Rolling element bearing systems are applied in rotating machinery, such as motors, vehicles, domestic appliances etc. For certain applications, the bearings should have a low noise level, such as for applications in electric motors which are used for driving domestic appliances.

Attempts have been made to reduce the vibrations and displacement by misalignment of bearings by means of elastic suspension blocks, e.g. of rubber. The rubber blocks however have the disadvantage that the radial bearing stiffness is greatly impaired, hence causing undesired high shaft vibrations. For many applications, such as electric motors, this is unacceptable.

The object of the invention is therefore to provide a bearing which has the required stiffness characteristics, but which also offers improved damping characteristics. This object is achieved in that at least one of the rings of the bearing engages a damping layer which at least partly consists of a visco-elastic material or a damping fluid, which damping layer is at a distance from the raceway of said ring.

The damping layer provides an increased damping due to its visco- elastic properties. Moreover, said layer can be rather thin, whereby virtually no displacements in radial direction are possible and thus the radial stiffness of the bearing is not, or only in a limited way influenced by the visco-elastic material. Despite the small deformation of the visco-elastic layer, a sufficient damping is obtained due to the shear stresses which are generated in said layer as a result of the relatively high pressures in the contact between the rolling element set and the ring.

In particular, the radial deformation of the damping layer is preferably of the same order of magnitude as the indentation or contact deformation of the raceway which occurs between rolling elements and said ring under nominal load without the presence of such

layer. Under these conditions, the radial stiffness of the bearing is about of the same order of magnitude to the radial stiffness of a standard bearing.

The layer of visco-elastic material may be applied in a number of positions. According to a first possibility, at least one of the rings comprises two concentric ring parts which are mutually connected by the damping layer. The visco-elastic layer thus may form an integral part of one of the bearing rings.

An increased damping effect may be obtained in case the damping layer is situated near the region of maximum shear stress which occurs in said ring as a result of the contact of the rolling elements over the raceway. The layer can be at sufficient distance from the outer surface of the outer ring or the bore of the inner ring, where reduced shear stresses are encountered so that the bearing and in particular the layer have sufficient fatigue life According to further possibilities, the damping layer may be situated on the outer circumference of the outer ring, and/or on the inner circumference of the inner ring.

The damping obtained can also be influenced by selecting a proper material or layout. For instance, the damping layer may be composed of at least two layers with different material characteristics and/or thickness. Furthermore, the annular damping layer need not be of a continuous character in axial direction. According to the invention, instead of a continuous layer of visco-elastic material, the damping layer may comprise segments of visco-elastic material separated by axial air gaps. The bearing may comprise at least two concentric damping layers.

Furthermore bearing systems are known which for instance may comprise a deep groove ball bearing. Under load, the rolling elements such as balls exert a force on the raceways of the rings. Thereby, these rings are locally deformed as a result of the contact forces.

The deformation is maximal in the middle region close to the contact area, and thus in the middle region of the rings. The adjacent edge regions of the rings are deformed in a far less degree.

Moreover, the rings and thereby the raceways may have form deviations such as out-of-roundness or waviness in circumferential direction. Such waviness induces vibrations in the bearing system,

which give rise to noise. In combination with the stiff coupling with the housing and shaft, these phenomena are aggravated.

According to the invention, a bearing system can be obtained which performs better with respect to such vibrations. This is achieved in that at least one of the cooperating surfaces of the outer ring and the bearing support, and/or at least one of the cooperating surfaces of the inner ring and the shaft have a circumferential groove directly opposite the raceway of said ring.

The ring in question of the bearing system according to the invention is able to locally deform in the contact area between raceway and rolling element, due to the fact that the ring is exposed to supporting forces only in the adjacent edge areas thereof. Thus, possible vibrations caused by waviness are less transmitted to the other elements of the bearing system, such as supports and shafts.

The damping effect can be obtained by filling at least one groove with a damping fluid or a solid visco-elastic material.

The grooves can be applied in various ways. For instance, the outer ring may have a groove in its outer circumference; in the alternative, the bearing support may have a groove in its contact surface.

Furthermore, the inner ring may have a groove in its outer circumference, or the shaft may have a groove in its contact surface.

Reference is made to a bearing system such as known from US-A- 5310268. The bearing applied therein has an outer ring with a circumferential groove, which groove is not directly opposite a series of rollers. Thereby, no reduction in vibrations is obtained.

Instead, the groove according to US-A-5310268 plays a role in fixing the outer ring of the bearing in a housing. To that end, the groove has protruding, sharp edges which cut into the housing wall.

Such layout is not suitable for attenuating vibrations, but would rather lead to fretting when subjected to vibrations.

According to a preferred embodiment of the invention, the edges between the groove and the surface in which the groove opens out, are rounded off in a cross-section through the axis of the bearing.

The smooth transition between groove and opposite wall of e.g. a housing provides a reduction of vibrations, without causing damage to the adjoining components in question.

The ratio of the minimum distance between the raceway of a bearing ring, and the damping layer, to the thickness of the damping layer is between 5-15.

In particular, the maximum distance between the raceway of a bearing ring, and the damping layer, to the thickness of the damping layer may be between 7-10.

Preferably, the ratio of the minimum distance between the raceway of a bearing ring, and the damping layer, to the thickness of the damping layer is 8.

The invention is also related to a rolling element bearing having an inner ring and an outer ring which enclose a bearing space containing at least one series of rolling elements; according to the invention, at least one of the rings comprises a damping layer which at least partly consists of a visco-elastic material, which damping layer is at a distance from the raceway of said ring. Furthermore, the outer ring and/or the inner ring may have a circumferential groove directly opposite the raceway of said ring, which ring is at least partly filled with a visco-elastic material.

The invention will further be described with reference to some embodiments shown in the figures.

Figure 1 shows a first embodiment of a bearing system according to the invention.

Figure 2 shows a second embodiment.

Figure 3 shows a cross-section through a third embodiment.

Figure 4 shows a cross-section through a fourth embodiment.

Figure 5 shows a fifth embodiment of the bearing system according to the invention.

Figure 6 shows a sixth embodiment.

Figure 7 shows a detail of figure 5.

In figure 1, a bearing 1 is shown which is supported in a support 2. The bearing 1 comprises an outer ring 3 and an inner ring 4, which is connected to a shaft 5. The outer ring 3 and the inner ring 4 are rotatable with respect to each other by means of balls 6.

According to the invention, a thin visco-elastic layer 7 is present around the outside of the outer ring 3, which visco-elastic layer 7 itself is supported in a correspondingly shaped hole in bearing support 2.

The deformation of the visco-elastic layer 7 is about the same order of magnitude as the contact deformation of the raceways of outer ring 3 and inner ring 4, as generated at the contact spots between rolling elements 6 and outer ring 3 or inner ring 4.

Thanks to the small thickness of the visco-elastic layer 7, the shaft 5 is supported in a relatively stiff way in the radial direction, as is the case with normal bearings. On the other hand, despite the small deformation of the visco-elastic layer 7, it is able to dampen out vibrations in the shaft 5.

In the embodiment of figure 2, a ball bearing is shown comprising an outer ring 8, an inner ring 9 connected to a shaft 10, and balls 11. The outer ring is split into two concentric outer rings halves 12, 13, which are interconnected by a thin visco-elastic layer 14. As in the embodiment of figure 1, the deformation of this intermediate visco-elastic layer 14 is of the same order of magnitude as the contact deformation.

Preferably, the distance of the visco-elastic layer 14 with respect to the raceway of the outer ring is selected such that the visco-elastic layer 14 finds itself where the shear stresses in the outer ring 8 are at a maximum. Some distance to the area of maximum shear stress is however required, such that the fatigue life of said layer is not reduced.

The shear stresses are generated by the rolling movement of the balls 11 over the raceway of the outer ring 12, 13. By selecting this location for the visco-elastic layer 14, its contribution to the damping characteristics of the bearing is maximized.

The embodiment of figure 3 shows a cross-section through the axis of a bearing. The bearing comprises an outer ring 15, and an inner ring 16 as well as balls 17. Outer ring 15 comprises two outer ring halves 18, 19, enclosing a thin visco-elastic layer 20.

In the cross-section of figure 3, the thin visco-elastic layer 20 is chevron-shaped, such that in the legs 21 of each chevron additional shear forces are generated under load.

The embodiment of figure 4 shows a bearing having an intermediate visco-elastic layer 24 which is sandwiched between metal foil layers 23. The sandwich of these layers 23, 24 forms a unity, and has been applied in the inner ring of a bearing. The outer ring of this bearing has been indicated by 26; the balls are indicated by 25.

Figure 5 shows a bearing system comprising a deep groove ball bearing 31, the outer ring 32 two of which is supported in a housing 33. The inner ring 34 in turn rotatably supports a shaft 35. Between the rings 32, 34, balls 36 have been accommodated.

According to the invention, the outer ring 32 has a circumferential groove 37, which at that area does not contact the housing 33, and which is filled with a visco-elastic material 45.

As a result, under a load, the outer ring 32 is able to flex a little bit under the forces exerted thereon by the balls 36, in such a way that vibrations, such as caused by the waviness of the outer and inner ring, are reduced by the visco-elastic material 45.

In the alternative embodiment of figure 6, the deep groove ball bearing 38 is of standard design, and has rings 39, 40 between which balls 41 are accommodated.

Instead, the housing 42 has a groove 43, at the spot directly opposite the raceway 44 of inner ring 50, which groove 43 is filled with a visco-elastic material 46.

Here as well, vibrations caused under load are reduced due to the ability of the outer ring 40 to flex at this spot of groove 43.

Of course, also the shaft 35 may have a circumferential groove, directly opposite the corresponding raceway of the inner ring 39.

The detail of figure 7 shows the edge area 44 of the groove 37.

This edge area 44 is rounded off somewhat, so as to provide a smooth transition zone. Such smooth transition is favourable with respect to vibration reduction, and prevents fretting under dynamic loads.