WO2015178823A1

WO2015178823A1 - Bearing arrangement and method for determining optical properties of a lubricant in a bearing

Info

Publication number: WO2015178823A1
Application number: PCT/SE2015/050508
Authority: WO
Inventors: Nicholas DITTES
Original assignee: Aktiebolaget Skf
Priority date: 2014-05-19
Filing date: 2015-05-07
Publication date: 2015-11-26

Abstract

There is provided a bearing arrangement comprising: a bearing having an inner race an outer race and a plurality of rolling elements arranged between said inner and outer race such that said first race is rotatable relative said second race; a bearing lubricant arranged within said bearing to lubricate said rolling elements; an optical emitter configured to emit light comprising wavelengths within a wavelength range, and an optical receiver arranged so that light emitted by said emitter passes through a portion of said grease before reaching said receiver; wherein said optical receiver is configured to determine an intensity of received light at a plurality of wavelengths within said wavelength range. There is also provided a method for determining a condition of a lubricant in a bearing.

Description

BEARING ARRANGEMENT AND METHOD FOR DETERMINING OPTICAL PROPERTIES OF A LUBRICANT IN A BEARING

Field of the Invention

The present invention relates to a bearing arrangement. In particular, the present invention relates to a bearing comprising an optical sensor for determining a characteristic of grease in the bearing.

Technical Background

Bearings and in particular rolling bearings are used in a wide range of applications comprising rotating shafts. Rolling bearings provide low friction rotation by means of the rolling elements arranged between the outer and inner race of the bearing. To further reduce the friction between the rolling elements and the races, thereby increasing the life length of the bearing, bearing grease is often used to lubricate the rolling elements.

In order to improve the availability/up-time and life span of, for example, a wind turbine, various bearing related parameters may be monitored, such as, for example, vibrations, lubricant pressure, lubricant quality, lubricant temperature and bearing load.

It has been found that bearing failures are often caused by water in the bearing grease. Water in the grease can cause surface erosion and cavitations within the bearing. Also, it can cause hydrogen embrittlement due to the extreme pressures in rolling element bearings, as high as 1 to 3 GPa, where water can break down into its constituent atoms causing hydrogen to penetrate the surface of the bearing elements and cause the surface to become more brittle. Water can also cause additives to precipitate and cause abrasive particles or sludge to form. Water also causes the base oil to oxidize much more quickly. Even without the addition of external water, water content in the grease may increase slightly due to oxidation of hydrocarbons in the grease. For example, a bearing exposed to the elements such as a wheel bearing on a train or in a car is more susceptible to contamination, both by water and by other contaminating particles.

Accordingly, it is important to be able to monitor the condition of the bearing so that a bearing can be replaced before it breaks.

Although the monitoring that is currently described in the art helps the scheduling of maintenance and prediction of failures before they occur, and thereby may improve the availability of rotating systems, many such systems are both complicated and impractical to use in many applications.

Summary of the Invention

In view of the above-mentioned desired properties of a bearing, and the above-mentioned and other drawbacks of the prior art, it is an object of the present invention to provide an improved bearing and a method for monitoring the properties of the bearing so that unexpected bearing failure may be avoided.

According to a first aspect of the invention, it is therefore provided a bearing arrangement comprising: a bearing having an inner race an outer race and a plurality of rolling elements arranged between said inner and outer race such that said first race is rotatable relative said second race; a bearing lubricant arranged within said bearing to lubricate said rolling elements; an optical emitter configured to emit light comprising wavelengths within a wavelength range, and an optical receiver arranged so that light emitted by said emitter passes through a portion of said grease before reaching said receiver; wherein said optical receiver is configured to determine an intensity of received light at a plurality of wavelengths within said wavelength range.

In the present context, a race of a bearing refers to a ring with a groove where the rolling elements rest, such that the outer and inner races are rotatable relative each other by means of the rolling elements. The rolling elements may be balls or any type of roller known to the skilled person, such as a cylindrical roller, spherical roller, tapered roller, or needle roller.

Commonly used lubricants for bearings include different types of grease and oil. Moreover, the intensity of the received light may also be described in equivalent terms such as the absorbance or transmittance of the light for emitted light having known properties.

An optical emitter may typically be a light emitting diode (LED). Light emitted by the optical emitter may include both visible light and light in the infrared (IR) and ultraviolet (UV) wavelength ranges. The optical emitter may thus be a broadband emitter configured to emit light within a wide range of wavelengths. The optical receiver may be any suitable optical receiver, such as a photodiode, configured to receive and determine the intensity of light at a plurality of predetermined wavelengths within the wavelength range of light emitted by the emitter.

The present invention is based on the realization that the properties of the bearing lubricant may be determined by allowing light to propagate through the lubricant and by monitoring the intensity of light having

propagated through the lubricant for different wavelengths of light. A change in optical properties of the lubricant is typically an indication of deteriorating lubrication properties. As an example, an increase in water content in the lubricant would lead to an increase in total absorption in near infrared wavelengths of light due to the higher absorption of light in water compared to in pure lubricant. Contaminants in the lubricant such as particles may also lead to a reduced intensity in the received light due to scattering and an increase of absorption. Moreover, by observing at which wavelengths the properties have changed further information can be determined, such as the nature or cause of the change. Thereby it can be determined if a change of properties is potentially dangerous and if there is an enhanced risk of bearing failure.

In one embodiment of the invention, the optical receiver may advantageously be a photodetector array comprising a plurality of

photodetectors each configured to detect a wavelength within the wavelength range, such that all wavelengths within the wavelength range are detected. The photodetector array may thus comprise a range of photodetectors detecting different sub-ranges within the wavelength range of emitted light, thereby separately determining the intensity of received light at different wavelength ranges.

According to one embodiment of the invention, the optical receiver may advantageously comprise: a photodetector array comprising a plurality of photodetectors each configured to determine an intensity of light within the wavelength range; and a wavelength dividing element configured to divide the received light into a plurality of distinct wavelengths, and to provide each of the distinct wavelengths to a respective photodetector. By means of the wavelength dividing element, only light of a specific selected sub-range of the total wavelength range is received by a corresponding photodetector.

Thereby, each photodetector only receives light in at the wavelengths at which is configured to detect, and a more sensitive measurement with less noise can be achieved. Various optical filters may also be used to the same effect or to further enhance the wavelength selectivity.

In one embodiment of the invention, the wavelength dividing element may advantageously be a diffraction grating. Similarly, in one embodiment of the invention the wavelength dividing element may be a dispersive prism. Both a diffraction grating and a dispersive prism can achieve the desirable wavelength division discussed above.

According to one embodiment of the invention, the optical emitter may advantageously comprise light sources configured to emit multiple

wavelengths of light ranging from ultraviolet, visible to infrared light. Visible light may for example be provided by a combination of red green and blue (RGB) optical emitters, typically in the form of light emitting diodes (LEDs), and the visible spectrum of light can be analyzed to determine the color of the lubricant. Such analysis is enabled by an optical receiver capable of detecting the intensity of light at different wavelengths of the visible spectrum. A determination of the color of the lubricant can be used to quantify solid or liquid contaminants, such as dust, particles or oxidation products. The wavelength of light emitted by an RGB LED can also be fine tuned making it possible to detect not only color, but also the presence of specific

components and substances in the lubricant by accurately quantifying changes in absorption at specific wavelengths. Moreover, near infrared light is advantageously used to observe the absorbance due to water to determine water content in the lubricant and near ultraviolet light may be used to detect oxidation of the lubricant. Accordingly, by providing an optical emitter capable of emitting light within a wide wavelength range or multiple single wavelength emitters, different properties of the lubricant can be determined. Near infrared light can be seen as light having wavelengths in the range of 900 nm to 1450 nm, and near ultraviolet as having wavelengths in the range of 200 to 400 nm.

According to one embodiment of the invention, the optical emitter and optical receiver is advantageously arranged so that light emitted by the emitter travels in a substantially straight path to the receiver. Such an arrangement allows for a straight forward mounting of components, where losses between the optical emitter and receiver can be kept as low as possible.

According to one embodiment of the invention, the bearing

arrangement may further comprise a redirecting element arranged to redirect light from the optical emitter towards the optical receiver. Utilizing one or more redirecting elements allows for more flexibility in positioning of the optical components as it is not necessary to have direct emission from the emitter to the receiver. Thereby, one or more redirecting elements for redirecting light from the emitter may be arranged in the bearing so that light may propagate through a larger portion of the lubricant before reaching the receiver, thereby increasing the accuracy of the measurement. The redirecting element may for example be a reflecting element such as a mirror or a prism.

According to one embodiment of the invention, the bearing

arrangement may comprise a first and a second sealing ring arranged on respective sides of said bearing, wherein said optical emitter and said optical receiver is arranged on one of said first and second sealing ring. A sealing ring is used to seal the bearing such that bearing lubricant remains within the bearing and to protect the rolling elements form contaminants. Where such a sealing ring is used, the optical emitter and receiver may be arranged on the sealing ring. Thereby, it can be ensured that emitted light propagates through the lubricant on its way to the receiver. Moreover, dedicated sealing rings may be provided which allow the incorporation of the optical components without the need to modify the remainder of the bearing. This would also facilitate retrofitting of existing bearings by replacing the sealing ring.

However, the optical components may equally well be arranged at other locations within the bearing, such as together with a bearing cage typically present within the bearing. Furthermore, in a bearing comprising two rows of rolling elements, the optical components may be arranged between the two rows.

In one embodiment of the invention, the bearing arrangement may further comprise a temperature sensor arranged to measure the temperature of the lubricant. During certain operational conditions, the temperature of the bearing lubricant may change, such as during startup or stop of a rotating machine connected to the bearing. As the absorption of light in water is temperature dependent, the intensity of the transmitted optical signal may vary as a result of changing temperature without any change of composition of the lubricant if there is water in the lubricant. Accordingly, by measuring the temperature the variations in absorbance due to temperature can be accounted for and the properties of the lubricant can be measured also during periods where the temperature of the lubricant is changing.

According to a second aspect of the invention, there is provided a method for determining a condition of a lubricant in a bearing comprising: a bearing having an inner race an outer race and a plurality of rolling elements arranged between the inner and outer race such that the first race is rotatable relative the second race; a bearing lubricant arranged within the bearing to lubricate the rolling elements; at least one optical emitter and an optical receiver arranged so that light emitted by the emitter passes through a portion of the lubricant before reaching the receiver, wherein the optical emitter is configured to emit light comprising a plurality of wavelengths within a predetermined wavelength range and the optical receiver is configured to determine an intensity of received light for at least one wavelength within the wavelength range; the method comprising: by the emitter, emitting light comprising a plurality of wavelengths within the predetermined wavelength range into the lubricant; by the receiver, receiving light having passed through a portion of the lubricant; determining a value indicative of an intensity of the received light for at least one wavelength within the wavelength range;

determining a difference between the value and a predetermined value; and if the difference is larger than a predetermined threshold value, determine that properties of the lubricant have changed.

The monitoring of properties of a bearing is commonly referred to as condition monitoring. Herein, to monitor the condition of the bearing lubricant refers to monitoring a change in properties of the lubricant which influenced the optical properties of the bearing.

Through the above described method, it is possible to determine if the properties of the lubricant have changed by observing the difference between the measured intensity value and a predetermined intensity value. The difference should be larger than a predetermined threshold value to be indicative of a change of properties. The threshold value may for example be set so that common measurement inaccuracies typical for the system are not mistaken for changed lubricant properties.

In one embodiment of the invention, the method may further comprise further comprise: in the optical receiver, determining a respective first and second value indicative of an intensity of the received light of a first and second wavelength within the wavelength range; determining a first and a second difference between the first and second value and a corresponding predetermined first and second value; and if the first difference is larger than a first predetermined threshold value, determine that a first property of the lubricant has changed, and if the second difference is larger than a second predetermined threshold value, determine that a second property of the lubricant has changed. By determining if there is a change in intensity of received light at different wavelengths, it is possible to determine different properties of the lubricant, as different contaminants absorb light differently at different wavelengths.

According to one embodiment of the invention, the method may further comprise emitting light comprising the wavelengths 970 nm or 1450 nm; in the receiver, determine an intensity of received light having a wavelength of 970 nm or 1450 nm; and if the difference is larger than the predetermined threshold value, determine that a water content of the lubricant have changed. It is well known that water has absorption peaks at approximately 970 nm and 1450 nm. Thus, by selecting an optical emitter emitting light comprising one of the aforementioned wavelengths, and a receiver capable of determining the intensity of received light for at least one of the wavelengths, it is possible to more accurately determine if there is any water in the lubricant since the sensitivity of the measurement is increased.

In one embodiment of the invention, the predetermined value may advantageously be a previously determined value for the same bearing in the same application. The predetermined value may for example be the most recently determined value such that the difference between consecutive measurements values is continuously monitored. The interval between consecutive measurement values may be arbitrarily set. Alternatively, or in combination, the measured value may also be compared with a reference value based on measurements on similar bearing. The reference value may also be based on analytical models and simulations. Furthermore, the predetermined intensity value may be determined based on known properties of the emitted optical signal and the lubricant. If the optical properties of the lubricant are known, the expected intensity of a received signal can be determined for a given intensity of the emitted light. The intensity of the received light may then be compared to the expected intensity, and a difference will indicates that the properties of the lubricant are other than what is expected.

Further effects and features of this second aspect of the present invention are largely analogous to those described above in connection with the first aspect of the invention.

Additional features of, and advantages with, the present invention will become apparent when studying the appended claims and the following description. The skilled person realizes that different features of the present invention may be combined to create embodiments other than those described in the following, without departing from the scope of the present invention.

Brief Description of the Drawings These and other aspects of the present invention will now be described in more detail with reference to the appended drawings showing a example embodiments of the invention, wherein:

Fig. 1 schematically illustrates a bearing arrangement according to an embodiment of the invention;

Figs. 2a-d schematically illustrates sensing units according to various embodiments of the invention;

Fig. 3 schematically illustrates a sensing unit according to an embodiment of the invention; and

Fig. 4 is a flow chart outlining the general steps of a method according to an embodiment of the invention.

Detailed Description of Preferred Embodiments of the Invention

In the present detailed description, various embodiments of bearing arrangement according to the present invention are mainly discussed with reference to a ball bearing. It should be noted that this by no means limits the scope of the present invention which is equally applicable to any type of bearing comprising a lubricant.

Fig. 1 is an exploded view of a bearing 100 comprising an inner race 102, an outer race 104 and a plurality of rolling elements 106 in the form of balls arranged between the inner and outer race. The bearing further comprises sealing rings 108a-b arranged on the respective sides of the bearing to seal the bearing. A lubricant 1 10 is arranged within the bearing to provide lubrication for the rolling elements 106. Bearings are also commonly provided with a cage for keeping the rolling elements in place. Such cages may be arranged and configured in many different ways known by the skilled person, and bearing cages are therefore not shown herein to avoid

complicating the drawings. An optical emitter 1 12 and an optical receiver 1 14 are arranged attached to one of the sealing rings 108a. Here, the optical emitter 1 12 and the optical receiver 1 14 are vertically arranged so that light can travel in a straight path from the emitter 1 12 to the receiver 1 14. The optical transmitter 1 12 is a light source configured to emit light of a plurality of wavelengths and may for example be a single light emitting diode (LED) or comprise a plurality of LEDs emitting light at different wavelength or wavelength ranges. The optical receiver 1 14 is arranged to receive light emitted from the optical emitter after the light has passed through a portion of the lubricant 1 10 and to detect an intensity of the received light at different wavelengths. The optical receiver 1 14 for example comprises a plurality of conventional photodiodes configured to determine the intensity of received light at different wavelengths to perform a spectroscopic analysis of the received light. The optical receiver 1 14 may also comprise an array of identical photodetectors combined with an optical filter arrangement so that each photodetector receives a unique subset of wavelengths from the emitted wavelength range. The optical emitter 1 12 and optical receiver 1 14 are here illustrated as being arranged on a printed circuit board (PCB) 1 16, forming a sensing unit 1 18. Thereby, a compact and easily adaptable sensing unit 1 18 is provided for arrangement in a bearing. Even though the sensing unit 1 18 is illustrated as being attached to a sealing ring, the sensing unit 1 18 may equally well be attached to a bearing cage or to one of the races 102, 104 of the bearing, depending on what is appropriate for the particular bearing. Through the flexible layout, the sensing unit 1 18 may be adapted to fit within a wide range of bearings having diameters from a few centimeters up to the very large bearings used in wind mills and the like having a diameter of a meter or more. The sensing device may communicate with a host system through various known communication means, such as through a wired connection through an opening in the bearing or wirelessly via for example RFID. The sensing device may for example be powered through such a wired connection or by means of an internal power supply. Since the sensing device will be arranged in a rotating system, various energy harvesting methods are also feasible for powering the device. The distance between the emitter and receiver is determined based on a range of parameters such as the intensity of light emitted by the emitter, the sensitivity of the receiver and the properties of the lubricant. For each specific application, a suitable configuration will have to be found. It is also possible to provide an optical emitter having variable intensity so that sufficient light can reach the receiver for a range of different circumstances. Figs. 2a-d illustrates various arrangements of optical components that may be used in a bearing arrangement according to embodiments of the invention.

Fig. 2a schematically illustrates a sensing unit 200 comprising an optical emitter 202 configured to emit light within a predetermined wavelength range, a diffraction grating 204 arranged between the optical emitter 202 and an optical receiver 206 comprising a photodetector array. The diffraction grating is configured and arranged to diffract light such that light of different subsets of the predetermined wavelength range falls on different

photodetectors in the photodetector array. To achieve this, the distance between the wavelength dividing element and the receiver must be controlled so that the correct subset of wavelengths fall on the corresponding

photodetector.

In Fig. 2b, a refractive prism 205 is configured and arranged to achieve the same wavelength dividing effect as the grating 204 in Fig. 2a.

Fig. 2c illustrates a sensing unit 210 where the optical emitter 212 and the optical receiver 214 are horizontally arranged on the PCB 1 16. The optical receiver is provided in the form of a photodetector array. A light reflecting element 216 such as a mirror is arranged to redirect emitted in a direction towards the receiver 214. Due to the optically translucent properties of commonly used lubricants such as grease, light traveling towards the receiver is scattered in the lubricant so that a portion of the scattered light reaches the receiver 214. Trough suitable arrangement of reflecting elements, one optical emitter may be used together with a plurality of receivers. Thereby, it is possible to determine properties of the lubricant at different locations in the bearing using only one optical emitter.

Fig. 2d schematically illustrates a sensing unit 220 comprising a prism- shaped light redirecting element 222.

Fig. 3 schematically illustrates an embodiment where the sensing unit comprises a plurality of optical emitters 302a-b and one optical receiver 304. The optical emitters 302 are arranged in a substantially circular configuration and a circular light redirecting element 306 is arranged to redirect light from all emitters towards the centrally arranged optical receiver 304 comprising a photodetector array.

The above described bearing arrangements may also comprise a suitably arranged temperature sensor for measuring the temperature of the lubricant.

Fig. 4 is a flow chart outlining the general steps of a method according to an embodiment of the invention. The method will be discussed with reference to the embodiment of the invention illustrated in Fig. 1 .

First, 402, light is emitted by the optical emitter 1 12 such that light propagates through the lubricant towards the receiver 1 14. Next 404, the emitted light is received by the receiver 1 14 and the intensity of received light is determined 406. The determined intensity is then compared 408 to a predetermined intensity. The predetermined intensity value may be a value based on previous measurements on a similar device, or it may be based on theoretical calculations. However, more preferably the predetermined value is a previously measured intensity for the same bearing under similar conditions. It may for example be the most recently measured intensity value prior to the current measurement.

A detected difference is compared 410 with a threshold value, and the difference should be larger than the threshold value to rule out variations caused for example by measurement inaccuracies. The threshold value can be set by performing calibration measurements. If the detected difference is larger than the threshold value, it is determined 412 that the properties of the lubricant have changed. A change may for example give rise to an alert to a host system. By only sending an alert to a host system when it is determined that properties have changed, an energy efficient monitoring system can be provide where a minimum of communication is required. A control unit for controlling the emitter and receiver and for performing the required analysis may for example be provided on the PCB or external to the bearing.

The control unit may include a microprocessor, microcontroller, programmable digital signal processor or another programmable device. The control unit may also, or instead, include an application specific integrated circuit, a programmable gate array or programmable array logic, a programmable logic device, or a digital signal processor. Where the control unit includes a programmable device such as the microprocessor,

microcontroller or programmable digital signal processor mentioned above, the processor may further include computer executable code that controls operation of the programmable device.

The above described comparison if performed for a number of different wavelengths or sub-ranges of the wavelength range of the emitted light to quantify the absorbance/transmittance spectra of the lubricant in order to determine if properties of the lubricant have changed.

By continuously observing the intensity to determine if there is a change in intensity, changing properties of the lubricant can be identified without knowing the specific transmission properties in a particular

application.

Moreover, the temperature may be monitored so that changes in temperature can be corrected for in the detected intensity.

Even though the invention has been described with reference to specific exemplifying embodiments thereof, many different alterations, modifications and the like will become apparent for those skilled in the art. For example, the described invention may be used in many different types of bearings. Moreover, many different arrangements of emitters and receivers are possible. Also, it should be noted that parts of the system may be omitted, interchanged or arranged in various ways, the bearing arrangement yet being able to perform the functionality of the present invention.

Additionally, variations to the disclosed embodiments can be understood and effected by the skilled person in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantage.

Claims

1 . A bearing arrangement (100) comprising:

a bearing having an inner race (102), an outer race (104) and a plurality of rolling elements (106) arranged between said inner and outer race such that said first race is rotatable relative said second race;

a bearing lubricant (1 18) arranged within said bearing to lubricate said rolling elements;

an optical emitter (1 12, 202, 212, 302a-b) configured to emit light comprising wavelengths within a wavelength range, and an optical receiver (1 14, 206, 214, 304) arranged so that light emitted by said emitter passes through a portion of said grease before reaching said receiver;

wherein said optical receiver is configured to determine an intensity of received light at a plurality of wavelengths within said wavelength range.

2. The bearing arrangement according to claim 1 , wherein said optical receiver is a photodetector array comprising a plurality of

photodetectors each configured to detect a wavelength within said

wavelength range, such that all wavelengths within said wavelength range are detected.

3. The bearing arrangement according to claim 1 , wherein said optical receiver comprises:

a photodetector array comprising a plurality of photodetectors each configured to determine an intensity of light within said wavelength range; and a wavelength dividing element (204, 205) configured to divide said received light into a plurality of distinct wavelengths, and to provide each of said distinct wavelengths to a respective photodetector.

4. The bearing arrangement according to claim 3, wherein said wavelength dividing element is a diffraction grating (204).

5. The bearing arrangement according to claim 3, wherein said wavelength dividing element is a dispersive prism (205).

6. The bearing arrangement according to any one of the preceding claims, wherein said optical emitter comprises light sources configured to emit ultraviolet, visible and infrared light.

7. The bearing arrangement according to any one of the preceding claims, wherein said optical emitter and said optical receiver is arranged so that light emitted by said emitter travels in a substantially straight path to said receiver.

8. The bearing arrangement according to any one of claims 1 to 6, further comprising a redirecting element (216, 222) arranged to redirect light from said optical emitter towards said optical receiver.

9. The bearing arrangement according to claim 1 , further comprising a first and a second sealing ring (108a-b) arranged on respective sides of said bearing, wherein said optical emitter and said optical receiver is arranged on one of said first and second sealing ring.

10. The bearing arrangement according to any one of the preceding claims, further comprising a temperature sensor arranged to measure the temperature of said lubricant.

1 1 . A method for determining a condition of a lubricant in a bearing comprising:

a bearing having an inner race an outer race and a plurality of rolling elements arranged between said inner and outer race such that said first race is rotatable relative said second race;

a bearing lubricant arranged within said bearing to lubricate said rolling elements; at least one optical emitter and an optical receiver arranged so that light emitted by said emitter passes through a portion of said lubricant before reaching said receiver, wherein said optical emitter is configured to emit light comprising a plurality of wavelengths within a predetermined wavelength range and said optical receiver is configured to determine an intensity of received light for at least one wavelength within said wavelength range;

said method comprising:

by said emitter, emitting light comprising a plurality of wavelengths within said predetermined wavelength range into said lubricant;

by said receiver, receiving light having passed through a portion of said lubricant;

determining a value indicative of an intensity of said received light for at least one wavelength within said wavelength range;

determining a difference between said value and a predetermined value; and

if said difference is larger than a predetermined threshold value, determine that properties of the lubricant have changed.

12. The method according to claim 1 1 , further comprising:

in said optical receiver, determining a respective first and second value indicative of an intensity of said received light of a first and second

wavelength within said wavelength range;

determining a first and a second difference between said first and second value and a corresponding predetermined first and second value; and if said first difference is larger than a first predetermined threshold value, determine that a first property of the lubricant has changed, and if said second difference is larger than a second predetermined threshold value, determine that a second property of the lubricant has changed.

13. The method according to claim 1 1 , comprising emitting light comprising the wavelengths 970 nm or 1450 nm;

in said receiver, determine an intensity of received light having a wavelength of 970 nm or 1450 nm; and if said difference is larger than said predetermined threshold value, determine that a water content of said lubricant have changed.

14. The method according to any one of claims 1 1 to 13, wherein said predetermined value is a previously determined value.

15. The method according to claim 14, wherein said predetermined value is the most recently determined value.