Figure 6-11.-Cable winch.
GEARS USED TO INCREASE
We use gear trains to increase mechanical
advantage. In fact, wherever there is a speed reduction,
you multiply the effect of the effort. Look at the cable
winch in figure 6-11. The crank arm is 30 inches long,
and the drum on which the cable is wound has a 15-inch
radius. The small pinion gear has 10 teeth, which mesh
with the 60 teeth on the internal spur gear. You will find
it easier to figure the mechanical advantage of this
machine if you think of it as two machines.
First, figure out what the gear and pinion do for you.
You find the theoretical mechanical advantage (T.M.A.)
of any arrangement of two meshed gears by using the
T. = number of teeth on driven gear;
To = number of teeth on driver gear.
In this case,
T. = 60 and T. = 10.
Now, figure the mechanical advantage for the other
part of the machine-a simple wheel-and-axle arrange-
ment consisting of the crank arm and the drum. Divide
the distance the effort moves (2nR) in making one
complete revolution by the distance the cable is drawn
up in one revolution of the drum (2nr).
Figure 6-12.-Camdriven valve.