Bearing unit with cage

Abstract

A bearing unit (30) having a central axis of rotation (X) and a cage (40) for a plurality of rolling bodies (32) interposed between a radially outer ring (31) and a radially inner ring (33) wherein the cage (40) is provided with:-a one-piece annular body (41) forming two rings (42) connected by a bridge (43); -a plurality of cavities (44) for accommodating and retaining respective rolling bodies (32), the cavities (44) being separated in pairs by bridges (43) and each being framed by two rings (42), the two rings (42) of the annular body (41) being provided with a plurality of protrusions (45) protruding radially outward to center the cage (40) on the radially outer ring (31), the protrusions (45):-being discontinuous relative to each other; -for each ring (42) of the annular body (41), arranged in two parallel columns in the circumferential direction; and-staggered with respect to each other.

Description

Technical Field

[0001] The present invention relates to a bearing unit having a retaining cage. In particular, the bearing unit is an angular contact bearing unit for applications requiring high precision, such as machine tools. Background Art

[0002] Known bearing units with cages include:

[0003] - radial outer ring,

[0004] - radial inner ring,

[0005] - a plurality of rolling elements (especially balls) interposed between the inner ring and the outer ring so that the inner ring and the outer ring can rotate relative to each other, and

[0006] - Cage, which holds the balls in their corresponding circumferential positions.

[0007] A known type of cage comprises a one-piece annular body defining a reference axis, the body of the cage delimiting substantially cylindrical cavities or pockets distributed around the reference axis. The balls are housed in these cavities, which define the positioning and retention of said balls.

[0008] These cages are often used in high-speed, high-vibration bearing units, particularly for applications in the industrial sector (e.g., in machine tool spindles), and are typically manufactured by injection molding polymer materials, including to compensate for poor lubrication conditions. Because the bearing units require a high degree of precision, the cages used in these applications must also meet these precision requirements. Therefore, the cages are precision-machined using machine tools and must have a very simple geometry.

[0009] One disadvantage of this type of cage is the friction losses between the outer surface of the cage and the inner surface of the outer ring of the bearing unit, on which the cage is centered. Summary of the Invention

[0010] It is an object of the present invention to provide a bearing unit with a cage which does not have the above-mentioned disadvantages.

[0011] The present invention thus describes a bearing unit with a cage having the features set out in the independent claims appended to this description.

[0012] Further preferred and / or particularly advantageous embodiments of the invention are described with reference to the features set out in the accompanying dependent claims. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The invention is described below with reference to the accompanying drawings, which show non-limiting example embodiments of the invention, in which:

[0014] - Figure 1 is an axonometric view of a first preferred embodiment of a bearing unit according to the present invention,

[0015] - Figure 2 yes Figure 1 Axonometric view of the cage of the bearing unit in

[0016] - Figure 3 Shown Figure 2 Details of the cage in the

[0017] - Figure 4 Shown Figure 2 Further details of the cage are given in the . DETAILED DESCRIPTION

[0018] exist Figure 1 In the drawings, reference numeral 30 denotes the entire bearing unit according to a preferred embodiment of the present invention.

[0019] The bearing unit 30 has a central axis of rotation X and comprises:

[0020] - a stationary radial outer ring 31,

[0021] - a rotating radial inner ring 33,

[0022] - an array of rolling elements 32 (in particular balls) between a radially outer ring 31 and a radially inner ring 33, and

[0023] A cage 40 for holding the rolling elements 32 in position, said cage being centered on the radially outer ring 31 .

[0024] Throughout the present specification and claims, unless otherwise indicated, terms and expressions indicating position and orientation (such as "radial" and "axial") are to be understood with reference to the central rotation axis X of the bearing unit 30. For simplicity, the term "ball" may be used by way of example in this specification and the accompanying drawings instead of the more general term "rolling element", and the same reference numerals will be used.

[0025] Reference Figure 2 and Figure 3 , the cage 40 according to the invention has an annular shape defining a main axis of inertia or reference axis, which coincides with the central axis of rotation X of the bearing unit 30 .

[0026] The cage 40 has a one-piece annular body 41 forming two rings 42 connected by bridges 43 , one ring 42 at each axial end.

[0027] The annular body 41 of the cage 40 may be made of any suitable material, in particular any material suitable for additive manufacturing, such as plastic, ceramic or metallic materials. An example of a preferably used material is a composite material based on cotton fibers impregnated with phenolic resin.

[0028] The two rings 42 and the bridges 43 define a plurality of through cavities 44 or pockets, which are separated in pairs by the bridges 43 and each of which is framed by the two rings 42. Radially outer surfaces 45 of the cavities 44 define the boundaries of the cavities relative to the two rings 42 and the pairs of bridges 43. The cavities 44 accommodate corresponding rolling elements 32 (particularly balls) to position and retain them.

[0029] Each cavity 44 has a polar plane PP and an equatorial plane PE, both perpendicular to the annular body 41. Each cavity 44 has a center O at the intersection of these two planes. The centers O of the cavities are all located at equal distances from the central axis of rotation X.

[0030] The object of the present invention is to optimize the topology of a cage 40 , in particular a cage for an ultra-precision angular contact bearing unit capable of operating optimally at high speeds (NDm speed factor of approximately 3 million).

[0031] In particular, the present invention aims to optimize the centering between the cage 40 and the radial outer ring 31. This centering is very important in high-speed and high-vibration angular contact bearing units because it ensures the stability of the cage (and therefore the rolling elements) at high speeds. More precisely, the centering is defined by a predetermined nominal gap between the radially outer cylindrical surface of the cage's annular body 41 and the radially inner surface of the outer ring, so that the movement of the cage relative to the outer ring is orbital. This means that the actual contact surface between the cage and the outer ring is a significant portion of the cage, but is limited to approximately one-third of the cage's axial width, because the orbiting motion causes contact to act only on one axial end of the cage's annular body 41 (i.e., one ring 42).

[0032] The solution provided by the present invention significantly reduces this contact surface with the outer ring. This is obviously intended to reduce the friction generated by the contact between the cage and the outer ring and, as explained further below, to facilitate the flow of grease using dedicated paths. According to the invention, the two rings 42 of the annular body 41 are provided with a plurality of protrusions 45 protruding radially outward, the plurality of protrusions 45 being discontinuous relative to one another, arranged in two parallel rows in the circumferential direction, and staggered relative to one another. Thus, each ring 42 has a first protrusion 45' arranged as a first, axially outer circumferential row and a second protrusion 45" arranged as a corresponding second, axially inner circumferential row. The protrusions 45 thus ensure that the cage 40 is centered on the radially outer ring 31.

[0033] Each projection 45 has a lateral surface 46 connected to the corresponding ring 42 of the annular body 41, and a radially outer semi-cylindrical surface 47. The combination of semi-cylindrical surfaces 47 is intended to contact the fixed outer ring 31 during the movement of the bearing unit to center and guide the cage 40. The multiple semi-cylindrical surfaces 47 represent only a portion of the inner cylindrical centering surface of the prior art, so the total contact surface of the cage 40 is reduced, thereby reducing friction losses caused by contact between the cage and the outer ring. All projections 45 (i.e., all semi-cylindrical surfaces 47) help to center the cage, of course not by contacting the outer ring simultaneously, but rather periodically according to the cage's orbital motion.

[0034] Along the entire circumference of the ring 42 of the annular body 41, there are exactly twice as many protrusions 45 as through-going cavities 44, and thus along the entire circumference of the entire annular body 41, there are four times as many protrusions 45 as cavities 44. For example, a typical cage for an angular contact bearing unit is provided with 19 through-going cavities 44 and thus 38 protrusions 45 per ring 42.

[0035] Also refer to Figure 4 Advantageously, the angular extension 2α of the projections 45 relative to the central axis of rotation X of the cage 40 and the bearing unit 30 is between 4° and 5°, for example, 4.6°. The projections 45 are equidistant in the circumferential direction. The angular distance β between the respective midplanes A, A' of two angularly consecutive projections is preferably between 9° and 10°, for example, 9.5°.

[0036] In total, the 38 protrusions 45 forming the two rows of each ring 42 cover an angular extent of 174.8°, representing 48.6% of the 360° of the entire circumference. In other words, according to the invention, the contact surface between cage and ring is reduced to slightly less than half compared to known solutions. In fact, the advantage in terms of reducing the contact surface is even greater because, according to the invention, the contact is established via the semi-cylindrical surface 47, which creates a linear contact rather than a surface contact.

[0037] Furthermore, this particular geometry defines a path P for the grease, which is defined between the radially outer surfaces 42' of the rings 42 that are not in contact with the radially outer ring 31 and the lateral surfaces 46 of the respective rings 42 that are connected to the protrusions 45. This tortuous path promotes the flow and / or redistribution of the grease in all contact areas between the cage and the outer ring.

[0038] This improved performance of the cage is due to the optimization of the cage topology, a process that combines design tools and FEM calculations and enables the realization of highly customized shapes. The principles of topology optimization have been applied to the cage of super-precision angular contact ball bearings, assuming that the component parts are manufactured using additive manufacturing technology. This enables the realization of all possible complex geometries resulting from the optimized topological design.

[0039] Thus, although the invention is applicable to any method of manufacturing a bearing unit cage, given the specific geometry of the cage, as described above, the aforementioned invention is particularly suitable for cages of bearing units whose bodies are obtained by additive manufacturing.

[0040] Finally, the present invention provides the following advantages:

[0041] - the entire cage can be obtained using the known injection moulding process, or preferably using other processes such as additive manufacturing,

[0042] - Reduce the actual area where there is contact between the cage and the outer ring. This mainly means reducing the friction between the two components,

[0043] - Improve the flow / redistribution of grease during cage movement, along the paths created between the cage surfaces that are not in contact with the outer ring.

[0044] In addition to the embodiments of the present invention described above, many other variations exist. The embodiments are provided by way of example only and do not limit the scope of the invention, its applications, or possible configurations of the invention. Indeed, although the description provided above enables one skilled in the art to implement the invention according to at least one example configuration of the invention, many variations of the described components may be used without departing from the scope of the invention as defined in the appended claims, interpreted literally and / or according to their legal equivalents.

Claims

1. A bearing unit (30) having a central axis of rotation (X) and comprising: - radial outer ring (31), - radial inner ring (33), - an array of rolling elements (32) placed between the radially outer ring (31) and the radially inner ring (33), and a cage (40) for retaining the rolling elements (32), the cage (40) being centered on the radially outer ring (31) and further comprising: - an annular body (41), in a single piece, forming two rings (42) connected by a bridge (43), - a plurality of cavities (44) for receiving and retaining corresponding rolling bodies (32), said cavities (44) being separated in pairs by said bridges (43) and each cavity being framed by said two rings (42), The bearing unit (30) is characterized in that the two rings (42) of the annular body (41) are provided with a plurality of protrusions (45) at the outer radial height for centering the cage (40) on the radial outer ring (31), wherein the protrusions (45): - are not continuous with each other, - for each ring (42) of the annular body (41), two parallel rows are arranged in the circumferential direction, and -intertwined with each other.

2. The bearing unit (30) according to claim 1, characterized in that Each protrusion (45) has a lateral surface (46) connected to the corresponding ring (42) of the annular body (41) and a radially outer semi-cylindrical surface (47).

3. The bearing unit (30) according to claim 1 or 2, characterized in that Along the entire circumference of the ring (42) of the annular body (41), the protrusion (45) is exactly twice the size of the cavity (44).

4. The bearing unit (30) according to any one of the preceding claims, characterized in that The angular amplitude (2α) of each protrusion (45) relative to the central rotation axis (X) of the bearing unit (30) ranges from 4° to 5°.

5. The bearing unit (30) according to claim 4, characterized in that Between two angularly consecutive protrusions (45), an angular width (β) of between 9° and 10° exists between the corresponding mid-planes (A, A′).

6. The bearing unit (30) according to claim 2, characterized in that A tortuous path P for grease is defined between the radially outer surface (42') of the ring (42) and the lateral surface (46) of the protrusion (45).

7. Bearing unit (30) according to any one of the preceding claims, characterized in that The annular body (41) of the retaining frame (40) is made of a material suitable for additive manufacturing technology.

8. The bearing unit (30) according to claim 7, characterized in that The material is a composite material based on phenolic resin impregnated with cotton fibers.

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