When the pump compresses this bubbly oil, the bubbles violently implode on the high-pressure side, creating loud noises, strong vibrations and wear of internal pump parts. This pressure reduction results in the creation of vaporous bubbles and causes air normally dissolved in the oil to be desorbed and become entrained as air bubbles. Such fluid immobility can starve a pump, cause damaging vaporous cavitation and produce high fluid and mechanical friction, not to mention lubricant starvation of bearing surfaces.Ĭertainly, the usefulness of a fluid as a lubrication medium at low temperature hinges upon its viscosity and pour-point characteristics.įor hydraulic circulating systems, high oil viscosity causes a drastic drop in the oil’s static pressure as suction draws the oil into the pump’s inlet. Such low temperatures can cause petroleum-based fluids to increase in viscosity and eventually reach the critical point where the fluid actually congeals and will no longer pour or flow. Very low fluid temperatures usually result from exposure of some system part to the external environment, particularly when operation takes place in arctic or high-altitude conditions. Low temperature can damage the temperature stability of a hydraulic fluid or lubricant just as much as high temperature. Operation and maintenance personnel should thoroughly investigate an occurrence of temperature instability to understand the effects on machine operation in order to optimize its performance and prolong equipment service life. High temperature also accelerates wear, destroys hydrodynamic lubrication regimes, increases the oxidation rate, fosters additive depletion and affects other critical aspects of the machine.įluid temperature instability is the result of various machine operating factors such as component integrity (design, selection, manufacture, application and maintenance), duty cycle severity (load application, magnitude and duration), environmental hostility and heat absorption/desorption. When temperature is too low, fluid viscosity is high.Īt low temperatures, the fluid often reaches the point where it actually congeals and will no longer flow (pour point). Temperature extremes have a pronounced effect on component materials as well as machine performance. If left unabated, the conditional failure ultimately results in both material and performance degradation of machine components. The machine loses stability and experiences conditional failure whenever the system’s fluid temperature violates these limits. All hydraulic and lubricating fluids have practical limits on the acceptable operating temperature range - both high and low levels. At any rate, I intend to make some measurements myself of the temperature coefficient of liquid viscosity, in the hope that they may throw some light on the old problem as to the force exerted on a single molecule by the molecules in its immediate neighbourhood, within the Lorentz sphere.Fluid temperature stability is essential to the success of mechanical systems. I am far from being satisfied with the theory as it stands: my hopes go no further than that the facts cited in my present letter may suggest to some that there is a germ of truth in the point of view put forward. The conception of a transitory and fluctuating ‘crystallisation’ of a liquid seems to fit in with other observations. 1 upon the subject of liquid viscosity called forth a number of letters, and seemed to make it advisable to say a little more of a theory which, little elaborated as it is, offers a picture which may prove helpful. Recently, a letter of mine published in NATURE of Mar. The new technique of X-rays, the Raman effect, and the depolarisation of light may do much to elucidate the structure of liquids, but the older and grosser property of viscosity must be at least as pertinent. A great body of more or less careful observations exist, but it has furnished remarkably little information as to the nature of the liquid state. AMONG the properties of liquids the viscosity is probably the one the investigation of which has suffered most from lack of any accepted theory, however crude and approximate, to guide it.
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