Aiding the Helmsman

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