Physical Properties of Hydraulic Transmission Oil
Posted on 20/03/2024
1. Viscosity
The physical quantity indicating the size of viscosity is called viscosity. The viscosity determines the fluidity of hydraulic oil and has a great impact on the flow characteristics and pressure loss of the hydraulic system.
1.Viscosity
(1) Dynamic viscosity μ Two layers of liquid with an area of one square centimeter each and one centimeter apart move relative to each other at a speed of 1 cm/second. The resistance generated at this time is dynamic viscosity, or absolute viscosity. The absolute unit is poise (dynes·second/cm2), and the engineering unit is kilogram·second/m2.
1 poise = 100 centipoise
1kg·m2=98.1≈100 poise
(2) Kinematic viscosity ν Kinematic viscosity is the ratio of dynamic viscosity to density μ/ρ, represented by the symbol ν. Its absolute unit is Tu (cm2/s) and the engineering unit is m2/s.
1 stoke = 100 centistokes
1 meter 2 / second = 104 turrets
(3) Relative viscosity °E is used to express the viscosity of a liquid relative to the viscosity of water. It is also called Engler's viscosity and is represented by the symbol °E. It is often used in engineering.
Engler's viscosity is measured as the ratio of the time t1 required to flow out 200 ml from a φ2.8 mm orifice at a certain temperature to the time t2 required for distilled water to flow out the same volume at 20°C. °E=t1/t2
Engler's viscosity and kinematic viscosity can be converted according to the following formula
ν=7.31°E-6.31/°E (centistokes)
2. The relationship between viscosity, temperature and pressure. When the temperature rises, the viscosity of the oil decreases. This phenomenon is more obvious below 50°C, and the change is relatively gentle between 50°C and 100°C. Good viscosity-temperature performance means that the smaller the change in viscosity with temperature, the better.
When the pressure increases, the viscosity of the oil increases. Generally, when it is below 300kgf/cm2, the viscosity and pressure are almost in a linear relationship with little change. When pressure is extremely high, viscosity increases dramatically.
3. The selection of oil viscosity is generally based on the type of hydraulic pump, operating temperature and system pressure, see the table below.
Recommended viscosity range Centistokes (50°C)
Operating temperature ℃ | 5～40 | 40～80 | |
Vane Pump | Below 70kgf/cm2 | 17～29 | 25～44 |
Above 70kgf/cm2 | 31～40 | 37～54 | |
Gear Pump | 17～40 | 63～88 | |
Axial piston pump | 25～44 | 40～98 | |
Radial plunger pump | 17～62 | 37～154 | |
Note: When the pressure is high, the temperature is high, and the movement speed is low, take the larger value.
2. Compressibility
The magnitude of hydraulic compressibility is expressed by the compression coefficient βV. It is equivalent to the change in liquid volume for every 1 kgf/cm2 increase in pressure.
βV＝(1/Δp)·(ΔV/V0) (cm2/kgf)
The reciprocal of the volume compressibility coefficient is called the volume elastic coefficient, represented by E, that is, E=1/βV (kgf/cm2)
When the pressure p≤150kgf/cm2 and the temperature t=20℃, the volume compressibility coefficient of various oils is generally βV=(5~7.5) (cm2/kgf).
The volume compressibility coefficient βV increases with the increase of temperature and decreases with the increase of pressure. The higher the viscosity, the greater the gas content, and the greater the βV value.
3. Expansion
The size of hydraulic expansibility is expressed by the volume expansion coefficient βt. It represents the relative change in liquid volume when the temperature increases by 1°C.
βt＝(1/Δt)·(ΔV/V0) (1/℃)
The oil used in the hydraulic system is generally βt=(8.5~9) ×10-4/℃
4. Hydraulic oil selection
The selection of hydraulic oil should take into account the requirements of each component and other working conditions.
1. Appropriate viscosity, good viscosity-temperature properties, and low compressibility.
2. Good oxidation stability and no deterioration during long-term operation.
3. Good lubrication performance and strong anti-corrosion ability.
4. Good anti-foaming and anti-emulsification properties.
5. Does not contain water-soluble acid and alkali, and has no corrosion to hydraulic components and sealing devices.
6. No impurities and sediments.
7. High ignition point, low-temperature oil requires low freezing point.
Generally, mechanical oil (No. 10, No. 20, No. 30), diesel engine oil (No. 8), and gasoline engine oil (No. 22, No. 30) are commonly used in hydraulic transmission systems. Hydraulic systems with higher requirements can use special hydraulic oil.
