BASIC PRINCIPLES OF HYDRAULICS
The basic principles of hydraulics are few and simple and are as follows:
Liquids have no shape of their own.
Liquids will NOT compress.
Liquids transmit applied pressure in all directions.
Liquids provide great increase in work force.
Pressure and Force
The terms force and pressure are used extensively in the study of fluid power. It is essential that we distinguish between these terms. Force means a total push or pull. It is push or pull exerted against the total area of a particular surface and is expressed in pounds or grams. Pressure means the amount of push or pull (force) applied to each unit area of the surface and is expressed in pounds per square inch (lb/ in 2 ) or grams per square centimeter (gm/ cm 2 ). Pressure may be exerted in one direction, in several directions, or in all directions.
Computing Force, Pressure, and Area
A formula is used in computing force, pressure, and area in hydraulic systems. In this formula, P refers to pressure, F indicates force, and A represents area.
Force equals pressure times area. Thus, the formula is written F = P x A
Pressure equals force divided by area. By rearranging the above formula, this state may be condensed into the following: P = F divided by A.
Since area equals force divided by pressure, the formula for area is written as follows: A = F divided by P
Figure 3-1 shows a memory device for recalling the different variations of the formula. Any letter in the triangle may be expressed as the product or quotient of the other two, depending on its position within the triangle.
Incompressibility and Expansion of Liquids
For all practical purposes, fluids are incom-pressible. Under extremely high pressures: the volume of a fluid can be decreased somewhat, though the decrease is so slight that it is considered to be negligible except by design engineers.
Liquids expand and contract because of temperature changes. When liquid in a closed container is subjected to high temperatures, it expands and this exerts pressure on the walls of the container; therefore, it is necessary that pressure-relief mechanisms and expansion chambers be incorporated into hydraulic systems. Without these precautionary measures, the expanding fluid could exert enough pressure to rupture the system.
Transmission of Forces through Liquids
When the end of a solid bar is struck, the main force of the blow is carried straight through the bar to the other end (fig. 3-2, view A). This happens because the bar is rigid. The direction of the blow almost entirely determines the direction of the transmitted force. The more rigid the bar, the less force is lost inside the bar or transmitted outward at right angles to the direction of the blow.
When a force is applied to the end of a column of confined liquid (fig. 3-2, view B), it is transmitted straight through the other end and also undiminished in every direction throughout the column- forward, backward, and sideways- so that the containing vessel is literally tilled with pressure.
An example of this distribution of force is shown in figure 3-3. The flat hose takes on a circular cross section when it is filled with water under pressure. The outward push of the water is equal in every direction.Pascal's Law
The foundation of modern hydraulics was established when Blaise Pascal, a French scientist,
discovered the fundamental law for the science of hydraulics. Pascal's law tells us that pressure on a confined fluid is transmitted undiminished in every direction, and acts with equal force on equal areas, throughout the confining vessel or system.
According to Pascal's law, any force applied to a confined fluid is transmitted in all directions throughout the fluid regardless of the shape of the container. Consider the effect of this in the systems shown in views A and B of figure 3-4. If there is resistance on the output piston (view A, piston 2) and the input piston is pushed downward, a pressure is created through the fluid which acts equally at right angles to surfaces in all parts of the container.
If the force 1 is 100 pounds and the area of input piston 1 is 10 square inches, then pressure in the fluid is 10 psi ( 100 ÷ 10). It must be emphasized that this fluid pressure cannot be created without resistance to flow, which, in this case, is provided by the 100-pound force acting against the top of the output piston 2. This pressure acts on piston 2, so for each square inch of its area, it is pushed upward with the force of 10 pounds. In this case, a fluid column of a uniform cross section is considered so the area of output piston 2 is the same as input piston 1, or 10 square inches; therefore, the upward force on output piston 2 is 100 pounds- the same as was applied to input piston 1. All that has been accomplished in this system was to transmit the 100- pound force around a bend; however, this principle underlies practically all-mechanical applications of fluid power.
At this point, it should be noted that since Pascal's law is independent of the shape of the container, it is not necessary that the' tubing connecting the two pistons should be the full area of the pistons. A connection of any size, shape, or length will do so long as an unobstructed passage is provided. Therefore, the system shown in view B of figure 3-4 (a relatively small, bent pipe connects the two cylinders) will act the same as that shown in view A.
Multiplication of Forces
Some hydraulic systems are used to multiply force. In figure 3-5, notice that piston 1 is smaller than piston 2. Assume that the area of the input piston 1 is 2 square inches. With a resistant force on piston 2, a downward force of 20 pounds acting on piston 1 creates 10 psi (20 ÷ 2) in the fluid. Although this force is much smaller than the applied forces in figure 3-4,
This pressure of 10 psi acts on all parts of the fluid container, including the bottom of output piston 2; therefore, the upward force on output piston 2 is 10 pounds for each of its 20 square inches of area, or 200 pounds (10 x 20). In this case, the original force has been multiplied tenfold while using the same pressure in the fluid as before. In any system with these dimensions, the ratio of output force to input force is always 10 to 1 regardless of the applied force; for example, if the applied force of input piston 1 is 50 pounds, the pressure in the system is increased to 25 psi. This will support a resistant force of 500 pounds on output piston 2.
The system works the same in reverse. Consider piston 2 as the input and piston 1 as the output; then the output force will always be one tenth of the input force.
Therefore, the first basic rule for two pistons used in a fluid power system is the force acting on each is directly proportional to its area, and the magnitude of each force is the product of the pressure and its area is totally applicable.
Volume and Distance FactorsIn the systems shown in views A and B of figure 3-4, the pistons have areas of 10 square inches. Since the areas of the input and output pistons are equal, a force of 100 pounds on the input piston will support a resistant force of 100 pounds on the output piston. At this point the pressure of the fluid is 10 psi. A slight force in excess of 100 pounds on the input piston will increase the pressure of the fluid, which, in turn, overcomes the resistance force. Assume that the output piston is forced downward 1 inch. This action displaces 10 cubic inches of fluid (1 in. x 10 sq. in. = 10 cubic inches). Since liquid is practically incompressible, this volume must go some place. This volume of fluid moves the output piston. Since the area of the output piston is likewise 10 square inches, it moves 1 inch upward to accommodate the 10 cubic inches of fluid. The pistons are of equal areas; therefore, they will move the same distance, though in opposite directions.
