H - HOLD
I - INCREASE OF SPEED
T - TURN
L - REDUCTION OF SPEED
R - REVERSE
Figure 19-7. Laws of Planetary Gearing
only do the bearings provide support, but they also serve
(ball, roller, or needle bearings). Bronze bushings are
to reduce the friction between the moving parts.
used extensively for piston pin bushings in internal
a. Sleeve-Type Bearings. These include bearings
b. Thrust Washers. A thrust washer is a form of
for rotating parts and for parts that merely slide over each
friction bearing designed to limit lateral movement.
other without turning. Most of the bearings of the
These washers generally are placed between moving
mechanisms described in this section are for rotating
parts or one moving part and a stationary member. They
parts. However, two examples of bearings for parts that
generally are not designed to absorb thrust constantly, as
do not rotate relative to each other are the shifter shafts
overheating can occur. Thrust washers generally are
in transmissions, which merely slide endwise in holes in
made of bronze or hardened steel.
the case, and the slip joints of propeller shafts.
The simplest type of sliding surface bearing applied to a
19-8. Ball and Roller Bearings (Antifriction). These
rotating part is one in which an accurately finished shaft,
bearings are used throughout automotive power
or journal, rotates in an accurately finished hole without
transmission systems. The usual locations of antifriction
any bushing, the two being separated by an oil film.
bearings in a truck chassis are shown in figure 19-8.
Such bearings may be used for differential pinions; the
They are known as antifriction bearings, because friction
holes are in the rotating pinions, which turn on the
in them is eliminated because they depend upon rolling
stationary differential spider or cross pin.
contact rather than sliding contacts. Ball bearings often
are referred to as having point contact between balls and
Such bearings usually are known as plain bearings and
raceways; in the same sense, roller bearings are said to
are used only for low speeds or light duty or both.
have line contact between rollers and races. These are
Probably the next simplest plain bearing, also of the
merely descriptive terms, however, because the elastic
sliding-surface type, is the bushing that is replaceable
deformation occurring under load results in substantial
when worn. A bushing is usually of bronze or similar
areas of contact in either type. The starting friction of ball
relatively soft material and is pressed into a hole and
and roller bearings is but slightly greater than their
reamed to fit, forming a lining in which a journal rotates.
running friction, an important advantage in machinery
Plain bushings are suitable for radial loads only. Bronze
that is required to start frequently under load. They also
bushings formerly were used to a considerable extent in
can sustain high overloads for short periods without
automotive power transmission systems but have been
failure. Ordinarily, a ball or roller bearing does not fail
superseded by antifriction bearings
suddenly, but gives warning by a gradual decrease in
smoothness of running; whereas, the plain bearing is
subject to an accelerated type of failure that often results