Patent Publication Number: US-6338691-B1

Title: Gearing for power sharing in planetary transmission

Description:
This application is a divisional application under 37 C.F.R. §1.53 (b) of prior application Ser. No. 09/354,981 filed Jul. 16, 1999, now U.S. Pat. No. 6,179,743. The disclosures of the specification, claims, drawings and abstract of application Ser. No. 09/354,981 are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention is directed to improvements in planetary transmissions. More particularly, the present invention is directed to planetary gear transmissions having multiple planetary gear sets employing helical cut gears for providing power sharing amongst the planetary gear sets. 
     BACKGROUND OF THE INVENTION 
     Planetary gear trains have the advantage over pinion type gearing by permitting higher power densities, large gear ratios, and concentric power input and output. Increased power requirements in planetary gear trains are usually accommodated by increasing the diameter and width of the gears. If there are restrictions on the diametrical size of the gear train, increases in power can be met only by increasing the width of the gears or upping the material and machining specifications. There are practical limits to both these approaches. 
     An apparent solution for increasing power capacity within a limited diametric size is to add more gear sets to the train so that power is shared between more than one gear set. This results in a lesser load for each gear set but a higher total power capacity. However, there are severe practical problems with such a solution. This solution requires nearly perfect power sharing among the several gear sets. Such perfect power sharing among the gear sets would require manufacturing tolerances for the gears which are not practical for the vast majority of commercial applications. Practical manufacturing tolerances for the gears would result in uneven power sharing. Such uneven power sharing or uneven loading results in one set of gears being loaded more heavily than its designed for. This results in excessive wear and/or premature failure. 
     Gear transmissions having pairs of helical gears mounted on a drive shaft for engagement with respective pairs of helical gears mounted on a driven shaft are disclosed in copending U.S. patent application Ser. No. 09/167,760 filed Oct. 7, 1998 entitled Improvements In Power Sharing Gear Sets. The disclosures, including the disclosures of the specification and drawings, of prior U.S. patent application Ser. No. 09/167,760 filed Oct. 7, 1998, are hereby expressly incorporated by reference into this present application. The use of paired helical gears in multi-speed automotive transmissions is disclosed in U.S. patent application Ser. No. 09/187,905 filed Nov. 6, 1998 entitled Multi-Speed Automotive Transmission Using Paired Helical Gearing. The disclosures, including the disclosures of the specification and drawings, of prior U.S. patent application Ser. No. 09/187,905 filed Nov. 6, 1998 are hereby expressly incorporated by reference into this present application. 
     OBJECTS OF THE INVENTION 
     It is an object of the present invention to provide a planetary transmission providing a balanced load or balanced power sharing between two or more planetary gear sets. 
     It is also an object of the present invention to provide a planetary transmission which is compact in diametrical size but has high power capacity. 
     It is a further object of the present invention to provide a planetary transmission using helical gears for providing a balanced load or balanced power sharing between two or more planetary gear sets. 
     These and other objects of the present invention will become apparent from the following description and claims read in conjunction with the drawings. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a planetary gear transmission having multiple planetary gear sets employing helical cut gears. 
     Each planetary gear set in the gear train comprises a sun gear mounted on a sun shaft with the sun gear engaging a plurality of planet gears mounted in a planet gear carrier with the plurality of planet gears engaging a ring gear mounted on the transmission housing. The planet gear carrier may be the drive member with the sun shaft being the driven member. Conversely, the sun shaft may be the drive member with the planet gear carrier being the driven member. 
     The planetary gear transmission of the present invention employs helical cut gears for the sun gears, ring gears, and planet gears of the planetary gear sets to obtain practical balanced power sharing and load between two or more planetary gear sets in the gear train. 
     Helical gears, due to the helical angle of the gear cut, experience axial thrust when loaded. The magnitude of this axial thrust is directly proportional to the torque load on the gear. Power sharing and balanced load between planetary gear sets of the planetary gear transmission of the present invention is achieved by employing this axial thrust reaction and the resulting axial movement of sun gears and/or ring gears of adjacent planetary gear sets wherein sun gears and/or ring gears of adjacent planetary gear sets are mounted for axial movement with respect to the planetary transmission housing. If one of the planetary gear sets is more heavily loaded than the others, the axial thrust on the helical sun gear and helical ring gear of that set is not balanced with the axial thrust loads on the other planetary gear sets. The sun gear and/or the ring gear that is more heavily loaded, and thus experiencing a greater axial thrust load, moves axially in response to this thrust imbalance so as to achieve equal load sharing between planetary gear sets and no axial thrust imbalances. 
     The planet gears do not move axially during the operation of the planetary gear transmission. The axial thrust on a given planet gear due to interaction with a respective sun gear is equal and opposite to the axial thrust due to interaction with a respective ring gear. Therefore, the axial forces acting on a planet gear are equal and opposite resulting in no tendency for planet gears to move axially. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings forming part hereof: 
     FIG. 1 is an illustrative schematic side elevation view of a planetary gear transmission in accordance with one embodiment of the present invention with parts removed for purposes of clarity of understanding. 
     FIG. 2 is a schematic cross-sectional view along line  2 — 2  of FIG.  1 . 
     FIG. 3 is a schematic cross-sectional view along line  3 — 3  of FIG.  1 . 
     FIG. 4 a  is an illustrative schematic side elevation view of the planetary gear transmission in accordance with the embodiment of FIG. 1 with additional parts removed illustrating gear locations prior to start up. 
     FIG. 4 b  is an illustrative schematic side elevation view of the planetary gear transmission in accordance with the embodiment of FIG. 1 with additional parts removed illustrating gear locations at equilibrium load sharing. 
     FIG. 5 is an illustrative schematic side elevation view of a planetary gear transmission in accordance with another embodiment of the present invention similar to that of FIG. 1 with parts removed for purposes of clarity of understanding illustrating a planetary transmission with two helical gear sets. 
     FIG. 6 is a partial schematic perspective view of a planet gear carrier removed from the transmission housing. 
     In order to provide a more complete understanding of the present invention and an appreciation of its advantages, a detailed description of preferred embodiments is provided below. 
    
    
     DETAILED DESCRIPTION 
     For a detailed explanation of the forces acting upon pairs of helical cut gears and load sharing and load balancing amongst pairs of helical cut gears, reference is made to the previously identified U.S. patent application Ser. No. 09/167,760 filed Oct. 7, 1998 entitled Improvements In Power Sharing Gear Sets, the disclosure of which is incorporated herein by reference. 
     Referring to FIG. 1 of the drawings, a planetary gear train with four planetary gear sets is illustrated in accordance with one embodiment of the present invention. It should be understood that the present invention may be practiced with two planetary gear sets or a plurality of planetary of planetary gear sets. 
     With reference to FIG. 1, planet gear carrier  4  is mounted by bearings  6   a , which may be, e.g., needle type bearings or roller bearings, in cylindrical transmission housing  7 . Planet gear sets  2   a ,  2   b ,  2   c , and  2   d  each comprise planet gears which are mounted for rotation in planet gear carrier  4 . Planet gear carrier  4  holds planet gear sets  2   a ,  2   b ,  2   c , and  2   d  in fixed relationship to one another. All the planet gears are free to turn or rotate independently of one another. With further reference to FIG. 3, in the illustrated embodiment, each planet gear has two planet shafts  12  which are received by a respective bore in planet gear holder  4  and the rotational mounting is accomplished by means, e.g., of journal bearings  6   d . In one practical embodiment, each planet gear and its associated planet gear shafts  12  would be an integral member machined from a common stock material. 
     Planet gears  2   a ,  2   b ,  2   c  and  2   d  all have helical cut gear teeth. The helical cut of planet gears  2   a  (four in number as shown in the embodiment illustrated in FIG. 2) has a sense or hand and an angle with respect to the axial center line of the gear which is the same for each planet gear  2   a . The helical cut for planet gears  2   b  has a sense or hand and an angle with respect to the axial center line of the gear which is the same for each planet gear  2   b , but which is opposite to the sense or hand and angle of the helical cut of planet gears  2   a . The helical cut for planet gears  2   c  has a sense or a hand and an angle with respect to the axial center line of the gear which is the same for each planet gear  2   c , but which is opposite to the sense or hand and angle of the helical cut of planet gears  2   b . As is apparent, the sense or hand and the angle of the helical cut of planet gears  2   c  are the same as the sense or hand and angle of the helical cut of planet gears  2   a . The helical cut for planet gears  2   d  has a sense or a hand and an angle with respect to the axial center line of the gear which is the same for each planet gear  2   d , but which is opposite to the sense or the hand and angle of the helical cut of planet gears  2   c.    
     Cylindrical transmission housing  7  has a longitudinal central axis  20 . Planet gear carrier  4  has a longitudinal central axis which is parallel to and coincident with the longitudinal central axis  20  of cylindrical transmission housing  7 . The planet gears are mounted for rotation in planet gear carrier  4  by shafts  12  and bearings  6   d  in a manner which substantially prevents, during operation, movement of the planet gears in the direction of the longitudinal central axis of planet gear carrier  4 . As will be apparent, planet gear carrier  4  rotates within cylindrical transmission housing  7  around longitudinal central axis  20 . Planet gear carrier  4  is mounted for rotation in cylindrical transmission housing  4  by bearings  6   a  in a manner which substantially prevents during operation, movement of planet gear carrier  4  in the direction of the longitudinal central axis  20  of cylindrical transmission housing  7 . Cylindrical transmission housing  7  has a first end  21  and a second end  22 . Arm members  9  of planet gear carrier  4 , located adjacent first end  21  of cylindrical transmission housing  7 , is a mechanism which is connected to a power source (not illustrated) for inputting power into planet gear carrier  4  in embodiments where planet carrier  4  is a drive member or, conversely, outputs power from planet gear carrier  4  to a power output unit (not illustrated) in embodiments where planet carrier  4  is a driven member. 
     FIG. 6 is a partial schematic perspective view of planet gear carrier  4  removed from cylindrical transmission housing  7 . 
     Sun shaft  5  has a first end adjacent to the first end  21  of cylindrical transmission housing  7  and is rotatably mounted in planet gear carrier  4  by bearing  6   b , which may be a needle type or roller bearing. Sun shaft  5  is also rotatably mounted in planet gear carrier  4  at a second end of sun shaft  5  adjacent the second end  22  of cylindrical transmission housing  7  by bearing  6   b , which may be a needle type bearing or a roller bearing. In some embodiments, a stop member or stop ring  24  may be mounted on sun shaft  5  at its first end and second end. Sun shaft  5  has a longitudinal central axis which is parallel to and coincident with longitudinal central axis  20  of cylindrical transmission housing  20 . Sun shaft  5  is mounted in planet gear carrier  4  by bearings  6   b  in a manner which substantially prevents, during operation, movement of sun shaft  5  in the direction of the longitudinal central axis of planet gear carrier  4 . If sun shaft  5  is a driven member, power is outputted to a power output unit (not illustrated) at the end of sun shaft  5  adjacent the second end  22  of cylindrical transmission housing  7 . If sun shaft  5  is a drive member, a power source (not illustrated) would be connected to the end of sun shaft  5  adjacent the second end  22  of cylindrical transmission housing  7 . 
     In the illustrated embodiment, sun shaft  5  is a splined shaft. Sun gears  1   a ,  1   b ,  1   c  and  1   d  are mounted on splined sun shaft  5  for movement in the axial direction of sun shaft  5 . 
     As can be seen in FIG. 2, which is a schematic cross-section at  2 — 2  of FIG. 1, sun shaft  5  is a spline shaft having teeth engaging corresponding notches in sun gear  1   b . The arrangement would be similar for sun gears  1   a ,  1   c , and  1   d . As will be appreciated, this spline shaft mounting results in the transmission of rotational movement and rotational power between the sun gears and the sun shaft. That is, the sun gears can drive the sun shaft  5  or the sun shaft  5  can drive the sun gears depending on the mode of operation. The spline shaft mounting also permits axial movement of the sun gears  1   a ,  1   b ,  1   c , and  1   d  on the sun shaft  5 . The illustrated embodiment of the use of a spline shaft for sun shaft  5  is by way of example and not limitation. Other mechanisms may be selected by one skilled in the art to mount the helical cut sun gears on the sun shaft for both transmission of rotational power and for permitting axial movement of the sun gears on the sun shaft, such as being keyed rather than splined. 
     Sun gears  1   a ,  1   b ,  1   c , and  1   d  all have helical cut gear teeth. The helical cut of sun gear  1   a  has a sense or a hand an angle with respect to the sun shaft axis which is opposite to the sense or the hand and the angle of the helical cut of planet gears  2   a  of the first planet gear set. The helical cut of sun gear  1   b  has a sense or a hand and an angle with respect to the sun shaft axis which is opposite to the sense or the hand and the angle of the helical cut of the planet gears  2   b  of the second planet gear set. It will be appreciated by one skilled in the art that the sense or the hand and angle of the helical cut of sun gear  1   a  is opposite to the sense or the hand and angle of the helical cut of sun gear  1   b . The helical cut of sun gear  1   c  has a sense or a hand and an angle with respect to the sun shaft axis which is opposite to the sense or the hand and the angle of the helical cut of planet gears  3   c  of the third planet gear set. It will be appreciated by one skilled in the art that the sense or the hand and the angle of the helical cut of sun gear  1   c  is the same as the sense or the hand and the angle of the helical cut of sun gear  1   a . The helical cut of sun gear  1   d  has a sense or a hand and an angle with respect to the axis of the sun shaft which is opposite to the sense or the hand and the angle of the helical cut of planet gears  2   d  of the fourth planet gear set. 
     The helical teeth of sun gears  1   a ,  1   b ,  1   c , and  1   d  respectively engage the helical teeth of the planet gears  2   a  of the first planet gear set, planet gears  2   b  of the second planet gear set, planet gears  2   c  of the third planet gear set, and planet gears  2   d  of the fourth planet gear set. FIG. 2 illustrates the gear teeth of sun gear  1   b  engaging the gear teeth of the four planet gears  2   b  of the second planet gear set. 
     In the embodiment of the present invention illustrated in FIG. 1, a cylindrical member  10  is disposed between sun gear  1   a  and sun gear  1   b  and a cylindrical member is disposed between sun gear  1   c  and sun gear  1   d . In the embodiment illustrated in FIG. 3, the spline shaft teeth of sun shaft  5  engage corresponding notches in cylindrical member  10 . It is not necessary for cylindrical member  10  to have notches engaging the spline of sun shaft  5 . The interior of cylindrical member  10  could be smooth resting on the spline of sun shaft  5 . Cylindrical member  10  is mounted so that it can move on sun shaft  5  in the axial direction of sun shaft  5 . 
     As will hereinafter be discussed, in one mode of operation, forces created by rotation and the helical cut of the gears cause sun gear la and sun gear  1   b  to tend to move toward one another and to move together on sun shaft  5  and sun gear  1   c  and sun gear  1   d  to tend to move toward one another and to move together on sun shaft  5 . Cylindrical member  10  between sun gear  1   a  and sun gear  1   b  restrains sun gears  1   a  and  1   b  from moving toward one another, while permitting sun gears  1   a  and  1   b  to move together as a unit on sun shaft  5 . Likewise, cylindrical member  10  between sun gear  1   c  and sun gear  1   d  restrains sun gears  1   c  and  1   d  from moving toward one another, while permitting sun gears  1   c  and  1   d  to move together as a unit on sun shaft  5 . By this movement, load is balanced between sun gear  1   a  and sun gear  1   b  and respective planet gears  2   a  of the first planet gear set and planet gears  2   b  of the second planet gear set. Load is also balanced between sun gear  1   c  and sun gear  1   d  and respective planet gears  2   c  of the third planet gear set and planet gears  2   d  of the fourth planet gear set. It will be appreciated that in such a mode of operation or such an embodiment, cylindrical body  10  does not have to be a separate member. Sun gears  1   a  and  1   b  and cylindrical member  10  could be machined from the same stock of material. The same would apply to sun gears  1   c  and  1   d  and associated cylindrical member  10 . 
     As a practical matter, one skilled in the art may find it useful to mount cylindrical member  10  to the planet carrier  4  by bearings  6   c , which may be, e.g., journal bearings. This provided additional support for sun shaft  5 . Ring-like retainer members  24  are shown mounted on the first end and the second end sun shaft  5 . Ring-like retainer members  24  are mounted on the ends of sun shaft  5  in a manner that they are restrained from movement in the axial direction of sun shaft  5 . 
     In the embodiment illustrated in FIG. 1, a cylindrical ring-like member  3   a  is illustrated machined in the interior circumferential surface of the cylindrical transmission housing  7 . The interior circumferential surface of ring-like member  3   a  has a helical gear cut with a helical sense or hand and angle with respect to the longitudinal central axis  20  of cylindrical transmission housing  7  which is opposite to the sense or hand and angle of the helical cut of planet gears  2   a  of the first planet gear set. Thus, ring-like member  3   a  may be said to be a cylindrical shaped ring gear. The structure of cylindrical shaped ring gear  3   a  restrains it from movement in the axial direction toward the first end  21  of cylindrical transmission housing  7 . The structure of cylindrical shaped ring gear  3   a  also prevents it from moving in the circumferential direction of the interior circumferential surface of the cylindrical transmission housing  7 . The gear teeth of ring gear  3   a  engage the gear teeth of the planet gears  2   a  of the first set of planet gears. It will be appreciated that cylindrical shaped ring gear  3   a  could be a separate machined cylindrical member which is connected to the interior circumferential surface of cylindrical transmission housing  7 , e.g., by bolting. 
     In the embodiment illustrated in FIG. 1, cylindrical ring shaped gear  3   d  is similar to cylindrical ring shaped gear  3   a  with the helical cut of cylindrical ring shaped gear  3   d  having a sense or a hand and an angle with respect to the longitudinal central axis  20  of cylindrical transmission housing  7  which is opposite to the sense or the hand and the angle of the helical cut of planet gears  2   d  of the fourth planet gear set. It will be apparent that in the embodiment illustrated in FIG. 1, the sense or the hand and the angle of the helical cut of cylindrical shaped ring gear  3   d  is opposite to the sense or the hand and the angle of the helical cut of cylindrical shaped ring gear  3   a . In the embodiment illustrated in FIG. 1, cylindrical shaped ring gear  3   d  is restrained from axial movement in the direction toward the second end  22  of cylindrical transmission housing  7 . Cylindrical shaped ring gear  3   d  is also restrained from movement in the circumferential direction of cylindrical transmission housing  7 . 
     FIG. 1 further illustrates cylindrical shaped ring gears  3   b  and  3   c . These cylindrical shaped ring gears  3   b  and  3   c  are machined on the interior circumferential surface of a cylindrical unit  8 . 
     The interior circumferential surface of cylindrical shaped ring gear  3   b  has a helical cut with a sense or a hand and an angle with respect to the longitudinal central axis  20  of cylindrical transmission housing  7  which is opposite to the sense or the hand and angle of the helical cut of planet gears  3   b  of the second planet gear set. It will be appreciated that the sense or the hand and the angle of the helical cut of cylindrical shaped ring gear  3   b  is opposite to the sense or the hand and the angle of the helical cut of cylindrical shaped ring gear  3   a.    
     The interior circumferential surface of cylindrical shaped ring gear  3   c  has a helical cut with a sense or a hand an angle with respect to the longitudinal central axis  20  of cylindrical transmission housing  7  which is opposite to the sense or the hand and the angle of the helical cut of planet gears  3   c  of the third planet gear set. It will be appreciated that the sense or the hand and the helical cut of cylindrical shaped ring gear  3   c  is opposite to the sense or the hand and the angle of cut of cylindrical shaped ring gear  3   d  and cylindrical shaped ring gear  3   b.    
     Cylindrical unit  8 , along with integral cylindrical shaped ring gear  3   b  and cylindrical shaped ring gear  3   c , in the embodiment illustrated in FIG. 1, is mounted on the interior surface of cylindrical transmission housing  7  for axial movement in the direction of the longitudinal central axis  20  by a splined mounting. As illustrated in FIG. 2, the splined teeth of the outer surface of cylindrical member  8 , including cylindrical shaped ring gears  3   b ,  3   c , engage splined teeth on an interior surface of cylindrical transmission housing  7 . That is, the outer surface of the cylindrical wall of cylindrical unit  8  is fitted with a spline surface. The inner cylindrical wall of cylindrical transmission housing  7  adjacent cylindrical unit  8  is fitted with a mating spline surface. The splined connection permits axial movement of cylindrical unit but prevents circumferential movement of cylindrical unit  8  with respect to cylindrical transmission housing  7 . 
     In the embodiment wherein rotation and the helical cut creates forces forcing cylindrical shaped ring gear  3   b  and cylindrical shaped ring gear  3   c  to tend to move toward one another, the portion of cylindrical unit  8  located between cylindrical shaped ring gear  3   b  and cylindrical shaped ring gear  3   c  transmits these forces in a direction parallel to the longitudinal central axis  20  and restrains ring gear  3   b  and ring gear  3   b  from moving together. As will hereinafter be discussed, in the embodiment wherein forces force ring gears  3   b ,  3   c  to tend to move toward one another, cylindrical shaped ring gear  3   b  and cylindrical shaped ring gear  3   c  move together in the direction of longitudinal central axis  20  on the splined mounting to balance load transmission between ring gear  3   b  and ring gear  3   c  and respective planet gears  2   b  of the second planet gear set and planet gears  2   c  of the third planet gear set. 
     It will be appreciated that cylindrical shaped ring gear  3   b  and cylindrical shaped ring gear  3   c  may each be a separate machined cylindrical member splined to the interior circumferential surface of cylindrical transmission housing  7  for axial movement in the direction of longitudinal central axis  20 . In such a case, in the embodiment illustrated in FIG. 1, a separate machined cylindrical element would be disposed between separate cylindrical ring shaped gear  3   c  and separate cylindrical ring shaped gear  3   d . This separate machined cylindrical element would also be splined to the interior circumferential surface of cylindrical transmission housing  7  for axial movement in the direction of longitudinal central axis  20 . It will also be appreciated that other mechanisms, in addition to splining, could be provided to mount cylindrical shaped ring gears and intermediate members to the interior circumferential surface of cylindrical transmission housing  7  for axial movement in the direction of longitudinal central axis  20  and prevent circumferential movement with respect to the cylindrical transmission housing  7 . 
     Operation of the embodiment illustrated in FIG. 1 will be explained for the embodiment where rotational power is inputted in the clockwise direction at arm members  9  of planet gear carrier  4  and rotational power is outputted by sun shaft  5  at second end  22  of cylindrical transmission housing  7 . In this embodiment, the transmission would be a speed increase transmission. 
     Rotation of planet gear carrier  4  in the clockwise direction by a rotational power input source (not illustrated) would cause planet gear sets  2   a ,  2   b ,  2   c  and  2   d  to rotate in respective cylindrical shaped ring gears  3   a ,  3   b ,  3   c  and  3   d  and further cause rotation of each planet gear of the planet gear sets. Cylindrical shaped ring gears  3   a ,  3   b ,  3   c  and  3   d , engaged with respective planet gears of planet gear sets  2   a ,  2   b ,  2   c  and  2   d , do not rotate because cylindrical shaped ring gears  3   a ,  3   b ,  3   c  and  3   d  are mounted to be restrained from rotation in the circumferential direction of cylindrical transmission housing  7 . 
     Rotation of the planet gears of planet gear sets  2   a ,  2   b ,  2   c  and  2   d , engaging sun gears  1   a ,  1   b ,  1   c  and  1   d , results in rotation of sun gears  1   a ,  1   b ,  1   c  and  1   d.    
     Rotation of sun gears  1   a ,  1   b ,  1   c , and  1   d , splined to sun shaft  5 , results in rotation of sun shaft  5  and the transmission of rotational power or torque to sun shaft  5 . In this embodiment, sun shaft  5  is a driven shaft. 
     Load sharing or load balancing amongst the gears occurs as follows. Helical gears, due to the angle of the helical cut, experience axial thrust when loaded. The magnitude of this axial thrust is directly proportional to the torque load on the gear. Power sharing between planetary sets results from this thrust reaction and consequent axial movement of sun gears and ring gears. That is, if one planetary set is more heavily loaded than the others, the axial thrust on the helical sun gear and the helical ring gear of that set is not balanced with the thrust loads on the helical sun gear and helical ring gear of the other planetary sets. The sun gear and/or the ring gear that is more heavily loaded moves axially in response to this load imbalance, as described in detail below, to ultimately result in load balancing or load sharing. 
     An example of load balancing or load sharing, in accordance with the present invention, is as follows. If due to manufacturing tolerances, the gears of planetary set “a” engage before the gears of the other three planetary sets of the embodiment illustrated in FIG. 1, the torque load it experiences results in an axial thrust load on both sun gear  1   a  and cylindrical shaped ring gear  3   a . In the above described embodiment, where rotational power is inputted to planet gear carrier  4  in the clockwise direction, the thrust load on sun gear  1   a  is directed to the second end  22  of cylindrical transmission housing  7  and the thrust load on ring gear  3   a  is directed to the first end  21  of cylindrical transmission housing  7 . Since planetary set “b” is not experiencing as much load, there is not an equal and opposite thrust axial thrust on sun gear  1   b . The imbalance in axial thrust, due to the imbalance in load, causes sun gear  1   a  to move on sun shaft  5  toward the second end  22  of cylindrical transmission housing  5 . Sun gear  1   a  thus pushes sun gear  1   b  in the axial direction on sun shaft  5  via cylindrical member  10  toward second end  22  of cylindrical transmission housing  5 . This axial movement of sun gear  1   a  due to axial thrust resulting from the helical gear cut results in sun gear  1   a  and hence the entire planetary set “a” to become less loaded, while at the same time forces sun gear  1   b , and hence the entire planetary set “b”, to become more loaded. As sun gear  1   b  becomes loaded, the helical cut on sun gear  1   b  results in an axial thrust on sun gear  1   b  in the direction toward the first end  21  of cylindrical transmission housing  7 . 
     As sun gear  1   b  becomes more loaded, the torque transmitted by sun gear  1   b  to ring gear  3   b  via planet gears  2   b  increases the load on ring gear  3   b . The helical cut on ring gear  3   b  results in an axial thrust on ring gear  3   b  proportional to the loading in the axial direction toward the second end  22  of cylindrical transmission housing  7 . If planetary set “c” is not yet loaded or is less loaded that planetary set “b”, the axial thrust exerted by ring gear  3   b  forces cylindrical unit  8 , and thereby ring gear  3   c , to axially move, along with ring gear  3   b , toward the second end  22  of cylindrical transmission housing  7 . This results in ring gear  3   c  becoming more loaded. 
     As ring gear  3   c  becomes more loaded, it transmits more load, via planet gears  2   c , to sun gear  1   c . Also as ring gear  3   c  becomes more loaded, it develops more axial thrust in the direction toward the first end  21  of cylindrical transmission housing  21 . 
     As sun gear  1   c  becomes more loaded, it exerts a greater thrust toward the second end  22  of cylindrical transmission housing  7 . If sun gear  1   d  is not loaded or less loaded than sun gear  1   c , sun gear  1   c  will move on sun shaft  5  in the axial direction toward second end  22  of cylindrical transmission housing  7 . Sun gear  1   c  thus pushes sun gear  1   d  via cylindrical member  10  toward the second end  22  of transmission housing  7 . This causes sun gear  1   d  to become more loaded and sun gear  1   c  to become less loaded. 
     As sun gear id becomes more loaded, it transmits more load to ring gear  3   d  via planet gears  2   d . In addition, as sun gear  1   d  becomes more loaded it exerts a greater axial thrust in the direction toward the first end  21  of cylindrical transmission housing  7 , with the axial thrust exerted by sun gear  1   d  again being proportional to the load on sun gear  1   d.    
     In the described embodiment illustrated in FIG. 1, the axial thrust developed by ring gear  3   a  will tend to cause ring gear  3   a  to move toward to the first end  21  of cylindrical transmission housing  7 . In the illustrated embodiment of FIG. 1, ring gear  3   a  does not move toward the first end  21  of cylindrical transmission housing  7  because ring gear  3   a  is an integral machined member on the interior circumferential surface of the cylindrical transmission housing  7 . It will be appreciated that cylindrical shaped ring gear  3   a  could be, for example, a separate cylindrical ring shaped member splined to the interior circumferential surface of cylindrical transmission housing  7  for axial movement in the direction of longitudinal central axis  20 . In this instance, a stop member would be provided to restrain axial movement of such a ring gear  3   a  in the axial direction toward first end  21  of cylindrical transmission housing  7 . 
     In the described embodiment illustrated in FIG. 1, the axial thrust developed by ring gear  3   d  will tend to cause ring gear  3   d  to move toward the second end  22  of cylindrical transmission housing  7 . In the illustrated embodiment of FIG. 1, ring gear  3   d  does not move toward the second end  22  of the cylindrical transmission housing  7  because ring gear  3   d  is also an integral machined member on the interior circumferential surface of the cylindrical transmission housing  7 . As with cylindrical shaped ring gear  3   a , it will be appreciated that cylindrical ring gear  3   d  could be, for example, a separate cylindrical ring shaped member splined to the interior circumferential surface of cylindrical transmission housing  7 , with a stop member provide to restrain axial movement of such a ring gear  3   d  toward the second end  22  of cylindrical transmission housing  7 . 
     The above-described loading and thrusts, with the thrusts being proportional to the loading, continues until sun gears  1   a  and  1   b , ring gears  3   b  and  3   c , and sun gears  1   c  and  1   d  move as pairs in the axial direction so as to balance load transmitted by all gears. The load transmission amongst gears is self balancing and self compensating. 
     As can be seen from the above description, any imbalance in the torque among the planetary sets results in an imbalance in lateral thrust of the various sun gears and ring gears. This imbalance in thrusts results in these gears moving in the axial direction in response to the direction of the imbalance of thrust. This axial movement will continue until all the thrusts are balanced. When the thrust forces are balanced, the torques or loads transmitted amongst the gears are also balanced. 
     The planet gears do not move axially during the operation of the transmission. The thrust on a planet gear due to interaction with a respective sun gear is equal and opposite to the thrust due to interaction with a respective ring gear. Hence, the axial forces acting on a planet gear are equal and opposite to one another. This results in no tendency for the planet gears to move in the axial direction. 
     FIG. 4 a  is a schematic illustration of the described embodiment of FIG. 1 illustrating an example of gear locations prior to start up or prior to rotating planet gear carrier  4 . FIG. 4 b  is a schematic illustration of the described embodiment of FIG. 1 illustrating an example of gear locations after start up when equilibrium load sharing or balanced load amongst gears has been achieved. It will be appreciated that a plurality of planetary gear sets employing the principles of the present invention may be used as dictated by the design criteria for the transmission. 
     FIG. 5 is a schematic illustration of an embodiment similar to FIG. 1 wherein there are only two planetary gear sets. The principles of operation of the present invention would be the same. 
     If sun shaft  5  were the drive shaft and rotational power was inputted to sun shaft  5  by a power source (not illustrated) to rotate sun shaft  5  in the counterclockwise direction, operation would take place as previously described in connection with FIG. 1 wherein planet gear carrier  4  is rotated in the clockwise direction by a power source. If sun shaft  5  is the drive shaft and planet carrier  4  is the driven member, the transmission becomes a speed reducing transmission. 
     If in the embodiment illustrated in FIG. 1, planet gear carrier  4  was rotated by a power source in the counterclockwise direction, i.e., the input torque rotational direction of planet gear carrier  4  is counterclockwise, the power sharing characteristics, in accordance with the present invention, would be the same. The following described modification would be made to the embodiment illustrated in FIG.  1 . 
     If planet gear carrier  4  was rotated in the counterclockwise direction by the outside power source, in the embodiment of FIG. 1, the thrusts created by the helical cut on the gears would cause sun gears  1   a  and  1   b  to tend to move apart in the axial direction of sun shaft  5  and cause sun gears  1   c  and  1   d  to tend to move apart in the axial direction of the sun shaft  5 . Likewise, sun gear  1   b  and sun gear  1   c  would tend to move toward one another in the axial direction of the sun shaft  5 . Thus, cylindrical member  10 , or some other type of spacer, would be placed between sun gear  1   b  and sun gear  1   c.    
     In the embodiment of FIG. 1 wherein planet carrier  4  is rotated in the counterclockwise direction, ring gears  3   a  and  3   b  would tend to move together in the axial direction of the cylindrical transmission housing and ring gears  3   c  and  3   d  would tend to move together in the axial direction of the cylindrical transmission housing. In such an embodiment, ring gears  3   a  and  3   b  could be a spline mounted unit such as the spline mounted ring gear unit  3   b ,  8 ,  3   c  illustrated in FIG.  1 . Likewise, in such an embodiment, ring gears  3   c  and  3   d  could be such a spline mounted unit. Alternatively, each of cylindrical shaped ring gears  3   a ,  3   b ,  3   c  and  3   d  could be a separate cylindrical member each splined to the interior circumferential surface of cylindrical transmission housing  7  with a spacer member disposed between ring gears  3   a  and  3   b  and another spacer member disposed between ring gears  3   c  and  3   d.    
     In the embodiment of FIG. 1 wherein planet carrier  4  is rotated counterclockwise, a stop member  24  would be mounted at the end of sun shaft  5  adjacent the first end  21  of cylindrical transmission housing  7  to restrain axial movement of sun gear  1   a  in the axial direction toward the first end  21  of cylindrical transmission housing  7  and maintain sun gear  1   a  in engagement with planet gears  2   a . Similarly, a stop member  25  would be provided at the second end of sun shaft  5  to restrain axial movement of sun gear  1   d  toward the second end  22  of cylindrical transmission housing  7  . 
     As previously stated, if planet carrier  4  was rotated in the counterclockwise direction, the power sharing characteristics and principles of operation of the present invention would be the same as previously described. 
     In practical embodiments, the distance of axial movement of sun gears or ring gears in the practice of the present invention would be, for example, about 0.1 mm to 1 mm. 
     It will be appreciated that one skilled in the art will be able to devise numerous mechanical variations employing the principles of the present invention described herein. 
     The present invention provides for a commercially practical, cost-effective planetary gear transmission having multiple planetary gear sets in the gear train by employing helical cut gears. The helical cut gears used in the planetary gear transmission of the present invention need only have commercially practical, cost-effective manufacturing tolerances. 
     Although preferred embodiments of the present invention have been described in detail, it is apparent that modifications may be made by those skilled in the art within the spirit and the scope of the present invention as defined in the claims.