You know from your experience with levers that you cant get something for nothing. Applying this knowledge to the simple system in figure 10-9, you know that you cant get a 10-pound force from a

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

    F_l x D_l = F₂ x D₂

By substituting

    l x D₁ = 10 x 1

you find that

    D₁ = 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 longand that wouldnt be practical. In addition, it isnt 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 youre beginning to wonder how the large piston gets back down after the process is finished. The fluid cant run back past check valve B-thats 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.

Youve 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

principles of their operation. As the hand steering wheel turns in a counterclockwise direction, its motion turns the pinion gear (g). This causes the left-hand rack (r_l) to move downward and the right-hand rack (r₂) to move upward. Notice that each rack attaches to a piston (p_l or p₂). The downward motion of rack r₁ moves piston p₁ downward in its cylinder and pushes the oil out of that cylinder through the line. At the same time, piston p₂ moves upward and pulls oil from the right-hand line into the right-hand cylinder.

If you follow these two lines, you see that they enter a hydraulic cylinder (S). One line enters above and one below the single piston in that cylinder. This piston and the attached plunger are pushed down toward the hydraulic pump (h) in the direction of the oil flow shown in the diagram. So far in this operation, hand power has been used to develop enough oil pressure to move the control plunger attached to the hydraulic pump. At this point, an electric motor takes over and drives the pump (h).

Oil is pumped under pressure to the two big steering rams (R₁ and R₂). You can see that the pistons in these rams connect directly to the rudder crosshead that controls the position of the rudder. With the pump operating in the direction shown, the ships rudder is thrown to the left, and the bow will swing to port. This operation shows how a small force applied on the steering wheel sets in motion a series of operations that result in a force of thousands of pounds.

Getting Planes on Deck

The swift, smooth power required to get airplanes from the hanger deck to the flight deck of a carrier is provided by a hydraulic lift. Figure 10-12 shows how this lifting is done. An electric motor drives a variable-speed gear pump. Oil enters the pump from the reservoir and is forced through the lines to four hydraulic rams. The pistons of the rams raise the elevator platform. The oil under pressure exerts its force on each square inch of surface area of the four pistons. Since the pistons are large, a large total lifting force results. Either reversing the pump or opening valve 1 and closing valve 2 lowers the elevator. The weight of the elevator then forces the oil out of the cylinders and back into the reservoir.