Pressure and Force in Fluid Power Systems

Since pressure is equal to the force per unit area, set A equal to 1. Then the formula pressure becomes P = h D Equation 2-5. It is essential that h and D be expressed in similar units. That is, if D is expressed in pounds per cubic foot, the value of h must be expressed in feet. If the desired pressure is to be expressed in pounds per square inch, the pressure formula, equation 2-5, becomes Equation 2-6. Pascal was also the first to prove by experiment that the shape and volume of a container in no way alters pressure. Thus in figure 2-8, if the pressure due to the weight of the liquid at a point on horizontal line H is 8 psi, the pressure is 8 psi everywhere at level H in the system. Equation 2-5 also shows that the pressure is independent of the shape and volume of a container. Pressure and Force in Fluid Power Systems Figure 2-9.—Force transmitted through fluid. of the shape of the container. Consider the effect of this in the system shown in figure 2-9. If there is a resistance on the output piston 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 force 1 is 100 pounds and the area of the input piston is 10 square inches, then the pressure in the fluid is 10 psi Recall that, according to Pascal’s law, any force applied to a confined fluid is transmitted in all directions throughout the fluid regardless NOTE: Fluid pressure cannot be created without resistance to flow. In this case, resistance Figure 2-8.—Pressure relationship 2-6 with shape.