Practical Applications of Pascal’s Law – Basic Of Fluid Mechanics
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.
Air-to-Hydraulic Pressure Booster
This device is used for converting shop air into higher hydraulic pressure needed for operating hydraulic cylinders requiring small to medium volumes of higher pressure oil. It consists of a cylinder containing a large-diameter air piston driving a small-diameter hydraulic piston that is actually a long rod connected to the piston. Any shop equipped with an airline can obtain smooth, efficient hydraulic power from an air-to-hydraulic pressure booster hooked into the air line. The alternative would be a complete hydraulic system including expensive pumps and high-pressure valves. Other benefits include space savings and low operating and maintenance costs.
Figure shows an application where an air-to-hydraulic pressure booster supplies high-pressure oil to a hydraulic cylinder whose short stroke piston is used to clamp a workpiece to a machine tool table. Since shop air pressure normally operates at 100 psi, a pneumatically operated clamp would require an excessively large cylinder to rigidly hold the workpiece while it is being machined.
The air-to-hydraulic pressure booster operates as follows. Let us assume that the air piston has 10 cm2 area and is subjected to a 10 bar air pressure. This produces a 1000 N force on the booster’s hydraulic piston. Thus, if the area of the booster’s hydraulic piston is 1 cm2
, the hydraulic oil pressure is 100 bar. As per Pascal’s law, this produces 100 bar oil at the short stroke piston of the hydraulic clamping
cylinder mounted on the machine tool table.
The pressure ratio of an air-to-hydraulic pressure booster can be found by using the following equation:
Pressure ratio = Output oil pressure /Input oil pressure
= Area of air piston/ Area of hydraulic piston
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