What are the classifications of hydraulic pumps

Hydraulic fluid


Hydraulic fluid is used as a lubricant for hydraulic systems. In addition to engine oil, hydraulic oil is the second largest area of ​​lubricants: 150,000 t are used throughout Germany every year. Find out all the facts about hydraulic fluids and why the viscosity of hydraulic oil is crucial for top performance.

Hydraulic oil - lubricant for hydraulic systems

Hydraulic oil is a lubricant that is required for power transmission in hydraulic systems. As with classic motor oil, additives are also used in hydraulic oil. These cause an improvement in the properties of the oil, e.g. B. in terms of viscosity or the reduction of friction and wear.

The main tasks of hydraulic oil are:

  • lubrication
  • Corrosion protection
  • Transmission of hydraulic forces

Hydraulic fluid sets machine parts in motion under pressure. A pump in the hydraulic system pumps oil through a line, moving a piston at the end of the line. The law of levers means that very large forces can be transmitted even with a comparatively low pump output. Hydraulic oil is particularly widespread in industry, e.g. for the lubrication of machine tools, in rolling mills or for the lubrication of commercial vehicles with hydraulic elements.

Probably the best-known example of areas of application is the use of hydraulic fluid in power steering systems.

Viscosity of hydraulic oil

As with gear oils, the viscosity of the hydraulic fluids plays a decisive role. While automotive oils are divided into viscosity classes, the viscosity of industrial lubricants is determined by so-called viscosity groups (VG).

Viscosity describes the flow behavior or the viscosity of fluids and thus also the internal friction. If the wrong oil is used, either a sufficient lubricating film cannot be formed or the oil reaches the points to be lubricated too late due to the high viscosity.

The wrong viscosity class or viscosity group can have far-reaching negative effects. If the lubrication behavior is disturbed, the components of the system to be lubricated can quickly be damaged or even completely destroyed. In addition, if there is insufficient lubrication, more energy is consumed due to the increased friction. The wrong viscosity therefore means: more wear, more friction and more energy consumption. Depending on the area of ​​application and the climate of the corresponding hydraulic systems, hydraulic oils of different viscosities should be used.

Classification of hydraulic fluids

All lubricants that are used in hydraulic systems are summarized under the term hydraulic fluids.

The hydraulic fluids are divided into the following main groups:

Main group

Designations (excerpt)


Hydraulic fluid based on mineral oil

Sum of all hydraulic oils

Hydraulic fluid for use in the food and feed industry

Hydraulic oils specially designed for use in the industries listed

Environmentally friendly hydraulic fluid

Biodegradable fluids

Flame retardant hydraulic fluid

Fluids for use in areas with an increased risk of fire


The names of the different oils are specified in the relevant standards and are based on the different properties of the special fluids.

Hydraulic fluids based on mineral oil, for example, are often enriched with active ingredients for increased corrosion protection or special substances for protection against aging. In the case of hydraulic oils for the food and feed industry, on the other hand, the focus is more on food compatibility, since contact within production cannot always be avoided here.

Certain areas require the use of environmentally friendly lubricants. This need is covered with biodegradable hydraulic oils in various designs. Flame-resistant hydraulic fluids are used for hydraulic systems in areas where there is a risk of fire.

Common industrial hydraulic oils

Hydraulic fluids based on mineral oil are the most frequently used pressure fluids in hydraulic systems (hydraulic oils). The oils of the HLP type are particularly widespread in the industrial sector.

The name HLP is based on the special properties that HLP oil brings:

HLP hydraulic oil contains substances for better corrosion protection, various additives for use in high pressure areas and certain active ingredients for increased resistance to aging. The hydraulic oil HLP 46 is the most frequently used oil of the HLP hydraulic oils. It is assigned to ISO VG 46, i.e. it has a viscosity of 46 mm² / s at a temperature of 40 ° C. In comparison, the hydraulic oil HLP 32 has a lower viscosity of 32 mm² / s at the same temperature. The HLP 32 is therefore thinner than the HLP 46.

Use proper oil to prevent damage

In order to enable problem-free operation of your hydraulic systems and plants, it is essential to choose the right hydraulic oil. The specifications of the manufacturer of your hydraulic systems regarding the hydraulic fluid to be selected must be strictly adhered to, otherwise premature wear and performance losses are quickly inevitable.