Abstract:
A bushing for use in a planetary gear system has a cylindrical body portion defining a bore extending along a central axis to be received on a planetary gear shaft. A tab extends axially beyond a nominal body portion of the bushing and is received in a notch in a thrust washer adjacent to the bushing to prevent rotation of the thrust washer. A gear cage and an air turbine starter incorporating the bushing, along with a method of installing the bushing are also disclosed.

Description:
BACKGROUND 
     This application relates to a retention feature for retaining a thrust washer against rotation in a planetary gear system. 
     Planetary gear systems are well known, and have been utilized to provide a gear change between an input and an output. A sun gear rotates about a central axis, and a ring gear rotates about the same center but outwardly of the sun gear. A plurality of planet gears are positioned to transmit rotation from the sun gear to the ring gear. 
     In one known type of planetary gear system, the planet gears are mounted on stationary shafts positioned inwardly of the ring gears. The planet shaft provides an inner race for bearings which support the planet gears. Also, thrust washers sit at both ends of the planet shaft, and provide axial thrust surfaces against the end surfaces of the planet gears. 
     One application of a planetary gear system is in an air turbine starter system. In an air turbine starter system, air is delivered across a turbine rotor to drive the rotor. The rotor drives a sun gear, which drives a ring gear through planet gears. The ring gear in turn drives a starter output shaft for a gas turbine engine. 
     In such applications, the asymmetry and clearances of the planet gears tend to induce an axial force, which reacts against the thrust washers. Those reacted gear forces attempt to rotate the thrust washers, which must remain stationary relative to the fixed housing. The thrust washers serve to provide a designated durable surface for that relative motion and loading. In the prior art, the thrust washers received a simple pin to limit rotation. 
     SUMMARY 
     A bushing for use in a planetary gear system has a cylindrical body portion defining a bore extending along a central axis to be received on a planetary gear shaft. A tab extends axially beyond a nominal body portion of the bushing and is received in a notch in a thrust washer adjacent to the bushing to prevent rotation of the thrust washer. 
     A gear cage and an air turbine starter incorporating the bushing, along with a method of installing the bushing are also disclosed and claimed. 
     These and other features of the present invention can be best understood from the following specification and drawings, the following of which is a brief description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows an air turbine starter assembly. 
         FIG. 2  shows a portion of a gear cage. 
         FIG. 3A  shows a perspective view of a flanged bushing. 
         FIG. 3B  is a cross-sectional view through the flanged bushing of  FIG. 3A . 
         FIG. 3C  is an end view of the flanged bushing of  3 A. 
         FIG. 4A  shows a thrust washer. 
         FIG. 4B  is a cross-sectional view of the  FIG. 4A  thrust washer. 
         FIG. 5A  is a cross-sectional view through a second bushing. 
         FIG. 5B  is an end view of the sleeve. 
         FIG. 6A  shows another thrust washer. 
         FIG. 6B  is a cross-sectional view of the  FIG. 6A  thrust washer. 
         FIG. 7A  shows yet another thrust washer. 
         FIG. 7B  is a cross-sectional view of the  FIG. 7A  thrust washer. 
     
    
    
     DETAILED DESCRIPTION 
     An air turbine starter assembly  20  may be associated with an aircraft, or other systems including a gas turbine engine as shown in  FIG. 1 . A source of hot air  22 , which may be from an auxiliary power unit, as typically utilized while on the ground, delivers hot, high pressure air into an inlet  24 . The high pressure air flows across a turbine rotor  26 , causing the turbine rotor  26  to rotate. As the turbine rotor  26  rotates, it rotates an output shaft  28  through a planetary gear system. The output shaft  28  may be utilized as a starter, to initiate operation of a main gas turbine engine  30 . 
     The planetary gear system includes a sun gear  34  that is driven by a rotor shaft  32  that rotates with the turbine rotor  26 . The sun gear  34  in turn drives a plurality of planet gears  40 . The planet gears  40  include output gear teeth  41 , which drive a ring gear  42 . The ring gear  42  drives the output shaft  28  through a mechanical clutch connection. 
     The planet gears  40  rotate about a stationary planet shaft  44 . The stationary planet shaft  44  includes an end flange  46  that is fixed to prevent rotation relative to a housing  38 . Needle bearings  300  support the gear  40  on stationary shaft  44 . 
     As shown in  FIG. 2 , a thrust washer  100  sits on one end face of the planet gear  40 , and the combined pair of thrust washers  134  and  136  sit on an opposed end. A bushing  102  includes a nominal body portion press-fit into the housing bore, and has a protruding axial tab  106  at one end extending into a notch  108  in the thrust washer  100 . The tab  106  prevents rotation of the thrust washer  100 . A flange  112  is formed at an outboard end of the bushing  102 , and serves to position the bushing against the counterbore surface  116  in the housing  38 . 
       FIG. 3A  shows a detail of the bushing  102 , including the flange  112  and tab  106 . 
     The bushing  102  is shown in cross-section in  FIG. 3B . A nominal body portion  111  is the portion which is force-fit into a housing bore. The flange  112  is also illustrated. The tab  106  extends axially from the nominal body portion  111 . As shown, the nominal body portion  111  extends for an axial length d 0 , while the tab extends for an axial length d 1 . The tab  106  has an inner curved surface positioned from a center axis by a radius R 1 . The outer periphery of tab  106  is defined by R 3 . The outer periphery of body  111  is defined by D 1  which is the surface contacting the housing bore. 
     As shown in  FIG. 3C , the bushing  102  has the tab  106  extending over a circumferential width d 2 . The d 2  dimension is a side-to-side dimension, generally extending circumferentially. In the embodiment shown, the sides are generally parallel to each other, and the distance would thus be measured between the parallel sides. An alternate embodiment could have those surfaces radiating from center, for which d 2  would be an arc length or an angular dimension. 
       FIG. 4A  shows the thrust washer  100  having oil grooves  118  on one face, and a notch  108 . A countersunk bore  114  can also be seen in  FIG. 4B . As shown, the notch  108  extends for a circumferential width d 3 . As also shown in  FIG. 4B , the inner most bore of the washer  100  (that length not including the countersunk bore  114 ) extends for an axial dimension d 4 . 
     In one embodiment, d 0  was 0.386″ (0.980 cm); d 1  was 0.100″ (0.254 cm); d 2  was 0.125″ (0.317 cm); d 3  was 0.196″ (0.498 cm); and d 4  was 0.108″ (0.274 cm). In that same embodiment, D 1  was 0.766″ (1.94 cm), and R 1  was 0.303″ (0.770 cm), and R 3  was 0.373″ (0.947 cm). 
     In embodiments, a ratio of d 1  to d 0  is between 0.2 and 1.0; a ratio of d 2  to d 3  is between 0.60 and 0.98; and a ratio of d 1  to d 4  is between 0.20 and 0.98. 
     Returning to  FIG. 2 , at an opposed end of the planet gear  40 , is a flangeless bushing  130 , which is also press-fit into the gear cage housing bore, and positioned by bottoming on the bushing end face  160 . Bushing  130  also has a tab or an extension  132  extending inboard, which engages into the notch  142  in thrust washer  136  and into notch  140  in thrust washer  134 . Thrust washers  134  and  136  function together as a spherical joint, in which thrust washer  134  nests into thrust washer  136 . Thrust washer  136  has a notch  142  extending through its entire axial length, while thrust washer  134  has notch  140  extending a finite axial distance. The tab  132  of the bushing  130  extends within the notches  140  and  142  in the washers  134  and  136  to prevent rotation of those washers. 
       FIG. 5A  shows the bushing  130 . Tab  132  extends from the nominal body portion  146 . As shown, the outer periphery of the nominal body portion  146  is at a diameter D 3 . The inner curved surface of the tab  132  is at a radius R 2 . The tab  132  extends for an axial distance of d 5 , while the nominal body portion  146  extends for an axial dimension of d 6 . 
       FIG. 5B  is an end view of the bushing  130 , and shows the tab  132 . Tab  132  extends for a side-to-side dimension d 7 , measured circumferentially. In the embodiment shown, the sides are generally parallel to each other, and the distance would thus be measured between the parallel sides. An alternate embodiment could have those surfaces radiating from center, for which d 2  would be an arc length or an angular dimension. 
     The washer  136  is illustrated in  FIG. 6A . As can be seen between  FIGS. 6A and 6B , a concave face  158  is formed on one side, and spaced from a perpendicular face  156 . A notch  142  extends through the entire axial thickness of the washer  136 . As shown in  FIG. 6A , the width of the notch  142  is defined as d 8 . 
       FIG. 6B  shows the washer  136  has the notch  142  extending for an axial length d 9 . 
       FIG. 7A  shows the washer  134 . Washer  134  has a notch  140  with a width d 10 . Also, oil grooves  301  can be seen. 
       FIG. 7B  shows the mating thrust washer  134 . Washers  134  and  136  nest together, as mentioned above. Washer  134  has a notch  140  extending to an end surface  138 , with a depth d 11 . A convex face  150  is spaced from a relatively perpendicular face  152 . 
     In one embodiment, d 5  was 0.191″ (0.485 cm); d 6  was 0.413″ (1.04 cm); d 7  was 0.125″ (0.317 cm); d 8  was 0.196″ (0.498 cm); d 9  was 0.070″ (0.178 cm); d 10  was 0.196″ (0.498 cm); and d 11  was 0.140″ (0.356 cm). In that same embodiment, D 3  was 0.746″ (1.90 cm) and R 2  was 0.323″ (0.820 cm). In embodiments, a ratio of d 5  to d 6  is between 0.20 and 1.0; a ratio of d 7  to d 8  is between 0.60 and 0.98; and a ratio of d 5  to the sum of d 9  and d 11  was between 0.2 and 0.98. 
     Both distances d 2  and d 7  could be defined as side-to-side distances measured circumferentially about an axis. Of course, the sides are generally parallel to each other, and the distance would thus be measured between the parallel sides. An alternate embodiment could have those surfaces radiating from center, for which d 2  would be an arc length or an angular dimension. 
     The tabs and grooves are sized such that there is a small clearance between the outer periphery of the tab and the inner periphery of the groove. Still, the tabs will prevent relative rotation of the grooves, and their respective component features. 
     This assembly provides a secure way of preventing rotation of the washers  134 ,  136  and  100 , without requiring extra components. 
     This method of assembly and rotation restriction provides increased contact area between the tab sides and slot edges, thereby reducing the unit load per area. This notably reduces the contact stresses, and improves wear life of the mating parts. 
     The disclosed combinations will increase the life of the assembly by increasing a contact area, thereby reducing stress and wear. The assembly of the embodiments is improved by reducing part count, thereby reducing inventory and related costs. In addition, there are reduced assembly operations and labor, and an ease of assembly benefits. As an example, the disclosed combinations eliminate blind assembly, such as inserting narrow pins into a blind hole. 
     Although an embodiment of this invention has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this invention. For that reason, the following claims should be studied to determine the true scope and content of this invention.