Abstract:
A torque limiter limits transmission of torque between an input shaft an output shaft. The torque limiter may be incorporated in a geared rotary actuator for actuating an aircraft control surface. The torque limiter is responsive to output torque associated with the output shaft instead of input torque associated with the input shaft. The torque limiter includes a structural ground and a gear assembly for transmitting rotational motion of the input shaft to the output shaft. The gear assembly includes a reference gear coupled to the structural ground such that movement of the reference gear relative to the structural ground is dependent upon an output torque at the output shaft. The reference gear is stationary relative to the structural ground when the output torque is below an output torque limit, and the reference gear moves relative the structural ground when the output torque exceeds the output torque limit.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to torque limiting mechanisms, especially those used in geared rotary actuators (“GRAs”) for actuating aircraft control surfaces. 
       BACKGROUND OF THE INVENTION 
       [0002]    GRAs are used, for example, in aircraft for actuating flaps, slats, and other aerodynamic control surfaces. GRAs typically incorporate a torque limiter for limiting transmission of torque between an input shaft and an output shaft of the GRA in the event of a malfunction. Conventional torque limiting devices include a disc brake pack having multiple brake discs utilizing frictional contact between adjacent discs for limitation of torque transmission. Such torque limiting devices have several inherent problems. Because the friction coefficient is very sensitive to lubrication, changes in the lubrication environment can cause the friction coefficient to drop below a critical value required to provide a positive torque limit. This can cause the torque limiter to exceed the maximum torque limit setting. If too little lubrication is present in the disc brake pack and moisture is present, the disc brake pack can freeze up, causing nuisance lock-ups. When adequate lubrication is provided to the disc brake pack, considerable viscous drag is present. The viscous drag is not a problem as long as it is accurately predicted and accounted for in the torque limiter setting and power control unit (“PCU”) sizing, however, such viscous drag causes inefficiency in the system and higher limit loads on components downstream of the torque limiter. 
         [0003]    Known torque limiting mechanisms respond to input torque to the GRA rather than GRA output torque. Consequently, the lock-up torque limit setting must be significantly higher than the maximum operating torque of the GRA, and therefore the GRA is designed with a relatively large limit output torque. As a result, each GRA has a greater weight associated therewith, and structure downstream from the GRA is increased. Given that an aircraft may have many GRAs, for example thirty or more, a cumulative weight cost is imposed on the aircraft design. 
         [0004]    There is a need for a torque limiter that solves the problems described above. 
       SUMMARY OF THE INVENTION 
       [0005]    The present invention provides a torque limiter that limits transmission of torque between an input shaft rotatable about an input axis and an output shaft rotatable about an output axis, and does so in a manner that solves the problems discussed above. In an illustrative embodiment of the present invention, the torque limiter is incorporated in a GRA for actuating an aircraft control surface, e.g. a flap or a slat movable relative to a fixed wing. The torque limiter of the present invention is characterized by the fact that it is responsive to output torque associated with the output shaft instead of input torque associated with the input shaft. 
         [0006]    A torque limiter of the present invention generally comprises a structural ground and a gear assembly for transmitting rotational motion of the input shaft to the output shaft. The gear assembly includes a reference gear coupled to the structural ground such that movement of the reference gear relative to the structural ground is dependent upon an output torque at the output shaft. The reference gear is stationary relative to the structural ground when the output torque is below an output torque limit, and the reference gear moves relative the structural ground when the output torque exceeds the output torque limit. 
         [0007]    In accordance with a specific embodiment of the invention, the gear assembly may also include an input gear rotated relative to the structural ground in response to rotation of the input shaft, a driven gear associated with the output shaft such that the output shaft is rotated in response to rotation of the driven gear, and at least one transmitting gear engaging the input gear, the reference gear and the driven gear such that rotation of the input shaft causes rotation of the output shaft without causing movement of the reference gear relative to the structural ground unless the torque limit is exceeded. The reference gear moves relative to the structural ground when the torque limit is exceeded, for example the reference gear may rotate about its axis relative to the structural ground. The gear assembly may be configured as a planetary gear assembly in which the input gear is arranged as a sun gear on the input shaft, the reference gear is arranged as a ring gear about the input gear, and the at least one transmitting gear includes a plurality of planet gears arranged between the input gear and the reference gear. The input gear, reference gear, and driven gear may be arranged coaxially along a main axis, and the planet gears may extend axially in a direction parallel to the main axis of the assembly. 
         [0008]    In a further aspect of the present invention, the torque limiter may comprises a lockout mechanism for preventing transmission of torque between the input shaft and the output shaft after the torque limit has been exceeded, wherein the lockout mechanism redirects torque from the input shaft to the structural ground after the torque limit has been exceeded. The lockout mechanism may comprise a pawl carrier arranged to rotate with the input shaft, and at least one pawl member pivotally coupled to the pawl carrier. The lockout mechanism may further comprise a lockout ring including at least one stop extending radially inward, wherein the lockout ring is arranged along the main axis and is axially displaceable from a non-lockout position wherein each stop is radially clear of each pawl member to a lockout position wherein each stop radially interferes with each pawl member. A spring may be arranged to urge the lockout ring toward the non-lockout position, and a plurality of ball bearings may be seated between the lockout ring and the reference gear. The ball bearings maintain the lockout ring in the non-lockout position when the lockout ring and the reference gear are in a predetermined angular orientation about the main axis relative to one another, and displace the lockout ring toward the lockout position when the reference gear rotates about the main axis relative to the lockout ring. When activated by rotation of the reference gear, the lockout mechanism may redirect input torque through the lockout ring to the structural ground. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING VIEWS 
         [0009]    The invention will be described in detail below with reference to the accompanying drawing figures, in which: 
           [0010]      FIG. 1  is a perspective view of a GRA formed in accordance with an embodiment of the present invention; 
           [0011]      FIG. 2  is a cross-sectioned perspective view showing the GRA of  FIG. 1 ; 
           [0012]      FIG. 3  is another cross-sectioned perspective view showing the GRA of  FIG. 1 ; 
           [0013]      FIG. 4  is a longitudinal cross-sectional view of the GRA shown in  FIG. 1 ; 
           [0014]      FIG. 5  is a transverse cross-sectional view of the GRA taken generally along the line A-A in  FIG. 4 ; 
           [0015]      FIG. 6  is a transverse cross-sectional view of the GRA taken generally along the line B-B in  FIG. 4 ; 
           [0016]      FIG. 7  is a transverse cross-sectional view of the GRA taken generally along the line C-C in  FIG. 4 ; 
           [0017]      FIG. 8  is a transverse cross-sectional view of the GRA taken generally along the line D-D in  FIG. 4 ; 
           [0018]      FIG. 9  is a perspective view showing a lockout mechanism of the GRA according to an embodiment of the present invention; 
           [0019]      FIGS. 10A and 10B  are enlarged cross-sectional views illustrating axial displacement of a lockout ring of the lockout mechanism from a non-lockout position to a lockout position when the torque limit is exceeded; 
           [0020]      FIGS. 11A-11E  are a series of transverse cross-sectional views illustrating operation of the lockout mechanism when the torque limit is exceeded; 
           [0021]      FIGS. 12A-12E  are a series of transverse cross-sectional views illustrating resetting of the lockout mechanism by counter-rotation of a pawl carrier of the lockout mechanism; and 
           [0022]      FIG. 13  is a detailed cross-sectional view illustrating an asymmetrical ball pocket in accordance with an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0023]      FIGS. 1-7  depict a GRA  10  embodying the present invention. GRA  10  may be used in an aircraft control surface actuation system or in other applications involving torque transmission. GRA  10  is configured to transmit torque between an input shaft  12  rotatable about an input axis and an output shaft  14  rotatable about an output axis. In the current embodiment, the input axis and output axis coincide with one another along a main axis  11 . 
         [0024]    GRA  10  comprises a structural ground in the form of an outer housing  16  that may include a housing shell  18 , a housing end plate  20  at an end of housing shell  18 , and a spacer ring  22  held in an axially fixed location adjacent housing end plate  20 . Spacer ring  22  may define a ring-shaped radial step surface  24 . Input shaft  12  may be rotatably supported at an input end of housing  16  by a rotary bearing  13 . Output shaft  14  may be rotatably supported at an end of input shaft  12  by another rotary bearing  15 . 
         [0025]    GRA  10  also comprises a gear assembly for transmitting rotational motion of input shaft  12  to output shaft  14 . As shown in the illustrated embodiment, the gear assembly may include an input gear  26 , a reference gear  28 , a driven gear  30 , and at least one transmitting gear  32 . Input gear  26  may be fixedly mounted on input shaft  12  or integrally formed with the input shaft such that it rotates relative to housing  16  in response to rotation of the input shaft. Reference gear  28  is coupled to housing  16  such that the reference gear does not move relative to housing  16  unless a torque limit is exceeded. For example, reference gear  28  may be in the form of an internally-toothed ring gear held within housing  16  such that the reference gear will not rotate about main axis  11  relative to housing  16  unless the reference gear is subjected to torque about main axis  11  that exceeds the torque limit. Driven gear  30  is associated with output shaft  14 , for example by fixedly connecting the driven gear to output shaft  14  or integrally forming the driven gear with output shaft  14 , wherein output shaft  14  is rotated in response to rotation of driven gear  30 . As shown in the illustrated embodiment, driven gear  30  may be an internally-toothed ring gear. 
         [0026]    The at least one transmitting gear  32  engages input gear  26 , reference gear  28  and driven gear  30  such that rotation of input shaft  12  causes rotation of output shaft  14  without causing movement of reference gear  28  relative to the structural ground provided by housing  16  unless the torque limit is exceeded. When the torque limit is exceeded, reference gear  28  moves relative to the structural ground (i.e. housing  16 ) by rotating about main axis  11  relative to housing  16 . 
         [0027]    As shown in the figures, the gear assembly may be a planetary gear assembly in which input gear  26  is arranged as a sun gear on input shaft  12 , reference gear  28  is arranged as a ring gear about the input gear, and the at least one transmitting gear  32  includes a plurality of planet gears arranged between input gear  26  and the reference gear  28 . In the depicted embodiment, the plurality of planet gears (i.e. transmitting gears  32 ) extend axially in a direction parallel to main axis  11 . Input gear  26  and driven gear  30  may be arranged coaxially with one another along main axis  11 . Furthermore, reference gear  28  may be arranged coaxially with input gear  26  and driven gear  30  along main axis  11 . Transmitting gears  32  may be arranged about input gear  26 , and each transmitting gear may include a first toothed portion  32 A meshing with input gear  26  and reference gear  30 , a second toothed portion  32 B meshing only with reference gear  28 , and a third toothed portion  32 C meshing only with driven gear  30 . 
         [0028]    As mentioned above, reference gear  28  moves relative to housing  16  when the torque limit is exceeded. Reference gear  28  may be coupled to housing  16  by frictional contact such that the torque limit corresponds to a torque necessary to overcome static friction associated with the frictional contact. The frictional contact may include frictional contact between a cylindrical exterior surface of reference gear  28  and a cylindrical interior surface of housing shell  18 . The frictional contact may also include an annular end surface  28 A of reference gear  28  and a radial step surface  24  of housing  16 . The frictional contact between end surface  28 A and radial step surface  24  may be spring-loaded, for example by an axially-loaded spring or spring pack  36 . Spring  36  may be a Belleville spring, for example. 
         [0029]    Additional reference is now made to  FIGS. 8 through 12E . GRA  10  may further comprise a lockout mechanism generally identified by reference numeral  40 , for preventing transmission of torque between input shaft  12  and output shaft  14  after the torque limit has been exceeded. Lockout mechanism  40  may operate by redirecting torque from input shaft  12  to the structural ground provided by housing  16  after the torque limit has been exceeded. 
         [0030]    Lockout mechanism  40  may comprise a pawl carrier  42  arranged to rotate with input shaft  12 , and at least one pawl member  44  pivotally coupled to pawl carrier  42 . Lockout mechanism  40  may also comprise a lockout ring  46  including at least one stop  48  extending radially inward, wherein the lockout ring is arranged along main axis  11 . In the described embodiment, lockout ring  46  is axially displaceable from a non-lockout position (see  FIG. 10A ) wherein each stop  48  of lockout ring  46  is radially clear of each pawl member  44  to a lockout position wherein each stop  48  of lockout ring  46  radially interferes with each pawl member  44  (see  FIG. 10B ). Lockout ring  46  may be mounted in housing shell  18  by axial slide pins  49  received in corresponding external axial grooves in lockout ring  46  and internal axial grooves within housing shell  18 , whereby lockout ring  46  is free to move axially through a range, but is prevented from rotating about main axis  11  relative to housing  16 . Exactly two pawl members  44 , or a different number of pawl members  44 , may be provided. If more than one pawl member  44  is provided, the pawl members  44  may be arranged at regular angular intervals about main axis  11 . Exactly four stops  48 , or a different number of stops  48 , may be provided. If more than one stop  48  is provided, the stops  48  may be arranged at regular angular intervals about main axis  11 . 
         [0031]    As best seen in  FIGS. 8 ,  9 , and  10 A, each pawl member  44  may be pivotally mounted on pawl carrier  42  by a pivot pin  50 , and releasably held in a neutral pivot position as shown in  FIG. 8  by a radially-directed spring-loaded ball plunger  52  seated in pawl carrier  42 . When pawl member  44  is in its neutral position, ball plunger  52  engages a central recess  54  of the pawl member. Each pawl member  44  may also include lateral recesses  56  on opposite sides of central recess  54  for engagement by ball plunger  52  when pawl member  44  pivots about an axis defined by pivot pin  50 , as will be described later below. Each pawl member  44  may have a pair of catch members  58  extending in opposite lateral directions relative to pivot pin  50 , and an outer tab  60  in radial alignment with ball plunger  52 . In the illustrated embodiment, tab  60  is adjacent a radial clearance surface  62  in a direction of main axis  11 . As will be understood, radial clearance surfaces  62  of pawl members  44  are axially aligned with stops  48  of lockout ring  46  when lockout ring  46  is in its non-lockout axial position, such that pawl carrier  42  is free to rotate relative lockout ring  46  without any of the pawl members  44  engaging any of the stops  48 . Thus, pawl carrier  42  is free to rotate with input shaft  12  about main axis  11  under normal operating conditions. 
         [0032]    Lockout mechanism  40  may also comprise spring  36  arranged to urge the lockout ring  46  toward the non-lockout position, and a plurality of ball bearings  64  seated between lockout ring  46  and reference gear  28 . Ball bearings  64  are seated so as to maintain lockout ring  46  in the non-lockout position when lockout ring  46  and reference gear  28  are in a predetermined angular orientation about main axis  11  relative to one another, and to displace lockout ring  46  toward the lockout position when the reference gear  28  rotates about main axis  11  relative to lockout ring  46 . For example, ball bearings  64  may be seated within a corresponding set of pockets  66  in lockout ring  46  and another corresponding set of pockets  68  in reference gear  28 , and the ball bearings  64  roll out of respective pockets  66  and  68  incident to rotation of reference gear  28  relative to lockout ring  46 . 
         [0033]    Operation of GRA  10  and lockout mechanism  40  is now described. Under normal operating conditions, torque applied to input shaft  12  rotates the input shaft about main axis  11 , thereby rotating input gear  26  about main axis  11 . The rotation of input gear  26  causes counter-rotation of transmitting gears  32 . The transmitting gears  32  are meshed with reference gear  28 , which remains stationary under normal loading conditions, such that the transmitting gears  32  orbit about input gear  26 . The rotation of transmitting gears  32  causes output gear  30  to rotate, which in turn causes output shaft  14  to rotate for displacing a load, e.g. moving an aircraft control surface. 
         [0034]    Under certain abnormal or unexpected operating conditions, such as the malfunction or jamming of a control surface panel, rotation of output shaft  14  is impeded while input torque continues to be applied, and a sudden increase in torque at the output shaft occurs. Consequently, transmitting gears  32  experience increased torque loading and thus transmit additional torque to reference gear  28 . When a designed torque limit is exceeded, static friction is overcome and reference gear  28  will move relative to housing  16  by rotating about main axis  11  in the illustrated embodiment. This slippage within GRA  10  helps to prevent structural damage to output shaft  14  and downstream components. 
         [0035]    After the torque limit has been exceeded, lockout mechanism  40  is activated to prevent transmission of torque between input shaft  12  and output shaft  14 . As reference gear  28  rotates relative to housing  16 , it also rotates relative to lockout ring  46 , which is prevented from rotation with respect to housing  16  by slide pins  49 . This relative angular displacement causes ball bearings  64  to roll out of their respective pockets  66  in lockout ring  46  and pockets  68  in reference gear  28 , thereby displacing lockout ring  46  axially toward its lockout position against the bias of spring  36 . 
         [0036]    Reference is now made to  FIGS. 11A-11E , which illustrate what happens once lockout ring  46  is in its lockout position and input shaft  12 . In  FIG. 11A , lockout ring  46  is still in its non-lockout position, whereas in  FIG. 11B , lockout ring has been axially displaced to its lockout position. When lockout ring  46  is in its lockout position, stops  48  interfere radially with the circular travel path of tabs  60  on pawl members  44 . As pawl carrier  42  rotates, tab  60  of a pawl member  44  engages a stop  48  as shown in  FIG. 11B . As depicted in  FIG. 11C , this engagement causes pawl member  44  to pivot about an axis defined by pivot pin  50 , thereby compressing ball plunger  52  as the ball plunger moves out of central recess  54  in the pawl member. Rotation of pawl carrier  42  continues, accompanied by further pivoting of pawl member  44 , until ball plunger  52  resiliently decompresses and is received within a lateral recess  56  in pawl member  44 , as may be seen in  FIG. 11D . At this stage, pawl member  44  is set in a lockout pivot position wherein one of its catch members  58  will radially interfere with stops  48  and the other catch member  58  will be braced against pivoting by engagement with pawl carrier  42 . As pawl carrier  42  continues to rotate about main axis  11 , the cocked pawl member  44  will engage the next stop  48  as shown in  FIG. 11E . Consequently, transmission of torque between input shaft  12  and output shaft  14  is prevented. In the embodiment described herein, torque from input shaft  12  is redirected by lockout mechanism  40  to the structural ground provided by housing  16 . 
         [0037]      FIGS. 12A-12E  illustrate how lockout mechanism  40  may be reset by commanding counter-rotation of input shaft  12  to thereby counter-rotate pawl carrier  42 . Initially, it will be understood that ball bearings  64  have already realigned with pockets  66  and  68 , and the bias of spring  36  has returned lockout ring  46  to its non-lockout axial position. Pawl carrier  42  and pawl members  44  begin from the full lockout condition depicted in  FIG. 12A  (this is the same condition shown in  FIG. 11E ). Pawl carrier  42  is counter-rotated until the trailing, radially outer catch member  58  engages the previous stop  48  as shown in  FIG. 12B . As may be understood from  FIG. 12C , this causes pawl member  42  to pivot about the axis of pivot pin  50 , thereby compressing ball plunger  52  as pawl carrier  42  continues its counter-rotation. Proceeding to  FIG. 12D , it will be seen that further counter-rotation of pawl carrier  42  causes pawl member  44  to continue pivoting until ball plunger resiliently decompresses and is received in central recess  54 . Consequently, as shown in  FIG. 12E , pawl member  44  is now reset with radial clearance relative to stops  48  of lockout ring  46 . 
         [0038]    As best seen in  FIG. 13 , the pockets  66  in lockout ring  46  and pockets  68  in reference gear  28  may have a first slope  70  associated with a first angular direction about main axis  11 , and a second slope  72  associated with a second angular direction about the main axis opposite the first angular direction, wherein the first slope differs from the second slope. In this way, the torque required to actuate lockout mechanism  40  may be made greater in one rotational direction, e.g. the rotational direction associated with flap or slat extension, than in the opposite rotational direction, e.g. the rotational direction associated with flap or slat retraction. 
         [0039]    Because the torque limiting mechanism of GRA  10  responds to output torque instead of input torque, the lock-up torque limit can be set closer to the maximum operating torque, resulting in a lower limit torque at the output of each GRA. This can result in significant weight savings of not only the GRA itself, but more importantly the downstream structure that it protects. 
         [0040]    The output torque sensing GRA described herein also solves the problems associated with the disc brake pack of the prior art. First, the invention eliminates the friction disc brake pack and replaces it with a pawl lockout mechanism. This change drastically reduces the viscous drag torque generated by brake plates and eliminates reliance on friction for positive torque limiting. With as many as thirty GRAs in an aircraft control surface system, this change also greatly reduces the power required by the PCU. Significant reduction in the weight of the entire drive system may be achieved. Second, the invention also has the potential to eliminate the requirement for a skew detection system on some aircraft control surface (e.g. flap and slat) actuation systems, resulting in dramatic improvements in cost, weight and system reliability. 
         [0041]    Embodiments of the present invention are described in detail herein, however those skilled in the art will realize that modifications may be made. As one example, it is noted that alternative configurations are possible in which only one ramp surface is provided, either on armature  26  or on opposing plate  14 . Such modifications do not stray from the spirit and scope of the invention as defined by the appended claims. 
       PARTS LIST 
       [0042]      10  Geared rotary actuator (“GRA”) 
         [0043]      11  Main axis 
         [0044]      12  Input shaft 
         [0045]      13  Rotary bearing 
         [0046]      14  Output shaft 
         [0047]      15  Rotary bearing 
         [0048]      16  Housing (structural ground) 
         [0049]      18  Housing shell 
         [0050]      20  Housing end plate 
         [0051]      22  Spacer ring 
         [0052]      24  Radial step surface 
         [0053]      26  Input gear 
         [0054]      28  Reference gear 
         [0055]      30  Driven gear 
         [0056]      32  Transmitting gear 
         [0057]      36  Spring 
         [0058]      40  Lockout mechanism 
         [0059]      42  Pawl carrier 
         [0060]      44  Pawl member 
         [0061]      46  Lockout ring 
         [0062]      48  Stop on lockout ring 
         [0063]      49  Slide pin 
         [0064]      50  Pivot pin 
         [0065]      52  Ball plunger 
         [0066]      54  Central recess in pawl member 
         [0067]      56  Lateral recess in pawl member 
         [0068]      58  Catch member of pawl member 
         [0069]      60  Tab of pawl member 
         [0070]      62  Radial clearance surface of pawl member 
         [0071]      64  Ball bearing 
         [0072]      66  Ball pocket in lockout ring 
         [0073]      68  Ball pocket in reference gear 
         [0074]      70  First slope of ball pocket 
         [0075]      72  Second slope of ball pocket