# Properties of Manifold Block hydraulic oil

Before the introduction of “hydraulic oil types and uses”, now introduce the properties of hydraulic oil.

1. Density of hydraulic fluid.

The mass per unit volume of a liquid is called density. Density varies with temperature or pressure, but the change is small and usually negligible. Industrial hydraulic oil is mineral oil with a density of 0.85 ~ 0.95g/cm3. Water-in-oil hydraulic oil contains more oil and its density is about 0.92-0.94 g/cm3. The oil-in-water hydraulic oil contains more water and its density is about 1.05 ~ 1.1g/cm3.

In general calculation, the hydraulic oil is mineral oil, density ρ=900kg/m3.

2. Viscosity of hydraulic fluid.

(1) Newton’s law of internal friction liquid flow under the action of external force, due to the cohesive attraction between liquid molecules and produce a resistance to the relative motion of liquid molecules of internal friction. The property of internal friction between the molecules of a fluid as it flows is called viscosity. A liquid is viscous only when it is flowing. The viscosity of hydraulic oil has great influence on mechanical efficiency, abrasion, pressure loss, volumetric efficiency, oil leakage and suction of pump. As shown in FIG. 1-4, the flow between parallel plates is taken as an example. It is assumed that the upper plate moves to the right with the velocity U, while the lower plate is fixed. The fluid attached to the upper plate adheres to the upper plate at the same rate as the upper plate. The fluid attached to the lower plate adheres to the lower plate with zero velocity. The velocity of the intermediate fluid is linearly distributed. We think of this flow as the movement of infinitely thin layers of fluid, and as the faster moving layer slides over the slower one, internal friction arises between the two layers due to viscosity.

According to the actual measured data, the internal friction force F between the fluid layers is directly proportional to the contact area A of the fluid layer and the relative velocity du of the fluid layer, and inversely proportional to the distance dy between the layers of the second flow body, namely:

τ=F/A to represent the internal friction shear stress, then:

Schematic diagram of the viscosity of a liquid

Figure 1-4 Schematic diagram of the viscosity of a liquid

This is Newton’s law of internal friction. In the formula, μ is the proportionality constant, also known as viscosity coefficient or dynamic viscosity.

(2) Viscosity The viscosity of a liquid is expressed in terms of viscosity. There are three commonly used types of viscosity: dynamic viscosity, kinemical viscosity and relative viscosity.

(1) Dynamic viscosity represents the internal friction coefficient of fluid viscosity, also known as absolute viscosity, expressed in μ, the unit is Pa·s (Pa· second).

The ratio of dynamic viscosity and liquid density is called kinemative viscosity, expressed by ν, the unit is m2/s, the commonly used unit is St (S), cSt (s), the conversion relationship is 1m2/s=104cm2/s(St) = 106mm2/s(cSt), as shown in Table 1-1 for the commonly used hydraulic oil grade and movement.

Grades and kinematic viscosity of commonly used hydraulic fluids

Table 1-1 Grades and kinematics viscosity of commonly used hydraulic oils

Viscosity is the main performance index of hydraulic oil. It is customary to use kinematic viscosity to calibrate the viscosity of a liquid. For example, the value of a mechanical oil grade is the value of its average viscosity of motion at 40℃ (cSt).

The preparation method and detailed meaning of hydraulic oil grades can be referred to the relevant hydraulic manual.

③ Relative viscosity, also known as conditional viscosity, is the viscosity of the liquid measured by a specific viscometer under specified conditions. China and Germany and other countries adopt Engler viscosity °E, the United States adopts Sayler viscosity, the United Kingdom adopts Reye viscosity. Engler viscosity °E was measured by engler viscometer. 200mL of the measured liquid into the vessel of the viscositeter, evenly heated to a certain temperature T, liquid from the bottom of the 2.8mm hole flow required time t1, and then measured the same volume of distilled water at 20℃ through the same hole required time T2, t1 and T2 ratio is the measured liquid at this temperature T engler viscosity °Et.

(3) The relationship between viscosity and pressure When the pressure of the liquid increases, the distance between the molecules decreases, the cohesion increases, and the viscosity also increases slightly. In the medium and low pressure system, the pressure of hydraulic oil changes very little, and the influence of pressure on the viscosity can be ignored. When the pressure is high (greater than 10MPa) or the pressure changes are large, the influence of pressure on the viscosity should be considered.

(4) The relationship between pressure and temperature Hydraulic oil is very sensitive to the change of temperature, temperature rise, viscosity decreases; As the temperature drops, the viscosity increases. The variation of viscosity with temperature of this oil is called viscosity-temperature property. FIG. 1-5 shows the viscosity-temperature curves of several commonly used domestic hydraulic oils. Viscosity decreases, resulting in increased leakage, wear, efficiency and other problems; Viscosity increase, resulting in difficult flow and pump rotation is not easy and other problems. If the oil temperature exceeds 60℃ when working, it is necessary to install a cooler, because the oil temperature is above 60℃, every more than 10℃, the deterioration rate of oil will double. We hope that the viscosity and temperature characteristics of hydraulic oil are good, that is, the viscosity changes with temperature as little as possible.

Viscosity – temperature curves of some domestic hydraulic oils

Figure 1-5 Viscosity-temperature curves of several domestic hydraulic oils

3. Compressibility of hydraulic oil.

Hydraulic oil at low and medium pressure can be regarded as incompressible liquid, but at high pressure, compressibility can not be ignored. The compressibility of hydraulic oil is 100 ~ 150 times that of steel, that is, comparable to the compressibility of wood. Compressibility will reduce the accuracy of the movement, increase the pressure loss, and make the oil temperature rise, in the pressure signal transmission, there will be time delay, poor response and other phenomena.

Hydraulic oil has other properties, such as stability, foam resistance, deemulsification, fire resistance, rust resistance, lubrication and compatibility. These properties are achieved by adding various additives to the hydraulic oil.