Principles of Hydraulic Pressure

Figure 10-5.-Pressure to a fluid transmits in all directions. PRINCIPLES OF HYDRAULIC PRESSURE A Frenchman named Pascal discovered that a pressure applied to any part of a confined fluid transmits to every other part with no loss. The pressure acts with equal force on all equal areas of the confining walls and perpendicular to the walls. Remember when you are talking about the hydraulic machine, you are talking about the way a liquid acts in a closed system of pipes and cylinders. The action of a liquid under such conditions is somewhat different from its behavior in open containers or in lakes, rivers, or oceans. You also should keep in mind that you cannot compress most liquids into a smaller space. Liquids don’t “give” the way air does when you apply pressure, nor do liquids expand when you remove pressure. Punch a hole in a tube of toothpaste. If you push down at any point on the tube, the toothpaste comes out of the hole. Your force has transmitted from one place to another through the toothpaste, which is a thick, liquid fluid. Figure 10-5 shows what would happen if you punched four holes in the tube. If you were to press on the tube at one point, the toothpaste would come out of all four holes. You have illustrated a basic principle of hydraulic machines. That is, a force applied on a liquid transmits equally in every direction to all parts of the container. We use this principle in the operation of four-wheel hydraulic automobile brakes. Figure 10-6 is a simplified drawing of this brake system. You push down on the brake pedal and force the piston in the master cylinder against the fluid in that cylinder. This push sets up a pressure on the fluid as your finger did on the toothpaste in the tube. The pressure on the fluid in the master cylinder transmits through the lines to the brake cylinders in each wheel. This fluid under pressure Figure 10-6.-Hydraulic brakes. Figure 10-7.-Liquid transmits force. pushes against the pistons in each of the brake cylinders and forces the brake shoes out against the drums. MECHANICAL ADVANTAGES OF HYDRAULIC PRESSURE Another aspect to understand about hydraulic machines is the relationship between the force you apply and the result you get. Figure 10-7 will help you understand this principle. The U-shaped tube has a cross-sectional area of 1 square inch. In each arm is a piston that fits snugly, but can move up and down. If you place a 1-pound weight on one piston, the other one will push out the top of its arm immediately. If you place a 10-5