Abstract:
A ball-detent torque-limiting assembly has 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:
FIELD OF THE INVENTION 
     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 
     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 EMA is rendered inoperative. A costly overhaul process is required to replace parts and return the unit to service. 
     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. 
     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&#39;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 
     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. 
     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. 
     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. 
     The torque limiting assembly of present invention protects surface and internal components of an EMA, and is easily resettable. The present invention finds application in both unidirectional and bidirectional torque transmission systems. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING VIEWS 
       Features and advantages of embodiments of the present disclosure will become apparent by reference to the following detailed description and drawings, in which: 
         FIG. 1  is a perspective view of a torque-limiting assembly formed in accordance with an embodiment of the present invention, wherein the torque-limiting assembly is shown in its normal operating condition; 
         FIG. 2  is a central cross-sectional view of the torque-limiting assembly shown in  FIG. 1 ; 
         FIG. 2A  is a cross-sectional view of the torque-limiting assembly shown in  FIGS. 1 and 2 , sectioned through a secondary ball thereof; 
         FIG. 3  is an exploded perspective view of the torque-limiting assembly shown in  FIG. 1 , looking generally in a first axial direction; 
         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; 
         FIG. 5  is a partially-sectioned perspective view of an input gear of the torque-limiting assembly shown in  FIG. 1 ; 
         FIG. 6  is a partially-sectioned perspective view of a ball cage of the torque-limiting assembly shown in  FIG. 1 ; 
         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; 
         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; 
         FIG. 8A  is a cross-sectional view of the torque-limiting assembly shown in  FIG. 8 , sectioned through a secondary ball thereof, wherein some structure is omitted for sake of clarity; 
         FIG. 9  is a schematic axial plan view of the torque-limiting assembly in its normal operating condition; 
         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; 
         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; and 
         FIG. 12  is an enlarged, sectioned side view illustrating full deployment of a plurality of secondary balls of the torque limiting assembly. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIGS. 1-4  depict a bidirectional torque-limiting assembly  10  formed in accordance with an 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. 
     Assembly  10  generally comprises an elongated shaft  12  supporting an input gear  14  and an output gear  16 . Shaft  12  includes a splined 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 . 
     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 . 
     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 splined 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 splined end  18  and toward output gear  16 . 
     Backing plate  28  includes a toothed opening  46  enabling the backing plate to be mounted on splined 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. 
     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 . 
     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 . 
     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. 
     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. 
     In the described 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 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 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 . 
     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 . 
     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. 
       FIG. 8  and  FIG. 8A , by contrast, illustrate 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  and  FIG. 8A , 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  and  FIG. 8A , 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. 
     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 . 
     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 splined 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 . 
       FIG. 11  depicts further rotation of input gear  14  relative to backing plate  28 . In  FIG. 11 , 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 . 
       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 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 . 
     If a breakout occurs, the control electronics will eventually command the EMA&#39;s motor to stop. The present invention will then allow a simple reset of the assembly  10  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. 
     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. 
     LIST OF REFERENCE SIGNS 
     
         
         
           
               10  torque-limiting assembly 
               12  shaft 
               14  input gear 
               16  output gear 
               18  splined end of shaft 
               19  retaining groove of shaft 
               20  spring 
               22  washer 
               24  collar 
               25  bushing 
               26  retainer clip 
               27  Belleville spring 
               28  backing plate 
               30  primary ball 
               31  retaining ring 
               32  cage 
               34  input gear mounting sleeve 
               36  input gear annular recess 
               38  input gear driving surface 
               40  input gear primary ball pocket 
               46  backing plate toothed opening 
               48  backing plate detent surface 
               50  backing plate primary ball pocket 
               52  cage mounting hole 
               54  cage driven surface 
               56  cage braking surface 
               58  cage ball opening 
               60  secondary ball 
               62  secondary ball pocket of input gear 
               63  exit ramp from secondary ball pocket of input gear 
               64  secondary ball terminal position of input gear 
               66  secondary ball stop of input gear 
               68  secondary ball pocket of cage 
               69  exit ramp from secondary ball pocket of cage 
               70  secondary ball terminal position of cage 
               72  secondary ball stop of cage