Abstract:
The toroidal-type continuously variable transmission comprises an input shaft, an input disk, an output disk, a plurality of trunnions, a plurality of power rollers, and power roller bearings, wherein the mutually opposed inner surfaces of the input and output disks are respectively formed as concave surfaces each having an arc-shaped section, the peripheral surfaces of the power rollers are respectively formed as spherical-shaped convex surfaces, and these peripheral surfaces are contacted with the above-mentioned inner surfaces. The retainer of the power roller bearing is formed of high strength brass, and the high strength brass has a micro structure in which the α phase thereof is 40% or more in the area ratio thereof, and the hardness of said retainer is set at HB80 or higher.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to a toroidal-type continuously variable transmission and, for example, to a toroidal-type continuously variable transmission for use in a vehicle such as a car.  
           [0002]    Conventionally, as a toroidal-type continuously variable transmission, for example, there is known a transmission which has such a structure as shown in FIG. 1.  
           [0003]    Specifically, in the interior of a housing (not shown), an input disk  1  and an output disk  2  are disposed in such a manner that they are concentric with each other but are opposed to each other. An input shaft  3  is penetrated through the axial core portion of a toroidal transmission part in which the input disk  1  and output disk  2  are disposed. On one end of the input shaft  3 , there is disposed a loading cam  4 . This loading cam  4  is structured such that it can transmit the power (rotational force) of the input shaft  3  to the input disk  1  through a cam roller  5 .  
           [0004]    Also, between the input disk  1  and output disk  2 , there are interposed a plurality of trunnions swingable about their respective support shafts (not shown) disposed at positions twisted with respect to the center axes of the two disks  1  and  2  (in other words, between the input disk  1  and output disk  2 , there are interposed a plurality of trunnions swingable about their respective support shafts (not shown) each of which is disposed at a positions that is perpendicular to an axial line of the input shaft  3  and does not intersect the axial line thereof (this physical relationship is referred as “a twisted position”.).  
           [0005]    The input disk  1  and output disk  2  are substantially the same in shape and are also arranged symmetric, while their respective opposed surfaces are respectively formed as toroidal surfaces which, when they are viewed in the axial-direction section thereof, cooperate together in forming a substantially semicircular shape. Within a toroidal cavity defined by the toroidal surfaces of the input disk  1  and output disk  2 , a pair of power roller bearings  6  and  7  are disposed for power transmission in such a manner that they are in contact with the input disk  1  and output disk  2 .  
           [0006]    By the way, the power roller bearing 6 comprises a power roller  6   a  (which corresponds to an inner ring forming the power roller bearing  6 ) capable of rolling on the toroidal surfaces of the input disk  1  and output disk  2 , an outer ring  6   b , and a plurality of rolling elements (steel balls)  6   c . On the other hand, the other power roller bearing  7  comprises a power roller  7   a  (which corresponds to an inner ring forming the power roller bearing  7 ) capable of rolling on the toroidal surfaces of the input disk  1  and output disk  2 , an outer ring  7   b , and a plurality of rolling elements (steel balls)  7   c.    
           [0007]    That is, the power roller  6   a  serves also as the inner ring which is a component of the power roller bearing  6 , and the power roller  7   a  serves also as the inner ring which is a component of the power roller bearing  7 . In this structure, the power roller  6   a  is mounted pivotally and rotatably on a trunnion  10  through a pivot shaft  8 , the outer ring  6   b  and the plurality of rolling elements  6   c , and also is supported so as to be inclinable about a shift shaft  0  which provides the center of the toroidal surfaces of the input disk  1  and output disk  2 .  
           [0008]    On the other hand, the power roller  7   a  is mounted pivotally and rotatably on a trunnion  11  through a pivot shaft  9 , the outer ring  7   b  and the plurality of rolling elements  7   c , and also is supported so as to be inclinable about a shift shaft  0  which provides the center of the toroidal surfaces of the input disk  1  and output disk  2 . And, lubricating oil (traction oil) having large viscous frictional resistance is supplied to the contact surfaces of the input disk  1  and output disk  2  with respect to the power roller  6   a  and power roller  7   a , in such a manner that the power input to the input disk  1  can be transmitted to the output disk  2  through the film of the lubricating oil, power roller  6   a  and power roller  7   a.    
           [0009]    By the way, the input disk  1  and output disk  2  are connected with the input shaft  3  through needle roller bearings  12  such that they are independent of the input shaft  3  (that is, they are held in a state where they are not influenced directly by the power of the input shaft  3 ). On the output disk  2 , there is disposed an output shaft  14  which is arranged in parallel to the input shaft  3  and also supported rotatably on the housing (not shown) through angular contact ball bearings  13 .  
           [0010]    In the toroidal-type continuously variable transmission  20 , the power of the input shaft  3  is transmitted to the loading cam  4 . In case where the loading cam  4  is rotated due to this power transmission, power generated by the rotation of the loading cam  4  is transmitted through the cam roller  5  to the input disk  1 , so that the input disk  1  can be rotated. Further, power generated by the rotation of the input disk  1  is transmitted through the power rollers  6   a  and  7   a  to the output disk  2 . As a result of this, the output disk  2  is rotated integrally with the output shaft  14 .  
           [0011]    In speed change, the trunnions  10  and  11  are moved by a slight distance in the directions of the shift shafts  0 , respectively. That is, due to the movements of the trunnions  10  and  11  in the axial direction thereof, the intersection between the rotation axes of the power rollers  6   a,    7   a  and the axes of the input and output disks  1 ,  2  is shifted slightly. This shift loses balance between the rotation peripheral speeds of the power rollers  6   a,    7   a  and the rotation peripheral speed of the input disk  1 . In addition, due to the component of the rotation drive force of the input disk  1 , the power rollers  6   a,    7   a  are allowed to inclinedly roll around the shift shafts  0 , whereby the power rollers  6   a ,  7   a  are allowed to incliningly roll on the curved surfaces of the input and output disks  1 ,  2 . As a result of this, the speed ratio is changed, so that the speed of the vehicle is decreased or increased. As a toroidal-type continuously variable transmission having the above structure, for example, there is known a conventional transmission which is disclosed in JP-B-2-49411U.  
           [0012]    In the above-mentioned toroidal-type continuously variable transmission, when driven, there is a fear that the power roller retainers of the power roller bearings interposed between the input and output disks can be damaged or broken. The reason for this is as follows: that is, when this toroidal-type continuously variable transmission is driven with a high load and high torque, the inner ring raceways of the power roller bearings, which have received loads from the input and output disks, are respectively deformed into an elliptic shape; and, when the rolling elements held by the retainers roll along the thus-deformed raceways, the rolling elements apply loads to the retainers. As a measure to deal with problem, there is known a method of enhancing the strength of the retainers. However, in this method, in case where the strength of the retainers is increased, seizure can occur between the contact portions between the retainers and power roller inner rings or outer rings, or the power roller inner rings or outer rings can be broken by the retainers.  
         SUMMARY OF THE INVENTION  
         [0013]    The present invention aims at eliminating the above problems found in the conventional toroidal-type continuously variable transmission. Accordingly, it is an object of the invention to provide a toroidal-type continuously variable transmission in which the retainers of the power roller bearings are made of high-strength brass to thereby be able to optimize the strength of the retainers.  
           [0014]    In attaining the above object, according to the invention, there is provided a toroidal-type continuously variable transmission, comprising: an input shaft; an input disk rotatable together with the input shaft; an output disk disposed concentrically with the input disk and supported rotatable with respect to the input disk; a plurality of trunnions respectively swingable about support shafts disposed at twisted positions with respect to the center axes of the input and output disks; a plurality of shift shafts respectively supported on their associated trunnions; a plurality of power rollers respectively rotatably supported on the shift shafts and interposed between the input and output disks; and thrust ball bearings for supporting thrust loads applied to the power rollers, wherein each of the thrust ball bearing comprises: an inner ring raceway formed in the outer peripheral surface of the power roller; an outer ring including an outer ring raceway formed in the inner peripheral surface thereof; a plurality of balls interposed between and rollable along the inner ring raceway and outer ring raceway; and, a retainer for holding the balls, wherein the retainer is formed of high strength brass and the high strength brass has a micro structure in which the α phase thereof is 40% or more in the area ratio thereof.  
           [0015]    Preferably, the hardness of the retainer may be set at HB80 or higher.  
           [0016]    Further, preferably, the α phase is set in the range of 40% to 80%. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    [0017]FIG. 1 is an explanatory view of a toroidal-type continuously variable transmission;  
         [0018]    [0018]FIG. 2 is an explanatory view of a power roller and a thrust ball bearing that are the main portions of a toroidal-type continuously variable transmission according to the invention;  
         [0019]    [0019]FIG. 3 is a sketch of microscope picture of the micro-structure of high strength brass;  
         [0020]    [0020]FIG. 4 is a characteristic diagram of the relationship between the hardness and a-phase of high strength brass; and,  
         [0021]    [0021]FIG. 5 is a characteristic diagram of the relationship between the fatigue strength and α phase of high strength brass. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0022]    Now, description will be given below in more detail of a toroidal-type continuously variable transmission according to the invention.  
         [0023]    [0023]FIG. 2 shows the main portions of a toroidal-type continuously variable transmission according to the invention: that is, a power roller  28 ; and, a thrust ball bearing  32  which is a power roller bearing for supporting a thrust load applied to the power roller  28 . This thrust ball bearing  32  is structured in the following manner. A plurality of balls  41 ,  41  held by a circular-ring-shaped retainer  44  are interposed between an inner ring raceway  42  formed in the outer surface (in FIG. 2, upper surface) of the power roller  28  and an outer ring raceway  43  formed in the inner peripheral surface of an outer ring  33 , in such a manner that the balls  41 ,  41  are allowed to roll on the inner ring raceway  42  and outer ring raceway  43 . And, the peripheral surface (traction surface)  28   a  of the power roller  28  is contacted with the inner surfaces  1   a ,  2   a  of the input side and output side disks  1  and  2  to thereby be able to transmit power between the two disks  1  and  2 .  
         [0024]    The retainer  44  is formed of high strength brass.  
         [0025]    The reason why high strength brass is used as the material of the retainer  44  is as follows. That is, in the power roller  28 , the traction surface  28   a  receives normal forces Fc from the two output and input side disks. At the same time, in the inner ring raceway  42 , the power roller  28  receives normal forces Fa from the balls  41 . In FIG. 1, the vertical-direction components of the normal forces Fc balance well with the normal forces Fa, whereas the horizontal-direction components of the normal forces Fc act as a force to crash the power roller  28 . This force provides a force to deform the power roller  28 , so that the raceway surface of the power roller  28  is slightly deformed into an elliptic shape.  
         [0026]    Balls  41  are held in a circular shape by the retainer  44  but, since the raceway surface of the power roller  28  is slightly deformed into an elliptic shape, the balls  41  are caused to roll on the elliptic-shaped raceway surface. Therefore, as the balls roll, the retainer is rotated while the retainer is being deformed into an elliptic shape. However, the retainer is not originally designed while taking such deformation into account. That is, for example, in case where the retainer is formed of plastics, the retainer is easy to break, or in case where the retainer is formed of steel such as SPCC, the retainer cannot be broken but, since the retainer is contacted with the power roller  28  or outer ring  33  while rotating, seizure can occur in the retainer. On the other hand, in case where the retainer is formed of high strength brass, the retainer is worn in the contact portion between the ball  41  and retainer  44  to thereby relieve the degree of contact, which can prevent the retainer from being broken.  
         [0027]    Also, in the case of the high strength brass, in case where the optimum-composition is selected, there can be obtained a retainer which is excellent in fatigue strength. In the case of the high strength brass, its hardness lowers as its α phase decreases (FIG. 4). On the other hand, the fatigue strength of the high strength brass increases greatly in case where its α phase exceeds 40% (FIG. 5). This disagrees with the tendency of ordinary steel material in which the fatigue strength increases as the hardness increases. The reason why the high strength brass provides the above tendencies seems that the high strength has a two-phase structure composed of α phase and β phase.  
         [0028]    For the power roller of a toroidal-type continuously variable transmission, from the viewpoint of the fatigue strength, the α phase is required to be 40% or more; and, from the viewpoint of the material strength, the hardness is required to be HB80 or higher. Also, preferably, the α phase may be set in the range of 40% to 80%.  
         [0029]    In case where the above-mentioned high strength brass is used as the material of the power roller retainer, there can be obtained a toroidal-type continuously variable transmission which can prevent the retainer against damage and can enjoy a long life.  
         [0030]    [Embodiments] 
         [0031]    Now, description will be given below of the embodiments of the invention. By the way, the basic structure of a toroidal-type continuously variable transmission according to the invention is similar to that shown in FIG. 1, but the present toroidal-type continuously variable transmission is different in the material of the power roller retainer from the conventional toroidal-type continuously variable transmission.  
         [0032]    Using two kinds of high strength brass HBsC1 and HBsC2, there were manufactured two kinds of high strength brass retainers for a power roller. Also, as comparison examples, there were manufactured a plastic retainer using nylon 66 and a steel-made retainer using SPCC.  
         [0033]    Using the above power roller retainers, there were assembled toroidal-type continuously variable transmissions. In the case of disks and power rollers, mass-produced disks and power rollers were used.  
         [0034]    A durability test was conducted under the following conditions 1; specifically, the test was conducted until the retainer was broken or seized, or the test was conducted up to 100 hr. and was stopped with the retainer not broken. The test results are shown in the following table 1. In the case of the high strength brass HBsC1 and HBsC2, the retainers were both found not broken. On the other hand, in the case of the plastic retainer according to the comparison example 1, the retainer was found broken; and, in the case of the comparison example 2, the power roller was found seized.  
         [0035]    Test condition 1:  
         [0036]    Number of rotation of input shaft: 4000 r.p.m.  
         [0037]    Input torque: 350 Nm  
         [0038]    Oil used: Traction oil  
         [0039]    Oil supply temperature: 100° C.  
                                                 TABLE 1                                   Kinds of   Test Time               Retainers   (hr)   Test Results                                    Embodiments 1   HBsC1   100   Not Broken       Embodiments 2   HBsC2   100   Not Broken       Comparison Examples 1   Nylon 66    5   Retainer was                   broken       Comparison Examples 2   SPCC    16   Power roller                   was seized                  
 
         [0040]    Next, such chemical compositions as shown in the following table 2 were dissolved and cast to thereby produce high strength brass. The structure of the thus-produced high strength brass was observed using a microscope and the area ratio of the α phase thereof was obtained. FIG. 3 shows a microscope picture of the micro structure of the high strength brass (×100 corrosion solution ferric chloride hydrochloride solution). Also, the hardness of the high strength brass was measured at the same time. A rotational bending fatigue test was conducted on the respective kinds of high strength brass and the fatigue strengths of the respective kinds of high strength brass at 10 7  cycle were obtained. The thus obtained results are shown in the following table 3.  
                                                                                                           TABLE 2                                       Chemical compositions (%)                Cu   Zn   Mn   Fe   Al   Sn   Ni   Pb   Si                        Embodiment 3   59.32   36.19   2.25   0.76   0.94   0.17   0.18   0.16   0.012       Embodiment 4   59.99   35.48   2.25   0.76   0.97   0.18   0.18   0.17   0.017       Embodiment 5   59.98   35.57   2.26   0.74   0.82   0.18   0.18   0.19   0.017       Embodiment 6   59.98   35.81   2.23   0.74   0.65   0.17   0.17   0.18   0.008       Embodiment 7   59.99   34.71   3.41   0.75   0.56   0.15   0.19   0.19   0.009       Comparison   56.38   39.21   2.21   0.70   0.97   0.16   0.17   0.17   0.010       examples 3       Comparison   57.51   38.05   2.22   0.74   0.96   0.17   0.16   0.18   0.008       examples 4       Comparison   58.52   37.01   2.21   0.75   0.95   0.18   0.18   0.18   0.012       examples 5       Comparison   59.99   34.22   3.99   0.76   0.51   0.15   0.19   0.18   0.009       examples 6                  
 
         [0041]    [0041]                                                 TABLE 3                                           Fatigue           Phase area   Hardness   Strength (Nm)           Ratio (%)   (HB)   10 cycle                                    Embodiment 3   41   121   152       Embodiment 4   49   113   178       Embodiment 5   60   105   182       Embodiment 6   71    95   174       Embodiment 7   80    87   151       Comparison examples 3   10   144    78       Comparison examples 4   21   136    83       Comparison examples 5   32   130    86       Comparison examples 6   89    71   120                    
         [0042]    In the results shown in Table 3, the relations between the α phase area ratios and hardnesses were put in order and the relations between the α phase area ratios and fatigue strengths were put in order. The hardness lowered as the α phase area ratio decreased. On the other hand, the fatigue strengths increased greatly in case where the α phase exceeded 40%.  
         [0043]    Next, using the high strength brass shown in Table 3, there were manufactured power roller retainers, there were assembled toroidal-type continuously variable transmission units, and a durability test was conducted on the retainers and transmission units. The test condition is the same as the test condition 1. The test was conducted until retainers and transmission units were broken, or, in the case of the test in which they were not broken, the test was conducted up to 300 hrs. and was then broken off.  
         [0044]    The following table 4 shows the test results. In the case of the embodiments, the test was ended in 300 hrs. with the retainers and transmission units not broken. In the case of the comparison examples 3-5, the test was ended in 123-131 hrs. where the retainers were found broken. Also, in the case of the comparison example 6, the test was broken off because the rotation accuracy could not be obtained due to the deformation of the retainer. That is, in the comparison example 6, the test was discontinued because the strength of the retainer was not sufficient for the power rollers of the toroidal-type continuously variable transmission.  
                                             TABLE 4                                   Test Time               (hrs.)   Test results                                        Embodiment 3   300   Not broken           Embodiment 4   300   Not broken           Embodiment 5   300   Not broken           Embodiment 6   300   Not broken           Embodiment 7   300   Not broken           Comparison examples 3   123   Retainer was broken.           Comparison examples 4   131   Retainer was broken.           Comparison examples 5   158   Retainer was broken.           Comparison examples 6    44   Test was discontinued                   due to deformed                   retainer.                      
 
         [0045]    As has been described heretofore in detail, according to the invention, by using the high strength brass as the material of the retainer of a power roller bearing, there can be supplied a toroidal-type continuously variable transmission which can provide high durability.  
         [0046]    While there has been described in connection with the preferred embodiment of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is aimed, therefore, to cover in the appended claim all such changes and modifications as fall within the true spirit and scope of the invention.