Figure 6-15.—A steering mechanism.
The advantage from C to D is 3 to 1. The sprocket
wheel to the far left, on the same shaft with D, is
called a wildcat. The anchor chain is drawn up over
this. Every second link is caught and held by the
protruding teeth of the wildcat. The overall
mechanical advantage of the winch is 4 x 3, or 12 to
1.
RACK AND PINION
Figure 6-15 shows you an application of the rack
and pinion as a steering mechanism. Turning the
ship’s wheel turns the small pinion (A). This pinion
causes the internal spur gear to turn. Notice that
this arrangement has a large mechanical advantage.
Now you see that when the center pinion (P)
turns, it meshes with the two vertical racks. When
the wheel turns full to the right, one rack moves
downward and the other moves upward to the
position of the racks. Attached to the bottom of the
racks are two hydraulic pistons that control the
steering of the ship. You’ll get some information on
this hydraulic system in a later chapter.
SUMMARY
These are the important points you should keep in
mind about gears:
Gears can do a job for you by changing the
direction, speed, or size of the force you apply.
When two external gears mesh, they always turn
in opposite directions. You can make them turn
in the same direction by placing an idler gear
between the two.
The product of the number of teeth on each of the
driver gears divided by the product of the
number of teeth on each of the driven gears
gives you the speed ratio of any gear train.
The theoretical mechanical advantage of any gear
train is the product of the number of teeth on the
driven gear wheels, divided by the product of the
number of teeth on the driver gears.
The overall theoretical mechanical advantage of a
We compound machine is equal to the product
of the theoretical mechanical advantages of all
the simple machines that make it up.
We can use cams to change rotary motion into
linear motion.
6-8
