Abstract:
A mechanism for reducing vibratory motion applied to a card cage mounted in a rack is disclosed. The cage accommodates a plurality of printed circuit boards arranged in parallel at regular intervals. The mechanism comprises dampers provided between the rack and the card cage. Each of the dampers is responsive to external vibrations applied to the rack in a transverse direction relative to a direction along which the plurality of printed circuit boards are arranged, and changes damping characteristics thereof in two stages depending on strengths and frequencies of the external vibrations.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates generally to a vibratory motion restrainer, and more specifically to a mechanism for reducing or depressing externally applied oscillations to a rack-mounted card cage by way of a rack, which cage accommodates a plurality of printed circuit boards.  
           [0003]    2. Description of Related Art  
           [0004]    As is known in the art, groups of printed circuit boards are interconnected by placing them in card cages (also known as shelves or sub-racks) which include hardware for housing the printed circuit boards.  
           [0005]    Prior to turning to the present invention, it is deemed preferable to briefly describe, with reference to FIGS. I and  2 , a conventional rack and card cages, to which the present invention is applicable.  
           [0006]    [0006]FIG. 1 is a perspective view schematically showing part of rack  10  and one card cage  12  to be housed therein. As shown, the rack  10  comprises four frames  14   a - 14   d  vertically provided at four corners of the rack  10 , two side panels  16   a  and  16   b  secured to the frames  14   a - 14   d,  and a back panel  16   c  fixed to the frames  14   b  and  14   c.  The rack  10  further comprises a plurality of pairs of card cage ledges (shelves), each pair of which is used to support the card cage at left and right bottoms thereof. More specifically, FIG. 1 shows that the card cage  12  is supported by a pair of cage ledges (only one ledge  20   a  is partially shown). On the other hand, another pair of cage ledges  22   a - 22   b  are secured to the opposite inner sides of the rack  10  so as to support another card cage (not shown) to be installed into the rack  10  above the cage  12 .  
           [0007]    The card cage  12  comprises four frames  24  (one frame is hidden in FIG. 1) transversely positioned to tops and bottoms of front and rear portions thereof. The cage  12  further comprises two side panels  26   a - 26   b  respectively secured to the right and left sides of the cage  12 , and a rear panel  26   c  fixed to the rear portion thereof. The side panels  26   a - 26   b  respectively have front edges  27   a - 27   b  which are respectively used to detachably fix the card cage  12  to the rack frames  14   a - 14   b.  Further, a plurality of guide rails  28  are mounted at the tops and bottoms of the four frames  24  at regular intervals between the side panels  26   a - 26   b.    
           [0008]    A plurality of card units  30  are detachably inserted into the corresponding guide rails  28 , leaving a predetermined space between the adjacent ones. The removing and extracting of the card units is for maintenance, upgrade or other purposes. Each card unit  30  is provided with a front panel  32 , a printed circuit board.  34 , a unit connector  36  mating with a corresponding connector  38  (FIG. 2) provided on the inner wall of the rear panel  16   c.  Further, each unit  30  comprises a pair of ejectors  40  in the vicinity of top and bottom portions of the front panel  32 . It is preferable to provide the ejector  40  with a suitable unit locking mechanism so as to prevent unintentional removal of the card unit  30  from the cage  12 . It is to be noted that FIG. 1 omits a plurality of resilient supporting members provided between the rack  10  and the card cage  12 , which will be referred to in FIG. 2.  
           [0009]    [0009]FIG. 2 is a sectional plan view taken along a section line A-A of FIG. 1. In order to simplify the drawing, the front panel  32 , the guide rails  28 , etc. are omitted in that they are impertinent to the present invention.  
           [0010]    As shown in FIG. 2, the cage  12 , which has accommodated the multiple printed circuit boards  34 , is housed in the rack  10  (FIG. 1), in the case of which each card unit connector  36  is electrically coupled to the corresponding cage connector  38 . The card cage  12  is detachably secured to the rack frames  14   a  and  14   d  respectively at the elongated front edges  27   a - 27   b  using a suitable fastening member  50  such as a screw. On the other hand, resilient support members  52  are respectively fixed to the rack frames  14   b  and  14   c  using screws  54  at the rear portions thereof. The above-mentioned support members  52 , however, may be welded to the corresponding rack frames  14   b - 14   c  in place of the screws  54 . When the card cage  12  is completely inserted into the rack  10  as illustrated, the resilient support members  52  press the rear side portions of the cage  12 , thereby rigidly supporting the same in the transverse direction, viz., a direction normal to that along which the printed circuit board  34  are inserted. The resiliently supporting technology, as just mentioned above, is disclosed in Japanese Laid-open Patent Application No. 10-93264. As an alternative, the resilient support members  52  may be fixed, using screws (for example), to the rear sides of the card cage  12 .  
           [0011]    When the rack  10  is transported by land, sea, or air while accommodating the card units  30  (viz., printed circuit boards  34 ) within the cage(s)  12 , the rack  10  is typically subject to vibrations or oscillations with a variety of frequencies and magnitudes. Further, even after the rack  10  has been set, the rack  10  may be subject to vibrations with low frequency and large amplitude due to earthquake (for example). Therefore, if the cage  12  is secured to the rack  10  using the rigid supporting members  52  at the rear portions, and fixed to the rack frames  14   a - 14   d  at the front edges  27   a - 27   b,  it is not expected to effectively reducing the externally applied vibrations with high frequencies and relatively low amplitude. On the other hand, if the supporting members  52  are made of material with high elasticity so as to high frequencies and low amplitude vibrations, it is in turn unable to effectively absorb the vibrations with low frequencies and large amplitudes.  
           [0012]    If the vibrations applied to the card cage  12  coincides with the characteristic (natural) frequency of the printed circuit board, the electronic components may be damaged due to sympathetic vibrations. Further, in such a case, the adjacent card units may collide with each other, resulting in damages of the card units and/or the printed circuit boards mounted thereon.  
         SUMMARY OF THE INVENTION  
         [0013]    It is therefore an object of the present Invention to provide a mechanism for reducing vibratory motions applied to the rack-mounted card cage by changing a spring constant of a resilient member in response to the magnitude and frequencies of the vibration.  
           [0014]    In brief, the object is achieved by the techniques wherein a mechanism for reducing vibratory motion applied to a card cage mounted in a rack is provided. The cage accommodates a plurality of printed circuit boards arranged in parallel at regular intervals. The mechanism comprises dampers provided between the rack and the card cage. Each of the dampers is responsive to external vibrations applied to the rack in a transverse direction relative to a direction along which the plurality of printed circuit boards are arranged, and changes damping characteristics thereof in two stages depending on strengths and frequencies of the external vibrations.  
           [0015]    One aspect of the present invention resides in a mechanism for reducing vibratory motion applied to a card cage mounted in a rack, the card cage accommodating a plurality of printed circuit boards arranged in parallel at regular intervals, the mechanism comprising: damping means, provided between the rack and the card cage, for responding to external vibrations applied to the rack in a transverse direction relative to a direction along which the plurality of printed circuit boards are arranged, and changing damping characteristics thereof in two stages depending on strengths and frequencies of the external vibrations. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    The features and advantages of the present invention will become more clearly appreciated from the following description taken in conjunction with the accompanying drawings in which like elements or portions are denoted by like reference numerals and in which:  
         [0017]    [0017]FIG. 1 is a perspective view showing part of a conventional rack and one card cage detachably inserted thereinto, having been referred to in the opening paragraphs of the instant disclosure;  
         [0018]    [0018]FIG. 2 is a horizontal sectional view of part of the rack and the card cage, taken along section line A-A of FIG. 1;  
         [0019]    [0019]FIG. 3 is a horizontal sectional view of part of the rack and the card cage, together with vibration depressing members according to a first embodiment of the present invention;  
         [0020]    FIGS.  4 (A) and  4 (B) show respectively vibration depressing members shown in FIG. 3,  
         [0021]    [0021]FIG. 5 is a sketch graphically showing the manner wherein a spring constant of the vibration depressing member of FIG. 3 is changed;  
         [0022]    FIGS.  6 (A) and (B) are diagrams respectively showing different operations of the vibration depressing member of FIG. 3;  
         [0023]    [0023]FIG. 7 is a diagram schematically showing a variation of the first embodiment;  
         [0024]    [0024]FIG. 8 is a horizontal sectional view of part of a rack and a card cage, together with vibration depressing members according to a second embodiment of the present invention; and  
         [0025]    [0025]FIG. 9 is a horizontal sectional view of part of a rack and a card cage, together with vibration depressing members according to a third embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0026]    A principle underlying the present invention resides in the fact that a plurality of vibration depressing members are provided between a rack and a card cage in a manner to change spring constants thereof (viz, damping characteristics thereof) in two stages depending on the strength and amplitude of the vibrations transversely applied to the rack from external.  
         [0027]    A first embodiment of the present invention will be described with reference to FIGS.  3  to  7 .  
         [0028]    [0028]FIG. 3 is a sectional view, which corresponds to FIG. 1, schematically showing vibration depressing members  60   a - 60   b  and  62   a - 62   b  according to the first embodiment and the portions or members associated therewith. In FIG. 3, the portions other than the members  60   a - 60   b  and  62 - 62   b  are substantially identical to those shown in FIG. 2, and accordingly, the descriptions thereof will be not given except for becoming necessary in context.  
         [0029]    As shown in FIG. 3, the vibration depressing member  60   a  is provided between the rack frame  14   a  and the right front portion of the cage  12 , while the other vibration depressing member  60   b  is positioned between the rack frame  14   b  and the left front portion of the case  12 . Similarly, the vibration depressing member  62   a  is provided between the rack frame  14   b  and the right rear portion of the cage  12 , and the other vibration depressing member  62   b  is provided between the rack frame  14   c  and the left rear portion of the case  12 .  
         [0030]    Each of the members  60   a - 60   b  and  62   a - 62   b  is a plate member. Since the members  60   a - 60   b  are configured in exactly the same manner, it is sufficient to refer only to the member  60   a,  and similarly, since members  62   a - 62   b  are formed in exactly the same manner, the description is made only to the member  62   a.  The vibration depressing members  60   a  and  62   b  will be discussed in detail with reference to FIGS.  4 (A) and  4 (B).  
         [0031]    As shown in FIG. 3, the rear portion of the member  60   a  is secured to the rack frame  14   a  by way of a screw  64   a,  and the front portion thereof is detachably fixed to the elongated front edge  27   a  using a screw  64   b  corresponding to the screw  50  of FIG. 2. On the other hand, the member  62   a  is secured, at the rear portion thereof, to the rack frame  14   b  by way of a screw  66 . It is to be noted that the rear portions of the members  60   a  and  62   b  may be welded in place of the aforesaid fixing using the screws.  
         [0032]    FIGS.  4 (A) and  4 (B) are respectively top views of the vibration depressing members  60   a  and  62   a  (FIG. 3) and sectional views of rack frames  14   a  and  14   b.  As shown in FIG. 4(A), the vibration depressing member  60   a  is provided with a rear portion P 1  rigidly attached to the rear side of the frame  14   a  using the screw  64   a,  an intermediate portion P 2  extending from the rear portion P 1  toward the front side of the rack  10  (i.e., the cage  12 ) at an angle with respect to the cage inserting direction, a protrusion P 3  outwardly projecting from the inner side of the intermediate portion P 2 , and a front portion P 4  detachably fixed to the front end plate  27   a  by means of the screw  64   b  (FIG. 3). On the other hand, as shown in FIG. 4(B), the vibration depressing member  62   a,  provided at the rear portion of the rack  10 , has a rear portion Q 1  rigidly attached to the rear side of the frame  14   b  using the screw  66 , an intermediate portion Q 2  extending from the rear portion Q 1  toward the front side of the rack  10  (i.e., the cage  12 ) at an angle with respect to the cage inserting direction, a protrusion Q 3  outwardly projecting from the inner side of the intermediate portion Q 2 , and a front portion Q 4  with a folded portion abutting the rear side of the cage  12 . Each of the intermediate portions P 2  and Q 2  functions as a plate spring as will be described later. The legends L 1 , L 2 , and L 3  attached to FIGS.  4 (A) and  4 (B) will be described with reference to FIG. 5.  
         [0033]    [0033]FIG. 5 is a diagram graphically showing the operation of the intermediate portion P 2  or Q 2  of FIG. 3. In FIG. 5, L 1  denotes an effective elastic length of the intermediate portion P 2  when the protrusion P 3  does not reach the frame  14   a  ( 14   b )(referred to as CASE  1 ) or touches the frame  14   a  without applying any pressure thereto, and L 2  is an effective elastic length when the protrusion P 3  presses the frame  14   a  ( 14   b ) (referred to as CASE  2 ) or vice versa. CASE  1  and CASE  2  respectively occur when the rack  10  is subject to the external vibrations less than and exceeding a threshold condition determined by magnitude and amplitude of the vibration, elasticity of the vibration depressing members, size and weight of the rack and the card cage, etc. In the instant disclosure, the threshold condition is used to determine whether or not the spring constant of the vibration depressing member is switched over. In the above description, the intermediate portion P 2  (or Q 2 ), which operates as a plate spring, can be regarded as a beam with one end fixed and the other end being free. In this case, when the plate spring is not forced onto a fixed corner  68 , a spring constant K 1  of the plate spring is determined by the following equation.  
         K1=3EI/L1 3    
         [0034]    where E denotes a modulus of longitudinal elasticity, and I denotes geometrical moment of inertia (i.e., second moment of area). On the other hand, when the plate spring is forced onto the fixed corner  68 , a spring constant K 2  of the plate spring is determined as follows.  
         K2=3EI/L2 3    
         [0035]    Since L 2 &lt;L 1 , then K 1 &lt;K 2 . This implies that the vibration depressing member  60   a  (or  62   a ) responds to the vibratory motion externally applied to the rack  10  (FIG. 1) and changes the spring constant thereon in two stages, thereby to effectively reduce the vibrating forces imparted onto the rack-mounted card cage  12 .  
         [0036]    FIGS.  6 (A) and  6 (B) show respectively the above-mentioned CASE  1  and CASE  2  with respect to the vibration depressing member  62   a.  As shown in FIG. 6(A). when the rack  10  (FIG. 1) is subject to the external vibrations less than the threshold condition, the protrusion Q 3  is kept apart from the rack frame  14   b.  Thus, in such a case, the member  62   a  absorbs the vibratory motion using the full length of the intermediate portion Q 2 . Contrarily, as illustrated in FIG. 6(B), when the rack  10  (FIG. 1) is subject to the external vibrations exceeding the threshold condition, the protrusion Q 3  abuts or strongly presses the rack frame  14  or vice versa, and the vibratory motion is absorbed using part of the intermediate portion Q 2 . The above discussion is applicable to the other member  60   a.    
         [0037]    Reference is made to FIG. 7, a variation of the first embodiment is illustrated. That is, the vibration depressing member  62   a  is provided with a suitable buffer member (tip)  70 , made of rubber for example, at the end of the projection Q 3 . Other than this, the member  62   a  is formed in exactly the same manner as mentioned above. Such a buffer member  70  is also attached to the end of the protrusion P 3  of the other member  60   a  (FIG. 3).  
         [0038]    A second embodiment of the present invention will be described with reference to FIG. 8. According to the instant embodiment, a protrusion for use in changing the above-mentioned spring constant is provided at the corresponding rack frame in place of the vibration depressing member. That is, the protrusions P 5  are respectively provided at the rack frames  14   a  and  14   d,  while the protrusions Q 5  are respectively provided at the rack frames  14   b  and  14   c.  Other than this, the second embodiment Is identical to the first embodiment, and as such, further description thereof will be omitted for the sake of simplifying the disclosure. It is clearly understood that the operation of reducing the externally applied vibrations according to the second embodiment is substantially identical to that of the first embodiment.  
         [0039]    A third embodiment of the present invention will be described with reference to FIG. 9. As shown, a vibration depressing member  80   a,  provided at the lower right hand corner in the figure, comprises a rear portion M 1  secured to the frame  14   a  using a screw  82 , an elastic portion M 2  extending substantially in parallel with the side wall of the card cage  12 , and a semi-cylindrical resilient member M 3  accommodated in a holder  84 . The card cage  12  is detachably secured, at the front edge  27   a,  to one end of a rod-like member  83  whose other end is fixed to the frame  14   a.  Likewise, another vibration depressing members  86   a,  provided at the upper right hand corner in the figure, comprises a rear portion N 1  secured to the frame  14   b  using a screw  88 , an elastic portion N 2  extending substantially in parallel with the side wall of the card cage  12 , and a semi-cylindrical resilient member N 3  accommodated in a holder  90 . A vibration depressing member  80   b,  provided between the rack frame  14   d  and the left front side of the cage  12  is formed in exactly the same manner as the member  80   a  although the member  80   b  is installed in a reversed manner relative to the member  80   a.  Further, a vibration depressing member  86   b,  provided between the rack frame  14   c  and the left rear side of the cage  12 , is formed in exactly the same manner as the. member  86   a,  which is however installed in a reversed manner relative to the member  86   a.    
         [0040]    Since the vibration reducing functions of the members  80   a - 80   b  and  86   a - 86   b  are substantially identical with one another, the operation of the member  80   a  is described on behalf of the remainders. When the vibratory motions applied to the rack  10  (FIG. 1) is less than the above-mentioned threshold condition, the vibratory motions are absorbed by the semi-cylindrical member M 3 . On the other hand, if the vibratory motions applied to the rack  10  exceeds the threshold condition, the semi-cylindrical member M 3  is pressed to an extent that the holder  84  abuts the side wall of the card cage  12  or vice versa, whereby the vibratory motions are absorbed by the resilient portion M 2 .  
         [0041]    The foregoing descriptions show one preferred embodiment and some modifications thereof. However, other various modifications are apparent to those skilled in the art without departing from the scope of the present invention which is only limited by the appended claims. Therefore, the embodiments and modification shown and described are only illustrated, not restrictive.