Figure 19-2. Power Takeoff Driving Winch
Section II. GEARS
gear is twice that of the small gear. On the other hand,
19-3. Principles of Gearing. Because gears play an
when the larger gear drives the smaller gear, the torque
important part in power trains, gear principles as well as
is reduced while the speed is increased.
various types of gears used in power trains will be
discussed. Gears are used to transmit rotary motion
To start an automotive vehicle moving, a great deal of
from one shaft to another. Sometimes the shafts are
tractive effort or torque must be applied to the driving
parallel to each other, at other times they are at an angle.
wheels or drive sprockets. This may be accomplished by
The gearing is selected to fit the application and provide
using a series of gears, or a gear train, which greatly
the desired drive ratio (ratio of revolutions per minute)
reduces the speed and thereby greatly increases the
between the driving and driven shafts.
torque between the engine and driving wheel or sprocket.
A common arrangement in passenger cars permits a
a. Torque Ratio. Torque has been defined as a
gear reduction, in first (or low) gear, of 12:1 between the
twisting, or turning, effort. When one gear drives the
engine and rear wheels.
This means the engine
other, it turns the other by the application of torque. The
crankshaft must turn 12 times to turn the rear wheels
torque ratio between two meshing gears varies with the
once. This speed reduction results in a torque increase.
mechanical advantage, that is, with the gear ratio of the
Ignoring friction, the torque increase would be twelvefold.
driving to the driven gear. When a small gear drives a
That is, if the torque at the crankshaft were 100 ft lb, then
larger gear, for example, the speed is reduced but the
the torque at the rear axles would be 1,200 ft lb. In large
torque delivered by the larger gear is increased. Thus,
trucks and heavy military vehicles, the
when a 12-tooth gear drives a 24-tooth gear, torque is
doubled; that is, the torque of the large