DE20301647U1 - Rolling bearings, in particular for an electric motor - Google Patents

Description

RÜGER, BARTHEL T "&' &Agr;'&Igr;&dgr;'&Egr;&Igr;'"

Patent attorneys ^&bgr; European Patent Attorneys

Dr.-lng.R. Rüger Dipl.-Ing. HP Barthelt Dr.-lng.T.Abel Patent Attorneys European Patent Rüger, Barthelt & Abel ■ R O. Box 10 04 61 ■ D-73704 Esslingen Attorneys

SKF K. Matthies Shareholders

Hornsgatan 1 ■ ^Brands

415 50 Gothenburg POBox 100461 M

Sweden D-73704 Esslingen &agr;. &Ngr;.

Webergasse 3 <R>

D-73728 Esslingen a. N. ^

Telephone (0711) 356539 Fax (0711) 35 99 E-mail ruba@ab-patent.com VAT DE 145 265

30 January 2 SKF GM 17

Rolling bearings, especially for an electric motor

The present invention relates to the field of rolling bearings, in particular those used, for example, in electric motors.

An electric motor generally comprises a housing in which a fixed stator and a rotor mounted for rotation within the stator are mounted. The rotor is mounted for rotation by means of two rolling bearings, which are generally of the rigid ball bearing type and are arranged at the two ends of the shaft of the rotor.

The rolling bearings, generally of the "rigid ball bearing" type, have annular bearing surfaces with a cross-sectional profile that is symmetrical with respect to a plane passing through the centre of the balls and are designed to withstand relatively large radial forces and moderate axial forces during operation.

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Accounts: Deutsche Bank AG, Esslingen branch 304 014 (bank code 611 700 76) · Postbank Stuttgart 62451-700 (bank code 600100 70)

To absorb the forces. The two rolling bearings are attached to the shaft via their inner rings with a press fit.

The outer rings of the rolling bearings are fixed in cylindrical seats in the engine casing, the casing being essentially made of a light alloy based on aluminium. The outer ring of one of the two rolling bearings is axially fixed in its seat by suitable means, such as retaining rings and/or shoulders, and is possibly clamped in the casing.

The outer ring of the other bearing is fixed in its seat "freely" in the axial direction, in other words with an acceptable axial clearance. In fact, the axial distance separating the second bearing from the first bearing, which is fixed axially with respect to the housing, may vary slightly during an increase in the temperature of the electric motor due to the differential expansion of the various parts of the motor. The second bearing must therefore be able to slightly vary its position in the axial direction with respect to its seat without causing an undesirable rotation of its outer ring with respect to the seat.

From the document FR-A-2 585 095 a rolling bearing is known which is equipped on its outer ring with rings made of synthetic resin which are injection-molded in annular grooves formed around the circumference of the outer ring. These rings protrude in the radial direction with respect to the cylindrical outer surface of the outer ring of the rolling bearing and make it possible to prevent the outer ring from moving when the

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Engine temperature could cause rotation relative to the housing seat.

The coefficient of expansion of the motor housing, which is made of a light metal alloy, is approximately twice as high as that of the steel used for the rolling bearing rings. The increase in temperature is therefore reflected in a radial clearance between the outer ring of the rolling bearing and the housing seat. The plastic rings have a high coefficient of expansion compared to the light metal alloy of the motor housing, which prevents the outer ring from accidentally rotating in relation to the seat due to the internal friction of the rolling bearing when the temperature rises. The plastic rings have a relatively large outer diameter compared to the outer diameter of the rolling bearing.

Thus, in the case of a rolling bearing with an external diameter of, for example, 32 mm, for a matching nominal value, the permissible deviation (tolerance) for the diameter of the plastic rings is between +20 and +50 μηι or even between +3 0 and +70 μηι, whilst the tolerance for the diameter of the outer ring of the rolling bearing is between 0 and -δΩμηι. This means that the plastic rings constantly protrude in the radial direction with respect to the surface of the outer ring of the rolling bearing.

Such a device is relatively satisfactory for preventing inadvertent rotation of the outer ring when the temperature rises, but it nevertheless has certain disadvantages.

The installation of the rolling bearing in its seat requires

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a considerable amount of force is required when pressing in and must therefore be carried out using a type of press. The plastic rings are arranged on the outer ring with a considerable radial projection in relation to its peripheral surface, which means that there is a risk that the rings will be damaged or torn out during pressing in. The clamping of the rolling bearing in its seat does not allow sufficient freedom for axial displacement of this rolling bearing when the temperature changes. Accordingly, the rolling bearing is subjected to unusually strong axial forces, particularly at low and medium temperatures, which can lead to a detrimental reduction in the service life of the rolling bearing.

The invention teaches how to solve these different problems.

The invention proposes an improved rolling bearing with compensating rings.

According to one aspect of the invention, the rolling bearing device includes an outer ring intended to fit in a cylindrical seat, a running surface formed on the outer ring, a series of rolling elements arranged in contact with the running surface, and at least one expansion compensation ring fixed in an annular groove formed in the cylindrical outer surface of the outer ring. The compensation ring is made of a material whose expansion coefficient is greater than that of the material from which the outer ring is formed. The compensation ring is sized with an outer diameter which is between the outer diameter of the outer ring minus 50 μιη and the outer diameter of the outer ring plus 2 0 μιη.

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Advantageously, the fit between the outer diameter of the outer ring of the rolling bearing and the housing seat made of a light metal alloy is suitably selected and a sliding fit is created at room temperature.

In one embodiment of the invention, the tolerance of the outer diameter of the compensating ring relative to the nominal diameter is between +10 and -50 μιη, preferably a tolerance between +10 and -30 μιη could be selected.

In one embodiment of the invention, the tolerance on the outer diameter of the outer ring compared to the nominal diameter is between 0 and -10 μηη.

Preferably, the rolling bearing contains two expansion compensation rings which are fastened in two grooves of the outer ring, these grooves being spaced apart from one another in the axial direction. Preferably, the rolling bearing is symmetrical with respect to a radial plane passing through the center of the rolling elements.

The rolling bearing may have a solid or sheet metal inner ring provided with an outer running surface that is in contact with the rolling elements.

In one embodiment of the invention, the compensating ring is made of plastic. In particular, the compensating ring can be made of polyamide.

The rolling bearing can be mounted in a seat which has a cylindrical bore and is part of a mechanical

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mechanical element, for example the stator part of a motor. The seat can be made of a light metal alloy.

Furthermore, according to the invention, an electric motor is provided with a housing, a stator fastened in the housing, a rotor and at least one rolling bearing of the type described above fastened between the housing and the rotor.

In one embodiment of the invention, the housing is made of a light metal alloy, in particular of a metal alloy containing aluminum and/or magnesium.

In one embodiment of the invention, the thermal expansion coefficient of the compensating ring is greater than that of the material from which the outer ring of the rolling bearing is made and greater than that of the material from which the seat of the outer ring is made.

In other words, the outer peripheral surface of the compensating ring(s) can protrude by 10 μιδ in the radial direction relative to the cylindrical outer surface of the outer ring of the rolling bearing or can be set back by up to 25 μιδ in relation to this. Since the outer diameter of the outer ring of the rolling bearing is dimensioned with a tolerance of 0 to -10 μιδ in relation to its nominal dimension, and the seat made of a light metal alloy is dimensioned with a tolerance of 0 to + 16 μιδ in relation to its nominal dimension, which is equal to that of the outer diameter of the outer ring of the rolling bearing, the rolling bearing can be designed with a tolerance of 0 to + 16 μιδ in relation to its nominal dimension, which is equal to that of the outer diameter of the outer ring of the rolling bearing, due to the small maximum possible radial projection of the projection.

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The compensating rings can be installed particularly easily in relation to the outer ring and without any risk of damaging the compensating rings in their seats.

At low temperatures, particularly below or close to 0°C, the mutual engagement between the outer ring of the bearing, made of steel, and the housing seat, made of a light alloy, is crucial. This engagement ensures that the outer ring of the bearing is centered in its seat and does not rotate in it, while slight axial displacements of the outer ring in its seat are possible under a sufficiently large axial load.

However, when the temperature increases, for example up to around 12 0 ⁰ C, the mutual engagement between the compensating rings and the seat increasingly ensures centering and resistance to rotation of the outer ring with respect to its seat. In fact, the coefficient of expansion of the steel for the outer ring is of the order of 1.5 x ICT ⁵ , that of the light alloy for the housing is in the region of 2.3 x ICT ⁵ and that of the compensating rings is of the order of 13 x ICT ⁵ . The compensating rings therefore expand considerably more than the outer ring and ensure excellent compensation of expansion at medium and high temperatures.

In particular, the compensation rings expand more rapidly in the radial direction than the housing seat and therefore adhere effectively to it, preventing any rotation of the outer ring of the rolling bearing in the housing and ensuring excellent centering between the rolling bearing and

the seat and therefore between the stator and the rotor of the motor. The latter point is particularly important because the performance of an electric motor depends in particular on the clearance between the stator and the rotor, with a reduction in clearance increasing the performance of the motor. Of course, a reduction in clearance increases the requirements for the accuracy of the centering between the rotor and the stator.

In certain applications, for example in the case of use in electric motors for the power steering of motor vehicles, where the requirements for precision are particularly high, the error in centring the rotor with respect to the stator must not exceed 50 μπα. The invention therefore makes it possible to meet these requirements.

In addition, in the high temperature range, the structure of the compensation rings temporarily changes and they become softer and more deformable, sufficiently to allow the small axial displacements required for the outer ring under load.

As an example, a polyamide of the PAIl type can be chosen for the production of the compensation rings, which has the following mechanical properties:

- Young's modulus of elasticity of 2200 MPa at 20 ⁰ C

- Poisson's coefficient (transverse contraction coefficient) 0.4

- Thermal expansion coefficient of 13 x 10~ ⁵ .

The present invention will be more clearly understood after reading the detailed description of some merely

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exemplary and in no way limiting embodiments, which are illustrated by the following drawings:

Fig. 1 shows a longitudinal section of an electric motor according to the prior art;

Fig. 2 shows a side view of a rolling bearing according to one aspect of the invention;

Fig. 3 shows an axial sectional view of the rolling bearing according to Fig. 2 mounted in an engine;

Fig. 4 and 5 show detailed sections of Fig.

3 at higher or lower temperatures; and

Fig. 6 shows a graph of the axial forces that must be exerted on the rolling bearing in order to move it in its seat as a function of temperature.

As can be seen from Fig. 1, an electric motor 1 comprises a housing 2 made of a light metal alloy, a stator 3 attached to the housing 2, a rotor

4 and a shaft 5 on which the rotor 4 is mounted. The housing 2 has a substantially cylindrical portion 6, an annular radial portion 7 arranged at one end of the cylindrical portion 6 and a circularly shaped radial portion 8 arranged at the opposite end.

An axial section 9, which has a cylindrical bore 10, a shoulder 11 adjacent to the cylindrical bore 10 and a shoulder 11 opposite to the shoulder 11,

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· · a

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the side of the cylindrical bore 10, is formed integrally with the annular radial section 7. An axial section 13 with a cylindrical bore 14 is formed integrally with the circular radial section 8 and projects axially into the interior of the electric motor 1. A rolling bearing 15 of conventional design is fastened via its outer ring in the cylindrical bore 10 of the axial edge 9 and is held in place in the axial direction by the shoulder 11 and a locking ring 16 arranged in the annular groove 12. The rolling bearing 15 is fastened via its inner ring to a cylindrical outer surface of the shaft 5 and is fixed in the axial direction by a shoulder 17 formed on the shaft 5 and a locking ring 18 which sits in an annular groove machined in the shaft 5.

A rolling bearing 19 is press-fitted via its inner ring to one end of the shaft 5 until it comes to abut against a shoulder 20 of the shaft 5. The press-fit ensures the firm connection of the rolling bearing 19 to the shaft 5, but additional means can be provided for connecting the inner ring of the rolling bearing to the shaft, which can be done for example by means of a locking ring fastened to the end of the shaft. The outer ring of the rolling bearing 19 is fastened in the bore 14 of the axial section 13 in such a way that the outer ring of the rolling bearing 19 can undergo a certain axial displacement with respect to the axial section 13. The rings of the rolling bearings and the balls are generally made of steel, while the housing 2 is made of a light metal alloy, for example based on aluminum.

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Since the light alloy has a considerably higher coefficient of expansion than, in particular, the steel from which the outer ring of the bearing 19 is made, it is understood that at low temperatures a large axial force is required to move the outer ring of the bearing 19 in the housing, which entails an axial load on the bearing which can prove damaging to the bearings. At high temperatures, on the other hand, a disproportionately large play can cause the outer ring to rotate with respect to the housing as a result of the torque transmitted due to the internal resistance of the bearing 19.

In order to prevent this phenomenon, it is intended to replace the rolling bearing 19 by a rolling bearing 21 of the type shown in Fig. 3. The rolling bearing 21 comprises a solid inner ring 22 with an annular running surface 23, a solid outer ring 24 with an annular running surface 25 and a plurality of rolling elements 26 arranged between the running surfaces 23 and 25 of the bearing, in the present example balls, which are held at a uniform distance around the circumference by a cage 27 made of plastic.

The outer ring 24 has two grooves 28 and 29 arranged on either side of the running surface 25, which serve as a support for protective flange seals 30 and 31 made of sheet metal, which extend to the vicinity of a cylindrical seating surface for the inner ring 22 in order to create a seal by forming a narrow gap.

Furthermore, the outer ring 24 has a cylindrical outer peripheral surface 32 in which two annular grooves 33 and 34 are formed, which are arranged in relation to a

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centric point of the rolling elements 2 6 are symmetrical to one another and can have a profile in the shape of a U in the axial cross section. In each of the grooves 33, 34 there is a ring 3 5 or 3 6 made of plastic, for example polyamide PAIl, which essentially ends flush with the cylindrical outer surface 32 of the outer ring 24 at the same height.

More precisely, each of the rings 35, 36 has a cylindrical outer surface whose outer diameter is between the outer diameter of the outer ring 24 minus 50 μm and the outer diameter of the outer ring 24 plus 20 μm. In order to achieve this, it could be ensured that the permissible deviation (tolerance) of the outer diameter of the rings 35, 36 with respect to the nominal diameter of the outer surface of these rings 35, 36 is between +10 μm and -50 μm , the tolerance advantageously being between +10 and -30 μm.

Furthermore, it could be ensured that the tolerance for the outside diameter of the outer ring with respect to its nominal diameter is between 0 and -10 μιη. In this way, the rings 35, 36 do not protrude with respect to the outer surface 32 of the outer ring 24 at room temperature, or at least only protrude so slightly that the rolling bearing 21 can be easily inserted into a cylindrical bore, such as the bore of the axial projection 13 of the housing 2, without the rings 35, 36 being burst or damaged.

The rings 35 and 36 are preferably injection molded into the ring grooves 33 and 34. These can therefore protrude slightly from the outer surface 32 after injection molding.

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as shown in Fig. 4, or slightly recessed or recessed, as shown in Fig. 5.

The grooves 33 and 34 are spaced apart in the axial direction in such a way that the cylindrical outer surface 32 of the outer ring 24 is divided into three sections, one of which is arranged between one of the radial end faces of the rolling bearing and the groove 33, the second between the grooves 33 and 34 and the third between the groove 34 and the radial end face opposite the first.

In order to facilitate the installation of the rolling bearing 21 in the bore 14 of the axial projection 13, it can also be provided that the cylindrical bore has a tolerance of between 0 and +16 /zm with respect to its nominal dimension. The installation of the rolling bearing can thus be carried out in a simple manner.

To give an example, in the case of a rolling bearing with an outside diameter of 50 mm or less, a tolerance on the outside diameter of between 0 and -10 μιη may be provided, while for a rolling bearing with an outside diameter of 50 to 80 mm, a tolerance on the diameter of between 0 and -15 μιη may be provided.

The inner ring 22 can be fixed on a shaft 5.

Fig. 6 shows the course of the axial forces that must be exerted on the rolling bearing at different temperatures in order to move it in the cylindrical bore of the axial projection that forms the seat of the rolling bearing.

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bearing. The basis here is a rolling bearing with a nominal outside diameter of 32 mm with diameter tolerances that ensure maximum mutual engagement between the seat and the rolling bearing with its rings. The curve therefore represents an envelope that delimits an area between the abscissa and this curve, within which curves run that correspond to other, weaker diameter-related engagements.

At temperatures below 0 ⁰ C, contact between the light alloy of the housing and the steel of the outer ring of the bearing predominates, while at temperatures above 0 ⁰ C, contact between the light alloy of the housing and the compensation rings is decisive. It can be seen that the axial load is less than 500 N at temperatures above 0 ⁰ C and is between 400 and 500 N at ambient temperatures from 0° to 3O ⁰ C.

From a temperature of 2 0 ⁰ C, the loading force decreases monotonically as a function of temperature until it reaches a value below 200 N near 8O ⁰ C. In the temperature range below ^{0 0} C, the loading force also decreases monotonically as a function of temperature, and with a much greater gradient.

It is understood that according to the invention, a rolling bearing with expansion compensation rings is created which is excellently suited for various applications, in particular in the field of electric motors, and which do not protrude at low temperatures with respect to the outer diameter of the outer ring of the rolling bearing and therefore enable easier and risk-free insertion, and which at higher temperatures due to the considerable expansion of the expansion

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Compensating rings ensure that this fit is maintained, thereby preventing the greater thermal expansion of the seat compared to the outer ring of the rolling bearing from leading to a separation of the rotationally fixed connection between these two elements.

This makes it possible to design electric motors with less radial clearance and therefore with improved performance. It also avoids excessive axial loads on the bearings, which reduce their service life, and also avoids rotation of the outer ring in relation to its seat, which is equally detrimental to the service life of the motor.

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Claims (11)

1. A rolling bearing device ( 21 ) comprising: an outer ring ( 24 ) intended to fit within a cylindrical seat, a running surface formed on the outer ring, a plurality of rolling elements ( 26 ) arranged in contact with the running surface, and at least one expansion compensation ring ( 35 ) seated in an annular groove ( 33 ) formed in the cylindrical outer surface of the outer ring, the expansion compensation ring being made of a material having a larger expansion coefficient than the material from which the outer ring is made, characterized in that the expansion compensation ring has an outer diameter which is between the outer diameter of the outer ring minus 50 µm and the outer diameter of the outer ring plus 20 µm. 2. Device according to claim 1, characterized in that the tolerance of the outer diameter of the compensating ring compared to the nominal diameter is between +10 to -50 µm. 3. Device according to claim 1 or 2, characterized in that the tolerance of the outer diameter of the compensating ring compared to the nominal diameter is +10 to -30 µm. 4. Device according to one of the preceding claims, characterized in that the tolerance of the outer diameter of the outer ring compared to the nominal diameter is between +0 and -10 µm. 5. Device according to one of the preceding claims, characterized in that the device includes two compensating rings ( 35 , 36 ) which are fixed in two grooves ( 33 , 34 ) of the outer ring at a distance from one another in the axial direction. 6. Device according to one of the preceding claims, characterized in that the outer ring is made of steel. 7. Device according to one of the preceding claims, characterized in that the compensating ring is made of plastic. 8. A mechanical element comprising a bearing housing having a cylindrical interior and a device according to any preceding claim mounted in the bearing housing. 9. Mechanical element according to claim 8, characterized in that the bearing housing is made of a light metal alloy. 10. Mechanical element according to claim 9, characterized in that the bearing housing is made of an alloy based on aluminum or magnesium. 11. Mechanical element according to one of claims 8 to 10, characterized in that the thermal expansion coefficient of the compensating ring is greater than that of the material from which the outer ring of the rolling bearing is made and is greater than that of the material from which the seat of the outer ring is made.