Bearing unit with retaining cage

Abstract

A bearing unit (30) having a central rotation axis (X) and provided with a retaining cage (4) for a plurality of rolling bodies (32) placed between a radially outer ring (31) and a radially inner ring (33), wherein: - the cage (40) is centred on the radially outer ring (31) and has an annular body (45) inside which there is formed a plurality of pockets (50, 51, 52), each of which is able to contain with clearance a respective rolling body 32, the pockets (50, 51, 52) being arranged side by side in a circumferential direction, and - the cage (40) is configured to create an axial clearance between pocket (50, 51, 52) and rolling body (32) smaller than a circumferential clearance between said pocket (50, 51, 52) and rolling body (32).

Description

[0001] 1 202400150

[0002] BEARING UNIT WITH RETAINING CAGE

[0003] DESCRIPTION

[0004] Technical Sector of the Invention

[0005] The present invention relates to a bearing unit with a retaining cage. In particular, the bearing unit is of the angular contact type for applications which require a high degree of precision, for example machine tools.

[0006] Prior Art

[0007] The bearing units with retaining case of the known type comprise:

[0008] - a radially outer ring,

[0009] - a radially inner ring,

[0010] - a plurality of rolling bodies, in particular balls, placed between the two - inner and outer - rings so as to allow relative rotation thereof, and

[0011] - a retaining case which retains the balls in respective circumferential positions and is centred on the radially outer ring.

[0012] The retaining cage of the known type comprises an annular body made as one piece which defines a reference axis, the body of the cage delimiting substantially cylindrical cavities or pockets distributed around the reference axis. The balls are seated inside these cavities which determine the positioning and retaining of the said balls.

[0013] Generally speaking, these retaining cages are used in high-speed bearing units with a high degree of rotational precision, in particular for applications in the industrial sector, for example in spindles of machine tools, and are typically made by means of injection-moulding using polymer material, among other things in order to rectify poor lubricating conditions. Since the bearing unit requires a significant degree of precision, the cages for these applications must satisfy said requirement for precision.

[0014] During operation of the bearing, also in view of the high rotational 2 202400150 speeds which are reached, the retaining cages may both vibrate and be subject to impacts with the rolling bodies and with the rings, and these vibrations and impacts, in particular within specific speed ranges, may generate clicking sounds, namely acute and repetitive noises, which, although tolerated by the end users since this defect does not result in premature malfunctions, are in any case perceived as indicating a lack of quality.

[0015] In an attempt to reduce the clicking sound produced by the cages, namely also the vibrations affecting them, in some applications to which the present description will make reference to without thereby losing its general character, the radially outer rings of some bearings are provided with annular guides. However, in particular within certain specific speed ranges, these annular guides have not led to the desired results. On the contrary, it has been possible to note that the friction between the retaining cages and the annular guides generates circular, vortical or unstable movements in the retaining cages themselves, with even a further increase in the clicking noise.

[0016] At the moment a solution to this technical problem is not known. It is possible to design a cage so that it is guided by the balls and not by the outer ring. This solution prevents the clicking sound of the cage, but sometimes the guide on the balls limits the operating conditions of the bearing itself.

[0017] In general, the oil lubrication results in less probability of clicking of the cage then grease lubrication, but this choice also cannot be regarded as providing a definitive solution to the technical problem.

[0018] Summary of the Invention

[0019] An object of the present invention is to provide a bearing unit with retaining cage, where there is no clicking noise of the cage or where it is at least much less loud.

[0020] Therefore, according to the present invention a bearing unit with 3 202400150 retaining cage having the characteristic features indicated in the independent claim attached to the present description is described.

[0021] Further preferred and / or particularly advantageous embodiments of the invention are described in accordance with the characteristic features indicated in the attached dependent claims.

[0022] Brief Description of the Drawings

[0023] The invention will now be described with reference to the accompanying drawings which illustrate a number of non-limiting examples of embodiment thereof, in which:

[0024] - Figure 1 shows a cross-sectional view of an embodiment of a bearing unit provided in accordance with the principles of the present invention, and

[0025] - Figure 2 shows a detail of a retaining cage according to the prior art (Fig. 2a) and, respectively, of a first embodiment (Fig. 2b), a second embodiment (Fig. 2c) and a third embodiment (Fig. 2d) of retaining cages of the bearing unit shown in Figure 1.

[0026] Detailed Description

[0027] With reference to Figure 1, 30 denotes overall a bearing unit according to a preferred embodiment of the invention.

[0028] The bearing unit 30 has a central rotation axis X and comprises:

[0029] - a stationary radially outer ring 31;

[0030] - a rotatable radially inner ring 33;

[0031] - a row 32 of rolling bodies 32, in particular balls, placed between the radially outer ring 31 and the radially inner ring 33; and

[0032] - a cage 40 for keeping in position the rolling bodies 32, which is centred on the radially outer ring 31.

[0033] In particular, the cage 40 has a cylindrical surface 40a which is centred on a corresponding, cylindrical and radially inner, guide surface of the radially 4 202400150 outer ring 31.

[0034] The cage 40 is provided with an annular body 45 which has, formed inside it, a plurality of pockets 50, each of which is able to contain a respective rolling body 32, the pockets 50 being arranged side by side in the circumferential direction.

[0035] In the whole of the present description and in the claims, the terms and the expressions indicating positions and orientations such as "radial" and "axial" are understood as referring 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 likewise the same reference numbers will be used).

[0036] In order to reduce the clicking noise of the cage, the pockets 50 of the cage 40 according to the present invention are optimized as regards the geometry and / or the form and / or the position inside the annular body 45.

[0037] The clearance between pocket 50 and ball 32 is an important parameter of the geometry of the cage and directly influences the clicking noise of the cage. Reducing the clearance of the ball inside the pocket reduces the probability of clicking of the cage occurring and also its loudness.

[0038] The simplest way of reducing to a minimum the possible movements of the cage between the balls, and therefore of reducing the probability of the vortical or unstable movement which causes the clicking noise, is to reduce the dimensions of the pocket. A first series of experimental tests is shown in Table 1 in which cages tested with different dimensions of the pocket are listed. The cage according to the prior art, or standard cage, has a pocket diameter (0pocket) equal to 7.6 mm, the diameter (0ball) of the balls 32 being equal to 7.14 mm. For the same diameter of the balls, the other cages tested 5 202400150 have a larger or smaller diameter (and consequently a greater or smaller clearance between pocket and ball) compared to the cage with standard pockets, as shown in Table 1.

[0039] TABLE 1

[0040] In particular, the cage with a larger pocket is characterized by a pocket size which is bigger than that of the standard cage and it generates vibrations of the cage greater than those of the standard cage. By reducing the diameter of the cage pocket compared to the standard dimension, the vortical movement of the cage is reduced and / or avoided, but other noisy instability of the cage, in particular of the cage with smaller pocket 2, namely that with the smallest pocket diameter, was noted. Most probably, excessively reducing the size of the pocket triggers an instability of the cage between balls and cage, instead of the vortical movement of the cage which occurs mainly between the cage 40 and the guiding surface 31a of the radially outer ring 31.

[0041] Therefore, the reduction in the diameter of the pocket risks triggering further noise phenomena if this reduction is excessive, as in the case of the cage with smaller pocket 2.

[0042] It was also noticed that an excessive reduction of the clearance between pocket and ball also results in an increase in the temperature of the bearing unit, limiting the maximum speed at which the bearing unit itself may operate without the risk of malfunctioning.

[0043] Therefore, with reference to Figure 2, the clearance between pocket 50, 51, 52 and ball is reduced only in the axial direction X, while the original clearance in the circumferential direction Y is maintained.

[0044] Figure 2 shows a number of embodiments of the invention according to 6 202400150 the present solution, namely cages with axial clearance between pocket and ball smaller than the circumferential clearance between pocket and ball. In particular, Figure 2a shows in schematic form a cage according to the prior art, namely with clearance unchanged between pocket and ball. As regards the other figures:

[0045] - Figure 2b shows a cage according to a first embodiment of the invention with circular pockets 50 which are unaligned in the axial direction X so as to have an effective axial dimension which is smaller;

[0046] - Figure 2c shows a cage in a second embodiment with slotted pockets 51 having an axial dimension (0axial) which is smaller and a circumferential dimension 0circ which is unchanged compared to the diameter of the pockets of the standard cage;

[0047] - Figure 2d shows a cage according to a third embodiment with slotted pockets 51 having an axial dimension (0axial) which is smaller and a circumferential dimension 0circ which is unchanged. Moreover, the pockets 52 have conical side surfaces 52a formed through the thickness of the cage in the radial direction, i.e. direction perpendicular to the plane formed by the axes X and Y.

[0048] Therefore, the solution of cages with varying axial and circumferential clearance of the pockets may be obtained by arranging the circular pockets 50 of the cage axially unaligned according to the first embodiment or by forming slotted pockets 51, 52 for the cages in the second and third embodiments of the invention, respectively.

[0049] Both solutions, suitably tested experimentally, show that the reduction of the axial movement range of the cage improves its performance since it was observed that the instability of the cage also has an axial component. The parameters of the cages experimentally tested with varying axial and 7 202400150 circumferential clearance of the pocket are listed in Table 2. All of the parameters refer to the cage according to the prior art. Circumferentially, all the cages tested maintain the same clearance as the standard cage, but axially have a smaller clearance with the balls (diameter of the balls always equal to 7.14 mm) and preferably the same value as that of the cage with a smaller pocket 2 (7.45 mm as in Table 1).

[0050] TABLE 2

[0051] All the cages with varying axial and circumferential clearance of the respective pockets had an improved vibration performance compared to that of the standard cage.

[0052] In percentage terms and in relation to a standard cage, therefore, the axial clearance, based on the data shown in Table 2, is 33% less 0axial / 0ball = 4.3 / 6.4 = 67%).

[0053] Finally, in Table 3 two further variants of cages and balls were considered. In these two variants, the axial clearance between pocket and ball (diameter of the balls again equal to 7.14 mm) is reduced further and at the same time the circumferential clearance is also reduced, even though by a smaller amount compared to the axial clearance.

[0054] TABLE 3 8 202400150

[0055] The test data have shown, therefore, that the axial clearance may be reduced down to a value 0axial / 0ball equal to 3.5%, while the circumferential clearance may be reduced down to the value 0circ / 0ball 5.2%, maintaining good performance results in terms of vibration, i.e. thus achieving the objects of the present invention. In percentage terms and in relation to a standard cage, therefore, the axial clearance, is 45% less 0axial / 0ball = 3.5 / 6.4 = 55%), while the circumferential clearance is 19% less 0circ / 0ball = 5.2 / 6.4 = 81%). The reduction of the axial clearance, in percentage terms and in relation to the standard, therefore lies within a range of between -33% and - 45%, while the reduction of the circumferential clearance, again in percentage terms and in relation to the standard, lies within a range of between 0% and - 19%.

[0056] Therefore, with the cages according to the present invention, which have between pocket and ball an axial clearance which is smaller and a circumferential clearance which is unchanged compared to the standard value or smaller but to a lesser degree compared to the reduction of the axial clearance, two advantages may be achieved:

[0057] - by reducing the amount of the movement of the cage in the axial direction, the space available for the cage to develop clicking noise is limited, thereby reducing the noise level, if not completely preventing it;

[0058] - by keeping the circumferential clearance between pocket and ball unchanged or slightly smaller, the increase in the temperature of the bearing unit is avoided along with the triggering of instability between the balls and the cage.

[0059] In addition to the embodiments of the invention, as described above, it 9 202400150 is to be understood that numerous further variants exist. It must also be understood that said embodiments are only examples and do not limit either the scope of the invention, nor its applications, nor its possible configurations. On the contrary, although the description provided above enables the person skilled in the art to implement the present invention at least in one of its examples of configuration, it must be understood that numerous variations of the components described are feasible, without thereby departing from the scope of the invention, as defined in the accompanying claims, interpreted literally and / or in accordance with their legal equivalents.

Claims

10 202400150C LA I M S1. Bearing unit (30) having a central rotation axis (X) and comprising :- a stationary radially outer ring (31),- a rotating radially in ner ring (33),- a row (32) of rolling bodies, placed between the radially outer ring (31) and the radially inner ring (33), and- a retaining cage (40) for retaining the rolling bodies (32), centred on the radially outer ring (31) and having an annular body (45), inside which there is formed a plurality of pockets (50, 51, 52), each of which is a ble to contain with clea ra nce a respective rolling body (32), the pockets (50, 51, 52) being arranged side by side in a circu mferential direction, the bearing unit (30) being characterized in that the cage (40) is configured to create an axial clea rance between pocket (50, 51, 52) and rolling body (32) smaller than a circumferential clearance between said pocket (50, 51, 52) and rolling body (32) .

2. Bearing unit (30) according to Claim 1, wherein the percentage reduction, in relation to the standard, of the axial clearance between pocket (50, 51, 52) and rolling body (32) is between -33% and -45% .

3. Bearing unit (30) according to Claim 2, wherein the percentage reduction, in relation to the standard, of the circumferential clearance between pocket and rolling body is between 0% a nd - 19%.

4. Bearing unit (30) according to any one of the preceding claims, wherein the pockets (50) are circular and unaligned in the axial direction .

5. Bearing unit (30) according to any one of Claims 1 to 3, wherein11 202400150 the pockets (51, 52) are slotted.

6. Bearing unit (30) according to Claim 5, wherein the pockets (52) have conical side surfaces (52a) .

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