WO2009092390A1

WO2009092390A1 - Bearing and sensor unit

Info

Publication number: WO2009092390A1
Application number: PCT/EP2008/000453
Authority: WO
Inventors: Xiao Bo Zhou
Original assignee: SKF AB
Current assignee: SKF AB
Priority date: 2008-01-22
Filing date: 2008-01-22
Publication date: 2009-07-30
Anticipated expiration: 2010-07-22

Abstract

The present invention concerns a rolling element bearing (1) comprising an inner ring (3), an outer ring (5) and a plurality of rolling elements (7) disposed therebetween on at least one set of opposing raceways (8, 9), the bearing being provided with at least one piezoelectric sensor (10). The at least one piezoelectric sensor is adapted to operate in bending mode, to sense deformation of the bearing due to bending strain, and is capable of sensing dynamic loads and/or vibrations in the bearing.

Description

22 January 2008

BEARING AND SENSOR UNIT

TECHNICAL FIELD

The invention concerns a rolling element bearing provided with at least one piezoelectric sensor.

BACKGROUND

Bearings are used in many different types of machinery to retain and support rotating components such as, for example, a wheel on a vehicle, a vane on a windmill or a drum in a washing machine. During use, the bearing is subjected to different loads, both static and dynamic. The static load is due to the weight supported by the bearing, and may also be due to a preload with which the bearing is mounted. The dynamic loads are time dependent and are due to the operating conditions.

In many systems, it is desirable to be able to monitor the load acting on a bearing. In modern vehicles, for example, load data from the wheel bearings are used in the control of vehicle stability systems. Vibration is another physical parameter that is important with regard to bearings, as it provides an indication of the 'health' of a bearing. Excessive vibration may be a sign that a bearing is nearing the end of its life, and so bearings in vital machinery are often provided with sensors to monitor vibration.

Many examples of bearings provided with sensors are known in the art. Displacement sensors such as strain gauges are commonly applied to measure load and deformation, while thin film piezoelectric transducers may be used as vibration sensors. Piezoelectric elements can also be used as load sensors. One example of a sensor assembly that is capable of sensing both loads and vibrations in a bearing is disclosed in US 6845672. The sensor assembly comprises a piezoelectric plate mounted in series with a spring, where the piezoelectric plate and spring are mounted between opposing faces of a cavity in a bearing ring.

There is still room for improvement, however.

SUMMARY An object of the invention is to define a bearing and sensor unit that is capable of sensing loads and/or vibrations in the bearing, where the sensor is compact, easy to mount and produces an excellent linear response over a large frequency range.

The aforementioned objects are achieved according to the invention in a rolling element bearing comprising an inner ring, an outer ring and a plurality of rolling elements disposed therebetween on at least one set of opposing raceways, where the bearing is provided with at least one piezoelectric sensor that is adapted to operate in bending mode and sense deformation due to bending strain. The objects of the invention are further achieved in that the piezoelectric bending sensor comprises two or more layers of suitably polarized piezoelectric material, with a central electrode layer disposed between each of the two or more piezoelectric layers.

A suitable material for the piezoelectric layers is lead zirconate titanate (PZT), which has the advantages of being readily available and inexpensive to process. Moreover, PZT may be subjected to a polarization field to obtain an excellent response in bending mode. The term bending mode (also known as d31 mode) is used when the resulting forces on a piezoelectric material are exerted perpendicular to the direction of polarization. Other suitable materials include piezoelectric aluminium nitride and zinc oxide. These materials have the advantage of being self-polarizing, and thus do not need to be subjected to a polarization field. For example, aluminium nitride may be deposited by means of a physical vapour deposition process like reactive sputtering. The process parameters may be controlled such that the required polarization direction is obtained, i.e. a crystallographic orientation that predominantly lies in the c-axis.

In a first embodiment of the invention, the bending sensor of the bearing and sensor unit comprises two piezoelectric layers separated by a central electrode layer and sandwiched between first and second external electrodes. The sensor may be mounted e.g. on an outer circumference of the bearing outer ring. When the suitably polarized two-layer sensor is subjected to a to bending moment, due to the flexures imparted to the bearing ring as the rolling elements pass by, charge develops across each layer in an attempt to counteract the imposed strains. Because charge develops across each layer, the charge that may be collected is greater than the charge generated by a single piezoelectric layer.

In a preferred embodiment of the invention, the bending sensor comprises a stack of several piezoelectric layers with a central electrode layer between opposing piezoelectric layers. The charge across each layer is collected, and the total charge generated is of a magnitude that enables a signal from the sensor to be measured without the aid of a charge amplifier. The bending sensor may suitably comprise between four and ten piezoelectric layers, and the sensing range is from 0.001 micron up to 1000 microns of bending movement.

A bending sensor used in the bearing and sensor unit according to the invention has a frequency response in the sub-Hz range up to the MHz range. For the measurement of dynamic bearing loads, i.e. deformation of a bearing ring due to the passage of rolling elements, the frequency of the sensor signal is in the order of the rotational frequency of the bearing, e.g. 10 - 1000 Hz. For condition monitoring, i.e. vibration sensing to provide an indication that a bearing is approaching failure due to a surface defect like a crack or spall in a raceway, the frequency of the vibrations produced lies in the range of 10 -100 times the rotational frequency of the bearing. As will be clear to persons skilled in the art, suitable pass-band filters and processing circuitry may be employed to process the signal in the desired frequency range or ranges. Depending of the stiffness, damping and mass characteristics of the system in which the bearing is mounted, it may also be advantageous to filter out certain resonances from the system.

The thickness of each piezoelectric layer may be between 0.002 and 1 mm, preferably between 0.01 to 0.2 mm. A sensor according to the invention, even if it comprises several piezoelectric layers, therefore takes up very little space. The sensor may take the form of a strip that is mounted circumferentially on an outer surface of the bearing outer ring or an inner surface (bore) of the inner ring. Such an arrangement is suitable for load sensing and vibration sensing. When mounted on the inner ring bore, it is advantageous to machine a shallow notch in the bore and attach the sensor in this notch, to avoid damaging the sensor when the bearing is mounted on e.g. a shaft. Likewise, a notch may also be provided in the bearing outer ring if the outer ring must be tightly enclosed in a housing. A bearing and sensor unit according to the invention may also comprise more than one row of rolling elements, such as a hub bearing unit for a vehicle, and piezoelectric bending sensors may be suitably mounted to sense the deformation caused by the rolling elements in each row. For condition monitoring purposes, the piezoelectric sensor may be mounted on a side face of the bearing inner or outer ring. A suitable method of attaching the bending sensor to a bearing ring is by means of adhesive bonding. The sensor could also be clamped to a bearing ring.

In a further embodiment of the invention, the bending sensor mounted on the bearing is curved and has a curvature equal to the curvature of the surface on which it is mounted. This is advantageous if the length of the sensor is relatively large compared with the circumference of the surface on which it is mounted. If the sensor were bent around the surface, the piezoelectric layers would probably fracture. If the length of the sensor is relatively small in comparison with the circumference of the bearing surface, the sensor may be flat. A flat sensor has the advantage of being cheaper to produce and may be applied on a range of bearing sizes. In a further embodiment of a bearing and sensor unit according to the invention, the sensor could also be made with a curvature somewhat greater than the curvature of the bearing ring surface on which the sensor is to be mounted, e.g. 4 - 8 % greater. The advantage of this embodiment is that the sensor may be clamped to a bearing ring, whereby a clamping force is applied only at the centre of the sensor.

Thus, a bearing and sensor unit according to the invention is able to sense deformation of the bearing the ring due to bending strain, which data may be used to determine the loads acting on the system in which the bearing is mounted. A bearing and sensor unit according to the invention is also capable of sensing vibrations in the bearing, which data is indicative of the condition of the bearing. This enables other components of a machine or system to be protected, since a bearing that is nearing failure may be replaced before damage is caused. Other advantages of the present invention will become apparent from the detailed description and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail for explanatory and in no sense limiting purposes with reference to the following figures, in which

Fig. 1 illustrates a bearing and sensor unit according to an embodiment of the invention;

Fig. 2 illustrates an example of a sensor used on a bearing and sensor unit according to the invention;

Fig. 3 illustrates a bearing and sensor unit according to a further embodiment of the invention. DETAILED DESCRIPTION

Fig. 1 illustrates a rolling element bearing according to an embodiment of the invention. The bearing 1 comprises an inner ring 3 and an outer ring 5 rotatably coupled by means of rolling elements 7. The rolling elements are disposed between the inner and outer rings on at least one set of opposing raceways 8, 9. Many applications exist where it is desirable to determine the loads acting on a bearing.

According to the invention, this is achieved by mounting at least one piezoelectric bending sensor 10 on an inner circumference of the inner ring 3 or an outer circumference of the outer ring 5. During operation, the rolling elements impart minute flexures to the bearing rings. The bending sensor 10 also flexes when a rolling element 7 passes by and the amplitude of the signal from the bending sensor 10 (commonly referred to as the ball pass signal) is a measure of the load on the rolling element. This in turn provides an indication of the load on the bearing. By mounting a plurality of piezoelectric bending sensors 10 on a bearing ring, as shown in Fig. 1 , the complete load vector on the bearing may be calculated by a method such as disclosed in WO2005/008204.

A bending sensor should be understood as a sensor which measures deformation due to bending moment. According to the invention, a multilayer piezoelectric sensor is used that is adapted to produce optimal response in bending mode. When employed in bending mode, a piezoelectric sensor develops charge in response to forces applied perpendicular to the direction of polarization.

An example of a piezoelectric bending sensor that may be used on a rolling element bearing according to the invention is shown in Fig. 2. The sensor 10 in this example comprises a multilayer structure of three piezoelectric layers 20 with a central electrode layer 22 interposed between each of the piezoelectric layers. The sensor also comprises a first external electrode 24 and a second external electrode 26, which may be coupled to a suitable processing unit via sensor connections 29. The advantage of a multilayer piezoelectric sensor is that under deformation, charge is developed across each piezoelectric layer, thereby increasing the strength of the signal generated. Preferably, the sensor comprises 4 or more piezoelectric layers. To collect the charge generated across each layer, the sensor 10 further comprises a third external electrode 28. This third external electrode 28 is disposed at one side of the multilayer structure and connects the central electrode layers 22 to each other. A sensor connection is then attached to the third external electrode 28. It will be understood that if the sensor comprises only two piezoelectric layers 20, a third external electrode is not needed; a sensor connection 29 is simply attached to the one central electrode layer 22.

A preferred material for the piezoelectric layers 20 is lead zirconate titanate (PZT), as this material may be polarized to obtain an excellent response in bending mode. A method of producing such a bending sensor will be briefly described.

A suitable composition for the PZT is 48 mole% PdTiO₃ and 52 mole% PbZrO₃. The PZT is initially in powder form and is mixed with a suitable binder and solvent and then milled to form an homogeneous suspension slurry. A piezoelectric layer 20 is created by tape casting the powder slurry on a carrier foil. The PZT layer may have a thickness of between 0.002 and 1.0 mm, with a preferred value of between 0.01 and 0.2 mm. An electrode layer 22 is then screen printed on the PZT layer. Suitable materials for the electrode layer 22 include nickel, silver, platinum and palladium, which are screen printed in a thickness of between 1 and 5 microns.

To obtain a multilayer sensor, the required number of PZT layers 20 (with electrode layer 22) are laminated under pressure and heated to a temperature slightly above the glass transition temperature of the binder material, e.g. 70 °C when the binder is polyvinyl butyral (PVB). The resulting multilayer structure is cut to the required dimensions of the bending sensor, heated to a temperature of 500 - 700 °C to burn out the binder, and then sintered at temperature of 1000

- 1300 °C, depending on the electrode material. In a next step, the first, second and third outer electrode layers 24, 26, 28 are screen printed on the multilayer structure. Finally, a polarization field of 2 - 3 kV/mm is applied for between 1 and 10 minutes at a temperature of between 100 and 150 °C. The polarization field is applied in a direction transverse to the layer thickness of the PZT layer 20. The resulting sensor generates maximal piezoelectric response under bending and has a piezoelectric coefficient of 300 - 700 pC/N.

In an advantageous development, the piezoelectric layers 20 may be formed in a curved mould, so that the bending sensor 10 obtains a curvature equal to the curvature of the surface on which it will be mounted. The bending sensor may also be produced with a curvature somewhat greater than the curvature of the surface on which it will be mounted, to enable the sensor to be clamped to the surface at a single clamping location in the centre of the sensor. Moreover, the bending sensor 10 may be covered in a film layer of plastic packaging, to provide mechanical protection.

The bending sensor has a frequency response which extends from the sub- Hz range up to the Mega-Hz range. A bearing and sensor unit according to the invention is therefore capable of sensing cyclical deformation of the bearing ring due to the passage of rolling elements, at frequencies of e.g. 20

- 200 Hz. A bearing and sensor unit according to the invention sensor may also be used for condition monitoring, to sense high-frequency vibrations as a result of e.g. the rolling elements over-rolling a defect on a raceway surface. These vibrations may be in the order of e.g. 10,000 Hz. The actual frequency ranges for load sensing and vibration sensing depend on the rotational frequency of the bearing, the number of rolling elements in the bearing and the stiffness and mass characteristics of the system. Suitable pass-band filters and processing circuitry are selected accordingly. For condition monitoring, the bending sensor may be mounted against a side face of the bearing inner or outer ring, or on the outer surface of the outer ring or the bore of the inner ring. Fig. 3 shows an embodiment of the invention, where the bending sensor 10 is mounted in a notch 30 in the outer ring 5. One or more of these notches could also be machined in the inner ring 3. The advantage of this embodiment is that the geometry of the bearing remains unchanged.

In a bearing and sensor unit according to the invention, the one or more bending sensors are preferably mounted on the bearing ring that is fixed in relation to the rotatable bearing ring. If the circumstances of the application make this desirable, however, one or more bending sensors could be mounted on the rotatable bearing ring and the electrical connection with the sensors could be realized by means of e.g. slip rings. Alternatively, the bending sensor could also be provided with wireless functionality to transmit the sensed signal to a suitable receiver.

A number of aspects/embodiments of the invention have been described. It is to be understood that each aspect/embodiment may be combined with any other aspect/embodiment. Moreover the invention is not restricted to the described embodiments, but may be varied within the scope of the accompanying patent claims.

REFERENCE NUMERALS

1 rolling element bearing

3 bearing inner ring, 5 bearing outer ring,

7 rolling elements,

8, 9 raceways,

10 bending sensor,

20 piezoelectric layer, 22 central electrode layer,

24 first external electrode,

26 second external electrode,

28 third external electrode,

29 sensor connections, 30 notch.

Claims

22 January 2007CLAIMS

1. A rolling element bearing (1 ) comprising an inner ring (3), an outer ring (5) and a plurality of rolling elements (7) disposed therebetween on at least one set of opposing raceways (8, 9), the bearing (1 ) being provided with at least one piezoelectric sensor (10), characterized in that the at least one piezoelectric sensor (10) is adapted to operate in bending mode and sense deformation due to bending strain.

2. A rolling element bearing according to claim 1 , wherein the at least one piezoelectric sensor (10) comprises a stack of two or more layers of piezoelectric material (20) with a central electrode layer (22) disposed between each of the two or more piezoelectric layers (20).

3. A rolling element bearing according to claim 2, wherein the two or more piezoelectric layers (20) consist of aluminium nitride with a crystallographic orientation which predominantly lies in the c-axis.

4. A rolling element bearing according to claim 2, wherein the two or more piezoelectric layers (20) consist of lead zirconate titanate that is polarized in a suitable electric field to obtain maximal piezoelectric effect in bending mode.

5. A rolling element bearing according to any of the preceding claims, wherein the at least one piezoelectric sensor (10) is mounted on a side face of the bearing inner ring (3) and/or a side face of the bearing outer ring (5).

6. A rolling element bearing according to any of claims 1 to 4, wherein the at least one piezoelectric sensor (10) is mounted on a radially outer surface of the bearing outer ring (5) and/or on a radially inner surface of the bearing inner ring (3).

7. A rolling element bearing according to claim 6, wherein the at least one piezoelectric sensor (10) has a curvature substantially equal to the curvature of the surface on which it is mounted.

8. A rolling element bearing according to claim 6, wherein the at least one piezoelectric sensor (10) has a curvature that is four to eight percent greater than the curvature of the surface on which it is mounted.

9. A rolling element bearing according to any of claims 6 to 8, wherein the at least one piezoelectric sensor (10) is mounted in a notch (30) in the radially inner surface of the bearing inner ring (3) and/or the radially outer surface of the bearing outer ring (5).