The subject of the present invention is a method for minimal lubrication of mechanical units in motion by means of a two-phase fluid according to the preamble of the main claim. The subject of the invention is also a system for minimal lubrication of mechanical units in motion by means of a two-phase fluid according to the preamble of the main claim.

Systems of the aforementioned type, and by means of which it is possible to implement a method as previously described, have been known for some time. For example, DEI 02006030651 describes a system for obtaining minimal lubrication (with nebulised oil) of a tool, comprising a nebuliser for emission of a first fluid, containing lubricant and water, within a duct where a second, transport fluid, usually air, moves. The mixture thus obtained is sent to a tool, for lubrication of its parts. The detection of the flow rate of the lubricant fluid is included and the nebulisation of this fluid is regulated by means of the flow rate information detected, so as to keep this flow rate at a predetermined value.

In particular, these known systems permit minimal lubrication, by means of a nebulised air-oil mixture, of mechanical components and elements which are subject to friction, such as, for example: ball bearings of high-speed mandrels of machine tools used in workstations, milling machines, boring machines, grinders, drills, lathe centres, and of all the rotary bearings which support high-speed rotary shafts, as well as the tools used in processes for removal of shavings. In the case of lubrication of the tools, the flow rates of the (nebulised) lubricant are in the order of millilitres per minute, so as to obtain an efficient lubrication effect. On the other hand, in the case of lubrication of mechanical units such as ball bearings, the lubricant fluid is a two-phase fluid (mixture of air and oil), wherein the oil remains in the liquid state with flow rates of microlitres per minute. In the latter case, the lubricant is present in the form of liquid streaks in pressurised ducts.

EP2333396 describes a device for minimal lubrication of tools by means of nebulised oil (not a two-phase mixture), comprising a tank for the lubricant and means to supply this lubricant under pressure to at least one modular element. The latter comprises a duct on which a flow regulator and a compressed air circuit are placed. The lubricant duct and the compressed air duct are coupled to an air-lubricant mixing element. EP2333396 does not describe any sensor for measuring the flow rate of the lubricant .

In the known systems where the lubrication is carried out with a two-phase (air-oil) fluid dosing a small volume of lubricant, this lubrication takes place cyclically over a period of time, in a continuous flow of pressurised air. Nevertheless, this procedure gives rise to discontinuity in the supply of the lubricant which, contrary to what happens in these systems, should be distributed in a minimal, continuous quantity, in order to obtain optimum lubrication of the mechanical elements with which these systems cooperate. In lubrication with a two-phase fluid, the reason for this discontinuity is associated with the difficulty of dosing small quantities of oil below the limit of 10 mm ³ (i.e. of 10 μΐ = 10 microlitres) . This gives rise to the need to dilute the doses of oil so that they are smaller than this limit, in a continuous flow of compressed air, which flow has an effect of distribution and homogenisation of the quantities dosed cyclically. However, this distribution and homogenisation is associated with the length of the duct which conveys the air-oil mixture to the unit to be lubricated, which duct must therefore be sufficiently long (at least 3 m) . However it is not always possible for this to be carried out.

Consequently, although the length of the piping for supply of the air-oil mixture constitutes a means for diluting and regularising the flow rate of lubricant at the point of lubrication, the lubrication procedure implemented in all of the known air-oil systems involves on the other hand a discontinuity in terms of flow of lubricant supplied at the point of lubrication. This constitutes a constraint for the high-speed performances of the unit lubricated, for example the lubricated ball bearing of the corresponding high-speed mandrel on which the bearings are fitted.

Also, an additional problem of the known solutions is the lack of precision of the dosing devices implemented with volumetric pumps and the like. Any variations or abnormalities of supply of the lubricant or of the air-oil lubricant mixture, especially in the field of bearings for high speed, cause problems in terms of excess or lack of lubrication .

For this reason, the most highly-developed lubrication systems are controlled by optical sensors (for example the one described in DE 102006030651) placed on the piping for supply of the air-oil mixture, also known as streak sensors, which permit regulation of the flow rate of the oil or mixture which contains it. However, these known systems also have applicative limits, for example the minimum value of the dose of lubricant which can be regulated and also the maximum diametral size of the regulated piping.

In addition, the optical regulation of a streak of lubricant which moves within a transparent pipe does not really involve regulation of the flow rate of the dosed lubricant, but only a check on the functioning of the lubricant fluid supply system, i.e. whether or not the oil is present in the transport fluid (air) . For example, WO2010/128380 describes a method and a device for minimal lubrication which includes the use of an optical sensor to detect an air-oil mixture within a duct, which mixture is to be conveyed to a unit to be lubricated.

Devices are also known for the micro-supply of low- viscosity fluids, such as, for example, piezoelectric micro-pumps which, although they guarantee a minimal supply of approximately Ιμΐ/sec (microlitres/second) , are not able to supply fluids of a certain viscosity, such as lubricant oils, and at pressures higher than 1 bar (0.9 bar or 90 kPa) . These devices are therefore not able to supply a high pressure fluid, as is the case for a lubricant fluid which must necessarily be supplied at pressures of at least 5-6 bars and immersed in a compressed air duct which can also have a pressurisation value of 6 bars.

Furthermore, the supply pressure obtained with micro- pumps of this type is not always constant, but depends on the characteristics and counter-pressures generated in the downstream section of the air-oil lubrication supply circuit .

An objective of the present invention is to provide a method and a system which permit minimal lubrication of the aforementioned type which is improved in comparison with that obtained with the known methodologies, systems and devices .

In particular, an objective of the invention is to provide a system and a method for minimal lubrication which permits a continuous or repeated minimal, almost constant, lubricant dosing, (e.g. 10 μΐ/min or microlitres/minute) with precise, regular and laminar motion at an appropriate regulated supply pressure, i.e. which is measured with precision .

Another objective is to provide a method and a system of the aforementioned type which prevent the need to have lengthy piping for the dilution/distribution of relatively large quantities of 10 mm ³ (or 10 μΐ ) dosed cyclically, but above all to guarantee the efficient measured regulation of minimal flow rates of lubricant. Another objective of the present invention is to provide a system and a method for minimal lubrication which permit stable and continuous supply of the lubricant, independently of the variable pressure of the duct for supply of the air-oil mixture to the ball bearing to be lubricated .

A further objective of the present invention is to provide a system and a method for minimal lubrication with supply of the phases (air-oil) which can be modulated both in terms of air pressure and in terms of volume of lubricant, by means of a device for supply of lubricant which is controlled based on measurement of the flow rate of the lubricant.

Another objective of the present invention is to provide a system and a method for minimal air-oil lubrication, in particular for lubrication of ball bearings, designed to indicate automatically any deficits of lubricant or pressure in the air-oil mixture, which could cause functional abnormalities, with seizing up of the bearing to be lubricated.

These objectives and others, which will be apparent to persons skilled in the art, will be achieved by a method and a system for minimal lubrication of mechanical units according to the appended claims. For better understanding of the present invention, purely by way of non-limiting example, the following drawings are appended, in which:

figure 1 shows a first embodiment of a system according to the invention;

figure 2 shows a second embodiment of a system according to the invention;

figure 3 shows a schematic view of part of the system in figures 1 and 2;

figure 4 shows a schematic view of the part in figure 3 during use; and

figure 5 shows a graph showing the measurements of lubricant fluid flow rate (relating to amplitude and frequency of an electrical signal generated by a self- mixing laser interferometer) when there is variation in the speed of movement of the lubricant fluid in a transport duct 3.

With reference to figure 1, a system according to the invention for minimal air-oil lubrication of a mechanical unit is indicated generally as 1. This system comprises a tank 2 containing a first fluid, i.e. lubricant or oil, from which there extends a first duct 3 which is connected by means of a one-way valve 4 to a second duct 5 (for mixing) in which a second, transport fluid moves; the arrangement at right-angles of the ducts 3 and 5 is purely by way of non-limiting example of the invention, as these ducts can be arranged at different angles.

The tank 2 is connected to a pressure regulator 7 which generates a pressure in the tank 2 in order to obtain a predetermined flow rate of first fluid or lubricant within the pipe 3. The pressure regulator is connected to a command and control unit 8, preferably with a microprocessor, which controls the functioning of the system 1.

This unit is also connected to a pressure sensor 10 which detects the pressure of the lubricant in the tank 2, and to a pressure sensor 12 associated with the second duct 5 downstream from the valve 4, in the direction of the motion of the second, transport fluid, which is generally air. In particular, this sensor 12 is placed along this second duct corresponding to a section of the latter in which the two fluids, i.e. oil and air, are mixed. The sensor 12 then detects the pressure in the air-oil mixture.

The second duct 5 is connected to an electronic pressure regulator 13 for the second fluid (compressed air), which is collected in any known manner from a compressed air network. The air is clearly also the element which generates the pressure on the surface 15 of the oil (indicated as 17) present in the tank 2, such as to pressurise it for its entry into the first duct 3; the end 3A of the latter which is immersed in the tank 2 is provided with one-way valves 20.

A flow sensor 21 is present on the first duct. According to the invention, this is a generating flow sensor with a laser ray which can detect the rate of flow in the duct 3 extremely accurately. This sensor 21 is connected to the unit 8, which, according to the data received from this sensor and from the sensors 10 and 12, regulates the supply of two-phase fluid to the mechanical unit accordingly.

According to the invention, this sensor, or means for detecting the flow, is a known self-mixing interferometer (see figures 3 and 4) . This is fitted on the duct 3, such that the source of emission 21A of the laser ray 21B is in direct contact with, or in the immediate vicinity of, the duct 3 (usually defined by a capillary tube) . This position, whether it is at right-angles to the duct 3 or inclined (with variable inclination) , is optimum for the control of the quantity of dosed lubricant. This source of emission is usually and preferably a laser diode.

As is known, the self-mixing interferometer is based on measurement of the disturbances induced when a small fraction of the light emitted by a semiconductor laser is backscattered from a remote target. A type of consistent revelation of the radiation takes place, by means of the laser oscillator, such that the power emitted is modulated by an interferometric wave form, which is a periodic function of the backscattered field phase φ = 2ks : where k=27T/ is the wave number at a set wave length λ and s is the distance between the laser and the target.

In the case in question, the interferometric signal is revealed by means of a monitoring photodiode in an electronic circuit 21C of the sensor 21. In the voltage signal photo-generated, there is generation of an interferometric fringe whenever the target undergoes displacement equal to λ/2.

The laser diode or source of laser emission 21A is simply supported on (or placed in the immediate vicinity of) the outer surface of the duct 3 which contains the flow of oil. This avoids the use of optics, and therefore of complex alignments and assemblies, since a measurement Doppler signal 21E is generated by the natural divergence of the laser diode 21A (typically between 10° and 30°), as shown in figure 4.

The self-mixing interferometer 21A produces a type of sum of the signal contributions induced by the various diffusers in motion in the oil: in the case of viscous lubricant fluids, the diffusers are the particles of dye which are advantageously present. At the output a signal is observed, frequency development of which depends on the speed of the flow of oil.

It should be noted that in the field of minimal lubrication, a feature of the lubricant is that it is particularly clean and well-filtered (for example to 3 pm) , in other words, the fluid does not contain pollutant particles or lack of uniformity or dirt, which would give rise to lack of uniformity. This is a fundamental feature for the satisfactory functioning of the system according to the invention. The maximum reliability of use and a long service life of the lubricated mechanical unit are thus guaranteed. Consequently, in order to detect the movement of the fluid, known coloured lubricants or colouring activation pigments are advantageously used.

The interferometer or sensor 21 thus permits accurate measurement of the flow of oil in the duct 3, which oil can by this means be continuously supplied (in a controlled manner) to the duct 5. This is also because of the fact that this oil moves with laminar motion (even with a minimal flow rate), and has a minimal viscosity, and thanks to its clearness, which permits determination of the coloured pigments, as well as its feature of long-term homogeneity. The feature of laminar motion characterises the use of the self-mixing interferometer as a flow measurer, since the latter carries out a superficial speed measurement, which is reliable only in the case of laminar motion. The use of dyes makes it possible to "encode" the oil, thus making the system reliable and repeatable.

Figure 5 shows some measurements when the speed of the fluid varies. In this figure: the curve A indicates a base signal or noise, the curve B relates to a flow of 0.4 drops/s; the curve C relates to a flow of 1 drop/s; the curve D relates to a flow of 2 drops/s; the curve E relates to a flow of 3 drops/s.

According to one embodiment, on the basis of the signals in accordance with the curves in figure 5, a development algorithm has been produced according to which the unit 8 operates. By this means it evaluates a type of barycentre of the logarithmic spectrum, after having subtracted the noise base. The frequency of the barycentre is calculated as:

where the integral extends to fnoise , corresponding to the frequency at which the signal exceeds the noise base of a pre-set threshold (for example 1 dB) . To the algorithm there is added an experimental calibration coefficient which associates f with the flow, thus obtaining a measurement in real-time.

The algorithm thus provides the flow measurement directly .

By this means, by obtaining the precise value of the flow of lubricant 17 in the duct 3, it is possible to regulate the flow precisely (and continuously), whilst keeping it constant in the long term, and thus obtaining continuous lubrication of the mechanical unit.

Figure 1 also shows an air flow regulator 40 placed in the mixing duct 5. In addition, it shows a fine filter (3 pm) for the lubricant.

Figure 2, where parts corresponding to those in figure 1 are indicated with the same numerical references, shows a different embodiment of the invention. It shows the system 1 comprising the oil tank 2 connected to a pump 30, driven by its own motor 31, which thrusts the oil 17 towards a known volumetric dosing element 33, from which there extends the duct 3 where the self-mixing interferometer flow sensor 21 is positioned.

The latter is connected to the control unit 8, which, on the basis of the flow data detected by this sensor and the data detected by the pressure sensor 12, intervenes on the motor 31 of the pump 30 and on the pressure regulator 13, in order to obtain a continuous flow of lubricant to be introduced into the second, transport or air, fluid.

Figure 2 also shows an air flow regulator 40 placed in the mixing duct 5. It also shows a fine (3 pm) filter for the lubricant.

By means of the invention, it is possible to provide a minimal supply of lubricant to a mechanical unit, obtained with repeated, substantially constant dosing of lubricant (with frequency which can be modified) , even in periods of time which are very close together, at a rate of at least 1-2 seconds. The lubricant moves with regular, precise laminar motion, with a predefined supply pressure, which is also regulated and controlled precisely. This permits optimum mixing of the oil (or first fluid or lubricant fluid) dosed in sufficiently small quantities in the air (or second fluid or transport fluid), without needing lengthy piping for the dilution of relatively large quantities (for example 10 mm ³ or 10 μΐ) dosed cyclically, as takes place in the solution according to the prior art.

A supply of the two-phase mixture (air-oil) is also obtained which is independent from the variable pressure present in the duct for supply of this mixture to the mechanical unit.

Descriptions have been provided of different embodiments of the invention. Others are however possible; for example, the laser sensor 21 can also be different from an interferometer or self-mixing unit, but use the same methods for detection of a fluid with stable optical properties in motion on a regular and laminar basis. These variants can also be considered to be included within the scope of the invention as defined by the appended claims.