Types of working fluids. Physical properties of liquids
Working fluids
one . REQUIREMENTS FOR WORKING LIQUIDS.
Normal operation of the hydraulic drive is possible when using such working fluids that can simultaneously perform various functions.
First of all, the working fluid in the hydraulic drive is the working fluid, i.e. is a carrier of energy, ensuring the transfer of the latter from the energy source (motor) to its consumer (actuators). In addition, the working fluid acts as a lubricant in the friction pairs of the hydraulic drive, being a lubricating and cooling agent, and a medium that removes wear products. The functions of the working fluid include the protection of parts of the hydraulic drive from corrosion.
In this regard, versatile requirements are imposed on working fluids, to some extent contradictory and the fulfillment of which is not always possible in full. These include:
Good lubricating properties;
Small change in viscosity with changes in temperature and pressure;
Inertness in relation to structural materials of hydraulic drive parts;
Optimal viscosity, ensuring minimal energy losses and normal functioning of the seals;
Low toxicity of the working fluid itself and its vapors;
Little tendency to foam;
Anti-corrosion properties; the ability to protect hydraulic drive parts from corrosion;
Optimum density;
Durability;
Optimum solubility of water in the working fluid: poor for pure mineral oils; good for emulsions etc.
Flammability;
Low ability to absorb or dissolve air;
Good thermal conductivity;
Small coefficient of thermal expansion;
The ability to be well cleaned of contaminants;
Compatibility with other brands of working fluid;
Low price;
Failure to comply with these conditions leads to various violations in the functioning of the hydraulic drive. In particular, poor lubricating or anti-corrosion properties lead to a reduction in the service life of the hydraulic actuator; non-optimal viscosity or its too high dependence on the operating modes of the hydraulic drive reduce the overall efficiency. etc.
The normal and long-term operation of the hydraulic drive is determined equally by the correct choice of the brand of the working fluid during design, and by the competent operation of the hydraulic drive.
2. PROPERTIES AND CHARACTERISTICS OF THE WORKING LIQUID
2.1 GENERAL PHYSICAL PROPERTIES
The density of the working fluid is a physical quantity that characterizes the ratio of the mass m of the fluid to its volume:
Density unit - kg / m3.
The density value is of great importance for energy performance hydraulic drive. The value of hydraulic losses depends on it, which is determined as
where C is the velocity of the fluid.
The change in the density of the working fluid when the temperature changes from t1 to t2 is described by the expression:
rt2 =r n1 / 1+b(t2-t1).
where b is the volume expansion coefficient.
The relative change in the volume of a liquid with a change in temperature is characterized by the temperature coefficient of volume expansion b.
where V and DV are the initial volume and the increment in volume as the temperature rises by Dt. The unit of coefficient b is 1/°c.
The change in the volume DV and the volume of the working fluid when the temperature changes from t1 to t2 can be determined by the formulas:
Vt2=Vt1.
The value of the coefficient of volumetric expansion is small. However, this change should still be taken into account when calculating hydraulic drives with closed flow circulation in order to avoid destruction of the hydraulic drive elements during heating.
The possibility of destruction of parts of the hydraulic drive is due to the difference in the values of the temperature coefficient of volume expansion of the working fluid and the metal of the parts of the hydraulic drive. The increase in pressure due to heating is usually estimated by the formula:
Dp = (b-bm)DtE / k
where bm is the coefficient of volumetric expansion of the material of the hydraulic drive parts;
E is the modulus of elasticity of the liquid;
k- coefficient characterizing the volumetric elasticity of the material of the hydraulic drive elements.
A rough estimate of the pressure increase in a closed vessel upon heating by 10°C and the accepted average values of b=8.75 10-4, bm=5.3 10-5, E=1.7 103 MPa and k=1 gives a value of about 15 MPa. Therefore, in a hydraulic drive with closed circulation, operated with a wide range of changes in the temperature of the working fluid, should be installed safety valves or other devices that compensate for the temperature increase in the volume of the liquid.
The compressibility of a liquid is its ability to change its volume in a reversible way under the action of external pressure, i.e. so that after the termination of the external pressure, the initial volume is restored.
The compressibility of a liquid is characterized by the modulus of elasticity of the liquid E with the dimension Pa (or MPa).
The decrease in the volume of a liquid under pressure is determined by the formula
With increasing pressure, the modulus of elasticity increases, and when the liquid is heated, it decreases.
Typically, the oil of a working hydraulic drive contains up to 6% of undissolved air. After settling for a day, the air content decreases to 0.01-0.02%. In this case, the working fluid is a gas-liquid mixture, the elastic modulus of which is calculated by the formula:
Egzh \u003d E (Vzh / Vp + 1) / (V well / Vp + E p0 / p 2)
where Vl, Vp are the volumes of the liquid and gas phases, respectively, at atmospheric pressure Р0.
The working fluid also contains a certain amount of dissolved air (proportional to the pressure), which practically does not affect the physico-chemical properties of the oil, but contributes to the occurrence of cavitation, especially in the suction lines of pumps, in throttles and other places of the hydraulic drive, where there is a sharp change in pressure.
2.2 VISCOSITY
Viscosity - the property of a liquid to resist the shear of one layer relative to another under the action of a tangential force of internal friction. The friction stress according to Newton's law is proportional to the velocity gradient dC/dy
The coefficient of proportionality h is called dynamic viscosity
The unit of dynamic viscosity is 1 Pa.s. (pascal second).
More common is another indicator - kinematic viscosity, which takes into account the dependence of internal friction forces on the inertia of the fluid flow. Kinematic viscosity (or dynamic viscosity coefficient) is given by
The unit of kinematic viscosity is 1m2/s. This value is large and inconvenient for practical calculations. Therefore, a value of 104 less than -1 cm2 / s = 1Cst (stokes) is used, or 1 hundredth of St - cSt (centistokes). The regulatory and technical documents usually indicate the kinematic viscosity at 100 ° C - (g100) or at 50 ° C - (g50). For new brands of oils, in accordance with international standards, the viscosity is indicated at 40 ° C (more precisely at 37.8 ° C) - g40. The indicated temperature corresponds to 1000 Fahrenheit.
In practice, other parameters characterizing the viscosity of liquids are also used. Often used is the so-called conditional or relative viscosity, determined by the flow of a liquid through the small hole of a viscometer (a device for determining viscosity) and comparing the flow time with the flow time of water. Depending on the amount of liquid being tested, the diameter of the hole and other test conditions, different indicators are used. In Russia, to measure viscosity conditions, conventional degrees Engler (°E) are adopted, which are viscometer readings at 20, 50 and 100°C and are designated respectively °E20; °E50 and °E100 . The value of viscosity in degrees Engler is the ratio of the time of flow through the hole of the viscometer 200 cm3 of the test liquid to the time of flow of the same amount of distilled water at t = 20 C ..
The viscosity of a liquid depends on the chemical composition, temperature and pressure. The most important factor affecting viscosity is temperature. The dependence of viscosity on temperature is different for different liquids. For oils in the temperature range from t = +50 0C to the pour point, the formula is applied:
nzh= n50 exp (A / Tzha)
where nl is the value of kinematic viscosity at temperature Tl (° K), in cCm;
A and a are empirical coefficients.
For some working fluids, the values of the coefficients A and a are given in Table. one.
Table 1.
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Working fluids are essential integral part hydraulic drive, performing the most important function - the role of the working fluid. It is the working fluids that largely determine the possible operating parameters, technical resource and reliability indicators of drives. Mistakes in the choice of working fluids and lubricating media entail increased wear of hydraulic equipment, and in some cases lead to its premature failures. In addition, enterprises using hydraulic equipment suffer serious economic losses associated with leakage of the working fluid, which can occur not only due to wear and aging of seals, but also due to overheating of the working fluid caused by its contamination. The range of issues to be resolved related to the rational choice and operation of working fluids is extremely wide and requires a comprehensive consideration of complex problems that are at the junction, on the one hand, of mechanical engineering, hydraulics, and economics, and, on the other hand, tribology, petrochemistry, and heat engineering. Such a complex of issues is difficult to solve for both a mechanical engineer and a petrochemical engineer with traditional training. In connection with these relatively recently a new scientific and technical direction - chemmotology.
Chemmotology is the science of the properties, quality and rational use of fuels, lubricants and special fluids, studying, in particular, from a unified logical position, processes in the elements of mechanisms that come into contact with a working fluid or a lubricating medium. The word "chemmotology" is formed by the abbreviation of three words: chemistry (chemia - Greek); motor (motor - lat.); logy (logos - Greek - science).
The practice of operating hydroficated machines and mechanisms has revealed the feasibility of training service personnel in the field of chemmotology, since a qualified choice, competent Maintenance and operation of the working fluid not only increase its service life, but also increase the technical life of hydraulic drives. It is very important to consider working fluids and lubricants in conjunction with the operation of hydraulic systems and their components.
Here, the processes in the elements of hydraulic machines are considered and the fundamental mathematical relationships describing these processes, as well as the existing empirical formulas, are given. In addition, the physical properties of liquids are considered in more detail in connection with the possibility of operating with them using modern methods for calculating hydraulic systems. Particular attention is paid to the mechanism of aging of working fluids and its relationship with the molecular structure of the latter.
Operating conditions of the working fluid can be very difficult:
This is a wide temperature range (-60…+90С);
High flow rates during throttling - more than 50 m/s;
High pressures reaching 32 MPa and above;
Contact of the working fluid with various structural materials.
The listed operating conditions increase the level of requirements for the working fluids of hydraulic systems.
Working fluids are divided into two groups:
Group 1 - with normal flammability. These are working fluids on a mineral (petroleum) basis;
Group 2 - with reduced flammability or fire resistance. These are aqueous and synthetic working fluids.
Petroleum-based working fluids have a relatively low upper limit of the operating temperature range and contain antioxidant and anti-corrosion additives. The upper temperature limit of mineral oils is from 80 to 90C with a short-term increase in temperature to 110 ... 120C.
Synthetic working fluids have high temperature properties and provide fire safety at temperatures up to 350C. However, they are relatively expensive, which limits their use. The following classes of synthetic fluids are used in hydraulic systems:
1) diesters (esters);
2) siloxanes (liquids based on organosilicon polymers);
3) phosphates (liquids based on esters of phosphoric acid);
5) fluorine - and organochlorine (halocarbon).
Diester-based fluids are used in hydraulic systems with particularly high loads on the elements in the operating temperature range from -30 to +180, subject to a thorough check of their compatibility with the materials of the hydraulic system. Sleeves and seals made of nitrile rubber, electrical insulating materials, metals containing lead, cadmium and zinc coatings do not work well in the environment of diesters.
Siloxanes and polysiloxanes have the flattest viscosity-temperature characteristic of all working fluids. They are highly compressible, but have a minimum surface tension. The latter allows them to be used as antifoam additives. These fluids are resistant to oxidation and temperatures up to 190C, however, when exposed to a temperature of 200C for a long time, they decompose to form silica, which is an abrasive. Group 2 lubricity is poor, especially on steel, so siloxanes are only used in mixtures with diesters or petroleum oils.
Phosphates have increased fire resistance and good lubricity. However, their viscosity-temperature characteristics are worse than those of oils. Phosphates are prone to hydrolysis, so they should not be used in hydraulic systems with possible flooding. Many phosphates are toxic. In addition, they have an increased tendency to foam, as well as incompatibility with conventional seal materials and worse radiation resistance than oils. Upon hydrolysis, phosphates form phosphate compounds that can react with glassy materials, enamels and metals.
Aqueous liquids of group 4 do not ignite when sprayed on a flame or on a surface with a temperature up to 700. Other liquids have increased fire resistance, but are combustible, that is, they can ignite when exposed to fire or hot objects. Only organofluorine liquids are completely incombustible, they are also chemically inert, have thermal stability.
Water-glycol fluids are toxic, so water-glycerin fluids with additives are more often used. Group 4 fluids have satisfactory viscosity-temperature characteristics, lubricating and anti-corrosion properties.
A great advantage of aqueous fluids is their compatibility with seal materials based on nitrile rubbers. In addition, they have low compressibility and the highest heat capacity. The disadvantages of group 4 include their electrical conductivity and possible incompatibility with paint coatings.
Aqueous liquids are non-combustible as long as they contain at least 30% water by mass, so they are used in pressurized hydraulic systems that ensure that there are no losses due to water evaporation. Due to the low boiling point of water, the vapor pressure of group 4 is high. Therefore, it is recommended to use water-containing liquids in the operating temperature range from 65 to 70. If water evaporates, glycerin or glycol may ignite. In domestic practice, water-glycol liquids are used only for cooling systems (antifreezes, antifreezes). Water-glycerin liquid PGV is used for hydraulic systems of mobile objects and ship hydraulic drives in the operating temperature range from -30 to 65…70. She has a characteristic Blue colour. The results of long-term operation of PGV in hydraulic drives without replacement of materials were positive. However, a thorough analysis of the compatibility of PGV with hydraulic system materials, especially with paint and varnish and electroplated coatings, is first necessary. Water losses due to evaporation (for relatively hermetic hydraulic systems 3...4% per year) are compensated by the addition of distilled or soft water. When hard water is added, as well as when lubricants and oils get into the PGV liquid, precipitation may occur.
For industrial hydraulic systems operated in conditions of possible fire hazard, Promhydrol water-glycerin fluids (grades P20, P20M1, M20M2, color - light yellow) are used. Promhydrol differs from PGV fluid by a high content of thickening additive. The self-ignition temperature of Promhydrol is 420, which made it possible to use it in the hydraulic system of a blast furnace.
Organofluorine liquids chemical composition divided into three main groups:
Fluorochlorocarbon - low molecular weight polymers of trichlorofluoroethylene (in domestic practice, grades 11F, 12F, 13F, 14F; in the USA - kelef, fluorolyub);
Perfluorocarbons obtained by fluoridation of petroleum oils;
So, mineral oils have a limited temperature range of application. In addition, they are flammable. These shortcomings are less pronounced in synthetic working fluids. They have a flatter viscosity-temperature characteristic, have greater fire resistance. The disadvantages of synthetic fluids include high cost, poor lubricity, and the need to switch to special seal materials.
Another type of working fluids is water-containing emulsions. They have low cost, low compressibility, higher heat capacity and fire resistance. In hydraulic drives of forging and pressing machines, oil-in-water emulsions are used, which consist of 2 ... 5% emulsol containing mineral oil and 95 ... 98% water. Emulsol is in water in the dispersed phase. The disadvantages of such liquids are low lubricity, high corrosiveness and the inability to use at low temperatures. More promising emulsion "water in oil", the water content of which is about 40%. It combines the positive properties of oil-in-water emulsions and mineral oils. However, water-containing working fluids have not yet become widespread, since switching to them leads to an increase in the cost of individual hydraulic devices by about 1.5–5 times and an increase in the power consumed by pumps by about 1.5 times. Currently, they are used in hydraulic systems for which fire safety issues are especially important, for example, in mining and metallurgical equipment.
AT last years Intensive work is underway on the use of environmentally friendly working fluids in hydraulic drives, and, first of all, of vegetable origin. The best known in this regard is rapeseed oil, which, in terms of its tribological characteristics, is not only not inferior, but in some parameters, for example, the wear of rubbing surfaces, is superior to oil-based working fluids. To fight aging vegetable oils special antioxidant additives are added to them. The viscosity of vegetable oils is much less dependent on temperature than mineral oils. But for vegetable oils, the ingress of water, which leads to their decay, is unacceptable.
From an environmental point of view, it is also of interest to use pure water as a working fluid. Despite the understandable shortcomings of water for the operation of hydraulic machines and hydraulic devices, which, at certain costs, can be compensated for by constructive measures and the choice of appropriate materials, positive traits make it a convenient working fluid. Significantly decrease hydraulic losses, it becomes possible in many cases to abandon the cooling systems of the working fluid. A lower volumetric compression ratio contributes to an increase in the rigidity of the hydraulic drive. Danfoss (Denmark) has developed the Nessie hydraulic system capable of operating on clean water.
In the installation and maintenance of technical mechanisms, the greatest attention is paid to functional elements, auxiliary equipment and various fixation and support systems. But at the same time, the quality of equipment operation largely depends on them. They perform different functions, but all of them ultimately come down to one task - to extend the service life of the serviced object. A special place in this group is occupied by hydraulic fluid, which also acts as a functional component, putting pressure on the working elements of the mechanism.
Where are hydraulic fluids used?
Oils of this type are used in various technical devices and mechanisms. A classic example of their application is pipeline shut-off valves. By themselves, hydraulic devices are widely used in various fields of industry, manufacturing and construction. These can be press machines, units in factory lines, hydraulic processing systems, etc. It is important to note that hydraulic fluid can also be used in household equipment. Some models of pneumatic stations, pumping equipment and power units may also use such fluids. Moreover, the functions of this type of oil are also different - they should be considered in more detail.
Fluid Functions
The main task of the hydraulic fluid is to transfer pressure to the working component of the system. It can be a piston or a valve, the main thing is that the volume of oil acts as a dynamic force transmitter and at the same time performs a number of auxiliary functions. For example, as already noted, technical oil provides lubrication of the rubbing elements of the working system, extending their life. Depending on the operating conditions, special tasks may also be required.
For example, if the installation is planned to be operated in an environment subject to thermal effects or close contact with moisture, then the hydraulic fluid is replaced with a composition with suitable protective qualities. In this case, the technologist will recommend an oil with anti-corrosion properties and thermal stability. At the same time, by default, each composition of the hydraulic fluid provides for cleaning. Pipelines are regularly washed, as a result of which their internal surfaces get rid of precipitation and other destructive substances.
Properties of oils for hydraulic systems
The quality of the above functions is determined by the properties of a particular composition. The basic performance properties of hydraulic fluids include thermal resistance, viscosity, inertness, and density. But special working qualities, including protective ones, are of increasing importance. For example, anti-corrosion allows you to withstand liquid and humid environments without negative rusting processes. Also important is the liquid, which determines the intensity of the working function of the composition. That is, the lower the resistance index, the easier it is to transfer the force from the power unit. As a result, less energy is spent to ensure the operation of the installation. Another thing is that the achievement of optimal resistance indicators is rarely carried out without loss in other technical and physical qualities of hydraulic oils.
Classifications of hydraulic fluids
Experts classify such liquids according to several criteria. For example, the main division is carried out on the basis of purpose - a separate place in the assortment is occupied by hydrostatic and hydrodynamic compositions. Liquids are also released depending on the application. In particular, ISO 15380 marked lubricant formulations provide fast biodegradation processes. There are also modifications that are more environmentally friendly. They are often used in aggregates. Food Industry. Hydraulic fluid labeled STOU is also common. It is usually involved in the maintenance of mobile systems. At the same time, a wide group of auxiliary fluids is in demand, which do not work in the main part of the hydraulic piston mechanism, but are used in technical support individual components such as couplings, bearing groups and converters.
Varieties of liquid on the basis of working qualities
In this classification, it is appropriate to consider three main groups. The first is represented by the main compositions, which differ in balanced indicators of viscosity, compressibility and pressure. It can be said that these are typical universal means of providing a liquid hydraulic function. The second group covers products that are resistant to oxidation processes. This includes thermally stable types of hydraulic fluids that are able to circulate under high pressure contact with metal surfaces, water and air. The third group provides for a more perfect performance of the thermal protection function. These are compounds that are not subject to fire hazards even in close contact with sources of fire.
Hydraulic fluid compositions
The output product is usually concentrates based on industrial oils and additives. A classic example is one made with mineral oil and emulsifiers, and diluted with rust inhibitors. Actually, such a combination in itself can serve as the basis for the preparation of more technological modifications, which can also be combined with a huge range of elastomers. For example, to improve hydraulic performance, manufacturers introduce sealants into formulations. Conversely, if a higher degree of elasticity of the working component is to be achieved, emulsion lubricating oils are added.
Basic foundation
Paraffin compositions, naphthenic mixtures and various combined solutions can be used as base mineral oil. There are also special modifications with improved basic working qualities. These are synthetic fluids that use hydrocracking components, ester compounds and polyglycols, which are most often used for fire-resistant mixtures. Natural base bases from which biodegradable hydraulic oils are produced also find their application. Liquids of this type may contain vegetable processed products that are environmentally friendly.
Regardless of the type of base oils, the quality of their purification also matters. There are different categories, differing in the degree of preliminary preparation of the composition. There are mixtures of coarse cleaning, and there are also oils that have undergone repeated filtration. This is not to say that the second option will be the best in all use cases. In some areas, it is precisely liquids that are based on a rough elemental combination that manifest themselves optimally.
Additives and fluid modifiers
It is often the additional components that play a decisive role in the performance. They are mutually exclusive or complementary, so it is impossible to get a completely universal tool suitable for any need. To varying degrees, the base can be given properties such as anti-corrosion, aging resistance, extreme pressure and anti-wear properties.
At the same time, additives are divided according to the nature of the application. There are components that are added as an addition to the mineral base oil, and there are also surfactants. For example, hydraulic is obtained as a result of the inclusion of surface friction modifiers, which can be introduced into the composition already during the operation of the mechanism.
Basic ones are usually included in the factory. This category includes antifoaming elements, antioxidants, etc. Active additives against this background will be beneficial in that they do not require special treatment of the liquid after addition.
How to choose hydraulic fluid?
To a large extent, the choice of one or another composition is determined by the operating conditions. In particular, the range of operating temperatures, type hydraulic system, pressure, environmental safety requirements and external influences. It is desirable to pay special attention to the viscosity index. If the task is to reduce leakage and increase sealing, then mixtures with a minimum level of viscosity should be preferred. The temperature of the working environment is also taken into account in a separate order. When deciding which hydraulic fluid to choose for a stationary system, you can give preference to compositions designed for a regime of 40-50 ° C. For mobile and dynamic systems, highly specialized fluids are often selected.
How to change hydraulic fluid?
First of all, it is necessary to open access to the liquid storage tank, as a rule, these are special metal tanks. Further, space is freed up for work with the communication infrastructure. Usually, the supply hoses are supplied with clamps, which should be unclenched. This will check the hydraulic fluid level, pressure and general condition. Next, the oil is pumped out. This operation can be performed using syringes or pumps with compressors, depending on the design feasibility.
Then you can start pouring a new mixture. This operation is also performed using an improvised tool or directly if it is possible to disconnect the supply hose. Proper hydraulic fluid replacement is also done with air evacuation. Excessive airing can lead to losses in terms of unit efficiency, therefore, removing excess gas mixtures is indispensable.
Conclusion
Hydraulic mechanisms often perform critical tasks that require the connection of high powers. In turn, the hydraulic fluid acts as a full-fledged functional component of such systems, ensuring the stable operation of the units. On condition right choice With this oil, maintenance personnel will be able not only to extend the service life of the operating installation, machine or tool, but also to increase the energy efficiency of the equipment. This is due to the fact that the same indicators of the resistance of the working fluid can increase or alleviate the load on the drive mechanism, which will directly affect the amount of resource consumed.
The working fluid in a hydraulic transmission is a fluid whose properties determine the working process of hydraulic energy transfer. The physical properties of the working fluid are characterized by specific gravity, compressibility, viscosity. In addition to these parameters, to assess the fluid as a working fluid in hydraulic transmissions, it is necessary to take into account its resistance to mechanical stress, chemical resistance at high and low temperatures operating range of the hydraulic system, lubricating qualities and stability of lubricating properties, degree of aggressiveness to metals and sealing elements of the structure, levels fire hazard and toxicity when exposed to humans (the liquid itself and its vapors).
Consider the properties of the two most common working fluids: oils - AMG-10 and liquid 7-50S-3, used in modern aircraft hydraulic systems. Their densities p (specific gravity γ) are respectively 833 kg / m 3 (8163.94 N/m 3) and 921 kg / m 3 (9031.92 N / m 3). For comparison, the density (specific gravity) of water is 999 kg / m 3 (9796.84 N / m 3).
When heated, hydraulic fluid expands, like all fluids, changing specific gravity and density. Mendeleev's equation establishes a relationship between temperature change and the mass per unit volume of liquid
,
where is the desired specific gravity at a given temperature t, - specific gravity at t= 15°С; - coefficient of volumetric expansion (for hydraulic fluids = 0.0007).
Fig.10.1.Dependences of the density of working fluids on temperature.
According to the graphs of changes in the density of AMG-10 oil and working fluid 7-50S-3 depending on temperature (Fig. 10.1), it is possible to determine the increase in the volume of liquid poured into the hydraulic system and evaluate the change in the liquid level in the tank during heating. The expansion of the liquid during heating must be taken into account in the tank when it is locked in the cylinder by a hydraulic valve, since the pressure in the closed system can exceed the allowable stresses in the pipelines and the cylinder and lead to their destruction. The density of a hydraulic fluid changes by approximately 7% per 100°C change in temperature.
Fluid compressibility determined by the bulk modulus of elasticity E, which for hydraulic fluids is in the range from 1350 - 1750 MPa. For water at relatively low pressures, the modulus of elasticity is assumed to be 1962 MPa. The compressibility of a liquid is characterized by the relative compression ratio β
where V- liquid volume; - volume change with pressure change R.
Hence coefficient = 1 /E.
For accepted pressures in hydraulic systems, we can assume = 0.00007. This means that when the pressure changes by 10 5 Pa (about 1 at), the relative change in volume V/V= 0.00007. Therefore, in many calculations, the compressibility of a liquid can be neglected due to its small value.
One of the most important properties of a liquid is called viscosity. Viscosity- this is the ability of a liquid to resist the sliding of its layers relative to each other when moving.
The friction force that falls on the unit of the contact surface of two sliding layers of liquid, provided that the velocity gradient along the normal is equal to unity, is called coefficient of dynamic viscosity μ.
Dynamic viscosity ratio μ density ρ is called coefficient of kinematic viscosity ν. Quantities ν, μ and ρ are related by the relation ν = μ/ρ .
The viscosity of a liquid is due to the forces of molecular cohesion, which decrease with increasing temperature, and the viscosity also decreases (Table 10.1).
The physical-mechanical, lubricating and other properties of mineral oils and their mixtures used in hydraulic systems deteriorate during operation due to their oxidation upon contact with air, emulsification and foaming when air and moisture enter them. This deterioration in the properties of working fluids is manifested in a decrease in their viscosity, contamination with deposits in the form of resins, metal particles, dust, etc. At the same time, the most effective way extending the performance of the liquid is its continuous and thorough filtration using periodically replaced cleaning filters.
In addition, hydraulic fluids dissolve gases that, in a dispersed state, have practically no mechanical effect on the operation of the hydraulic system. However, when the pressure in any zone decreases, the dissolved gases are released in the form of small bubbles, uniting into larger ones and forming gas cavities, which worsen the mechanical properties of the hydraulic system. Different gases have different solubility in fluids used in hydraulic systems. So, the solubility of air is about 11% of the liquid volume; nitrogen - 13%; carbon dioxide (exhaust gases) - 85%.
Clogging of the liquid with air worsens the operating conditions of the pumps and the entire hydraulic system as a whole, disrupts the smoothness of the movement of hydraulic drives, impairs lubrication and causes corrosion of parts of hydraulic units.
In addition to the above properties of AMG-10 oil and hydraulic fluid 7-50S-3, we present the following technical data for them. AMG-10 oil is prepared by thickening a low-viscosity oil fraction. It contains an antioxidant additive; it is non-corrosive and non-toxic. Oil is efficient at temperatures from -60 to 125 ° C in contact with air or nitrogen and for a short time up to 150 ° C only in contact with technical nitrogen. As sealants when working with oil, rubbers made of nitrile rubber grades V-14, IRP-1078, IRP-1353 are used. Liquid AMG-10 - homogeneous, transparent, red.
Working fluid 7-50C-3 is a mixture of synthetic products - polyxyloxanes and organic ethers. Contains antioxidant and anti-corrosion additives. She is R Operates in the temperature range from -60 to 175°C in contact with air and technical nitrogen and for a short time up to 200°C in contact with nitrogen. The liquid has low toxicity, has an increased effect on copper, cadmium and phosphate coatings. It is used with sealing rubber brand IRP-1353 and fluoroelastomer IRP-1287. Liquid 7-50C-3 - transparent, color is not regulated.
2. HYDRAULIC FLUIDS
2.1. Appointment of working fluids and the basic requirements for them
The fluid used in hydraulic drives is their working fluid. As a result, it is called working. The working fluid provides the transfer of energy from the pump to the hydraulic motor and control signals in the hydraulic system. In addition, it provides lubrication of the friction surfaces of hydraulic devices, removal of wear products from friction pairs, protection of metal parts from corrosion and removal of heat generated in the hydraulic drive.
Working fluids are exposed to a wide range of pressures, temperatures and velocities. The correct choice of working fluid ensures the performance of the hydraulic drive and largely determines its operating parameters.
The following requirements apply to the working fluid.
1. Good lubricity, ensuring reliable operation of friction pairs.
2. A small change in viscosity is possible over a wide temperature range, which also determines the low variability of the characteristics of hydraulic devices and the hydraulic drive as a whole.
3. High fire resistance.
4. Stability of mechanical and chemical properties under conditions of long-term operation and storage. The stability of mechanical properties is understood, first of all, as the ability of a liquid to resist the “crumpling” process, which is the process of destruction of molecules during its long-term throttling in narrow slots, mixing of the liquid and exposure to vibrations, which leads to a decrease in viscosity. The stability of chemical properties is understood as the ability to resist oxidation under the influence of the environment and hydrolysis reactions due to the presence of water in the liquid, as well as the chemical reaction of the liquid with the materials of the walls of hydraulic devices and seals.
5. Low toxicity of the working fluid and its vapors.
6. High bulk elasticity.
7. High thermal conductivity.
8. Small coefficient of thermal expansion.
9. Radiation resistance.
10. Resistance to foaming.
11. Low solubility of gases, providing high elasticity of the liquid.
12. Low cost.
These requirements are largely incompatible. Therefore, the choice of working fluid is a certain difficulty.
2.2. Basic physical properties of working fluids
Of the numerous properties of fluids, we will focus only on those that are most important from the point of view of the operation of hydraulic drives, determine their operating parameters and which must be taken into account by the developer. These properties are determined by the requirements listed above.
Density,, characterized by the mass ratio m to its volume
For practical calculations, the density of mineral working fluids can be taken .
The density of the working fluid characterizes the pressure loss during its flow through throttles, valves and hydraulic lines. So in turbulent flow
where Q is the fluid flow rate; pressure loss; Density decreases with increasing temperature
, (2.2)
where, respectively, the density at temperatures, the coefficient of volumetric expansion. For mineral fluids at
This property must be taken into account when designing a hydraulic drive with closed circulation of the working fluid. In such a drive, with an increase in temperature, an increase in volume and an increase in pressure occurs, which can lead to the destruction of the hydraulic system. To avoid this, a thermal compensator is attached to the hydraulic tank, for example, a bellows type. The change in its volume must be sufficient to compensate for the thermal expansion of the working fluid in the entire hydraulic system.
Viscosity- the property of a liquid to resist the relative displacement of its layers. This property is essential for the operation of the hydraulic drive.
The influence of viscosity is ambiguous. On the one hand, high viscosity increases the reliability of lubrication of rubbing surfaces. Reduces leakage in hydraulic devices and improves the stability of the hydraulic drive. On the other hand, it increases friction losses, increases hydraulic resistance in hydraulic lines and reduces drive speed.
The viscosity of a liquid is characterized by the coefficients of dynamic and kinematic viscosity. The coefficient of dynamic viscosity, Pa, is determined from the equation expressing Newton's law of fluid friction:
where T - force arising between moving layers of liquid; S – the area of contact of the surfaces of the layers; – speed gradient.
The coefficient of kinematic viscosity, is determined by the ratio
It is also measured in stokes (St)
1 St=100 cSt=1
Due to the fact that it is difficult to directly measure the viscosity in a moving liquid, the conditional viscosity is determined using special instruments called viscometers. The Engler viscometer, which measures viscosity as the ratio of the time of flow of a liquid through a hole with a diameter of 2.8 mm under the action of its own weight, to the time of flow of the same volume of distilled water at a temperature of 4 ° C, has found the greatest application. The unit of viscosity determined in this way is called the degree of conditional viscosity). In some countries, this unit is called the Engler degree ().
The conversion to cSt is carried out according to the formula
The viscosity of the working fluid depends significantly on its temperature. For mineral oils, this effect can be determined empirically.
where is the viscosity at 50C; temperature. This dependence is valid in the temperature range 30С150. For oils in the interval = 1050cCt.
Viscosity versus pressure p can be presented in the following form:
where is the coefficient of dynamic viscosity at p=0 ; piezo viscosity coefficient. The expression is valid at . The presence of air in the working fluid leads to some decrease in viscosity.
1+0.015V, (2.8)
where is the viscosity of a pure fluid; the viscosity of the working fluid that takes in air from the total volume.
Compressibility The property of a liquid to change its volume under pressure. The compressibility of the working fluid should be minimal, since its presence leads to a decrease in the supply of pumps, disrupts the smoothness of movement of machine units moved by the hydraulic drive, reduces the implementation of movements, and reduces the stability of the hydraulic drive.
Compressibility, , is characterized by the volumetric compression ratio
, (2.9) where the relative change in volume with a change in pressure by
The reciprocal value is called the volumetric elasticity modulus of the liquid, Pa:
For mineral oils, the modulus of elasticity lies within MPa. Pipelines, especially hoses, reduce the "reduced" modulus of elasticity.
The process of compression of the working fluid can take place at different speeds. Compression during slow processes, in which heat exchange with environment, is characterized by isothermal modulus of elasticity . Compression during fast processes, in which heat transfer does not have time to complete, is characterized by an adiabatic modulus of elasticity. The experimental method for determining this modulus is based on measuring the speed of propagation of sound waves in a liquid
where is the speed of sound in the liquid.
It has been established that when calculating fast processes in a hydraulic drive, it is possible to take . The bulk modulus depends on pressure and temperature. Elasticity increases with increasing pressure and decreases with increasing temperature
where is the bulk modulus without the presence of a gaseous medium in the liquid at C,.
The presence of undissolved air in the form of small bubbles has a great influence on the compressibility of the working fluid. The compressibility in this case is many times higher than the compressibility of a pure liquid. Let us consider this effect under the conditions of an isothermal compression process. The undissolved air in the volume forms a two-phase mixture with the volume of pure liquid.
Differentiating (2.12) with respect to pressure and assuming that the mixture compression law has the same character as for a pure liquid, and the air compression law obeys the Boyle-Marriott law
, (2.13)
where are the moduli of the volumetric mixture and the pure liquid; the volume is the pressure. With an isothermal compression process, n=1. From (2.13) and (2.12) we get
(2.14)
Dividing the right side of (2.14) by the initial volume of liquid in the mixture and substituting, we have
. (2.15)
In real systems, the air content can vary widely (). The dependence of the modulus of bulk elasticity on the pressure of the working fluid at different air content is shown in Fig.
As can be seen from the figure, the influence of pressure manifests itself in more at small values. To eliminate this zone, pressure valves should be installed in the drain hydraulic lines of the hydraulic pipelines, creating a backwater of the order of 0.5-1 MPa. Due to this, the compressibility of the working fluid in the drain cavities of the hydraulic motors decreases and the smoothness of the movement of the working bodies of the machines increases, especially when using hydraulic cylinders. At a pressure of more than 15 MPa, the effect of air on compressibility has practically no effect, since it passes into a dissolved state. This circumstance also determines the usefulness of switching to higher pressures of the working fluid in the pressure hydraulic lines of the drives. To reduce the amount of undissolved air, it is necessary to know the main ways of its penetration into the hydraulic system. The most intensive air leakage occurs in the suction line through leaks in the attachment points of the pump flanges and intake filters, through shaft seals, etc. Air leakage also occurs when the liquid level in the hydraulic tank decreases relative to the suction pipe. Undissolved air can form from dissolved air in areas with reduced pressure. In this case, the reverse process proceeds much more slowly.
Measurement of the amount of undissolved air is carried out either by measuring the volume of liquid before and after its separation, or by measuring some properties of the working fluid (density, modulus of elasticity, etc.), depending on its amount.
The amount of air in the hydraulic system can be reduced by using elastic diaphragms that exclude contact with liquid in the hydraulic tanks or by creating a back pressure in the suction line. Removal of air in dead-end hydraulic systems and at the upper points of hydraulic devices is carried out using air bleed plugs (breathers) or valves.
Thermal properties. Of greatest interest are specific heat capacity and thermal conductivity. The specific heat capacity characterizes the intensity of the increase in the temperature of the working fluid in the hydraulic system. Compared to water, the specific heat capacity of mineral oils is half that. Thermal conductivity characterizes the amount of heat transferred per unit time through a unit surface at a temperature difference between the liquid and the wall of one degree. For better heat dissipation, working fluids must have high thermal properties.
The temperature range for the use of working fluids is related to the flash and pour points. The flash point is the temperature at which the vapors of a liquid form a mixture with air that flashes when an open flame is brought up. The flash point makes it possible to judge the fire safety of hydraulic systems. The pour point is the temperature at which the working fluid thickens to such an extent that when the test tube is tilted at 45 ee, the level remains unchanged for 1 min. For the most common industrial oils, the flash point is 160 - 200C, and the pour point is 30 - 15C.
Electrical properties are important for working fluids used in electro-hydraulic devices of hydraulic drives. To avoid short circuits, insulation failure or sparking as a result of possible ingress of the working fluid, its electrical conductivity must be minimal.
2.3. Characteristics of working fluids
The main type of working fluids that have received the greatest use are mineral oils. Industrial oils I12A, I20A, I30A, I40A, I50A are used in general industrial hydraulic drives operating in heated rooms at an air temperature of 0 to +35C. The number in the oil designation indicates its viscosity in centistokes at t = 50C. Industrial oils are the cheapest, non-toxic, as they do not contain additives. However, on the other hand, they have an increased tendency to oxidize and release resins, due to which their service life is very limited. Industrial oils are used in hydraulic systems operating at a liquid temperature not exceeding 60C.
In hydraulic drives operating at temperatures above 60 ° C, turbine oils Tp-22, Tp-30, Tp-46 are used, which differ from industrial ones in higher performance properties (antioxidant and lubricity, anti-foam resistance, increased service life). Such properties are provided by the introduction of various types of additives () phenols, fatty acids, polysiloxanes, etc.).
Hydraulic drives operating at a pressure of 16-35 MPa are recommended to be operated on oils of the IGP series, which have even higher performance properties.
In hydraulic drives installed on machines operating in the field, oils are used that have a lower dependence of viscosity on temperature. Among them is multigrade oil MGE-10A, designed for operation without replacement for 10 years at ambient temperature from -55 to +55C. VMGZ oil is the main type of working fluid for hydraulic drives of road-building machines operating in the Far North, and is also used as a winter grade in temperate climates. MG-30 oil is used in similar drives as a summer oil.
Aviation hydraulic systems of subsonic aircraft use aviation oil AMG-10, which is easily distinguished by its red color.
Mineral oils have a limited temperature range of application. The upper limit usually does not exceed 80-90C. In addition, they are flammable. These shortcomings are less pronounced in synthetic working fluids. They have a flatter viscosity characteristic, have greater fire resistance. These include diesters, phosphates, siloxanes, water-glycol and water-glycerin liquids. From this class of working fluids, one can name fluid 7-50C-3, used in aviation hydraulic systems operating in the temperature range from -60 to + 175C. The disadvantages of synthetic fluids are high cost, poor lubricity and the need to switch to special seal materials.
Another type of working fluids are water-retaining emulsions. They have low cost, higher heat capacity, fire resistance. In hydraulic drives of forging and pressing machines, “oil-in-water” emulsions are used, in which 2-5% emulsol containing mineral oil and 95-98% water. Emulsol is in water in the dispersed phase. The disadvantages of such fluids are low lubricity, high corrosiveness and inability to use at low temperatures. More promising emulsion "water in oil", the water content of which is about 40%. It combines the positive properties of oil-in-water emulsions and mineral oils. But so far, water-containing working fluids have not received wide application, since switching to them leads to an increase in the cost of individual hydraulic devices by about 1.5-5 times and an increase in the power consumed by pumps by about 1.5 times. Currently, they are used in hydraulic systems, for which fire safety issues are especially important, for example, in mining and metallurgical equipment.