/L10 = 1, a point on the 0.90
reliability contour. The design point is at D, with the coordinates (the logs of )FD
andxD, a point that is on the R = RD reliability contour.