The mission of oleohydraulic fluid is the one to transmit Hydraulic Power produced by Pump to one or several receiving organs, at the same time as it must lubricate the movable pieces and protect to the system of Corrosion, to clean and to cool or to dissipate the heat. Besides these fundamental functions, the fluid oleohydraulic it must fulfill other requirements of quality.



It is the most important characteristic of a fluid, by means of which its physical conditioning is obtained of lubrication. It is possible to be defined as the internal resistance that to each other offers molecules when sliding some on others. This definition comes to be the expression of Law of Sir Isaac Newton, formulated in 1668, by which determines the necessity to use a force to overcome the resistance of flow of Liquid, that is similar to the resistance to the sliding of Solid . The flow of a liquid is denominated to laminate when the sliding of the liquid laminae that they conform the fluid in movement behave as the laminae (letters) of a deck, when sliding on others. If these slid without no friction (in the case of liquid laminae), the fluid would be perfect, that is to say, without viscosity. When it exists frotamiento between neighboring laminae appears viscosity. In rest state it is not distinguished a perfect liquid of one viscous one. Viscosity pronounces if an internal movement is caused to him by means of some means: draining, fall of a heavy body or ascent of one light one in liquid, etc., where the fluid offers a resistance to the internal sliding of its molecules. Viscosity depends essentially on the nature or bases of the lubricant ( Nafténica, Parafínica, mixed, etc.), like also of the temperature and the pressure, being these two last parameters those that more affect the oil.

Dynamic viscosity

It represents the real viscosity of a liquid and it is obtained by means of a system of precision depression. it measures the time necessary to fill of upwards a united to a capillary down, but located cavity over him, so that the analyzed fluid happens first through the tube (by aspiration) to enter a continuation in the cavity.

Kinematic viscosity

It is defined as the time that delays in passing the liquid of above downwards (by its own mass). The measurement of viscosity is realized by means of denominated apparatuses viscometers. There are them of different types: fall of ball, Engler, Saybolt, Redwood, etc. All of them are based on the fall of the fluid to a certain temperature. In all, the down time of a certain amount of the fluid to testear, multiplied by the constant of the apparatus, it will provide viscosity in Engler degrees directly, seconds Saybolt, seconds Redwood, etc. The figure shows a viscometer of ball fall, in which the relative viscosity of fluid a is obtained to testear based on viscosities known other fluids.
  • An increase of viscosity indicates a polymerisation of the fluid, probably due to a high temperature or to an acidification, by oxidation with lacquer formation.
  • A viscosity fall, indicates a polymer rupture (accompanied by a reduction of the viscosity index), or, a possible dilution of other products (dissolvent, gasoline, etc.) with
appreciable reduction of the flash point. In both cases one is due to consider in the successive ones filled up of the circuit, in which, by error, it is had been able to introduce a fluid with major or minor viscosity. Generally a fallen increase/of maximum viscosity, of the order from the 20 to 25%, according to cases, must to consider itself like use limit.

Index of Viscosity

It is defined as a coefficient that allows to judge the behavior of the viscosity of a fluid; it is in function of the elevation or diminution of the temperature that the fluid is put under. In the common language, denomination S., followed of a number, is used commonly to designate the index of viscosity of an oil. Whichever major is the index of viscosity, as much smaller will be the variation of the viscosity of this one with the temperature variations.

Flash point

It is denominated thus to the temperature in which the steam of the surface of the fluid become inflamed in contact with a flame, and that disappears when retiring the flame. If it is continued raising the temperature, it will be arrived at a point in that the oil will continue burning after retiring the flame: it is the combustion point. If the fluid is warmed up until the suitable temperature, it is arrived at a point in which the oil begins to burn spontaneously, without needing approaching no flame to him: spontaneous combustion is the point of or point of spontaneous combustion, which is far beyond the previous ones. A reduction accused of the flash point indicates a contamination with dissolvents, gasolines, gasohol, etc., and also comes accompanied by a reduction of viscosity. This case usually is rare in fluids hydraulic engineers, but frequent in motor oils. The data of the flash point is important since it gives an idea on the security of the use of a fluid, as much as far as fire risk, like of volatileness and evaporation (smoke).

Freezing point

This characteristic is one of most important when the hydraulic systems are destined to work a very low room temperatures. Indeed, since the engine of the machine starts, the fluid it must be able to circulate immediately through the pipes. The freezing point comes bound to dewaxed from oils, that is to say, oils of nature nafténica has freezing points inferiors to oils of parafínica nature. In order to obtain the coagulation or freezing point of an oil, it is possible to be obtained by cooling (reduction of the temperature) or by progressive heating. The oils submissive a gradual reduction of temperature arrive at a point in which they begin a to cloud due to the formation of microcrystals of Paraffin . To this temperature point is denominated to him of fog (cloud point). Although the fluid still maintains its mobility, this point must consider in certain applications such as compressing refrigerators, since from this point can exist difficulties with the valves and discontinuity of lubricant film. If it is continued lowering the temperature, the paraffin crystals will increase of size, until arriving at a point in that the fluid does not present/display mobility some: it is the freezing point. Generally a mineral oil does not have to be used to a temperature inferior to 10º C over its point of freezing. That is to say, if an oil has of freezing point -30º C, it will not be used to temperatures inferiors to -20º C.

Index of Neutralization

It indicates a state of degradation of the fluid. The hydraulic oils own, initially, a certain one acidity that comes from the own oil bases (minimum and practically despicable) and of the additives that it entails. With the use, when being submissive pressures and lifted temperatures, the oils can suffer oxidation process, which is going to degenerate in an acidification. This originating acidity of the oxidation that, as well, it is going to produce a corrosive attack to the pieces of the system. The mineral oils, by its own one nature, is resistant to the oxidation. Diverse the fluid rest behave of way. No, all the good hydraulic fluids take to built-in additives antioxidants with the purpose of to slow down to the maximum this effect. An increase of the acid value of the order of the 100% on the initial acidity is reason for a study of his causes since the increase can progressive and be accelerated (period of induction).

To be able Anticorrosive

The anticorrosive additives fight the action of the humidity and oxide on the organs pilots of circuit. These additives interpose between the metallic surfaces and the water. It is necessary to recognize that by very well that is conceived a circuit, equal will enter the water. It has been observed that the circuits of intermittent operation are seen more affected by this action that continuous operating systems. A hydraulic fluid, besides offering a great resistance to oxidize, must own protective qualities for the system. The fluid will have to protect of the corrosion to the steel and the yellow metals (brass, bronze) that it could have the system, as well as its inertia against the material sealants (together), sleeves and clich3es.

Aniline point

The aniline point of a fluid defines its dissolvent power and allows approximately to anticipate its action on the seals and the inner trimmings of the pipes (flexible). It bears one close relation to the swelling from synthetic rubbers by immersion. According to it is the material whereupon the coating or watertightness devices are made, we examine the phenomena that can be observed superficially or below the ideal aniline point. For a value superior to the ideal aniline point:
  • Softening
  • Swelling
  • Disintegration of the material.
For a value inferior:
  • Hardening of the material.

Index of Desemulción

The minor or major defines themselves as facility that presents/displays an oil to separate of the water that could to contain. The water and the oil give rise to the emulsions, from which muds on which are derived they are determining the impurities and different particles dragged by the oil. The mineral oil water presence is always pernicious, and is very critical in the fluids of transforming, refrigerating machines, precision instruments, etc. Generally are rejectable in all the cases by the corrosion problems that produce, breakage of lubricant film and variations of viscosity. A water content, in stable emulsion, from the 0.5%, can be critical for the system and it will have to come to its elimination. In new oils, its separation potential of water is very important, of form that, case of entrance to the system quickly separates, it by difference of densidades. The desemulsión index is determined putting in a cylindrical test tube 27 cc of the oil to analyze and 53 cc of distilled water. Next the liquids are put under and maintained to the temperature of 54,4°C for little viscous oils and for viscous 82,2°C. By means of a rectangular test tube of 120 xs 20 xs 1.5 mm, turning to 1500 rpm, is shaken the mixture during 5 min. Once finished the milkshake, the volume of oil is written down each 60 s that floats on the mixture (oil that it separates of the water). These indications allow to determine the index:
  • separated oil 1 cc in 1 min = I60 cc/h
  • separated oil 5 cc in 1 min = I300 cc/h

To be able Antiespumante

It is defined as the facility whereupon the air separate of the fluid not to form bubbles, since the fluid not it would tolerate as a steel bar (the fluid would be compressible). Nevertheless, all oil contains air. The foam form in the circuits by a milkshake of the fluid: spheres or air globules consist of, of very diverse sizes, that they can cause a discontinuity of lubricant film, an increase of oxidation of the oil, a corrosion of the metallic surfaces and considerable differences of compressibility in the hydraulic fluid, besides forming a superficial layer that prevents the normal one cooling of the lubricant. By these reasons the fluids with antiespumantes agents are aditivan.


Diverse tests exist on the characteristics that must present/display a fluid in front of a filter. filterability is the capacity or facility that presents/displays a fluid to be filtered. According to the type of filtering medium and the temperature of work, the water presence or other additives can reduce the filterability remarkably of oil.

Volumetric change

The coefficient of expansion of the oil is relatively great; this factor is had to consider very when oil volume of and in component or watertight system are facilities with a great ( oil stays pressurized in an element or line of time for a long period). The increase of volume by each 10º of increase in the temperature, is of approximately a 0.


In the majority of applications it is not necessary to consider the fluid compressibility; nevertheless, in some circumstances this factor must be considered to avoid possible problems of operation of system:
  • Great distance between the control element and the receiver.
  • Cylinders of long-hauls with low speeds.
  • Drive of parallel or motor cylinders in rotation with unequal loads.
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