DECOUPLER SHAFT

The field of the present invention is that of decoupler devices capable of driving in one direction only, and decoupling in response to a torque load in the opposite direction. More particularly, the present invention is in the field of air turbine starters having a decoupling device of the character described above.

Air turbine starters are known in the aviation field, and are commonly used to start propulsion turbine engines of modern aircraft. A persistent problem with high speed sprag-type overrunning clutches of the type used in air turbine starters is wear to the clutch caused by overrunning of the clutch in operation. Such overrunning occurs continuously during operation of a turbine engine, and particularly results in wear of the sprag members of the clutch. Consequently, one or more of the worn sprags may be forced over center during torque transmission through the clutch. Once over center, these sprags will possibly cause the clutch to transmit torque in the direction which should result in overrunning of the clutch. Were the clutch allowed to transmit reverse torque instead of overrunning, the turbine engine would drive the air turbine starter to a high and destructive speed. In order to prevent such destructive back driving of the air turbine starter by the turbine engine, a decoupler mechanism is conventionally provided in the power train between the starter and engine.

Unfortunately all presently known decoupler devices suffer from one or more of several shortcomings. That is, the conventional decoupler mechanisms may be overly large or complex in their construction, they may not be reliable in their operation, some may undesirably reset automatically to a torque transmitting condition after they are tripped by a reverse torque, others may require extensive time consuming disassembly to reset after being tripped by a reverse torque incident.

In view of the above, it is recognized in the pertinent art that an improved decoupler device is needed for use with air turbine starters. In comparison to the structure of U.S. Patent 2,964,931 having a frangible tensile member subject to both tension and torque predictability, the present invention offers enhanced repeatability by use of a tensile bar for fracturing in substantially only pure and predictable tension.

In view of the above, the present invention provides a decoupler shaft wherein first and second axially adjacent shaft portions cooperatively define interengaging driving and ramp surfaces. The driving surfaces transmit torque in a first sense between the shaft portions substantially without a resultant axial force between the shaft portions. On the other hand, reverse torque in a second sense opposite the driving torque results in generation of an axial separating force between the shaft portions by virtue of the cooperative ramp surfaces. This axial separating force is resisted only by a frangible tensile bar member extending between the two shaft portions. Should the reverse torque reach a predetermined level, the tensile bar member fractures to allow axial separation and decoupling of the shaft portions from one another. Once decoupled, the shaft portions are relatively rotatable to prevent transmission of both driving and reverse torque via the decoupler shaft. A dual function structure is provided which on the one hand provides a bearing surface for allowing relative rotation of the shaft portions without allowing them to flail, while on the other hand also positively preventing reengagement of the shaft portions.

The present invention also provides an air turbine starter including a decoupler shaft of the above-described character. The air turbine starter disposes the decoupler shaft in an easily accessible location such that removal thereof and resetting of the decoupler shaft does not require disassembly of the air turbine starter.

Resetting of the decoupler shaft, that is, restoring it to a condition for use as a torque transmitting shaft with a determined reverse torque decoupling function, involves returning the shaft portions axially to their interengaged relative position, and replacement of the fractured tensile bar member with a new intact tensile bar member.

An advantage of the present invention is the crisp, reliable, and highly repeatable decoupling action which is provided by fracture of the tensile bar member. That is, once the determined reverse torque level is imposed upon the decoupler shaft, decoupling action is very rapid, and without a delayed or slowed break away which could result in damage to the air turbine starter.

Other advantages of the present decoupler shaft are its small size, relatively simple and rugged construction which result in a comparatively low cost of manufacture, and the ease with which a tripped decoupler shaft can be restored to service condition by replacement of the tensile bar member. No special tools are required for this replacement, and the tensile bar member is itself a relatively inexpensive consumable component part.

Additional objects and advantages of the present invention will be apparent from the reading of the following detailed description of a single preferred embodiment thereof taken in conjunction with the appended drawing figures of which;

• FIG. 1 presents a fragmentary partially cross sectional view of an air turbine starter embodying the present invention;
• FIG. 2 depicts an enlarged fragmentary cross sectional view of a selected decoupler shaft portion of the air turbine starter depicted more fully in FIG. 1; and
• FIG. 3 presents an exploded perspective view of selected parts of the decoupler shaft of the present invention.

FIG. 1 depicts an air turbine starter (10) embodying the present invention. The air turbine starter (10) includes a housing (12) defining an inlet (14) (only a portion of which is depicted viewing FIG. 1) and an outlet (16). The housing (12) defines a flow path (18) extending between the inlet (14) and the outlet (16). An axial flow turbine member (20) is rotatably journaled by the housing (12) in the flow path (18) for extracting mechanical energy from a flow of pressurized fluid conducted between the inlet (14) and the outlet (16) via the flow path (18). The turbine member (20) is carried by a rotatable shaft (22) journaled by bearings (24) carried by the housing (12). Secured to the shaft member (22) is a gear member (26) engaging a speed-reducing gear train generally referenced with the numeral (28). The gear train (28) includes a gear portion (30) of an output member (32) which is also journaled by the housing (12) by a bearing member (34) carried thereby. The output member (32) drivingly connects with a sprag clutch of the inner-race-overrunning type generally referenced with the numeral (36), which in turn drivingly connects with a tubular output shaft (38). The output shaft (38) is rotatably carried by the output member (32) via bearing members (40)

A small oil pump (42) is carried by the housing (12) concentrically with the output shaft (38). Pump (42) is drivingly connected with output shaft (38) via a splined drive disk (44) at its outer perimeter engaging drivingly with a spline surface portion (46) of shaft (38). Lubricating oil from a sump portion (not shown) of housing (12) is admitted to pump (42) via a passage (48) defined by housing (12). The pump (42) discharges oil therefrom via a central axially extending passage (50) opening at (52) within the output shaft (38). Within shaft (38) a circumferentially extending groove (54) is radially disposed outwardly of the opening (52) to define a basin for receiving the oil from pump (42). A radially extending passage (56) extends outwardly from the groove (54) to the clutch (36) to supply oil thereto.

Carried by the tubular output shaft (38) is a decoupler shaft assembly generally referenced with the numeral (58). Decoupler shaft (58) drivingly connects with output shaft (38) via respective interengaging male and female spline surfaces generally indicated at (60). The decoupler shaft (58) also defines a male spline surface (62) by which the air turbine starter (10) couples in driving relation with a combustion turbine engine (not shown).

Viewing now FIGS. 2 and 3 in conjunction, it will be seen that the decoupler shaft assembly (58) includes a first axial portion (64) and a second axially adjacent axial portion (66). The portion (64) defines the male spline surface (60) which couples with output shaft (38), while the portion (66) defines spline surface (62). The portions (64) and (66) cooperatively define a stepped axial through bore (68). That is, each of the portions (64) and (66) defines a respective part of the through bore (68).

Disposed within bore (68) and extending between portions (64) and (66), is an elongate tubular guide sleeve member (70) formed of material having favorable bearing qualities. For example, the sleeve (70) may be formed of brass or bronze. Sleeve (70) defines a flange portion (72) engaging a step (74) on bore (68) within portion (64), and an end edge (76) in a first position disposed rightwardly (viewing FIG. 2) of a groove (78) defined on the bore (68) within portion (66). A radially resilient metal ring (80) is captured within the groove (78) outwardly of the sleeve (70).

Also received slidingly in the bore (68), and within sleeve member (70), is an elongate tensile bar member (82). The tensile bar member (82) at one end thereof defines a head portion (84) of larger diameter than the remainder thereof, and trapping the flange portion (72) of sleeve (70) against step (74) of bore (68). At the opposite end of tensile bar member (82), the latter defines an axially extended threaded bore (86) receiving a threaded fastener (88). An annular disk member (90) is carried upon the fastener (88) and defines an axially extending shoulder portion (92) having an axially disposed end face (94). The end face (94) of disk member (90) axially confronts in a first position thereof a step (96) on bore (68) to define an axial gap (98). A coil compression spring (100) extends between a step (102) on bore (68) and the disk member (90) to urge the tensile bar member (82) rightwardly in bore (68) (viewing FIG. 2). Consequently, the spring (100) insures that head portion (84) seats against flange (72) of the sleeve member (70), and that the gap (98) is ordinarily maintained. Thus, the portions (64) and (66) of decoupler shaft assembly (58) are biased axially together and are maintained in coaxial alignment by sleeve (70), while the tensile bar (82) is isolated from all axial stress except that caused by the relatively light preload of spring (100). It will be noted that tensile bar member (82) includes a portion (102) of reduced diameter, or neck-defining portion. The importance of this neck-defining portion (102) will be further described below.

Viewing particularly FIG. 3, it will be seen that the portions (64) and (66) each include matching interengageable radially extending driving and ramp surfaces, (104) and (106), respectively. As depicted, each of the driving surfaces (104), and each of the ramp surfaces (106) are circumferentially spaced apart. Each driving surface and each ramp surface of portion (64) is engageable with one of the correspondingly numbered surfaces of portion (66). The driving surfaces (104) each extend substantially axially so that opposing torques applied in respective first directions to the portions (64) and (66) and urging these surfaces into contact result in virtually no axial force being developed between the portions. On the other hand, the ramp surfaces (106) extend both axially and circumferentially so that opposing torques applied to the portions (64) and (66) in respective second directions opposite the first directions results in an axial force tending to axially separate the portions (64) and (66), as is depicted by arrow (108).

Having observed the structure of the air turbine starter (10), attention may now be given to its method of operation. During a normal start cycle of a combustion turbine engine with both the air turbine starter (10) and the engine being stationary, a supply of pressurized fluid is connected to the inlet of the air turbine starter (10), as is depicted by the fluid flow arrow (110). Viewing FIG. 1, it will be seen that a flow of pressurized fluid through the housing (12) via the flow path (18) will cause the turbine (20) to extract mechanical power therefrom and to deliver this power to the output member (32) via the gear train (28). The sprag clutch connects power from output member (32) to output shaft (38) while the latter drivingly connects with decoupler shaft portion (64). Engine starting torque applied to portion (64) urges surfaces (104) into engagement with each other so that decoupler shaft portion (66) conveys the engine starting power to the turbine engine substantially without axial force within the decoupler shaft (58). The air turbine starter (10) delivers mechanical power in this way to the decoupler shaft (58) thereof, and to the combustion turbine engine connected thereto to accelerate the latter towards its self-sustaining speed.

Upon the combustion turbine engine obtaining its self-sustaining speed, the shaft thereof will accelerate ahead of the output shaft (38) of the air turbine starter (10). Consequently, the torque loading within the sprag clutch (36) will be eliminated. Thus, shaft (38) overruns with respect to output member (32). Shortly after the turbine engine reaches self-sustaining speed, the supply of pressurized air to flow path (18) is shut off. As a result, the air turbine starter (10) coasts to a stop and remains stopped during operation of the turbine engine. The sprag clutch (36) continues overrunning as long as the engine operates. Oil pump (42) driven by shaft (38) continuously provides lubricating oil to clutch (36).

In the event of a physical failure in starter (10), for example, if one or more of the sprags of clutch (36) were to flip over center so that the turbine engine could back drive the starter (10) to a high and destructive speed, or alternatively, if one of the bearings (40) were to fail and seize so that a high resisting torque load were imposed on the turbine engine, the decoupler shaft (58) then functions to prevent further damage. Viewing FIGS. 2 and 3, it will be seen that reverse torque applied to portion (66) urges ramp surfaces (106) into engagement to result in an axial separating force (108) between the portions (64), (66). A relatively low reverse torque is sufficient to overcome the preload of spring (100) and to bring surfaces (94) and (96) into engagement eliminating gap (98). Thereafter, further increased reverse torque can be transmitted from portion (66) to portion (64) only with the development of a directly related tensile force imposed on tensile bar member (82). The neck portion (102) of tensile bar member (82) is sized so that the member will fail in tension at a predetermined stress level. That is, the tensile bar member (82) fractures at neck (102) when a predetermined level of reverse torque is imposed upon decoupler shaft (58).

As a result of the fracturing of tensile bar member (82) at neck (102), the portions (64) and (66) are allowed to rotate relative to one another in response to the reverse torque. Ramp surfaces (106) consequently move portion (64) leftwardly, viewing FIG. 1, to a position depicted in dashed lines. In this position, the surfaces (104) (106) are disengaged from one another. Leftward movement of portion (64) also causes the sleeve member (70) to move leftwardly relative to portion (66) so that the end edge (76) of sleeve (70) moves leftwardly of resilient ring (80). In this second position of the sleeve member (70), the ring (80) moves partially out of the groove (78) to block rightward movement of the sleeve member (70). Because the sleeve member (70) is fitted in bore (68) with an interference fit with portion (64), the latter cannot move rightwardly to once again engage the surfaces (104), (106). However, the sleeve member (70) is relatively rotatably received in the portion of bore (68) defined by decoupler shaft portion (66) so that the latter portion may rotate in output shaft (38) without excessive wear thereof. That is, the sleeve member (70) serves as a bearing to center the portion (66) and to protect the output shaft (38) from wear while the turbine engine continues to operate.

It will be easily appreciated that in conjunction with repair of the air turbine starter (10), the decoupler shaft assembly (58) may be removed from output shaft (38) so that a new tensile bar member (82) can be installed in the decoupler shaft latter. With replacement of the tensile bar member (82), the decoupler shaft assembly (58) may be reinstalled in the air turbine starter (10) for reuse to protect the starter against reverse torque.