THE WORM AND WORM WHEEL
Figure 6-7.Worm gears.
Worm and worm-wheel combinations, like those in
figure 6-7, have many uses and advantages. However,
its better to understand their operating theory before
learning of their uses and advantages.
Figure 6-7, view A, shows the action of a
single-thread worm. For each revolution of the worm,
the worm wheel turns one tooth. Thus, if the worm
wheel has 25 teeth, the gear ratio is 25:1.
Figure 6-7, view B, shows a double-thread worm.
For each revolution of the worm in this case, the worm
wheel turns two teeth. That makes the gear ratio 25:2 if
the worm wheel has 25 teeth.
A triple-thread worm would turn the worm wheel
three teeth per revolution of the worm.
A worm gear is a combination of a screw and a spur
gear. You can obtain remarkable mechanical advantages
with this arrangement. You can design worm drives so
that only the worm is the driver-the spur cannot drive
the worm. On a hoist, for example, you can raise or
lower the load by pulling on the chain that turns the
worm. If you let go of the chain, the load cannot drive
the spur gear; therefore, it lets the load drop to the deck.
This is a nonreversing worm drive.
GEARS USED TO CHANGE DIRECTION
The crankshaft in an automobile engine can turn in
only one direction. If you want the car to go backwards,
you must reverse the effect of the engines rotation. This
is done by a reversing gear in the transmission, not by
reversing the direction in which the crankshaft turns.
A study of figure 6-8 will show you how gears are
used to change the direction of motion. This is a
schematic diagram of the sight mounts on a Navy gun.
If you crank the range-adjusting handle (A) in a
clockwise direction, the gear (B) directly above it will
rotate in a counterclockwise direction. This motion
causes the two pinions (C and D) on the shaft to turn in
the same direction as the gear (B) against the teeth cut
in the bottom of the table. The table is tipped in the
direction indicated by the arrow.
As you turn the deflection-adjusting handle (E) in a
clockwise direction, the gear (F) directly above it turns
Figure 6-8.-Gears change direction of applied motion.