Method for manufacturing a composite material bearing cage, bearing cage and associated bearing unit

Abstract

Method for manufacturing a bearing cage made of composite material, bearing cage and associated bearing unit. Method for producing a bearing cage (3) comprising an annular body (7) and a plurality of pockets or housings (8) for bearing bodies; the annular body (7) comprising a plurality of superimposed layers (18) of reinforcing fibers (11) embedded in a synthetic plastic material (13); a preform tube (15) is produced by arranging the layers (18) on a mandrel (14), hardening the preform tube (15), radially cutting the preform tube (15) into slices consisting of axial sections (16) thereof, each having pockets or housings (8) to form a desired cage (3);a shrink band (21) is wound on the preform forming tube (15) and the curing process involves inserting the preform forming tube (15) and the mandrel (14) into an oven (20), the preform forming tube (15) being wrapped by the shrink band, and heating the assembly consisting of the mandrel (14), the preform forming tube (15) and the shrink band (21) to a polymerization temperature of the synthetic resin material (13). [Figure 3];

Description

Title of the invention: Method for manufacturing a bearing cage made of composite material, bearing cage and associated bearing unit. Technical field of the invention

[0001] The present invention relates to a method for manufacturing a bearing cage made of a fiber-reinforced composite synthetic plastic material, as well as a bearing cage manufactured according to the method and an associated bearing unit comprising such a cage. The invention relates, in particular, to an improved manufacturing method for obtaining a cage made of fiber-reinforced composite synthetic plastic material with high dimensional accuracy, so as to minimize the machining required to obtain the final cage. Prior art

[0002] It is well known that a rolling bearing unit comprises a rolling bearing having an outer ring, an inner ring and a plurality of rolling bodies (for example balls) interposed between the inner and outer rings to make them relatively rotatable with low friction, and a rolling bearing cage to retain the rolling bodies in position, the cage being arranged in the radial space delimited between the inner ring and the outer ring.

[0003] A bearing housing retainer cage comprises an annular body delimited between inner and outer cylindrical lateral surfaces and a plurality of pockets or housings, each configured to house and freely retain a respective bearing body of the roller bearing. The cage body is generally made of a synthetic plastic material, for example, a phenolic resin or a polyamide or other suitable synthetic materials, and supports the pockets or housings, which are located radially through it, for example, consisting of radial through holes.

[0004] A preferred manufacturing method consists of obtaining a preform in the form of a hollow tube, for example by molding the synthetic material, then the hollow tube is cut radially into a plurality of slices, each of which constitutes a cage body. Before or after the cutting operation, the pockets or recesses are drilled through the cage body.

[0005] To improve the operating performance of the cage, it is also known to obtain the cage body from a fiber-reinforced synthetic material, for example phenolic resins reinforced with cotton fibers embedded in the synthetic material matrix, or any other suitable composite material.

[0006] In these cases, the hollow tube constituting the preform can be produced by any known fiber placement process or by a process known as "continuous filament winding" (CFW), by tightly winding one or more filaments of composite material consisting of continuous fibers impregnated with a synthetic plastic resin onto a metal mandrel tool.

[0007] Here and hereafter, "plastic resin" means a synthetic material that is either thermosetting or thermoplastic, for example the impregnation of the fibers can be carried out either by a liquid thermosetting resin or by a solid thermoplastic powder.

[0008] After a predefined number of superimposed radial layers of pre-impregnated fibers have been obtained, the preform is hardened in a way known to cause the consolidation of the synthetic material impregnating the fibers into a solid matrix, in which the wound fibers remain embedded to constitute a reinforcing material.

[0009] Hardening can occur as disclosed, for example, in FR3053624A1.

[0010] Other fibre placement methods can be used to obtain the preform forming tube, for example by layering pre-impregnated fibre (pre-preg) in the form of sheets or mats with their pre-impregnated reinforcing fibre carefully arranged according to predefined patterns.

[0011] In any case, regardless of the manufacturing process, but more frequently when CFW processes are used, fiber-reinforced plastic cages may require extensive machining after the preform tube has been obtained, to ensure that the cage dimensions are within the prescribed operating tolerances. In particular, when a CFW process is used to obtain the preform tube, the outside and inside diameters of the preform tube, or those of the resulting cages cut from it, must be subjected to a turning or grinding process.

[0012] In fact, composite tubes produced by standard filament winding or conventional fiber placement processes have corrugated tube surfaces, which are not suitable for directly meeting the tight tolerances for the inner and outer diameters of the bearing cages. Consequently, the semi-finished tubes, namely the preformed tubes, are produced with a thickness greater than the nominal thickness (i.e., the thickness assigned at the design stage), and additional turning and / or grinding processes are required to obtain the correct inner and outer diameters of the bearing cage within the required tolerances.

[0013] Consequently, in the art, there is a need to obtain fiber-reinforced preform tubes having smooth inner and outer lateral surfaces and already having the correct inner and outer diameters designed for the final bearing cage. In fact, the need to machine the preform tube (or the cage preforms obtained from it) increases production costs, not only because of the machining operation itself, but also, and especially, due to the better use of the valuable raw materials involved, which can be expensive, and due to higher energy consumption during the curing stage, because of the larger quantity of material to be processed. Moreover, repeated machining can introduce errors that can lead to scrap. Summary of the invention

[0014] The objective of the present invention is to overcome the disadvantages of the prior art by providing a method for manufacturing a bearing cage made of composite material consisting of a synthetic plastic material reinforced by fibers, substantially free from the need to machine the inner and outer diameters of the tube forming the preform and / or the cage, in order to achieve the design diameters and to remain within the required operating tolerances.

[0015] An objective of the invention is also to provide a manufacturing process that requires less raw material and leads to reduced energy consumption.

[0016] An objective of the invention is finally to produce a bearing cage made of composite material consisting of a synthetic plastic material reinforced by fibers having reduced production costs and a high precision bearing unit equipped with such a cage, while maintaining good performance during use, especially in applications under particularly high stress, such as those requiring high rotational speeds and / or subjected to high loads.

[0017] According to the invention, a method for manufacturing a bearing cage made of composite material, and an associated bearing cage and a bearing unit equipped with such a cage, as defined in the attached claims, is provided.

[0018] The present invention allows for preform composite tubes configured for the production of bearing cages using any of the processes known in the art and having a correct dimension of the final inner and outer diameters of the cage to be obtained already during the placement of the fibers or the filament winding processes.

[0019] In embodiments of the invention, a defined number of layers of composite material are wound with regulated tension onto a smooth metal mandrel / tool ​​whose outer diameter is identical to the required inner diameter of the cage to be produced. The fiber placement or filament winding (CFW) process is stopped when the required outer diameter of the cage is obtained at the top of the stack of superimposed fiber layers impregnated with a suitable polymer resin, wound onto the mandrel to form a tube.

[0020] In a second step, a commercially available shrink band is placed or wound under regulated tension on the outer surface of such a tube formed by stacked layers of impregnated polymer fibers, which fibers have been oriented in the previous step in each layer according to a predefined pattern.

[0021] In a third step, the metal mandrel / tool, still carrying the tube of superimposed layers of impregnated polymer fibers wound on it, is placed, together with the layers of impregnated fibers and the shrink band wound around the outermost radially impregnated fiber layer, in an oven and cured at a defined temperature and for a defined time, for example according to FR3053624A1.

[0022] During the hardening stage, the shrinkage of the shrink band due to the temperature increase leads to a defined smoothing and compression / compaction of the composite preform forming tube made up of superimposed layers of impregnated polymer fibers, which makes it possible to obtain, after hardening and after removing the metal mandrel / mandrel from the hardened preform forming tube, and after cutting, in the radial direction, the hardened preform forming tube freed from the mandrel into slices, a number of cages, all having the same required cage outside diameter established in the design stage.

[0023] Consequently, any additional grinding or turning process of the hardened composite preform forming tube to obtain the required inner and outer bearing cage diameters within the required tolerances can be eliminated or at least greatly reduced, leading to a reduction in material and manufacturing time as well as a reduction in carbon footprint and costs. Brief description of the figures

[0024] Other features and advantages of the present invention will become clear from the following description of non-limiting examples thereof, made with reference to the figures in the accompanying drawings, in which:

[0025] - Figure [1] schematically represents a rolling bearing unit equipped with a bearing body retaining cage made according to the invention;

[0026] - [Fig.2] schematically represents, at an enlarged scale, a restraint cage bearing body for a bearing housing provided according to the present invention;

[0027] - Figure 3 schematically represents a preferred embodiment of certain steps of the process according to the invention, by which a preform-forming tube for obtaining the retaining cage of the [Fig.2] can be produced; and

[0028] - [Fig. 4] schematically represents a detailed perspective view of the tube forming a preform of [Fig. 3] where certain layers of composite material have been removed, for clarity, and other steps of the process according to the invention after the steps shown in [Fig. 3] have been carried out. Detailed description of the invention

[0029] With reference to Figures 1 to 4, reference number 1 indicates a rolling bearing unit ([Fig.1]) comprising a rolling bearing 2 of any known type and a rolling bearing cage 3, made of a composite material.

[0030] The bearing assembly comprises an inner ring 4, an outer ring 5 and a plurality of elements or bearing bodies 6, in the non-limiting embodiment shown, consisting of balls.

[0031] The bearing bodies 6 are arranged, in the example shown, in a ring of balls around an axis of symmetry of the bearing housing, which is also the axis of symmetry A of the cage 3 ([Fig. 2]). In different embodiments, not shown for simplicity, the bearing housing 2 may comprise two rings of bearing bodies arranged side by side, and the bearing bodies may be, interchangeably, balls, cylindrical or tapered rollers, or small cylinders, depending on the operating requirements.

[0032] In any case, the bearing cage 3 ([Fig.2]) comprises an annular body 7 and a plurality of pockets or housings 8, each configured to freely house, during use, a respective bearing body 6 of the bearing 2 to properly maintain the bearing bodies 6 spaced from each other by a predefined pitch.

[0033] The annular body 7 has an axis of symmetry A and a predefined axial width or length. The pockets or recesses 8 are located radially throughout the annular body 7, through respective inner and outer cylindrical lateral surfaces 9 and 10 ([Fig. 2]) of the annular body 7, substantially perpendicular to them, and, in the example shown, consist of simple radial cylindrical holes. The lateral surfaces 9 and 10 radially delimit the annular body 7 from each other.

[0034] The annular body 7 is made of a fiber-reinforced synthetic plastic material and, in a preferred embodiment of the present invention, can be obtained by a process known in the art as CFW (winding continuous filamentary), schematically represented in a non-limiting manner in [Fig.3], solely for illustrative purposes and for a better understanding of the invention.

[0035] With reference to [Fig.3], in a CFW production process, a plurality of reinforcing fibers 11 are unwound in a known manner from reels 12, are impregnated in a known manner with a resin / synthetic material 13, and then the impregnated reinforcing fibers 11b are wound around a mandrel or mandrel 14 at a predefined inclination with respect to the axis of symmetry Al of the mandrel 14, until a tube forming preform 15 is obtained (Figures 3 and 4).

[0036] Alternatively, carefully ordered pre-preg (prepreg) fibers or fiber sheets can be used to obtain the preform forming tube 15, for example according to any fiber placement process known in the art, always ultimately obtaining a preform forming tube 15, in this case composed of a number of sheets or mats of impregnated polymer fibers, tightly packed together and around the mandrel or mandrel-forming tool 14, in each sheet or mat the impregnated polymer fibers being carefully arranged according to a predefined pattern.

[0037] In any case, the final preform forming tube 15 ([Fig.4]) is composed of a plurality of layers 18 of impregnated polymer fibers 11b stacked one on top of the other and tightly wound (i.e. without any radial play) around the mandrel / mandrel-forming tool 14.

[0038] The axis of symmetry Al of the mandrel 14 coincides with the axis of symmetry A of the cage 3 to be obtained and, in the case of a CFW process, with the winding axis of the fibers 11 around the mandrel 14.

[0039] To obtain a plurality of annular bodies 7 from a single preform tube 15, the latter is hardened in any known and suitable way (for example according to FR3053624A1) in any known and suitable oven 20 ([Fig.4]), in order to polymerize the plastic material or synthetic resin 13 to form a solid matrix, and is then cut (in a way known in the art and not shown for reasons of simplicity) radially into slices each consisting of an axial section 16 ([Fig.4]) of the preform tube 15 cut in a radial direction in the preform tube 15, for example along the dashed lines ([Fig.4]), such that each axial section 16 of the preform tube 15 has the same axial width / length as a cage 3 to be obtained.

[0040] Before or after the cutting step, but generally after the hardening step, a plurality of radial holes configured to form pockets or housings 8 are drilled through each axial section 16 of the preforming tube 15 (a well-known process step in the technique, not shown for reasons of simplicity).

[0041] Alternatively, the pockets or housings 8 can be obtained, in a known manner, during the winding step as shown in [Fig.3], by correctly arranging the axial position of the fibers 11b and by equipping the mandrel 14 with a plurality of radially protruding pins (not shown) configured to form, each, a hole corresponding to a pocket or housing 8 in the preforming tube 15 directly during its formation.

[0042] Consequently, as shown in [Fig.4], each hardened section 16 comes to constitute, after the cutting step, an annular body 7 ([Fig.2]).

[0043] Each annular body 7, therefore, comprises a plurality of layers superimposed 18 of reinforcing fibers 11 embedded in a synthetic plastic material 13 and arranged with respect to the axis of symmetry A / Al according to a predefined pattern.

[0044] The preforming tube 15 could be obtained either from a polymerized fiber-reinforced thermosetting resin or from a polymerized thermoplastic resin. In the latter case, the hardening step of the preforming tube 15 would no longer be absolutely necessary, since the thermoplastic powder for impregnating / coating the fibers could be at least partially polymerized directly on the mandrel 14.

[0045] According to a first main feature of the invention, and with reference to Figures 3 and 4, the preform forming tube 15 is obtained, preferably by the CFW process shown in [Fig.3], but other known fiber placement processes can be used, with an outside diameter of these substantially identical, or very similar, to the design outside diameter of the cage 3 to be obtained, then a commercially available shrink band 21 is tightly wound under tension over the entire outer lateral surface of the radially outermost layer 18 (indicated by 18b in Figures 3 and 4) of the preform forming tube, before carrying out the curing step.

[0046] Here and hereafter, the expression "tightly wound" means a winding without leaving any radial play and carried out in such a way as to expel outwards any air possibly trapped between the outermost radial layer 18 of the preform forming tube 15 and the shrink band 21.

[0047] Accordingly, the present invention consists of a method for producing a roller bearing cage made of composite material 3 comprising an annular body 7 and a plurality of pockets or housings 8, each configured to house, during use, a respective bearing body 6 of a roller bearing 2, the annular body 7 having an axis of symmetry A and a predefined axial width and the pockets or housings 8 being located radially throughout the annular body 7, through internal and external cylindrical lateral surfaces 9, 10 respectively of the annular body 7 radially delimiting it; the method comprising the steps of:

[0048] a) production of a preform forming tube 15 made of a synthetic plastic material 13 reinforced by fibers 11 by arrangement on a mandrel 14 having an axis of symmetry Al coinciding with the axis of symmetry A of the bearing cage 3 to be obtained, and around this mandrel, of a plurality of layers 18 ([Fig.4]) of reinforcing fibers 11b impregnated with / embedded in a synthetic resin material 13;

[0049] b) hardening of the preform forming tube 15 to completely polymerize the synthetic resin material 13 in order to form a synthetic plastic matrix in which the reinforcing fibers 11 are embedded according to a predefined pattern;

[0050] c) radial cutting, in the hardened preform tube 15, of a plurality of axial sections 16 thereof, each having an axial width identical to that of the bearing cage 3 to be obtained, each said axial section 16 of the preform tube 15 having a plurality of pockets or housings 8 situated through it and configured to house, when used, the bearing bodies of a bearing; the pockets or housings 8 being obtained in step a) or being drilled in the preform tube 15 after step b).

[0051] According to the main feature of the invention, the method further comprises the step of winding without radial backlash and under tension a shrink band 21 around the preform forming tube 15, on a radially outermost layer 18b of it.

[0052] In combination with this last step, step b) of hardening the preform forming tube 15 is always carried out without removing the preform forming tube 15 from the mandrel 14 and by inserting the latter and the mandrel 14 into a furnace 20 of known type, the preform forming tube 15 being wrapped by the shrink band 21.

[0053] Next, the assembly formed by the mandrel 14, the preform forming tube 15 and the shrink band 21 is heated to a polymerization temperature of the synthetic plastic material 13 ([Fig.4]).

[0054] Then, once the polymerization of the synthetic plastic material 13 is complete, the preform forming tube 15 is cut in the radial direction to separate its axial sections 16 from each other, before or after obtaining the pockets or housings 8 through it, a ring of these through each section 16, so that the only machining to which the preform forming tube 15 is subjected in order to obtain the desired bearing cages 3 is the drilling operation to obtain the pockets or housings 8 and the cutting operation to separate the sections 16, each of them coming to constitute a bearing cage 3.

[0055] In embodiments of the process of the invention, the reinforcing fibers 11 are synthetic fibers in a material of high tensile strength and rigid and are impregnated by / embedded in a synthetic plastic material 13 having a glass transition temperature of at least 90 °C and preferably of 120 °C.

[0056] In preferred embodiments of the process of the invention, the synthetic resin material 13 is an epoxy resin.

[0057] In embodiments of the process of the invention, the reinforcing fibers 11 are selected from the group consisting of: carbon fibers, glass fibers, Kevlar® fibers, any synthetic fiber similar to these in terms of tensile strength and stiffness.

[0058] In some embodiments, the reinforcing fibers may consist of mineral fibers such as basalt and quartz fibers and also of ceramic fibers, such as A12O3 or SiC fibers, and even of metallic fibers such as steel or aluminum fibers.

[0059] In some embodiments, the reinforcing fibers may consist of other organic fibers such as cotton, cellulose, flax, jute, hemp and sisal fibers.

[0060] In preferred embodiments of the process of the invention, the preform forming tube 15 is obtained by a continuous filament winding technique, by winding on the mandrel 14 at least one continuous reinforcing fiber 11 impregnated with the synthetic resin material 13, the synthetic resin material 13 having a glass transition temperature, after hardening, of at least 90 °C and preferably of 120 °C.

[0061] In embodiments of the method of the invention, step a) is carried out until the outermost radial layer 18b (Figures 3 and 4) of the preform forming tube 15 which is arranged around the mandrel 14 reaches an outside diameter substantially identical to the design outside diameter of the bearing cage 3 to be obtained.

[0062] In different embodiments of the method of the invention, step a) is carried out until the radially outermost layer 18b of the preform tube 15 arranged around the mandrel 14 reaches an outside diameter substantially identical to the design outside diameter of the bearing cage 3 to be obtained, less the thickness of the shrink band 21 wound around it. In this case, the shrink band 21 wound around it cannot be removed after completion of step b).

[0063] In embodiments of the process of the invention, the shrink band 21 is sensitive to heat.

[0064] The shrink band 21 preferably consists of endless polyester silk, optionally colored, wound with solid edges and has a thickness of between 0.15 and 0.22 mm, a tear force of between 230 and 900 N and a shrinkage rate in hot air at 160 °C of at least 9%.

[0065] For example, according to embodiments of the invention, a commercially available shrink band 21 marketed by SinFlex® can be used.

[0066] In view of what has been described, it is evident that the present invention extends to a bearing cage made of composite material 3 comprising an annular body 7 and a plurality of pockets or housings 8, each configured to freely accommodate, during use, a respective bearing body 6 of a bearing 2, the annular body 7 having an axis of symmetry A and a predefined axial width, and the pockets or housings 8 being located radially throughout the annular body, through respective internal and external cylindrical lateral surfaces 9, 10 of the annular body 7 radially delimiting it, the annular body 7 being made of a fiber-reinforced synthetic plastic material comprising a plurality of superimposed layers 18 of reinforcing fibers 11 embedded in a synthetic resin material 13 and arranged with respect to said axis of symmetry A according to a predefined pattern; wherein:

[0067] i)- the consolidated synthetic plastic material 13 (after hardening) has a glass transition temperature greater than or equal to 90 °C and preferably greater than or equal to 120 °C;

[0068] ii)- the synthetic plastic material 13 preferably consists of an epoxy resin;

[0069] iii)- the reinforcing fibers 11 are selected from the group consisting of: carbon fibers, glass fibers, Kevlar® fibers, mineral fibers such as basalt and quartz fibers, ceramic fibers, for example A12O3 or SiC fibers, metallic fibers, for example steel or aluminum fibers, organic fibers including cotton, cellulose, flax, jute, hemp and sisal fibers, any synthetic, organic or inorganic fiber similar to these in terms of tensile strength and stiffness;

[0070] iv)- the reinforcing fibers 11 are continuous fibers impregnated with the synthetic resin material 13 and wound around said axis of symmetry A according to predefined winding angles, in order to form the superimposed layers 18, the continuous fibers of each layer forming with the axis of symmetry A, in a plan view, an angle corresponding to the winding angle with respect to it;

[0071] v)- the bearing cage 3 having been obtained by the process as disclosed here above, in such a way that the inner and outer cylindrical side surfaces 9, 10 of it are not machined and have a smooth finish.

[0072] According to one aspect of the invention, the bearing unit 1 of [Fig. 1] therefore comprises a bearing, for example bearing 2 or any other type of bearing having a plurality of bearing bodies 6 arranged in a defined radial space between the inner ring 4 and the outer ring 5 to make them relatively rotatable with low friction, and a bearing cage 3 as described above for retaining the bearing bodies 6 in a spaced manner. The bearing 2 is preferably of the high-precision bearing type, characterized by high-speed and / or high-load operation.

[0073] Based on this invention, additional grinding or turning processes for filament-wound epoxy / carbon tubes to obtain the required inner and outer cage diameters within the required tolerances can be eliminated or ultimately reduced to a minimum, depending on the circumstances. The number of layers, the type of tape material used, and the layer orientation offer sufficient flexibility to achieve any diameter with high accuracy.

[0074] It is not necessary, moreover, to produce semi-finished preform tubes of greater thickness compared to the final cage thickness.

[0075] Reducing the amount of epoxy / carbon strip material required leads to a reduction in waste, a reduction in costs and a reduction in carbon footprint.

[0076] A lower thickness of tube forming preform required leads to a reduction in the tube curing time in the furnace and to a reduction in energy and therefore in costs.

[0077] Finally, an overall reduction in the total duration of the cage production cycle and the total cost of manufacturing the cage is achieved.

[0078] All the objectives of the invention are therefore achieved.

Claims

1. Demands A method for producing a roller bearing cage made of composite material (3) comprising an annular body (7) and a plurality of pockets or housings (8), each configured to house, during use, a respective bearing body (6) of a roller bearing (2), the annular body (7) having an axis of symmetry (A) and a predefined axial width, and the pockets or housings (8) being located radially throughout the annular body, through respective internal and external cylindrical lateral surfaces (9, 10) of the annular body radially delimiting it; the method comprising the steps of: a) production of a preform forming tube (15) made of a synthetic resin material (13) reinforced by fibers (11) by arranging on a mandrel (14) having an axis of symmetry (Al) coinciding with the axis of symmetry (A) of the bearing cage (3) to be obtained, and around this mandrel, a plurality of layers (18) of reinforcing fibers (11b) impregnated with / embedded in said synthetic resin material (13); b) hardening of the tube forming preform (15) to polymerize the synthetic resin (13) in order to form a synthetic plastic matrix in which the reinforcing fibers (11) are embedded according to a predefined pattern; (c) radial cutting, in the preform tube (15), of a plurality of axial sections (16) thereof, each having an axial width identical to that of the bearing cage (3) to be obtained, each said axial section (16) of the preform tube (15) having a plurality of pockets or recesses (8) situated through it and configured to accommodate, when in use, the bearing bodies of a roller bearing; the pockets or recesses (8) being obtained in step (a) or being drilled in the preform tube (15) after step (b); characterized in that: (d) the method further comprises the step of radially backlash-free and tension-controlled winding of a shrink strip (21) around the preform tube (15), on a radially outermost layer (18b) thereof; and e) step b) of hardening the preforming tube (15) being carried out without removing the preforming tube (15) from the mandrel (14) and by inserting the preform forming tube (15) wrapped by the shrink band into a furnace (20) and, together with the mandrel (14), heating the assembly formed by the mandrel, the preform forming tube and the shrink band to a polymerization temperature of the synthetic resin.

2. A method according to claim 1, characterized in that said reinforcing fibers (11) are synthetic fibers in a material of high tensile strength and rigid and are impregnated by / embedded in a synthetic resin material (13) having a glass transition temperature of at least 90 °C, preferably 120 °C.

3. Method according to claim 2, characterized in that said synthetic resin material (13) is an epoxy resin or a thermoplastic powder.

4. A method according to any one of the preceding claims, characterized in that the reinforcing fibers (11) are selected from the group consisting of: carbon fibers, glass fibers, Kevlar® fibers, mineral fibers, preferably basalt and quartz fibers, ceramic fibers, for example A12O3 or SiC fibers, metallic fibers, for example steel or aluminum fibers, organic fibers including cotton, cellulose, flax, jute, hemp and sisal fibers, any synthetic, organic or inorganic fiber similar to these in terms of tensile strength and stiffness.

5. A method according to any one of the preceding claims, characterized in that the preform forming tube (15) is obtained by a continuous filament winding technique, by winding on said mandrel (14) at least one continuous reinforcing fiber (11) impregnated with said synthetic resin material (13), the synthetic resin material (13) having a glass transition temperature, after hardening, of at least 90 °C and preferably of 120 °C.

6. A method according to any one of the preceding claims, characterized in that said step a) is carried out until the radially outermost layer (18b) of said preform tube (15) arranged around said mandrel (14) reaches an outside diameter substantially identical to the outside diameter of design of the bearing cage to be obtained, said retractable strip (21) being wound on it.

7. A method according to any one of the preceding claims 1 to 5, characterized in that said step a) is carried out until the radially outermost layer (18b) of said tube forming preform (15) arranged around said mandrel (14) reaches an outside diameter substantially identical to the design outside diameter of the bearing cage (3) to be obtained less the thickness of the shrink band wound around it; said shrink band (21) wound around it not being removed after completion of step b).

8. A method according to any one of the preceding claims, characterized in that the heat-sensitive shrink band (21) preferably consists of endless polyester silk, optionally colored, wound with solid edges and has a thickness of between 0.15 and 0.22 mm, a tear force of between 230 and 900 N and a shrinkage rate in hot air at 160 °C of at least 9%.

9. A composite material bearing cage (3) comprising an annular body (7) and a plurality of pockets or housings (8), each configured to freely house, during use, a respective bearing body (6) of a bearing (2), the annular body (7) having an axis of symmetry (A) and a predefined axial width and the pockets or housings (8) being located radially throughout the annular body, through respective inner and outer cylindrical lateral surfaces (9, 10) of the annular body radially delimiting it, the annular body (7) being made of a fiber-reinforced synthetic plastic material comprising a plurality of superimposed layers (18) of reinforcing fibers (11) embedded in a synthetic plastic material (13) and arranged with respect to said axis of symmetry (A) according to a predefined pattern;characterized in that i) the synthetic plastic material (13) has a glass transition temperature greater than or equal to 90 °C, preferably 120 °C; ii) the synthetic plastic material (13) preferably consists of an epoxy resin; iü)- the reinforcing fibres (11) are selected from the group consisting of: carbon fibres, glass fibres, Kevlar® fibres, mineral fibres such as basalt and quartz fibres, ceramic fibres, for example A12O3 or SiC fibres, metallic fibres, for example steel or aluminium fibres, organic fibres including cotton, cellulose, flax, jute, hemp and sisal fibres, any synthetic, organic or inorganic fibres similar to these in terms of tensile strength and stiffness;(iv) said reinforcing fibers (11) are continuous fibers impregnated with said synthetic resin (13) and wound around said axis of symmetry (A) at predefined winding angles, to form said superimposed layers (18), the continuous fibers (11) of each layer forming with the axis of symmetry (A), in a plan view, an angle corresponding to the winding angle with respect to it; (v) said bearing cage (3) having been obtained by the process of claim 5 in such a way that said internal and external cylindrical lateral surfaces (9, 10) thereof are substantially not machined and have a smooth finish.

10. A bearing unit (1) comprising a bearing (2) comprising an outer ring (5), an inner ring (4) and a plurality of bearing bodies (6) arranged in a radial space defined between the inner ring and the outer ring to make them relatively rotatable with low friction, and a bearing cage (3) for retaining the bearing bodies in a spaced manner by means thereof, characterized in that the bearing cage (3) has the characteristics of claim 9.

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