Pascal’s law states that the pressure exerted on a confined fluid is transmitted undiminished in all directions and acts with equal force on equal areas and at right angles to the containing surfaces. In Fig. , a force is being applied to a piston, which in turn exerts a pressure on the confined fluid. The
pressure is equal everywhere and acts at right angles to the containing surfaces. Pressure is defined as
the force acting per unit area and is expressed as
where F is the force acting on the piston, A is the area of the piston and p is the pressure on the fluid.
Practical Applications of Pascal’s Law
The practical applications of Pascal’s law are numerous. In this section, two applications of Pascal’s
law are presented:
(a) The hand-operated hydraulic jack and
(b) the air-to-hydraulic pressure booster.
Hand-Operated Hydraulic Jack
This system uses a piston-type hand pump to power a hydraulic load cylinder for lifting loads, as
illustrated in Fig..The operation is as follows:
1. A hand force is applied at point A of handle ABC which is pivoted at point C. The piston rod of the pump cylinder is pinned to the input handle of the pump piston at point B.
2. The pump cylinder contains a small-diameter piston that is free to move up and down. The piston and rod are rigidly connected together. When the handle is pulled up, the piston rises and creates a vacuum in the space below it. As a result, the atmospheric pressure forces the oil to leave the oil tank and flow through check valve 1 to fill the void created below the pump piston. This is the suction process.
3. A check valve allows flow to pass in only one direction, as indicated by the arrow. When the handle is pushed down, oil is ejected from the small-diameter pump cylinder and it flows through check valve 2 and enters the bottom end of the large-diameter load cylinder.
4. The load cylinder is similar in construction to the pump cylinder and contains a piston connected to a rod. Pressure builds up below the load piston and equals the pressure generated by the pump piston. The pressure generated by the pump piston equals the force applied to the pump piston rod divided by the area of the pump piston.
5. The load that can be lifted equals the product of the pressure and the area of the load piston. Also, each time when the input handle is cycled up and down, a specified volume of oil is ejected from the pump to raise the load cylinder a given distance.
6. The bleed valve is a hand-operated valve, which, when opened, allows the load to be lowered by bleeding oil from the load cylinder back to the oil tank.
|Fig. Application of Pascal law|