square inches. With a resistant force on piston 2, adownward force of 20 pounds acting on piston 1 creates10 psi (20÷2) in the fluid. Although this force is muchsmaller than the applied forces in figure 10-1, thepressure is the same because the force is concentratedon a relatively small area.This pressure of 10 psi acts on all parts of the fluidcontainer, including the bottom of the output piston 2;therefore, the upward force on the output piston 2 is 10pounds for each of its 20 square inches of area, or 200pounds (10 x 20). In this case, the original force hasbeen multiplied tenfold while using the same pressurein the fluid as before. In any system with thesedimensions, the ratio of output force to input force isalways 10 to 1 regardless of the applied force; forexample, if the applied force of the input piston 1 is50 pounds, the pressure in the system is increased to 25psi. This will support a resistant force of 500 pounds onthe output piston 2.The system works the same in reverse. Considerpiston 2 as the input and piston 1 as the output; then theoutput force will always be one-tenth the input force.Sometimes such results are desired.Therefore, the first basic rule for two pistons usedin a fluid power system is the force acting on each isdirectly proportional to its area and the magnitude ofeach force is the product of the pressure and its area, istotally applicable.Volume and Distance FactorsIn the systems shown in views A and B of figure10-1, the pistons have areas of 10 square inches. Sincethe areas of the input and output pistons are equal, aforce of 100 pounds on the input piston will support aresistant force of 100 pounds on the output piston. Atthis point, the pressure of the fluid is 10 psi. A slightforce, in excess of 100 pounds, on the input piston willincrease the pressure of the fluid, which will, in turn,overcome the resistance force. Assume that the inputpiston is forced downward 1 inch. This displaces 10cubic inches of fluid. Since liquid is practicallyincompressible, this volume must go some place. In thecase of a gas, it will compress momentarily but willeventually expand to its original volume at 10 psi. Thisis provided, of course, that the 100 pounds of force isstill acting on the input piston. Thus this volume of fluidmoves the output piston. Since the area of the outputpiston is likewise 10 square inches, it moves 1 inchupward to accommodate the 10 cubic inches of fluid.The pistons are of equal areas; therefore, they will moveequal distances, though in opposite directions.Applying this reasoning to the system in figure 10-2,it is obvious that if the input piston 1 is pushed down 1inch, only 2 cubic inches of fluid is displaced. Theoutput piston 2 will have to move only one-tenth of aninch to accommodate these 2 cubic inches of fluid,because its area is 10 times that of the input piston 1.This leads to the second basic rule for two pistons in thesame fluid power system, which is the distances movedare inversely proportional to their areas.While the terms and principles mentioned above arenot all that apply to the physics of fluids, they aresufficient to allow further discussion in this trainingmanual. It is recommended that Fluid Power,NAVEDTRA 12964 (latest edition), be studied for amore detailed and knowledgeable coverage of thephysics of fluids and basic hydraulic/pneumaticsystems.COMPONENTSSince fluids are capable of transmitting force and atthe same time flow easily, the force applied to the fluidatone point is transmitted to any point the fluid reaches.Hydraulic and pneumatic systems are assemblies ofunits capable of doing this. They contain a unit forgenerating force (pumps), suitable tubing and hoses forcontaining and transmitting the fluid under pressure, andunits in which the energy in the fluid is converted tomechanical work (cylinders and fluid motors). Inaddition, all operative systems contain valves andrestrictors to control and direct the flow of fluid and limitthe maximum pressure in the system.Because of the similarities of hydraulic andpneumatic systems (that is, from a training point ofview), only the components of hydraulic systems arecovered in this section. Remember that most of theinformation is also applicable to pneumatic systems andtheir components.PUMPSThe heart of any hydraulic system is its pumps; it isthe pump that generates the force required by theactuating mechanisms. The pump causes a flow of fluid;thus, the amount of pressure created in a system is notcontrolled by the pump but by the workload imposed onthe system and the pressure-regulating valves.Basically, pumps may be classified into two groupsbased on performance: (1) fixed delivery when running10-3

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