Figure 5-9.—The quadrant davit.
wheel turns a long threaded shaft. Half the threads—
those nearer the wheel end of this shaft-are right-hand
threads. The other half of the threads-those farther
from the wheel—are left-hand threads. Nut A has a
right-hand thread, and nut B has a left-hand thread.
Notice that two steering arms connect the crosshead to
the nuts; the crosshead turns the rudder. If you stand in
front of the wheel and turn it in a clockwise direction—
to your right—arm A moves forward and arm B moves
backward. That turns the rudder counterclockwise, so
the ship swings in the direction you turn the wheel. This
steering mechanism has a great mechanical advantage.
Figure 5-9 shows you another practical use of the
screw. The quadrant davit makes it possible for two men
to put a large lifeboat over the side with little effort. The
operating handle attaches to a threaded screw that passes
through a traveling nut. Cranking the operating handle
in a counterclockwise direction (as you face outboard),
the nut travels outward along the screw. The traveling
nut fastens to the davit arm by a swivel. The davit arm
and the boat swing outboard as a result of the outward
movement of the screw. The thread on that screw is the
self-locking type; if you let go of the handle, the nut
remains locked in position.
SUMMARY
You have learned the following basic information
about the screw from this chapter; now notice the
different ways the Navy uses this simple machine:
The screw is a modification of the inclined plane—
modified to give you a high mechanical
advantage.
The theoretical mechanical advantage of the screw
can be found by the formula
As in all machines, the actual mechanical advantage
equals the resistance divided by the effort.
In many applications of the screw, you make use of
the large amount of friction that is commonly
present in this simple machine.
By using the screw, you reduce large amounts of
circular motion to very small amounts of
straight-line motion.
5-4
