Abstract:
A planetary gear set includes a sun gear including a first set of gear teeth having a helix angle and second set of gear teeth formed as a single unit with the first set of gear teeth and having a second helix angle of opposite hand relative to the first helix angle. A ring gear includes a third set of gear teeth having a third helix angle, and fourth set of gear teeth and having a fourth helix angle of opposite hand relative to the third helix angle. Planet pinions, each include a fifth set of gear teeth having a fifth helix angle, and sixth set of gear teeth formed as a single unit with the fifth set of gear teeth and having a sixth helix angle of opposite hand relative to the fifth helix angle.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates generally to a power transmission, particularly to a planetary gear set having helical gear teeth. 
         [0003]    2. Description of the Prior Art 
         [0004]    Due to noise associated with the operation of spur gears, it has become conventional for gear assemblies of automotive transmissions to include helical gearing to provide quieter operation. However, a recognized design constraint associated with helical planetary gear sets is the axial thrust component of the tangential loads that transmit torsion between the mating gear teeth. The axial component is caused by the helix angles of the mating gear teeth. 
         [0005]    In a planetary gear unit, a sun gear and ring gear mesh with each planet pinion. Thrust loads, applied to each planet pinion in opposite axial directions at the lines of contact due to its two meshing engagements, induce an overturning moment on each planet pinion. The reaction to the overturning moment causes additional loading of the bearing, as well as increasing the probability of end loading of the needles on which each planet pinion is supported on its pinion shaft. The additional loading reduces bearing life. In some cases, pinions are added to the assembly to increase bearing life even though the gear life is adequate. 
         [0006]    The axial forces cause unwanted loading of other components both inside and outside of the planetary assembly Within the assembly, the axial/moment loads produce an adverse effect on bearings, washers, pinion shafts, carrier surfaces, and pinion bores caused by the overturning moment. Multiple thrust bearings, which are added to most transmission assemblies to react the thrust loads, increase the package space required for the assembly. 
         [0007]    In addition, the thrust loads must be reacted outside the assembly, thereby requiring use of expensive external thrust bearings. These requirements increase the manufacturing and assembly cost of the transmission assembly and increase the length of the assembly, which is at a premium particularly in vehicles having front wheel drive, in which the transmission and engine are arranged laterally with respect to the longitudinal axis of the vehicle. 
         [0008]    In the automotive industry, it is known that herringbone gears can be used to address the thrust loads associated with conventional helical gearing. Because herringbone/double helical gears are difficult and costly to manufacture, as well as difficult to assemble, they are rarely used in automotive drive trains, transmissions or transfer cases. 
         [0009]    A need exists for a technique to limit or avoid use of trust bearings in transmission assemblies and to eliminate the overturning moment induced in planetary transmission planet pinions due to the thrust loads, yet maintain the quiet gear operation associated with helical gear teeth. 
       SUMMARY OF THE INVENTION 
       [0010]    The planetary gear set includes a sun gear including a first set of gear teeth having a first helix angle and second set of gear teeth formed as a single unit with the first set of gear teeth and having a second helix angle of opposite hand relative to the first helix angle. A ring gear includes a third set of gear teeth having a third helix angle, and fourth set of gear teeth having a fourth helix angle of opposite hand relative to the third helix angle. Planet pinions, each include a fifth set of gear teeth having a fifth helix angle, and sixth set of gear teeth formed as a single unit with the fifth set of gear teeth and having a sixth helix angle of opposite hand relative to the fifth helix angle. 
         [0011]    The axial force component or thrust forces developed at each mesh on the planet pinions have equal magnitude and opposite direction. The thrust force component developed at a mesh on each planet pinion is cancelled within the respective planet pinion by the thrust force component developed at the other mesh on the respective planet pinion. Therefore, there is substantially no unbalanced net thrust force component on any planet pinion that requires a reaction force to place the pinion in structural equilibrium, and no provision for a reaction force need be provided at the pinions. 
         [0012]    Similarly, the overturning moment that is induced by the axial force component developed at each mesh is cancelled within the respective pinion by the overturning moment developed at the other mesh on the respective planet pinion. Therefore, there is substantially no unbalanced net overturning moment on any planet pinion that would requires a reaction moment/load to place the pinion in structural equilibrium. Consequently, the bearing that supports a respective pinion on the pinion shaft has reduced loading due to the elimination of the overturning moment caused by the axial load component of the mesh helix angle and reducing the risk of end loading as well. 
         [0013]    The size of the gear set is smaller and its weight is less compared to those of a conventional planetary gear set having the same torque capacity, yet it compares favorably with respect to noise, vibration and harshness. The costs to manufacture and assemble the gear set are lower than those costs of a conventional planetary gear set. 
         [0014]    The scope of applicability of the preferred embodiment will become apparent from the following detailed description, claims and drawings. It should be understood, that the description and specific examples, although indicating preferred embodiments of the invention, are given by way of illustration only. Various changes and modifications to the described embodiments and examples will become apparent to those skilled in the art, such as both simple and compound Ravigneaux carriers. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0015]    These and other advantages will become readily apparent to those skilled in the art from the following detailed description of a preferred embodiment when considered in the light of the accompanying drawings in which: 
           [0016]      FIG. 1  is a partial sectional view of a planetary transmission gear set; 
           [0017]      FIG. 2  is side view of a sun gear having two helical gear teeth sets; 
           [0018]      FIG. 3  is side view of the sun gear of  FIG. 2  showing the sets of gear teeth offset mutually about the axis of the sun gear; 
           [0019]      FIG. 4  is a side view of the planet pinion and pinion shaft of  FIG. 2  showing the sets of pinion teeth offset mutually about the axis of the pinion; 
           [0020]      FIG. 5  is a perspective view showing pinion teeth engaged with the teeth of the sun gear and ring gear; 
           [0021]      FIG. 6  is a perspective side view of a first portion of the ring gear; 
           [0022]      FIG. 7  is a perspective side view of a second portion of a ring gear; and 
           [0023]      FIG. 8  is a perspective side view of the first and second ring gear portions assembled as shown in the transmission of  FIG. 1 . 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0024]    Referring now to  FIG. 1 , a planetary transmission  10  includes a housing  12  containing an input shaft  14  and output shaft  16  and a planetary gear assembly  18 . The gear assembly  18  includes a sun gear  20 , a ring gear  22 , a carrier  24 , and a set of planet pinions  26 , supported on the carrier and meshing with the sun gear and ring gear. Each planet pinion  22  is supported by a bearing  28  for rotation on a pinion shaft  30 , which is secured to the carrier  24 . Each pinion shaft  30  is spaced angularly about a central axis  32  from other pinion shafts on the carrier  24 , but that spacing need not be uniform about axis  32 . Carrier  24  is driveably connected by a spline  34  to input shaft  14 . Ring gear  22  is driveably connected by a disc  36  to output shaft  16 . 
         [0025]    A helix is a curve wound around the outer surface of a cylinder or cone that advances uniformly along the axis of the cylinder or cone as it winds around. A helix angle is the angle that a straight tangent to the helix at any point makes with an element of the cylinder or cone, such as a diametric plane perpendicular to the axis or a diametric plane parallel to the axis. 
         [0026]      FIG. 2  shows that sun gear  20  includes two sets of gear teeth formed with mutually oppositely directed helices, a first set of teeth  40  having a right-hand helix and a face width located at the right-hand side of an annular groove  44 , a second set of teeth  42  having a left-hand helix located at the left side of groove  44 , and spline teeth  46 , which connects sun gear  20  to a structural member  48 . The annular groove  44  allows the shaper/cutters that form the gear teeth  40 ,  42  to travel through one set of helical teeth and to stop before reaching the other set of helical teeth. The width of groove  44  is adjustable, and in some cases may not be required depending on the manufacturing method. 
         [0027]    Sun gear  20  is an integral unit, i.e., a unitary component formed in one piece without any connections, such as mechanical attachments, frictional engagements, interference fits or chemical bonds among constituent parts. Preferably sun gear  20  is formed as an integral unit of sintered powdered metal. 
         [0028]      FIG. 3  shows that teeth  40  are preferably indexed or offset one-half tooth pitch with respect to teeth  42  uniformly about the axis  32  of sun gear  20 . The pitch offset between the sets of teeth  40 ,  42  may be uniform about axis  32 , but different than one-half tooth pitch, or the teeth of gear sets  40 ,  42  may be aligned mutually with no pitch offset. 
         [0029]      FIG. 4  shows that each planet pinion  26  is also an integral gear, which includes two sets of gear teeth formed with mutually oppositely directed helices, a first set of teeth  50  having a left-hand helix, and a second set of teeth  52  having a right-hand helix. A short annular groove  54  is located between gear teeth  50  and  52 . The teeth of gear set  50  mesh with and engage the teeth of set  40  on sun gear  20 , and the teeth of gear set  52  mesh with and engage the teeth of gear set  42  on the sun gear. 
         [0030]    Each planet pinion  26  is an integral unit, i.e., a unitary component formed in one piece without any connections, such as mechanical attachments, frictional engagements, interference fits or chemical bonds among constituent parts. Preferably each planet pinion  26  is formed as an integral unit of sintered powdered metal. 
         [0031]      FIG. 4  shows that teeth  50  are preferably indexed or offset one-half tooth pitch with respect to teeth  52  uniformly about the axis  56  of pinion  26 . The pitch offset between the sets of teeth  50 ,  52  may be uniform about axis  56 , but different than one-half tooth pitch, or the teeth of gear sets  50 ,  52  may be aligned mutually with no pitch offset. 
         [0032]    Similarly,  FIG. 1  shows that ring gear  22  includes two sets of gear teeth formed with mutually oppositely directed helices, a first set of teeth  60  having a left-hand helix and a face width located at the right-hand side of an annular groove  62 , a second set of teeth  64  having a right-hand helix located at the left-hand side of groove  64 . Preferably ring gear  22  is formed of sintered powdered metal. 
         [0033]    The first set of gear teeth  60  of ring gear  22  are preferably indexed or offset one-half tooth pitch with respect to the teeth of the second set  64  uniformly about the axis  32  of ring gear  22 . The pitch offset between the sets of teeth  60 ,  64  may be uniform about axis  32 , but different than one-half tooth pitch, or the teeth of gear sets  60 ,  64  may be aligned mutually with no pitch offset. The teeth of gear set  50  mesh with and engage the teeth of gear set  60  on ring gear  22 , and the teeth of gear set  52  mesh with and engage the teeth of gear set  62  on the sun gear. 
         [0034]      FIG. 5  shows a planet pinion  26  meshing with sun gear  20  and ring gear  22 . The set of pinion teeth  50  mesh with gear teeth  60  of sun gear  20  and with gear teeth  60  of ring gear  22 . The set of gear teeth  52  mesh with gear teeth  62  of sun gear  20  and with gear teeth  64  of ring gear  22 . 
         [0035]    As  FIGS. 6-8  illustrate, ring gear  22  is formed in two parts: a first annular portion  70  formed with the internal gear set of helical teeth  60 , and a second annular portion  74  formed with the second internal gear set of helical teeth  64 . 
         [0036]    The outer surface of a circular cylinder  82  of the first ring gear portion  70  is formed with a series of angularly-spaced, radial teeth  78  and radial slots  80  angularly spaced about axis  32 , each slot being located between adjacent teeth  78 . 
         [0037]    Cylinder  82  extends along axis  32  from the radial slots  80  and radial teeth  78  to a series of axial teeth  84  and spaces  86 , each space being located between adjacent teeth  84  and having on open axial end. The profile of each tooth  84  is that of a trapezoid having an axial surface  88  directed along axis  32 , a surface  90  extending from an axial edge  92  of surface  82  and inclined axially and circumferentially, and a circumferential surface  92  connecting surfaces  88  and  90  at the base of each tooth  84 . 
         [0038]    The outer surface of a circular cylinder  100  of the second ring gear portion  74  is formed with a series of axial teeth  102  and axial recesses  104 , angularly-spaced about axis  32 . The profile of each recess  104  is complementary to the profile of each tooth  84  on the first ring gear portion  70  and has an open end  106  for accepting a tooth  84  when it is inserted axially into a recess  104 . Similarly, the profile of each axial tooth  102  is complementary to the profile of each axial space  86  on the first ring gear portion  70 . Preferably, the number of recesses  104  is equal to the number of teeth  84 , the number of spaces  86  is equal to the number of teeth  102 , each axial recesses  104  is aligned with a corresponding axial tooth  84 , and each axial space  86  is aligned with a corresponding axial tooth  102 . When the recesses  104 , spaces  86  and teeth  84 ,  102  engage mutually they form a drive connection between the portions  70  and  74  of ring gear  22 . That connection provides torsion continuity between portions  70 ,  74 , permits their mutual disengagement, and is indexed such that the gear teeth sets  50 - 60  and  52 - 66  are engaged with the proper pitch offset, if a pitch offset is employed. 
         [0039]      FIG. 7  shows the teeth  102  on ring gear portion  74  located in and engaged with the spaces  86  of ring gear portion  74 , and the teeth  84  on ring gear portion  70  located in and engaged with the recesses  104  on ring gear portion  74 . 
         [0040]    The left-hand or right-hand helix angle of the helical gear sets  40 ,  50 ,  60  are reversed with respect to that of the corresponding helical gear sets  42 ,  52 ,  64 , and they are phased or indexed, i.e., circumferentially offset mutually. For example, if the helix angle is right-handed for gear teeth  40 , then the helix orientation is left-handed for gear set  42 . The helical gear teeth  50  of pinions  26  are circumferentially indexed by one-half tooth pitch relative to the helical gear teeth  52  of pinions  26 , and the helical gear teeth  40  of sun gear  20  are circumferentially indexed by one-half tooth pitch relative to the helical gear teeth  42  of sun gear  20 . Similarly, helical gear teeth  60  of ring gear  22  are circumferentially indexed by one-half tooth pitch relative to the helical gear teeth  64  of the ring gear  22 . 
         [0041]    The axial force or thrust force component transmitted to the gear sets  50   52  of the planet pinions  26  due to their engagement with the ring gear  22  and sun gear  20  are of equal magnitude and opposite direction. For example, the thrust force component on each gear set  50 ,  52  of planet pinion  26  is cancelled within the respective planet pinion by the thrust force component developed at the other gear set on the respective planet pinion. Therefore, substantially no unbalanced net thrust force component is present on any planet pinion that requires a reaction force to place the pinion in structural equilibrium, and no provision for a reaction force is provided at the pinions  26 . 
         [0042]    Similarly, any overturning moment that is induced by the thrust force component transmitted to each gear set  50 ,  52  of pinions  26  is cancelled within the respective pinion by the overturning moment transmitted to other gear set on the respective planet pinion. Therefore, substantially no unbalanced net overturning moment is applied to any planet pinion  26  that would require a reaction moment to place the pinion in structural equilibrium. Consequently, the bearing that supports a pinion  26  on the pinion shaft  30  has virtually no end loading due to an unbalanced overturning moment on the pinion. 
         [0043]    Sun gear  20  and ring  48  are alternately held on case  12  against rotation and released by a hydraulically-actuated brake  120 , which is actuated by a servo  122  that includes a cylinder  124 , a piston  124  located in the cylinder, return spring  128  for restoring the piston to the disengaged position of  FIG. 1 , and a hydraulic line  130 , which alternately pressurizes and vents cylinder. The piston forces friction discs  132 , secured to a cylinder  134 , ring  48  and sun gear  20 , and pressure plates  136 , secured to case  12 , into mutual frictional contact when the brake  120  is engaged by hydraulic pressure in cylinder  124 . When the brake  120  is disengaged, spring  128  forces piston  126  leftward permitting discs  132  and pressure plates  136  to disengage mutually. 
         [0044]    Similarly a drive connection of sun gear  20  and ring  48  to another component  138  of transmission  10  can be opened and closed alternately by a hydraulically-actuated clutch  140 , which is actuated by a servo  142  that includes the cylinder  134 , a piston  146  located in the cylinder, return spring  148  for restoring the piston to the disengaged position of  FIG. 1 , and a hydraulic line  150 , which alternately pressurizes and vents cylinder  144 . The piston forces friction discs  152 , secured to component  138 , and pressure plates  154 , secured to ring  48  and sun gear  20 , into mutual frictional contact when the clutch  142  is engaged by hydraulic pressure in cylinder  134 . When clutch  142  is disengaged, spring  148  forces piston  146  leftward, permitting discs  152  and pressure plates  154  to disengage mutually. 
         [0045]    In accordance with the provisions of the patent statutes, the preferred embodiment has been described. However, it should be noted that the alternate embodiments can be practiced otherwise than as specifically illustrated and described.