Manufacturing method of a composite material rolling bearing cage, rolling bearing cage and associated rolling bearing unit

Abstract

Manufacture method of a composite fibre-reinforced rolling bearing cage by winding fibres around a mandrel 14 into multiple layers 18 (e.g. continuous filament winding) of reinforcing fibres impregnat

Description

Technical Sector of the Invention The present invention relates to a manufacturing method of a rolling bearing cage obtained in a fiber reinforced composite synthetic plastic material, as well as to a rolling bearing cage manufactured according to the method and to an associated rolling bearing unit, including such a cage. The invention relates, in particular, to an improved manufacturing method that allow a fiber reinforced composite synthetic plastic material cage to be obtained with high dimensional precision, so as to limit to a minimum the machining necessary to obtain the final cage. Background of the Invention As it is well known, 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 in position the rolling bodies, the cage being arranged in the radial space delimited between the inner ring and the outer ring. A rolling bearing retaining cage comprises an annular body delimited between an inner and an outer lateral cylindrical surface and a plurality of pockets or seats, each configured to house and retain in a freely rotatable manner a respective rolling body of the rolling 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 carries the pockets or seats, which are provided radially therethrough, e.g. consisting in passing-through radial holes. A preferred manufacturing method consists in obtaining a preform in the shape of a hollow tube, e.g., by molding the synthetic material, then the hollow tube is cut radially in a plurality of slices, each one constituting a cage body. Before or after the cutting operation the pockets or seats are drilled through the cage body. To improve the operative performance of the cage, is also known to obtain the cage body in a fiber-reinforced synthetic material, e.g. phenolic resins reinforced with cotton fibers embedded in the synthetic material matrix, or any other suitable composite material. In these cases, the hollow tube constituting the preform may be produced by any known process of fiber placement or by a process known as "continuous filament winding" (CFW), by tightly winding on a metal mandrel tool one or more filaments of composite material consisting in continuous fibers impregnated with a synthetic plastic resin. Here and in the following, for "plastic resin" it is to be understood either a thermoset or thermoplastic synthetic material, e.g., impregnation of fibers can either be made by a liquid thermoset resin or by a solid thermoplastic powder. After a prefixed number of superimposed radial layers of preimpregnated fibers are obtained, the preform is cured in a known manner to cause the consolidation of the synthetic material impregnating the fibers in a solid matrix, in which the winded fibers remain embedded to constitute a reinforcing material. Curing may occur as disclosed, e.g., in FR3053624A1. Other methods of fiber placement may be used to obtain the preform tube, e.g., by superimposing to each other layers of pre-impregnated fiber (pre-peg) in form of sheets or mats having the pre-impregnated reinforcing fibers thereof neatly arranged according to prefixed patterns. In any case, irrespective the obtention method thereof, but anyway more frequently when CFW manufacturing methods are employed, fiber reinforced plastic cages may require an intense machining following the obtention of the preform tube, to bring the dimension of the cage withing the prescribed working tolerances. In particular, when a CFW method is used to obtain the preform tube, the outer and inner diameter of the preform tube, or those of the resulting cages radially cut therefrom, are to be subjected to a turning or grinding process. In fact, composite tubes produced by conventional standard filament winding or fiber placement processes show wavy undulated tube surfaces, which are not able to directly fulfil the narrow tolerances for the bearing cages inner and outer diameter. Consequently, the semi-finished tubes, namely the preform tubes, are produced with a thickness greater than the nominal one (i.e. that one assigned in the design step) and additional turning and / or grinding processes are needed to achieve the correct bearing cage inner and outer diameter within requested tolerances. Accordingly, there is the need in the art to obtain fiber reinforced preform tubes with smooth inner and outer lateral surfaces and having already the correct inner and outer diameters designed for the final rolling bearing cage. In fact, the need of machining the preform tube (or the cages preform obtained therefrom) increases the production costs, not only due to the machining operation per se, but also, and above all, for the greater use of valuable raw materials involved, which may be expensive, and for an higher energy consumption in the curing step, due to the larger quantity of material to be treated. Moreover, repeated machining may introduce errors that may bring to scraps. Summary of the invention The aim of the present invention is to overcome the drawbacks of the prior art by providing a manufacturing method of a composite material rolling bearing cage made of a fiber reinforced synthetic plastic material, substantially free from the necessity of machining the outer and inner diameter of the preform tube or / and of the cage, in order to reach the design diameters and staying within the required working tolerances. It is also an aim of the invention to provide a manufacturing method which needs lower quantities of raw materials and brings to a reduced energy consumption. It is finally an aim of the invention to provide a composite material rolling bearing cage made of a fiber reinforced synthetic plastic material having reduced production costs and a high precision rolling bearing unit equipped with such a cage, at the same time maintaining good performances in use, especially in particularly stressful applications, like those requiring high rotation speeds and / or subjected to high loads. According to the invention, there are provided a manufacturing method of a composite material rolling bearing cage, and an associated rolling bearing cage and a rolling bearing unit provided with such a cage, as defined in the appended claims. The present disclosure allows composite preform tubes configured for production of rolling bearing cages by means of any of the methods known in the art and having a correct dimension of the final inner and outer diameter of the cage to be obtained already during the fiber placement or filament winding processes. In embodiments of the invention, a defined number of layers of composite material are wound with controlled tension onto a smooth metal mandrel / tooll having its outer diameter 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 cage outer diameter is achieved at the top of the stack of superimposed fiber layers impregnated with a suitable polymer resin, wound on the mandrel to form a tube. In a second step, a commercial shrink tape is placed or wound under controlled tension onto the outer surface of such tube formed by the stacked layers of polymer impregnated fibers, which fibers have been oriented in the preceding step in each layer according to a prefixed pattern. In a third step, the metallic mandrel / tooll, still carrying the tube of superimposed layers of polymer impregnated fibers wound thereupon, is placed, together with the layers of impregnated fibers and the shrink tape wound around the radially outermost layer of impregnated fibers into an oven and cured under defined temperature and time, e.g., according to FR3053624A1. During the curing step, the shrinkage of the shrink tape due to the increase of temperature leads to a defined smoothening and compression / compaction of the composite preform tube made of superimposed layers of polymer impregnated fibers allowing to obtain, after curing and after having extracted the metallic mandrel / mandrel from the cured preform tube, and after cutting in radial direction the cured preform tube freed from the mandrel in slices, a number of cages, all having the required and the same cage outer diameter established in the design stage. Consequently, any additional grinding or turning processes of the composite cured preform tube to achieve the required inner and outer bearing cage diameters within requested tolerances can be eliminated or at least strongly reduced, which leads to material and manufacturing time reduction as well as carbon footprint and cost savings. Brief Description of the Drawings Further characteristics and advantages of the present invention will become clear from the following description of non-limiting examples thereof, carried out with reference to the figures of the attached drawings, in which: - Figure 1 schematically shows a rolling bearing unit provided with a rolling bodies retaining cage made according to the invention; - Figure 2 schematically shows in enlarged scale a rolling bodies retaining cage for a rolling bearing provided according to the present invention; - Figure 3 schematically shows a preferred embodiment of some steps of the method of the invention, by which a preform tube for obtaining the retaining cage of figure 2 may be produced; and - Figure 4 schematically shows a detailed perspective view of the preform tube of figure 3 where some layers of composite material have been removed, for a better comprehension, and of further steps of the method of the invention after having carried out the steps shown in figure 3. Detailed description With reference to Figures from 1 to 4, the reference number 1 indicates a rolling bearing unit (figure 1) comprising a rolling bearing 2 of any known type and a rolling bearing cage 3, made of a composite material. The rolling bearing comprises an inner ring 4, an outer ring 5 and a plurality of rolling elements or bodies 6, in the non-limiting embodiment shown consisting of balls. The rolling bodies 6 are arranged, in the example shown, in one crown of balls around an axis of symmetry of the rolling bearing, which is also the axis of symmetry A of cage 3 (figure 2). In different embodiments, not shown for sake of simplicity, the rolling bearing 2 may comprise two crown of rolling bodies arranged side by side and the rolling bodies may be indifferently, balls, cylindrical or conical rolls, small cylinders, according to the operation necessity. In any case, the rolling bearing cage 3 (figure 2) comprises an annular body 7 and a plurality of pockets or seats 8, each configured to freely house in use a respective rolling body 6 of the rolling bearing 2 to correctly keep the rolling bodies 6 spaced apart to each other by a prefixed pitch. The annular body 7 has an axis of symmetry A and a prefixed axial width or length. The pockets or seats 8 are provided radially throughout the annular body 7, through respective inner and outer cylindrical lateral surfaces 9 and 10 (figure 2) of the annular body 7, substantially perpendicularly thereto and, in the example shown, consist in simple cylindrical radial holes. The lateral surfaces 9 and 10 radially delimit the annular body 7 therebetween. The annular body 7 is made of a fiber-reinforced synthetic plastic material and in a preferred embodiments of the present invention may be obtained by a method known in the art as CFW (continuous filament winding), schematically shown in a non-limitative manner in figure 3, merely for illustrative purposes and for a better understanding of the invention. With reference to figure 3, in a CFW production method a plurality of reinforcing fibers 11 are unwound in known manner from spools 12, are impregnated in known manner with a synthetic resin / material, 13 and then the impregnated reinforcing fibers lib are wound around a mandrel tool or mandrel 14 with a prefixed inclination with respect to the axis of symmetry Al of mandrel 14, up to obtain a preform tube 15 (figures 3 and 4). In alternative, pre-peg (pre-impregnated) fibers or sheets of neatly ordered fibers may be used to obtain the preform tube 15, e.g., according to any fiber placement method known in the art, in the end still obtaining a preform tube 15, in this case composed by a number of sheet or mats of polymer impregnated fibers, strictly wound onto one another and around the mandrel or mandrel tool 14, in each sheet or mat the polymer impregnated fibers being neatly arranged according to a prefixed pattern. In any case, the final preform tube 15 (figure 4) is composed by a plurality of layers 18 of polymer impregnated fibers lib stacked onto each other and strictly (i.e., without any radial play) wound around the mandrel / mandrel tool 14. The axis of symmetry Al of mandrel 14 coincides with the axis of symmetry A of the cages 3 to be obtained and, in case of a CFW process, to the axis of winding of the fibers 11 around the mandrel 14. To obtain a plurality of annular bodies 7 from a single preform tube 15, the latter is cured in any known and suitable manner (e.g. according to FR3053624A1) within any known and suitable oven 20 (figure 4), in order to polymerize the synthetic plastic material or resin 13 to form a solid matrix, and is thereafter cut (in a manner known in the art and not shown for sake of simplicity) radially in slices constituted each by an axial stretch 16 (figure 4) of the preform tube 15 cut away in a radial direction from the preform tube 15, e.g., along the dotted lines (figure 4), such as each axial stretch 16 of the preform tube 15 has the same axial width / length of a cage 3 to be obtained. Before or after the cutting step, but generally after the curing step, a plurality of radial holes configured to constitute the pockets or seats 8 are drilled through each axial stretch 16 of the preform tube 15 (method step well known in the art and not shown for sake of simplicity). In alternative, the pockets or seats 8 may be obtained, still in known manner, during the winding step as shown in figure 3, by properly arranging the axial position of the fibers lib and by providing the mandrel 14 with a plurality of radially outstanding pins (not shown) configured to form, each one, a hole corresponding to a pocket or seat 8 in the preform tube 15, directly during its obtention. Accordingly, as shown in figure 4, each cured stretch 16 comes to constitute, after the cutting step, an annular body 7 (figure 2). Each annular body 7, therefore, comprises a plurality of superimposed layers 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 prefixed pattern. The preform tube 15 could be obtained either in a polymerized fiber reinforced thermoset resin or in a polymerized thermoplastic resin. In this latter case, the curing step of the preform tube 15 would be no longer strictly necessary, since the thermoplastic powder for impregnating / embedding the fibers could be at least partially polymerized directly on the mandrel 14. According to a first and main feature of the invention, and with reference to figures 3 and 4, the preform tube 15 is obtained, preferably by the CFW method shown in figure 3, but other known methods of fiber placement may be used, with an outer diameter thereof substantially identical, or very close, to the design outer diameter of the cage 3 to be obtained and then a commercial shrink tape 21 is strictly wound under tension upon the complete outer lateral surface of the radially outermost layer 18 (indicated as 18b in figures 3 and 4) of the preform tube, before the carrying out of the curing step. Here and herein below, the expression "strictly wound" means a winding without leaving any radial play and carried out such as to expel outside any air possibly trapped between the radially outermost layer 18 of the preform tube 15 and the shrink tape 21. Accordingly, the present invention consists of a method for producing a composite material rolling bearing cage 3 comprising an annular body 7 and a plurality of pockets or seats 8 each configured to house in use a respective rolling body 6 of a rolling bearing 2, the annular body 7 having an axis of symmetry A and a prefixed axial width and the pockets or seats 8 being provided radially throughout the annular body 7, through respective inner and outer cylindrical lateral surfaces 9, 10 of the annular body 7 radially delimiting the same; the method comprising the steps of: a) producing a preform tube 15 made of a synthetic plastic material 13 reinforced with fibers 11 by arranging onto and around a mandrel 14 having an axis of symmetry Al coinciding with the axis of symmetry A of the rolling bearing cage 3 to be obtained, a plurality of layers 18 (figure 4) of reinforcing fibers lib impregnated with / embedded in a synthetic resin material 13; b) curing the preform tube 15 in order to completely polymerize the synthetic resin material 13 to form a synthetic plastic matrix in which the reinforcing fibers 11 are embedded according to a prefixed pattern; c) radially cutting from the cured preform tube 15 a plurality of axial stretches 16 thereof, each having an axial width identical to that of the rolling bearing cage 3 to be obtained, each said axial stretch 16 of the preform tube 15 having a plurality of pockets or seats 8 provided therethrough and configured to house in use rolling bodies of a rolling bearing; the pocket or seats 8 are obtained during step a) or are drilled in the preform tube 15 after step b); According to the main feature of the invention, the method further comprises the step of winding without radial play and under tension a shrink tape 21 around the preform tube 15, upon a radially outermost layer 18b thereof. In combination with this latter step, the step b) of curing the preform tube 15 is always carried out and without removing the preform tube 15 from the mandrel 14 and by inserting it and the mandrel 14 into an oven 20 of known type, the preform tube 15 being wound by the shrink tape 21. Thereafter, the assembly formed by the mandrel 14, the preform tube 15 and the shrink tape 21 is heated to a polymerization temperature of the synthetic plastic material 13 (figure 4). Thereafter, upon completion of the polymerization of the synthetic plastic material 13, the preform tube 15 is cut in radial direction to separate its axial stretches 16 from each other, before or after having obtained therethrough the pockets or seats 8, one crown of them through each stretch 16, so as the only machining to which the preform tube 15 is subjected in order to obtain the desired rolling bearing cages 3 is the drilling operation to obtain the pockets or seats 8 and the cutting operation to separate the stretches 16, each one of them comeing to constitute a rolling bearing cage 3. In embodiments of the method of the invention, the reinforcing fibers 11 are synthetic fibers of a high tensile strength and stiff material 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. In preferred embodiments of the method of the invention, the synthetic resin material 13 is an epoxy resin. In embodiments of the method of the invention, the reinforcing fibers 11 are selected in the group consisting of: carbon fibers, glass fibers, Kevlar® fibers, any synthetic fiber similar thereto in tensile strength and stiffnesss. In some embodiments, the reinforcing fibers may consist in mineral fibers like basalt and quartz fibers and also in ceramic fibers, like AI2O3 or SiC fibers and even in metal fibers like steel or aluminum fibers. In some embodiments, the reinforcing fibers may consist in other organic fibers like cotton, cellulose, flax, jute, hemp and sisal fibers. In preferred embodiments of the method of the invention, the preform 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 curing, of at least 90°C and preferably 120°C. In embodiments of the method of the invention, the step a) is carried out until the radially outermost layer 18b (figures 3, 4) of the preform tube 15 which is arranged around the mandrel 14 reaches an outer diameter substantially identical to the design outer diameter of the rolling bearing cage 3 to be obtained. In different embodiments of the method of the invention, the step a) is carried out until the radially outermost layer 18b of the preform tube 15 arranged around the mandrel 14 reaches an outer diameter substantially identical to the design outer diameter of the rolling bearing cage 3 to be obtained unless the thickness of the shrink tape 21 wound therearound. In this case, the shrink tape 21 wound therearound may not be removed after competition of step b). In embodiments of the method of the invention, the shrink tape 21 is sensitive to heat. The shrink tape 21 consists preferably in an endless, possibly colored polyester silk wound with solid edges and has a thickness of between 0.15 and 0.22 mm, a tear force between 230 and 900 N and a shrinkage rate in hot air at 160°C of at least 9%. For example, according to embodiments of the invention, a commercial shrink tape 21 marketed by company SinFlex® may be used. From what described, it is evident that the present invention extends to composite material rolling bearing cage 3 comprising an annular body 7 and a plurality of pockets or seats 8 each configured to freely house in use a respective rolling body 6 of a rolling bearing 2, the annular body 7 having an axis of symmetry A and a prefixed axial width and the pockets or seats 8 being provided radially throughout the annular body, through respective inner and outer cylindrical lateral surfaces 9,10 of the annular body 7 radially delimiting the same, 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 arranger with respect to said axis of symmetry A according to a prefixed pattern; wherein: i)- the consolidated (after curing) synthetic plastic material 13 has a glass transition temperature equal to or greater than 90°C and preferably equal to or greater than 120°C; ii)-the synthetic plastic material 13 consists preferably in an epoxy resin; ill)- the reinforcing fibers 11 are chosen in the group consisting of: carbon fibers, glass fibers, Kevlar® fibers, mineral fibers like basalt and quartz fibers, ceramic fibers, e.g., AI2O3 or SiC fibers, metal fibers, e.g., steel or aluminum fibers, organic fibers including cotton, cellulose, flax, jute, hemp and sisal fibers, any synthetic, organic or inorganic fiber similar thereto in tensile strength and stiffness; iv)- the reinforcing fibers 11 are continuous fibers impregnated in the synthetic resin material 13 and wound around said axis of symmetry A according to prefixed winding angles, 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 thereto; v)- the rolling bearing cage 3 having been obtained by the method as disclosed herein above, in such a manner that the inner and outer cylindrical lateral surfaces 9,10 thereof are unmachined and having a smooth finishing. According to one aspect of the invention, the rolling bearing unit 1 in figure 1 comprises therefore a rolling bearing, e.g., the rolling bearing 2 or any other model of rolling bearing having a plurality of rolling bodies 6 arranged in a radial space delimited between the inner ring 4 and the outer ring 5 to render them relatively rotatable with low friction, and a rolling bearing cage 3 as described above for retaining the rolling bodies 6 spaced apart. The rolling bearing 2 is preferably of the high precision bearing type, characterized by high speed and / or high load of operation. Based on this invention, additional grinding or turning processes of 5 filament wound epoxy / carbon tubes to achieve required inner and outer cage diameters within requested tolerances can be eliminated or eventually reduced to a minimum, depending on the cases. The number of layers, the type of tape material used, and the orientation of the layer give enough flexibility to reach any diameter with high precision. 10 There is no further need to produce semi-finished preform tubes with higher thickness compared to final cage thickness. Reduction of needed epoxy / carbon tape material leads to waste reduction, cost reduction and carbon footprint reduction. Lower required preform tube thickness leads to reduction of tube 15 hardening oven time and energy and therefore cost reduction. Finally, an overall reduction of total cage production cycle time and total cage manufacturing cost is obtained. All the aims of the invention are therefore achieved.

Claims

1. A method for producing a composite material rolling bearing cage (3) comprising an annular body (7) and a plurality of pockets or seats (8) each configured to house in use a respective rolling body (6) of a rolling bearing (2), the annular body (7) having an axis of symmetry (A) and a prefixed axial width and the pockets or seats (8) being provided radially throughout the annular body, through respective inner and outer cylindrical lateral surfaces (9,10) of the annular body radially delimiting the same; the method comprising the steps of:a) producing a preform tube (15) made of a synthetic resin material (13) reinforced with fibers (11) by arranging onto and around a mandrel (14) having an axis of symmetry (Al) coinciding with the axis of symmetry (A) of the rolling bearing cage (3) to be obtained, a plurality of layers (18) of reinforcing fibers (lib) impregnated with / embedded in said synthetic resin material (13);b) curing the preform tube (15) in order to polymerize the synthetic resin (13) to form a synthetic plastic matrix in which the reinforcing fibers (11) are embedded according to a prefixed pattern;c) radially cutting from the preform tube (15) a plurality of axial stretches (16) thereof, each having an axial width identical to that of the rolling bearing cage (3) to be obtained, each said axial stretch (16) of the preform tube (15) having a plurality of pockets or seats (8) provided therethrough and configured to house in use rolling bodies of a rolling bearing; the pocket or seats (8) being obtained during step a) or being drilled in the preform tube (15) after step b); characterized in that:d) the method further comprises the step of winding without radial play and under tension a shrink tape (21) around the preform tube (15), upon a radially outermost layer (18b) thereof; ande) the step b) of curing the preform tube (15) being carried out without removing the preform tube (15) from the mandrel (14) and by inserting into an oven (20) the preform tube (15) wound by the shrink tape and together with the mandrel (14), heating the assembly formed by the mandrel, the preform tube and the shrink tape to a polymerization temperature of the synthetic resin.

2. The method according to claim 1, characterized in that said reinforcing fibers (11) are synthetic fiber of a high tensile strength and stiff material 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. The method according to claim 2, characterized in that said synthetic resin material (13) is an epoxy resin or a thermoplastic powder.

4. The method according to anyone of the preceding claims, characterized in that the reinforcing fibers (11) are selected in the group consisting of: carbon fibers, glass fibers, Kevlar® fibers, mineral fibers preferably basalt and quartz fibers, ceramic fibers, e.g., AI2O3 or SiC fibers, metal fibers, e.g., steel or aluminum fibers, organic fibers including cotton, cellulose, flax, jute, hemp and sisal fibers, any synthetic, organic or inorganic fiber similar thereto in tensile strength and stiffness.

5. The method according to anyone of the preceding claims, characterized in that the preform tube (15) is obtained by a continuousfilament 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 curing, of at least 90°C and preferably of 120°C.

6. The method according to anyone 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 outer diameter substantially identical to the design outer diameter of the rolling bearing cage to be obtained, said shrink tape (21) wound therearound.

7. The method according to anyone of the preceding claims from 1 to 5, 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 outer diameter substantially identical to the design outer diameter of the rolling bearing cage (3) to be obtained unless the thickness of the shrink tape wound therearound; said shrink tape (21) wound therearound not being removed after competition of step b).

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

9. A composite material rolling bearing cage (3) comprising an annular body (7) and a plurality of pockets or seats (8) each configured tofreely house in use a respective rolling body (6) of a rolling bearing (2), the annular body (7) having an axis of symmetry (A) and a prefixed axial width and the pockets or seats (8) being provided radially throughout the annular body, through respective inner and outer cylindrical lateral surfaces (9,10) of the annular body radially delimiting the same, 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 prefixed pattern; characterized in thati)- the synthetic plastic material (13) has a glass transition temperature equal to or greater than 90°C, preferably 120°C;ii)-the synthetic plastic material (13) consists preferably in an epoxy resin;iii)- the reinforcing fibers (11) are chosen in the group consisting of: carbon fibers, glass fibers, Kevlar® fibers, mineral fibers like basalt and quartz fibers, ceramic fibers, e.g., AI2O3 or SiC fibers, metal fibers, e.g., steel or aluminum fibers, organic fibers including cotton, cellulose, flax, jute, hemp and sisal fibers, any synthetic, organic or inorganic fiber similar thereto in tensile strength and stiffness;iv)- said reinforcing fibers (11) are continuous fibers impregnated in said synthetic resin (13) and wound around said axis of symmetry (A) according to prefixed 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 thereto;v)- said rolling bearing cage (3) having been obtained by the method of claim 5 such as said inner and outer cylindrical lateral surfaces thereof (9,10) being substantially unmachined and having a smooth finishing.

10. Rolling bearing unit (1) comprising a rolling bearing (2) in turn5 comprising an outer ring (5), an inner ring (4) and a plurality of rolling bodies (6) arranged in a radial space delimited between the inner ring and the outer ring to render them relatively rotatable with low friction, and a rolling bearing cage (3) for retaining the rolling bodies spaced apart therewith, characterized in that the rolling bearing cage (3) has the 10 features of claim 9.

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