Abstract:
There is described a motion control device for preventing free-fall of electronics modules during vertical movement into or out of a supporting structure. The system comprises a rotatable element on the module or structure engageable with surface on the structure or module, respectively, for rolling movement therealong. The rotatable element is provided with a clutch or brake to provide resistance to rotation at least when the module is moving downward relative to the structure. The rotatable element may be a gear engageable with a toothed rack, or a friction roller engageable with a friction surface.

Description:
FIELD OF INVENTION 
   The present invention relates to modular assemblies and is particularly concerned with electronic equipment comprising a supporting structure and a plurality of circuit sub-assemblies or modules mounted to the supporting structure wherein the modules require periodic removal from and replacement into the supporting structure. 
   BACKGROUND OF THE INVENTION 
   In conventional rack-mounted computer systems, a number of substantially planar electronics modules are arranged in horizontal or vertical planes extending from a front face of the rack to a rear face thereof. The modules are all connected to a vertical back plane by means of connectors arranged on a back edge of each module mating with co-operating connectors on the backplane. Installation and removal of individual modules from the system is effected by moving the modules in a horizontal direction towards or away from the backplane to connect or disconnect the connectors. The modules are received in horizontally-extending guides to ensure correct alignment and support for the modules. 
   The components of the electronics modules generate heat when they operate, and in order to remove this heat cooling air is caused to flow over the electronic components of the modules, with air being drawn in at one side of the module and expelled at the opposite side. The cooling air flows are preferably arranged to be in the same direction for all of the modules of the system, so that heated air expelled from one module is not drawn into a neighbouring module. Since a typical installation of a rack-mounted system will comprise a plurality of racks situated side-by-side, the direction of the cooling airflow is usually arranged to be from the “front” of the system to the “back”. The vertical backplane, however, presents a barrier to such a cooling airflow and necessitates a change in airflow direction, which reduces cooling efficiency. 
   An improved front-to-back airflow is achieved by arranging the modules in vertical planes extending from front to back relative to the supporting structure in the rack-mounted system, with the interconnection of the modules being made by a horizontally arranged “baseplane” rather than a vertically arranged backplane. The front to back cooling airflow thus runs parallel to the plane of the baseplane, and heated air can be expelled from the back of each module without having to change direction to exit the system. 
   Mounting the baseplane in a horizontal orientation however requires the insertion and removal of modules from the system to be effected by moving the module vertically rather than horizontally to connect it to and disconnect it from the baseplane. For the modules to be insertable and removable with a single linear movement relative to the system supporting structure, the modules may be guided in vertical guides relative to the supporting structure, but to prevent the module from falling during insertion or removal an operative must support the weight of the module during insertion and removal procedures. The modules may weigh several kilograms, typically from 5 to 15 kg, and may have to be lifted or lowered at arm&#39;s length. 
   The modules and their exposed connectors are easily damaged if dropped, and the baseplane of the system may also be damaged if a module is dropped during insertion and falls on to the baseplane. 
   The present invention seeks to provide a motion control device and a modular assembly for housing a computer system which comprises the motion control device, and modules for an assembly, for controlling the movement of detachable modules relative to the supporting structure of the modular assembly. 
   SUMMARY OF THE INVENTION 
   According to a first aspect of the invention, there is provided an assembly for housing a modular electronic circuit, the assembly comprising a supporting structure having vertically extending guides, and at least one circuit module engageable with the guides for movement in a vertical direction between a mounted position and a dismounted position, the circuit assembly further comprising a motion control device having a first component provided on the module and a second component provided on the supporting structure, the first and second components of the motion control device being cooperable to resist downward movement of the module in the guides relative to the supporting structure. 
   According to a second aspect of the invention, there is provided a motion control device for providing a first degree of resistance to relative movement between first and second objects in a first movement direction, and a second, a smaller, degree of resistance to relative movement between the objects in a second movement direction opposite to the first. 
   A third aspect of the invention provides a motion control device for a modular assembly comprising a supporting structure and at least one module movable relative to the supporting structure in a movement direction into and out of a mounted position, the motion control device comprising an elongate engagement surface provided on either the supporting structure or module to extend in said movement direction, a rotatable element mountable to the other of the supporting structure and the electronic module to engage the engagement surface for rolling movement therealong, and a retarding device operable to resist rotation of the rotatable element in at least one rotation direction. 
   A fourth aspect of the invention provides a component for a motion control device, the component comprising a mounting plate, a swinging arm pivotally attached at one of its ends to the mounting plate for swinging movement towards and away from the mounting plate, a rotatable element mounted to a second end of the swinging arm, and a friction clutch or brake operable between the swinging arm and the rotatable element to resist rotation of the rotatable element in at least one rotation direction. 
   The invention provides, in further aspects, an electronic circuit module comprising a component of a motion control device, and a supporting structure for a modular circuit comprising a component of a motion control device. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the invention will now be described in detail with reference to the accompanying drawings, in which like parts are given like reference numbers. In the drawings: 
       FIG. 1  is a perspective view of a modular computer system with vertically-arranged modules; 
       FIG. 2  is a schematic front view of a first motion control device according to the invention; 
       FIG. 3  is a side view of the motion control device of  FIG. 2 ; 
       FIG. 4A  is a side view showing a module, module guides, and the motion control device; 
       FIG. 4B  is a detail perspective view of the upper end of the left-hand guide of  FIG. 4A ; 
       FIG. 4C  a sectional view taken in the plane X—X of  FIG. 4B ; 
       FIG. 5  is a schematic perspective view of a second motion control device of the present invention; 
       FIG. 6  is a schematic perspective view of a third motion control device of the present invention; 
       FIG. 7  is a schematic perspective view of a fourth motion control device of the present invention; 
       FIG. 8  is a schematic perspective view showing alternative engagements between the motion control device of the present invention and an electronics module; and 
       FIGS. 9A ,  9 B and  9 C are side, front, and perspective views, respectively, of a fifth motion control device according to the invention; and 
       FIG. 10  is a schematic perspective view of a sixth motion control device according to the invention. 
   

   DETAILED DESCRIPTION 
   Referring now to the Figures,  FIG. 1  shows a perspective view of modular assembly comprising a plurality of substantially planar electronic modules mounted in vertical orientations in slide-out drawers of a rack chassis to form a computing circuit. 
   In the embodiment shown, the chassis  1  comprises three drawers  2 , each drawer housing a number of electronic modules  3 . 
   The modules  3  are generally planar in configuration, and are arranged in vertical planes in the drawers  2 . 
   The drawers  2  are slidingly mounted to the chassis  1  for movement in the direction of arrow A between a retracted, operating position and an extended, maintenance position. The lowermost drawer  2  is shown in the extended, maintenance position, while the upper two drawers are in the operating position. 
   In the embodiment illustrated, the modules  3  are mounted in vertical planes within the drawers, vertical guides  4  being provided to receive opposing edges of the modules  3  in sliding engagement. In the lowermost drawer  2  of the chassis  1  shown in  FIG. 1 , the modules  3  are arranged in planes extending across the drawer, i.e. perpendicularly to the movement direction (A) of the drawer. It is, however, also possible that the modules may be arranged in planes extending from the front to the back of the drawer, as illustrated in relation to the uppermost drawer in  FIG. 1 . 
   To provide cooling airflow over the components of the electronic modules, cooling fans  5  may be provided on the chassis  1  or on the drawer  2 . When the modules are arranged to extend across the drawer, as in the lowermost drawer in  FIG. 1 , then cooling fans  5  may be mounted on the side of the chassis  1 . An array of fans may be provided on each side of the chassis, one array drawing air from the outside and blowing it between the modules  3  within the drawer  2 , and the other array of fans extracting air from within the drawer and exhausting it to the outside. 
   When the modules are mounted front-to-back in the drawer, as in the uppermost drawer in  FIG. 1 , the cooling fans  5  may be mounted to the drawer front  6 , and/or either to a back wall of the drawer or to a back wall of the chassis  1  to provide a horizontal air flow between the modules  3  within the drawer  2 . In this arrangement, the modules  3  are mounted in front-to-back orientation within the drawer, and cooling air is drawn in through the drawer front  6  and expelled through the rear of the chassis  1 . 
   Electrical connections between the modules  3  are made through a horizontal base plane  7  situated in the bottom of each drawer  2 , the base planes  7  of the drawers  2  being optionally connected to each other by cabling. The electronic modules  3  are electrically connected to the base planes by means of connectors situated on the lower edges of the modules  3  which cooperate with connecting sockets  8  on the base planes. 
   The electronic modules  3  are supported within the drawers  2  by means of vertically-extending guides  4  into which opposite edges of the electronic module  3  engage. In order to insert or remove a module  3 , the module is moved vertically in the guides  4  towards or away from the base plane  7 . In order to prevent the module  3  from freely falling in the guides  4  if the operative should release his grip on the module during insertion or removal, a motion control device is provided between the module  3  and the drawer  2 . A first embodiment of the motion control device is shown schematically in  FIGS. 2 and 3 . 
     FIG. 2  is a front view and  FIG. 3  is a side view of a motion control device and a module, showing only the operative parts of the assembly. The motion control device comprises a toothed wheel  10  mounted to a swinging arm  11  which is pivotally attached to a support  2   a  of the drawer  2 , and a toothed rack  12  fixed to the module  3  and engageable with the toothed wheel  10  as the module moves along the guides  4 . 
   The module  3  comprises a main circuit board  3   a  (“motherboard”) to which subsidiary circuit boards  3   b  (“daughterboards”) are attached by means of connectors  3   c . The main circuit board  3   a  has a pair of opposing vertical side edges  3   d  which are engageable with the vertical guides  4  mounted to the drawer  2 . The lower edge  3   e  of the main circuit board is provided with protruding connector portions  3   f  which are engageable with connection sockets  8  disposed on the base plane  7 . 
   The upper edge of the main circuit board  3   a  is attached to a handle assembly  9  which provides a handling grip  9   a  and latching levers  9   b , which will be described later. 
   The toothed rack  12  is attached to the main circuit board  3   a  to extend parallel to the side edge  3   d . The rack  12  has its toothed face in a plane perpendicular to the plane of the main circuit board  3   a , facing towards the side edge  3   d.    
   The swinging arm  11  of the motion control device is pivotally mounted to a support  2   a  of the drawer  2  by means of a pivot pin  13 . A biassing spring  14  extends between attachment points  15  and  16  on a support  2   b  of the drawer  2  and the swinging arm  11 , respectively, to apply a tensile force causing the swinging arm  11  to rotate anti-clockwise as shown in  FIG. 3 . This biassing force urges the free end of the swinging arm  11  towards the toothed face of the rack  12 . A limit stop  17  is provided on the drawer  2 , to limit the clockwise rotation of the swinging arm  11 . 
   At the free end of the swinging arm  11 , the toothed wheel  10  is mounted for rotation about an axle  18 . In the embodiment shown, the swinging arm  11  is in the form of a clevis, having a pair of slide plates  11   a  between which the toothed wheel  10  is mounted. The axle  18  extends through aligned bores in the side plates  11   a.    
   To apply a frictional force resisting the rotation of the toothed wheel  10 , a friction disk  19  is interposed between the toothed wheel  10  and one of the side plates  11   a  of the swinging arm  11 . A compression spring  20  bears at one end on an adjusting nut  21  attached to the axle  18 , and bears at its other end against the side plate  11   a  of the swinging arm  11 . The compression spring  20  applies a tensile force to the axle  18 , which draws the toothed wheel  10  and friction disk  19  into close contact with the inside face of the side plate  11   a . The urging together of the side plate, friction disk and toothed wheel provides a friction force on the toothed wheel which resists relative rotation of the toothed wheel and the swinging arm. The magnitude of the frictional resistance can be adjusted by adjusting the compression in the compression spring  20 , by moving the adjusting nut  21  along the axle  18 . 
   With the module  3  removed from the drawer  2 , the biassing spring  14  urges the swinging arm  11  to rotate until the swinging arm  11  contacts the limit stop  17 . As a module is inserted into the drawer  2 , by engaging the side edges  3   d  of the main circuit board of the module  3  with the vertical guide channels  4 , the lower end of the toothed rack  12  will contact the toothed wheel  10 . The lower end of the toothed rack  12  may have an oblique lead-in surface  22 . As the toothed wheel  10  engages the lead-in surface  22 , downward movement of the module  3  will cause the swinging arm  11  to rotate in a clockwise direction as seen in  FIG. 3 , tensioning the biassing spring  14 . Further downward movement of the module  3  will cause the teeth of the toothed wheel  10  to engage with the teeth of the rack  12 , so that the toothed wheel  10  will thereafter be rotated by continued downward movement of the module  3 . This rotation of the toothed wheel  10  is resisted by the friction disk  19 , and thus provides a retarding force against movement of the module  3 . 
   Preferably, the amount of compression in the compression spring  20  is adjusted so that the frictional resistance to rotation of the toothed wheel  10  is not overcome unless the module  3  is subjected to a downward force applied by an operative. In other words, the frictional resistance of the toothed wheel  10  is capable of supporting the self-weight of the module  3  when the module is at rest, and is also capable of arresting downward movement of the module when an externally-applied downward force is removed. 
   In order that the toothed wheel  10  is retained in close engagement with the rack  12 , the flank angles of the teeth of the toothed wheel  10  and the rack  12  are so arranged that the reaction force acting on the toothed wheel  10  during insertion of the module  3  results in a moment about the pivot pin  13  which urges the swinging arm  11  to rotate anti-clockwise, bringing the toothed wheel  10  into closer engagement with the rack  12 . 
   As the operative applies a downward force to the module  3 , frictional resistance of the toothed wheel  10  is overcome and the module moves down the guides  4 . The length of the rack  12  is so arranged that resistance to downward movement is provided until the electrical contacts  3   f  at the lower edge of the main circuit board  3   a  are engaged with the connection socket  8  of the base plane  7 . 
   The final movement of the module  3  to connect the module with the connection socket is preferably achieved by means of the latching levers  9   b  of the handle assembly  9  of the module  3 . Latching levers  9   b  are moved to a raised position (shown in broken lines in  FIG. 3 ) for removal and insertion of the module, and are rotated to a lowered position (shown in solid lines in  FIG. 3 ) to lock the module into the drawer. With the latching lever in its raised position, the module  3  is moved into the guides  4  until the connecting portion  3   f  is adjacent the connection socket  8  of the base plane  7 . The latching lever  9   b  is then rotated (anti-clockwise as seen in  FIG. 3 ) so that a detent  9   b   1  of the lever  9   b  engages with an abutment surface  22  of the drawer  2 . As the latching lever  9   b  is rotated to its closed position, a lever action urges the module  3  downwards so that the connection portion  3   f  enters the connecting socket  8  to complete the installation of the module. 
   In order to remove the module from the drawer, the latching levers  9   b  are raised to disengage the detent  9   b   1  from the abutment surface  22 . 
   The detent  9   b   1  may engage an upward-facing abutment (not shown) so that raising the latching levers  9   b  also raises the module  3  to disengage the electrical connection portion  3   f  from the connecting socket  8 . The handle  9   a  is then grasped and a vertical upward force applied to the module. As the module moves upwardly, the friction disk  19  provides a resistance to rotation of the toothed wheel  10 . However, the reaction force between the rack  12  and the toothed wheel  10  is now acting to rotate the swinging arm  11  in the clockwise sense as seen in  FIG. 3 , tending to disengage the toothed wheel  10  from the rack  12 . The toothed wheel  10  preferably does not rotate, but “jumps” over the teeth of the rack  12 , providing a ratchet-type engagement. Should the operative cease to apply the vertically upward force withdrawing the module  3 , the module is prevented from falling back in the guides by the engagement of the toothed wheel  10  and the rack  12 . As the direction of the reaction force changes, the toothed wheel  10  is brought into close engagement with the rack  12  to prevent downward movement of the module. 
   In  FIGS. 1 to 3 , the module  3  is provided with a single rack  12  extending vertically adjacent one of its side edges  3   d . Since the centre of gravity of the module  3  is spaced horizontally from the rack  12 , the reaction between the toothed wheel  10  and the rack  12  causes a moment to be applied to the module, tending to rotate it about a horizontal axis perpendicular to the plane of the module. This rotation can cause the edges  3   d  of the module to become jammed in the guides  4 .  FIG. 4  shows an arrangement for overcoming this difficulty by providing a pair of reaction rollers to counteract the moment caused by the asymmetric vertical forces. 
     FIG. 4A  illustrates schematically a module  3  having a rack  12 , and a pair of vertical guides  4 . The right-hand guide is separated into upper and lower portions, between which is mounted a swinging arm  11  and toothed wheel  10  biassed towards the rack  12  by a biassing spring  14 . Adjacent the lower edge  3   e  of the module, at its side adjacent the rack  12 , a first reaction roller  23  is mounted to the main circuit board  3   a  of the module  3 . In the embodiment shown, the first reaction roller  23  engages an inwardly-facing surface of the guide  4 , to produce a horizontal reaction on the module  3  indicated by the arrow R 1  in  FIG. 4A . At the upper end of the left-hand guide, a second reaction roller  24  is provided to engage the edge  3   d  of the main circuit board  3   a . The reaction between the second reaction roller  24  and the module  3  produces a second horizontal reaction force on the module  3 , indicated by the arrow R 2 . 
   As the module  3  is moved downwardly between the guides  4 , the reaction forces R 1  and R 2  provide a moment to counteract the moment generated by the horizontal offset of the reaction force on the rack  12  and the downward force on the module  3 . The module  3  is thus prevented from rotating and jamming in the guides  4 . 
   As an alternative to the reaction rollers  23  and  24 , the module may be provided with a symmetrical arrangement of two racks  12 , to cooperate with respective toothed wheels  10  mounted on swinging arms  11  adjacent each of the guides  4 . Such an arrangement will provide a symmetrical force distribution, and prevent jamming of the main circuit board  3   a  in the guide  4 . It will be appreciated that if the centre of gravity of the module  3  is offset relative to the vertical centre line of the module, this will generate a moment tending to jam the module in the guides if the reaction forces exerted by both of the toothed wheels are equal. It is therefore foreseen that the reaction forces of the two toothed wheels may be different from each other, in order to compensate for horizontal offsetting of the centre of gravity of the module. To achieve this, the compression spring  20  of the motion control device nearest to the centre of gravity of the module is adjusted to increase the frictional resistance of its toothed wheel  10 , so as to provide a larger reaction force at the side edge nearest the centre of gravity of the module. 
     FIG. 5  shows an alternative arrangement for the motion control device. In the arrangement shown in  FIG. 5 , the rack  12  of the module  3  is engaged by an idler wheel  25 , which meshes with a toothed wheel  10  mounted on a support  2   c  of the drawer  2  for rotation about an axis fixed in relation to the drawer  2 . Link arms  26  connect the centres of the toothed wheel  10  and the idler wheel  25 , and a biassing spring  14  coupled to a support  2   d  of the drawer  2  urges the link arms to rotate in order to bring the idler wheel  25  into close engagement with the rack  12 . A friction resistance device operates on the toothed wheel  10  to resist rotation of the toothed wheel  10  in both directions, and this resistance is transmitted to the rack  12  during insertion of the module. As described previously, the flank angles of the teeth of the idler wheel  25  and the toothed rack  12  are so arranged that reaction forces on the idler wheel  25  urge the idler wheel  25  into closer engagement with the rack  12  during insertion of the module, and cause the idler wheel  25  to “jump” over the teeth in the rack  12  when the module is removed. 
   As before, the motion control device may be provided at one side only of the drawer, and the module may be provided with a first reaction roller  23  as before, with a second reaction roller  24  being provided in the guide opposite the motion control device. Alternatively, two motion control devices may be provided, as described above. 
   A further alternative embodiment of the motion control device is illustrate in  FIG. 6 . In this arrangement, the toothed rack  12  is positioned on the module  3  with the toothed face of the rack oriented to face away from the plane of the module  3 . 
   The toothed wheel  10  is mounted to axle  18 , and rotation of the axle  18  relative to the swinging arm  11  is resisted in one direction only by a unidirectional clutch. In the embodiment shown, a “Spragg” clutch is used, wherein a coil spring  27  is wrapped around the axle  18 , and a tangentially-extending free end  28  of the coil spring  27  is captured between abutments  29  on the swinging arm  11 . The direction of wrapping of the coil spring  27  round the axle  18  is such that rotation of the toothed wheel  10  caused by insertion of the module  3  is resisted, due to the frictional engagement between the spring  27  and the axle  18  tending to wrap the spring  27  more tightly about the axle  18 . Conversely, as the module  3  is removed from the drawer  2 , rotation of the toothed wheel  10  is in the sense which causes friction between the spring  27  and the axle  18  to loosen the coils of the spring  27 , thus reducing resistance to rotation of the toothed wheel  10 . The module is therefore removable from the drawer with substantially no resistance being applied by the engagement of the toothed wheel  10  and the rack  12 . The coiled spring  27  shown in  FIG. 6  may be replaced by any other suitable unidirectional friction element. 
   It will be appreciated that, with the toothed surface of the rack  12  facing away from the main circuit board  3   a , the reaction between the toothed wheel  10  and the rack  12  will produce a reaction force on the module which is perpendicular to the plane of the module  3 , increasing frictional resistance to movement between the edges of the main circuit board  3   a  and the guides  4 . To counteract this, a second motion control device and a second rack  12  may be provided on the rear face of the module  3 , aligned with the rack  12  on the front face so that the reaction forces produced by the respective motion control devices will cancel each other out. Alternatively, a supporting bearing may be provided on the near face of the module. 
     FIG. 7  shows a further alternative arrangement of the motion control device. The arrangement is similar to that shown in  FIG. 5 , with the toothed wheel  10  being mounted to the drawer  2  with friction means to resist rotation of the toothed wheel  10 . An idler wheel  25  is provided, with the axle  30  of the idler wheel  25  received in a slot  31  in a guide  32  mounted to the drawer  2 . The slot  31  may be straight, as shown in  FIG. 7 , or may be arcuate and concentric with the axis of the toothed wheel  10 . The slot  31  is arranged so as to be downwardly convergent towards the rack  12 , so that the reactions on the idler wheel  25  as the module  3  is inserted into the drawer cause the idler wheel  25  to be drawn into close engagement with the rack  12  and with the toothed wheel  10 . When the module  3  is lifted out of the drawer  2 , the idler wheel  25  is moved upward in the slot  31  and engagement between the idler wheel  25  and the rack  12  is released, allowing the module  3  to be removed substantially without resistance, as the idler wheel  25  “jumps” across the teeth of the rack  12 . Should the module start to reenter the drawer, the idler wheel  25  is immediately engaged with the teeth of the rack  12  to prevent the module  3  from falling. 
     FIG. 8  illustrates schematically alternative methods of engaging the motion control device and the module, otherwise by than the rack-and-pinion arrangements shown in  FIGS. 1 to 7 . 
   At the upper part of  FIG. 8 , there is shown a friction engagement wherein a friction surface  33  adjacent the edge  3   d  of the main circuit board  3   a  is engaged by a friction roller  34 . The friction roller  34  may be supported on a shaft  35  so as to resist relative rotation of the roller  34  and shaft  35  in both directions by means of a friction clutch. Alternatively, the roller  34  may be mounted to the shaft  35  to resist relative rotation only in the insertion direction of the module  3 . In order to provide sufficient pressure between the friction roller  34  and the friction surface  33 , a back up roller  36  may be provided behind the module  3 . In the arrangement shown, the back up roller  36  is mounted on a shaft  37 , and the shafts  35  and  37  are mounted adjacent the free ends of a pair of swinging arms  38   a  and  38   b  pivotally mounted to a support  2   e  of the drawer  2 . A tension spring  39  urges the ends of the arms  38  together, so that the friction roller  34  and the back up roller  36  form a nip through which the edge region of the main circuit board  3   a  passes during insertion and removal of the module. 
   The lower part of  FIG. 8  shows a further alternative arrangement, wherein a series of recesses or openings  40  formed in an edge region of the main circuit board  3   a  are engaged by projections  41  on a sprocket roller  42 . The sprocket roller  42  may be controlled by a unidirectional or bidirectional friction clutch, either directly or via a transmission, and may be urged into engagement with the main circuit board by being mounted on a swinging arm as illustrated in relation to the friction roller  34 . 
   The recesses or openings  40  may be circular, as shown in  FIG. 8 , or may be square or rectangular. The recesses or openings may have edges shaped to cooperate with a gear wheel, or sprocket wheel. 
     FIG. 9  illustrates a motion control device for attachment to a drawer of chassis to provide motion control for a vertically movable module  3 . The motion control element comprises a fixing plate  45  providing with fixing locations  46  and  47  to accept fasteners such as screws, bolts of the like. Upstanding edges  48  of the fixing plate  45  support a pivot pin  49  on which is supported a gear housing  50 . A biassing spring  51  is mounted between the fixing plate  45  and the gear housing  50 , to urge the gear housing  50  away from the base plate  45 . A heel part  52  of the gear housing  50  is contactable with the fixing plate  45  to limit the movement of the gear housing  50  away from the fixing plate. 
   The gear housing  50  supports a gear shaft  53  on which is mounted a gear wheel  54 . Within the gear housing  50  is contained a friction clutch which provides frictional resistance to rotation of the gear wheel  54  in the clockwise sense as seen in  FIG. 9A . 
   In use, the motion control device of  FIG. 9  is attached to the drawer  2 , so that the gear housing  50  is supported in such a position as to engage the rack  12  of a module  3  insertable in guides in the drawer. The motion control device may be provided only at one side of a module, adjacent one of the guides. Alternatively, two motion control devices may be provided for each module, one mounted adjacent each of the guides. The racks  12  and motion control devices may be arranged with the axis of the gear wheel  54  extending either parallel to or perpendicular to the plane of the main circuit board  3   a  of the module. The friction device within the gear housing  50  may provide frictional resistance to rotation of the gear wheel  54  in both rotation directions, or only in one. 
   A further embodiment of the invention is illustrated in  FIG. 10 , again with like parts being given like reference numerals. 
   In this embodiment, the motion control device is mounted to the main circuit board  3   a  of the module, and engages a rack fixed to the supporting chassis or drawer. In this embodiment, the vertical guide which receives the edge of the main circuit board is formed with a toothed rack surface. In  FIG. 10 , the main circuit board  3   a  of the module  3  is provided with a mounting pin  55  on which is pivotally mounted a swinging arm arrangement  56  to support a toothed wheel  57 . A tension spring  58  is fixed to the swinging arm  56  and to an anchor pin  59 , to apply a moment to the swinging arm  56  to urge the toothed wheel  57  towards the edge  3   d  of the main circuit board  3   a . The swinging arm  56  is arranged to extend downwardly and outwardly relative to the main circuit board  3   a  (when the main circuit board is in its insertion position). The guide  4  is provided with a rack surface  60  engageable by the toothed wheel  57 . It will be understood that the rack surface  60  may be provided as a separate component to the guide  4 . 
   During insertion of the module  3  into the drawer  2 , the toothed wheel  57  engages the rack surface  60  and a friction device resisting rotation of the toothed wheel  57  provides an upward reaction force on the mounting pin  55  to support the weight of the module  3 . When the operative exerts a downward pressure on the module  3 , the toothed wheel  57  is rotated in engagement with the rack surface  60  and the module is moved into the drawer  2 . When an operative applies an upward force to the module  3 , the toothed wheel  57  may “jump” over the teeth of the rack surface  60 , if, the rotation of the toothed wheel  57  is resisted in both directions. If a unidirectional clutch is provided, then the toothed wheel  57  may remain in engagement with the rack surface  60  but provide no resistance to removal of the module. 
   It will be understood that any of the motion control devices described above may be mounted to the module  3  rather than to the drawer  2 , with the rack  12  mounted to the drawer  2  for engagement with the motion control device. 
   An advantage to mounting the motion control device on the module is that the frictional resistance provided by the motion control device may be tailored to suit the particular module to which it is mounted, with different degrees of frictional resistance being provided for modules of different mass. Each module may be provided with a single motion control device, or they may be mounted symmetrically in pairs. When a single motion control device is provided, the module and its guide may also be provided with reaction rollers. However, if the motion control device is mounted adjacent the lower edge of the main circuit board  3   a , then the horizontal reaction produced by the motion control device may be sufficient to prevent jamming of the module in the guides. 
   While in the embodiments shown the electronic modules  3  are moved downwards to their installed positions, it is possible that the modules  3  may be mounted in “overhead” positions in which the modules are moved vertically upwardly during insertion to their mounted positions, and are removed by withdrawing them vertically downwards. Such a situation may arise in avionic applications where systems must be accommodated in limited space. It will be appreciated that the motion control devices are usable in such “overhead” mounting systems, but the direction of the resistance force applied to the module will be such as to prevent the module from falling out of its mount, rather than to prevent the module from falling into its mounted position. 
   It is also contemplated that the motion control devices may be provided to control insertion and/or removal of modules mounted for horizontal insertion and removal, to prevent an accidental application of excessive force resulting in the module being moved too quickly into or from its mounted position. 
   The movement speed of the module may be controlled in any of the above-described embodiments or situations by providing a resistance force which increases as the speed of the module relative to the supporting assembly increases, for example by using centrifugal clutch or a fluid damper device in addition to, or instead of, the friction clutch.