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 10-1, the pressure is the same because the force is concentrated on a relatively small area.
This pressure of 10 psi acts on all parts of the fluid container, including the bottom of the output piston 2; therefore, the upward force on the 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 the 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 the 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 the input force. Sometimes such results are desired.
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.
In the systems shown in views A and B of figure 10-1, 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 will, in turn, overcome the resistance force. Assume that the input piston is forced downward 1 inch. This displaces 10 cubic inches of fluid. Since liquid is practically incompressible, this volume must go some place. In the case of a gas, it will compress momentarily but will eventually expand to its original volume at 10 psi. This is provided, of course, that the 100 pounds of force is still acting on the input piston. Thus 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 equal 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 1 inch, only 2 cubic inches of fluid is displaced. The output piston 2 will have to move only one-tenth of an inch 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 the same fluid power system, which is the distances moved are inversely proportional to their areas.
While the terms and principles mentioned above are not all that apply to the physics of fluids, they are sufficient to allow further discussion in this training manual. It is recommended that Fluid Power, NAVEDTRA 12964 (latest edition), be studied for a more detailed and knowledgeable coverage of the physics of fluids and basic hydraulic/pneumatic systems.
Since fluids are capable of transmitting force and at the same time flow easily, the force applied to the fluid atone point is transmitted to any point the fluid reaches. Hydraulic and pneumatic systems are assemblies of units capable of doing this. They contain a unit for generating force (pumps), suitable tubing and hoses for containing and transmitting the fluid under pressure, and units in which the energy in the fluid is converted to mechanical work (cylinders and fluid motors). In addition, all operative systems contain valves and restrictors to control and direct the flow of fluid and limit the maximum pressure in the system.
Because of the similarities of hydraulic and pneumatic systems (that is, from a training point of view), only the components of hydraulic systems are covered in this section. Remember that most of the information is also applicable to pneumatic systems and their components.
The heart of any hydraulic system is its pumps; it is the pump that generates the force required by the actuating mechanisms. The pump causes a flow of fluid; thus, the amount of pressure created in a system is not controlled by the pump but by the workload imposed on the system and the pressure-regulating valves.
Basically, pumps may be classified into two groups based on performance: (1) fixed delivery when runningContinue Reading