Bearing unit with retaining cage

Abstract

Bearing unit (30) having a central rotation axis (X) and a retaining cage (40, 41) for a plurality of rolling bodies (32) interposed between a radially outer ring (31) and a radially inner ring (33), where, alternatively or in combination: - the cage (40, 41) has a radially outer cylindrical surface (40a, 41a) provided with a texture, obtained by means of reliefs or scoring, - the radially outer ring (31) has a cylindrical and radially inner guide surface (31a) provided with a texture, obtained by means of reliefs or scoring, wherein the texture is obtained by correlating an autocorrelation length (SAL) and an aspect ratio (STR).

Description

[0001] 1 202400196

[0002] BEARING UNIT WITH RETAINING CAGE DESCRIPTION

[0003] Technical field of the invention

[0004] The present invention relates to a bearing unit with a retaining cage. In particular, the bearing unit is an angular-contact bearing unit for applications requiring high precision, for example machine tools.

[0005] Prior art

[0006] Known bearing units with a retaining cage comprise:

[0007] - a radially outer ring,

[0008] - a radially inner ring,

[0009] - a plurality of rolling bodies, in particular balls, interposed between the inner ring and the outer ring to enable the relative rotation thereof, and

[0010] - a retaining cage, which holds the balls in their respective circumferential positions and is centred on the radially outer ring.

[0011] The retaining cage of a known type comprises a one-piece annular body that defines a reference axis, the body of the cage delimiting substantially cylindrical cavities or pockets distributed about the reference axis. The balls are housed in these cavities, which define the positioning and holding of said balls.

[0012] These retaining cages are usually used in high-velocity, high-rotational-precision bearing units, in particular for applications in the industrial sector, for example in machine tool spindles, and are typically made by injection moulding a polymer material, including to compensate for poor lubrication conditions. As the bearing unit requires a high degree of precision, the cages for these applications must also meet this accuracy requirement.

[0013] When the bearing is in use, including in consideration of the high rotational speeds reached, the retaining cages can both vibrate and impact with the rolling bodies and the rings, and such vibrations and impacts, especially within specific speed ranges, can cause clicking, i.e. repetitive high-pitched noises, which, 2 202400196

[0014] although tolerated by end users because this defect does not lead to premature failure, are nonetheless perceived as a lack of quality.

[0015] In order to try to reduce the clicking caused by the cages, and therefore also the vibration thereof, the radially outer rings of some bearings were provided with ring guides in some applications discussed herein, this measure nonetheless being generally applicable. However, especially in some specific speed ranges, these ring guides did not have the desired effects. On the contrary, it was found that friction between the retaining cages and the ring guides caused circular, whirling or unstable motion in the retaining cages themselves, and even more clicking.

[0016] There is currently no known solution to this technical issue. It is possible to design a cage that is guided by the balls and not by the outer ring. This solution stops the cage from clicking, but using the balls for guidance can in some instances limit the operating conditions of the bearing itself.

[0017] In general, oil lubrication exhibits a lower likelihood of clicking in the cage compared to grease lubrication, but this choice also cannot be considered a permanent solution to the technical problem.

[0018] Summary of the invention

[0019] One purpose of the present invention is to provide a bearing unit with a retaining cage in which the clicking noise is absent or at least very reduced.

[0020] Consequently, the present invention describes a bearing unit with a retaining cage, having the features set out in the independent claim attached to the present description.

[0021] Other preferred and / or particularly advantageous embodiments of the invention are described according to the features set out in the attached dependent claims.

[0022] Brief description of the drawings

[0023] The invention is described below with reference to the attached drawings, 3 202400196

[0024] which illustrate some non-limiting example embodiments thereof, in which:

[0025] - Figure 1 is a sectional view of an embodiment of a bearing unit according to the present invention,

[0026] - Figure 2 illustrates a detail of a first embodiment (Figure 2a) and a second embodiment (Figure 2b) of retaining cages of the bearing unit in Figure 1,

[0027] - Figure 3 illustrates a detail of the cage in Figure 2a,

[0028] - Figure 4 is an autocorrelation graph for a retaining cage according to the prior art,

[0029] - Figures 5 and 6 illustrate the same autocorrelation graph for the cage in Figure 2a and the cage in Figure 2b respectively.

[0030] Detailed description

[0031] In Figure 1, reference sign 30 denotes a bearing unit as a whole, according to a preferred embodiment of the invention.

[0032] The bearing unit 30 has a central rotation axis X and comprises:

[0033] - a stationary radially outer ring 31,

[0034] - a rotary radially inner ring 33,

[0035] - a row of rolling bodies 32, in particular balls, interposed between the radially outer ring 31 and the radially inner ring 33, and

[0036] - a cage 40 to hold the rolling bodies 32 in place, said cage being centred on the radially outer ring 31.

[0037] Throughout the present description and in the claims, terms and expressions indicating positions and orientations, such as "radial" and "axial", are to be understood with reference to the central rotation axis X of the bearing unit 30, unless otherwise specified. For the sake of simplicity, the term "ball" may be used by way of example in the present description and in the attached drawings instead of the more generic term "rolling body", and the same reference signs shall be used. 4 202400196

[0038] With reference to Figure 2, the cage 40, 41 according to the present invention, made of metal or polymeric material, has a radially outer cylindrical surface 40a, 41a with a morphology or texture provided by dense reliefs and / or very small scoring that decisively influence both the tactile qualities and the visible qualities. The cylindrical surface 40a, 41a is the surface centred on a corresponding cylindrical radially inner guide surface 31a of the radially outer ring 31.

[0039] The texture provided on the cage is the preferred embodiment of the present invention. However, as an alternative, the same texture can be provided on the guide surface 31a of the radially outer ring 31, without thereby departing from the scope of the present invention. In addition, the texture could be provided on both components (the cage and outer ring) to achieve a synergistic effect. The present description focuses below on the first and preferred embodiment of the invention (texture on the cage) while remaining generally applicable.

[0040] The texture provided on the surface of the cage improves the characteristics of the friction between the two components, as evidenced by tests carried out following morphological changes to the cylindrical radially outer surface 40a of the cage.

[0041] The cage according to the invention may be:

[0042] - a cage 40, the cylindrical radially outer surface 40a of which is provided with an isotropic texture (Figure 2a), or

[0043] - a cage 41, the cylindrical radially outer surface 41a of which is provided with an anisotropic texture (Figure 2b).

[0044] The key parameters responsible for reducing friction and thus mitigating clicking in the cage are the autocorrelation length (SAL), in the order of microns, and the aspect ratio (STR) of the texture. Autocorrelation length is the distance within which the autocorrelation function of a surface texture decays to a 5 202400196

[0045] specified value (typically 0.2) in the fastest way. Essentially, it measures how quickly the surface texture loses its similarity over a distance. On the other hand, the aspect ratio of the texture is a measure of the uniformity or isotropy of the texture of the surface. It is calculated by dividing the minimum autocorrelation length by the maximum autocorrelation length. The value of the aspect ratio of the texture ranges from 0 to 1, where a value close to 0 indicates strong directionality (anisotropy), and a value close to 1 indicates a uniform texture independent of direction (isotropy).

[0046] In particular, an autocorrelation length greater than the original texture-free surface reduces friction, resulting in smoother surface profiles, which promote the formation of stable lubricating films and minimize direct contact with the surface.

[0047] Figure 3 for example shows a detail 50 of the cylindrical surface 40a, which is the subject of the graphs in Figures 4 to 6. In particular, Figures 4 to 6 show the autocorrelation graphs of a cage according to the prior art (Figure 4), of the isotropic cage 40 (Figure 5) and of the anisotropic cage 41 (Figure 6), respectively. These graphs illustrate the detail 50 of the cylindrical radially outer surface of the respective cages. They have length scales in the Y direction, the circumferential direction or direction of the circular sliding motion of the cage with respect to the radially outer ring, and in the X direction, the axial direction or direction perpendicular to the circular motion of the cage. These graphs represent the autocorrelation of the cylindrical surface of the corresponding cage. The chromatic scale is in fact the autocorrelation value, represented by a pure number.

[0048] The cage according to the prior art (Figure 4) is characterized by a short autocorrelation length and an aspect ratio of the isotropic texture.

[0049] The isotropic cage 40 (Figure 5) exhibits a near-isotropic aspect ratio STR of the texture (with STR > 0.5) but the autocorrelation length SAL is significantly 6 202400196

[0050] greater: preferably SAL > 60 μm, even more preferably SAL > 70 μm.

[0051] Finally, the anisotropic cage 41 (Figure 6) is characterized by a highly directional and therefore strongly anisotropic aspect ratio STR of the texture (STR < 0.1), which results in a maximum autocorrelation length SAL in the axial direction (X axis) and a minimum autocorrelation length in the circumferential direction (Y axis), which, as already mentioned, is the direction of the circular motion of the cage 41 with respect to the radially outer ring 31. Preferably the autocorrelation length SAL is greater than 100 μm in the axial direction and less than 30 μm in the circumferential direction.

[0052] Based on these observations on the autocorrelation length and the aspect ratio of the texture, the superior performance of the two textured cages, in terms of friction reduction, can be explained as follows:

[0053] - rough cylindrical surfaces with a shorter autocorrelation length (in the case of the cage according to the prior art) have more pronounced peaks and valleys which mechanically interlock with the mating surface of the radially outer ring, resulting in increased friction,

[0054] - the cage 40 with an isotropic surface and an autocorrelation length greater than the cages according to the prior art, reduces the aforementioned interlocking effect, and consequently friction. In addition, the greater autocorrelation length reduces the real contact area and therefore the adhesion forces, resulting in less friction. Finally, the lower, more uniform friction is also due to better retention and distribution of the lubricating grease in the contact area;

[0055] - in addition, the cage 41 with an anisotropic surface and an autocorrelation length that differs significantly between the vertical and horizontal directions, exhibits low friction because the perpendicular orientation of its characteristics with respect to the sliding direction contributes to generating hydroplaning, significantly reducing direct contact with the radially outer ring and 7 202400196

[0056] therefore friction.

[0057] Ultimately, according to the present invention, providing the texture on the radially outer cylindrical surface of the cage keeps the friction with the radially outer ring centring the cage low, preventing the generation of whirling or unstable motion of the cage that causes clicking in the bearing.

[0058] In addition to the embodiments of the invention as described above, it should be understood that there are numerous other variants. Said embodiments are provided solely by way of example and do not limit the scope of the invention, its applications or its possible configurations. Indeed, although the description provided above enables the person skilled in the art to carry out the present invention at least according to one example configuration thereof, numerous variations of the components described could be used without thereby departing from the scope of the invention, as defined in the attached claims interpreted literally and / or according to their legal equivalents.

Claims

8 202400196CLAI M S1. Bearing unit (30) having a central rotation axis (X) and comprising: - a stationary radially outer ring (31),- a rotatable radially inner ring (33),- a row of rolling bodies (32), interposed between the radially outer ring (31) and the radially inner ring (33), and- a retaining cage (40, 41) for retaining the rolling bodies (32), centred on the radially outer ring (31),the bearing unit (30) being characterized in that, alternatively or in combination:- the cage (40, 41) has a radially outer cylindrical surface (40a, 41a) provided with a texture, obtained by means of reliefs and / or scoring,- the radially outer ring (31) has a cylindrical and radially inner guide surface (31a) provided with a texture, obtained by means of reliefs and / or scoring,wherein the texture is obtained by correlating an autocorrelation length (SAL) and an aspect ratio (STR).

2. Bearing unit (30) according to claim 1, wherein the texture has a nearly isotropic aspect ratio (STR) with a high autocorrelation length (SAL).

3. Bearing unit (30) according to claim 2, wherein the aspect ratio (STR) is greater than 0.5 and the autocorrelation length (SAL) is greater than 60 μm.

4. Bearing unit (30) according to claim 1, wherein the texture has an anisotropic aspect ratio (STR) wherein:- the autocorrelation length (SAL) is maximum in a direction perpendicular to the circular motion of the cage (41) with respect to the radially outer ring (31), - the autocorrelation length (SAL) is minimum in the direction of the circular motion of the cage (41).

5. Bearing unit (30) according to claim 4, wherein the aspect ratio (STR)202400196is less than 0.1 and the autocorrelation length (SAL):- is greater than 100 μm in the direction perpendicular to the circular motion of the cage (41), - is less than 30 μm in the direction of the circular motion of the cage (41).

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