WO2013057276A1

WO2013057276A1 - Method and device for determining a load zone in a roller bearing

Info

Publication number: WO2013057276A1
Application number: PCT/EP2012/070800
Authority: WO
Inventors: Jeroen Bongaerts; Jordi JIMENEZ VIADER
Original assignee: Aktiebolaget Skf
Priority date: 2011-10-21
Filing date: 2012-10-19
Publication date: 2013-04-25
Also published as: WO2013057273A1; WO2013057275A1

Abstract

The invention relates to a method for determining a load zone (A) in a roller bearing (1), wherein the roller bearing (1) has at least two bearing rings (2, 3) and roller elements (4) between the bearing rings (2, 3). To allow a non-invasive and precise determination of the load zone in the bearing in an easy manner, the invention is characterized in that the method comprises the steps: a) emitting radially directed ultrasonic signals (S) from a radial inner or outer surface (5) of one of the bearing rings (2, 3) into the roller bearing (1) by means of at least two ultrasonic sound generators (6 ', 6", 6 ' ",...) at at least two distanced locations (11', 11 ", 11 ",...); b) sensing the ultrasonic signals (S') at the radial inner or outer surface (5) which are reflected at the transition zone (7) between the bearing ring (2, 3) and the roller elements (4) by means of at least two receivers (8); c) determining and forwarding the contact status between the bearing ring (2) having the ultrasonic sound generators (6', 6", 6"', ) and the receivers (8', 8", 8"',...) and the roller element (4) at each of the distanced locations (11', 11 ", 11",...) by evaluating the reflected ultrasonic signals (S') received by the receivers (8', 8", 8"',...). Furthermore, the invention relates to a device for determining the load zone in the roller bearing.

Description

Method and Device for determining a

Technical Field

The invention relates to a method for determining a load zone in a roller bearing, wherein the roller bearing has at least two bearing rings and roller elements between the bearing rings. Furthermore, the invention relates to a device for determining the load zone in the roller bearing.

Background

In many applications where a roller bearing is used for supporting a machine element the load distribution between the bearing rings and the roller elements is of interest. This information is for example relevant to evaluate the lifetime of the bearing. An unequal load distribution with respective load peaks reduces the lifetime of it. Similarly, location of loaded zones at less favorable positions on the bearing raceway, e. g. due to misalignment and/or unforeseen load forces can have a negative influence on bearing life. Thus, roller bearings have been equipped with measurement devices (e. g. with strain gauges) to detect the load distribution.

Especially, for research and condition monitoring purposes it is very useful to know exactly where the contacts are between the bearing rings and the roller elements. For example, it is very advantageous to be able to determine the mentioned load distribution occurring in a wind turbine bearing.

The known methods to detect the load distribution cannot always be used due to different reasons. Sometimes, strain gauges as well as other devices for the determination of the load acting onto the bearing cannot be employed.

A method using an acoustic signal for stress detection in a bearing is known from US 6,571,632 Bl. Here, an acoustic signal unit comprises a signal generation unit to generate a first electrical signal, a first transducer to generate an acoustic signal in response to the first electrical signal, and a second transducer to generate a second electrical signal in response to the received acoustic signal. A calculation unit is provided to compare the first and second electrical signals to determine the time of flight of the acoustic signal, wherein the time of flight corresponds to the stress in the roller bearing. The stress is then calculated according to a formula by the distance traveled by the acoustic signal across the rolling element bearing in an axial direction, i. e. transverse to the radial direction, and by the time of flight determined by said comparison unit. That is, changes of the speed of sound in the material due to the stress situation are used to determine the stress in the bearing.

US 7,066,027 B2 describes a method for determining the thickness of a lubricant film, which is disposed between two bodies. Here a frequency spectrum on a reflected ultrasound wave is measured and analysed which is sent towards the film which thickness is to be measured.

It is an o bj e c t of the present invention to propose a method and a corresponding device by which the load distribution in a roller bearing between a bearing ring and the roller elements can be determined in another way to provide an additional possibility for load distribution control in a roller bearing. Specifically, the method should be used in a non-invasive manner, i. e. the function of the roller bearing should not be disturbed in any way.

Summary of the invention A s o l u t i o n according to the invention is characterized in that the method comprises the steps: a) emitting radially directed ultrasonic signals from a radial inner or outer surface of one of the bearing rings into the roller bearing by means of at least two ultrasonic sound generators (also referred to as the pulser) at at least two distanced locations; b) sensing the ultrasonic signals at the radial inner or outer surface which are reflected at the transition zone between the bearing ring and the roller elements by means of at least two receivers; c) determining and forwarding the contact status between the bearing ring having the ultrasonic sound generators and the receivers and the roller element at each of the distanced locations by evaluating the reflected ultrasonic signals received by the receivers.

Preferably, in mentioned step c) contact between the bearing ring and a roller element at a location is assumed when the magnitude of the reflected ultrasonic signal below a given reference value; furthermore, no contact between the bearing ring and a roller element is assumed when the magnitude of the reflected ultrasonic signal is above the given reference value. The ultrasonic sound generators and the receivers can be are arranged along an axial direction of the bearing. Emitting and receiving of ultrasonic signals takes place preferably at at least five, specifically preferred at at least 10 distanced locations. So, a high resolution of the load distribution can be obtained. The distanced locations are preferably distanced between 0.05 mm and half the width of the raceway, specifically between 0.1 mm and 5.0 mm. The distance of the location of course depends on the number of sound generators and receivers which are employed. The employed sound generators and receivers respectively are normally equidistantly distributed along the axial extension of the bearing ring. So, a preferred distance is the width of the raceway divided by the number of sound generators / receivers which are used.

Emitting and receiving of ultrasonic signals is preferably done at each position by one and the same element, preferably by a piezo-active element. This allows a small design of the required elements and consequently a high resolution of the load distribution.

The method is preferably used during the regular operation of the roller bearing. Preferably, an ultrasonic signal is emitted into the roller bearing which has a frequency between 10 MHz and 200 MHz, specifically between 20 MHz and 100 MHz, most preferably between 30 MHz and 60 MHz.

An acquisition frequency of the receivers is preferably at least 500 MHz, specifically preferred at least 1 GHz, most preferably at least 2 GHz.

The device for determining a load zone in a roller bearing is characterized in that it comprises at least two ultrasonic sound generators and at least two receivers for an ultrasonic signal, wherein the sound generators and the receivers are located at a radial inner or outer surface of one of the bearing rings and wherein the sound generators and the receivers are arranged at at least two distanced locations.

Preferably, a plurality of ultrasonic sound generators and receivers are arranged in an array forming a one-piece component. The array can have at least five, preferably at at least 10, sound generators and receivers. The sound generators and receivers can be distanced between 0.05 mm and 100.0 mm, preferably between 1 mm and 50.0 mm along an axial extension of the array. In practice, the optimum separation will be determined by the size of the bearing. For very large bearings, such as in wind turbine applications, separations larger than those proposed here can be entirely suitable.

The device can further comprise time measuring means for measuring the time between the emission of an ultrasonic signal by the sound generators and a signal coming back by the receiver. This is useful for determining in addition the deformation of the contact zones between the bearing rings and the roller elements and the load which is transmitted between the bearing ring and the rolling element as described later on.

The sound generators can be designed to emit an ultrasonic signal between 10 MHz and 200 MHz, preferably between 20 MHz and 100 MHz, most preferably between 30 MHz and 60 MHz.

The ultrasonic sound generators and the receivers, specifically the array forming those elements, can be fixed at the bearing ring by an adhesive.

The ultrasonic sound generators and the receivers are preferably piezo-active elements. Those piezo-active elements can form the ultrasonic sound generators and the receivers in one and the same element. The piezo-active elements can be attached onto the bearing ring surface opposite to the raceway.

Accordingly, the invention suggests that an array of sensors is applied to the outer ring or inner ring of a bearing. The sensors can be used to determine whether the opposing surface they are probing is in contact with another solid body (roller element) or not.

A contact will result in a reduced reflectivity that can be picked up by the receivers. If the probed area is not in contact (almost) 100% of the reference echo is returned. If it is in a contact status, then less than 100% is returned and it can be determined that the probed location is in contact with a solid body (roller element). By this way the location of contacts of the roller elements with the raceway in the bearing can be determined on-line in bearing applications.

The described method is very beneficial as it becomes possible to detect the contact status in a non-invasive manner. Also the measurement can be made in a very cost efficient way. The required sound generators / receivers are mounted during the initial assembly of the system. Then, the status of the bearing can be surveyed in an easy way. In the state of the art it is not possible in many applications due to cost reasons to dismount the bearing from the applications, e. g. in wind turbine gear box bearings, to learn something about the contact situation between the roller elements and bearing rings in an early stage of wear. Now, the invention allows a better and cheaper survey.

Thus, potential bearing failure due to inappropriate lading conditions can now be detected directly during the regular operation of the bearing.

It was already mentioned that also the deformation of the bearing rings and the roller elements and also the load can be determined which is transmitted between the bearing ring and the roller elements. For doing so, the time of flight of the ultrasound signal is measured; from the time of flight the location of the transition zone between the bearing ring and the roller element can be calculated. Then, a comparison is made between the location of the transition zone in a loaded status with the location of the zone in an unloaded (load-free) status. Knowing this difference it becomes possible to determine the load by using the Hertzian theory.

Specifically, for doing so the process steps are carried out as follows:

The ultrasonic signal is emitted as explained above radially from the radial inner or outer surface of a bearing ring by means of the ultrasonic sound generators. - The ultrasonic signal which is reflected at the transition zone between the bearing ring and the roller elements is measured by means of the receivers.

Measurement takes place of the time of flight between the emission of the ultrasonic signal and the return of the reflected ultrasonic signal; from this the calculation of the location of the transition zone takes place from the measured time of flight.

Then, a calculation of the radial displacement can be made by comparison of the calculated location of the transition zone and the location of the transition zone in a load free status of the bearing and/or its rings.

By this information the calculation of the load between the bearing ring and the roller element can be done by use of radial displacement.

With respect to last step a preferred approach is that the calculation of the load is done by using a correlation between the radial displacement and the load onto the bearing which correlation is gained empirically, wherein all measured data from all receivers are taken into consideration, i. e. all loads detected by the receivers are added together to get the whole load of the bearing. Thus, prior to the regular operation of the bearing a function (curve) is determined by an experiment, by which the radial displacement is measured in dependence of the load acting onto the bearing. Later on - during the regular operation of the bearing - the calculation of the load onto the bearing in accordance with last mentioned above step takes place be using the measured function, that is, the corresponding load is determined from the function by specification of the measured radial displacement.

But there are also other possibilities for the calculation of the load onto the bearing from the radial displacement according to the last mentioned step above: Another preferred embodiment suggests that the load onto the bearing is calculated from the formula:

with

Ti : radius of the roller element;

r₂: radius of the raceway of the bearing ring (negative value) and

with Vi : Poisson ration of the material of the roller element;

Ei: elastic modulus of the material of the roller element;

v₂: Poisson ration of the material of the bearing ring;

E₂: elastic modulus of the material of the bearing ring.

The mentioned formula is basing on the Hertzian theory concerning the deformation of two spherical bodies under load. For most bearings the equations for appropriate elliptical contacts are more suitable and can be employed as well.

The measurement of the transition zone in a load free status of the bearing and/or its rings is done preferably by carrying out above-mentioned steps for a load free bearing. But there is also another possibility according to the invention to measure and thus to determine the transition zone in a load free status of the bearing and/or its rings: This can be done by carrying out the above mentioned steps in a status, in which no roller element is arranged between the two bearing rings in a radial direction starting from the ultrasonic sound generators, i. e. the ultrasonic sound generators are not on the radial beam where the roller elements are. This means, the measurement of the load free status is done in that time interval when no roller element is arranged in radial direction - seen from the sound generators. During this time interval probed local area (section) of the bearing ring is free from any load as the load transmitting roller element are distanced from the location where the displacement is measured by the sound generators and the receivers.

The central frequency of the ultrasonic pulse determines the resolution of the time-of-flight (TOF) method. The higher the frequency, the higher the resolution, in principle. However, at higher frequencies, absorption of the ultrasonic wave increases significantly in bearing materials, such as steel. This puts a practical upper limit on the central frequency that can be employed. Thus, the above mentioned frequencies of the ultrasound are preferred as well as the above mentioned acquisition frequencies of the electronics on the receiving elements. This is especially relevant in the combination of high-frequency ultrasonic signal with high frequency of the acquisition electronics as this determines the accuracy of the time-of-flight method. This allows with respect of the speed of sound in a metallic material (steel) a resolution of several micrometers concerning the radial displacement.

In a preferred embodiment, the acoustic sound wave is focused onto the bearing raceway to increase temporal and spatial resolution. This can be done by acoustic lenses, by shaping the piezoelements, by using phase modulation methods or similar focusing methods known to experts in the art.

The sound generators are preferably designed to emit an ultrasonic signal (pulse) in the above mentioned and preferred frequency ranges with a narrow pulse length of several oscillations. Equipment known in the art can be used to generate such pulses.

The method described above determines the load of a single rolling-element- raceway contact and here for the different receivers. Therefore, it is a local load determination, not the determination of the overall load applied onto the bearing. To determine the total load on the bearing at first the sum of the partial loads detected by each receiver must be calculated; furthermore all roller elements must be taken into account. For doing so, there are several options: A first possibility is as follows: The loads of all loaded rolling elements need to be determined at several positions around the inner or outer ring. The total load can then be determined by adding the load (vectors) of all elements employed. So, if say three rolling elements are at any point in time in the loaded zone, than the loads of these three elements need to be added to obtain the total load on the bearing. An added advantage is that, in principle, multiple load vectors can be obtained, such as the radial and axial load vector.

A second possibility is as follows: The relationship of the load in the single contact zone in the raceway to the overall load is known through either available calculation models or through an empirically determined relationship. So, the total load is calculated from the load which is determined for one measured roller. Another method to determine load is to determine the contact width of the roller-raceway contact. This can be done in several ways. One way is to evaluate the contact width from the number and location of load sensors that indicate that their probing area of the raceway is in a contact condition. Another method is to monitor the amplitude of the reflected ultrasonic signal as the rolling element passes the sensitive area of any of the acoustic elements. At increasing load the contact area will increase along the rolling direction according to the suitable contact mechanics equations.

The reflected intensity will decreases when the probed area holds the contact and this situation will last until the rolling element leaves the probed area. Therefore, if the surface speed of the moving contact area is known, one can calculate the contact length. At increasing load, this contact length will increase. This method is especially suitable if the contact length is of a magnitude of the order of the sensitive region of the acoustic beam on the raceway. More preferably, the acoustic beam is smaller than the contact area. The latter can be achieved by acoustic focusing. In the latter situation the contact width is equal to the surface speed of the contact on the raceway, multiplied by the time that the contact is in the 'sensitive zone'.

If the sensitive zone is not much smaller than the loaded zone, a deconvolution method needs to be employed. Alternatively, an empirical relation is determined before using the system that links the observed time in contact to the applied load.

Brief description of the drawings

The drawings show an embodiment of the invention.

Fig. 1 shows a radial cross section through a roller bearing and

Fig. 2 shows a magnified part of the upper center part of the roller bearing according to figure 1.

Detailed description of the invention

In Fig. 1 a roller bearing 1 is shown which has an outer bearing ring 2 and an inner bearing ring 3. Between the bearing rings 2, 3 roller elements 4 are arranged. The roller elements 4 are equidistantly distributed around the circumference of the bearing rings 2, 3. The roller elements 4 are held and guided by a cage (not shown). So, when a load F is to be transferred from one bearing ring to the other via the roller elements 4 the specific location is moving in circumferential direction where the transfer of the force takes place.

To determine the load distribution between the outer bearing ring 2 and a rolling element 4 the following process and arrangement is used:

At a circumferential position along the most predominant loading direction - in the (not-limiting) embodiment at the 12 o'clock position - an array 9 is arranged, which contains a plurality of ultrasonic sound generators 6', 6", 6"', ... and receivers 8', 8", 8' ", ... for sound waves. The array 9 is fixed at a radial outer surface 5 of the outer bearing ring 2 and is extending in an axial direction (see direction 'a' in the figures) of the bearing 1. The fixation can take place by means of an adhesive.

In the array 9 a number of ultrasonic sound generators 6', 6", 6" ', ... and a corresponding number of receivers 8', 8", 8"' for sound waves are arranged, see Fig. 2. So, a corresponding number of locations 1 , 11 ", 1 ", ... is covered with a generator 6 and a receiver 8.

It should be mentioned that it is also possible that the two components sound generator 6 and receiver 8 are forming a single element, especially a piezo- active element what is a preferred embodiment. Thus, the emitting and receiving element are one and the same piezo-active element. An electrical signal is used to create an acoustic signal when it is emitting and the returning acoustic signal results in turn in an electric signal at the same element. Thus, in this preferred design the emitter and receiver are the same object.

The ultrasonic sound generator 6 is designed to emit an ultrasound wave between 30 MHz and 60 MHz. This emitted ultrasonic signal S is emitted in radial direction r from the radial outer surface 5 into the outer bearing ring 2. The signal S travels with the speed of sound in radial direction of the material of the bearing ring 2 until it reaches the transition zone 7 between the outer bearing ring 2 and the roller elements 4.

At the transition zone 7 a reflection of the signal S takes place in the case that there is no contact between the bearing ring 2 and the rolling element 4. Thus the reflected ultrasonic signal S' runs back to the surface 5 and is here detected by the receiver 8.

If there is a contact between the bearing ring 2 and the rolling element 4 a significant reduction in the reflection of the signal takes place and consequently a reduced respective signal is sent back to the surface. By measuring the signals coming back from the transition zone 7 by a plurality of sound generators 6 and receivers 8 it becomes thus possible to obtain information concerning the load profile between the bearing ring and the roller elements. More specifically, the width of the load zone A (see Fig. 2) can be determined and it can then easily be said if the contact situation between the bearing ring and the roller element is in order or if problems with the bearing are to be expected due to a unfavorable load distribution or an overall too high load. For the purpose of illustration "+" and "-" signs in a circle have been depicted in Fig. 2 above each generator - receiver unit 6/8. The "+" sign means that a reflected signal S' has been detected by the respective receiver 8; the "-" sign means that no reflected signal was detected. Consequently, by using a sufficient number of generators/receivers 6/8 the width of the load zone A can easily be measured.

For the evaluation by a software if a reflection took place or not, a reference value for the magnitude of the reflected signal S' can be set. If the measured reflected signal S' is below this reference value, the software decides that "no reflection" took place. If a measured signal S' is above this reference value the software decides that "reflection" took place.

Also time measurement means 10 are arranged to measure the time of flight of the signal S from its emission till its return as signal S'. By this information deformations and thus (partial) loads can be determined for a single sound generator/receiver-unit 6/8. As the speed of sound in the material of the bearing ring 2 is known (e. g. v = 5,900 m/s in steel) the location of the transition zone 7 can be calculated by evaluating the time-of-flight of the emitted signal till its return to the outer surface 5. This can be done for a loaded status of the area of the probed raceway of the roller bearing 1 (i. e. when a rolling element is located at the probed raceway) and for a load-free status of the same. It should be mentioned that it is of course also possible to place the measurement means 10 outside the housing 9 at a different location, i. e. in this case this electronic is placed outside the bearing. By combining the described method for determining the load distribution in axial direction of the bearing and by determining the deformations / partial loads by using the time-of-flight method as described it becomes possible to determine the loading profile of the raceway as a function of the axial position of the raceway. This is a valuable information for the assessment of the status of the roller bearing.

Reference Numerals:

1 Roller bearing

2 Outer bearing ring

3 Inner bearing ring

4 Roller element

5 Radial inner or outer surface

6' Ultrasonic sound generator

6" Ultrasonic sound generator

6" ' Ultrasonic sound generator

7 Transition zone between bearing ring and roller element

8' Receiver

8" Receiver

8" ' Receiver

9 Array

10 Time measurement means

1 Γ First location

11 " Second location

11 " ' Third Location

A Load zone

S Emitted ultrasonic signal

S' Reflected ultrasonic signal

F Load onto the bearing

r Radial direction

a Axial direction

Claims

Patent Claims:

1. Method for determining a load zone (A) in a roller bearing (1), wherein the roller bearing (1) has at least two bearing rings (2, 3) and roller elements (4) between the bearing rings (2, 3), characterized in that the method comprises the steps: a) emitting radially directed ultrasonic signals (S) from a radial inner or outer surface (5) of one of the bearing rings (2, 3) into the roller bearing (1) by means of at least two ultrasonic sound generators (6', 6", 6"', ...) at at least two distanced locations (1 1 ', 11 ", 11 "',

.. ·); b) sensing the ultrasonic signals (S') at the radial inner or outer surface (5) which are reflected at the transition zone (7) between the bearing ring (2, 3) and the roller elements (4) by means of at least two receivers (8\ 8", 8" ', ...); c) determining and forwarding the contact status between the bearing ring (2) having the ultrasonic sound generators (6', 6", 6' ", ...) and the receivers (8', 8", 8' ", ...) and the roller element (4) at each of the distanced locations (1 , 11 ", 1 ", ...) by evaluating the reflected ultrasonic signals (S') received by the receivers (8', 8",

2. Method according to claim 1 , characterized in that in step c) of claim 1 contact between the bearing ring (2) and an roller element (4) at a location (1 , 11 ", 1 ", ...) is assumed when the magnitude of the reflected ultrasonic signal (S') is below a given reference value and that no contact between the bearing ring (2) and an roller element (4) is assumed when the magnitude of the reflected ultrasonic signal (S') is above the given reference value.

3. Method according to claim 1 or 2, characterized in that the ultrasonic sound generators (6', 6", 6" ', ...) and the receivers (8', 8", 8' ", ...) are arranged along an axial direction (a) of the bearing (1).

4. Method according to one of claims 1 to 3, characterized in that emitting and receiving of ultrasonic signals (S, S') takes place at at least five, preferably at at least 10, distanced locations (1 , 11 ", 1 ", ...).

5. Method according to claim 4, characterized in that the distanced locations (1 1 ', 11 ", 1 ", ...) are distanced between 0.05 mm and half the width of the raceway, preferably between 0.1 mm and 5.0 mm.

6. Method according to one of claims 1 till 5, characterized in that emitting and receiving of ultrasonic signals (S, S') is done by one and the same element, preferably by a piezo-active element.

7. Method according to one of claims 1 till 6, characterized in that it is used during the regular operation of the roller bearing (1).

8. Method according to one of claims 1 to 7, characterized in that an ultrasonic signal is emitted into the roller bearing which has a frequency between 10 MHz and 200 MHz, preferably between 20 MHz and 100 MHz, most preferably between 30 MHz and 60 MHz.

9. Method according to one of claims 1 to 8, characterized in that an acquisition frequency of the receivers (8', 8", 8' ", ...) is at least 500 MHz, preferably at least 1 GHz, most preferably at least 2 GHz.

Device for determining a load zone (A) in a roller bearing (1), wherein the roller bearing (1) has at least two bearing rings (2, 3) and roller elements (4) between the bearing rings (2, 3), characterized in that the device comprises at least two ultrasonic sound generators (6', 6", 6"') and at least two receivers (8 ', 8", 8' ") for an ultrasonic signal, wherein the sound generators (6', 6", 6" ', ...) and the receivers (8', 8", 8"', ...) are located at a radial inner or outer surface (5) of one of the bearing rings (2, 3) and wherein the sound generators (6', 6", 6" ', ...) and the receivers (8', 8", 8" ', ...) are arranged at at least two distanced locations (11 11 ", 11 "', ...).

11. Device according to claim 10, characterized in that a plurality of ultrasonic sound generators (6', 6", 6" ') and receivers (8', 8", 8"') are arranged in an array (9) forming a one-piece component.

Device according to claim 1 1 , characterized in that the array (9) has at least five, preferably at at least 10, sound generators (6', 6", 6" ') and receivers (8\ 8", 8"').

Device according to claim 12, characterized in that the sound generators (6', 6", 6" ') and receivers (8', 8", 8"') are distanced between 0.05 mm and 10.0 mm, preferably between 0.1 mm and 5.0 mm along an axial extension (a) of the array (9).

Device according to one of claims 10 to 13, characterized in that it further comprises time measuring means (10) for measuring the time between the emission of an ultrasonic signal (S) by the sound generators (6', 6", 6" ', ...) and a signal (S') coming back by the receiver (8', 8",

Device according to one of claims 10 to 14, characterized in that the sound generators (6', 6", 6' ", ...) are designed to emit an ultrasonic signal between 10 MHz and 200 MHz, preferably between 20 MHz and 100 MHz, most preferably between 30 MHz and 60 MHz.

Device according to one of claims 10 to 15, characterized in that the ultrasonic sound generators (6', 6", 6" ', ...) and the receivers (8', 8", 8"', ...), specifically the array (9) forming those elements, is fixed at the bearing ring (2, 3) by an adhesive.

Device according to one of claims 10 to 16, characterized in that the ultrasonic sound generators (6', 6", 6" ', ...) and the receivers (8', 8", 8" ') are piezo-active elements.

18. Device according to claim 17, characterized in that the piezo-active elements form the ultrasonic sound generators (6', 6", 6' ", ...) and the receivers (8', 8", 8' ") in one and the same element.

Device according to claim 17 or 18, characterized in that the piezo-active elements are attached onto the bearing ring surface opposite to the raceway.