Physical properties of hydraulic fluid There are many basic properties of hydraulic fluid, and three of them are closely related to hydraulic transmission performance for an introduction.
(1) Density The mass per unit volume of liquid is called the density of the liquid, that is, the density of the p-liquid in the formula P=mv; v-—the volume of a liquid; m – the mass of the liquid. The density values of commonly used hydraulic transmission hydraulic fluids are shown in Table 2-2. Table 2_2 Density of commonly used hydraulic transmission hydraulic fluid (20 °C) hydraulic fluid density / (kg.m-3) hydraulic fluid density / (kg.m-3) anti-wear hydraulic fluid L-HM32 anti-wear hydraulic fluid L-HM46 water-in-oil emulsion L-HFB oil-in-water emulsion L-HFAE 0. 87 x l03 0. 875 x l03 0. 932 x l030. 9977 x l03 Water Ethylene Glycol Hydraulic Fluid L-HFC General Phosphate Ester Hydraulic Fluid L-HFDR Aircraft Phosphate Ester Hydraulic Fluid L-HFDR No. 10 Aviation Hydraulic Fluid 1. 06 x l031. 15 x l031. 05 x l030. The density of an 85 x l03 liquid varies with pressure or temperature, but the amount of change is generally small and negligible in engineering calculations.
(2) Compressibility The property of volume reduction of liquid due to increased pressure is called compressibility. If the volume of the liquid is vo when the pressure is Po, the pressure increases △p, and the volume of the liquid decreases △y, then the relative change of the volume of the liquid under the change of unit pressure is in the formula, and k is called the liquid compression ratio. Since the volume of the liquid decreases as the pressure increases, the two change in opposite directions, and in order for k to be positive, a negative sign must be added to the right of the above formula. The reciprocal of the liquid compression ratio K is called the liquid volumetric elastic modulus (hereinafter referred to as the bulk modulus), and Table 2-3 shows the bulk modulus of various hydraulic fluids. From the value of the bulk modulus of petroleum-based hydraulic oil in the table, it can be seen that its compressibility is 100-150 times that of steel (the elastic modulus of steel is 2.1×l05 MPa).
Bulk modulus of various hydraulic fluids (20. C, atmospheric pressure) hydraulic fluid bulk modulus K/lVfPa hydraulic fluid bulk modulus K/MPa petroleum-based hydraulic oil-in-water emulsion, water-in-oil emulsion(1.4 ~2) xl031. 95 x l032. 3 x L03 water. Ethylene glycol hydraulic fluid, phosphate ester, hydraulic fluid water 3. 45 x l032. 65 x l032. 4 x L03 The liquid enclosed in the container is very much like a spring under the action of external force: the external force increases, the volume decreases; The external force decreases and the volume increases. After the stiffness of this spring.
(3) Viscosity
1. The performance of viscosity When the liquid flows under the action of external force, the presence of intermolecular cohesion forces restricts its flow, thereby generating internal friction along its interface, which is called the viscosity of the liquid. Figure 2-2 is used as an example to illustrate the viscosity of liquids. If the distance of ^ between two parallel plates is filled with liquid, the lower plate is fixed, and the upper plate is at speed II. Translate to the right.
Due to the adhesion between the liquid and the solid wall and the viscosity of the liquid, the velocity of each liquid layer inside the flowing liquid will be unequal: the velocity of the liquid layer next to the lower plate is zero, the velocity of the liquid layer next to the upper plate is "0", and the speed of the liquid in the middle layer is distributed linearly decreasing from top to bottom when the distance between the layers ^ is small. Among them, the fast liquid layer drives the slow speed; The slow liquid layer has a blocking effect on the fast speed. Experimental measurements show that the internal friction Ff between adjacent liquid layers of the flowing liquid is proportional to the contact area A of the liquid layer and the velocity gradient du/dy between the liquid layers, and the scaling coefficient p is called absolute viscosity or dynamic viscosity. If the following represents the shear stress between the liquid layers, that is, the internal friction force per unit area, then the above equation can be expressed as Newton's law of internal friction in liquids. Figure 2-2 Schematic diagram of liquid viscosity It can be seen from the above equation that in a still liquid, the velocity gradient du/dy = 0, so its internal friction is zero, so the still liquid is not viscous, and the liquid only shows its viscosity when flowing.
2. Measurement of viscosity A physical quantity that measures the amount of viscosity is called viscosity. There are three commonly used viscosities, namely absolute viscosity (dynamic viscosity), kinematic viscosity, and relative viscosity.
(1) Absolute viscosity (dynamic viscosity) u From equation (2-6), it can be seen that absolute viscosity anus is a viscous coefficient that characterizes the friction in a flowing liquid. Its magnitude is equal to the internal friction force per unit area when the liquid flows at a unit velocity gradient, that is, u=r/y (2-7) In China's legal unit of measurement system and SI system, the unit of absolute viscosity p is Pa.s (Pa·s) or expressed by N.s/m2 (N·s/m2). If the absolute viscosity is only related to the type of liquid and not to the velocity gradient, then the liquid is called a Newtonian liquid, otherwise it is a non-Newtonian liquid. Petroleum-based hydraulic fluids are generally Newtonian fluids.
(2) Kinematic viscosity p The ratio of the absolute viscosity of the liquid to its density is called the kinematic viscosity z of the liquid, that is, in China's legal unit of measurement system and SI system, the unit of kinematic viscosity ∥ is -2/s (m2/s). Because there are only dimensions of length and time, it is called kinematic viscosity. The international standard ISO divides the viscosity grade (VG) of the oil according to the kinematic viscosity value, as shown in Table 2-4. Table 2-4 Commonly used hydraulic oil kinematic viscosity grades (×10 -6II12/S) viscosity average at 40 °C viscosity range, viscosity range at 40 °C, viscosity grade at 40 °C, viscosity range VG10VC15VG22VC32101522329. 00 ~11. 013. 5 ~ 16. 519. 8 ~ 24. 228. 8 ~ 35. 2VG46VG68VC100VG150466810015041. 4 ~ 50. 661. 2 ~ 74. 890. O ~ 110135 ~165 Note: The main one is VG15~ VC68.
(3) Relative viscosity Relative viscosity is formulated according to specific measurement conditions, so it is also called conditional viscosity. Depending on the measurement conditions, the relative viscosity units used are different. Such as En's viscosity oE (some European countries), general S. S. Sus (United States, United Kingdom), commercial Ray's second RiS (United Kingdom, United States and other countries) and Babbitt oB (France) and so on. The International Organization for Standardization ISO has specified the uniform use of kinematic viscosity to indicate the viscosity of oils.
