Patent Publication Number: US-2023160426-A1

Title: Bearing unit with reinforced retention cage

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based on and claims priority to Italian Patent Application No. 102021000029609 filed on Nov. 24, 2021, under 35 U.S.C. § 119, the disclosure of which is incorporated by reference herein. 
     FIELD 
     The present disclosure relates generally to bearing units. In particular, the present disclosure relates to bearing units having a retention cage for a row of rolling bodies that is especially suited for a high-speed bearing unit, although not required. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The disclosure will now be described with reference to the attached drawings, which show some non-limiting embodiments thereof, in which: 
         FIG.  1    is a partial cross section of a bearing unit with a cage for containing and retaining one or more rolling bodies according to an exemplary embodiment; 
         FIG.  2    is an axonometric view of a cage for one or more rolling bodies according to an exemplary embodiment of this disclosure; 
         FIG.  3    is a second axonometric view of the cage of  FIG.  2   ; 
         FIG.  4    is a detail view of the cage of  FIG.  2   , illustrating an armature for reinforcing the cage according to an exemplary embodiment of this disclosure; 
         FIG.  5    is an axonometric view of a tenon of reduced mass of the cage of  FIG.  2   , according to an exemplary embodiment of this disclosure; 
         FIG.  6    is a partial cross section of the bearing unit of  FIG.  1   , according to an exemplary embodiment of this disclosure; 
         FIG.  7    is an end-on view of the reinforcing armature of the cage of  FIG.  2   , according to an exemplary embodiment; and 
         FIG.  8    is an axial view of the reinforcing armature of the cage of  FIG.  2   , according to an exemplary embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Known retention cages for one or more rolling bodies, e.g., balls, of a bearing unit according to a first type are formed of a rib with a circular base and a plurality of circumferentially spaced fingers or tenons extending from both sides of the rib. The base rib and the fingers have spherically concave surfaces that define between them a plurality of spherical cavities or recesses for holding a respective rolling body of the one or more rolling bodies. 
     A second known type of retention cage differs from the first type in that the plurality of tenons of the retention cage of the second type extend from just one side of the base rib. 
     The base rib may be a continuous structural element that extends circumferentially along the cage and constitutes a solid base, giving the cage a rigidity needed to hold the rolling bodies along raceways of the bearing unit, by means of the plurality of tenons. 
     These cages are referred to as “snap” cages because the rolling bodies are contained and retained as a result of the elastic behavior of the tenons, which can open and close for insertion and retention of the one or more rolling bodies. 
     In particular, this second type of retention cage is especially used in high-speed bearing units, e.g., Deep Groove Ball Bearing (DGBB) bearing units for applications in the automotive sector (e.g., power units) and industry (e.g., mandrels of machine tools). Specifically, increased use of electrically powered motor vehicles requires ever more efficient performance in terms of, for example, rotational speed of the bearing unit. 
     The main drawback of a “snap” type cage is limitation on speed due to the rigidity of the polymer used to produce the cage. In particular, during use and at high speeds, the structure of the cage undergoes deformation in all directions. For example, the cage expands in the radial direction, risking contact between a radially outer diameter of the cage and a radially outer ring of the bearing unit and contact between a radially inner diameter of the cage and a radially inner ring of the bearing unit. In contrast, the cavities or recesses open in a direction tangential to the radial direction, resulting in an increased diameter of the cavity or recess. The enlarged diameter can reduce the force that is retaining the rolling body, which can result in the rolling body falling out of the cage. 
     Furthermore, cages made of a polymeric material are sensitive to temperature and are therefore less efficient at temperatures above the glass transition temperature. For a polymer such as polyamide (e.g., PA 66), this means a temperature of around 70° C. to 80° C. 
     In bearing units, capacity to reach higher speeds is often measured by a speed index that is obtained by multiplying the number of rotations of the rotating ring of the bearing unit by the mean diameter (pitch diameter) of the bearing unit. This speed index is measured in “mndm”, m standing for million, n representing the number of rotations, and dm representing the mean diameter pf the bearing unit. 
     To counter the increase in deformation of the cage resulting from speed, a first solution is to increase the thickness of the rib. This countermeasure increases the rigidity of the supporting structure of the cage and thus reduces the extent of the deformation resulting from the high speed. A solution of this type is not always feasible, however, since it may require an increase in the axial dimensions of the bearing unit. This can increase cost and make the solution infeasible in light of limited axial space in a layout of assembly for the bearing unit. 
     Another solution is to reduce the mass of the tenons to reduce the centrifugal force acting on the cage. This second solution, however, has a drawback in a reduced rigidity of the cage, resulting in a lower force of retention of the balls. Therefore, this second solution may not be feasible for high-speed applications, at least when used independently of other countermeasures. 
     There is therefore a need to design a cage for retaining rolling bodies (e.g., balls, rollers) of bearing units that do not have the aforementioned disadvantages. 
     An aim of the present disclosure is to provide a retention cage which can be used in bearing units for high-speed applications. As described in greater detail throughout this disclosure, this aim may be achieved with a retention cage of reinforced structure having an armature or the like on which a polymeric material of the rib and of the tenons is over-molded. 
     Purely by way of non-limiting example, exemplary embodiments of a bearing unit according to the present disclosure will now be described. The bearing unit may be described with reference to a Deep Groove Ball Bearing (“DBGG”) bearing unit for high-speed applications. A person or ordinary skill in the art will understand that reference to a DBGG bearing unit is merely illustrative and that alternative types of bearing units may be used without departing from the scope of this disclosure. 
     With reference to  FIG.  1   , a bearing unit  30  having a central axis of rotation X may include a stationary radially outer ring  31 , a rotatable radially inner ring  33 , a row of rolling bodies  32 , e.g., balls, inserted between radially outer ring  31  and radially inner ring  33 , and a cage  40  for holding the rolling bodies of the row of rolling bodies  32  in place. 
     Throughout the present disclosure and in the claims, terms and expressions indicating positions and orientations such as “radial” and “axial” are intended with reference to central axis of rotation X of bearing unit  30 , where not 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.” Furthermore, the numeral  32  may be used to refer to the row of rolling bodies or an individual rolling body in the row of rolling bodies. 
     With reference to  FIGS.  2  and  3   , cage  40  may include a rib  41  and a plurality of circumferentially spaced tenons  42  about a circumference of rib  41  at a constant pitch and extending from one side of rib  41 . In alternative embodiments, tenons  42  may not be evenly spaced around a circumference of rib  41 . 
     In various embodiments, rib  41  and tenons  42  may be made of a polymeric material and include one or more spherical concave surfaces that define between them a plurality of spherical cavities or hollows  43  to hold a respective ball  32  of the plurality of rolling bodies  32 . Each cavity  43  may thus be defined by the spherical concave surfaces of the rib  41  and of the spherical concave surfaces of a pair of tenons  42 . 
     Cage  40  may further include an armature  50  rigidly secured to rib  41 . In various embodiments, rib  41  may be over-molded around armature  50 . 
     An armature  50  according to this disclosure may take the form of an undulated washer having a box-like shape in which portions of a circular crown alternate circumferentially in two different planes and are connected to one another. A plurality of first portions  50   a  of circular crown rests on a first plane Y and a plurality of second portions  50   b  of circular crown rests on a second plane W. The box-like shape creates a very rigid armature, and in any case an armature having a greater rigidity than an armature having a shape of a flat washer, and reduces a bending moment arm of the centrifugal force acting on tenons  42 . 
     Advantageously, in various embodiments, tenons  42  may have at least one shaped and tapered edge  42 ′ such that an overall mass of tenon  42  is streamlined. 
     Tapered edge  42 ′ may have an axial length G that is a function of an axial half-width ORL 11  of radially outer ring  31  and of an axial width ORL 42  of a discharge edge  31 ′ of radially outer ring  31 , according to: 
       0.5&lt; G &lt;( ORL 11− ORL 42)
 
     Tapered edge  42 ′ may extend radially as far as a radially outer surface  42 ″ having a diameter DL that is a function of a mean diameter BCD 50  of bearing unit  30  and of a diameter Dw of the rolling bodies  32  according to the following formula: 
         DL=BCD 50+0.05× Dw  
 
     Cage  40  may thus assume a substantially “L” shape with the web of the “L” including tapered tenons  42 . This arrangement reduces cage  40 &#39;s sensitivity to centrifugal forces. Any resulting reduction of overall strength of cage  40  may be compensated for by the increase in strength resulting from the presence of armature  50 . 
     The reduction in the bending moment arm of the centrifugal force acting on tenons  42  acts in synergy with the substantially “L” shape of cage  40  as a whole which, with respect to known cages, in turn reduces a mass of a tenon  42  subjected to the centrifugal force. Creating rigidity in cage  40  in this way, i.e., by including an armature  50  with a box-like structure, is particularly advantageous because armature  50  is kept inside rib  41  of cage  40  and, therefore, does not enter space between cage  40  and rolling bodies  32 . This maintains a degree of flexibility between cage  40  and rolling bodies  32  so as to ensure fitting of the cage/rolling bodies assembly in bearing unit  30  without risk of breaking tenons  42 . 
     In various embodiments, armature  50  has an overall height H that can range from 0.3 mm to 1.2 mm. First and second portions  50   a  and  50   b  may have a thickness not less than 0.2 mm while the difference Z in axial position between the first portions  50   a  and second portions  50   b  may vary between 0.1 mm and 0.25×Dw, Dw being the diameter of the rolling bodies  32 . Armatures having a thickness less than 0.2 mm may compromise the rigidity of cage  40 , and a thickness that is too large may require axial dimensions of a cage that would be incompatible with the physical constraints of a cage  40  according to the present disclosure. 
     Positioning of armature  50  inside a mold in which the polymeric material of cage  40  is over-molded and the attachment resulting from the over-molding are both important. 
     In various embodiments, armature  50  may be a distance E from radially outer surface  41 ′″ of rib  41  that is a function of diameter Dw of rolling bodies  32 , but may be no greater than 0.45×Dw mm. 
     In embodiments in which distance E is zero, armature  50  may be centered on radially outer surface  41 ′″ of rib  41 . 
     In various embodiments, armature  50  may include a plurality of circumferentially spaced through holes  51 . A center of each through hole  51  may lie along a circumference of armature  50  defined by a diameter Ph that is a function of the mean diameter BCD 50  of bearing unit  30  and of the diameter Dw of rolling bodies  32  according to: 
         Ph=BCD 50/2+0.1× Dw  
 
     A diameter of through holes  51  may vary from a minimum of 0.1 mm to a maximum calculated as a function of the diameter Dw of the rolling bodies  32  and equal to 0.2×Dw mm. 
     A total number of through holes  51  may preferably be equal to a number of rolling bodies  32 , or to a number of tenons  42  of cage  40 , or may be a multiple of the number of such elements. These through holes  51  act as a guide for the axial and radial positioning of armature  50  in the mold for subsequent over-molding of the polymeric material. 
     Through holes  51  may also assist with the mechanical gripping for the polymer of a cage  40  being over-molded. 
     To improve the position of armature  50  inside the mold, a further guide may be defined a radially inner rounding  41 ″ on a surface of a base  41 ′ of rib  41 , the surface on a side of rib  41  axially opposite to tenons  42 . 
     Furthermore, to improve attachment of the polymeric material and, consequently, prevent relative rotation of armature  50  with respect to rib  41  of cage  40 , armature  50  may have a plurality of slots  52  into which the polymeric material may flow during over-molding. Slots  52  may be arranged at or near a radially outer edge  50   c  of armature  50 . 
     In various embodiments, pairs of slots  52  may be circumferentially spaced about armature  50  and be positioned on either side of each through hole  51 . In other words, each through hole  51  will be flanked by a pair of slots  52 , and therefore the number of slots  52  will be double the number of through holes  51 . 
     An armature  50  according to the present disclosure may be made of metal material. In various embodiments, second portions  50   b  may be obtained by deep-drawing from a flat washer according to known processes. Slots  52  are included to prevent detachment of the polymeric material from the metal material of armature  50 , as a chemical connection may be formed with the polymeric material in slots  52 . Forming a chemical connection of the polymeric material in slots  52  eliminates relative movements between cage  40  and armature  50 . 
     Alternatively, an armature  50  may be made of plastic material that is reinforced with resistant fibers. For example, a plastic armature  50  may consist of polyamide PA66 reinforced with glass fibers at a ratio of 2:1 (i.e. at a rate of 50%). In this case, the box-like shape of the armature  50  that includes first and second portions  50   a ,  50   b  of the circular crown alternating in two different planes may be obtained by injection-molding according to known processes. The remainder of cage  40  may be made of polyamide (e.g., PA66) as pure polymer or polymer slightly charged with reinforcing fibers. In the latter case, it may have a lower density than pure polymer. A resulting centrifugal force acting on tenons  42  of such cages may be reduced due to the lower density of the reinforced polyamide material. 
     Because the armature may be made of plastic material, as the cage is, it may be easier to recycle such cages, as there is no need to separate any metal from plastic. It is however important to be careful when selecting only cages of this type (i.e. with reinforcing armatures) to avoid mixing them up with other cages that do not have a reinforcing armature. 
     Embodiments of this disclosure are compatible with polymeric materials used for cages, namely, as mentioned above, polyamides. These materials include, but are not limited to, PA66 GF25 and PA46 GF30. 
     Deformation of a cage  40  according to this disclosure caused by centrifugal forces is reduced by 50% to 70% with respect to deformation of a cage made entirely of polymeric material and without a reinforcing armature  50  as described herein. By decreasing the deformation of cage  40 , contact with rolling bodies  32  is also reduced, resulting in less heating of tenons  42  from such contact that can reduce the operating life of cage  40 . 
     All of this therefore makes it possible to reach higher speeds and have a greater capacity for those speeds (e.g., being able to operate a higher speeds for a longer period of time). Embodiments of a bearing unit according to this disclosure can enable speeds corresponding to a speed index of greater than 1 mndm. 
     Reinforcement of the rib, i.e. of the supporting structure of the cage, offers a very big increase in rigidity and thus allows the cage to reach very high speeds. 
     Embodiments of a bearing unit, e.g., bearing unit  30 , described herein do not require an increase in an axial length of the bearing unit and therefore are compatible with, existing applications of bearing units. 
     In addition to the embodiments described herein, many other variants exist. It must be understood that these embodiments are simply examples and do not limit the subject matter of this disclosure, its applications, or its possible configurations. On the contrary, although the description above makes it possible for a person skilled in the art to implement the present disclosure according to at least one exemplary configuration thereof, it must be understood that many variations of the components described may be envisaged without thereby exceeding the subject matter of the disclosure as defined in the attached claims, interpreted literally and/or according to their legal equivalents.