Thrust and Pressure - Hydraulic Machines




Hydraulic Machines:

Pascal’s law for transmission of fluid pressure states that the pressure exerted anywhere in a confined incompressible fluid is transmitted undiminished and equally in all directions throughout the fluid.

The above law means that if we consider a fluid that is restricted within a specific region in space and if the volume of the fluid doesn’t change with the pressure, then the amount of pressure exerted will be the same as the amount of pressure transmitted.

A number of devices, such as hydraulic lift and hydraulic brakes, are based on Pascal’s law. In these devices, fluids are used for transmitting pressure. In a hydraulic lift, as shown in Fig, two pistons are separated by the space filled with a liquid. A piston of small cross-section `"A"_1` is used to exert a force `"F"_1` directly on the liquid. The pressure `"P" = "F"_1/"A"_1` is transmitted throughout the liquid to the larger cylinder attached with a larger piston of area `"A"_2`, which results in an upward force of P × `"A"_2`. Therefore, the piston is capable of supporting a large force (large weight of, say a car or a truck placed on the platform `"F"_2 = "PA"_2 = "F"_1/"A"_1"A"_2`. By changing the force at `"A"_1` the platform can be moved up or down. Thus the applied force can be increased by a factor of `"A"_2/"A"_1` and this factor is the mechanical advantage of the device.

Hydraulic brakes work on the principle of Pascal’s law.

  • According to this law whenever pressure is applied on fluid it travels uniformly in all the directions.

  • Therefore when we apply force on a small piston, the pressure gets created which is transmitted through the fluid to a larger piston. As a result of this larger force, uniform braking is applied on all four wheels.

  • As braking force is generated due to hydraulic pressure, they are known as hydraulic brakes.

  • Liquids are used instead of gas as liquids are incompressible.


  • The fluid in the hydraulic brake is known as brake fluid.

  • It consists of a master cylinder, four-wheel cylinders, and pipes carrying brake fluid from the master cylinder to wheel cylinders.

  • The master cylinder consists of a piston that is connected to pedal through the connecting rod.

  • The wheel cylinders consist of two pistons between which fluid is filled.

  • Each wheel brake consists of a cylinder brake drum. This drum is mounted on the inner side of the wheel. The drum revolves with the wheel.

  • Two brake shoes that are mounted inside the drum remain stationary.


  • When we press the brake pedal, the piston in the master cylinder forces the brake fluid through a linkage.

  • As a result, pressure increases and gets transmitted to all the pipes and to all the wheel cylinders according to Pascal’s law.

  • Because of this pressure, both the pistons move out and transmit the braking force on all the wheels.


  • Equal braking effort to all the four wheels.

  • Less rate of wear due to the absence of joints.

  • By just changing the size of one piston and cylinder, the force can be increased or decreased.


  • Leakage of brake fluid spoils the brake shoes.

  • Even the slightest presence of air pockets can spoil the whole system.

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