WO2013043822A2

WO2013043822A2 - Compact planetary gear system with stiffened carrier

Info

Publication number: WO2013043822A2
Application number: PCT/US2012/056259
Authority: WO
Inventors: Gerald P. Fox
Original assignee: The Timken Company
Priority date: 2011-09-22
Filing date: 2012-09-20
Publication date: 2013-03-28
Also published as: WO2013043822A3

Abstract

A planetary gear system (A) includes a sun gear (2), a ring gear (4), planet gears (6), and a straddle-type carrier (8) having two end walls (12, 14) and pockets (18) between them. The planet gears form part of the planet assemblies (B) that are installed as units in the pockets where the planet gears rotate in engagement with the sun and ring gears. Each planet assembly includes a pin (36) about which the planet gear rotates on a bearing (46) that is integrated into the pin and planet gear in that its raceways (48, 50) form surfaces on the planet gear and pin, thus allowing for a planet gear of reduced pitch diameter. The pin is clamped tightly between the two end walls by a cap screw (30) that extends through the pin, and this serves to further stiffen the carrier and resist carrier wind-up.

Description

COMPACT PLANETARY GEAR SYSTEM WITH STIFFENED CARRIER

CROSS REFERENCE TO RELATED APPLICATIONS

This application derives priority from and otherwise claims the benefit of U.S. provisional application 61 /537,841 filed September 22, 201 1 , which is incorporated herein by reference.

BACKGROUND ART

This invention relates in general to a planetary gear systems and, more particularly, to a planetary gear system that utilizes a straddle-type carrier and to assemblies for use in the gear system.

Planetary transmissions have the capacity to transmit large amounts of power in small packages, that is to say, they have high power densities. Even so, in some applications, such as in the transmissions of wind turbines, it is desirable to have even smaller packages.

The typical planetary transmission, which is organized about a main axis, includes a sun gear, a ring gear surrounding the sun gear, and planet gears located between and engaged with the sun and ring gears where they rotate about offset axes that are parallel to the main axis. In addition, the typical transmission has a carrier that establishes the offset axes about which the planet gears rotate. Irrespective of whether the carrier rotates about the main axis or remains fixed, it is subjected to torque when power is transmitted. A straddle-type carrier (as opposed to a single-wall carrier) has a pair of end walls and webs connecting the walls such that they create pockets in which the planet gears are positioned. Torque is applied to the carrier at one of the walls through a hub or some other device on that wall.

The planet gears rotate in the pockets of the carrier on antifriction bearings that are supported on pins that in turn extend between the end walls of the carrier. Several bearing arrangements currently exist, each with its own deficiencies. In one bearing arrangement (nonintegrated) both the inner and outer races are discrete components, the inner races, such as cones, being fitted over the pins and the outer races, such as cups, being fitted into the planet gears. The rolling elements roll along opposed raceways on the inner and outer races. The torque must transfer through the pins to the planet gears, and hence the pins must be thick enough in cross section to withstand the forces imposed by the torque. Likewise, the planet gears require a rim thickness sufficient to provide the strength required to transfer the torque. This type of bearing arrangement, with its separate inner and outer races, consumes considerable space. It may be used with a single-piece carrier or a split carrier in which the two end walls are on separate carrier sections.

In another bearing arrangement (semi-integrated) the outer races are integrated into the planet gears, in that their raceways are actually surfaces on the planet gears. Yet the inner raceways remain on separate inner races that fit over the pins. This bearing arrangement enables the planet gears to assume a lesser diameter, which in turn reduces the size of the planetary system. See US 2003/0123984. Even so, the separate inner races require that pin diameter must remain large enough to enable the pin to withstand the forces generated in the torque transfer. Each pin in the carrier has one end mechanically connected to one of the end walls and its other end capable of floating in the other end wall in the sense that, while it may be press-fitted into the other end wall, no mechanical connection exists between that end and the end wall. The semi-integrated bearing arrangement may be used in either a single-piece or split carrier.

Still another bearing arrangement (fully integrated) has its outer raceway as surfaces on the planet gears and its inner raceways as surfaces on the pins. See US 6,770,007. This arrangement enables the outer races to assume a lesser diameter and the pin to assume a larger diameter. Thus, the planetary system can transmit high torque while occupying minimum space.

All three bearing arrangements leave the planetary systems of which they are a part susceptible to carrier wind up. In this regard, irrespective of the bearing arrangement, torque is applied to the carrier at one end wall but passes to the pins and planet gears through both end walls. It takes two paths through the carrier - one directly through the end wall to which it is applied and the other through the webs and the other end wall. The former is stiffer then the latter, and as a consequence, the second end wall tends to lag the first end wall, that is to say displaces angularly with respect to the first end wall. This skews the pins and disrupts the mesh between the planet gears and the sun and ring gears. To compensate for carrier wind up many systems build in lead correction and modify gear profiles.

The reduction of pitch diameter for the planetary gears is oftentimes limited by design criteria placed on the components that support the planetary gears, such as required bearing rating, bearing contact stress, bending stress in the center of the pin, and minimum required gear rim thickness. Reduction of planetary gear diameter is made especially difficult when a solid planetary carrier is used because of the need to insert each pin through the carrier end walls and bearing inner races after its planet gear and bearing have been inserted into the carrier pockets and aligned with holes in the two end walls of the carrier. BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a perspective view, partially broken away and in section, of a planetary gear system constructed in accordance with and embodying the present invention;

Fig. 2 is a partial sectional view of the carrier assembly for the system and showing the carrier, one of the planet gears, the pin about which it rotates, and its bearing;

Fig. 3 is a perspective view of the carrier;

Fig. 4 is a sectional view of a planet assembly forming part of the system with its bearing partially exploded;

Fig. 5 is a perspective view of a separator for one of the rows of the bearing and two of the rollers that it separates; and Fig. 6 is a fragmentary end view of the bearing showing rollers spaced apart with separators, with two of the separators configured to accommodate insertion of the last roller. BEST MODES FOR CARRYING OUT THE INVENTION

Referring now to the drawings, a planetary or epicyclic gear system A (Fig. 1 ), which is organized about a main central axis X, has a high power density, that is to say it transmits a large amount of power in a highly compact configuration. The planetary system A includes a sun gear 2 located along the axis X, a ring 4 surrounding the sun gear 2 with its axis coincident with the axis X, and planet gears 6 located between and engaged with the sun gear 2 and ring gear 4 for rotating about offset axes Y that lie parallel to the main axis X. In addition, the system A includes a carrier 8 that surrounds the axis X and holds the planet gears 6, establishing the axes Y about which they rotate. Typically, one of the sun gear 2, the ring gear 4, or the carrier 8 remains fixed while the other two rotate. In any event, all three are subjected to torque as the system A transmits power. The system A is ideally suited for use in wind turbines to effect an increase in angular velocity between a rotor, which carries vanes against which the wind impinges, and an electrical generator. When so employed, the ring gear 4 typically remains fixed, the carrier 8 is coupled to and rotated by the wind-driven rotor, and the sun gear 2 delivers power through a shaft 10 to the generator at a greater angular velocity.

The carrier 8 possesses (Figs. 2 & 3) a straddle configuration in that it has two end walls 1 2 and 14 that are spaced apart to accommodate the planet gears 6, which rotate between them. The carrier 8 also includes webs 16 that extend between and connect the two end walls 1 2 and 14, and, while maintaining a fixed spacial relationship between the end walls 12 and 14, they provide pockets 18 which receive the planet gears 6. However, the planet gars 6 project out of the pockets 18 between the webs 16 to engage the sun gear 2 and ring gear 4. The end wall 1 2 has a hub 20 or a flange or some other device on it for applying torque to the carrier 8. At each pocket 18 in the carrier 8 the end walls 12 and 14 have planar inside faces 21 that are machined perpendicular to the offset axis Y for the pocket 18 and are located, within set tolerances, a prescribed distance apart. At the machined inside faces 21 the end walls 12 and 14 have bores 22 that pass through the walls 12 and 14 and align with each other along the axes Y.

The bores 22 at each pocket 18 contain stub shafts 24 that project beyond the inside faces 21 of the side walls 1 2 and 14 and thus into the pocket 18. At their opposite ends the stub shafts 24 have flanges 26 that bear against the outside surfaces of the end walls 12 and 14. The stub shafts 24 contain through bores 28 that likewise align along the axis Y that passes through the pocket 18. The bores 28 for the stub shafts 24 in the wall 1 2 are threaded, whereas the bores 28 in the stub shafts 24 for the end wall 14 are smooth, although that may be reversed. The stub shafts 24 fit tightly into the bores 22 in the end walls 12 and 14, and this may be achieved by manufacturing them slightly oversize for the bores 22, then reducing their temperature to contract them enough to be press-fitted easily into the bores 22, and finally allowing them to reach ambient temperature while in the bores 22. Upon expanding with the rise in temperature, the stub shafts 24 acquire tight interference fits in their bores 22. The aligned through bores 28 in the stub shafts 24 at each pocket 18 receive a high strength cap screw 30 that extends through the smooth bore 28 in the stub shaft 24 at one end of the pocket 18 and engages the threads of the stub shaft 24 at the other end of the pocket 18,. As such, the cap screw 30 passes through the pocket 18 along the offset axis Y for the pocket 1 8. The carrier 8 may be formed with its walls 12 and 14 and webs 16 integral so as exist as a single piece or it may be formed in two pieces, with the walls 12 and 14 on separate pieces. Preferably, it takes the form of a single-piece casting.

Each planet gear 6 forms part of a planet assembly B (Figs. 2 & 4) that occupies the pocket 18 in which the planet gear 6 rotates, and the several planet assemblies B together with the carrier 8, the stub shafts 24 and the cap screws 30 form a carrier assembly C which is a component of the overall planetary system A. In addition, to its planet gear 6, each planet assembly B includes a pin 36 that extends between the two side walls 12 and 14 of the carrier 8 and at its ends is squared off perpendicular to the axis Y to provide end faces 38 along which the pin 36 abuts the inside faces 21 of the side walls 1 2 and 14 Actually, the length of the pin 36, that is the distance between its end faces 38, is slightly less, on the order of 0.10 to 0.040 mm, than the spacing between the inside faces 21 of the carrier 8 when the end walls 12 and 14 are not deflected toward each other. The pin 36 contains a center bore 40 through which the cap screw 30 for the pocket 1 8 extends with radial clearance. The pin 36 also has counterbores 42 that open out of its end faces 38 at the center bore 40 and receive the inwardly projecting ends of the stub shafts 24 for the pocket 18. Indeed, the stub shafts 24 likewise fit into the counterbores 42 with interference fits. Hence, the stub shafts 24 for the pocket 1 8 position the pin 36 - and the planet assembly B of which it is a part -- radially within the pocket 18. When tightened, the cap screw 30 eliminates the initial clearance between the inside faces 21 of the end walls 12 and 14 and the end faces 38 of the pin 36, so that the pin 38 becomes clamped tightly between the end walls 1 2 and 14.

The planet assembly B for each pocket 18 also includes an antifriction bearing 46 that enables the planet gear 6 to rotate about the pin 36 and offset axis Y with minimal friction and further confines that planet gear 6 axially between the two walls 12 and 14 of the carrier 8. Preferably the bearing 46 takes the form of a double row tapered roller bearing that is integrated into both the planet gear 6 and the pin 36 about which it rotates. To this end, the bearing 46 includes two outer raceways 48 that taper downwardly toward each other so that their least diameters are generally midway between the end faces of the planet gear 6. The raceways 48 form surfaces on the planet gear 6 itself and are thus integrated into the gear 6. In addition, the bearing has two inner raceways 50 that are carried by and actually form surfaces on the pin 36 itself, so that they are integrated into the pin 36. The inner raceways 50 lie opposite the outer raceways 48 on the planet gear 6 and taper in the same directions such that the raceways 50 likewise have their least diameters generally midway between the ends of the pin 36. The large end of the inner raceway 50 that is closest to the end wall 12 lies along an outwardly directed thrust rib 52 that leads out to the end face 38 that abuts the wall 12. The rib 52 forms part of the pin 36, and is indeed formed integral with the pin 36. The large end of the inner raceway 50 that is closest to the end wall 14 leads out to a short cylindrical surface 54 which in turn leads into a rabbet 56 that opens out of the end face 38 that abuts the end wall 14. The rabbet 56 contains a rib ring 60 having a thrust rib 62 that projects over the cylindrical surface 54 and presents a rib face along the large end of the inner raceway 50 that leads up to the surface 54, the position of that rib face being determined by the location of a shoulder 64 on the rib ring 62, in that the shoulder 64 abuts the end of the rabbet 56 when the rib ring 60 is properly positioned. The rabbet 56 also contains a spacer 66 that lies between the rib ring 60 and the inside surface 21 of the end wall 14 and is preferably bonded with an adhesive to the rib ring 60. The rib ring 60 and spacer 66 fit into the rabbet 56 with a press fit and are further prevented from rotating in the rabbet 56 by a key 68.

The bearing 46 further includes rolling elements in the form of tapered rollers 70 organized in two rows between the outer raceways 48 and inner raceways 50, there being one row of rollers 70 located along the integral thrust rib 52 and another row along the thrust rib 62 that forms part of the rib ring 60. The spacing between the two rows of rollers 70 determines the setting for the bearing 46, and preferably that setting is a slight preload, so that no internal clearances exist in the bearing 46. This prevents free motion, both axial and radial, between the planet gear 6 and the pin 36, but of course the planet gear 6 can rotate easily on the pin 36 which serves as a journal for the planet gear 6. The raceways 48 and 50 are on apex so that the tapered rollers 70 experience pure rolling contact along them.

The rollers 70 that lie along the integral thrust rib 52 are separated by a cage 72 which may be a stamped metal cage of conventional design. It not only separates the rollers 70 of the row along which it lies, but also retains them around the inner raceway 50 in the absence of the planet gear 6. The row along the thrust rib 62 of the rib ring 60 may contain a full complement of rollers 70, and those rollers 50 may have tribological coatings to retard adhesion between adjacent rollers and otherwise reduce friction. US 6,764,21 9, which is incorporated herein by reference, discloses suitable coatings and procedures for applying them.

In lieu of a full complement of rollers 70, the row along the rib ring 60 may have less than a full complement, with all but one of those rollers 70 being spaced from adjacent rollers 70 by separators 74 (Figs. 5 & 6) that are detached from one another. They can be made from a variety of materials including polymers, sintered steel with or without oil impregnation, and sintered brass, to name a few. Each separator 74 includes a bridge 76 having concave side faces 78 that conform to the tapered side faces of the rollers 68 that they separate. At their ends opposite from the rib ring 60 the separators 74 have flanges 80 (Fig. 5) that project circumferentially beyond the bridges 76 to partially overlie the small end faces of the rollers 70 that they separate. At their ends closest to the rib ring 60 the separators 74 have flanges 82 that also project circumferentially beyond the bridges 76 and overlie the large end faces of the rollers 70 that they separate. The remaining roller 70 - a final roller - is spaced from the two rollers 70 on the sides of it by truncated separators 90 (Fig. 6) that resemble the separators 74, but are truncated in the sense that each has a single flange 82, and that flange 82 does not overlie the large end face of the final roller 70, but does overlie the large end face of the adjacent roller 70.

To assemble the planetary system A, the planet assemblies B are first completed so that they may be installed in the pockets 18 of the carrier 8. The assembly of a planet assembly B begins with its pin 36 preferably placed in a upright position, its integral thrust rib 52 being located downwardly. Thereupon, the rollers 70 for row that runs the raceway 50 at the integral thrust rib 52 are placed over that raceway 50, resting on the thrust rib 52. Next the cage 72 in an expanded condition is placed over the rollers 70. By a pressing operation the cage 72 is deformed to reduce its diameter and thus close it about the rollers 70. When closed, the cage 72 retains the rollers 70 around the raceway 50 at the integral thrust rib 52. With that row of rollers 70 and its cage 72 in place along the integral rib 52, the planet gear 6 is advanced over those rollers 70 until its leading outer raceway 48 seats against the in-place rollers 70. This leaves an annular space between the other outboard raceway 48 and other inboard raceway 50 opposite it, that is to say, the raceways 48 and 50 that lead up to the rabbet 56. The rollers 70 for the other row are inserted into this space. Indeed, this annular space may contain a full complement of rollers 70 or less than a full complement with the separators 74 and 90 utilized to space those rollers 70 apart

Where less than a full complement is utilized along the rabbet 56, the separators 74 and 90 and rollers 70, with the exception of the final roller 70, are installed in such a way that the flanges 82 at the large ends of the separators 74 and 90 overlie the large end faces of the rollers 70 along which they lie. The arrangement of separators 74 and 90 is such that the two separators 90 which have only a single large flange 82 lie next to each other, and provide unobstructed access to the cavity between concave side faces 78 of the bridges 76 on those separators 90. The final roller 70 is inserted into that cavity.

Once the second row of rollers 70 is installed, either as a full complement or with separators 74 and 90, or with some other type of cage arrangement, the rib ring 60 is fitted to the pin 36. The position that the rib face of the thrust rib 62 assumes determines the setting for the bearing 46, whether it be end play or preload. And that position is controlled by grinding the shoulder 64 on the rib ring 60 so that when the shoulder 64 abuts the end of the rabbet 56, the bearing 46 will have the desired setting. Thereupon, a spacer 66 is selected that will occupy the remainder of the rabbet 56, such that when the shoulder 64 of the rib ring 60 abuts the end of the rabbet 56 and the spacer 64 is against the rib ring 60, the exposed end of the spacer 66 will lie flush with the end face 38 of the pin 36. In other words, the width y of the rib ring 60 at its shoulder 64 plus the width z of the spacer 66 should equal the width x of the rabbet 56 (Fig. 4). The correctly sized spacer 66 is then bonded to the rib ring 62. The press fits between the rib ring 60 and spacer 66 and the pin 36 at the rabbet 56, retain the rib ring 60 and spacer 64 in place and thus hold the planet assembly 36 together.

As an alternative, the rib ring 62 may be manufactured with the axial dimension x between its shoulder 64 and end face oversize. Once the shoulder 64 is ground to provide the proper setting for the bearing 46, the opposite end of the rib ring 60 is ground down so that the end lies flush with the end face 38 of the pin 36.

Thereupon, the planet assemblies B are installed in the pockets 18 of the carrier 8 to form the carrier assembly C. To this end, the pin 36 of each planet assembly B is advanced generally radially into its pocket 18 in the absence of the stub shafts 24 and cap screw 30 for that pocket 18. The squared off end faces 38 of the pin 36 move along the machined inside faces 21 on the walls 12 and 14 of the carrier 8 until the counterbores 42 that open out of the end faces 38 align with the bores 22 in the two end walls 12 and 14. Next the stub shafts 24 are press fitted into the bores 22 from the outside surfaces of the walls 12 and 14. To this end, the stub shafts 24 may be cooled to a temperature low enough to enable them to advance easily through the bores 22. Indeed, the stub shafts 24 advance through the bores 22 and then into the counterbores 42 that open out of the ends of the pin 36. Afterwards the temperature of the stub shafts 24 rises to ambient, and the stub shafts 24 expand to fit tightly in the bores 22 and counterbores 42. With the stub shafts 24 projecting into the ends of the pin 36 the end faces 38 of the pin 36 lie along the machined inside faces 21 on end walls 12 and 14 with a slight clearance. Next the cap screw 30 for the pocket 18 is inserted into the through bore 28 in the stub shaft 24 for the wall 14 and is advanced through the center bore 40 in the pin 36. Its threads are engaged with the internal threads in the bore 28 of the stub shaft 24 in the end wall 12. When the cap screw 30 is tightened, it brings the flanges 26 of the stub shafts 24 tightly against the outside faces of the end walls 12 and 14 and deflects the walls 12 and 14 elastically against the end faces 38 of the pin 36. The cap screws 30 maintain the pins 36 of the planet assemblies B compressed firmly between the machined inside faces 21 on end walls 1 2 and 14. The rib ring 60 and spacer 66 lie captured between the end of the rabbet 56 and the inside face 21 of the end wall 14, and this ensures the bearing 46 will remain intact with its setting preserved.

Finally, the sun gear 2 is inserted into the carrier 8 and engaged with the planet gears 6, while the ring gear 4 is installed over and engaged with the planet gears 6.

Irrespective of where power is introduced into the planetary gear system A and delivered from it, the carrier 8 is subjected to torque that is applied at its hub 20. The torque transfers through the system A basically through two paths. In one path the torque passes through the end wall 12 to the pins 36 and thence to the planet gears 6. In the other path torque transfers from the end wall 12 through the webs 16 to the end wall 14 and then to the pins 36 and planet gears 6. The second path is longer than the first and torque transferred in that path seeks to distort the carrier 8 such that the end wall 14 offsets angularly with respect to the end wall 12. This will skew the pins 36 so that the planet gears 6 do not mesh properly with the sun gear 2 and ring gear 4. In a conventional planetary system with a straddle- type carrier only the webs that connect the two end walls resist the tendency to deflect. In the planetary system A, the pins 36 in addition to the webs 16 resist the tendency of the end wall 14 to become angularly offset and lag behind the end wall 12. This derives from the fact that each pin 36 abuts the inside faces 21 of the end walls 12 and 14 and is clamped tightly between the end walls 12 and 14 by the cap screw 30 that passes through it. Despite the presence of the center bore 40 in the pin 36, the pin 36 has a substantial cross-sectional modulus owing to the integration of the bearing 46 with the pin 36, that is to say, the presence of the inner raceways 50 on the pin 36 itself instead of on separate cones or inner races fitted over a pin of considerably reduced diameter. Thus, the pins 36 serve as additional webs to further stiffen the carrier 8 and thereby enable it to better resist torsional wind-up.

Actually, the integration of the inner raceways 50 into the pins 36 allows the inner raceways 50 to assume a smaller diameter, and this affords a reduction in the size of the bearings 46 and the pitch diameter of the planet gears 6 that they support. This in turn permits an overall reduction in the diameter of the planetary system A, resulting in a more compact configuration and having a greater power density.

The planet assemblies B need not and are not assembled in the carrier

8, but are instead assembled at a location, such as a factory, where the components of the bearings 46 may be machined with considerable precision and where the bearings 46 may be properly adjusted. Only then are the planet assemblies B fitted to the carrier 8 to form the carrier assembly C. The installation is quite simple, allowing little opportunity for error. Basically, each planet assembly B is introduced into its pocket 1 8 in the carrier 8 with the end faces 38 of its pin 36 passing along the machined inside surfaces 21 on the end walls 12 and 14 until the counterbores 42 in the pin 36 align with the bores 22 in the end walls 1 2 and 14. Then, the stub shafts 24 are inserted into the bores 22 and advanced through them and into the counterbores 40 of the pin 36. Finally, the cap screw 30 is installed and tightened to clamp the pin 36 tightly between the end walls 12 and 14.

The bearings 46 need not be tapered roller bearings, but may be some other type of antifriction bearing with rolling elements. For example, they may take the form of angular contact ball bearings, spherical roller bearings or cylindrical roller bearings. Also, the cap screws may engage nuts along the end wall 12 instead of threads in the stub shafts 24 for that end wall 12. The carrier 8 may be of unitary or split construction, but the former normally can be produced with less cost and offers the greatest advantage when used with the planet assembly B. In lieu of a single cap screw 30 passing through each pin 36, a separate cap screw may extend through each end wall 12 and 14 and engage threads within the pin 36 itself, so that when turned down they deflect the walls 12 and 14 toward each other and tightly against the end faces 38 of the pin 36, but this arrangement places the pin 36 in tension which is not as effective as the compression imparted by the cap screw 30. The separators 74 and 90 have utility in other types of antifriction bearings, such as cylindrical roller bearings, and in applications other than planetary gear systems.

Claims

CLAIMS:

1 . A carrier assembly for a planetary gear system having a main axis, said carrier assembly comprising:

a carrier located around the main axis and having spaced apart end walls and webs connecting the end walls, the carrier being configured to have torque applied to it at one of its end walls; and

a planet assembly comprising:

a planet gear;

a pin extended through the planet gear along an offset axis and being located tightly between the end walls of the carrier so as to prevent the end walls from displacing axially with respect to the pin; an antifriction bearing located between the pin and the planet gear to enable the planet gear to rotate about the offset axis.

2. A carrier assembly according to claim 1 and further comprising a screw extended through the pin parallel to the offset axis and exerting forces on the end walls to clamp the pin between the end walls.

3. A carrier assembly according to claim 2 and further comprising stub shafts that extend from the end walls into the ends of the pin and locate the planet assembly radially with respect to the main axis.

4. A carrier assembly according to claim 2 wherein the screw exerts forces on the end walls through the stub shafts.

5. A carrier assembly according to claim 4 wherein the screw extends through one of the stub shafts and threads into the other stub shaft.

6. A carrier assembly according to claim 5 wherein the stub shafts have flanges that bear against the outside surfaces of the end walls.

7. A carrier assembly according to claim 3 wherein the stub shafts are displaceable in the end walls along the offset axis so that they can be withdrawn from the pin.

8. A carrier assembly according to claim 1 wherein the antifriction bearing comprises:

at least one outer raceway that is carried by the planet gear; at least one inner raceway presented toward the outer raceway and forming a surface on the pin; and

rolling elements located between and contacting the outer and inner raceways.

9. A carrier assembly according to claim 8 wherein the outer raceway forms a surface on the planet gear.

10. A carrier assembly according to claim 1 wherein the antifriction bearing comprises:

first and second outer raceways carried by the planet gears and tapering downwardly toward each other such that they have their least diameters are where they are closest;

a first inner raceway forming a surface on the pin and presented toward and tapering in the same direction as the first outer raceway; a second inner raceway forming a surface on the pin and presented toward and tapering in the same direction as the second outer raceway;

a first thrust rib carried by the pin at the large end of the first inner raceway;

an initially separate rib ring carried by the pin and providing a second thrust rib at the large end of the second inner raceway;

first tapered rollers located between the first raceways with their large ends against the first thrust rib;

second tapered rollers located between the second raceways with their large end faces against the second thrust rib.

1 1 . A carrier assembly according to claim 10 wherein the first thrust rib is formed integral with the pin, and the pin at the large end of the second inner raceway contains a rabbet which receives the rib ring.

12. A carrier assembly according to claim 1 1 wherein the rib ring has a shoulder from which the second thrust rib projects axially, the shoulder bearing against the end of the rabbet and its location controlling the setting of the bearing.

13. A carrier assembly according to claim 1 2 wherein the rib ring is captured between the end of the rabbet and one of the end walls of the carrier.

14. A carrier assembly according to claim 10 wherein the bearing further comprises discrete separators separating the second tapered rollers in the space between the second tapered raceways, each separator including:

a bridge located between adjacent rollers and having concave side faces that guide the tapered rollers;

first flanges that project circumferentially beyond the bridge and overlie the small ends of adjacent rollers;

at least one second flange that projects circumferentially beyond the side faces of the bridge and overlies the large end of an adjacent roller;

the second flanges on most of the separators projecting beyond both side faces of the bridge and overlying the large ends of adjacent rollers, but at least two of the separators having second flanges that project circumferentially beyond only one side face of the bridge on the separator so that the two separators allow access to a cavity into which a final roller may be inserted before installation of the rib ring during assembly of the bearing assembly.

15. A planetary gear system organized about a main axis; said gear system comprising:

a sun gear located along the main axis;

a ring gear located around the main axis;

a carrier also located around the main axis and having spaced apart end walls and webs connecting the end walls;

a pin clamped tightly between the end walls along an offset axis; a planet gear located around the offset axis and engaged with the sun gear and ring gear;

an antifriction bearing located between the pin and planet gear to enable the planet gear to rotate about the offset axis.

16. A planetary system according to claim 15 wherein the bearing includes raceways that form surfaces on the planet gear and pin and rolling elements between the raceways.

17. A process for assembling the bearing of claim 3, said process comprising:

fitting the planet assembly between the end walls of the carrier;

inserting the stub shafts through the end walls and into the ends of the pin for the planet assembly;

extending the screw through the stub shafts;

tightening the screw to urge the end walls together and tightly against the ends of the pin.

18. An antifriction bearing comprising:

a first raceway;

a second raceway presented toward the first raceway;

an initially separate rib along an end of the first raceway;

rollers located between the first and second raceways and having first and second ends, with the first ends being along the rib and the second ends being presented away from the rib;

first separators located between most of the rollers to separate adjacent rollers, one from the other, each first separator including:

a bridge that lies between the rollers,

first flanges projecting from the bridge and overlying the first ends of adjacent rollers;

second separators separating one of the rollers— a final roller - from its adjacent rollers, each second separator including:

a bridge located between the final roller and an adjacent roller; a first flange overlying the first end of the adjacent roller, but not the first end of the final roller;

a second flange overlying the second ends of the adjacent roller and the second end of the final roller; whereby a cavity unobstructed by first flanges is exposed between the second separators at the rib and receives the final roller.

19. An antifriction bearing according to claim 18 wherein the raceways taper toward each other away from the rib.

20. A process for assembling the bearing of claim 1 8, said process comprising:

installing all of the rollers, except the final roller, between the first and second raceways,

installing the first separators, between adjacent rollers;

installing the second separators along two of the rollers so that the cavity that is unobstructed by first flanges is between the bridges on the second separators;

inserting the final roller into the cavity; and

installing the initially separate rib ring along the end of first raceway.