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Guide to hydraulic systems and fluids
Guide to hydraulic systems and fluids
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Guide to hydraulic systems and fluids

03.09.2024

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What is hydraulic energy?

Hydraulic energy is energy obtained from the movement or pressure of liquids. It is often associated with water, used to produce electricity (hydroelectricity), but can also include other liquids in various industrial systems to transmit power or mechanical energy.

Where can you find hydraulic systems?

Thanks to their ability to produce large forces, hydraulic systems are used in a variety of sectors to lift, drive or move massive items with precision.
In the manufacturing industry, they are used for pressing, moulding and assembling parts. In construction, they power equipment such as excavators, bulldozers and cranes. In agriculture, tractors and combine harvesters incorporate hydraulic systems in front loaders, the rear lift for mounted machines and hydrostatic gearboxes. Heavy vehicles use them for brakes, power steering and other control mechanisms. The maritime sector uses hydraulics to operate rudders and lift anchors. Finally, this type of energy is essential in materials handling to lift heavy loads.

How does a hydraulic system work?

A hydraulic system is made up of a number of components: the tank, the pump, the hydraulic hoses, the directional control valve and the actuators.
  1. The hydraulic pump draws fluid from the reservoir. Inside, a mechanism such as a gear, piston or vane pressurises the fluid and pumps it to the distributor.
  2. The directional valve receives the fluid under pressure and directs it to the receiving components according to the operator's commands. For example, in a hydraulic cylinder, the directional valve directs the fluid to one of the chambers. The pressure in the chamber increases, pushing the piston and then the rod, generating a movement. This movement compresses the fluid in the 2nd chamber, forcing it back into the reservoir.
  3. Return to the reservoir : After activating the actuator, the fluid is returned to the reservoir to be reused.
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Hydraulic fluids

Their functions

Hydraulic fluid (also known as "hydraulic oil") plays many roles in this type of system:
  • Power transmission: Hydraulic power transmission always requires an electric motor (stationary applications) or an internal combustion engine (mobile applications) to drive the pump. It is this pump that generates the hydraulic energy. The hydraulic power is transmitted to a receiver (cylinder) through a circuit. The receiver moves a mechanical component, such as the arm of an excavator.
  • Lubrication: These oils lubricate and maintain a film between the surfaces of the various parts of the hydraulic system, reducing friction and therefore premature wear on moving mechanical components.
  • Cooling friction parts: As the fluid moves through the hydraulic circuit, it distributes the heat generated by friction and the operation of mechanical components. A water-oil heat exchanger (radiator) can be fitted to the hydraulic circuit for optimum fluid temperature control.
  • Protection against corrosion: Most hydraulic oils have properties that guarantee internal protection of mechanical components against corrosion and oxidation.
  • Evacuation of pollutants: The fluid collects and transports the pollutants to the system's filters.

The different families of hydraulic oil

Hydraulic oils are divided into four main families, adapted according to the manufacturer's recommendations and the conditions of use.
  • Mineral oils: These are economical but short-lived, sensitive to extreme temperatures, polluting and flammable. Used mainly in the automotive, agricultural, industrial and old equipment sectors.
  • Synthetic oils: Versatile, durable and resistant to extreme temperatures, they are nevertheless more expensive. Commonly used in industry, aeronautics, mining and public works.
  • Biodegradable oils: These are less toxic and break down naturally, making them ideal for applications where environmental impact needs to be reduced (marine equipment, forestry, hydroelectric power stations). More expensive, they are also vulnerable to high temperatures and water.
  • Water-based oils: Fire-resistant and less harmful to the environment, they are used in industries with a high risk of fire, such as metallurgy and mining. Less effective in cold weather and potentially corrosive, they require anti-corrosion additives.

Their important properties

The properties of a hydraulic fluid include viscosity, resistance to temperature variations and fire resistance.
  • Viscosity refers to resistance to flow and is determined by the viscosity index, based on the change in thickness of the fluid according to temperature variations. It is essential to use a fluid with a viscosity suited to the equipment: too viscous, it can cause loss of efficiency and cavitation; too fluid, it increases the risk of seizure and breakage.
  • Resistance to temperature variations corresponds to the temperature range within which the fluid can be used without losing its properties.
  • Fire resistance refers to a fluid's ability to minimise the risk of combustion, assessed by its flash point, ignition point and self-ignition point.

A hydraulic fluid also has environmental characteristics, as well as oxidation and water resistance properties.
 
These hydraulic fluids are susceptible to contamination by particles, water or gas, which can compromise system performance. To minimise these risks, installing suitable filters on the various parts of the hydraulic circuit is an effective solution. To find out more, please consult our dedicated article: [...]