HYDRAULIC MOTOR EXPLANATION GENERALLY ACCEPTED CONDITIONS The accepted Laws of Thermodynamics describe the working capability of pressurized fluid. This is addressed in the text, "Fundamentals of Engineering Thermodynamics", written by Shapiro and Moran; section 2.2.3, "Work in Quasiequilibrium Expansion or Compression Processes". The Laws of Thermodynamics establish that a piston will move in the direction of lesser force until the forces on both sides of the piston are equal. If the force is reduced on one side, the piston movement will continue until equilibrium has been regained. Shapiro and Moran used a conventional piston, with incremental masses as a load to illustrate this fact. (FIGURE 1) (Page 13.293) A piston with a ten square inch boundary face and a fluid pressure of 10 PSIG would reach equilibrium if the mass it was lifting weighed one hundred pounds. The potential work in the lifted mass has a linear relationship to the work required to put the pressurized fluid into the piston "B" and they are equal at all times equilibrium has been reached. The same fact can be demonstrated by linking the drive rods of two identical pistons. (FIGURE 2) (Page 13.293) If both pistons have the same pressure applied to them the force of each piston would be equal and there would be no movement. If either piston's pressure were reduced, the piston with the higher pressure would attain a force advantage and start to push back the other piston. If the pressures become equal at some point, the movement will stop because the forces on the pistons will have become equal; therefore, the system will have regained equalibrium. At mid-point of the pistons' stroke, the amount of work to provide the pressurized fluid to both pistons is equal and the work relationship potential is linear. EXAMPLE: Consider both pistons to have a rectangular drive face of one inch by ten inches and a maximum stroke of ten inches. When 10 PSIG pressure is applied to the pistons, they would both exert a force of one hundred pounds. Equalibrium, at the middle of their strokes, would require the same volume of fluid applied to each piston. Consider the pistons arranged as in FIGURE 3 (Page 13.294). Piston "A" is filled with fluid and connected to a cushion tank "C" that is able to accept piston "A's" volume at a constant pressure of 10 PSIG. Piston "B" receives fluid from another source. When piston "B" receives fluid pressurized to about 10.01 PSIG it will start to displace piston "A's" fluid into cushion tank "C". 13.291
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