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What is Hydraulic System?

Hydraulic systems are used in large range of mechanical applications, from cranes, lifters, assembly processes to steel mills, paper mills, uses and many others. Hydraulic systems enable man to do significant work such as lifting heavy loads, turning a shaft, drilling precision holes, etc… with least investment in linkage of mechanical type, referring to Pascal’s law, which states:

Confined Liquid

Pressure applied to a confined fluid at any point is transmitted undiminished throughout the fluid in all directions and acts upon every part of the confining vessel at right angles to its interior surfaces and equally upon equal areas

Pascal’s law

A hydraulic drive system is a drive or transmission system that uses pressurized hydraulic fluid to power hydraulic machinery in hydraulic. The term hydrostatic refers to the transfer of energy from flow and pressure, not from the kinetic energy of the flow.

History of Hydraulic System

In the start of the industrial revolution, a British mechanic name of Joseph Bramah applied the rule of Pascal’s law in the design/development of the first hydraulic press in the history. In 1795, he patented the hydraulic press, by the name of the Bramah press. Bramahfound that if a smaller force on a smaller area would cause a proportionally larger force on a bigger area, then the only limit/extent to the force that a machine can apply will be the area the pressure is applied.

A hydraulic drive system in hydraulic system comprises of three components:


  • A hydraulic pump, driven by electric motor
  • An engine, valves, filters, piping etc… (to guide and control the system); and
  • The actuator (e.g. a cylinder-hydraulic or hydraulic motor) for driving the machines.

By using the Pascal’s law and Brahma’s using this, it is clear that an input force of 100 pounds on 10 square inches will cause a pressure of 10 pounds per square inch throughout the confined vessel/container. This pressure wills hold/support 1000-pounds of weight if the area of the weight is 100 square inches.

The Pascal’s law principle is realized/used in a hydraulic system by the means of hydraulic fluid/liquid which is utilized to transmit the energy from one point to another point. Because the hydraulic fluid is almost incompressible, it is able to transmit instantaneously the power to other point.


Hydraulic System Components

Hydraulic Reservoirs, Filters, Pumps, Accumulators and Motors

The key components that comprise a hydraulic system are the reservoir / tank, pump, valve(s) and the actuator(s) (cylinder, motor, etc...). A means of storing hydraulic fluid and minimizing contamination is necessary to any hydraulic system. These functions are performed by reservoirs and filters. The component which causes fluid flow in a hydraulic system. The heart of any hydraulic system can be a hand pump, power-driven pump, accumulator, or any combination of the three. Finally, a means of converting hydraulic pressure to mechanical rotation is sometimes necessary, and this is accomplished by a hydraulic motor.

Hydraulic System Reservoir


The hydraulic reservoir is a container for holding the fluid required to supply the system, including a reserve to cover any losses from minor leakage and evaporation. The reservoir can be designed to provide space for fluid expansion, permit air entrained in the fluid to escape, and to help cool the fluid.

Hydraulic System reservoirs are either vented to the atmosphere or closed to the atmosphere and pressurized. A description of each type follows.

Vented Reservoir. A vented reservoir is one that is open to atmospheric pressure through a vent line. Because atmospheric pressure and gravity are the forces which cause the fluid to flow to the pump, a vented reservoir is mounted at the highest point in the hydraulic system. Air is drawn into and exhausted from the reservoir through a vent line. A filter is usually installed in the vent line to prevent foreign material from being taken into the system.

Pressurized Reservoir. A pressurized reservoir is sealed from the atmosphere. This reservoir is pressurized either by engine bleed air or by hydraulic pressure produced within the hydraulic system itself. Pressurized reservoirs are used on aircraft intended for high altitude flight, where atmospheric pressure is not enough to cause fluid flow to the pump.

Pump-Driven Hydraulic Pumps


In hydraulic system the power-driven pumps receive their driving force from an external power source. This force is converted into energy in the form of fluid pressure. The four basic types of power-driven hydraulic pumps are gear, vane, diaphragm, and piston. Of these, the piston type is most commonly found in Army aircraft. The reason for this is that it operates more efficiently at higher pressures and has a longer life than any of the others. The Piston pumps used in hydraulic systems are further categorized as either constant delivery or variable delivery. Pumps in the hydraulic system are coupled to their driving units by a short, splined coupling shaft, commonly called a drive coupling.

Hydraulic Accumulators

The purpose of a hydraulic accumulator in a hydraulic system is to store hydraulic fluid under pressure. It may be used to: Dampen hydraulic shocks which may develop when pressure surges occur in hydraulic systems and/or Add to the output of a pump during peak load operation of the system, making it possible to use a pump of much smaller capacity than would otherwise be required to:

  • Absorb the increases in fluid volume caused by increases in temperature.
  • Act as a source of fluid pressure for starting aircraft auxiliary power units (APUs).
  • Assist in emergency operations.

Hydraulic Motors


Hydraulic motors are installed in a hydraulic system to use hydraulic pressure in obtaining powered rotation. A hydraulic motor does just the opposite of what a power-driven pump does. A pump receives rotative force from an engine or other driving unit and converts it into hydraulic pressure. A hydraulic motor receives hydraulic fluid pressure and converts it into rotative force in the hydraulic.


The basic components of any hydraulic system are reservoirs, filters, and pumps (hand or power-driven). The reservoir holds the fluid supply for the system and helps cool the fluid. Filters are used to ensure that no contamination reaches the components in a hydraulic system. The pleated micronic filter is the most common in the hydraulic system. The pump converts mechanical energy to fluid flow. The most common power-driven pump is the piston pump. In all but the simplest hydraulic systems, variable-delivery pumps are used. A variable delivery pump delivers only the amount of fluid demanded by the system. This is accomplished through the use of a compensator. Depending on the application, hydraulic accumulators and hydraulic motors can also be found in the hydraulic system. Accumulators are used primarily to supply pressure for starting auxiliary power units and emergency hydraulic pressure. Hydraulic motors in the hydraulic system perform a variety of functions, e.g. in case of aircraft; raising and lowering cargo doors, operating rescue hoists, and positioning wing flaps.


Basic Hydraulic System With A Linear Hydraulic Actuator

Updated: Sep 24, 2015