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Wednesday, 27 November 2013

Hydraulic Oil

Hydraulic Oil Points : Hydraulic Oil, Hydraulic Oil Requirements, Chemical Stability, Demulsibility, Film Strength, Proper Viscosity Hydraulic Oil Requirements Since the average hydraulic system is a more or less complex assembly of operating cylinders, valves, cams, stops, etc, and since most of these parts are operated by the hydraulic pressure developed by pump, sluggish or contaminated oil can interfere with their proper functioning and seriously impair the efficient operation of the machine. Maximum economical production, therefore, implies the use of a fluid medium that will act uniformly, promptly, and with undiminished effectiveness at all times. A properly selected hydraulic oil must have: 1. Chemical Stability Its mean to resist oxidation and thus the formation of sludge or gummy deposits. Oils differ widely in their chemical composition and, therefore, in their chemical stability. In a hydraulic system, constant circulation and churning in the presence of oxygen tend to produce chemical changes in the oil. Oils that cannot resist this tendency thicken and become sluggish in service. Sluggish oils retard the operating sequence and slow down production. Eventual, sludge or gummy deposits form and interfere with the reliable action of the machine tool. Therefore, oil that resist chemical change and retain .its original characteristics longer will render the most satisfactory service. 2. Demulsibility Maximum demulsibility, to separate readily from water, and thus to minimize the formation of emulsions. Moisture is often present in the hydraulic system of a machine tool. This moisture may result from leakage of cutting fluid into the system or may result from condensation of the moisture in the atmosphere as air surges in and out of the reservoir breather pipe. Of course, every effort should be made to seal out cutting fluid and to eliminate this contamination. When water mixes intimately with oil, emulsions are formed. As a result of local operating conditions, these may be of a thin, slimy nature; of a sticky, pasty consistency; or in the form of heavy, gummy deposits. Such emulsions may interfere with the proper functioning of valves and other delicately adjusted parts.

The resistance of an oil to emulsification depends upon its ability to separate quickly from moisture in order that water which enters the system shell settle to the bottom of the reservoir and not be circulated with the oil. To maintain quick separation in services the oil must be able to resist oxidation. In other words, it must be chemically stable, so that it does not change under operating conditions.
3. Film Strength Adequate film strength, to minimize the wear of pumps, valves, cylinders, ‘pistons, etc, and to some machines. to prevent chatter of tables and sticking of slides. Hydraulic oils not only serve as the means for transmitting pressure, but also act as lubricants for the moving parts of the pumps, cylinders, valves, etc., and sometimes, of ways. Pressures between some of these moving parts may be extremely high. In order to prevent excessive wear, particularly where fluid pressures are high, hydraulic oils must be capable of providing strong lubricating films that resist the pressures and wiping action between moving parts at whatever operating temperatures may be met. Since a lubricating-oil film under these conditions is only microscopic in thickness, it must possess unusual film strength.

The lack of adequate film strength results in excessive wear and unnecessary power consumption. Wear inside a machine increases internal clearances and therefore internal leakage, while wear at glands and stuffing boxes increases external leakage. Both reduce the over-all efficiency of the machine. Moreover, when wear occurs, the metallic contact between the moving parts develops excessive frictional heat, which increases the oil temperatures and thins the oil.

Hydraulic oils must possess the necessary film strength to resist the severe wiping action that occurs between some of the moving parts. The development of such oils is the result of many years of research made under working conditions similar to those of the machines themselves.
4. Proper Viscosity Proper viscosity and minimum change of viscosity, to minimize leakage; and at the same time assure ready flow and prompt response to all controls. and to assure uniformly high production during the warming-up periods. In all hydraulic pumps there is always more or less interval leakage. Frequently referred to as slippage. Although this leakage does not involve actual loss of oil from the system, it does lower the capacity of the pump and increase oil temperature. In variable stroke piston pumps, moderate slippage can be compensated for by lengthening the strokes of the pistons.

With a gear or a vane pump, in which a by-pass relief valve discharge pressure, the volume of oil discharged is always greater than the demands of the working cylinders, and the excess flows through the relief valve back to the reservoir. Moderate slippage in the pump merely reduces the flow of oil lost through the relief valve. It does not change the pressure against which the pump operates or require that the speed of the pump be increased. Slippage does increase the temperature of the oil at the pump. In order to minimize slippage and maintain maximum pump capacity with minimum power consumption and low oil temperature, it is most important to use an all of viscosity (resistance to leakage) which is suited to the particular design of the pump. Viscosity, however, must be considered also from the standpoint of the, ready flow of the oil through the system, and the prompt response of valves and other parts. Light-bodied oils assure ready flow and quick response, but their use may result in excessive internal leakage and high power consumption. Heavier-bodied oils offer a higher resistance to leakage but they are sluggish and, therefore, require more power for circulation through the pipes, valves, and openings.

Since the choice of viscosity is influenced principally by the design of the pump and, to some extent, by the nature of the system, it is always wise to consult the instructions issued by the various manufacturers of hydraulic-machine tools, who specify the most suitable oil viscosity for their particular pump or pumps.

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