Figure 7-9.—Roller bitt saves line.
The roller bitt in figure 7-9 is another example of
how you can cut down the wear and tear on lines or
cable and reduce your frictional loss.
When you need one surface to move over another,
you can decrease the friction with lubricants such as
oil, grease, or soap. You can use a lubricant on flat
surfaces and gun slides as well as on ball and roller
bearings. A lubricant reduces frictional resistance
and cuts down wear.
In many situations friction is helpful. However,
many sailors have found out about this the hard
way—on a wet, slippery deck. You’ll find rough grain
coverings are used on some of our ships. Here you
have friction working for you. It helps you to keep
your footing.
EFFICIENCY
To make it easier to explain machine operations,
we have neglected the effect of friction on machines
up to this point. Friction happens every time two
surfaces move against one another. The work used in
overcoming the frictional resistance does not appear
in the work output. Therefore, it’s obvious that you
have to put more work into a machine than you get
out of it. Thus, no machine is 100 percent efficient.
Take the jack in figure 7-6, for example. The
chances are good that a 2-pound force exerted on the
handle wouldn’t do the job at all. You would need a
pull of at least 10 pounds. This shows that only 2
pounds out of the 10 pounds, or 20 percent of the
effort, is employed to do the job. The remaining 8
pounds of effort was is in overcoming the friction in
the jack. Thus, the jack has an efficiency of only 20
percent. Most jacks are inefficient. However, even
with this inefficiency, it is possible to deliver a huge
push with a small amount of effort.
A simple way to calculate the efficiency of a
machine is to divide the output by the input and
convert it to a percentage:
Output
Efficiency = Input
Now go back to the block-and-tackle problem
illustrated in figure 7-5. It’s likely that instead of
being able to lift the load with a 120-pound pull, the
sailor would have to use a 160-pound pull through
the 100 feet. You can calculate the efficiency of the
rig by the following method:
Output F2 x S2
Efficiency = Input = F1 x S1
and, by substitution,
600 x 20
Efficiency = 160 x 100 = 0.75 0r 75 percent.
Theoretically, with the mechanical advantage of
12 developed by the cable winch in figure 6-11, you
can lift a 600-pound load with a 50-pound push on the
handle. If the machine has an efficiency of 60
percent, how big a push would you actually have to
apply? Actually, 50 + 0.60 = 83.3 pounds. You can
check this yourself in the following manner:
Output
Efficiency = Input
F2 x S2
= F1 x S1
One revolution of the drum would raise the
600-pound load a distance S2 of 2
p
r, or 7.85 feet. To
make the drum revolve once, the pinion gear must
rotate six times by the handle, and the handle must
turn through
7-5
