WO2014116333A2

WO2014116333A2 - Non-chattering ball detent torque limiter

Info

Publication number: WO2014116333A2
Application number: PCT/US2013/069174
Authority: WO
Inventors: Kerry Randall KOHUTH; Derek Pedersen; Scott A. LEE
Original assignee: Moog Inc.
Priority date: 2012-11-11
Filing date: 2013-11-08
Publication date: 2014-07-31
Also published as: WO2014116333A3

Abstract

The present invention improves a ball-detent torque-limiting assembly by providing breakout means for maintaining an axial separation distance between opposing pocketed surfaces of the assembly once the primary balls of the assembly have rolled out of their pockets, wherein the axial separation distance maintained by the breakout means is at least as great as the diameter of the balls. The breakout means may include a plurality of secondary balls deployed in a breakout event. The breakout means assumes the axially directed spring load that urges the opposing pocketed surfaces together, thereby preventing the primary balls from entering and exiting the pockets in quick and violent succession following breakout and avoiding damage to the torque-limiting assembly. The torque-limiting assembly is resettable by counter-rotation following a breakout event.

Description

NON-CHATTERING BALL DETENT TORQUE LIMITER

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present application claims priority of U.S. Provisional Patent

Application No. 61/724,989 filed November 11, 2012, U.S. Non-Provisional Patent Application No. 13/829,867 filed March 14, 2013, and U.S. Non-Provisional Patent

Application No. 14/058,625 filed October 21, 2013. The entire contents of each of the foregoing applications are incorporated herein by reference.

FIELD OF THE INVENTION

[0002] The present invention relates generally to electromechanical actuation of aircraft control surfaces, and more particularly to torque limiters designed to prevent transmission of excessive torque and load after an electromechanical actuator for moving an aircraft control surface has encountered a hard mechanical stop.

BACKGROUND OF THE INVENTION

[0003] Aircraft control surfaces, for example flaps located on the trailing edge of a fixed wing, slats located on a leading edge of a fixed wing, spoiler panels, aileron surfaces, and the like, have traditionally been actuated by hydraulic actuation systems. More recently, electromechanical actuators ("EMAs") have gained acceptance in the aviation industry for adjusting the position of control surfaces. EMAs are designed to sweep through a given stroke, linear or rotary, but must have definite points where the stroke must start and end. In practice, two sets of endpoints are defined: one set defines the electrical stroke and the other the mechanical stroke. In normal operation, EMAs are controlled by sophisticated integral or remote electronics over the electrical stroke. However, conditions may arise where an errant command results in the EMA being driven beyond the normal electrical stroke endpoint into a mechanical stroke endpoint. The endpoints that define the mechanical stroke are usually hard

mechanical stops. Aircraft manufacturers require that the EMA contain the EMA stroke to prevent possible damage to the airframe or control surfaces. Because of usual space constraints in aircraft, extra room to include "soft" mechanically cushioned stops is not available. If an EMA is driven at sufficient rate into a mechanical end stop either during an in-flight event or as a result of a rigging error during assembly, significant damage usually occurs. After a "shearout" device is employed, and after an event, the EM A is rendered inoperative. A costly overhaul process is required to replace parts and return the unit to service.

[0004] It is known to use a rotary ball detent mechanism in an EMA system to limit the torque transmitted from an input gear to an output gear to a chosen maximum torque. The input and output gears are axially aligned on a drive shaft. After a stop is encountered, the rotary ball detent mechanism disconnects the driving inertia from the load path at levels that prevent damage. Conventional ball detent mechanisms employ a series of metal balls all in the same plane that are equally spaced around a circumference about the drive shaft. The balls are held between two circular plates each having an array of pockets to hold the balls. The spacing between the plates is therefore the ball diameter less the depth of the opposing ball pockets. A cage between the plates having a thickness slightly less than the plate spacing is usually employed to maintain even angular ball spacing. The plates and balls are held on the drive shaft by relatively heavy axial spring loading. Under normal operation, all parts rotate together at a commanded speed. The magnitude of the spring loading, the size and number of balls, and depth and shape of pocket dictate the torque limit of the device.

[0005] The breakout load or torque limit is selected to be greater than the maximum operating load so that it never "trips" during normal operation, but less than loads that would cause damage to the EMA. With the conventional ball detent mechanism described above, after a breakout or hard stop condition is encountered, one plate is brought to an abrupt stop while the other continues to rotate as the set of balls, in unison due to the cage, roll out of the pockets and onto the flat opposing surfaces of the two circular plates. The shaft is usually rotating at least several hundred - and often up to several thousand - revolutions per minute. The control electronics cannot sense a problem or act on a problem instantaneously, so the EMA's motor is driven for some fraction of a second after breakout. For example, if initial speed is 2400 RPM and six balls are used, with an assumed time of 200 msec before the motor can be turned OFF, 8 revolutions occur. Therefore, the balls that breakout of the initial pockets then encounter 48 more events of rolling into and out of subsequent pockets in the direction of rotation. With the high spring force and the abrupt shape of the pockets, the continued motion of the balls rolling into and out of pockets results in a very violent series of events. The balls experience very high and repeated impact loading and may fracture. Also, the edges of the pockets in the plates may generate harmful debris. Tests have shown significant damage to ball pockets after several encounters. The audible noise from the conventional approach is a loud chatter that may be described as "machine-gun-like."

SUMMARY OF THE INVENTION

[0006] The present invention solves the damage and noise problems associated with a breakout event experienced by a conventional torque-limiting assembly.

Moreover, the present invention provides a torque-limiting assembly that is easily reset for continued operation after a breakout event.

[0007] The present invention provides a ball-detent torque-limiting assembly with breakout means for maintaining an axial separation distance between opposing pocketed surfaces of the assembly once the primary balls of the assembly have rolled out of their pockets as a result of relative rotation between the opposing pocketed surfaces when a torque limit of the assembly is exceeded. The axial separation distance maintained by the breakout means is at least as great as the diameter of the primary balls, and may be greater than the diameter of the primary balls. The breakout means may assume the axially directed spring load that urges the opposing pocketed surfaces together, thereby preventing the primary balls from entering and exiting the pockets in quick and violent succession following breakout and avoiding damage to the torque-limiting assembly.

[0008] The breakout means may comprise a plurality of secondary balls deployed in a breakout event to keep the opposing pocketed surfaces separated by an axial distance that may be slightly greater than the diameter of the primary balls. In an embodiment of the invention, the opposing pocketed surfaces are respective surfaces of an input gear and a backing plate, the primary balls are radially retained with angularly spaced openings in a ball cage located between the input gear and the backing plate, and the secondary balls are situated between the input gear and the cage. [0009] In another embodiment, the breakout means may comprise an angular array of cooperating pairs of ramp members respectively protruding from one of the pocketed surfaces and from a facing surface of the ball cage retaining the primary balls.

[0010] In a further embodiment, the breakout means may comprise a plurality of rollers in an angular array spaced radially relative to the primary balls and opposing primary ball pockets to avoid alignment with the ball pockets.

[0011] The torque limiting assembly of present invention protects surface and internal components of an EMA, and is easily resettable by commanding a reverse rotation in an angular direction opposite the breakout direction. The present invention finds application in both unidirectional and bidirectional torque transmission systems.

BRIEF DESCRIPTION OF THE DRAWING VIEWS

[0012] Features and advantages of embodiments of the present disclosure will become apparent by reference to the following detailed description and drawings, in which:

[0013] Fig. 1 is a perspective view of a torque-limiting assembly formed in accordance with a first embodiment of the present invention, wherein the torque- limiting assembly is shown in its normal operating condition;

[0014] Fig. 2 is a cross-sectional view of the torque-limiting assembly shown in Fig. 1;

[0015] Fig. 3 is an exploded perspective view of the torque-limiting assembly shown in Fig. 1, looking generally in a first axial direction;

[0016] Fig. 4 is another exploded perspective view of the torque-limiting assembly shown in Fig. 1, looking generally in a second axial direction opposite the first axial direction;

[0017] Fig. 5 is a partially-sectioned perspective view of an input gear of the torque-limiting assembly shown in Fig. 1;

[0018] Fig. 6 is a partially-sectioned perspective view of a ball cage of the torque- limiting assembly shown in Fig. 1;

[0019] Fig. 7 is a side view of the torque-limiting assembly shown in Fig. 1, wherein the torque-limiting assembly is shown in its normal operating condition; [0020] Fig. 8 is a side view similar to that of Fig. 7, wherein the torque-limiting assembly is shown in its final breakout operating condition after its torque limit has been exceeded;

[0021] Fig. 9 is a schematic axial plan view of the torque-limiting assembly in its normal operating condition;

[0022] Fig. 10 is a schematic axial plan view similar to that of Fig. 9, wherein the torque-limiting assembly is shown during breakout just after its torque limit has been exceeded;

[0023] Fig. 11 is a schematic axial plan view similar to those of Figs. 9 and 10, wherein the torque-limiting assembly is shown in its final breakout operating condition;

[0024] Fig. 12 is an enlarged, sectioned side view illustrating full deployment of a plurality of secondary balls of the torque limiting assembly;

[0025] Fig. 13 is an exploded perspective view of a torque-limiting assembly formed in accordance with a second embodiment of the present invention, looking generally in a first axial direction;

[0026] Fig. 14 is an exploded perspective view of the torque-limiting assembly shown in Fig. 13, looking generally in a second axial direction opposite the first axial direction;

[0027] Fig. 15 is a perspective view of an input gear of the torque-limiting assembly shown in Fig. 13;

[0028] Fig. 16 is an axial plan view of the input gear shown in Fig. 15;

[0029]

[0030] Fig. 17 is a perspective view of a cage, balls, and backing plate of the torque-limiting assembly shown in Fig. 13;

[0031] Fig. 18 is an axial plan view of the cage, balls, and backing plate shown in Fig. 17;

[0032] Fig. 19 is a cross-sectional view of the torque-limiting assembly shown in Fig. 13, in normal operating condition;

[0033] Fig. 20 is a cross-sectional view of the torque-limiting assembly shown in

Fig. 13, in breakout operating condition; [0034] Fig. 21 is an elevational view illustrating the torque-limiting assembly of Fig. 13 after breakout;

[0035] Fig. 22 is a perspective view illustrating the torque-limiting assembly of Fig. 13 after breakout;

[0036] Fig. 23 is an exploded perspective view of a torque-limiting assembly formed in accordance with a third embodiment of the present invention, looking generally in a first axial direction;

[0037] Fig. 24 is an exploded perspective view of the torque-limiting assembly shown in Fig. 23, looking generally in a second axial direction opposite the first axial direction;

[0038] Fig. 25 is a perspective view of an input gear of the torque-limiting assembly shown in Figs. 23-24;

[0039] Fig. 26 is an axial plan view of the input gear shown in Fig. 25;

[0040] Fig. 27 is an enlarged perspective view of the backing plate shown in Fig. 23;

[0041] Fig. 28 is a perspective view of an outer cage, balls, inner cage, rollers and backing plate of the torque-limiting assembly shown in Figs. 23-24;

[0042] Fig. 29 is an axial plan view of the outer cage, balls, inner cage, rollers and backing plate shown in Fig. 28; and

[0043] Figs. 30-36 are a sequential series of schematic axial views showing the torque-limiting assembly of the third embodiment as it experiences breakout and then reset.

DETAILED DESCRIPTION OF THE INVENTION

[0044] Figs. 1-4 depict a bidirectional torque-limiting assembly 10 formed in accordance with a first embodiment of the present invention. Assembly 10 has utility in an EMA drive system for actuating an aircraft control surface, e.g. a spoiler panel, flap, slat, horizontal stabilizer, or other aircraft control surface.

[0045] Assembly 10 generally comprises an elongated shaft 12 supporting an input gear 14 and an output gear 16. Shaft 12 includes a sp lined end 18 provided with a circumferential retaining groove 19. Assembly 10 also comprises a spring 20, washers 22, a roller bearing 23, a collar 24, and retainer clips 26 all mounted on shaft 12. Assembly 10 further comprises a backing plate 28 mounted on shaft 12 and a cap 29 covering retainer clips 26.

[0046] Output gear 16 is mounted on shaft 12 for rotation with the shaft. In the context of the present specification, "mounted on" is meant in a broad sense to include a part that is separately manufactured and slid onto shaft 12, as well as a part that is integrally formed on shaft 12.

[0047] Input gear 14 is mounted on shaft 12 so as to be rotatable about the shaft axis relative to the shaft, and axially displaceable along the shaft in first and second opposite axial directions. For example, input gear 14 may be mounted on shaft 12 by a cylindrical bushing 25. Input gear 14, shown in greater detail in Fig. 5, includes a driving surface 38 facing in a first axial direction toward sp lined end 18 of shaft 12. Driving surface 38 may be an integral surface of input gear 14 as shown in Figs. 2-4, or it may be a surface of a drive plate (not shown) that is manufactured separately from input gear 14. Integrating driving surface 38 with input gear 14 is advantageous because it saves axial space. Driving surface 38 includes a plurality of primary ball pockets 40 angularly spaced about the axis of shaft 12. As best seen in Fig. 3, input gear 14 may include an annular recess 36 on the side opposite from driving surface 38, and a cylindrical mounting sleeve 34 extending in a second axial direction away from sp lined end 18 and toward output gear 16.

[0048] Backing plate 28 includes a toothed opening 46 enabling the backing plate to be mounted on sp lined end 18 of shaft 12 such that the backing plate rotates with the shaft about the shaft axis. Backing plate 28 is constrained against axial displacement along shaft 12 in the first axial direction by C-shaped retainer clips 26 received in retaining groove 19. Backing plate 28 includes a detent surface 48 opposing driving surface 38 and having a plurality of primary ball pockets 50 angularly spaced about the shaft axis.

[0049] Spring 20, which may be embodied as a Belleville spring pack, may be mounted over cylindrical sleeve 34 of input gear 14 for partial receipt within annular recess 36 for an axially-compact biasing arrangement. One end of spring 20 bears against axially-fixed output gear 16 by way of washers 22, roller bearing 23, and collar 24, while the other end of spring 20 bears against axially-displaceable input gear 14. As may be understood, spring 20 is arranged to provide an axially-directed load urging input gear 14 in the first axial direction toward backing plate 28.

[0050] Assembly 10 further comprises a cage 32, shown in Fig. 6, having a central mounting hole 52 for mounting the cage on shaft 12. Cage 32 is mounted on shaft 12 between driving surface 38 and detent surface 48. Cage 32 includes a driven surface 54 facing driving surface 38, and a braking surface 56 facing detent surface 48. Cage 32 further includes a plurality of primary ball openings 58 therethrough. Primary ball openings 58 are angularly spaced about the axis of shaft 12. Assembly 10 may comprise an axially slidable Belleville spring 27 and retaining ring 31 between a flanged end of bushing 25 and cage 32.

[0051] Assembly 10 also includes a plurality of primary balls 30 of uniform diameter received in primary ball openings 58. The diameter of primary balls 30 is greater than the axial thickness of cage 32 (i.e. the distance from driven surface 54 to braking surface 56), such that protruding spherical caps of each primary ball 30 project into a primary ball pocket 40 in driving surface 38 and an opposing primary ball pocket 50 in detent surface 48. Under normal torque loading conditions not exceeding a predetermined torque limit, the bias of spring 20 maintains the assembly in the described arrangement.

[0052] When a hard mechanical stop event results in abrupt rotational stoppage of shaft 12 and output gear 16, the motor of the EMA momentarily continues to drive input gear 14. When this occurs, assembly 10 is designed to allow slippage between input gear 14 and shaft 12 to prevent torque transmission to shaft 12 in excess of a predetermined torque limit. As relative rotation occurs between input gear 14 and shaft 12 during a mechanical stop event, primary balls 30 roll out of primary ball pockets 40 and 50 in gear 14 and backing plate 50, respectively, thereby causing axial separation of driving surface 38 from detent surface 48 by a distance corresponding to the diameter of primary balls 30. In accordance with the present invention, a plurality of secondary balls 60 are arranged to keep the opposing surfaces 38, 48 separated by an axial distance slightly greater than the diameter of the primary balls 30 during intermittent alignment of the primary balls with the opposing ball pockets during the relative rotation, such that primary balls 30 are not repeatedly slammed into pockets 40 and 50 as input gear 14 continues to rotate. [0053] In the described first embodiment, the plurality of secondary balls 60 are arranged between driving surface 38 of input gear 14 and driven surface 54 of cage 32. As best seen in Fig. 5, driving surface 38 has a plurality of secondary ball pockets 62 therein. As may be understood from the drawing, the plurality of primary ball pockets 40 in driving surface 38 are angularly spaced about the shaft axis at a first radius, and the plurality of secondary ball pockets 62 in driving surface 38 are angularly spaced about the shaft axis at a second radius different from the first radius. In the first embodiment described herein, six primary ball pockets 40 are provided for six primary balls 30, and three secondary ball pockets 62 are provided for three secondary balls 60. A different number of primary balls 30 and primary ball pockets

40 may be used, and a different number of secondary balls 60 and secondary ball pockets 62 may be used. In the first embodiment described herein, the first radius associated with the primary ball pockets 40 is greater than the second radius associated with the secondary ball pockets 62, however the second radius may be greater than the first radius without straying from the invention. Each of the secondary ball pockets 62 in driving surface 38 may have associated therewith a pair of ball terminal positions 64 and on opposite angular sides of the secondary ball pocket 62, and a pair of exit ramps 63 each leading from the secondary ball pocket 62 to a respective one of the terminal positions 64. A pair of secondary ball stops 66 may be arranged on input gear 14 respectively adjacent the pair of terminal positions

64.

[0054] As seen in Fig. 6, driven surface 54 of cage 32 has a plurality of secondary ball pockets 68 therein. Secondary ball pockets 68 in driven surface 54 are angularly spaced about the shaft axis at the same "second radius" associated with secondary ball pockets 62 in driving surface 38 of input gear 14. Similar to secondary ball pockets

62, each of the secondary ball pockets 68 in driven surface 54 may have associated therewith a pair of ball terminal positions 70 on opposite angular sides of the secondary ball pocket 68 and a pair of exit ramps 69 each leading from the secondary ball pocket 68 to a respective one of the terminal positions 60. Likewise, a pair of secondary ball stops 72 may be arranged on cage 32 respectively adjacent the pair of terminal positions 70. [0055] Operation of torque-limiting assembly 10 will now be described with reference to Figs. 7-12. Fig. 7 shows assembly 10 in its normal operating condition, wherein torque not exceeding the torque limit is transmitted from input gear 14 to output gear 16 via shaft 12. In the normal operating condition, axially directed force provided by spring 20 urges input gear 14 in a first axial direction (to the right in Fig.

7) toward axially fixed backing plate 28. Primary balls 30, not visible in Fig. 7, are retained by primary ball openings 58 in cage 32. Spherical caps at opposite ends of primary balls 30 are received within aligned primary ball pockets 40, 50 in input gear 14 and backing plate 28. Secondary balls 60, also not visible in Fig. 7, are held within aligned secondary ball pockets 62, 68 in input gear 14 and cage 32. For example, one hemisphere of a given secondary ball 60 may reside within secondary ball pocket 62 in driving surface 38, and the other hemisphere of the secondary ball may reside within secondary ball pocket 68 in driven surface 54. This arrangement may be seen in the cross-sectional view of Fig. 2. Under normal operating conditions, the torque limit is not exceeded and assembly 10 remains axially compact.

[0056] Fig. 8, by contrast, illustrates assembly 10 in an axially extended state after the torque limit is exceeded and a breakout event occurs. After breakout, input gear 14 is displaced in a second axial direction, to the left in Fig. 8, away from backing plate 28. As will be explained in detail below, the axial displacement of input gear 14 is initially caused by primary balls 30 rolling out of primary ball pockets 40, 50, and is incrementally furthered and maintained by deployment of secondary balls 60 from secondary ball pockets 62, 68, against the axially-directed urging of spring 20. In the breakout state shown in Fig. 8, primary balls 30 do not bear the axial load imposed by spring 20. In accordance with the present invention, the spring load is borne by secondary balls 60 and is transmitted through cage 32 to backing plate 28. Thus, primary balls 30 do not repeatedly roll into and out of subsequent pockets in the direction of rotation, and the violent "machine-gun-like" chatter is eliminated.

[0057] A breakout event will now be described with reference to Figs. 9-12 which provide sequential axial plan views of cage 32 and input gear 14. Fig. 9 illustrates an initial normal operating condition prior to breakout. In the normal operating condition, primary balls 30 are received by primary ball pockets 40, and secondary balls 60 are received by aligned secondary ball pockets 62 and 68. [0058] When a hard mechanical stop is encountered, backing plate 28 stops rotating together with shaft 12 and output gear 16. However, input gear 14 continues to be driven momentarily due to delay in stopping the EMA motor, and torque is transmitted to shaft 12. When the torque limit is exceeded, input gear 14 will rotate relative to shaft 12 and backing plate 28. As this happens, primary balls 30 will roll out of primary ball pockets 40 in driving surface 38, as may be seen in Fig. 10. The primary balls 30 will also roll out of primary ball pockets 50 in detent surface 48 of backing plate 28 because the backing plate is rotationally stopped with shaft 12. As primary balls 30 roll out onto the flat driving surface 38 and flat detent surface 48, they displace input gear 14 in the second axial direction (away from sp lined end 18) against the bias of spring 20. Because cage 32 is situated between input gear 14 and fixed backing plate 28 and retains primary balls 30, cage 32 will rotate about the central shaft axis in the same angular direction as input gear 14, but only through an angle that is half the angle through which the input gear has rotated. In Fig. 10, the secondary balls 60 have rolled out of secondary ball pockets 62 in input gear 14, over ramps 63, to terminal positions 64, where they are stopped from further travel by a secondary ball stop 66 (not shown in Fig. 10). At this point, the secondary balls 60 remain in secondary ball pockets 68 in cage 32. Thus, in Fig. 10, terminal positions 64 and secondary ball pockets 68 are in overlapping alignment with secondary balls 60.

[0059] Fig. 11 depicts further rotation of input gear 14 relative to backing plate 28. In Fig. 1 1, primary balls 30 have continued rolling on flat driving surface 38 of input gear 14 and flat detent surface 48 of backing plate 28, and are now situated at an angle midway between adjacent primary ball pockets 40. Cage 32 has also rotated through half the angle of rotation of input gear 14, causing secondary balls 60 to roll out of secondary ball pockets 68, over ramps 69, to terminal positions 70, where they are stopped from further travel by a secondary ball stop 72 (not shown in Fig. 11). At this point, the secondary balls 60 are in a terminal position 64 on input gear 14 and an aligned terminal position 70 on cage 32, and are now fully deployed. Thus, in Fig. 11, terminal positions 64 and 70 are in overlapping alignment with secondary balls

60. [0060] Fig. 12 provides an enlarged, sectioned side view illustrating full deployment of secondary balls 60. Each secondary ball 60 is confined between a secondary ball stop 66 associated with input gear 14 and a secondary ball stop 72 associated cage 32 such that the ball 60 is seated at terminal positions 64 and 70 on input gear 14 and cage 32, respectively. The terminal positions 64, 70 and secondary balls 60 are configured and sized such that when secondary balls 60 are fully deployed, the secondary balls 60 and cage 32 maintain an axial separation distance between driving surface 38 and detent surface 48 that is at least as great as the diameter of primary balls 30. The terminal positions 64, 70 and secondary balls 60 may be configured and sized such that when secondary balls 60 are fully deployed, input gear 14 is displaced an incremental axial distance away from backing plate 28 against the bias of spring 20, as shown in Fig. 12. In the illustrated first embodiment, the distance between driving surface 38 of input gear 14 and detent surface 48 of backing plate 28 becomes slightly greater than the diameter of primary balls 30, and the primary balls no longer bear any force of spring 20. The invention eliminates the repeated slamming of primary balls 30 into and out of aligned primary ball pockets 40, 50 during continued rotation of the input gear 14 relative to backing plate 28 immediately after a breakout event. Also, cage 32 is forced axially toward backing plate 28 such that frictional resistance to the relative rotation is increased by surface- to-surface engagement of braking surface 56 against detent surface 48.

[0061] If a breakout occurs, the control electronics will eventually command the EMA's motor to stop. A simple reset of the assembly 10 is achieved by commanding a reverse rotary motion of input gear 14 to cause balls 30 to roll back into the original pockets 40, 50. The invention handles a breakout event with little or no damage to the system.

[0062] Fig. 13-14 depict a torque-limiting assembly 110 formed in accordance with a second embodiment of the present invention. Assembly 110 has utility in an EMA drive system for actuating an aircraft control surface, e.g. a spoiler panel, flap, slat or other aircraft control surface.

[0063] Assembly 110 generally comprises an elongated shaft 112 supporting an input gear 114 and an output gear 116. Shaft 112 includes a sp lined end 118 provided with a circumferential retaining groove 119. Assembly 110 further comprises a spring 120, washers 122, a collar 124, retainer clips 126, and a backing plate 128 all mounted on shaft 112. Output gear 116 is mounted on shaft 112 for rotation with the shaft.

[0064] Input gear 114 is mounted on shaft 112 so as to be rotatable about the shaft axis relative to the shaft, and axially displaceable along the shaft in first and second opposite axial directions. Input gear 114 includes a driving surface 138 facing in a first axial direction toward sp lined end 118 of shaft 112. Driving surface 138 may be an integral surface of input gear 114 as shown in Fig. 14, or it may be a surface of a drive plate (not shown) that is manufactured separately from input gear 114.

Integrating driving surface 138 with input gear 14 is advantageous because it saves axial space. Driving surface 138 includes a plurality of ball pockets 140 angularly spaced about the axis of shaft 112. As best seen in Fig. 13, input gear 114 may include an annular recess 136 on the side opposite from driving surface 138, and a cylindrical mounting sleeve 134 extending in a second axial direction away from sp lined end 118 and toward output gear 116.

[0065] Backing plate 128 includes a toothed opening 146 enabling the backing plate to be mounted on sp lined end 118 of shaft 112 such that the backing plate rotates with the shaft about the shaft axis. Backing plate 128 is constrained against axial displacement along shaft 112 in the first axial direction by C-shaped retainer clips 126 received in retaining groove 119. Backing plate 128 includes a detent surface 148 opposing driving surface 138 and having a plurality of ball pockets 150 angularly spaced about the shaft axis.

[0066] Spring 120, which may be embodied as a Belleville spring pack, may be mounted over cylindrical sleeve 134 of input gear 114 for partial receipt within annular recess 136 for an axially-compact biasing arrangement. One end of spring

120 bears against axially- fixed output gear 116 by way of washers 122 and collar 124, while the other end of spring 120 bears against axially-displaceable input gear 114. As may be understood, spring 120 is arranged to provide an axially-directed load urging input gear 114 in the first axial direction toward backing plate 128.

[0067] Assembly 110 further comprises a cage 132 having a central mounting hole 152 for mounting the cage on shaft 112. Cage 132 is mounted on shaft 112 between driving surface 138 and detent surface 148. Cage 132 includes a driven surface 154 facing driving surface 138, and a braking surface 156 facing detent surface 148. Cage 132 further includes a plurality of ball openings 158 therethrough. Ball openings 158 are angularly spaced about the axis of shaft 112. Assembly 110 also includes a plurality of balls 130 of uniform diameter received in ball openings 158. The diameter of balls 130 is greater than the axial thickness of cage 132 (i.e. the distance from driven surface 154 to braking surface 156), such that protruding spherical caps of each ball 130 project into a ball pocket 140 in driving surface 138 and an opposing ball pocket 150 in detent surface 148. Under normal torque loading conditions, the bias of spring 120 maintains the assembly in the described

arrangement.

[0068] When a hard mechanical stop event results in abrupt rotational stoppage of shaft 112 and output gear 116, the motor of the EMA momentarily continues to drive input gear 114. When this occurs, assembly 110 is designed to allow slippage between input gear 114 and shaft 112 to prevent torque transmission to shaft 112 in excess of a predetermined torque limit. In accordance with the present invention, assembly 110 comprises breakout means for causing and maintaining axial separation of driving surface 138 from detent surface 148 by a distance at least as great as the diameter of balls 130 during a mechanical stop event, whereby balls 130 are not repeatedly slammed into pockets 140 and 150 as input gear 114 continues to rotate.

[0069] Reference is made to Figs. 15-22 for explanation of the breakout means of the second embodiment. In the second embodiment, the breakout means includes a circular series of peaked ramps 142 protruding out of driving surface 138, and a corresponding circular series of peaked ramps 160 protruding out of driven surface 154. Peaked ramps 142 are angularly spaced about the axis of shaft 112 and are separated from one another by arc-shaped slots 144. Likewise, peaked ramps 160 are angularly spaced about the axis of shaft 112 and are separated from one another by arc-shaped slots 162. The circle defined by ramps 142 and slots 144, and the circle defined by ramps 160 and slots 162, have the same radius. In the depicted second embodiment, the ramp-slot circles are radially within a circle defined by balls 130, however an arrangement in which the ramp-slot circles are radially outside the ball circle is within the scope of the invention. Under normal condition, ramps 142 are received in slots 162 and ramps 160 are received in slots 144; this condition can be seen in the cross-sectional view of Fig. 19.

[0070] When a hard mechanical stop is encountered, backing plate 128 stops rotating along with shaft 112 and output gear 116. However, input gear 114 continues to be driven momentarily due to delay in stopping the EMA motor, and toque is transmitted to shaft 112. When the torque limit is exceeded, input gear 114 will rotate relative to shaft 112 and backing plate 128. As this happens, balls 130 will roll out of pockets 140 in driving surface 138; this is best seen in Figs. 20 and 22. The balls will also roll out of pockets 150 in detent surface 148 of backing plate 128 because the backing plate is rotationally stopped with shaft 112. As the balls 130 roll out onto the flat driving surface 138 and flat detent surface 148, they displace input gear 114 slightly in the second axial direction (away from sp lined end 118) against the bias of spring 120.

[0071] Figs. 20 and 21 show that simultaneously with the breakout of balls 130 from pockets 140, complementary sloped surfaces of ramps 142 and 160 engage one another, thereby converting the relative rotary motion between input gear 114 and cage 132 into further axial displacement of input gear 114 in the second axial direction. The cooperative engagement of ramps 142 and 160 causes the driving surface 138 and detent surface 148 to be separated by an axial distance greater than the diameter of balls 130, such that the balls do not bear the load of axial spring 120.

The engaged ramps 142 and 160 also cause cage 132 to rotate in unison with input gear 114 (or with a separate driving plate, if a separate driving plate is used as mentioned above). This prevents the balls from reaching another pocket 140. The balls 130 are unloaded and rotate with input gear 114 (or with a separate driving plate) and with cage 132. Cage 132 is also displaced in the first axial direction such that its braking surface 156 comes into frictional contact with detent surface 148 of stationary backing plate 128, thereby providing braking action which gently slows the rotating parts.

[0072] If a breakout occurs, the control electronics will eventually command the EMA's motor to stop. The present invention will then allow a simple reset of the assembly 110 by commanding a reverse rotary motion of input gear 114 to cause balls 130 to roll back into the original pockets 140, 150. The invention handles a breakout event with little or no damage to the system.

[0073] Figs. 23 and 24 illustrate a torque-limiting assembly 210 formed in accordance with a third embodiment of the present invention that employs another alternative breakout means. Assembly 210 comprises an input gear 214, output gear

216, a backing plate 228, a composite cage 232, and a plurality of balls 230 arranged and mounted on drive shaft 212 and biased by spring 220 in a manner similar to the second embodiment.

[0074] Figs. 25 and 26 show input gear 214 in detail. Input gear 214 includes a driving surface 238 facing in the first axial direction toward sp lined end 218 of shaft

212. As in the second embodiment, driving surface 238 may be an integral surface of input gear 214 as shown in Fig. 24, or it may be a surface of a separately- manufactured drive plate (not shown). Driving surface 238 includes a plurality of ball pockets 240 angularly spaced about the axis of shaft 212. In contrast to driving surface 138 of the second embodiment, driving surface 238 does not have ramps and slots.

[0075] Backing plate 228, shown in Fig. 27, includes a detent surface 248 opposing driving surface 238 and having a plurality of ball pockets 250 angularly spaced about the shaft axis. Detent surface 248 is also provided with a plurality of curved roller pockets 251 angularly spaced about the axis of shaft 212 radially inward from ball pockets 250.

[0076] Reference is now made to Figs. 28-29. Cage 232 of the third embodiment is a two-piece assembly comprising a radially outer cage 233 and a radially inner cage 235, wherein inner cage 235 is slidably received within an axial hole 252 of outer cage 233 to permit relative rotation between the inner and outer cages. A plurality of ball openings 258 are provided through outer cage 233 for receiving and retaining balls 230 in an angularly spaced arrangement around the shaft axis. A plurality of arc-segment coupling recesses 259 are arranged around an edge of axial hole 252 facing driving surface 238.

[0077] Inner cage 235 has a central mounting hole 264 for mounting the inner cage on shaft 212. Inner cage 235 also has a plurality of roller openings 266 angularly spaced about the shaft axis for receiving a plurality of rollers 231. In the figures, rollers 231 are illustrated as being cylindrical rollers to readily distinguish them from balls 230, however rollers 231 may also be embodied as spherical rollers (balls). Regardless of the shape that rollers 231 take, the diameter of rollers 231 is selected to be the same as or slightly greater than the diameter of balls 230. Finally, inner cage 235 includes a plurality of coupling tabs 268 each projecting radially outward for receipt within an associated coupling recess 259 of outer cage 233.

[0078] Operation of the breakout means of the third embodiment will now be explained with reference to Figs. 30-36. Fig. 30 shows the relative arrangement of input gear 214, outer cage 233, inner cage 235, and balls 230 in an initial angular "set" position about the axis of shaft 212 prior to a breakout event. Balls 230 are aligned with pockets 240 of input gear 214 and also with pockets 250 of backing plate 228 (not shown in Figs. 30-36). Outer cage 233 is arranged to contain balls 230 within ball openings 258. Inner cage 235 is arranged such that its coupling tabs 268 extend into respective coupling recesses 259 of outer cage 233 with clearance in both angular directions from ends of the recess 259. Shaft 212 is rotating CW about its axis at high RPM, e.g. in the neighborhood of 2400 RPM.

[0079] Fig. 31 illustrates the onset of a breakout event when output gear 216, shaft 212, and backing plate 228 are unexpectedly and suddenly stopped from rotation when the EMA hits a hard mechanical stop. Input gear 214 continues to rotate in the CW direction (a 30° CW rotation is illustrated). Outer cage 233, situated between rotating input gear 214 and stationary backing plate 228 and carrying balls 230, rotates 15° CW. Balls 230 roll out of pockets 240 and 250 and come into rolling contact with driving surface 238 and detent surface 248. As may be understood, balls

230 now carry the axial load of spring 220, and input gear 214 is displaced slightly in the second axial direction against the spring force. Inner cage 235 carrying rollers

231 remains in the same angular position.

[0080] The breakout event continues in Fig. 32. Input gear 214 continues its CW rotation (a further 22° CW rotation is illustrated). Outer cage 233 and balls 230 rotate another 11° in the CW direction. At this point, respective ends of coupling recesses 259 come into contact with coupling tabs 268 of inner cage 235, which heretofore has been stationary. [0081] Fig. 33 illustrates continuation of the breakout event. Input gear 214 continues its CW rotation (a further 52° CW rotation is illustrated; total rotation is now 104 ° CW). Outer cage 233 and balls 230 rotate an additional 26° in the CW direction, for a total rotation of 52° CW. As outer cage 233 rotates, the engagement of coupling tabs 268 with ends of coupling recesses 259 causes inner cage 235 to rotate together with outer cage 233. Thus, Fig. 33 illustrates 26° CW rotation of inner cage 235 and confined rollers 231. As may be understood, the rotation of inner cage 235 relative to stationary backing plate 228 causes rollers 231 to roll out of roller pockets 251 in backing plate 228. When this happens, rollers 231 come into rolling contact with driving surface 238 and detent surface 248. The diameter of rollers 231 is chosen to be the same as or slightly greater than the diameter of balls 230 so that rollers 231 will assume axial loading of spring 220 from balls 230.

[0082] As may be understood, input gear 214 will continue to rotate in the CW direction until the EMA's control electronics have received a signal that actuator output is not moving and sent a motor stop command to cease driving input gear 214.

This may take on the order of 100 - 200 msec. Assuming an initial speed of 2400 RPM (40 revs per second), approximately eight revolutions of input gear 214 may be expected. During these revolutions, outer cage 233 and inner cage 235 will also rotate about shaft 212 such that rollers 231 will periodically reenter roller pockets 251 and spring loading will be momentary transferred back onto balls 230. Thus, balls 230 and rollers 231 will alternate in taking up the spring load during post-breakout rotations. In order to prevent damage or at least reduce the risk of damage, it may be advantageous to use special non-galling stainless steel (Nitronic 60) or another material suitable for braking or sustained frictional heating for inner cage 235, which is spring loaded against the backing plate 228 with about 600 pounds of force. An oil bath lubrication of assembly 210 may also be used to prevent or minimize damage to moving parts.

[0083] Fig. 34 shows an arbitrary rotational position at which rotation of input gear 214 is stopped by the EMA control electronics. Input gear 214 is at an angular position 120° CW from its original set position. Outer cage 233 and balls 230 are at a an angular position 60° CW from their original set position. Inner cage 235 and rollers 231 are at an angular position 34° CW from their original set position. In the position, outer cage 233 and balls 230 are centered over both sets of pockets 240 and 250, and rollers 231 carry all the axial spring load. With input gear 214 stopped, the breakout event is complete. In accordance with the present invention, assembly 210 can be reset in a relatively simple manner by commanding reverse rotation of input gear 214.

[0084] Fig. 35 depicts the beginning of the reset process in which input gear 214 is rotated CCW by 60° from its stopped position in Fig. 34 by commanding the EMA. Outer cage 233, balls 230, inner cage 235 and rollers 231 are rotated CCW by 30° from their stopped position in Fig. 34. At this point, rollers 231 return to roller pockets 251 and balls 230 assume the axial spring load.

[0085] Fig. 36 shows the completed reset position achieved by commanding an additional 60° CCW rotation of input gear 214. Outer cage 233 and balls 230 rotate another 30° CCW, whereas inner cage 235 is left in the position shown in Fig. 35, thereby substantially centering coupling tabs 268 in the associated coupling recesses 259. The outer cage 233 and balls 230 are aligned with ball pockets 240 and 250, and spring 220 resets the mechanism so that the EMA is once again operational.

[0086] It will be appreciated that the present invention prevents repeated events in which the balls roll out of their pockets and are then slammed back into another pocket. This improvement is accomplished in a very compact space envelope. Other approaches may accomplish the same functionality, but they use mechanisms requiring larger physical volume, weight, and inertia.

Claims

WHAT IS CLAIMED IS:

1. In a torque-limiting assembly wherein a plurality of primary balls roll out of respective opposing ball pockets in opposing surfaces of a gear and a backing plate when a torque limit is exceeded to enable relative rotation between the gear and the backing plate by rolling engagement of the primary balls with the opposing surfaces, wherein the opposing surfaces are biased toward one another by axially directed spring loading, whereby the primary balls separate the opposing surfaces by an axial separation distance corresponding to the primary ball diameter against the spring loading, the improvement comprising: breakout means for maintaining an axial separation distance of the opposing surfaces at least as great as the diameter of the primary balls while the relative rotation between the gear and the backing plate continues once the torque limit has been exceeded.

2. The improvement according to claim 1, wherein the breakout means

comprises a plurality of secondary balls deployed when the torque limit is exceeded.

3. The improvement according to claim 2, wherein the plurality of secondary balls keep the opposing surfaces separated by an axial separation distance greater than the diameter of the primary balls during intermittent alignment of the primary balls with the opposing ball pockets during the relative rotation.

4. The improvement according to claim 2, wherein the plurality of primary balls are retained within respective openings in a cage located between the gear and the backing plate, and the plurality of secondary balls are arranged between facing surfaces of the gear and the cage.

5. The improvement according to claim 4, wherein the facing surfaces of the gear and the cage include corresponding sets of secondary ball pockets for receiving the plurality of secondary balls.

The improvement according to claim 5, wherein each of the secondary ball pockets has associated therewith a pair of ball terminal positions on opposite angular sides of the secondary ball pocket and a pair of exit ramps each leading from the secondary ball pocket to a respective one of the terminal positions.

The improvement according to claim 6, wherein a pair of secondary ball stops are arranged respectively adjacent the pair of terminal positions associated with each of the secondary ball pockets.

The improvement according to claim 1 , wherein the gear is an input gear driven by a motor and a shaft connects the input gear to an output gear rigidly mounted on the shaft, wherein the torque limit is exceeded in response to a mechanical stop event halting rotation of the shaft about the shaft axis.

The improvement according to claim 1 , wherein the gear is axially

displaceable along a shaft in first and second opposite axial directions, the spring loading urges the gear in the first axial direction toward the backing plate, and the backing plate is constrained against axial displacement along the shaft in the first axial direction.

The improvement according to claim 4, wherein the deployment of the plurality of secondary balls urges the cage into surface-to-surface factional contact with the backing plate.

The improvement according to claim 1 , wherein the plurality of primary balls are retained within respective openings in a cage located between the gear and the backing plate, and wherein the breakout means comprises:

a circular series of ramps protruding out of a driving surface of the gear and angularly spaced about an axis of rotation of the gear, wherein the ramps protruding out of the driving surface are separated from one another by arc-shaped slots in the driving surface; and a corresponding circular series of ramps protruding out of a driven surface of the cage, wherein the ramps protruding out of the driven surface are separated from one another by arc-shaped slots in the driven surface;

wherein as the gear rotates about the rotational axis relative to the backing plate incident to the torque limit being exceeded, the ramps protruding out of the driving surface engage the ramps protruding out of the driven surface to maintain at least the axial separation distance corresponding to the primary ball diameter.

12. The improvement according to claim 11 , wherein the engagement of the ramps protruding out of the driving surface with the ramps protruding out of the driven surface causes the opposing surfaces of the gear and the backing plate to be separated by an axial distance greater than the diameter of the primary balls, wherein the primary balls are freed from the axially-directed spring load.

13. The improvement according to claim 1 , wherein the breakout means

comprises a plurality of rollers angularly spaced about an axis of rotation of the gear, wherein the plurality of rollers are deployed when the torque limit is exceeded such that the plurality of rollers engage the opposing surfaces of the gear and the backing plate.

14. The improvement according to claim 13, comprising a cage located between the gear and the backing plate, wherein the cage includes a first radial portion in which the plurality of primary balls are retained within respective openings and a second radial portion in which the plurality of rollers are retained within respective openings, the first and second radial portions of the cage being coupled to one another such that relative rotation between the first and second cage portions is permitted through a limited angle and when the angle is reached in a given rotational direction the first and second cage portions rotate together with one another.

The improvement according to claim 1 , wherein the breakout means is resettable by commanding a reverse rotation of the gear.