Figure 10-10.-Hydraulic press.
1-pound effort without sacrificing distance. You must
apply the 1-pound effort through a much greater
distance than the 10-pound force will move. To raise the
10-pound weight a distance of 1 foot, you must apply
the 1-pound effort through what distance? Remember,
if you neglect friction, the work done on any machine
equals the work done by that machine. Use the work
formula to find how far the smaller piston will have to
move.
Work input = Work output
FlxDl=F~xD2
By substituting
lXD1=lOX1
you find that
D] = 10 feet
The smaller piston will have to move a distance of
10 feet to raise the 10-pound load 1 foot. It looks then
as though the smaller cylinder would have to be at least
10 feet long—and that wouldn’t be practical. In
addition, it isn’t necessary if you put a valve in the
system.
The hydraulic press in figure 10-10 contains a valve.
As the small piston moves down, it forces the fluid past
check valve A into the large cylinder. As soon as the
small piston moves upward, it removes the pressure to
the right of check valve A. The pressure of the fluid on
the check valve spring below the large piston helps force
that valve shut. The liquid that has passed through the
valve opening on the down stroke of the small piston is
trapped in the large cylinder.
The small piston rises on the upstroke until its
bottom passes the opening to the fluid reservoir. More
fluid is sucked past check valve B and into the small
cylinder. The next downstroke forces this new charge of
fluid out of the small cylinder past the check valve into
the large cylinder. This process repeats stroke by stroke
until enough fluid has been forced into the large cylinder
to raise the large piston the required distance of 1 foot.
The force has been applied through a distance of 10 feet
on the pump handle. However, it was done through a
series of relatively short strokes, the total of the strokes
being equal to 10 feet.
Maybe you’re beginning to wonder how the large
piston gets back down after the process is finished. The
fluid can’t run back past check valve B-that’s obvious,
Therefore, you lower the piston by letting the oil flow
back into the reservoir through a return line. Notice that
a simple globe valve is in this line. When the globe valve
opens, the fluid flows back into the reservoir. Of course,
this valve is shut while the pump is in operation.
Aiding the Helmsman
You’ve probably seen the helmsman swing a ship
weighing thousands of tons almost as easily as you turn
your car. No, helmsmen are not superhuman. They
control the ship with machines. Many of these machines
are hydraulic.
There are several types of hydraulic and electro-
hydraulic steering mechanisms. The simplified diagram
10-7
