INNER RACE ROTATES
OUTER RACE ATTEMPTS TO
FASTER THAN OUTER RACE.
ROTATE FASTER THAN INNER
SPRAGS ARE FREE.
RACE. SPRAGS ARE WEDGED.
Figure 2-56.--Typical sprag assembly, overrunning clutch.
essential for an understanding of the automatic
outer race rotation is with the slight angle of the sprag,
the unit will overrun. However, if the inner or outer
race slows down and tends to rotate against the angle of
the sprag, it will jam or wedge between the two races
and cause them to turn as one unit.
One of the first things that should be noted about
gears is the direction of rotation. The direction that the
PLANETARY GEAR SYSTEM
driven gear turns is opposite to that of the driving gear
when only two meshing gears are used. To have the
Automatic transmissions use a system of planetary
driven gear turn in the same direction as the driving
gears to enable the torque from the engine, coupled
gear, an idler gear is used between the driving and
used as efficiently as possible in meeting the power
driven gears. Figure 2-57 shows the direction of gear
demands placed on the vehicle either by the operator or
travel using an idler gear. By simply covering one of
pull loads. The planetary gear units are the heart of the
the outside gears, shown in figure 2-57, direction of
modern automatic transmission. Therefore, an
gear travel can be seen for two meshed gears. The gear
understanding of gears and the planetary gear system is
ratio for the gears in figure 2-57 is 1 to 1, since each