Instructions for using hydraulic oil in hydraulic systems

Instructions for using hydraulic oil in hydraulic systems/1. Viscosity of fluids
When a liquid flows under external forces, the cohesive force between liquid molecules (known as internal friction) prevents relative sliding between liquid layers, and this property of the liquid is called viscosity. The magnitude of viscosity is represented by viscosity.

1) Dynamic viscosity μ: It is the internal friction coefficient that represents the viscosity of a liquid. It has been experimentally determined that the magnitude of the internal friction force between liquid layers in a flowing liquid is directly proportional to the contact area between the layers, the dynamic viscosity μ of the liquid, and the relative velocity between the layers, and inversely proportional to the relative distance between the layers. The higher the dynamic viscosity, the greater the frictional force inside the flowing liquid.

2) Kinematic viscosity υ: The ratio of dynamic viscosity to the density of the liquid is called kinematic viscosity, which is υ=μ/ρ. The unit of kinematic viscosity in SI is m2/s. The grade of hydraulic oil is labeled based on the average value of its kinematic viscosity (mm2/s) at 40 ℃. For example, the average kinematic viscosity of L-HL32 ordinary hydraulic oil at 40 ℃ is 32 mm2/s.

3) The relationship between viscosity, temperature, and pressure: The viscosity of hydraulic oil is highly sensitive to temperature changes, and as the temperature increases, the viscosity will significantly decrease. The property of viscosity of hydraulic oil changing with temperature is called viscosity temperature characteristic, and different types of hydraulic oil have different viscosity temperature characteristics. When the pressure on a liquid increases, its intermolecular distance decreases, cohesion increases, and viscosity also increases.

2、 Compressibility of liquids
The property of a liquid that undergoes a change in volume under pressure is called its compressibility. For general medium and low-pressure hydraulic systems, the compressibility of the liquid is very small, and it can be considered that the liquid is incompressible.
Selection of hydraulic oil

3、 Types of hydraulic oil
Hydraulic oil is mainly divided into three categories: petroleum type, emulsion type, and synthetic type. Petroleum hydraulic oil is divided into ordinary hydraulic oil, hydraulic rail oil, anti-wear hydraulic oil, low-temperature hydraulic oil, high viscosity index hydraulic oil, and specialized hydraulic oil. Petroleum hydraulic oil has the advantages of good lubrication performance, low corrosiveness, high viscosity, and good chemical stability, and is widely used in hydraulic transmission systems.
Synthetic hydraulic oil mainly includes water ethylene glycol, phosphate ester liquid, and silicone oil, while emulsion hydraulic oil is divided into two categories: oil in water emulsion (L-HFAE) and water in oil emulsion (L-HFB).

4、 Selection of hydraulic oil
When selecting hydraulic oil, it is generally recommended to choose the working medium based on the hydraulic component product samples and instructions. Alternatively, it can be comprehensively considered based on the working conditions of the hydraulic system (system pressure, movement speed, working temperature) and environmental conditions. Usually, the viscosity range is determined first, and then the type of hydraulic oil is selected. Please also pay attention to the special requirements of the hydraulic system. For systems operating under low temperature conditions, it is advisable to use oils with lower viscosity, while for high-pressure systems, oils with good anti-wear properties should be selected. For the overall power loss, it is advisable to use hydraulic oil with lower viscosity.