Basic Principle of a Hydraulic System Operation

In a hydraulic system, a liquid fluid functions as a power transmission.  The liquid fluid commonly used in hydraulic systems is mineral oil.  In principle, hydromechanics is divided into two branches of study, namely:


Hydrodynamics is also called hydrokinetics concerning the use of fluids in power transmission at an open place.  One of the applications is the use of fluid flow on a turbine.


Hydrostatics concerns any pressure working on a confined area or place containing a fluid (in hydraulic systems).

The basic principle of hydrostatics is Pascal’s Law.  The main difference between the two branches of hydromechanics is seen from the liquid fluid itself.  In a hydrodynamic system, the pressure is generated by a prime unit, that is a hydraulic pump.  In a hydrostatic system, the pressure is generated by the difference in the liquid fluid level (liquid power generator).

Diagram 2-2 shows the movement of a hydraulic oil that converts the heat or electric energy into hydraulic or mechanical energy.

A hydraulic system functions to transmit an amount of power by pushing a certain amount of fluid.  The pressurized oil is pushed by a pump and converted into mechanical movements by working elements.

The working elements can produce two kinds of movement, namely:

  1. Linear movement, produced by the cylinder
  2. Rotary movement, produced by the hydraulic motor.



  1. Rotary mover or motor provide a rotary movement to the system. It can be connected to a pulley, a wheel, a conveyor, etc.
  2. Rotary drive shaft

Diagrams 2-9 and 2-10 show the hydraulic oil flow in a hydraulic system.  The hydraulic oil is accumulated in a reservoir and pumped out at a certain pressure.  This pressure pushes the piston in the cylinder and push the oil out and return to the reservoir.

Based on the two diagrams, a hydraulic system seems very simple.  In principle, a hydraulic system cannot work automatically by itself.  It requires accurate movements, security and safety, energy saving.

All of the above requirements must be met by providing the hydraulic system with control valves.  Detailed information on control valves will be discussed in the next unit.


In principle, a hydraulic system cannot drive the working elements without a loss of heat.

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