Abstract:
A turntable is a table mounted to a shaft ( 4 ) of a spindle motor and the turntable supports a laminated disc thereon and drives the disc in rotation. The laminated disc includes a lower disc and an upper disc stacked on the lower disc, and is supported on a disc-supporting surface. When the disc is loaded on the turntable, a fitting member fits into the center hole of the laminated disc for centering the laminated disc with respect to the shaft. The fitting member fits the center hole of the lower disc when the laminated disc is supported on said disc-supporting surface. The turntable may include a tapered member which guides the laminated disc to the disc-supporting surface. The laminated disc includes lower and upper discs each having an edge which defines the center hole. The corners of the edges are cut away along a circumference of the center hole. The upper and lower discs are placed together so that the cut-away corner directly face each other.

Description:
This application is a divisional of application Ser. No. 09/089,395, filed on Jun. 3, 1998, now U.S. Pat. No. 6,049,522, which is a Div. of Ser. No. 08/863,620 filed May 27, 1997, now U.S. Pat. No. 5,889,757, the entire contents of which are hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Technical Field of the Invention 
     The present invention relates to a laminated disc such as a DVD (Digital Video Disc) and a turntable on which laminated disc Is placed for reproduction of information. 
     2. Description of Prior Art 
     FIG. 20 is a cross-sectional view of a prior art turntable which is disclosed in Japanese Patent Preliminary Publication No. 8-7425. A turntable  102  has a hub  105  into which a motor shaft  104  is press-fitted or securely bonded to drive the turntable  102  in rotation. A disc  101  is placed on the turntable  102  and clamped by a clamper  103  so that the disc  101  is firmly held between the turntable  102  and the clamper  103  in a sandwiched relation. The hub  105  fits into a center hole  106  in the disc  101  with a very small clearance, thereby concentrically positioning the disc  101  with respect to the rotational axis of the motor shaft  104 . The disc  101  is rotated by a driving force of a spindle motor  100 . 
     FIG. 21 is a perspective view of a conventional disc  101  on which information is recorded. The disc  101  has a center hole  106  in its center and placed on the turntable  102 . 
     A DVD is a laminated disc which includes two discs bonded together back-to-back. When the two discs are bonded together, they are placed one over the other slightly eccentric due to alignment error. This alignment error varies within a predetermined tolerance and results in a small step in the wall of the center hole of the discs. 
     When a DVD is loaded onto the aforementioned conventional turntable, the stepped wall of the center hole of the disc may be caught by the hub  105  so that the disc  101  will not descend to the supporting surface of the turntable. 
     A DVD is usually loaded onto the positioning hub of the turntable so that the hub extends through the center hole with a very small clearance between the DVD. Burrs projecting from the the bonded interface of the disc into the center hole also cause a problem similar to that caused by the stepped wall or may present a problem that the disc is not concentric with the positioning hub. A further problem with the conventional disc is that the bonding agent spreads from between the two discs into the center hole during the manufacture of the disc or during operation of the turntable at elevated temperatures. 
     SUMMARY OF THE INVENTION 
     The present invention was made in view of the aforementioned problems. 
     An object of the invention is to provide a turntable which allows a disc to be smoothly loaded without burrs or a step in the wall of the center hole caught by the positioning hub. 
     Another object of the invention is to provide a laminated disc which can be loaded onto a turntable without the step and burrs of the wall of the center hole of the disc caught by the hub and can be adjusted to be concentric with the positioning hub upon loading. 
     Still another object of the invention is to provide a laminated disc which prevents the bonding agent from spreading into the center hole from the bonded interface when the disc is manufactured or operated at elevated temperatures. 
     A turntable is a table which is mounted to a shaft of a spindle motor and supports a laminated disc thereon so as to drive the disc in rotation. The laminated disc includes a lower disc and an upper disc bonded on the lower disc, and is supported on a disc-supporting surface of the turntable. When the disc is loaded on the turntable, a fitting member of the turntable fits into the center hole of the laminated disc for centering the laminated disc with respect to the shaft. The fitting member fits the center hole of the lower disc when the laminated disc is supported on the disc-supporting surface. 
     The turntable may include a tapered member in the form of a truncated circular cone which is continuous with the fitting member and guides a laminated disc to the disc-supporting surface. The tapered member has a surface at an angle (θ) with the shaft of the spindle motor. The surface meets three conditions expressed by, 
     
       
         Condition I: tan θ&gt; d 1/( m 1− m 2) 
       
     
     which defines a maximum center-to-center distance d1 between the disc and the shaft; 
     
       
         Condition II: tan θ≧{( n 1 −n 2)/ n 3} 
       
     
     which defines a lower limit of θ so that the tapered member is prevented from interfering the laminated disc when the laminated disc is loaded onto the turntable; and 
     
       
         Condition III: tan θ&lt;1/μ 
       
     
     which defines an upper limit of the angle (θ) of the tapered member; 
     where 
     θ is the angle (θ) which the tapered surface of the tapered member makes with the shaft; 
     d1 is a center-to-center distance between the disc and the shaft; 
     m1 is a distance between the disc-supporting surface and an upper end of the tapered surface of the tapered member; 
     m2 is the distance between the disc-supporting surface and a lower end of the tapered surface of the tapered member; 
     n1 is a center-to-center distance between the upper and lower discs of the laminated disc; 
     n2 is a clearance between the laminated disc and the fitting member; 
     n3 is a difference between the distance m2 and a thickness of the lower disc; and 
     μ is a coefficient of friction between the laminated disc  11  and a surface  15   a  of the tapered member. 
     The laminated disc includes lower and upper discs, each having an inner edge which defines the center hole of the disc. The corners of the edges are cut away along the circumference of the center hole. The upper and lower discs are placed together in such a way that the cut-away corners of the upper and lower discs face each other. 
     Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of-the present invention, and wherein: 
     FIG. 1 is a cross-sectional view of a turntable according to a first embodiment when a laminated disc is loaded thereon; 
     FIG. 2 is a fragmentary cross-sectional view of the turntable and a disc loaded on the turntable, the disc having a burn; 
     FIG. 3 is a fragmentary cross-sectional view of the turntable and a disc with a bonding agent spreading out of an interface between the two discs; 
     FIG. 4 illustrates the positional relation between the disc and the turntable; 
     FIG. 5 is a fragmentary side view of the turntable and the disc loaded to the turntable; 
     FIG. 6 illustrates the the disc smoothly guided along the inclined surface of the truncated cone; 
     FIG. 7 illustrates the positioning hub of the turntable having a horizontal surface formed parallel to the disc surface placed on the turntable; 
     FIG. 8 is a top view of the disc and turntable according to a second embodiment; 
     FIG. 9 is a cross-sectional view taken along lines J-O and O-K of FIG. 8; 
     FIG. 10 is an exploded perspective view when the turntable is provided with a ball-chucking construction; 
     FIG. 11 is a cross-sectional view when the disc is loaded onto the chucking type turntable; 
     FIG. 12 is a fragmentary cross-sectional side view of a laminated disc according to a third embodiment; 
     FIG. 13 is a cross-sectional view of the laminated disc when it is loaded to a supporting surface of a conventional turntable; 
     FIG. 14 is a cross-sectional view of the laminated disc of the third embodiment when it is loaded onto the conventional ball chucking type turntable having a metal chucking ball; 
     FIG. 15 is a cross-sectional view of a disc according to a fourth embodiment; 
     FIG. 16 is a top view of the lower disc; 
     FIG. 17 is a cross-sectional view of another laminated disc; 
     FIG. 18 is a fragmentary cross-sectional view of a laminated disc with a beveled corner of the center hole and an annular grooved therein; 
     FIG. 19 is a fragmentary cross-sectional view of a laminated disc having a recess closer to the center hole than the groove of FIG. 18; 
     FIG. 20 is a cross-section view of a prior art turntable; and 
     FIG. 21 is a perspective view of a conventional disc on which information is recorded. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of the present invention will be described in detail with reference to the drawings. 
     First Embodiment 
     FIG. 1 is a cross-sectional view of a turntable  10  according to a first embodiment, showing the turntable  10  when a laminated disc  11  is loaded thereon. 
     Referring to FIG. 1, a positioning hub  5  fits into a center hole  6  in the disc  11  with a very small clearance, for example, several microns, accurately positioning the disc with respect to the rotational axis S-S′ of the motor shaft  4 . The disc  11  is supported on the turntable  10 . A motor shaft  4  of a spindle motor, not shown, is press-fitted into or securely bonded to a truncated cone  15  of the turntable  10  in a similar manner to the prior art turntable shown in FIG.  20 . The spindle motor drives the turntable  10  in rotation via the motor shaft  4 . 
     The laminated disc  10  includes two discs placed together, slightly eccentric due to alignment error within a predetermined tolerance. The relative eccentricity of the center holes of the two discs results in a small step  50  in the wall of the center hole  6  of the laminated disc  1 . The positioning hub  5  is integrally continuous with the cone  15  having an inclined surface  15   a.  The inclined surface  15   a  makes an angle θ with the axis S-S′ so that the inclined surface  15   a  will not interfere with the stepped edge of the wall of the center hole  6 . 
     FIG. 2 is a fragmentary cross-sectional view of a disc  11  which has a burr  12  and is loaded on the turntable  10 . FIG. 3 is a fragmentary cross-sectional view of the turntable  10  and the disc  11  when a bonding agent  13  spreads out of an interface  14  between the two discs. 
     The truncated cone  15  will now be described in more detail with reference to FIGS. 4-7. FIG. 4 illustrates the positional relation between the disc  11  and the turntable  10 . 
     The disc  11  is assumed to have been held above the turntable  10  with its center line D-D′ offset a distance d1 from the axis S-S′ of the motor shaft  4 . Then, the disc  11  is lowered in a direction shown by arrow A, thereby being loaded onto the inclined surface  15   a  of the turntable  10 . The surface  15   a  of the truncated cone  15  makes an angle of θ with the shaft  4  as shown in FIG.  1 . In order that the disc  11  is comfortably loaded onto the turntable  10  without deformation, the disc  11  must first be placed on the inclined surface  15   a  of the truncated cone  15 . This condition is achieved by the following relation. 
     
       
           d 2 &gt;d 1  (1) 
       
     
     where d2 is a dimension of the truncated cone parallel to the disc  11 . 
     The dimension d2 may be expressed as follows: 
     
       
           d 2=( m 1 −m 2) tan θ  (2) 
       
     
     where m1 is the distance between the disc supporting surface  16  of the turntable  11  and the top flat surface  60  of the truncated cone  15 , and m2 is the distance between the supporting surface  16  and the upper end of the positioning hub  5 . 
     From Equations (1) and (2), the following relation is derived. 
     
       
         tan θ≧ d 1/( m 1− m 2)  (3) 
       
     
     In other words, in order to ensure that d2&gt;d1, it is necessary that the lower limit of the angle θ must satisfy Equation (3). 
     The distance m1 is selected to be about 2.5 mm since the turntable  11  is usually of thin construction. The distance m2 between the full thickness of the lower disc and the half the full thickness since the disc  11  is placed in position by having the center hole of the lower disc  11   b  engage the positioning hub  5 . A DVD has a thickness of 0.6 mm, and therefore the distance m2 is selected to be 0.3 mm which is a half of the thickness of the lower disc. When manufacturing a DVD, the upper and lower discs are placed together in such a way that the outer diameter of one disc is registered with that of the other. Therefore, the offset d1 of about 0.5 mm usually results from the eccentricity of the outer diameter with respect to the center hole and the dimensional errors of the outer diameters of the upper and lower discs. Consequently, the angle θ greater than 13° is obtained by putting m1=0.5 mm, m2=0.3 mm, and d1=0.5 mm into Equation (3). 
     FIG. 5 is a fragmentary side view of the turntable  10  and the disc  11  loaded to the turntable  10 . The disc  11  has a step  50 . There is a small clearance  51  between the lower disc  11   b  and the positioning hub  5 . In order that the disc  11  smoothly fits to the turntable  10  without the truncated cone  15  interfering with the step  50 , the lower limit of the angle θ must be determined by the following relation. 
     
       
         tan θ≧{( n 1− n 2)/ n 3}  (4) 
       
     
     where n1 is the size of the step  50 , n2 is the size of the clearance  51 , and n3 is the difference between the thickness t and the distance m2. The step  50  or n1 is usually less than 0.1 mm and therefore the distance m2 longer than the half of the thickness t is enough. For DVDs, m2 is 0.3 mm and thickness t is 0.6 mm. The clearance  51  is usually several tens microns and is assumed to be 30 microns in this embodiment. By putting m2=0.3 mm, n1=0.1 mm, n2=30 μm, and t=0.3 mm into Equation (4), the angle θ greater than 13° is obtained. 
     FIG. 6 illustrates the the disc  11  smoothly guided along the inclined surface  15   a  of the truncated cone  15 . The total weight P of the disc  11  is a force acting downwardly. The weight P is resolved into a component P 2  normal to the inclined surface  15   a  and a component P 1  acting downward along the inclined surface  15   a.  In order for the disc  11  to smoothly slide along the inclined surface  15   a  of the truncated cone  15 , the following relation must be satisfied. 
     
       
         P 1 &gt;P 3   (5) 
       
     
     where P 3  is a friction acting in a direction opposite to the component P 1 . 
     The angle θ is determined by putting P 1 =P cos θ, P 2 =P sin θ, and P 3 =μ P 2 =μ P sin θ into Equation (5). 
     
       
         tan θ&lt;1/μ  (6) 
       
     
     where μ is a coefficient of friction between the disc  11  and the inclined surface  15   a  of the truncated cone  15 . 
     Optical discs are usually made of thermoplastic plastics and turntables are made of materials such as thermoplastic plastics or metal. With discs and turntables made of such materials, the coefficient of friction μ is in the range of 0.2-0.5 and is the largest when both the disc and turntable are made of plastics. For μ=0.5, the angle θ is less than 63.5° from Equation (6). 
     Therefore, the angle θ of the truncated cone  15  should be in the range from 13 to 63.5 degrees, and preferably in the range of 15-60 degrees. 
     The aforementioned construction offers the same advantages for discs having the burr  12  or the sag  13  of an adhesive on the wall of the center hole of the disc. 
     In the aforementioned embodiment, the angle θ of the surface of the truncated cone  15  is selected such that the wall defining the center hole  6  of the disc will  11  not touch the inclined surface  15   a  of the truncated cone  15  if the upper and lower discs of the disc  11  are eccentric with each other within a predetermined tolerance. As shown in FIG. 7, the turntable  10  may be formed with a horizontal surface  17  parallel to the disc surface placed on the turntable  10  so that the truncated cone is continuous with the positioning hub  5  via the horizontal surface  17 . The horizontal surface  17  has a radial distance less than about several hundred microns. 
     Second Embodiment 
     FIG. 8 is a top view of the disc  11  and turntable  10  according to a second embodiment and FIG. 9 is a cross-sectional view taken along lines J-O and O-K of FIG.  8 . Referring to FIGS. 8 and 9, a spring  18  has three spring legs  18   a-   18   b  angularly 120° spaced apart and extending outwardly and downwardly. The spring leg has an abutting surface  19  at its free end. The abutting surface  19  is a vertically extending surface. It is to be noted that the abutting surface  19  abuts the circumferential wall of the center hole  6  of the lower disc  11   b  with a predetermined urging force. This construction is advantageous in that the step  50  or the burr  12  such as shown in FIG. 2 will not touch any part of the turntable  10  and the disc  11  can be positioned in place with no clearance between the abutting surface  19  and the wall defining center hole  6 . 
     FIG. 10 is an exploded perspective view of a chucking type turntable  10   a  which is provided with a ball-chucking construction. The spring  18  in FIGS. 8-9 is also used in the construction. The chucking construction includes chucking balls  20 . FIG. 11 is a cross-sectional view when the disc  11  is loaded onto the chucking type turntable  10   a . As is clear from FIGS. 9 and 10, the step  50  in the wall of the center hole  6  of the disc  11  still will not touch the turntable  10   a  of the chucking construction. 
     Third Embodiment 
     FIG. 12 is a cross-sectional view of a laminated disc  11  according to a third embodiment. The upper and lower discs  11   a  and  11   b  are beveled at their edges  21  defining the center hole  6 , and are placed together so that the beveled edges directly face each other. 
     FIG. 13 is a cross-sectional view of the laminated disc  11  when it is loaded to a supporting surface  116  of a conventional turntable  102 . With the aid of the beveled edges, the turntable  102  can fit into the center hole  6  of the laminated disc  11  without the head corner  52  of the turntable  102  caught by the step  50 . 
     The laminated disc  11  is usually made of plastics by molding and has a thickness larger than 0.5 mm. This thickness is a reasonable thickness that can be achieved with a sufficient dimensional accuracy by molding. Accordingly, the upper and lower discs need to be beveled by a width less than 0.5 mm. For example, the corners of the edges are rounded or radiused by less than R=0.5. 
     FIG. 14 is a cross-sectional view of the laminated disc  11  of the third embodiment when it is loaded onto the conventional ball chucking type turntable  102  having a metal chucking ball  20 . The beveled edge facilitates the smooth passage of the disc  11  over the ball  20  along the turntable  102 . The disc  11  can be loaded onto the disc supporting surface  116  of the turntable  102  without being caught by the ball  20 . 
     Fourth Embodiment 
     FIG. 15 is a cross-sectional view of a disc  11  according to a fourth embodiment. The upper and lower discs  11   a  and  11   b  are formed with annular grooves  22   a  and  22   b , respectively, in the bonding surface  14  thereof. The grooves  22   a  and  22   b  have a cross section of a semicircle and are close to and concentric with the center hole  6 . The upper and lower discs  11   a  and  11   b  are placed together so that the grooves  22   a  and  22   b  define a toroidal space that acts as a reservoir  22  for holding a bonding agent therein. 
     FIG. 16 is a top view of the lower disc  11   b . The bonding agent  13  is applied to the bonding surface  14  as depicted by hatched portion except an area inner than the annular groove  22   b.    
     When the upper and lower discs  11   a  and  11   b  are placed together after applying the bonding agent  13 , excessive bonding agent  13  spreads into the reservoir  22 , being prevented from further spreading toward the center hole  6 . 
     The reservoir  22  also serves to prevent the bonding agent  13  from spreading toward the center hole  6  when the laminated disc  11  is placed in an environment of elevated temperatures. 
     FIG. 17 is a cross-sectional view of another laminated disc  11 . The upper and lower discs  11   a  and  11   b  are formed with annular, shallow steps  14   a  and  14   b , respectively, in an area closer to the center hole  6  than the annular groove  22   a  and  22   b.  When the upper and lower discs are placed together, the steps  14   a  and  14   b  define a space that communicates with the reservoir  22  to accommodate the bonding agent spreading out of the reservoir  22  toward the center hole  6 . 
     The laminated disc may have both a beveled corner of the center hole and an annular groove therein as shown in FIG.  18 . The laminated disc shown in FIG. 18 may further be formed with a shallow recess, as shown in FIG. 19, closer to the center hole than the groove. 
     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.