Abstract:
A power drive unit (PDU) for cargo handling systems comprising a frame arranged to be mounted in use for raising and lowering movement relative to a supporting structure, a drive motor carried by the frame and having an output shaft, a drive roller assembly carried by said frame for engagement in use with a unit load device (ULD) or the like to be moved by the PDU, a first gear train transmitting drive from said motor output shaft to a drive roller of said drive roller assembly, a rotatable lifting cam assembly carried by said frame and driven in use relative to said frame to lift and lower the frame on said mounting, a second gear train for transmitting drive from said motor output shaft to said lifting cam assembly to operate said lifting cam assembly, clutch means operable to connect and disconnect said motor output shaft to and from said second gear train, a torque limiting device limiting the torque transmitted through said clutch, when said clutch is engaged, to said lifting cam assembly, and a brake mechanism between said torque limiting device and said lifting cam assembly for braking said lifting cam assembly against rotation relative to said frame.

Description:
RELATED APPLICATION 
     This application claims priority to United Kingdom Patent Application Number 0212354.5, filed May 29, 2002. 
     TECHNICAL FIELD 
     The present invention relates to a power drive unit for cargo handling systems, particularly drive units for use in a cargo handling system in the cargo compartment of an aircraft. 
     BACKGROUND ART 
     Conventional Power Drive Units (PDUs) include rotationally driven rollers which can be raised through an aperture in a deck panel of the cargo compartment frictionally to engage the under-surface of a cargo unit, conventionally a Unit Load Device (ULD) to move the ULD within the cargo compartment. It would be understood however that such PDUs can be used to handle ULDs and other cargo units in cargo handling systems external to the cargo compartment of an aircraft or other vehicle or vessel. 
     In the accompanying drawings FIG. 1 is a cross-sectional representation of a conventional PDU. The conventional PDU includes a rigid metallic frame  11  pivotally mounted at  12  to a chassis (not shown in FIG. 1) rigidly secured beneath a deck panel of the floor or deck of a cargo compartment. The axis  13  of pivotal movement of the frame  11  relative to the chassis is parallel to, and spaced below, the plane of the deck panel. Adjacent its end remote from the pivot axis  13 , the frame  11  rotatably supports a roller assembly  14  for rotation about an axis  15  parallel to the axis  13 . The roller assembly is disposed in alignment with an aperture in the deck panel and in a rest position of the frame  11  relative to the deck panel rubber tyred rollers  15   a  of the roller assembly  14  lie just beneath the plane of an array of Ball Transfer Units (BTUs) carried on the upper surface of the deck panel to provide a low friction support for a ULD on the deck panel. There is provided an arrangement for raising the PDU by pivoting the frame  11  about the axis  13  to raise the periphery of the rollers  15   a  through the aperture in the deck panel to engage the under-surface of a ULD seated on the BTUs. 
     Mounted within the frame  11  is an electric drive motor  16  the rotor shaft  17  of which is equipped, at one end of the shaft  17 , with an electro-magnetically operable brake assembly  18 . The brake assembly  18  when operative brakes the shaft  17  of the motor against rotation. 
     The opposite end of the shaft  17  from the brake  18  is equipped with a small diameter pinion gear wheel  19  which meshes with the teeth of a large diameter internal gear wheel  21  mounted to the frame  11  for rotation about an axis parallel to the axes of the shaft  17  and roller assembly  14 . A shaft  22  extending from the gear wheel  21  and rotatable therewith is formed with a small diameter pinion gear wheel  23  meshing with a larger diameter gear wheel  24  on a shaft  25  mounted to the frame for rotation about an axis co-extensive with the axis of rotation of the shaft  17 . A train of gears  43 ,  44 ,  45  all rotatable about parallel axes transmit drive from the gear wheel  24  to a shaft  46  carrying the gear wheel  45 . The shaft  46  carries the axially aligned rollers  15   a  for rotation therewith about the axis  15 , the shaft  46  being journalled at its opposite axial ends respectively in bearings on the frame  11  for rotation relative thereto, and the gear wheel  45  being disposed adjacent the mid-point of the length of the shaft  46  between the rollers  15   a . It will be recognised therefore that when the brake  18  is de-energised to release the shaft  17  and the motor  16  is energised then the motor  16  drives the rollers  15   a  for rotation in unison about their common rotational axis  15 . 
     The end of the shaft  25  remote from the motor  16  is coupled to an axially co-extensive drive shaft  26  through the intermediary of a torque limiting device  27  and an electro-magnetically operable clutch  28 . A small diameter pinion gear wheel  29  on the shaft  26  meshes with a larger diameter gear wheel  31  on a shaft  32  parallel to the shaft  26 . A smaller diameter gear wheel  33  on the shaft  32  drives a larger diameter gear wheel  34  driving a co-axial gear wheel  35  meshing with a gear wheel  36  on the end of a cam shaft  37  journalled for rotation in the frame  11 . The cam shaft  37  extends the full width of the frame  11  and adjacent its opposite axial ends respectively carries first and second cams  38  which cooperate with fixed cam followers on the chassis of the PDU whereby angular movement of the shaft  37  about its longitudinal axis lifts and lowers the frame  11  about the axis  13  by virtue of the cam action between the cams  38  and the cam followers on the chassis. 
     The operation of the conventional PDU illustrated in FIG. 1 is as follows. Let us assume firstly that the brake  18  is operative, the clutch  28  and the motor  16  are de-energised, and the cam shaft  37  is in a rotated position such that the frame  11  is collapsed into its rest position below the level of the deck panel. In order to raise the rollers  15   a  through the aperture in the deck panel to engage a ULD, power is supplied to the motor  16  and at the same time the brake  18  is de-energised so that the shaft  17  is released for rotation. Simultaneously power is applied to the electromagnetic clutch  28  so that the clutch is engaged and rotational movement of the shaft  17  is transmitted through the torque limiter  27  and the engaged clutch  28  to the shaft  26 . 
     Simultaneously rotational movement of the motor shaft  17  is transmitted through the gear train  24 ,  43 ,  44 ,  45  to the rollers  15   a  to rotate the rollers  15   a  about their axis  15 . 
     Rotation of the shaft  26  drives the shaft  37  through the step-down gear train  29 ,  31 ,  33 ,  34 ,  35 ,  36  interconnecting the shafts  26  and  37 . The shaft  37  is thus moved angularly about its longitudinal axis causing the cams  38  to cooperate with the chassis and thus raise the frame  11  relative to the deck panel about the axis  13 . The periphery of the rollers  15   a  is thus caused to project upwardly through the aperture in the deck panel so as frictionally to engage the under-surface of a ULD supported on the deck panel. As the rollers  15   a  are being rotated by the motor  16  the ULD will be moved relative to the deck panel. 
     It will be noted that the roller assembly  14  includes a rubber tyred wheel  39  mounted for rotation about the axis  15  and having an outer diameter similar to the outer diameter of the rollers  15   a . The wheel  39  engages the under-surface of a ULD at the same time that it is engaged by the rollers  15   a . However, the wheel  39  is not driven with the rollers  15   a  and thus can detect slip between the rollers  15   a  and ULD since in such a situation the rollers  15   a  will continue to rotate but the wheel  39  will be stationary, or moving at a different speed, by virtue of its engagement with the ULD. A slip sensor detects any difference in the rotational speeds of the wheel  39  and the rollers  15   a  and either provides warning of slippage or alternatively de-energises the motor. 
     When the engagement between the cams  38  and the cam followers reaches its highest point (corresponding to the maximum lift position of the frame  11 ) the cams  38  engage stops which prevent further rotation of the shaft  37 . Thus the gear train and the shaft  26  become stalled since the shaft  37  cannot rotate any further, and the torque limiting device  27  slips so that the motor  16  can continue to operate and to drive the rollers  15   a  notwithstanding that the shaft  37  is now held against further rotation. It will be recognised that energy is dissipated within the torque limiting device  27  as the device  27  slips throughout the whole of the time that the rollers  15  are rotated in their fully raised position. 
     In the event that the rollers  15   a  are subjected to a shock loading in a vertical direction then they can be depressed relative to the deck to accommodate such a shock loading by reverse rotation of the cams  38  and the shaft  37  as permitted by slippage in the torque limiting device  27 . Immediately the loading is removed then the rollers will be returned to their fully raised position. This arrangement also accommodates unevenness in the under-surface of a ULD or other cargo unit. 
     When it is required to reverse the direction of rotation of the rollers  15   a  in order to drive a ULD in the opposite direction the polarity of the motor  16  is reversed so that the rotor shaft  17  of the motor is rotated in the opposite direction. The effect of this is to reverse the rotational direction of the shaft  37  so that the cams  38  are moved away from their stops lowering the frame  11  to its fully lowered position, and thereafter the cams, which are symmetrical about their rest point, start to raise the frame again by rotation of the cams  38  beyond their rest position. The rollers  15   a  are of course being rotated in the opposite direction during this movement. Rotation of the shaft  37  ceases when the cams  38  engage their stops with the frame  11  full raised, but with the rollers  15   a  now rotating in the opposite direction to the previous operation. 
     If it is desired to brake the motion of a ULD in contact with the rollers  15   a  the power supply to the motor  16  is broken and the power supply to the brake  18  is re-established to brake the shaft  17  against rotation. Thus as long as the ULD is moving in the direction in which it was driven by the rollers  15   a  then the cams  38  will remain against their stops, the shaft  37  will not rotate, and the rollers  15   a  will remain in their raised position but will not be rotated so applying a braking force to a ULD moving relative thereto 
     In order retract the rollers from their operative position to their rest position below the deck panel the clutch  28  is de-energised so that the shaft  26  can rotate freely irrespective of the shaft  17  being held against rotation. Torsion springs  41  acting on the shaft  37  can now rotate the shaft  37  in the reverse direction moving the cams  38  away from their stops and lowering the frame relative to the chassis. Reverse rotation of the shaft  37  is permitted by rotation of the gear train coupling the shaft  37  to the shaft  26 , and the freedom of rotation of the shaft  26  by virtue of de-energisation of the clutch  28 . 
     The conventional PDU suffers from a number of recognised disadvantages. Firstly, power dissipated within the torque limiting device  27  during normal operation of the PDU is wasteful of energy, and generates heat which may be a problem in some environments. Secondly, when it is necessary to reverse the drive provided by the rollers to the ULD the PDU must go through a sequence of being lowered to its rest position and then raised again fully to its operative position before the drive from the PDU to the ULD is reversed. This sequence is often referred to as “lift-lower-lift” and the time taken to do this can be several seconds leading to significant operator frustration. 
     An alternative known form of PDU utilizing a differential gear drive mechanism is disclosed in U.S. Pat. No. 5,938,003. Such PDUs are disadvantageous in that they are very complex to manufacture and assemble and thus are expensive to supply and maintain. Moreover the arrangement disclosed in U.S. Pat. No. 5,938,003 is disadvantageous in that the roller braking mechanism  90  is permanently operative and so consumes power and generates heat and wear in normal use; the lifting mechanism, if obstructed during lifting may not assume the fully raised position after the obstruction is removed; and the roller  30  being cantilevered from a bearing at one end only of its support shaft requires the use of heavy duty bearings to ensure a long working life. 
     It is an object of the present invention to provide a PDU wherein the aforementioned disadvantages are minimised or obviated. 
     DISCLOSURE OF INVENTION 
     In accordance with the present invention there is provided a power drive unit (PDU) comprising a frame arranged to be mounted in use for raising and lowering movement relative to a supporting structure, a drive motor carried by the frame and having an output shaft, a drive roller assembly carried by said frame for engagement in use with a unit load device (ULD) or the like to be moved by the PDU, a first gear train transmitting drive from said motor output shaft to a drive roller of said drive roller assembly, a rotatable lifting cam assembly carried by said frame and driven in use relative to said frame to lift and lower the frame on said mounting, a second gear train for transmitting drive from said motor output shaft to said lifting cam assembly to operate said lifting cam assembly, clutch means operable to connect and disconnect said motor output shaft to and from said second gear train, a torque limiting device limiting the torque transmitted through said clutch, when said clutch is engaged, to said lifting cam assembly, and a brake mechanism between said torque limiting device and said lifting cam assembly for braking said lifting cam assembly against rotation relative to said frame. 
     Preferably the power drive unit includes a control system which is arranged to receive control input from an operator and to effect control over operation of said motor, said clutch, and said brake mechanism. 
     Desirably said control system includes a timer which is arranged so that after the lapse of a pre-determined time period from the point at which the motor is operated to raise the frame, the control system initiates operation of said brake to lock said lifting cam assembly, and disengages said clutch to disconnect said second gear train from said motor output shaft. 
     Alternatively said control system includes a sensor detecting the fully raised position of the frame and in response thereto signalling the control system to initiate operation of said brake to lock said lifting cam assembly, and disengage said clutch to disconnect said second gear train from said motor output shaft. 
     Conveniently the control system includes both a timer which is arranged to produce a signal after the lapse of a pre-determined time period from the point at which the motor is operated to raise the frame, and a sensor detecting the fully raised position of the frame and producing a signal responsive thereto, the control system responding to the earliest of, or both, signals to initiate operation of said brake to lock said lifting cam assembly, and disengage said clutch to disconnect said second gear train from said motor output shaft. 
     Preferably the control system is so arranged that said clutch is disengaged fractionally after engagement of said brake. 
     Preferably a resiliently compliant mounting system is interposed between said frame and the fixed support structure supporting the power drive unit. Conveniently said power drive unit includes a chassis to which said frame is pivotally mounted, and a resiliently compliant coupling mechanism securing said chassis to the fixed support structure. 
     Preferably the roller assembly includes first and second axially aligned rollers carried by bearing supports at both axial ends of the assembly. 
     Preferably there is provided a further brake for braking the motor output shaft, said further brake also being under the control of said control mechanism. 
    
    
     BRIEF DESCRIPTION OF THE INVENTION 
     In the accompanying drawings: 
     FIG. 1 is a cross-sectional representation of a known power drive unit for an aircraft cargo handling system; 
     FIG. 2 is a view similar to FIG. 1 of a power drive unit in accordance with a first example of the present invention; 
     FIG. 3 is a cross-sectional representation of a mounting arrangement for the power drive unit of FIG. 2; and, 
     FIG. 4 is a diagrammatic representation of the PDU control system. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     It will be noted that the structure of the power drive unit of FIG. 2 is similar to that of FIG. 1, and like components bear the same reference numerals in both drawings. It can be seen therefore that the arrangement of the motor  16  together with its output shaft  17  and brake  18  is unchanged as is the roller assembly  14  and the gear train  24 ,  43 ,  44 ,  45  transmitting drive from the shaft  25  and pinion wheel  24  to the rollers  15   a . Furthermore, the second gear train which transmits rotation of the shaft  26  through the gears  29 ,  31 ,  33 ,  34 ,  35 , and  36  to the cam shaft  37  to move the cams  38  angularly is also unchanged. However significant changes have been made in the arrangement whereby drive from the shaft  25  (driven by the shaft  17  of the motor  16 ) is transmitted to the shaft  26 . 
     It can be seen in FIG. 2 that the shaft  25  carries, at its end remote from the motor  16  the input member  72  of an electro-magnetic clutch  71 . An output member  73  of the clutch  71  is positioned co-axial with the input member  72  and is coupled to a shaft arrangement  74  separate from, but having its axis co-extensive with, the shaft  25 . The shaft  74  is secured to the input of a torque limiting arrangement  75  of known form which may be similar in its structure and operation to the torque limiting device  27  of the arrangement described with reference to FIG. 1. A detailed understanding of the operation of the torque limiting device is not necessary to an understanding of the invention, and it is sufficient to recognise that rotation of the shaft  74  will be transmitted through the torque limiting device  75  to a further shaft  76  separate from, but having its axis co-extensive with the shaft  74 , provided that the torque to be transmitted does not exceed a pre-determined value. If the shaft  76  is held against rotation while the shaft  74  rotates then a point will be reached at which the torque applied to the device  75  exceeds the set value of the device  75  whereupon the device will slip so that the shaft  74  rotates relative to the shaft  76 . 
     The shaft  76  at the output of the torque limiting device  75  is an integral extension of the shaft  26  and has rotatable therewith a brake element  78  of an electro-magnetically operable brake  77 . A fixed brake element  79  secured to the frame  11  co-acts with the rotatable brake element  78  so that when the electro-magnetically operable brake is energised the elements  78  and  79  are held against relative rotation and thus the shaft  26  is held against rotation relative to the frame  11 . 
     As illustrated diagrammatically in FIG. 4 of the drawings the PDU includes an electronic control system  80  effecting control over energisation of the motor  16 , operation of the brake  18 , operation of the clutch  71 , and operation of the brake  77 . With the PDU of FIG. 2 in its rest position, that is to say with the frame collapsed so that the periphery of rollers  15   a  is below the load carrying plane of the cargo deck, a control input in the form of a “raise” command given by an operator by means of switches or the like is accepted by the control system  80  which then energises the electric motor  16  and de-energises the brake  18  so that the brake  18  is released. Simultaneously the clutch  71  is energised and the brake  77  is de-energised so that rotation of the output shaft  17  of the motor is transmitted from the shaft  25  through the clutch  71 , the torque limiting device  75 , and the shaft  26  to the second gear train connecting the shaft  26  to the shaft  37  of the cam arrangement. At the same time drive is transmitted from the drive pinion  24  of the shaft  25  through the first gear train to the rollers  15   a  to rotate the rollers. 
     Rotation of the shaft  37  in the frame is accompanied by movement of the cams  38  relative to their respective cam followers and thus the frame  11  is pivoted about the axis  13  to raise the rollers  15   a  through the aperture in the deck panel of the cargo compartment. The speed of operation of the motor  16  is pre-determined, and thus the time taken for the frame  11  to be raised from its rest position to its fully operative position is known. The control system  80  includes a timing arrangement  80   a  which, at the lapse of a pre-determined period of time from energisation of the motor  16 , signals the control system  80  to energise the brake  77  and de-energise the clutch  71 . 
     The period of time set by the timer  80   a  is in excess of the amount of time calculated for the motor  16  and cams  38  to raise the frame  11  from its rest position to its fully raised position. Thus as the frame reaches its fully raised position the cams  38  will engage the stops associated with the cam followers and further rotation of the shaft  37  will be prevented. The shaft  26  will thus cease to rotate and as the shaft  25  is continuing to rotate the torque limiting device  75  will slip permitting rotation of the motor  16  to continue, driving the rollers  15   a , even though the shaft  37  can rotate no further. The control system  80  energises the brake  77  to lock the shaft  26  against rotation in either direction, fractionally before de-energising the clutch  71 ; this of course does not have any impact on the operation of the PDU since the shaft  26  was already stationary, and the device  75  was slipping. However disengagement of the clutch  71  disconnects the shaft  25 , and therefore the motor  16 , from the shaft  26  and shaft  37 . The brake  77  in preventing rotation of the shaft  26  in either direction, locks the frame in its fully raised position by locking the cams  38  against movement. However, transmission of drive to the rollers  15   a  is not affected and no power is now being dissipated in the torque limiting device  75  since disengagement of the clutch  71  has disconnected the torque limiting device from the shaft  26  and thus no drive is applied to the torque limiting device. 
     It will be understood that in the PDU illustrated in FIG. 2, should it become necessary to reverse the direction of rotation of the rollers  15   a  then the polarity of the motor  16  can be reversed, if necessary using the brake  18  momentarily to arrest rotation of the shaft  17  of the motor, the first gear train, and the rollers  15   a , to reverse the drive to the rollers  15   a . As the clutch  71  is disengaged the reversal of rotation of the motor  16  has no effect whatsoever on the shaft  37  and the cams  38  and thus the frame  11  remains in its raised position throughout the change in drive rotation to the rollers  15   a.    
     Clearly, by comparison with the arrangement shown in FIG. 1 there are two immediate and very significant advantages. Firstly, during normal operation power is dissipated in the torque limiting device  75  for a brief period only. Thereafter the clutch  71  is de-energised and no further power dissipation in the device  75  is required while the frame is maintained in its raised position. Secondly, when drive reversal to the rollers  15   a  is required, such drive reversal can be achieve extremely quickly, and does not require the “lift lower lift” sequence necessitated by the mechanical arrangement of FIG. 1 as the disengagement of the clutch  71  has disconnected the raising mechanism from the motor drive to the rollers. 
     Where it is necessary to apply a braking action to a ULD or the like the motor  16  can be de-energised and the brake  18  can be applied so that rotation of the rollers  15   a  is braked. Again this has no bearing on the operation of the lifting and lowering cam mechanism of the frame  11  since the clutch  71  is disengaged. 
     When the operator wishes to lower the rollers  15   a  to their rest position a “lower” signal given to the control system  80  causes the control system  80  to de-energise the brake  77  so that the brake element  78  is free to rotate relative to the brake element  79  and thus the cam shaft  37  and the cams  38  can be rotated back to their rest position, to allow lowering of the frame  11  about the axis  13 , under the action of the return springs  41  acting on the shaft  37 . As the cams  38  rotate back towards their rest position the frame  11  pivots back to its rest position under gravity. 
     A possible area of difficulty noted with the mechanism of FIG. 2 is that when the clutch  71  is disengaged and the brake  77  is engaged then the position of the frame  11  is locked. Thus in the event that raising movement of the frame  11  is impeded, for example by the rollers  15   a  engaging a downwardly protruding of part of a ULD or the like, then raising movement of the frame will be obstructed at a point before the fully raised position is achieved. Before the obstruction is cleared the torque limiting device  75  will be slipping, and the timer of the control system  80  may well have signalled the application of the brake  77  and the disengagement of the clutch  71 . Thereafter, should the obstruction to full raising of the frame  11  be removed the frame will not achieve its fully erected position since the clutch  71  will be disengaged and the brake  77  will be preventing any further movement of the shaft  37  and cams  38 . Thus irrespective of removal of the obstruction frame  11  will not be fully raised. 
     An associated problem has also been noted in the situation where the frame  11  is fully raised, and is impacted by a “low-point” on an uneven ULD, or is impacted by a ULD or the like being overweight or dropped onto the rollers  15   a . Because the shaft  37  and cams  38  are locked by the brake  77  the only way in which such sudden shock loadings on the rollers  15   a  can be accommodated is by flexure of the rubber tyres of the rollers, and this may not be sufficient to prevent damage to the PDU in all circumstances. 
     Accordingly, in order to overcome these disadvantages noted in the construction of FIG. 2, the manner in which the frame  11  is supported from the fixed support structure of the deck is arranged to include resiliently compliant couplings. A preferred arrangement of such a coupling is illustrated with reference to FIG. 3 which shows one of a pair of chassis members  81  by means of which the frame  11  is secured to the rigid support structure  82  of the deck of the cargo area. Each chassis member  81  extends beneath the deck panel of the deck of the cargo area and is apertured at one end  83  to receive a pivot pin extending through a respective mounting  12  of the frame  11 . Thus the pivot pins pivotally secure the frame  11  to the two chassis members  81  for pivotal movement about the axis  13 . The chassis members  81  extend beneath the frame  11  in use and each has an upstanding cam follower  84  engageable by a respective cam  38  of the frame  11 . 
     At its end remote from the pivot axis  13  each chassis member  81  is bolted to an adaptor rail  85  which in turn is secured to the structure  82  through a resiliently compliant coupling  86 . It will however be understood that in some embodiments it will be appropriate to manufacture the chassis members  81  with the adaptor rails as integral parts of the members  81 . 
     Each coupling  86  includes a spring arrangement which may be a coil spring or, as shown in the drawings, a stack of “Bellville” spring washers  87 , one end of the stack engaging the support structure  82 , and the opposite end of the stack acting through a load washer  88  against the head  91  of an elongate bolt  89 . The shank of the bolt  89  extends through pack  87  and through a clearance aperture in the structure  82  into screw threaded engagement with a part-spherical nut  92  on the respective adaptor rail  85 . Normally the pre-stressing of the spring washers of the pack  87  holds the adaptor rail  85  in facial contact with a surface of the support structure  82 . However, a load applied to the chassis member  81  sufficient to overcome the pre-stressing of the spring pack  87  will cause the chassis member  81  and its associated rail  85  to deflect relative to the support structure  82  further compressing the spring pack  87 . It will be recognised that where the load on the chassis member  81  is removed then the restoring action of the spring pack  87  will return the chassis member  81  and the associated rail  85  to appropriate alignment with the structural member  82 . 
     In operation therefore when the frame  11  is being raised relative to the chassis members  81 , should the raising movement of the frame  11  be obstructed then sufficient torque can be applied by way of the torque limiting device  75  to deflect the or each chassis member  81  relative to the support structure  82  so permitting the frame  11  to reach its fully raised position relative to the chassis members  81 . Thereafter the brake  77  will lock the frame  11  in its fully raised position relative to the chassis members  81  and when the obstruction to raising movement of the frame  11  is removed the spring packs  87  will restore the alignment of the chassis members  81  and the structural supports  82  thus lifting the frame  11  to occupy the fully raised position relative to the deck panel. 
     Similarly, should the rollers  15   a  be subjected to impact loadings having a vector in the direction of lowering the frame  11  then the resiliently compliant mountings  86  will deflect to allow deflection of the frame  11  and rollers  15   a  rather than the mechanism being subject to the risk of damage. The chassis members  81  and frame  11  will be restored to their original positions upon removal of the impact loading by the restoring action of the spring packs  87 . 
     It will be understood therefore that although the PDU described in relation to FIG. 2 can be used without the resiliently compliant mountings, the mountings are used in a preferred embodiment to enhance the performance of the PDU. 
     In a modification of the PDU described above a sensor  90  of any convenient form monitors rotation of the shaft  37  and so can signal the control system  80  when the lift cams  38  are in a fully operational position. Such a signal can be used by the control system  80  to initiate operation of the brake  77  to lock the frame  11  in the fully raised position and to disengage the clutch  71 . If desired the sensor  90  can replace the timer  80   a  as the “lock” signal generator, but desirably the sensor is used in conjunction with the timer, the signal from the sensor  90  being gated by the control system  80  with the signal from the timer  80   a  to ensure that locking of the frame in the raised position occurs in response to receipt of the first of the two signals. If desired the gating can ensure that locking occurs only on receipt of both signals.