Abstract:
A tapered roller bearing has an increased load capacity and has a decreased maximum face pressure on the raceway surfaces without lowering the rigidity of the cage. The tapered roller bearing includes an inner ring, an outer ring, multiple tapered rollers rollably disposed between the inner ring  2  and the outer ring  3 , and a cage for holding the tapered rollers at predetermined circumferential intervals, wherein the roller coefficient γ thereof is larger than 0.94. Herein, γ=(the number of the rollers×the average diameter of the rollers)/(π×PCD).

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a tapered roller bearing, and more particularly, to a tapered roller bearing suitably incorporated into the gear device of an automobile transmission. 
     2. Description of the Related Art 
     Automobile transmissions are broadly classified into a manual type and an automatic type. Furthermore, they can also be classified according to the driving system of a vehicle into a trans-axle for front wheel drive (FWD), a transmission for rear wheel drive (RWD), and a transfer for four-wheel drive (4WD). These are used to speed-change the drive power delivered from the engine and to transmit it to the drive shaft or the like. 
       FIG. 7  shows a configuration example of an automobile transmission. This transmission is a synchronous meshing type, in which the left side in the figure is the engine side and the right side is the drive wheel side. A tapered roller bearing  43  is disposed between a main shaft  41  and a main drive gear  42 . In this example, the outer ring raceway surface of the tapered roller bearing  43  is directly formed on the inner circumference of the main drive gear  42 . The main drive gear  42  is supported using a tapered roller bearing  44  so as to be rotatable with respect to a casing  45 . A clutch gear  46  is engaged with and connected to the main drive gear  42 , and a synchro-mechanism  47  is disposed adjacent the clutch gear  46 . 
     The synchro-mechanism  47  comprises a sleeve  48  that is moved axially (in the left-right direction in the figure) by the action of a selector (not shown), a synchronizer key  49  installed in the inner circumference of the sleeve  48  so as to be movable in the axial direction, a hub  50  engaged with and connected to the outer circumference of the main shaft  41 , a synchronizer ring  51  slidably mounted on the outer circumference (the cone section) of the clutch gear  46 , and a urging pin  52  and a spring  53  for elastically pressing the synchronizer key  49  against the inner circumference of the sleeve  48 . 
     In the state shown in the figure, the sleeve  48  and the synchronizer key  49  are held at the neutral position using the urging pin  52 . At this time, the main drive gear  42  rotates idle with respect to the main shaft  41 . On the other hand, when the sleeve  48  is moved, for example, to the left in the axial direction, from the state shown in the figure by the operation of the selector, the synchronizer key  49  is moved to the left in the axial direction, following the sleeve  48 , whereby the synchronizer ring  51  is pressed against the inclined surface of the cone section of the clutch gear  46 . This decreases the rotation speed of the clutch gear  46  and increases the rotation speed of the synchro-mechanism  47 . Furthermore, at the time when the rotation speeds of the two have become synchronized, the sleeve  48  is further moved to the left in the axial direction, meshing with the clutch gear  46 . Hence, the main shaft  41  and the main drive gear  42  are connected to each other via the synchro-mechanism  47 . As a result, the main shaft  41  and the main drive gear  42  are rotated synchronously. 
     In recent years, low-viscosity oil tends to be used for automobile transmissions to meet the needs for automatic transmission (AT), continuously variable transmission (CVT), low fuel consumption, etc. In an environment where low-viscosity oil is used, surface-originated flaking, which causes a very short life, sometimes occurs in the inner ring raceway surface having high surface pressure due to improper lubrication when such adverse conditions as (1) high oil temperature, (2) low amount of oil and (3) loss of pressurization occur simultaneously. 
     A direct and effective solution to the problem of the short life due to the surface-originated flaking is to reduce the maximum surface pressure. For the purpose of reducing the maximum surface pressure, it is necessary to change the bearing size or to increase the number of the rollers or the bearing if the bearing size is not to be changed. For the purpose or increasing the number of the rollers without decreasing the roller diameter, it is necessary to narrow the distance between the pockets in the cage. However, for this purpose, the pitch circle of the cage must be increased so that the cage is shifted so as to be as close as possible to the outer ring. 
     As an example in which the cage is shifted so as to make contact with the inner diameter surface of the outer ring, there is a tapered roller bearing shown in  FIG. 8  (refer to Japanese Patent Laid-Open No. 2003-28165). In this tapered roller bearing  61 , the outer circumferential surface of the small diameter annular section  62   a  and the outer circumferential surface of the large diameter annular section  62   b  of the cage  62  are disposed in slide contact with the inner diameter surface of the outer ring  63  so as to guide the cage  62 . Furthermore, a recess  64  for suppressing drag torque is formed on the outer diameter surface of the pole section  62   c  of the cage  62 , thereby maintaining the non-contact state between the outer diameter surface of the pole section  62   c  and the raceway surface  63   a  of the outer ring  63 . The cage  62  has the small diameter annular section  62   a , the large diameter annular section  62   b , and the multiple pole sections  62   c  that connect the small diameter annular section  62   a  to the large diameter annular section  62   b  in the axial direction and are formed with the recess  64  on the outer diameter surface thereof. Furthermore, multiple pockets, in each so which a tapered roller  65  is rollably accommodated, are provided so that each pocket is disposed between two pole sections  62   c . The small diameter annular section  62   a  is provided with a flange section  62   d  integrally extending to the inner diameter side. The tapered roller bearing shown in  FIG. 8  is an example intended to improve the strength of the cage  62 , wherein the cage  62  is shifted so as to make contact with the inner diameter surface of the outer ring  63  in order to increase the circumferential width of the pole section  62   c  of the cage  62 . 
     In the tapered roller bearing  61  described in Japanese Patent Laid-Open No. 2003-28165, the cage  62  is shifted to the outer diameter side so as to make contact with the inner diameter surface of the outer ring  63  in order to increase the circumferential width of the pole section  62   c  of the cage  62 . Furthermore, because the recess  64  is provided in the pole section  62   c  of the cage  62 , the plate thickness of the pole section  62   c  becomes inevitably thin, and the rigidity of the cage  62  is reduced. Hence, the cage  62  may be deformed due to stress during the assembly of the bearing  61  or may also be deformed during the rotation of the bearing  61 . 
     On the other hand, a conventional typical tapered roller bearing with a cage, other than the tapered roller bearing described in Japanese Patent Laid-Open No. 2003-28165, is designed so that the roller coefficient γ (roller filling factor) defined by the following formula is usually 0.94 or less in order to securely obtain the pole width of the cage  72  and obtain appropriate strength of the pole of the cage  72  and smooth rotation while avoiding contact between the outer ring  71  and the cage  72  as shown in  FIG. 9 .
 
Roller coefficient γ=( Z·DA )/(π· PCD )
 
where Z is the number of the rollers, DA is the average diameter of the rollers, and PCD is the pitch circle diameter of the cage.
 
     In addition, in  FIG. 9 , numeral  73  denotes the tapered roller, numeral  74  denotes the surface of the pole, numeral  75  denotes the inner ring, and e denotes a window angle. 
     SUMMARY OF THE INVENTION 
     The present invention is intended to increase the load capacity of a tapered roller bearing and to prevent premature breakage due to excessive pressure on the raceway surfaces thereof. 
     The tapered roller bearing according to the first aspect of the invention comprises an inner ring, an outer ring, multiple tapered rollers rollably disposed between the inner and outer rings, and a cage for holding the tapered rollers at predetermined circumferential intervals, wherein the roller coefficient γ thereof is larger than 0.94. 
     The second aspect of the invention is characterized in that the window angle of the pocket of the tapered roller bearing according to the first aspect of the invention is in the range of 55° to 80°. The window angle is the angle formed by the guide surfaces of the pole sections making contact with the circumferential surface of each roller. The reason for setting the minimum value of the window angle at 55° is to secure a proper state of contact with the roller. In addition, the reason for setting the maximum value at 80° is that if the angle is larger than this value, the radial pressing force increases, causing a danger that smooth rotation cannot be obtained even if the cage is made of a self-lubricating resin material. In the case of ordinary cages, the window angle thereof is in the range of 25° to 50°. 
     The third aspect of the invention is characterized in that the cage of the tapered roller bearing according to the first or second aspect of the invention is formed of an engineering plastic superior in mechanical strength, oil resistance, and heat resistance. In comparison with a cage formed of a steel plate, the cage formed of a resin material is light-weight, self-lubricating, and low in friction coefficient. These features, together with the effect of the lubricating oil present in the bearing, make it possible to suppress occurrence of abrasion due to contact with the outer ring. 
     In comparison with a steel plate, such a resin is light in weight and low in friction coefficient, thereby being suitable for reducing torque loss and cage abrasion at the time of starting the rotation of the bearing. 
     Engineering plastics include general-purpose engineering plastics and super engineering plastics. Typical ones are given below. However, they are examples of engineering plastics, and engineering plastics are not limited to those described below. 
     [General-purpose engineering plastics] polycarbonate (PC), polyamide 6 (PA6), polyamide 66 (PA66), polyacetal (POM), modified polyphenylene ether (m-PPE), polybutylene terephthalate (PBT), GF-reinforced polyethylene terephthalate (GF-PET), ultra-high molecular weight polyethylene (UHMW-PE) 
     [Super engineering plastics] polysulfone (PSF), polyethersulfone (PES), polyphenylene sulfide (PPS), polyarylate (PAR), polyamideimide (PAI), polyetherimide (PEI), (polyetheretherketone (PEEK), liquid crystal polymer (LCP), thermoplastic polyimide (TPI), polybenzimidazole (PBI), polymethylpentene (TPX), poly 1,4-cyclohexane dimethylene terephthalate (PCT), polyamide 46 (PA46), polyamide 6T (PA6T), polyamide 9T (PA9T), polyamide 11, 12 (PA11, 12), fluororesin, polyphthalamide (PPA) 
     Because the roller coefficient γ of the tapered roller bearing is set so as to be greater than 0.94, not only the load capacity increases but also the maximum surface pressure on the raceway surfaces can be reduced. Therefore, it is possible to prevent surface-originated flaking, which causes a very short life, under severe lubrication conditions. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects and advantages of the invention will be more apparent from the following description and drawings, in which: 
         FIG. 1A  is a cross-sectional view showing a tapered roller bearing according to the present invention, and  FIG. 1B  is a vertical-sectional view showing the bearing; 
         FIG. 2  is a partially enlarged sectional view showing the tapered roller bearing having a minimum window angle; 
         FIG. 3  is a partially enlarged sectional view showing the tapered roller bearing having a maximum window angle; 
         FIG. 4  is table showing the results of bearing life tests. 
         FIG. 5  is a partially sectional view showing a tapered roller bearing according to a modified embodiment of the present invention; 
         FIG. 6  is a sectional view showing a pole section or the cage shown in  FIG. 5 ; 
         FIG. 7  is a sectional view showing a general automobile transmission; 
         FIG. 8  is a sectional view showing a conventional tapered roller bearing with the cage shifted to the outer ring; and 
         FIG. 9  is a partially enlarged sectional view showing another conventional tapered roller bearing. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An embodiment according to the present invention will be described hereinafter referring to  FIGS. 1 to 4 . A tapered roller bearing  1  according to the embodiment, shown in  FIGS. 1A and 1B , has a tapered raceway surface  2   a , and comprises an inner ring  2  having a small flange section  2   b  on the small diameter side and a large flange section  2   c  on the large diameter side of the raceway surface  2   a , an outer ring  3  having a tapered raceway surface  3   a , multiple tapered rollers  4  rollably disposed between the raceway surface  2   a  of the inner ring  2  and the raceway surface  3   a  of the outer ring  3 , and a cage  5  for holding the tapered rollers  4  equal circumferential intervals. The roller coefficient γ of the taper-roller bearing  1  is herein greater than 0.94. 
     The cage  5 , integrally molded of a super engineering plastic, such as PPS, PEEK, PA, PPA or PAI, comprises a small diameter side annular section  5   a , a large diameter side annular section  5   b , and multiple pole sections  5   c  that make axial connection between the small diameter side annular section  5   a  and the large diameter side annular section  5   b.    
     The minimum window angle θmin of the window angle θ of pole surfaces  5   d  is 55° as shown in  FIG. 2 , and the maximum window angle θmax thereof is 80° as shown in  FIG. 3 . The window angle in the typical tapered roller bearing with a cage that is spaced from the outer ring as shown in  FIG. 9  is approximately 50° at most. The reason for setting the minimum window angle θmin at 55° is to secure a proper state of contact with the roller. If the window angle is less than 55°, the state of contact with the roller becomes improper. That is, in the case that the window angle is 55° or more, γ can be made greater than 0.94, and a proper state of contact can be ensured while the strength of the cage is ensured. Furthermore, the reason for setting the maximum window angle θmax at 80° is that if it is larger than this value, the pressing force in the radial direction increases, and there is a danger that smooth rotation cannot be obtained even if the cage is made of a self-lubricating resin material. 
       FIG. 4  shows the results of bearing life tests. In  FIG. 4  “Comparative example 1” in the “Bearing” column is a typical conventional tapered roller bearing with a cage that is spaced from the outer ring. “Embodiment 1” is a tapered roller bearing according to the present invention, the roller coefficient γ of which is greater than 0.94, being different from the conventional bearing only in this respect. “Embodiment 2” is another tapered roller bearing according to the present invention, the roller coefficient γ of which is greater than 0.94, and the window angle of which is set in the range of 55° to 80°. The tests were conducted under severe lubrication and excessive load conditions. As clarified in the figure, the life of “Embodiment 1” is more than twice the life of “Comparative example 1.” Furthermore, the life of “Embodiment 2” is approximately five or more times the life of “Embodiment 1” although the roller coefficient thereof is the same (0.96) as that of “Embodiment 1.” “Comparative example 1”, “Embodiment 1” and “Embodiment 2” measure 45 (inner diameter)×81 (outer diameter)×16 overall width (unit: mm), the number of the rollers in “Comparative example 1” is 24, the number of the rollers in “Embodiment 1” and “Embodiment 2” is 27, and oil film parameter Λ is 0.2. 
     Next, a modified embodiment according to the present invention will be described referring to  FIGS. 5 and 6 . In the tapered roller bearing  1  shown in the figures, protruding sections  5   f  having a convex shape protruding to the outer ring raceway surface are formed on the outer diameter surfaces of the pole sections  5   c  of the cage  5  that is integrally molded of an engineering plastic. In other respects, the cage  5  is the same as that described above. The contour of the protruding section  5   f , in the cross-sectional direction of the pole section  5   c , is arc-shaped as shown in  FIG. 6 . The curvature radius R 2  of this arc shape is made smaller than the radius R 1  of the outer ring raceway surface. The shape is determined so that a proper wedge-shaped oil film is formed between the protruding section  5   f  and the outer ring raceway surface, and it is desirable that the curvature radius R 2  of the protruding section should be approximately 70 to 90% of the radius R 1  of the outer ring raceway surface. If the curvature radius is less than 70%, the inlet opening angle of the wedge-shaped oil film becomes so large that the dynamic pressure decreases. Furthermore, if it is more than 90%, the inlet angle of the wedge-shaped oil film becomes so small that the dynamic pressure also decreases. In addition, the width W 2  of the protruding section  5   f  is desirably 50% or more of the width W 1  or the pole section  5   c  (W 2 ≧0.5×W 1 ). The reason is that if the width is less than 50%, the height of the protruding section  5   f  for forming a proper wedge-shaped oil film cannot be secured sufficiently. In addition, because the radius R 1  of the outer ring raceway surface continuously changes from the large diameter side to the small diameter side, the curvature radius R 2  of the protruding section  5   f  is also changed continuously from the large curvature radius R 2  of the large diameter side annular section  5   b  to the small curvature radius R 2  of the small diameter side annular section  5   a  accordingly in a similar way. 
     Because the tapered roller bearing  1  shown in  FIGS. 5 and 6  is configured as described above, when the bearing  1  rotates and the cage  5  starts to rotate, a wedge-shaped oil film is formed between the outer ring raceway surface and the protruding section  5   f  of the cage  5 . This wedge-shaped oil film produces dynamic pressure substantially proportional to the rotation speed of the bearing  1 . Therefore, even if the pitch circle diameter (PCD) of the cage  5  is made larger than the conventional value so as to dispose the cage close to the outer ring raceway surface, the bearing  1  can be rotated without causing much abrasion or torque loss. Hence, the number of the rollers can be increased without trouble. 
     Although the embodiments according to the present invention have been described above, the present invention is not limited to the above-mentioned embodiments but can be modified variously. For example, although a super engineering plastic, such as PPS, PEEK, PA, PPA or PAI, is used as the material of the cage in the above-mentioned embodiments, it may be possible that glass fiber, carbon fiber or the like is mixed with such a resin material or other engineering plastics as necessary to increase the strength. 
     INDUSTRIAL APPLICABILITY 
     The tapered roller bearing  1  according to the present invention can be incorporated into automobile transmissions, and can also be used for automobile differential gears and automobile gear devices, and for other applications.