Patent Publication Number: US-11036186-B2

Title: Reduced-friction shaft support bearing

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention relates to a bearing for micromechanics in which a shaft is capable of pivoting, the shaft comprising, at at least one end thereof, a shoulder via which the shaft is in contact with an opposite surface of the bearing, said shoulder being extended by a pivot engaged in a hole provided in the bearing. Such a bearing is in particular used in the horological field. 
     TECHNOLOGICAL BACKGROUND OF THE INVENTION 
     In order to drive a wheel in rotation in a horological movement, the wheel is known to be secured to a shaft comprising, at at least one of the ends thereof, a shoulder extended by a pivot. The shaft is positioned between two bearings, each of which comprises a hole in which is housed a pivot of the shaft. According to the chosen construction method, the shoulder of one end of the shaft or the shoulder of each end of the shaft enters into contact with an opposite surface of the associated bearing. The two bearings guide the shaft in rotation while preventing the axial translation thereof. 
     One known method for reducing friction between the shoulder of the shaft and the bearing is to arrange a recess on an outer edge of the hole in which is housed the pivot of the shaft in order to create a reservoir, commonly called an oil-sink, and intended to receive a drop of oil. The drop of oil present in the reservoir infiltrates by capillarity between the shaft and the walls of the hole of the bearing, and between the shoulder of the shaft and the surface of the bearing facing said shoulder. This technique is used to reduce friction between the shaft and the bearing. However, the layer of oil between the shoulder of the shaft and the opposite surface of the bearing is particularly thin, resulting in the observation of an adherence effect between the shoulder of the shaft and the opposite surface of the bearing and, when the shaft turns, a shear stress of the layer of oil opposes the rotation of the shaft. These interference phenomena result in a loss of energy, which should be avoided. 
     SUMMARY OF THE INVENTION 
     The purpose of this invention is to improve the known technique, by proposing a solution for reducing the shear stress in the region of the layer of oil present between the shoulder of a shaft and the opposite surface of a bearing in which the shaft is pivoted. 
     To this end, the present invention relates to a bearing for micromechanics, in which pivots a shaft, the shaft comprising, at one end thereof, a shoulder via which the shaft is in contact with an opposite surface of the bearing, said shoulder being extended by a pivot engaged in a hole provided in the bearing, the surface of the bearing facing the shoulder of the shaft comprising at least one recess in order to reduce the surface of contact between the shoulder of the shaft and the bearing. 
     By reducing the surface of contact between the shoulder of the shaft and the bearing, the shear stress effect in the layer of oil is reduced. The shaft therefore pivots more easily in the bearing. Moreover, even when reduced, the surface of contact remains sufficient for guiding the shaft in the bearing. 
     According to one embodiment of the invention, the recess is present in the form of a ring centred around the hole of the bearing. The effect, i.e. the torque produced by the elementary shear stress, is the product of this shear stress multiplied by the distance between the rotational axis and the point of application of the elementary stress. By producing an annular recess distanced from the pivot of the shaft, the effects of the shear stress are even more reduced. 
     According to another embodiment of the invention, the surface facing the bearing via which the shoulder of the shaft is in contact with the bearing is provided with a plurality of hollows distributed around the hole of the bearing. The reduction in the effect of the shear stress is therefore more homogeneous between the shoulder of the shaft and the surface of the bearing facing the shoulder of the shaft. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       Other features and advantages of the present invention shall be better understood upon reading the detailed description given below of example embodiments of bearings according to the invention. These examples are given for illustrative purposes only and are not intended to limit the invention; they must be read with reference to the accompanying figures, in which: 
         FIG. 1  is a view of a wheel of a horology movement mounted such that it rotates between two bearings according to the invention; 
         FIG. 2  is an overhead view of a bearing according to the invention, and 
         FIG. 3  is an overhead view of another bearing according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF ONE EMBODIMENT OF THE INVENTION 
     The present invention is based on the general inventive idea consisting in reducing the surface of contact between the shoulder of a shaft and the opposite surface of a bearing supporting said shaft. For this purpose, the invention proposes a new bearing, the overall shape of which reduces said surface of contact. 
       FIG. 1  diagrammatically shows the implementation of bearings according to the invention for driving in rotation a toothed wheel of a horology movement. 
     The toothed wheel  20  is secured to a shaft  30  comprising, at each end thereof, a shoulder  31   a ,  31   b  extended by a pivot  32   a ,  32   b . The shaft  30  is mounted such that it rotates between two bearings  10   a ,  10   b  according to the invention. The bearings  10   a ,  10   b , preferably annular in shape, are immobilised in a frame  21 . Said bearings  10   a ,  10   b  comprise, in a known manner, a hole  11 , that is preferably centred, passing through said bearings from end to end. It is observed that the wall of the hole  11  can comprise an olive-cut intended to minimise contact with the pivots  32   a ,  32   b  and ease possible lubrication. 
     A pivot  32   a ,  32   b  of the shaft  30  guided in rotation by the corresponding bearing  10   a  or  10   b  is housed in the hole  11 . The shoulder  31   a ,  31   b  of the shaft  30  enters into contact with an opposite surface  12  of the bearing  10   a ,  10   b  such that the shaft  30  is immobilised in axial translation, to the nearest play, between the bearings  10   a ,  10   b.    
     On a side opposite the surface of contact  12  between the shoulder  31   a ,  31   b  of the shaft  30  and the bearing  10   a ,  10   b , the hole  11  opens out into a recess  13 , having a preferably conical shape. This recess  13 , commonly called an oil-sink in the horological field, can be intended to receive a drop of oil. The other end of the hole  11  is slightly flared in order to ease the infiltration of the oil between the bearing  10   a ,  10   b  and the shoulder  31   a ,  31   b  in the region of the surface of contact  12 . It is understood that this recess is optional and that it will only be provided in the event that the pivoting of the shaft  30  in the bearing  10   a ,  10   b  will be lubricated. 
     A bearing  10   a ,  10   b  according to the invention is characterised by a hollow  14  for reducing the surface of contact  12  between the shoulder  31   a ,  31   b  of the shaft  30  and the bearing  10   a ,  10   b . The hollow  14  is made on the side opposite the recess  13 , in the surface of contact  12  of the bearing  10   a ,  10   b  situated facing the shoulder  31   a ,  31   b  of the shaft  30 . 
     According to one embodiment shown in  FIG. 2 , the hollow  14  is annular in shape. The residual surface of contact  12  between the shoulder  31   a ,  31   b  of the shaft  30  and the bearing  10   a ,  10   b  thus has the shape of two inner  12   a  and outer  12   b  concentric rings. The inner radius R 0  of the inner ring  12   a  is substantially equal to the radius of the hole  11 , whereas the outer radius R 1  of the inner ring  12   a  is equal to the inner radius of the hollow  14 . The outer concentric ring  12   b  on the other hand lies between an inner radius R 2  equal to the outer radius of the hollow  14  and an outer radius R 3 . The radius R 1  must be sufficient to guarantee that the shaft  30  is correctly held inside the hole  11 . 
     According to another embodiment, a plurality of hollows  14  are hollowed out of the surface of the bearing  10   a ,  10   b  around the hole  11 . In the example shown in  FIG. 3 , six hollows  14  are evenly spaced apart, arranged in a concentric manner about the pivot  32   a ,  32   b  of the shaft  30  and opening out into the flared end of the hole  11 . 
     In the horological field, the dimensions of the bearings are small, from less than 1 mm to several millimetres for the largest dimension. The production of a bearing according to the invention is therefore delicate and requires specific tooling. 
     According to an alternative embodiment, the bearing  10   a ,  10   b  is made from a hard, monocrystalline material such as ruby, corundum, spinel or cubic zirconia and the hollows  14  are machined by material ablation using a laser beam, by spark erosion or by grinding. 
     According to another alternative embodiment, the bearing  10   a ,  10   b  is made from a hard, sintered material such as corundum, ruby, ceramics, alumina, zirconia or even a hard metal, and the hollows  14  are made by forming or are ablation-machined. This technique is in particular described in the document EP 2 778 801 A1 filed by the Applicant. 
     For reference, the method comprises a first step of forming a ceramic precursor from a ceramic-based powder dispersed in a binder. This ceramic-based powder can contain at least one metal oxide, one metal nitride or one metallic carbide. For the purposes of illustration, the ceramic-based powder can contain aluminium oxide in order to form synthetic sapphire or a mixture of aluminium oxide and chromium oxide to form synthetic ruby. The binder on the other hand can be a polymer binder or an organic binder. 
     The method comprises a second step that uses an upper die and a lower die, that are brought closer to each other, to compress the ceramic precursor in order to form a green body of the future bearing  10   a ,  10   b  with upper and lower surfaces respectively comprising at least one hollow  14  and, where applicable, a recess  13 . It is therefore understood that each green body thus formed already comprises the blanks of the hollow  14  and of the recess  13 . 
     In order to obtain these blanks of the hollow  14  and of the recess  13 , each substantially planar die comprises at least one punch intended to form the hollow  14  and, optionally, the recess  13 . To this end, the upper die comprises a punch with a substantially annular surface for forming the hollow  14 , and the lower die comprises a punch with a substantially conical surface for forming the recess  13 . 
     Finally, the green body is sintered in order to form a ceramic bearing  10   a ,  10   b  and the hole  11  is bored in order to connect the upper surface and the lower surface of the bearing  10   a ,  10   b  to each other. This step preferably takes place using destructive radiation of the laser type, in order to obtain very precise etching. However, this step can take place, for example, by mechanical boring or etching with high-pressure water. 
     It is evident that this invention is not limited to the embodiments described above and that various simple alternatives and modifications can be considered by one of ordinary skill in the art without departing from the scope of the invention as defined by the accompanying claims. It should in particular be noted that in the basic embodiment thereof, this invention applies in the event that only one of the two bearings  10   a ,  10   b  that guide the shaft  30  is equipped, in the surface of contact  12  thereof with the shoulder  31   a  or  31   b  of the corresponding pivot  32   a  or  32   b , with a hollow  14  according to the invention. The case in which both bearings  10   a ,  10   b  each have a hollow  14  to reduce the range of the surface of contact  12  with the shoulders  31   a  and  31   b  of the pivots  32   a  and  32   b  is also evidently considered. 
     NOMENCLATURE 
     
         
           10   a ,  10   b . Bearings 
           11 . Hole 
           12 . Surface of contact 
           12   a . Inner concentric ring 
           12   b . Outer concentric ring 
         R 0 , R 2 . Inner radii 
         R 1 , R 3 . Outer radii 
           13 . Recess 
           14 . Hollows 
           20 . Wheel 
           21 . Frame 
           30 . Shaft 
           31   a ,  31   b . Shoulders of the shaft 
           32   a ,  32   b . Pivots of the shaft