Figure 10-5.-Pressure to a fluid transmits in all directions.PRINCIPLES OF HYDRAULIC PRESSUREA Frenchman named Pascal discovered that apressure applied to any part of a confined fluid transmitsto every other part with no loss. The pressure acts withequal force on all equal areas of the confining walls andperpendicular to the walls.Remember when you are talking about thehydraulic machine, you are talking about the way aliquid acts in a closed system of pipes and cylinders. Theaction of a liquid under such conditions is somewhatdifferent from its behavior in open containers or in lakes,rivers, or oceans. You also should keep in mind that youcannot compress most liquids into a smaller space.Liquids don’t “give” the way air does when you applypressure, nor do liquids expand when you removepressure.Punch a hole in a tube of toothpaste. If you pushdown at any point on the tube, the toothpaste comes outof the hole. Your force has transmitted from one placeto another through the toothpaste, which is a thick, liquidfluid. Figure 10-5 shows what would happen if youpunched four holes in the tube. If you were to press onthe tube at one point, the toothpaste would come out ofall four holes. You have illustrated a basic principle ofhydraulic machines. That is, a force applied on a liquidtransmits equally in every direction to all parts of thecontainer.We use this principle in the operation of four-wheelhydraulic automobile brakes. Figure 10-6 is a simplifieddrawing of this brake system. You push down on thebrake pedal and force the piston in the master cylinderagainst the fluid in that cylinder. This push sets up apressure on the fluid as your finger did on the toothpastein the tube. The pressure on the fluid in the mastercylinder transmits through the lines to the brakecylinders in each wheel. This fluid under pressureFigure 10-6.-Hydraulic brakes.Figure 10-7.-Liquid transmits force.pushes against the pistons in each of the brake cylindersand forces the brake shoes out against the drums.MECHANICAL ADVANTAGES OFHYDRAULIC PRESSUREAnother aspect to understand about hydraulicmachines is the relationship between the force you applyand the result you get. Figure 10-7 will help youunderstand this principle. The U-shaped tube has across-sectional area of 1 square inch. In each arm is apiston that fits snugly, but can move up and down. If youplace a 1-pound weight on one piston, the other one willpush out the top of its arm immediately. If you place a10-5
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