Patent Publication Number: US-6655625-B2

Title: Magnetic tape cartridge

Description:
This is a divisional of application Ser. No. 09/622,573 filed Aug. 18, 2000, now U.S. Pat. No. 6,488,223; the disclosure of which is incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a magnetic tape cartridge having a lid that can open and closes an opening portion for drawing out a magnetic tape. 
     The present application is based on Japanese Patent Applications Nos. Hei. 10-36170, 10-79455 and 10-350136, the contents of which are incorporated herein by reference. 
     BACKGROUND ART 
     A magnetic tape cartridge has been used as recording medium for computers etc. As an example of such the magnetic tape cartridge, there has been known a magnetic tape cartridge which comprises: a reel; a magnetic tape wound around the reel; and a cartridge case including an upper cartridge and a lower cartridge, wherein the reel is accommodated in the cartridge case so as to be able to rotate. A lid is pivotably attached to the cartridge case so as to be able to open and close an opening portion formed in the cartridge case. When the magnetic tape cartridge is used, the opening portion is opened by the lid so that the magnetic tape may be drawn out of an opening portion. 
     When the magnetic tape cartridge is not used, the opening portion is closed by the lid. At this stage, a leader tape attached to a tape end is locked at an end surface of the cartridge case in such a state that the magnetic tape is fully wound up onto the reel while the reel is locked by such a lock component as a reel lock so as to prevent unexpected rotations of the reel. Incidentally, the leader tape is an engagement mechanism for such an apparatus as a computer to introduce the magnetic tape into a tape path. 
     The lid and the locking structure thereof will be described. In the cartridge case described above, the magnetic tape is wound around the reel rotatably mounted, and the leader tape is positioned at the opening portion when the lid closes the opening portion. The reel is comprised of a set of upper and lower reels, in which a gear portion is formed on the outer circumferential portion of the upper reel. 
     FIG. 1 shows a basic attachment structure of a lid  60 . As shown in FIG. 1, the lid  60  is mounted on an upper cartridge  2   a  through a supporting shaft  14 . The lid  60  is urged by a lid spring  65  in a closing direction as indicated by an arrow A. The lid spring  65  is attached to the supporting shaft  14  through a coil portion  65   c  thereof, and one leg portion  65   b  is supported by the frame  2   d  of the upper cartridge  2   a  while the other leg portion  65   a  is supported by a recessed portion  67  formed in the lid  60 . 
     The lid  60  has a hole  61  into which a lid lock member  70  for locking the opening or closing actions of the lid  60  is inserted. The lid lock member  70  can vertically slide in the hole  61  in a cartridge thickness direction. The lid lock member  70  is located adjacent to the supporting shaft  14  when the lid  60  is mounted on the upper cartridge  2   a . As shown in FIG. 1, the lid lock member  70  is urged by an upward force of a lid lock spring  16  in a direction indicated by an arrow U, to lock the lid  60  so as not to open by the upper end thereof being engaged with a recessed portion formed in the lower cartridge (not shown) when the magnetic tape cartridge is not used. 
     To ensure high accuracy and flame resistance, the lid  60  and the cartridge case are respectively made of a composite material of polycarbonate and glass fiber. The lid lock member  70  is made of a material excellent in sliding effect, such as POM, in order to reduce friction. The supporting shaft  14  is molded as a part of the upper cartridge  2   a  for the reasons of accuracy and costs. A shaft hole  63  through which the supporting shaft  14  is passed is molded as a part of the lid  60  for the reasons of accuracy and costs. 
     FIG. 2 shows positional and kinetic relationship between the lid  60  and the lid spring  65  in a static state where the lid is opened while a certain torque is being loaded on the lid spring  65 . As for the kinetic relationship while the lid  60  is opened, a torque Ts 1  of the lid spring  65  is applied to the spring bearing portion  68  of the lid  60  to generate a spring force F 6 . An opening force F 5  is applied to the lid  60  through a release lever (not shown) for opening the lid  60 . A load F 7  is applied on the supporting shaft  14 . The load F 7  is a difference between the forces F 6  and F 5 . In this magnetic tape cartridge, the Ts 1  is set as 140 gfcm in view of parameters of a lid spring which is generally utilized in this magnetic tape cartridge. A radius r of the supporting shaft  14  is 0.2 cm. A distance d 1  from the center of the supporting shaft  14  to a point of action of the force F 6  is 0.3 cm. A distance d 2  from the center of the supporting shaft  14  to a point of action of the force F 5  is 1.1 cm. 
     A relationship among T (torque), F (force), and d (distance) is defined as follows: 
     
       
           T  (torque)= F  (force)× d  (distance)  
       
     
     In view of the above, the specific values of F 6  and F 5  are defined as follows: 
     
       
           F   6 = Ts   1 / d   1 =467  gf    
       
     
     
       
           F   5 = Ts   1 / d   2 =127  gf    
       
     
     Further, the specific value of F 7  is defined based on balance of the forces as follows: 
     
       
           F   7 = F   6 − F   5 =340  gf    
       
     
     Accordingly, if a static friction coefficient μ of the resin is set as 0.52, a friction torque Tf 1  of the supporting shaft  14  is defined as follows: 
     
       
           Tf   1 = F   7 ×μ× r =35  gfcm    
       
     
     FIG. 3 shows positional and kinetic relationship between the lid  60  and the lid spring  65  in a static state where the lid  60  is closed. As for the kinetic relationship while the lid  60  is closed, a torque Ts 2  of the lid spring  65  is applied to the spring bearing portion  68  of the lid  60  to generate a spring force F 2 . A counter force Fe is applied from the leader tape  21  to the lid  60 . A load F 3  applied to the supporting shaft  14  is a sum of F 2  and Fl. Incidentally, in this magnetic tape cartridge, the Ts 2  is 60 gfcm and the force Fl is 8 gf while the distance d 3  from the center of the supporting shaft  14  to a point of action of the force Fl is 4.5 cm. 
     In view of the above, the specific value of F 2  is defined as follows: 
     
       
           F   2 = Ts   2 / d   1 =200  gf    
       
     
     Therefore, the specific value of F 3  is defined based on balance of the forces as follows: 
     
       
           F   3 = F   2 + Fl =208  gf    
       
     
     Accordingly, a friction torque Tf 2  of the supporting shaft  14  is defined as follows: 
     
       
           Tf   2 = F   3 ×μ× r =22  gfcm    
       
     
     As a result, a spare torque Tt of the lid spring  65  in the close state of the lid  60  is defined as follows: 
     
       
           Tt=Ts   2 −( Tf   2 + T   1 )=2  gfcm    
       
     
     (wherein T 1 =8×4.5=36 gfcm) 
     That is, the closing action of the lid  60  is maintained by the above torque Tt. 
     As described above, the material, represented by such as polycarbonate which can ensure the high accuracy and the flame resistance, has a large friction coefficient as a synthetic resin (friction coefficient μ=0.4 to 0.6). Therefore, the lid  60  may be stopped in the middle of the pivotal action due to the rotational resistance between the supporting shaft  14  and the shaft hole  63 . Further, there is a possibility in that the load is increased on the drive of an apparatus, such as a computer, when the lid  60  is pivoted. 
     Additionally, when the lid  60  is opened, since the lid lock member  70  moves while the lid lock member  70  is being pushed toward the upper cartridge  2   a  by the door releasing lever of the drive, thrust friction between the lid  60  and the upper cartridge  2   a  increases, and therefore the opening operation may require more force and friction powder may be generated. On the other hand, when the lid  60  is closed, since the lid lock member  70  pushes the lower cartridge, the friction occurs between the lid lock member  70  and the lower cartridge, and further, the thrust friction is caused between the lid  60  and the upper cartridge  2   a  by a counter force applied to the lid  60  by the lid lock member  70 . Since the lid  60  and the cartridge case are made of polycarbonate etc. having a large friction coefficient, an imperfect closure of the lid  60  may be caused due to the friction forces as described above, and further, such a serious trouble as a drop out may be caused by the generated friction powder etc. 
     As described above, such the small spare torque Tt of the lid spring  65  as 2 gfcm is considered to be the reason for the imperfect closure of the lid  60 . However, if the spring force of the lid spring  65  itself is increased as a countermeasure against the imperfect closure, the load on the supporting shaft  14  may be increased and the friction may be also increased. 
     When the force applied to the lid by the lid spring itself is merely increased, the force required for release of the lid is increased, and further, the friction is increased. In addition, the increase in the size of the lid spring itself also increases the storage space for the lid spring. Moreover, after the lid is opened and closed for a large number of times, the friction powder may be generated. The friction powder may result with such a serious trouble as a drop out. 
     It is possible to eliminate such possibilities as described above by utilization of a material with a better sliding effect, such as polyacetal, in the supporting shaft  14  and also in the shaft hole  63 . However, to prevent degradation in the flame resistance and accuracy of the cartridge and the lid, it is necessary to mold the upper cartridge  2   a  and the supporting shaft  14 , as well as the lid  60  and the shaft hole  63 , into separate structures or into two colors. In this case, however, there are disadvantages in the accuracy and the costs. 
     DISCLOSURE OF THE INVENTION 
     The present invention has been made in view of the background as described above, and an object of the invention is to provide a magnetic tape cartridge realizing smooth pivot of the lid at low cost. 
     To achieve the above object, according to the first aspect of the present invention, there is provided a magnetic tape cartridge which comprises: a cartridge case including an opening portion, an upper cartridge and a lower cartridge, the upper cartridge having a supporting shaft, and the lower cartridge having a recessed portion; a reel, around which a magnetic tape can be wound, accommodated in the cartridge case, wherein the magnetic tape can be drawn out from the opening portion of the cartridge case; a lid pivotably attached to the supporting shaft in order to open and close the opening portion; a lid spring being a torsion coil spring having a coil portion which is a compressive spring, the lid spring being attached to the supporting shaft through the coil portion so as to urge the lid to close the opening portion; and a lid lock member urged toward the lower cartridge by a lid lock spring so as to be engaged with the recessed portion of the lower cartridge, wherein, when the lid lock member engages with the recessed portion, the lid lock member locks the lid to close the opening portion, and, when the lid lock member is disengaged from the recessed portion, the lid can pivot. 
     According to the first aspect of the present invention, when the lid is opened, compression of the coil portion results with the counter force working to counter the thrusting force of the release lever and therefore the friction loss between the lid and the upper cartridge is reduced. On the contrary, when the lid is closed, since the lid lock member pushes the lower cartridge, although there is a slight friction between the lower cartridge and the lid lock member made of a low friction coefficient material excellent in sliding effect, a force is generated due to compression of the coil portion and counters the counter force in which the lid lock member pushes the lower cartridge, and therefore the friction loss between the lid and the upper cartridge is reduced. Therefore, the lid can pivot smoothly without being stopped halfway in the course of pivotal movement by the rotational resistance. Additionally, this is realized only by adoption of the compressive spring for the coil portion of the lid spring, and therefore no substantial increase in the cost will result. 
     The compressive spring herein is a spring which is compressible at the coil portion thereof even if it may resemble ordinary torsion coil springs in the appearance thereof. 
     Further, according to the second aspect of the present invention, it is preferable that a compressive force of the coil portion of the lid spring is set to be equal to or higher than an urging force in which the lid lock spring urges the lid lock member when the lid closes the opening portion of the cartridge case. 
     Further, according to the third aspect of the present invention, it is preferable that a compressive force of the coil portion of the lid spring is set to be 1.2 times to 1.5 times higher than an urging force in which the lid lock spring urges the lid lock member when the lid closes the opening portion of the cartridge case. 
     Still further, to achieve the above object, according to the fourth aspect of the present invention, there is provided a magnetic tape cartridge which comprises: a cartridge case including an opening portion, an upper cartridge and a lower cartridge, the upper cartridge having a supporting shaft; a frame formed on at least one of the upper cartridge and the lower cartridge; a reel, around which a magnetic tape can be wound, accommodated in the cartridge case, wherein the magnetic tape can be drawn out from the opening portion of the cartridge case; a lid pivotably attached to the supporting shaft in order to open and close the opening portion, the lid including a protruding portion; and a lid spring being a torsion coil spring having a coil portion through which the lid spring is attached to the supporting shaft so as to urge the lid to close the opening portion, one leg portion supported by the frame, and the other leg portion supported by the protruding portion of the lid so that a portion of the other leg portion adjacent to an end portion thereof is brought into contact with the protruding portion. 
     Accordingly, the load on the supporting shaft can be reduced. Consequently, the friction torque of the supporting shaft can be reduced to result with the reduction in the generation of friction powder and with the smooth pivot of the lid. Additionally, this is realized only by provision of the protruding portion on the lid alone and thus no substantial increase in the cost will result. Moreover, since the lid spring is supported at the portion adjacent to the leg end, the torque which applies a force in the direction to close the lid is increased, and therefore, the above-described imperfect closure of the lid can be eliminated. 
    
    
     Further, according to the fifth aspect of the present invention, the protruding portion may have an approximately square prism shape. Furthermore, according to the sixth aspect of the present invention, the lid may include a recessed portion in which the protruding portion is formed. 
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 shows a basic attachment structure of the lid; 
     FIG. 2 shows positional and kinetic relationship between the lid and the lid spring in a static state where the lid is opened; 
     FIG. 3 shows positional and kinetic relationship between the lid and the lid spring in a static state where the lid is closed; 
     FIG. 4 is an exploded perspective view showing a first embodiment of the magnetic tape cartridge according to the present invention; 
     FIG. 5 is a basic attachment structure of a lid of the first embodiment according to the present invention; 
     FIG. 6 shows a state in that a lid spring and the lid of FIG. 5 are attached onto a supporting shaft; 
     FIG. 7 is an enlarged view showing an assembled state of the magnetic tape cartridge; 
     FIG. 8 is a basic attachment structure of a lid of a second embodiment according the present invention; 
     FIG. 9 shows kinetic relationship when a lid spring of the second embodiment according to the present invention is attached onto a supporting shaft of an upper cartridge; 
     FIG. 10 is a basic attachment structure of a lid of a third embodiment according to the present invention; 
     FIG. 11 shows a state in that a lid spring and the lid of FIG. 5 are attached onto a supporting shaft; 
     FIG. 12 shows positional and kinetic relationship between the lid and the lid spring in a static state where the lid is opened; 
     FIG. 13 shows positional and kinetic relationship between the lid and the lid spring in a static state where the lid is closed; and 
     FIG. 14 is a graph showing a comparison between the friction torques respectively working on the supporting shafts of the embodiment shown in FIGS. 10-13 and the background art when the lids are opened and closed. 
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     The best mode for carrying out the present invention will now be described with reference to FIGS. 4 to  14 . FIG. 4 is an exploded perspective view showing a first embodiment of the magnetic tape cartridge according to the present invention. A magnetic tape cartridge  1  comprises: a magnetic tape  20  wound around a single reel  3 ; and a cartridge case accommodating the reel  3  so as to be able to rotate in the cartridge case. The reel  3  includes an upper reel  4  and a lower reel  5  which are combined by such a method as ultrasonic welding. The cartridge case includes an upper cartridge  2   a  and a lower cartridge  2   b  which are tightened with screws  19 . 
     The upper reel  4  of the reel  3  has an upper flange  4   a . A recessed portion is provided at the center of the upper flange  4   a . The upper flange  4   a  has a gear portion  4   b  formed on an outer circumferential portion thereof. A ring-shaped bearing  6  is press-fitted into and secured to a circular rib which is formed inside the recessed portion of the upper flange  4   a . A spring plug  7  is press-fitted into and secured to a hole formed at a center of the bearing. A reel spring  8  is attached to the spring plug  7  so that the reel  3  is pressed downward and is rotatably held. 
     When the magnetic tape cartridge  1  is not used, the reel  3  is locked to prevent unexpected rotation by reel locks  50  and  51  which are urged with some appropriate force by a reel lock spring  12 . In addition, a leader tape attached to a tape end is locked at a tip portion of a hook  18  which is attached at a position close to a lateral side of the lower cartridge  2   b  in such a state that the magnetic tape  20  is fully wound up onto the reel  3 . Incidentally, the leader tape is an engagement mechanism for such an apparatus as a computer to introduce the magnetic tape  20  into a tape path. 
     A lid  30  is attached to an opening portion  26  formed to lead the magnetic tape  20 . The lid  30  is urged with some appropriate force by a lid spring  15  which is a torsion coil spring, so that the lid  30  can open and close the opening portion  26  in a direction along a plane of the cartridge. When the magnetic tape cartridge  1  is not used, the lid  30  is locked so as not to be opened by a lid lock member  40  which is pushed with some appropriate force toward the lower half  2   b  by a lid lock spring  16 . The leader tape  21  is joined to the magnetic tape by such a method as splice, and engages with an engagement component for introducing the magnetic tape  20  into the tape path of a recording/reproducing apparatus. 
     FIG. 5 shows a basic attachment structure of the first embodiment of the lid according to the present invention. As shown in the FIG. 5, the lid  30  is mounted on a supporting shaft  14  and urged with a force in the closing direction indicated by an arrow A by the lid spring  15 . The lid spring  15  is a compressive spring in which a coil portion  15   c  thereof is formed in a spacing turn manner. The lid spring  15  is attached to the supporting shaft  14  through the coil portion  15   c  thereof, and one leg portion  15   a  is supported by a frame  2   d  of the upper cartridge  2   a  while the other leg portion  15   b  is supported by a recessed portion  37  formed in the lid  30 . Since this is realized by adoption of the compressive spring for the coil portion  15   c  of the lid spring  15 , no substantial increase in the cost will result in comparison with a background one. 
     FIG. 6 shows a state in that the lid spring  15  and the lid  30  are mounted onto the supporting shaft  14 . As shown in FIGS. 5 and 6, the lid  30  has a hole  31  into which the lid lock member  40  for locking the opening and closing actions of the lid  30  is inserted. The lid lock spring  16  is attached to the lid lock member  40  so that a boss  40   a  of the lid lock member  40  is inserted into the lid lock spring  16 . The lid lock member  40  can vertically slide in the hole  31 , and is urged by an upward force of the lid lock spring  16  in a direction indicated by an arrow U so that an upper end portion  40   b  of the lid lock member  40  protrudes from the hole  31 . Further, the protruded upper end portion  40   b  is engaged with the recessed portion formed in the lower cartridge  2   b , so that the lid  30  cannot pivot. 
     As shown in FIG. 7, when opening the lid  30 , the lid lock member  40  is pushed downward by a release lever of the drive in a direction of an arrow L (i.e., toward the upper cartridge  2   a ), and the lid  30  is pivoted in a direction of an arrow B. Although the urging force is applied to the lid  30  in the direction of the allow L due to the push of the release lever, since the coil portion  15   c  of the lid spring  15  is the compressive spring, a counter force in a direction opposite to the allow L is obtained due to the compression of the coil portion  15   c . Therefore, friction loss by contact of the lid  30  and the upper cartridge  2   a  can be reduced. 
     On the other hand, when closing the lid  30 , since the lid lock member  40  is abutted against the lower cartridge  2   b  by an urging force of the lid lock spring  16 , the lid  30  is pushed toward the upper cartridge  2   a  due to the urging force. However, since the coil portion  15   c  of the lid spring  15  can be compressed also in this case, the friction loss between the lid  30  and the upper cartridge  2   a , both of which are made of the material represented by such as polycarbonate etc. having a large friction coefficient, can be reduced by a counter force of the coil portion  15   c  countering the urging force of the lid lock spring  16 . According to this construction, since the friction between the lid  30  and the upper cartridge  2   a  can be reduced, there is no possibility in that the lid  30  is stopped halfway in the pivotal movement by the rotational resistance, and such a problem as imperfect closure will not occur, and therefore, the lid can pivot smoothly. Here, the lid lock member  40  is made of a low friction coefficient material excellent in sliding effect, such as POM, and therefore, the friction force is small and friction powder is not generated even if the lid lock member  40  is brought into contact with the lower cartridge  2   b.    
     It is preferable that the compressive force of the coil portion  15   c  is equal to or higher than an urging force in which the lid lock spring  16  urges the lid lock member  40  when the lid  30  closes the opening portion  26  of the cartridge case. In other words, it is preferable that the compressive force of the coil portion  15   c  is equal to or higher than the urging force generated by the lid lock spring  16  when the upper end portion  40   b  of the lid lock member  40  is engaged with the recessed portion formed in the lower cartridge  2   b  so that the lid  30  cannot pivot. Incidentally, it is more preferable that the compressive force of the coil portion  15   c  is 1.2 times to 1.5 times higher than the urging force generated by the lid lock spring  16 . Additionally, since the lid lock member  40  is made of a material excellent in sliding effect, such as POM, the friction coefficient thereof is smaller than the lid  30 . This is intended because it is preferable that the contact surface of the lid  30  is away from the contact surface of the upper cartridge  2   a . Namely, it is preferable that the lid  30  is urged toward the lower cartridge  2   b  so that the contact surface of the lid  30  is away from the contact surface of the upper cartridge  2   a . This is also effective in ensuring the lock of the lid  30 . However, it is necessary to adjust the compressive force of the coil portion  15   c  appropriately in the movable range of the lid lock member  40 . 
     With reference to FIGS. 8 and 9, a second embodiment of the present invention will be described in detail. As shown in FIG. 8, although the construction of the lid  30  is the same as in the first embodiment, the inner diameter of a coil portion  25   c  of the lid spring  25  is 1.5 times or more than the outer diameter of the supporting shaft  14  of the upper cartridge  2   a.    
     That is, the inner diameter of the coil portion  25   c  of the lid spring  25  is larger than the lid of the background art, when compared. The lid spring  25 , which is a torsion coil spring, includes one leg portion  25   a  supported by a frame  2   d  of the upper cartridge  2   a , and the other leg portion  25   b  supported by a recessed portion  37  formed in the lid  30 , and therefore the coil portion  25   c  itself is tilted. 
     As shown in FIG. 9, when the coil portion  25   c  is tilted, an upward (in the figure) component force Fv is generated. The friction loss due to the contact between the lid  30  and the upper cartridge  2   a  can be reduced by the component force Fv. To ensure the component force Fv, it is desirable to provide a notch or a step in the recessed portion  37  for ensuring retainment of the leg portion  25   b  of the lid spring  25  and also it is desirable that the inner diameter of the coil portion  25   c  of the lid spring  25  is 1.7 to 3 times larger than the outer diameter of the supporting shaft  14 . 
     Next, a basic attachment structure of a lid of a third embodiment according to the present invention will be described with reference to FIGS. 10 to  14 . As shown in FIG. 10, the lid  30  is attached to the supporting shaft  14 , and is urged with a force in the closing direction indicated by the arrow A by a lid spring  65 . The lid spring  65  herein is similar to those shown in FIGS. 1 to  3 . As shown in FIG. 11, the lid spring  65  is attached to the supporting shaft  14  through the coil portion  65   c  thereof, and one leg portion  65   b  is supported by the frame  2   d  of the upper cartridge  2   a  while the other leg portion  65   a  is supported by a protruding portion  33  which is formed in a recessed portion  67  of the lid  30 . The protruding portion  33 , which has an approximately square prism shape, supports the leg portion  65   a  at a portion adjacent to the end portion of the leg portion  65   a . The portion adjacent to the end portion is defined by a position where the distance between a point of action of a force of the lid spring  65  and the supporting shaft  14  is not less than twice the radius of the supporting shaft  14  and also is within the range between ½ and ⅔ of the distance between the supporting shaft  14  and a contact point of which the release lever (not shown) for opening the lid  30  contacts with the lid  30 . 
     FIG. 11 shows a state in which the lid spring  65  and the lid  30  are attached onto the supporting shaft  14 . As shown in FIG. 11, the lid lock spring  16  is stored between an inner wall of the recessed portion  67  and the protruding portion  33 . The lid lock member  40  is inserted into the hole  31  of the lid  30 . The lid lock spring  16  is attached to the lid lock member  40  so that the boss  40   a  of the lid lock member  40  is inserted into the lid lock spring  16 . The lid lock member  40  can vertically slide in the hole  31 , and is urged by the upward force of the lid lock spring  16  in the direction indicated by the arrow U so that an upper end portion  40   b  of the lid lock member  40  protrudes from the hole  31 . When the protruded upper end portion  40   b  engages with the recessed portion formed in the lower cartridge  2   b , the lid  30  cannot pivot. 
     FIG. 12 shows positional and kinetic relationship between the lid  30  and the lid spring  65  in a static state where the lid  30  is opened while a certain torque is being loaded on the lid spring  65 . As for the kinetic relationship while the lid  30  is opened, a torque Ts 1  of the lid spring  65  is applied to the protruding portion  33  of the lid  30  to generate a spring force F 6 ′. An opening force F 5 ′ is applied to the lid  30  through the release lever (not shown) for opening the lid  30 . A load F 7 ′ is applied on the supporting shaft  14 . The load F 7 ′ is a difference between the forces F 6 ′ and F 5 ′. Similar to the magnetic tape cartridge discussed in the background art section, the Ts 1  is set as 140 gfcm and a radius r of the supporting shaft  14  is 0.2 cm. A distance d 1 ′ from the center of the supporting shaft  14  to a point of action of the force F 6 ′ is 0.9 cm. A distance d 2  from the center of the supporting shaft  14  to a point of action of the force F 5 ′ is 1.1 cm. 
     As described in the background art section, the relationship among T (torque), F (force), and d (distance) is defined as follows: 
     
       
           T  (torque)= F  (force)× d  (distance)  
       
     
     In view of the above, the specific values of F 6 ′ and F 5 ′ are defined as follows: 
     
       
           F   6 ′= Ts   1 / d   1 ′=155  gf    
       
     
     
       
           F   5 ′= Tsa   1 / d   2 =127  gf    
       
     
     Further, the specific value of F 7 ′ is defined based on balance of forces as follows: 
     
       
           F   7 ′= F   6 ′− F   5 ′=28  gf    
       
     
     Accordingly, if a static friction coefficient μ of the resin is set as 0.52, a friction torque Tf 1 ′ of the supporting shaft  14  is defined as follows: 
     
       
           Tf   1 ′= F   7 ′×μ r =2.9  gfcm    
       
     
     FIG. 13 shows positional and kinetic relationship between the lid  30  and the lid spring  65  in a static state where the lid is closed. As for the kinetic relationship while the lid  30  is closed, a torque Ts 2  of the lid spring  65  is applied to the protruding portion  33  of the lid  30  to generate a spring force F 2 ′. A counter force Fl is applied from the leader tape  21  to the lid  30 . A load F 3 ′ applied to the supporting shaft  14  is a sum of F 2 ′ and Fl. Incidentally, similar to the magnetic tape cartridge discussed in the background art section, the Ts 2  is 60 gfcm and the force Fl is 8 gf while the distance d 3  from the center of the supporting shaft  14  to a point of action of the force Fl is 4.5 cm. 
     In view of the above, the specific value of F 2 ′ is defined as follows: 
     
       
           F   2 ′= Ts   2 / d ′ 1 =67  gf    
       
     
     Therefore, the specific value of F 3 ′ is defined as follows: 
     
       
           F   3 ′= F   2 ′+ Fl =75  gf    
       
     
     Accordingly, a friction torque Tf 2 ′ of the supporting shaft  14  is defined as follows: 
     
       
           Tf   2 ′= F   3 ′×μ r =7.8  gfcm    
       
     
     As a result, a spare torque Tt′ of the lid spring  65  in the close state of the lid  60  is defined as follows: 
     
       
           Tt′=Ts   2 −( Tf   2 ′+ T   1 )=16.2  gfcm    
       
     
     (wherein T 1 −8×4.5=36 gfcm) 
     FIG. 14 is a graph showing a comparison between the friction torques respectively working on the supporting shafts of the third embodiment and the background art when the lids are opened and closed. As shown in FIG. 14, the friction torque according to the third embodiment has decreased in either case of opening and closing. Thus, generation of friction powder can be reduced, and therefore, the lid can pivot smoothly. Additionally, as described above, such a sufficient spare torque as 16.2 gfcm is obtained when the lid is being closed, and therefore the possibility of imperfect closure of the lid will not occur. Moreover, since the protruding portion  33  functions also as a guide for the lid lock spring  16 , the positional stability at insertion of the lid lock spring can be enhanced. 
     INDUSTRIAL APPLICABILITY 
     As described above, according to the first embodiment of the present invention, the coil portion of the lid spring is the compressive spring. Further, according to the second embodiment of the present invention, the inner diameter of the coil portion is 1.5 times or more than the outer diameter of the supporting shaft of the upper cartridge. Therefore, when the lid is opened, compression of the lid spring results with the counter force working to counter the thrusting force of the release lever. Accordingly, the friction loss between the lid and the upper cartridge can be reduced. Furthermore, when the lid is being closed, compression of the coil portion results, in the same way, with the counter force working to counter such force as applied by the lid lock member, and therefore, the friction loss between the lid and the upper cartridge is reduced. 
     Therefore, the lid can pivot smoothly without being stopped halfway in the course of pivotal movement by the rotational resistance. Additionally, since this is realized only by adoption of the compressive spring for the coil portion of the lid spring, no substantial increase in the cost will result. 
     Furthermore, according to the third embodiment of the present invention, the load on the supporting shaft can be reduced since the lid which is disposed at the opening portion that is formed in the frame structure for drawing out this magnetic tape and which is mounted on the supporting shaft that is disposed on the upper cartridge is provided with the protruding portion that supports the leg portion of the lid spring in the vicinity of the end portion thereof. Therefore, the friction torque of the supporting shaft can be reduced to result with the reduction in the generation of friction powder and with the smooth pivot of the lid. Additionally, this is realized only by provision of the protruding portion on the lid alone and thus no substantial increase in the cost will result. Moreover, since the lid spring is supported at the portion in the vicinity of the leg end, the torque which applies a force in the direction to close the lid is increased, the problem of imperfect closure of the lid will not occur.