Hydraulic power transmission naturally lends itself to a wide range of purposes
where multiplication of force is required, or where accurate and dependable control gear must be provided. The immense forces that may be developed by the application of hydraulic principles and precision and flexibility of hydraulic controls, could in many cases be provided only with extreme difficulty by any other means. There is therefore, an almost endless variety of hydraulic gear, but the hydraulic media employed necessarily have many features in common.
The obvious prime requirements of a hydraulic medium are, that it should be relatively incompressible and sufficiently fluid to permit efficient transmission of power. These stipulations alone are met by many liquids and in fact, may have been employed more or less satisfactorily in the past. But conditions of service in modern hydraulic machinery are so precise that few liquids are able to meet their complex requirements.
Water for instance, suffers from a number of disadvantages; it promotes rusting and possesses negligible lubricating properties. Furthermore it is liable to freeze in cold weather and to boil at a temperature which is low compared with that at which oil can be used. Emulsions of soluble oil in water are sometimes used in a partially successful attempt to overcome the two disadvantages mentioned earlier, but even such emulsions are, in general, quite unsuitable for modern applications. As hydraulic media, mineral oils are the most satisfactory products available in sufficiently large quantities and at reasonably low cost.
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Where hydraulic medium with greater fire resistance than mineral oil is required, various types of synthetic and water containing fluids are available. In particular, the fire resistant emulsion fluid, a specially prepared water-in-oil type emulsion, is gaining popularity in this field. Extremely fine clearances are usual in modern hydraulic gear, and to avoid the excessive wear, the hydraulic medium must possess lubricating ability of high order. Furthermore, the designs of hydraulic systems commonly involve the lubrication of associated mechanical gear by the hydraulic fluid. A hydraulic oil must therefore perform dual function -power transmission and lubrication. This must be considered in determining the viscosity, most suitable in a hydraulic oil for a specific application. Rapid and efficient transmission of power is less readily attained with the oils of high viscosity than with those of low viscosity. Greater internal friction with oils of higher viscosity may result in considerable loss of power and increase in operating temperature. In addition, the ability of the oil to maintain fluid-film lubrication of working surface is affected by its penetrating and spreading ability. Oil of too high viscosity may fail in this respect also. On the other hand, the duties of the oil as a lubricant require that its viscosity should not be too low; moreover, an oil of too low viscosity might fail to provide an effective seal at the clearance between moving parts - for example, failure of the oil to seal pump clearances would impair the efficiency of transmission. A further requirement of a hydraulic oil is that its change of viscosity over the service temperature range must not be too great. Some hydraulic systems, e.g. on machinery subjected to wide variation in ambient temperatures and on certain machine tools, require oils with superior viscosity temperature characteristics. Correctly chosen mineral oils are entirely satisfactory in this respect.
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In many hydraulic systems it is impossible to completely prevent the entry of moisture into the oil and where unsuitabile oils are used, contamination with water may lead to the formation of oil and water emulsions. These emulsions vary in consistency but frequently take the form of viscous sludge which could restrict the oil flow and even lead to breakdown. Ability to separate readily from water is therefore an essential requirement for a hydraulic oil to be used in a system where water contamination is liable to occur. Adequate provisions should, of course, be made to draw off separate water from the system. Where emulsions are employed as hydraulic media, the oils from which they are made are specially produced to give the desired emulsion characteristics with a controlled amount of water. A further important consideration is that the characteristic of a hydraulic medium should remain relatively unchanged after long periods of service, so an essential requirement of hydraulic oil is stability towards oxidation. The oil is generally subjected over long periods of oxidising conditions including a combination of some or all of the following influences : high temperatures, high pressures, metallic catalysts and agitation in the presence of air. These will cause rapid breakdown of unsuitable oil, with the formation of oil soluble and insoluble degradation products. This would lead to increase in viscosity of the oil, deposition of sludge, and impairment of the oil's demulsibility and anti-foaming properties. Corrosive attack by acidic oxidation products is also a possibility.
Special hydraulic oils are now available with exceptional stability towards oxidation. They are prepared from carefully selected base oils by special refining treatment and their natural stability is improved still further by the incorporation of oxidation inhibitors.
Another requirement to be considered is that a hydraulic oil must protect the system from corrosion or rusting. That the components of the system are at all times flooded with oil is no safeguard against corrosion unless the oil itself is non-corrosive. Experience has shown that only highly refined oils of high oxidation stability remain free from corrosive acidity for long periods. However, rusting is liable to occur, irrespective of whether the oil is fresh or in a state of degradation, if the ferrous surfaces of the system are in contact with moisture and air entrained in the oil. The oil therefore, must have good metal-wetting properties and must be able to maintain a protective film over the surfaces of the system. Oils incorporating a special rust inhibitor can provide good protection against rust.
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With the advent of modern hydraulic systems it has become necessary for the hydraulic fluid to have special antiwear properties. Oils have been developed having high load carrying capacity, besides having excellent thermal and oxidation stability.
A special requirement for machine tools with very slow moving tables lubricated from the hydraulic systems is that the lubricant should be able to prevent stick-slip or jerky motion on the table. This requirement can be met by the use of special additive.