III. Read Text 3.4 and answer the following questions:

1. What is compressibility mean?

2. How can characterize it?

3. Which properties reflect the quality of hydraulic oils?

4. How can you determine Oxidation Stability?

5. Is Lubricating Value an important factor in hydraulic oils?

Iy. Translate the other texts in written form: Pour Point

This is the lowest temperature at which oil flows when chilled under given conditions. It is of concern if the system is exposed to low temperature. Pour point of the oil should be about 20 F below the lowest temperature to which oil will be exposed.

Compressibility: This is the degree to which a fluid undergoes a reduction in volume under pressure. It can be expressed in terms of reduction in volume produced of a given pressure increase or in terms of increase in pressure required for a specified reduction in volume. The bulk modulus, or more properly the bulk modulus of elasticity of a fluid is a measure of its resistance to compression and is the reciprocal of compressibility.

Fluid compressibility must be considered in designing for operation at high pressure. It affects power absorbed by and time required for, generation of pressure by the pump, energy release on decompression, and stiffness of power transmission and servo systems The effect on sonic velocity of resistance to compression is an important factor in cavitation phenomena.

Oils of different hydrocarbon types show measurable differences in compressibility. Paraffinic oils are less compressible than naphthenic oils. Differences between mineral-based Hydraulic oils however are negligible.

Expressed as percentage reduction in volume of oil under pressure, compressibility may be considered about 0,5 per cent for each 1000 psi pressure increase up to 4000 psi.

Indicators of Quality: Properties which reflect the quality of hydraulic oils include oxidation stability, rust prevention, foam resistance, water separation and film strength. Premium-quality hydraulic oils are obtained from selected crudes, which are refined to remove unsuitable compounds and then blended with selected additives to supplement and improve properties such as oxidation stability, rust protection and foam resistance.

Oxidation Stability: This is usually the most important property for longevity of service. It is the ability of an oil to resist both polymerization and decomposition reactions induced by air or oxygen. These reactions are aided by heat, water and the catalytic action of metals. Oxidation stability depends on type of oil, method of refining and use of inhibitors. One of the most important factors governing the ultimate life of hydraulic fluid is its operating temperature. Oxidation reactions approximately double for each increase of 18 F. Control of bulk oil temperature to 130 F maximum and the removal of contaminants, including such metallic catalysts as copper, zinc and lead together with water, help extend the useful life of the oil. The use of premium-quality, oxidation-inhibited hydraulic oils decreases the influence of factors which promote oil deterioration.

Lubricating Value: Lubricating ability is an important quality factor in hydraulic oils, since the fluid must lubricate moving parts of the system to reduce wear to an absolute minimum. Wear increases leakage and reduces efficiency. Petroleum oils satisfy this requirement for most current types of pumps, motors and valves.

In certain types of hydraulic pumps and motors, however, load-carrying ability of oil under conditions of boundary lubrication is important. Gear and vane-type pumps and motors are sometimes used at least intermittently, at speeds and loads which exceed design, and which can shorten pump life significantly. In such cases antiwear additives should be included.

Aeration and Foaming: Oil in hydraulic system always contains air as entrained bubbles as well as in solution. Although air in solution has little or no effect on system operation, entrained air can cause system malfunction.

Since compressibility is increased enormously, the fluid becomes elastic and the system noisy and erratic. Compression of entrained air generates considerable heat and can accelerate oxidation. Volumetric efficiency of the pump is reduced because air bubbles in the oil on the suction side of the pump expand as the oil enters the pump. Aeration is generally associated with cavitation. Collapse of air bubbles within the pump on the discharge side can produce damage similar to cavitation erosion.

Prevention of aeration requires care to system design to reduce entrainment of air and to promote its liberation the oil reservoir. In addition however the oil must be capable of releasing air readily.

Demulsibility: This is the ability of an oil to separate from water in an emulsion. It is impossible to keep all moisture out of a hydraulic system and to prevent completely the formation of emulsions. New hydraulic oils separate readily from water. However, as oil deteriorates with use, small quantities of decomposition products are formed with lessen this ability to “break loose” from the emulsified state.

However, petroleum oils can be prepared to minimize effects of these emulsion-promoting substances and retain adequate demulsibility. A readily demulsifiable oil quickly loses its water contents in the reservoir and can then be returned to the system in a comparatively water-free condition.

Corrosion Prevention: Since some moisture and air are always present in hydraulic systems, ferrous surfaces are subjected to rusting. To prevent the moisture from reaching the metal surfaces, the oil must have good metal-wetting properties. This characteristic is provided by inclusion of rust inhibitors in the oil. The inhibitor “plates out” on the metal surfaces and forms adherent protective films.

Compressibility with System Materials: Some antirust additives suitable for use in hydraulic systems, as well as many other lubricating oil additives, may attack zinc. Therefore, galvanized and other zinc-coated surfaces should be avoided in hydraulic systems. Zinc is also objectionable on the grounds that products of oxidation of the oil can react with it to form metal soaps.

Other metals susceptible to corrosion are magnesium-base alloys and lead. Magnesium alloys generally suffer heavy corrosion when water is present. Lead, like zinc, is attacked by products of oxidation, forming soaps which can cause emulsification of the oil. Copper should also be avoided in hydraulic systems, but for a different reason. It is one of the most effective catalysts in promoting oxidation of all types of oils. Thus, heat exchangers should be of steel rather than copper or brass. If paint is required for the inside coating of the oil tank, it should be oil-resistant.

The effect of hydraulic oils on seal materials must also be considered. Natural rubber is not oil resistant and is unsuitable for use in oil-hydraulic equipment.

Synthetic rubbers vary widely in their behavior when exposed to different fluids. In contact with a given fluid, some are almost unaffected, but others swell or shrink. Sealing elements for use with synthetic hydraulic fluids may be of synthetic rubber which is not oil resistant.

Most seals in contact with lubricating oil are made from nitrile (butadiene acrylonitrile) rubbers. Other materials suitable for lubricating oil systems include neoprene, thiokol, silicones, fluorocarbon rubbers and Hypalon (chlorosulphonated polyethylene).

Effect of mineral oils on conventional rubber depends mainly on aromaticity; the higher the aromaticity of the oil the more swelling it causes. High viscosity-index oils in general are “low-swelling”, medium viscosity-index oils are intermediate and low viscosity-index oils are “high-swelling”.

The aniline point of the oil indicates its behavior in this respect. The aniline point is the lowest temperature at which equal volumes of the oil and aniline are miscible. Relatively high aniline values are associated with low-swelling tendencies or tendency to cause shrinkage and relatively low aniline values with high swelling.

Flammability: Flash point of an oil is the temperature to which it must be heated to give off sufficient vapor to ignite on application of a test flame. Fire point is the temperature to which the oil must be heated to burn continuously after the test flame has been removed. Although flash and fire points of an oil are of some value in determining fire hazard, operating a temperature in hydraulic system should never approach these values. Flash points for petroleum oils range from 350 to 450 F.

Spontaneous ignition temperature is that temperature at which an oil ignites without the aid of an external ignition source. Petroleum-base hydraulic oils of the viscosity range normally used in industrial hydraulic systems have spontaneous ignition temperatures of 500 to 700 F. 8535

Text 4