Abstract:
A power drive unit for propelling cargo containers and pallets in a desired direction over a plurality of cargo deck roller elements includes an elongated yoke having a pivot end and an opposite end. The pivot end is pivotally connected to a deck support structure. A drive roller is rotatably mounted to the yoke. At least one resilient member is disposed between the opposite end of the yoke and the support portion of the deck structure. A drive motor coupled to the drive roller selectively rotates the drive roller in two opposed directions. A reaction member connected to the drive motor is at least partially movably engaged with the deck support structure to permit at least some pivotal movement of the yoke, and to substantially prevent rotation of the drive motor relative to the yoke.

Description:
FIELD OF THE INVENTION 
   The invention relates to onboard cargo handling systems for aircraft, and more particularly relates to a power drive unit that is capable of maintaining driving, braking, and holding contact with an irregular underside of a cargo container or pallet, and that is capable of providing substantially equal driving traction in two opposed directions. 
   BACKGROUND 
   Items that are shipped by air typically are loaded first onto specially configured pallets or into specially configured containers. In the airfreight industry, these various pallets and containers commonly are referred to as Unit Load Devices (“ULDs”). ULDs are available in various sizes, shapes and capacities. 
   A ULD typically is loaded with cargo at a location other than the immediate vicinity of an aircraft. Once a ULD is loaded with cargo items, the ULD is weighed, transferred to the aircraft, and is loaded onto an aircraft through a doorway or hatch using a conveyor ramp, scissor lift, or the like. Once inside the aircraft, a ULD is moved within the cargo compartment to its final stowage position. Multiple ULDs are brought onboard the aircraft, and each is placed in its respective stowed position. Once the aircraft reaches its destination, the ULDs are unloaded from the aircraft in a manner that is the reverse of the loading procedure. 
   To facilitate movement of a ULD within an aircraft cargo compartment as the ULD is loaded, stowed, and unloaded, the deck of an aircraft cargo compartment typically includes a number of raised roller elements. These roller elements often include elongated roller trays that extend longitudinally along the length of the cargo deck, ball panel units, and the like. For example, roller trays typically include elongated rows of cylindrical rollers that extend in a fore and aft direction. Ball panel units include plates with upwardly protruding rotatable spherical balls. The ULDs sit atop these roller elements, and the roller elements facilitate rolling movement of the ULDs within the cargo compartment. Cargo decks also commonly are equipped with a plurality of power drive units (PDUs). PDUs are electrically powered rollers that can be selectively energized to propel or drive a ULD in a desired direction over a cargo deck&#39;s roller elements. 
   Generally, PDUs can be one of two basic types. A first type of PDU is secured to a cargo deck structure such that the powered drive roller can only rotate in fore and aft directions within a cargo hold. Such a “fixed” PDU typically is installed within a cargo roller tray such that the PDU&#39;s drive roller protrudes above a plane defined by the uppermost portions of adjacent roller elements when the drive roller is in an active position. The drive roller can be either an inflated tire or a rigid roller having an elastomeric rim. The rotating tire or roller contacts and grips the bottom of an overlying ULD such that the ULD is driven in a desired direction by traction between the roller and the underside of the ULD. Such stationary PDUs often are configured such that the drive roller can be selectively moved between an active raised position, and a retracted inactive or stowed position. The lifting of the drive roller from the retracted position can be actuated by springs, by an electrically powered lift mechanism, or the like. Such fixed PDU&#39;s typically are installed at cargo deck locations remote from an aircraft&#39;s cargo door, where a ULD&#39;s movement can be substantially limited to the fore and aft directions. 
   A second type of PDU is known as a “steerable PDU”. In a typical steerable PDU, the drive roller is mounted to a rotatable frame or turntable that can be selectively oriented to align the drive roller in a desired direction within a cargo hold. Like the fixed PDUs described above, a steerable PDU can be configured to lift and retract the drive roller between its active raised position and its inactive retracted position. Steerable PDUs usually are installed at cargo deck locations that are proximate to an aircraft&#39;s cargo door, where a ULD may require movement in a direction other than the fore or aft directions as the ULD is being loaded and/or unloaded. 
   The bottom surfaces of ULDs can be irregular either due to their original construction or due to damage or deformation from prior use. Accordingly, when a ULD with an irregular bottom surface moves over an active PDU, the degree of contact between the unit&#39;s drive roller and the ULD can vary between full contact, partial contact, and zero contact. Once contact between the drive roller and the irregular surface of the ULD is lost or substantially reduced, the traction force between the drive roller and the ULD can be lost or reduced. When such lost or reduced traction occurs, the movement of the ULD within the cargo hold can be slowed or stopped, which detrimentally affects the cargo loading or unloading process. Though drive rollers that include resilient inflated tires can accommodate a certain amount of variation in contact between the drive roller and a ULD, non-inflated drive rollers are substantially less compliant to variations in the geometry of a ULD&#39;s undersurface. 
   One solution to this problem of lost or reduced contact and traction between a drive roller  20  and an irregular bottom surface  42  of a ULD  40  is illustrated in  FIG. 1 . In  FIG. 1 , a load-compliant PDU lift system  10  includes a drive roller  20  on a drive shaft  22  that is rotatably mounted to a yoke  12 . As used herein, the term “load-compliant” means capable of automatically adapting to substantial variations or irregularities in the geometry of the undersurface of a ULD that contacts a PDU&#39;s drive roller. A first end  14  of the yoke  12  is pivotally mounted to a base  30  about a pivot axis  18 . A second end  16  of the yoke  12  is vertically supported by one or more springs  50  disposed between the second end  16  and the base  30 . Accordingly, as the spring is compressed by a vertical load “L” on drive roller  20 , the second end  16  of the yoke  12  moves downward, the yoke  12  pivots downward, and the attached drive roller  20  also moves downward. Conversely, as the spring  50  pushes the second end  16  upward, the yoke  12  pivots upward, and the attached drive roller  20  also moves upward. Thus, the spring  50  permits the drive roller  20  to move up and down as necessary to maintain contact with an irregular bottom surface  42  of the ULD  40  as the ULD  40  is propelled by a traction force “F T ” applied by the roller  20 . The spring (or springs)  50  is sized such that the vertical force applied by the spring  50  is sufficient to maintain frictional contact between the drive roller  20  and the bottom surface of the ULD under load L. The PDU lift system  10  shown in  FIG. 1  can be adapted to mount to a stationary support, frame or base  30 , or to mount to a steerable rotating support or frame. In addition, the PDU lift system  10  can be configured such that the drive roller  20  is selectively retractable. 
   Though the load-compliant PDU lift system  10  depicted in  FIG. 1  may effectively maintain contact between the drive roller  20  and a ULD&#39;s irregular bottom surface  42 , such a PDU lift system  10  has some shortcomings. As shown in  FIG. 1 , the drive roller  20  is selectively operable to be driven and rotate in a counterclockwise driving direction “I”, and to be driven and rotate in an opposite clockwise driving direction “II”. The drive shaft  22  and drive roller  20  are rotated by a drive motor (not shown in  FIG. 1 ) that is affixed to the yoke. When the drive roller  20  is driven in a counterclockwise direction “I” under vertical load L, the drive roller  20  is subjected to a traction force F T  (acting left to right in  FIG. 1 ) due to the frictional drag between the drive roller  20  and the bottom surface  42  of the overlying ULD  40 . Because the drive roller  20  is connected to a drive motor that is affixed to the yoke  12 , this traction force F T  results in a clockwise torque T CW  acting on the yoke  12  that is equal to the traction force F T  times the vertical distance “H” between the top of the drive roller  20  and the yoke pivot axis  18  (T CW =F T ·H). The clockwise torque T CW  in turn forces the yoke  12  to rotate in a clockwise direction, thus compressing the spring  50 , and causing the yoke  12  and drive roller  20  to move away from the ULD  40 , and thus lessening the degree of contact between the drive roller  20  and the ULD&#39;s bottom surface  42 . Because the traction force F T  applied to the ULD  40  by the drive roller  20  is dependent upon the degree of contact between the drive roller  20  and the ULD  40 , the additional compression of the spring  50  that results from the counterclockwise rotation of the drive roller  20  is detrimental to the magnitude of the driving force F T  that is effectively applied to the ULD&#39;s bottom surface  42 . 
   In contrast, when the drive roller  20  is driven in a clockwise direction “II”, the direction of the frictional traction force F T  is opposite from that shown in  FIG. 1  (i.e. right to left in  FIG. 1 ), and the resulting torque T CCW  on the yoke  12  thus acts in a counterclockwise direction. This counterclockwise torque T CCW  forces the yoke  12  to rotate in a counterclockwise direction about the yoke pivot axis  18  and to move upward, thereby increasing the degree of contact between the drive roller  20  and the underside  42  of the ULD  40 . Because the degree of contact between the drive roller  20  and the ULD&#39;s underside  42  is increased by this movement, the effective traction force F T  on the underside  42  of the ULD  40  is enhanced by the counterclockwise movement of the yoke  12 . Thus, the load-compliant PDU  10  has a “strong” driving direction (left to right in  FIG. 1 ), and a “weak” driving direction (right to left in  FIG. 1 ). One possible solution to this problem is to provide larger and stiffer springs  50  to minimize the amount of additional spring compression that results from the traction force F T . But such large springs  50  can undesirably increase the size and weight of the PDU  10 . 
   Accordingly, there is a need for a load-compliant PDU, and more specifically, for a load-compliant PDU lift system that is equally effective in driving ULDs in two opposite directions. 
   SUMMARY 
   In one embodiment, the invention includes a power drive unit of a type that is mountable to a support portion of an aircraft deck structure. The power drive unit can include an elongated yoke having a pivot end and an opposite end, the pivot end being constructed and arranged for pivotal connection to the support portion of the aircraft deck structure along a pivot axis. The power drive unit further can include a drive roller rotatably mounted to the yoke about a roller axis, and at least one spring member constructed and arranged to be disposed between the opposite end of the yoke and the support portion of the aircraft deck structure. In addition, the power drive unit can include a drive motor coupled to the drive roller that is operable to selectively rotate the drive roller about the roller axis in two opposed directions. Furthermore, the invention can include a reaction member connected to the drive motor that is constructed and arranged to be at least partially movably engaged with the support portion of the aircraft deck structure to permit at least some pivotal movement between the yoke and the support portion of the aircraft deck structure, and to substantially prevent rotation of the drive motor relative to the yoke. 
   In addition, the invention includes a power drive unit having a drive roller powered by a drive motor, and means for supporting the drive roller proximate to a floor of a cargo deck. The power drive unit can further include a means for resiliently biasing the powered drive roller in an upward direction, and a means separate from the means for biasing for substantially preventing the transmission of torsional loads from the drive roller to the yoke. 
   The invention also includes a power drive unit for an aircraft having a cargo deck structure. The power drive unit can include a selectively rotatable frame constructed and arranged for mounting to the cargo deck structure, and an elongated yoke having a pivot end and an opposite end, the pivot end being pivotally connected to a first portion of the selectively rotatable frame along a pivot axis. The power drive unit also can include a drive roller rotatably mounted to the yoke, and a drive motor coupled to the drive roller that is engaged with the selectively rotatable frame in a manner that permits at least some pivoting movement of the yoke about the pivot axis, and that prevents substantial rotation of the drive motor relative to the yoke. In addition, the power drive unit can include at least one spring member disposed between the opposite end of the yoke and a second portion of the selectively rotatable frame. 
   These and other aspects of the invention will be apparent from a reading of the following detailed description together with the drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic side view of a prior art load-compliant PDU. 
       FIG. 2  is a perspective view of one embodiment a load-compliant lift system for a power drive unit according to the invention. 
       FIG. 3A  is side elevation view of the load-compliant lift system shown in  FIG. 2  with the drive roller in a retracted inactive position. 
       FIG. 3B  is a side elevation view of the load-compliant lift system shown in  FIGS. 2 and 3A  with the drive roller in a raised active position. 
       FIG. 4  is a detail perspective view of a reaction member portion of the load-compliant lift system shown in  FIGS. 2-3B . 
       FIG. 5  is a top perspective view of one embodiment of a steerable load-compliant power drive unit according to the invention. 
       FIG. 6  is another top perspective view of the steerable load-compliant power drive unit shown in  FIG. 5  with a top cover removed. 
       FIG. 7  is a bottom perspective view of the steerable load-compliant power drive unit shown in  FIGS. 5 and 6 . 
       FIG. 8  is a side perspective view of a lift system portion of the steerable load-compliant power drive unit shown in  FIGS. 5-7 . 
       FIG. 9  is a side elevation view of the lift system shown in  FIG. 8  with the drive roller in the raised active position. 
   

   DESCRIPTION 
   One embodiment of a load-compliant PDU with an improved lift system  100  according to the invention is shown in  FIGS. 2-4 . A shown in  FIGS. 2-3B , the PDU lift system  100  can include an elongated yoke  112  having a first end  114  and an opposed second end  116 . As shown in  FIGS. 3A and 3B , the first end  114  can be pivotally connected to a base or frame  190  by one or more hinge pins  118 . As shown in FIGS.  2  and  3 A- 3 B, the second end  116  of the yoke  112  includes a bearing plate  117  having a bottom surface that engages a plunger  152  of a coil spring pack  150 . In the embodiment shown, the spring pack  150  includes a housing  156  having a bottom  154 , and a movable top or plunger  152 . One or more coil springs  158  are vertically disposed within the housing  156  between the bottom  154  and plunger  152 . As shown in  FIGS. 3A and 3B , the bottom  154  of the spring pack housing  156  is fixed to base or frame  190 , such as with bolts, or the like. The distance between the bottom  154  and plunger  152  and the length of the coil springs  158  can be selected to such that the springs are pre-compressed and preloaded when the plunger  152  is at an uppermost position. 
   As shown in  FIGS. 3A and 3B , a drive roller  120  is affixed to a drive shaft  121  that is rotatably supported by the yoke  112 . As shown in  FIG. 2 , the drive roller can include a substantially rigid hub  122  surrounded by a rubber or polymeric rim  124 . Alternatively, the drive roller  120  can be an inflatable tire mounted on the hub  122 . A drive motor  160  is coupled to the drive shaft  121 , and is configured to selectively drive the drive shaft  121  and the connected drive roller  120  in two opposed rotating directions, such as fore and aft directions, for example. The drive motor  160  can be an electric motor, for example. In this embodiment and unlike previous PDUs, the drive motor  160  is not connected to the yoke  112 . Thus, the drive roller  120  is incapable of applying torsional loads to the yoke  112  through the rotatably connected drive shaft  121  and unconnected drive motor  160 . Because the drive motor  160  is not connected to the yoke, the drive motor  160  must otherwise be supported against rotation. For this purpose, a flange  174  affixed to the motor  160  rotatably supports a roller  172 . As shown in  FIG. 4 , the roller  172  is received in a substantially vertical slot  192  in the base or frame  190  of the lift unit  100 , which is fixed to or is an integral part of an aircraft structure that is beneath or forms a part of a cargo deck. As shown in  FIG. 2 , a lift post  180  can outwardly extend from the yoke  112 . The springs  158  of PDU lift system  100  permit the yoke  112  to upwardly and downwardly pivot about pin  118  in response to variations in the underside surface of a ULD, thereby maintaining substantial contact between the drive roller  120  and an overlying ULD when the PDU is in an active arrangement and engaged with the ULD. The roller  172  on the drive motor  160  permits the drive shaft  121  and drive roller  120  to upwardly and downwardly move with the yoke  112 , but substantially prevents rotation of the drive motor  160  relative to the base or frame  190 . 
   The PDU lift system  100  is shown in a retracted, inactive position in  FIG. 3A , in which the top of the drive roller  120  is positioned beneath the underside of an overlying ULD (indicated by line C-C). In this inactive position, the yoke  112  is downwardly pivoted about hinge pin  118  such that the coil springs  158  are further compressed by the bearing plate  117  and plunger  152 . The yoke  112  can be moved to this retracted position and/or maintained in this position by a lift actuator  182  of a type known in the art. The lift actuator  182  can engage the lift post  180  shown in  FIG. 2 , and cause the yoke  112  and drive roller  120  to be positioned such that there is no substantial contact between the drive roller  120  and the underside of an overlying ULD (indicated by line C-C). 
   The PDU lift system  100  is shown in a raised, active position in  FIG. 3B , in which the top of the drive roller  120  contacts the underside of an overlying ULD (again indicated by line C-C). In this active position, the yoke  112  and bearing plate  117  are pivoted upward about hinge pin  118  such that the coil springs  158  are extended, and thus compressed less than in their maximally compressed state shown in  FIG. 3A . The compressed springs  158  can assist in moving the yoke  112  from the retracted position shown in  FIG. 3A  to the active position shown in  FIG. 3B . The yoke  112  also can be released and/or moved to this active position by the lift actuator  182 . In the active position, the top of the drive roller  120  is in substantial contact with the lower surface of an overlying ULD. In addition, the actively positioned drive roller  120  supports at least a portion “L” of the weight of an overlying ULD. 
   As discussed above and as best shown in  FIG. 4 , the roller  172  on the drive motor  160  is received in a slot  192  in the base or frame  190  of the lift unit  100 , which is fixed to or is an integral part of an aircraft structure beneath or forming part of the cargo deck. The roller  172  and slot  192  cooperate to permit at least some vertical movement of the yoke  112  relative to the base or frame  190 , and to substantially prevent rotation of the drive motor  160  relative to the yoke  112 . The roller  172  and slot  192  also cooperate to react against any torque load on the drive roller  120  and connected drive shaft  121  caused by the traction force F T  on the drive roller  120 . Because the roller  172  is laterally constrained within the slot  192  and can move only vertically within the slot  192 , the roller  172  is effective to provide a substantially horizontal reaction force “R” (shown in  FIG. 3B ) that acts to oppose clockwise rotation of the drive motor  160  induced by the traction force F T  on the drive roller  120 . Accordingly, unlike previous PDU designs, the traction force F T  is borne by the drive motor  160  and base  190  rather than by the yoke  112 . Therefore, unlike previous PDUs, the traction force F T  does not cause downward movement of the yoke  112  and additional compression of the springs  158  when the drive roller  120  rotates in a counterclockwise direction against a lower surface of a ULD. Thus, the effective drive force on a ULD will not be substantially diminished by undesired further compression of the springs  158  as a result of clockwise downward rotation of the yoke  112  in response to a traction force F T . As shown in  FIG. 3B , the roller axle  176  can be positioned at a position that is coincident with the outer radius of the drive roller  120 , such that the reaction force R is substantially equal to the traction force F T . 
   A load compliant PDU lift system  100  like that described above can be configured to mount to either a fixed base  190 , as described above, or can be configured to mount to a steerable base. Another embodiment of the invention that is mounted to a steerable base or frame is described below. 
   As shown in  FIGS. 5-7 , a steerable, load-compliant PDU  200  includes a fixed frame or mounting ring  202 . In the embodiment shown, the mounting ring  202  supports a rotatable inner frame or pivot plate  208 . The pivot plate  208  can incorporate a removable cover  204 , and includes an opening that permits a drive roller  320  to upwardly extend to a raised, active position. Like the fixed embodiment  100  described above and as shown in  FIG. 5 , the PDU  200  also includes a drive motor  360  and a lift actuator  382 . The steerable load-compliant PDU  200  also can be provided with a power supply cable  203 . As shown in  FIG. 6 , the pivot plate  208  supports a PDU lift mechanism  300  within the stationary outer frame  202 . As shown in  FIGS. 6 ,  8  and  9 , the lift mechanism  300  can include a pair of opposed yoke members  312   a ,  312   b  respectively having first ends  314   a ,  314   b , and second ends  316   a ,  316   b . Alternatively, the lift mechanism  300  can include a one-piece yoke. The yoke members  312   a ,  312   b  can be mirror images of each other. In this embodiment, the first ends  314   a ,  314   b  of the yoke members  312   a ,  312   b  are pivotally connected to the rotatable inner frame  208  by one or more hinge pins, or the like. A drive roller  320  is rotatably mounted via a drive shaft  321  (shown in  FIG. 9 ) between the opposed yoke members  312   a ,  312   b , and is powered by a reversible drive motor  360  coupled to the drive shaft  321 . 
   Now referring to  FIGS. 6 and 7 , the steerable PDU  200  can include a stationary support frame  202  and a rotatable frame  208 . The stationary support frame  202  can be attached to an aircraft structure such that the frame  202  is fixed relative to a cargo deck of an aircraft. An actuator  210  can selectively rotate the rotatable frame  208  relative to the support frame  202 , such that the drive roller  320  can be selectively oriented in a desired direction on the cargo deck. The PDU lift portion  300  of this PDU  200  can be substantially the same as the PDU lift system  100  described above, except as further described below. 
   As shown in  FIG. 6 , the second ends  316   a ,  316   b  of yokes  312   a ,  312   b  of lift system  300  respectively include rollers  317   a ,  317   b . As shown in  FIGS. 6 and 7 , the rollers  317   a ,  317   b  respectively cooperate with first and second cams  386   a ,  386   b  which are rotatably mounted to the frame  208 . A first coil spring  350   a  is interconnected between the frame  208  and the first cam  386   a , and a second coil spring  350   a  is interconnected between the frame  208  and the second cam  386   a . Selective tandem rotation of the cams  386   a ,  386   b  by lift actuator  382  cause the rollers  317   a ,  317   b  and the second ends of yokes  312   a ,  312   b  to be raised and lowered as desired. The coil springs  350   a ,  350   b  permit at least some resilient movement between the frame  208  and the cams  386   a ,  386   b , thereby permitting the rollers  317   a ,  317   b , the second ends of yokes  312   a ,  312   b , and the drive roller  320  to move up or down in response to contact with an irregular bottom surface of a ULD. Accordingly, the coil springs  350   a ,  350   b  act to maintain frictional contact between the drive roller  320  and the bottom surface of an overlying ULD, even if different portions of the bottom surface vary in elevation relative to the cargo deck. 
   As shown in  FIGS. 8 and 9 , the PDU lift system  300  further includes a link  390  having a first end  392  and second end  394 . As shown in  FIG. 9 , the first end  392  of the link  390  is pivotally connected to a flange  374  on the drive motor  360 . The second end of the link  390  is pivotally connected to the frame  208 . The link  390  is configured such that the link permits the yokes  312   a ,  312   b  to pivot upwardly and downwardly about hinge points  318   a ,  318   b , while also preventing rotation of the drive motor  360  relative to the yokes  312   a ,  312   b . Accordingly, as shown in  FIG. 9 , the link  390  is capable of providing a reaction force “R” that is substantially parallel to its longitudinal axis. In operation, when the drive roller  320  is engaged with an overlying ULD, the drive roller experiences a traction force F T  that is parallel to a circumference of the roller  320 . As indicated in  FIG. 9 , when the drive roller  320  is driven in a counterclockwise direction, the traction force F T  acts in a left-to-right direction at the top of the roller  320 . If the drive motor  360  was connected to one or both yokes  312   a ,  312   b  (as in previous designs) rather than to the frame  208  via link  320 , the cams  386   a ,  386   b  and the coil springs  350   a ,  350   b  would necessarily react to at least a substantial portion of the resultant counterclockwise torque, the cams and springs would deflect, and the yokes  312   a ,  312   b  and drive roller  320  would move downward. Such downward movement would reduce or eliminate contact between the drive roller  320  and an overlying ULD. Such a reaction could substantially diminish the amount of traction force F T  being applied to the overlying ULD. 
   The link  390 , however, reacts against the traction force F T  by resisting rotation of the drive motor  360 . Because the drive roller  320  is incapable of imparting torsional loads to the yokes  312   a ,  312   b  through the drive shaft and connected drive motor  360 , there is no unwanted resultant downward movement of the yokes  312   a ,  312   b  and drive roller  320  in response to a torsional traction load F T  on the drive roller  320 . Accordingly, the effective drive force F T  between the drive roller  320  and a ULD will not be substantially diminished by undesired retraction of the drive roller in response to a traction force F T  when the roller  320  is driven in a counterclockwise direction. 
   The above descriptions of various embodiments of the invention are intended to illustrate various aspects and features of the invention. Persons of ordinary skill in the art will understand that certain modifications can be made to the specifically described embodiments without departing from the invention. All such changes and modifications are intended to be within the scope of the appended claims.