Patent Publication Number: US-9406284-B2

Title: Keyboard device and key unit

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present application claims priority from Japanese Patent Application Nos. 2014-227870 filed on Nov. 10, 2014, and 2015-050746 filed on Mar. 13, 2015, the contents of which are herein incorporated by reference. 
     BACKGROUND 
     1. Technical Field 
     The following disclosure relates to: a keyboard device including a plurality of key units superposed on and secured to one another on a key frame and each including (i) a common mounting portion elongated in a right and left direction and (ii) a plurality of keys having rear ends connected to the common mounting portion; and the key unit. 
     2. Description of the Related Art 
     There has been known a keyboard device including a plurality of key units assembled to a key frame and each formed of resin in one piece. Each of the plurality of key units includes a plurality of keys extending in a front and rear direction and arranged next to each other in a right and left direction; a common mounting portion; a plurality of hinges each shaped like a flat plate and provided between the common mounting portion and rear ends of the respective keys in a state in which upper surfaces of the respective hinges are horizontal. Patent Document 1 (Japanese Patent Application Publication No. 2002-62876) and Patent Document 2 (Japanese Patent Application Publication No. 2005-92233) disclose keyboard devices each including: a first key unit including a plurality of white keys respectively corresponding to notes D, F, and A; a second key unit including a plurality of white keys respectively corresponding to notes C, E, and B; and a third key unit including a plurality of black keys respectively corresponding to notes C#, D#, F#, G#, and A#. The first through third key units are superposed on each other in this order at their respective common mounting portions and placed on and secured to a rear end portion of a key frame by male screws. 
     In each of the first through third key units, each hinge includes: a hinge thick portion which is elastically deformed by a small amount when a corresponding white or black key is depressed and released; and hinge thin portions which are elastically deformed by a large amount, when a corresponding white or black key is depressed and released, so as to allow pivotal movement of a front end portion of the key in an up and down direction. A front end of the hinge thick portion is integrally connected to a rear surface of the corresponding key, and front ends of the respective hinge thin portions are connected to a rear end of the hinge thick portion, and rear ends of the respective hinge thin portions are connected to a front surface of the common mounting portion. In particular, in the third key unit including the black keys and located at the uppermost position among the first through third key units, the width of each hinge thick portion in the right and left direction is greater than that of a rear end of the corresponding black key in the right and left direction. Furthermore, in the third key unit, the pair of hinge thin portions having the same width in the right and left direction are spaced apart from each other in, the right and left direction and extend rearward from rear surfaces of respective opposite end portions of the hinge thick portion in the right and left direction. For each key of the third key unit, a right outer end of the hinge thick portion and a right outer end of a right one of the pair of hinge thin portions are located at the same position in the right and left direction, and a left outer end of the hinge thick portion and a left outer end of a left one of the pair of hinge thin portions are located at the same position in the right and left direction. Furthermore, an extremely small clearance is formed between the hinge thick portions for adjacent two of the black keys. 
     SUMMARY 
     In the above-described keyboard device, the extremely small clearance is merely formed between the adjacent hinge thick portions of the black keys in a state in which an upper cover covers a space formed behind positions located slightly at the rear of the respective rear ends of the white keys and the black keys. Thus, the hinge thick portions of the first and second key units including the white keys are not inconspicuous when viewed from above, resulting in good external appearance. 
     However, the pair of hinge thin portions are respectively located at opposite outer end portions of the hinge thick portion, and the plurality of black keys are arranged at different distances in the right and left direction. Thus, positions of the pair of hinge thin portions with respect to the corresponding black key in the right and left direction vary among the black keys, and the pair of hinge thin portions are not symmetric with respect to a center line of the corresponding black key in the right and left direction. As a result, variations may be caused, in stiffness (elasticity) of the hinge thin portions and the hinge thick portion in the right and left direction, between the black keys and between portions of each of the black keys in the right and left direction. In particular, this phenomenon occurs remarkably in the case where the length of the hinge thick portion is small in the front and rear direction and/or in the case where the lengths of the hinge thick portions in the right and left direction greatly differ from each other. As a result, key touch feelings during depression and release of the key may vary among the plurality of black keys, unfortunately. Also, an imbalance may be caused in each black key in the right and left direction, leading to deterioration of key touch feelings. Furthermore, this imbalance may cause twists in the hinge thin portions and the hinge thick portion, leading to deterioration of durability of the hinge. 
     Accordingly, an aspect of the disclosure relates to a keyboard device and a key unit with good external appearance and improved key touch feelings. 
     In one aspect of the disclosure, a keyboard device includes: a plurality of key units each including a plurality of keys, a plurality of hinges, a common mounting portion which are molded in one piece, a plurality of the common mounting portions of the plurality of key units being superposed on each other and secured to a key frame. The plurality of keys are arranged in a right and left direction and each extends in a front and rear direction. The common mounting portion is elongated in the right and left direction. Each of the plurality of hinges is provided between the common mounting portion and a rear end of a corresponding one of the plurality of keys and has a planar plate shape in which a plate surface of each of the plurality of hinges is substantially horizontal. The plurality of key units include a particular key unit including an uppermost common mounting portion of the plurality of the common mounting portions. Each of at least one of the plurality of hinges of the particular key unit includes: a hinge thick portion connected to a rear surface of a corresponding one of the plurality of keys; and a hinge thin portion connected to a rear surface of the hinge thick portion and connected to a front surface of the uppermost common mounting portion, the hinge thin portion being less in thickness than the hinge thick portion in an up and down direction. A protrusion protruding outward is provided on at least one of opposite outer ends of the hinge thick portion in the right and left direction. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The objects, features, advantages, and technical and industrial significance of the present disclosure will be better understood by reading the following detailed description of the embodiment, when considered in connection with the accompanying drawings, in which: 
         FIG. 1  is a perspective view generally illustrating a keyboard device including keys corresponding to one octave, according to one embodiment; 
         FIG. 2  is a plan view of the keyboard device in  FIG. 1 ; 
         FIG. 3A  is a plan view of the keyboard device, with an upper cover removed from the keyboard device in  FIG. 1 , and  FIG. 3B  is a side view of the keyboard device in  FIG. 3A ; 
         FIG. 4A  is a plan view of a first key unit of the keyboard device in  FIGS. 1 through 3B , and  FIG. 4B  is a side view of the first key unit; 
         FIG. 5A  is a plan view of a second key unit of the keyboard device in  FIGS. 1 through 3B , and  FIG. 5B  is a side view of the second key unit; 
         FIG. 6A  is a plan view of a third key unit of the keyboard device in  FIGS. 1 through 3B , and  FIG. 6B  is a side view of the third key unit; and 
         FIG. 7A  is a partially enlarged plan view of  FIG. 3A , and  FIG. 7B  is an enlarged cross-sectional view taken along line b-b in  FIG. 7A . 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENT 
     Hereinafter, there will be described one embodiment by reference to the drawings.  FIG. 1  is a perspective view generally illustrating a keyboard device including keys corresponding to one octave, according to one embodiment.  FIG. 2  is a plan view of the keyboard device.  FIG. 3A  is a plan view of the keyboard device, with an upper cover removed from the keyboard device, and  FIG. 3B  is a side view of the keyboard device in  FIG. 3A . In the following description, a player side (the lower left side in  FIG. 1  and the lower side in  FIG. 2 ) is defined as a front side of the keyboard device, and an opposite side (the upper right side in  FIG. 1  and the upper side in  FIG. 2 ) of the keyboard device from the player side is defined as a rear side of the keyboard device. The right and left direction with respect to the player (the direction directed from the upper left side toward the lower right side in  FIG. 1 , and the right and left direction in  FIG. 2 ) is defined as the right and left direction with respect to the keyboard device. It is noted that the following explanation will be provided for construction corresponding one octave for simplicity. 
     The keyboard device includes a keyboard and a key frame  10  disposed under the keyboard and fixed in a keyboard instrument. The key frame  10  is formed of a material such as metal or resin in one piece so as to have a planar plate shape extending horizontally in the right and left direction and the front and rear direction. A plurality of supports  11  are provided on an intermediate portion of the key frame  10  in the front and rear direction, and a plurality of supports  12  are provided on a rear end portion of the key frame  10  in the front and rear direction. In the present embodiment, the three supports  11  are provided respectively at opposite end portions and an intermediate portion in the right and left direction, and likewise the three supports  12  are provided respectively at opposite end portions and an intermediate portion in the right and left direction. Each of the supports  11 ,  12  has a circular cylindrical shape and is formed integrally with the key frame  10 . Each of the supports  11 ,  12  protrudes upward from an upper surface of the key frame  10 . An electric circuit board  20  is placed on and secured to upper surfaces of the supports  11 ,  12  respectively by male screws, not shown. In addition to the electric circuit board  20 , first through third key units  30 ,  40 ,  50  which will be described below in detail are also placed on and secured to the upper surfaces of the respective supports  12  respectively by male screws, not shown. Each of the supports  11 ,  12  has a screw hole which extends downward from its upper surface. Front and rear end portions of the electric circuit board  20  have through holes, not shown, into which the respective male screws are inserted. These through holes are formed respectively at positions corresponding to the supports  11 ,  12  in the right and left direction. 
     The electric circuit board  20  is formed of a material such as resin in one piece so as to have a planar plate shape. Key switches (e.g., rubber dome switches), not shown, electric circuit components and other similar components are disposed on an upper surface of the electric circuit board  20 . It is noted that the electric circuit components may be disposed on a lower surface of the electric circuit board  20 . In order to reduce a degree of contact or collision between white keys  31 ,  41  and black keys  51 , a cushioning material  21 , such as felt, elongated in the right and left direction is affixed to a lower surface of a front end portion of the electric circuit board  20  which is located in front of the support  11 . The electric circuit board  20  and the cushioning material  21  limit upward movement of a front end portion of each of the white keys  31 ,  41  and the black keys  51  when each of the white keys  31 ,  41  and the black keys  51  is released. 
     The key frame  10  is provided with a lowerlimit stopper  13  at a position located slightly in front of the supports  11 . This lowerlimit stopper  13  limits downward movement of the front end portion of each of the white keys  31 ,  41  and the black keys  51  when each of the white keys  31 ,  41  and the black keys  51  is depressed. The lowerlimit stopper  13  extends in the right and left direction so as to be formed integrally with the key frame  10 . When viewed in the right and left direction, this lowerlimit stopper  13  protrudes upward so as to have a three-sided rectangular shape opening downward, and a front end portion of the lowerlimit stopper  13  is slightly lower in height than a rear end portion of the lowerlimit stopper  13 . In order to reduce a degree of contact or collision between the white keys  31 ,  41  and the black keys  51 , a cushioning material  14 , such as felt, elongated in the right and left direction is affixed to an upper surface of the lowerlimit stopper  13 . An upper cover  15  extending in the right and left direction is provided over the rear end portion of the key frame  10 . For good external appearance, the upper cover  15  covers rear end portions of the respective first through third key units  30 ,  40 ,  50  (noted that the rear end portions are located slightly at the rear of respective rear ends of the white keys  31 ,  41  and the black keys  51 . 
     There will be next explained the first key unit  30  with reference to  FIGS. 4A and 4B .  FIG. 4A  is a plan view of the first key unit  30 , and  FIG. 4B  is a side view thereof. The first key unit  30  includes the four white keys  31  not next (adjacent) to each other and respectively corresponding to notes C, E, G, and B. Each of the white keys  31  is constituted by an upper plate, a front wall, right and left side walls, and a rear wall, which define a space opening downward and having a rectangular shape in cross section. The upper plate and the right and left side walls have one or two inside corner portions (step portions) each for arrangement of a corresponding one of black keys  51 . 
     Each of the white keys  31  is provided with an engaging piece  31   a  which is formed integrally with a lower surface of the white key  31  at its intermediate position in the front and rear direction. The engaging piece  31   a  extends downward and has a protrusion protruding rearward at a lower end portion of the engaging piece  31   a . When the white key  31  is depressed, a lower surface of the engaging piece  31   a  is brought into contact with the cushioning material  14  to limit downward movement of a front end of the white key  31 . When the white key is released, the protrusion of the engaging piece  31   a  is brought into contact with the cushioning material  21  at an upper surface of the protrusion to limit upward movement of the front end of the white key  31 . The lower surface of the white key  31  has a pressing portion  31   b  which is located at an intermediate position on the lower surface of the white key  31  in the front and rear direction and at a position opposed to a corresponding one of the key switches provided on the electric circuit board  20 . The pressing portion  31   b  presses a corresponding one of the key switches when the white key  31  is depressed. 
     The rear walls of the respective four white keys  31  are connected to a common mounting portion (a rail portion)  33  via respective hinges  32 . The hinges  32  and the common mounting portion  33  are molded integrally with the white keys  21 . Each of the hinges  32  is constituted by a hinge thick portion  32   a  and a pair of hinge thin portions  32   b ,  32   c . The hinge thick portion  32   a  is connected at its front end to a lower end portion of a rear surface of the rear wall of a corresponding one of the white keys  31 . The hinge thick portion  32   a  has a relatively large thickness in the up and down direction and is shaped like a planar plate having a substantially rectangular shape elongated in the right and left direction. With this construction, an amount of elastic deformation of the hinge thick portion  32   a  is small in the up and down direction. The center position of the hinge thick portion  32   a  in the right and left direction is aligned with that of the front end portion of the white key  31  in the right and left direction. The width of the hinge thick portion  32   a  in the tight and left direction is generally equal to that of the front end portion of the white key  31  in the right and left direction (strictly, the width of the hinge thick portion  32   a  in the right and left direction is extremely slightly greater than that of the front end portion of the white key  31  in the right and left direction). Opposite ends of the hinge thick portion  32   a  in the right and left direction are substantially aligned in position respectively with opposite ends of the front end portion of the white key  31  in the right and left direction. Each of hinge thick portions  32   a  has a gate corresponding portion  32   a   1  in its upper surface at a substantially central position of a rear end portion of the corresponding white key  31  in the right and left direction. The gate corresponding portion  32   a   1  is a gate from which resin is injected when the first key unit  30  is molded in one piece. 
     Each of the hinge thin portions  32   b ,  32   c  is connected at its front end to a lower end portion of a rear surface of a corresponding one of opposite outer end portions of the hinge thick portion  32   a  in the right and left direction. Each of the hinge thin portions  32   b ,  32   c  is thinner than the hinge thick portion  32   a  in the up and down direction and is shaped like a planar plate having a rectangular shape in which its length in the right and left direction is slightly greater than that in the front and rear direction. With this construction, an amount of elastic deformation of each of the hinge thin portions  32   b ,  32   c  is large in the up and down direction. A left end of the hinge thin portion  32   b  is substantially aligned in position with a left end of the front end portion of the white key  31  (and a left end of the hinge thick portion  32   a ). A right end of the hinge thin portion  32   b  is located to the left of the center of the front end portion of the white key  31  in the right and left direction by a corresponding predetermined distance. A right end of the hinge thin portion  32   c  is substantially aligned in position with a right end of the front end portion of the white key  31  (and a right end of the hinge thick portion  32   a ). A left end of the hinge thin portion  32   c  is located to the right of the center of the front end portion of the white key  31  in the right and left direction by a corresponding predetermined distance. That is, the hinge thin portions  32   b ,  32   c  are spaced apart from each other by a predetermined distance so as to be symmetric with respect to the center line of the front end portion of the white key  31  in the right and left direction. In terms of a positional relationship between the hinge thick portion  32   a  and each of the hinge thin portions  32   b ,  32   c , the hinge thin portions  32   b ,  32   c  are preferably provided at such positions that lateral stiffness of the hinge thin portions  32   b ,  32   c  and the hinge thick portion  32   a  is appropriate (for example, the stiffness is the greatest), in accordance with the length of the hinge thick portion  32   a  in the front and rear direction. 
     The common mounting portion  33  is elongated in the right and left direction and has a substantially rectangular shape in vertical cross section in the front and rear direction. Rear ends of the respective hinge thin portions  32   b ,  32   c  are connected to an upper portion of a front surface of this common mounting portion  33 . The common mounting portion  33  includes: a recessed portion  33   a  formed in a lower surface of the common mounting portion  33  so as to extend in the right and left direction; and a protruding portion.  33   b  formed on an upper surface of the common mounting portion  33  so as to extend in the right and left direction. Protrusions  33   c  extending downward are provided on the lower surface of the common mounting portion  33  at positions spaced apart from each other in the right and left direction. In the present embodiment, the protrusions  33   c  are respectively provided on opposite end portions and an intermediate portion of the lower surface of the common mounting portion  33  in the right and left direction. Each of the protrusions  33   c  is shaped like a circular cylinder whose lower end is chamfered. These protrusions  33   c  are pressed into holes, not shown, of the electric circuit board  20  when the common mounting portion  33  is assembled to the rear end portion of the electric circuit board  20 . A through hole  33   d  is formed in the common mounting portion  33 . In the present embodiment, the through hole  33   d  is formed at an intermediate portion of the common mounting portion  33  in the right and left direction. The through hole  33   d  has a round shape. A protrusion  43   c , which will be described below, provided on the second key unit  40  is pressed into the through hole  33   d  when the second key unit  40  is assembled onto the first key unit  30 . The common mounting portion  33  has through holes  33   e  spaced apart from each other in the right and left direction so as to correspond to the positions of the supports  12  of the key frame  10 . Each of the through holes  33   e  has a round shape slightly larger in size than the through hole  33   d . Male screws, not shown, are inserted through the respective through holes  33   e  when the first through third key units  30 ,  40 ,  50  are assembled to the key frame  10 . Protruding pieces  33   f  extending downward are provided on the lower surface of the common mounting portion  33  at positions spaced apart from each other in the right and left direction. In the present embodiment, the two protruding pieces  33   f  are provided on an intermediate portion of the lower surface of the common mounting portion  33  in the right and left direction. When the common mounting portion  33  is assembled to the rear end portion of the electric circuit board  20 , each of the protruding pieces  33   f  is inserted into a corresponding one of through holes, not shown, of the electric circuit board  20  with a clearance so as to function as a guide. 
     There will be next explained the second key unit  40  with reference to  FIGS. 5A and 5B .  FIG. 5A  is a plan view of the second key unit  40 , and  FIG. 5B  is a side view thereof. The second key unit  40  includes the three white keys  41  not next (adjacent) to each other and respectively corresponding to notes D, F, and A. Each of the white keys  41  is constituted by an upper plate, a front wall, right and left side walls, and a rear wall, which define a space opening downward and having a rectangular shape in cross section. The length of each white key  41  in the front and rear direction and the height of each white key  41  are equal to those of each white key  31 . The white key  41  differs from the white key  31  in inside corner portion or portions, provided on, the upper plate and the right and left side walls, each for arrangement of a corresponding one of black keys  51 . An engaging piece  41   a  and a pressing portion  41   b  are provided on an intermediate position of a lower surface of the white key  41  in the front and rear direction. The engaging piece  41   a  and the pressing portion  41   b  are similar in construction to the engaging piece  31   a  and the pressing portion  31   b  of the white key  31 , respectively. 
     The rear walls of the respective three white keys  41  are connected to a common mounting portion  43  via respective hinges  42 . The hinges  42  and the common mounting portion  43  are molded integrally with the white keys  41 . Each of the hinges  42  is constituted by a hinge thick portion  42   a  and a pair of hinge thin portions  42   b ,  42   c  which are similar in construction to the hinge thick portion  32   a  and the pair of hinge thin portions  32   b ,  32   c  of the first key unit  30 , respectively. Each of the hinge thick portions  42   a  has a gate corresponding portion  42   a   1  in its upper surface at a substantially center position of a corresponding one of the white keys  41  in the right and left direction. This gate corresponding portion  42   a   1  is similar in construction to the gate corresponding portion  32   a   1  of the first key unit  30 . The common mounting portion  43  is also substantially similar in construction to the common mounting portion  33  of the first key unit  30 . The common mounting portion  43  includes a recessed portion  43   a , a protruding portion  43   b , the protrusion  43   c , a through hole  43   d , and through holes  43   e  which are similar to the recessed portion  33   a , the protruding portion  33   b , the protrusion  33   c , the through hole  33   d , and the through holes  33   e , respectively. 
     In this second key unit  40 , however, each of the hinge thick portions  42   a  is connected at its front end to a lower end portion of a rear surface of the rear wall of a corresponding one of the white keys  41 , but each of the hinge thin portions  42   b ,  42   c  is connected at its front end to an upper end portion of a rear surface of a corresponding one of the hinge thick portions  42   a  and is connected at its rear end to a lower portion of a front surface of the common mounting portion  43 . In a state in which the first and second key units  30 ,  40  are assembled to the key frame  10  and the electric circuit board  20 , the hinge thick portion  42   a  and the hinge thick portion  32   a  of the first key unit  30  are located at the same height, and the common mounting portion  43  and the hinge thin portions  42   b ,  42   c  are located above the common mounting portion  33  and the hinge thin portions  32   b ,  32   c  of the first key unit  30 . Each of the hinge thick portions  42   a  is spaced apart from an adjacent one of the hinge thick portions  32   a  of the first key unit  30  with a slight clearance therebetween in, the right and left direction (strictly, with a clearance which is slightly smaller than a clearance between adjacent two of the white keys  31 ,  41  in the right and left direction). 
     The length of the recessed portion  43   a  in the front and rear direction is substantially equal to that of the protruding portion  33   b  of the first key unit  30  in the front and rear direction. The protruding portion  33   b  of the first key unit  30  is fitted into the recessed portion  43   a  in a state in which the common mounting portion  43  of the second key unit  40  is assembled onto the common mounting portion  33  of the first key unit  30 . The protrusion  43   c  is provided at a position opposed to the through hole  33   d  of the first key unit  30  in the right and left direction. The outside diameter of the protrusion  43   c  is substantially equal to the inside diameter of the through hole  33   d . The protrusion  43   c  is pressed into the through hole  33   d  in the state in which the common mounting portion  43  of the second key unit  40  is assembled onto the common mounting portion  33  of the first key unit  30 . The through hole  43   d  is formed in the common mounting portion  43 . In the present embodiment, the through hole  43   d  is formed at an intermediate portion of the common mounting portion  43  in the right and left direction. The through hole  43   d  has a round shape. A protrusion  53   c , which will be described below, provided on the third key unit  50  is pressed into the through hole  43   d  when the third key unit  50  is assembled onto the second key unit  40 . The through holes  43   e  are formed in the common mounting portion  43  at positions corresponding to the respective supports  12  of the key frame  10  and the respective through holes  33   e  of the first key unit  30  in the right and left direction. When the first through third key units  30 ,  40 ,  50  are assembled to the key frame  10 , the male screws, not shown, are inserted through the respective through holes  43   e.    
     There will be next explained the third key unit  50  with reference to  FIGS. 6A and 6B .  FIG. 6A  is a plan view of the third key unit  50 , and  FIG. 6B  is a side view thereof. The third key unit  50  includes the five black keys  51  not next (adjacent) to each other and respectively corresponding to notes C#, D#, F#, G#, and A#. Each of the black keys  51  is constituted by an upper plate, a front wall, right and left side walls, and a rear wall, which define a space opening downward and having a rectangular shape in cross section. The length of each black key  51  in the front and rear direction is less than that of each white key  31 ,  41  in the front and rear direction, and the height of each black key  51  is greater than that of each of the white keys  31 ,  41 . An upper portion of the front wall of the black key  51  is inclined so as to be lower at its front end than at its rear end. An engaging piece  51   a  is provided on a front end of the lower surface of the black key  51 , and this engaging piece  51   a  is similar to the engaging pieces  31   a ,  41   a  of the respective white keys  31 ,  41 . A pressing portion  51   b  is provided on an intermediate position of the lower surface of the black key  51 , and this pressing portion  51   h  is similar to the pressing portions  31   b ,  41   b  of the respective white keys  31 ,  41 . The engaging piece  51   a  has a gate corresponding portion  51   a   1  at a lower portion (which does not form external appearance) of a front surface of the engaging piece  51   a . This gate corresponding portion  51   a   1  is similar to the gate corresponding portions  32   a   1 ,  42   a   1  of the respective first and second key units  30 ,  40 . 
     Each black key  51  is provided with a protruding piece  51   c  which extends downward from a lower surface of a rear end portion of the black key  51 . This protruding piece  51   c  has a rectangular planar plate shape. The length of protruding piece  51   c  in the right and left direction is substantially equal to the width of the black key  51  in the right and left direction. In the state in which the first through third key units  30 ,  40 ,  50  are assembled to the key frame  10  and the electric circuit board  20 , each of the protruding pieces  51   c  is positioned in a space defined in front of a corresponding pair of the hinge thick portions  32   a ,  42   a  of the respective first and second key units  30 ,  40  between corresponding adjacent two of the white keys  31 ,  41  other than the white keys  31 ,  41  respectively corresponding to E and F. 
     The rear walls of the respective five black keys  51  are connected to a common mounting portion  53  via respective hinges  52 . The hinges  52  and the common mounting portion  53  are molded integrally with the black keys  51 . Each of the hinges  52  is constituted by a hinge thick portion  52   a  and a pair of hinge thin portions  52   b ,  52   c  which are similar in construction to the hinge thick portions  32   a ,  42   a  and the pair of hinge thin portions  32   b ,  32   c ,  42   b ,  42   c  of the first and second key units  30 ,  40 . The common mounting portion  53  is also substantially similar in construction to the common mounting portions  33 ,  43  of the respective first and second key units  30 ,  40 . The common mounting portion  53  includes a recessed portion  53   a , a protruding portion  53   b , the protrusion  53   c , and a through holes  53   e  which are similar to the recessed portions  33   a ,  43   a , the protruding portions  33   b ,  43   b , the protrusions  33   c ,  43   c , and the through holes  33   e ,  43   e , respectively. 
     In this third key unit  50 , however, each of the hinge thick portions  52   a  is connected at its front end to a lower end portion of a rear surface of the rear wall of a corresponding one of the black keys  51 , but each of the hinge thin portions  52   b ,  52   c  is connected at its front end to a lower end portion of a rear surface of a corresponding one of the hinge thick portions  52   a  and is connected at its rear end to a lower portion of a front surface of the common mounting portion  53 . In the state in which the first through third key units  30 ,  40 ,  50  are assembled to the key frame  10  and the electric circuit board  20 , the common mounting portion  53 , the hinge thick portion  52   a , and the hinge thin portions  52   b ,  52   c  of the third key unit  50  are respectively located above the common mounting portion  43 , the hinge thick portion  42   a , and the hinge thin portions  42   b ,  42   c  of the second key unit  40 . 
     The shape of the hinge thick portion  52   a  is substantially the same as that of each of the hinge thick portions  32   a ,  42   a  of the respective first and second key units  30 ,  40 . The thickness of the hinge thick portion  52   a  in the up and down direction is substantially equal to that of each of the hinge thick portions  32   a ,  42   a  of the respective first and second key units  30 ,  40  in the up and down direction. However, the length of the hinge thick portion  52   a  in the front and rear direction is extremely slightly less than that of each of the hinge thick portions  32   a ,  42   a  in the front and rear direction, and the length of the hinge thick portion  52   a  in the right and left direction is slightly greater than the length of each of the hinge thick portions  32   a ,  42   a  in the right and left direction. The center position of the hinge thick portion  52   a  in the right and left direction is the same as the center position of the black key  51  in the right and left direction. The shape of the hinge thin portions  52   b ,  52   c  is substantially the same as that of the hinge thin portions  32   b ,  32   c ,  42   b ,  42   c  of the first and second key units  30 ,  40 . The thickness of each of the hinge thin portions  52   b ,  52   c  in the up and down direction and the length thereof in the front and rear direction are substantially equal to the thickness of each of the hinge thin portions  32   b ,  32   c ,  42   b ,  42   c  of the first and second key units  30 ,  40  in the up and down direction and the length thereof in the front and rear direction, respectively. However, the length of each of the hinge thin portions  52   b ,  52   c  in the right and left direction is slightly less than the length of each of the hinge thin portions  32   b ,  32   c ,  42   b ,  42   c  in the right and left direction. The hinge thin portions  52   b ,  52   c  are also arranged so as to be symmetric with respect to the center line of the black key  51  in the right and left direction. However, a distance between the hinge thin portions  52   b ,  52   c  is greater than each of a distance between the hinge thin portions  32   b ,  32   c  and a distance between the hinge thin portions  42   b ,  42   c.    
     Protrusions (tabs)  52   d ,  52   e  are respectively provided on right and left ends of the hinge thick portion  52   a  so as to protrude in the right and left direction. Each of the protrusions  52   d ,  52   e  has a rectangular shape with a chamfered outer end. The thickness of each of the protrusions  52   d ,  52   e  in the up and down direction is equal to the thickness of the hinge thick portion  52   a  in the up and down direction, and the length of each of the protrusions  52   d ,  52   e  in the length of the front and rear direction is less than the length of the hinge thick portion  52   a  in the front and rear direction. These protrusions  52   d ,  52   e  are provided on a front portion of the hinge thick portion  52   a  such that front surfaces of the respective protrusions  52   d ,  52   e  are flush and integral with a front surface of the hinge thick portion  52   a . Thus, rear surfaces of the respective protrusions  52   d ,  52   e  are located in front of a rear surface of the hinge thick portion  52   a . The protrusions  52   d ,  52   e  of the plurality of black keys  51  have different lengths in the right and left direction. A slight clearance is formed between the protrusion  52   d  and the protrusion  52   e  of adjacent two of the black keys  51 . Specifically, for each of the black keys  51  respectively corresponding to C# and F#, an amount of protrusion of the protrusion  52   d  in the left direction is large, and an amount of protrusion of the protrusion  52   e  in the right direction is small. For each of the black keys  51  respectively corresponding to D# and A#, an amount of protrusion of the protrusion  52   d  in the left direction is small, and an amount of protrusion of the protrusion  52   e  in the right direction is large. For the black key  51  corresponding to G#, an amount of protrusion of each of the protrusions  52   d ,  52   e  in the right and left direction is small. 
     These protrusions  52   d ,  52   e  will be further explained with reference to  FIGS. 7A and 7B .  FIG. 7A  is an enlarged plan view of an area X indicated by the broken line in  FIG. 3A , and  FIG. 7B  is an enlarged cross-sectional view taken along line b-b in  FIG. 7A . It is noted that  FIG. 7B  illustrates a state in which a large gate mark is formed at the gate corresponding portion  32   a   1 . Each of the rear surfaces  52   d   1 ,  52   e   1  of the respective protrusions  52   d ,  52   e  is inclined so as to be lower at its front portion than at its rear portion (noted that the rear surface  52   d   1  is not illustrated). That is, a lower end of each of the rear surfaces  52   d   1 ,  52   e   1  is located hi front of its upper end. This is for increasing the area of upper surfaces of the respective protrusions  52   d ,  52   e  to make it difficult for the player to view the hinge thick portions  32   a ,  42   a  of the first and second key units  30 ,  40  from an upper side thereof and for increasing the area of upper surfaces of rear portions of the respective hinge thick portions  32   a ,  42   a  which are not opposed to lower surfaces of the respective protrusions  52   d ,  52   e.    
     The protrusions  52   d ,  52   e  are constructed as described above. Thus, in the state in which the first through third key units  30 ,  40 ,  50  are assembled to the key frame  10  and the electric circuit board  20 , a space is formed, between a pair of the hinge thick portions  52   a ,  52   a  of each adjacent two of the black keys  51 , over a portion of an upper surface of a rear portion of a corresponding one of the hinge thick portions  32   a ,  42   a  of the respective first and second key units  30 ,  40 . Each of the gate corresponding portions  32   a   1 ,  42   a   1  is formed in the upper surface of a corresponding one of the hinge thick portions  32   a ,  42   a  which corresponds to the space. This construction is for avoiding contact between gate marks (e.g., protrusions) remaining on the gate corresponding portions  32   a   1 ,  42   a   1  and the lower surfaces of the respective protrusions  52   d ,  52   e  when each of the first and second key units  30 ,  40  is molded in one piece by injecting resin into a corresponding one of the gate corresponding portions  32   a   1 ,  42   a   1 . When each of the black keys  51  is depressed and released, in particular, the hinge thick portion  52   a  corresponding to the black key  51  is pivoted as indicated by the broken-line arrows in  FIG. 7B . If the gate mark is large, a rear end of the lower surface of each of the protrusions  52   d ,  52   e  may be brought into contact with the gate mark. As described above, however, since each of the rear surfaces  52   d   1 ,  52   e   1  of the respective protrusions  52   d ,  52   e  are inclined so as to be lower at its front portion than at its rear portion, it is possible to avoid a contact between the gate mark and the rear end of the lower surface of each of the protrusions  52   d ,  52   e.    
     The length of the recessed portion  53   a  in the front and rear direction is substantially equal to that of the protruding portion  43   b  of the second key unit  40  in the front and rear direction. The protruding portion  43   b  of the second key unit  40  is fitted into the recessed portion  53   a  in a state in which the common mounting portion  53  of the third key unit  50  is assembled onto the common mounting portion  43  of the second key unit  40 . The protrusion  53   c  is provided at a position opposed to the through hole  43   d  of the second key unit  40  in the right and left direction. The outside diameter of the protrusion  53   c  is substantially equal to the inside diameter of the through hole  43   d . The protrusion  53   c  is pressed into the through hole  43   d  in the state in which the common mounting portion  53  of the third key unit  50  is assembled onto the common mounting portion  43  of the second key unit  40 . The through holes  53   e  are formed in the common mounting portion  53  at positions corresponding to the respective supports  12  of the key frame  10 , the respective through holes  33   e  of the first key unit  30 , and the respective through holes  43   e  of the second key unit  40  in the right and left direction. When the first through third key units  30 ,  40 ,  50  are assembled to the key frame  10 , the male screws, not shown, are inserted through the respective through holes  53   e.    
     Hooks  53   f  extend downward from a lower surface of a rear end portion of the common mounting portion  53 . The hooks  53   f  are spaced apart from each other in the right and left direction. In the present embodiment, the two hooks  53   f  are provided on an intermediate portion of the common mounting portion  53  in the right and left direction. A lower end portion of each of the hooks  53   f  has an engaging portion, in the form of an overhang, extending frontward and having a triangle shape in elevational cross section with its upper side made horizontal. When the first through third key units  30 ,  40 ,  50  are assembled to the electric circuit board  20 , the engaging portions of the respective hooks  53   f  are engaged with a rear end portion of the lower surface of the electric circuit board  20 . However, the engaging portions of the respective hooks  53   f  are not engaged or held in contact with the rear end portion of the lower surface of the electric circuit board  20  in a state in which the first through third key units  30 ,  40 ,  50  and the electric circuit board  20  are secured to the key frame  10  by screws, not shown. The common mounting portion  53  has rectangular through holes  53   g  extending therethrough in the up and down direction. These through holes  53   g  are formed at positions corresponding to the respective hooks  53   f  so as to be opposed to the upper surfaces of the respective engaging portions. These through holes  53   g  are formed in order to easily mold the engaging portions of the respective hooks  53   f  using a pair of upper and lower molds when the third key unit  50  is molded by resin in one piece. 
     There will be next explained assembly of the electric circuit board  20  and the first through third key units  30 ,  40 ,  50  to the key frame  10 . The common mounting portion  43  of the second key unit  40  is placed onto the common mounting portion  33  of the first key unit  30  to assemble the second key unit  40  onto the first key unit  30 . In this assembly, the protruding portion  33   b  of the common mounting portion  33  is fitted into the recessed portion  43   a  of the common mounting portion  43 , and the protrusion  43   c  of the common mounting portion  43  is pressed into the through hole  33   d  of the common mounting portion  33 . The common mounting portion  53  of the third key unit  50  is then placed onto the common mounting portion  43  of the second key unit  40  assembled onto the first key unit  30 , to assemble the third key unit  50  onto the second key unit  40 . In this assembly, the protruding portion  43   b  of the common mounting portion  43  is fitted into the recessed portion  53   a  of the common mounting portion  53 , and the protrusion  53   c  of the common mounting portion  53  is pressed into the through hole  43   d  of the common mounting portion  43 . As a result, a keyboard unit constituted by the first through third key units  30 ,  40 ,  50  is completed. 
     The electric circuit board  20  is then placed onto the supports  11 ,  12  such that through holes formed in a front portion of the electric circuit board  20  are aligned with the respective supports  11  of the key frame  10 . The male screws, not shown, are then inserted into and engaged with the respective screw holes of the supports  11  from the upper surface of the electric circuit board  20 , to assemble the electric circuit board  20  to the key frame  10 . It is noted that the cushioning materials  14 ,  21  are in advance fixed respectively to the lowerlimit stopper  13  of the key frame  10  and the lower surface of the front end portion of the electric circuit board  20 . 
     The keyboard unit constituted by the first through third key units  30 ,  40 ,  50  is then assembled to the electric circuit board  20 . In this assembly, the protruding pieces  33   f  of the first key unit  30  are inserted through the respective through holes, not shown, formed in the electric circuit board  20  in a state in which the engaging pieces  31   a ,  41   a ,  51   a  of the white keys  31 ,  41  and the black keys  51  are engaged with the lower surface of the front end portion of the electric circuit board  20 . Furthermore, the common mounting portion  33  of the first key unit  30  is placed onto the rear end portion of the electric circuit board  20  to engage the engaging portions of the respective hooks  53   f  of the third key unit  50  with the lower surface of the rear end portion of the electric circuit board  20 . In this state, the through holes  33   e ,  43   e ,  53   e  formed in the common mounting portions  33 ,  43 ,  53  of the first through third key units  30 ,  40 ,  50  and the through holes, not shown, formed in the electric circuit board  20  are substantially opposed to the respective screw holes formed in the supports  12  of the key frame  10 . The male screws, not shown, are inserted through the through holes  33   e ,  43   e ,  53   e  and the through holes of the electric circuit board  20  from an upper side thereof and further inserted into and engaged with the respective screw holes formed in the supports  12 . It is noted that, as illustrated in  FIG. 3 , the plurality of hinges  32 ,  42 ,  52  of the first through third key units  30 ,  40 ,  50  are arranged such that respective plate surfaces of the hinge thick portions and the hinge thin portions of the plurality of hinges  32 ,  42 ,  52  are substantially horizontal in the state in which the keyboard unit constituted by the first through third key units  30 ,  40 ,  50  is assembled to the key frame  10 . That is, the respective plate surfaces of the plurality of hinges  32 ,  42 ,  52  may be horizontal and may be inclined with respect to a horizontal plane. 
     The upper cover  15  is then connected to joints, not shown, of the key frame  10  to cover a rear portion of the third key unit  50 . As a result, the keyboard unit constituted by the first through third key units  30 ,  40 ,  50  is assembled onto the key frame  10  and the electric circuit board  20 , and the keyboard device is completed. In this state, a front surface of the upper cover  15  is located over the front portions of the respective hinge thick portions  52   a  of the third key unit  50  and the protrusions  52   d ,  52   e , and a slight clearance is formed between the front surface of the upper cover  15  and the rear end surfaces of the white keys  31 ,  41  and the black keys  51 . 
     In the keyboard device completed as described above, when depressed by the player, each of the white keys  31 ,  41  and the black keys  51  is swung about the common mounting portions  33 ,  43 ,  53  by elastic deformation of the hinge thin portions  32   b ,  32   c ,  42   b ,  42   c ,  52   b ,  52   c , such that the front end portion of each key is moved downward. When the lower surface of each of the respective engaging pieces  31   a ,  41   a ,  51   a  of the white keys  31 ,  41  and the black keys  51  is brought into contact with the upper surface of the cushioning material  14 , the swing of a corresponding one of the white keys  31 ,  41  and the black keys  51  is stopped. When the depressed one of the white keys  31 ,  41  and the black keys  51  is released, the front end portion of the key is moved upward by resilience forces of the corresponding hinge thin portions  32   b ,  32   c ,  42   b ,  42   c ,  52   b ,  52   c  and a corresponding one of the key switches (e.g., rubber dome switches), not shown. When the upper surface of the protrusion of each of the engaging pieces  31   a ,  41   a ,  51   a  of the white keys  31 ,  41  and the black keys  51  is brought into contact with a lower surface of the cushioning material  21 , a corresponding one of the white keys  31 ,  41  and the black keys  51  is returned to its original position, that is, a released key state is established again. 
     When each of the white keys  31 ,  41  and the black keys  51  is depressed and released as described above, a corresponding one of the pressing portions  31   b ,  41   b ,  51   b  turns on and off the corresponding key switch, not shown, provided on the electric circuit board  20 . Turning each of the key switches on and off is used for control of production of a musical sound signal corresponding to the corresponding one of the white keys  31 ,  41  and the black keys  51  which is depressed and released. 
     In the keyboard device constructed and operated as described above, the protrusions  52   d ,  52   e  protruding outward are provided on the respective right and left outer ends of the hinge thick portion  52   a  of the third key unit  50  having the uppermost common mounting portion  53 , and adjacent two of the protrusions  52   d ,  52   e  are spaced apart from each other with a small space therebetween in the right and left direction. With this construction, even in the case where the black keys  51  are next to each other at different distances, a space formed near a rear side of the black keys  51  between each adjacent two of the black keys  51  can be made extremely small regardless of the hinge thick portions  52   a . As a result, the hinges  32 ,  42  of the first and second key units  30 ,  40  having the respective common mounting portions  33 ,  43  arranged under the uppermost common mounting portion  53  can be made inconspicuous when viewed from an upper side, resulting in good external appearance in a state in which the upper cover  15  covers a space formed behind positions located slightly at the rear of the respective rear ends of the white keys  31 ,  41  and the black keys  51 . 
     In the plurality of hinges  32 ,  42  of the first and second key units  30 ,  40 , the hinge thick portions  32   a ,  42   a  have substantially the same thickness in the up and down direction, substantially the same length in the front and rear direction, and substantially the same width in the right and left direction, and the pair of hinge thin portions  32   b ,  32   c ,  42   b ,  42   c  have substantially the same thickness in the up and down direction, substantially the same length in the front and rear direction, and substantially the same width in the right and left direction. Also, in the plurality of hinges  32 ,  42 , the center position of each of the hinge thick portions  32   a ,  42   a  in the right and left direction is aligned with the center position of the front portion of a corresponding one of the white keys  31 ,  41  in the right and left direction, and each pair of hinge thin portions  32   b ,  32   e ,  42   b ,  42   c  are arranged so as to be symmetric with respect to the center line of the front portion of a corresponding one of the white keys  31 ,  41  in the right and left direction. This construction can eliminate variations, in stiffness (elasticity) of the hinge thin portions  32   b ,  32   c ,  42   b ,  42   c  and the hinge thick portions  32   a ,  42   a  in the right and left direction, between the white keys  31 ,  41  and between portions of each of the white keys  31 ,  41  in the right and left direction. As a result, the same key touch feeling can be provided for the white keys  31 ,  41  during depression and release thereof, and it is possible to eliminate an imbalance in each of the white keys  31 ,  41  in the right and left direction, resulting in increased key touch feeling. The elimination of the imbalance in the right and left direction can also eliminate twists of the hinge thin portions  32   b ,  32   c ,  42   b ,  42   c  and the hinge thick portions  32   a ,  42   a , resulting in improved durability. 
     Likewise, in the plurality of hinges  52  of the third key unit  50 , the hinge thick portions  52   a  have substantially the same thickness in the up and down direction, substantially the same length in the front and rear direction, and substantially the same width in the right and left direction, and the pair of hinge thin portions  52   b ,  52   c  have substantially the same thickness in the up and down direction, substantially the same length in the front and rear direction, and substantially the same width in the right and left direction. Also, in the plurality of hinges  52 , the center position of each of the hinge thick portions  52   a  in the right and left direction is substantially aligned with the center position of a corresponding one of the black key  51  in the right and left direction, and each pair of hinge thin portions  52   b ,  52   c  are arranged so as to be substantially symmetric with respect to the center line of a corresponding one of the black keys  51  in the right and left direction. This construction can eliminate variations, in stiffness (elasticity) of the hinge thin portions  52   b ,  52   c  and the hinge thick portion  52   a  in the right and left direction, between the black keys  51  and between portions of each of the black keys  51  in the right and left direction. As a result, the same key touch feeling can be provided for the black keys  51  during depression and release thereof, and it is possible to eliminate an imbalance in each of the black keys  51  in the right and left direction, resulting in increased key touch feeling. The elimination of the imbalance in the right and left direction can also eliminate twists of the hinge thin portions  52   b ,  52   c  and the hinge thick portion  52   a , resulting in improved durability. 
     It is noted that the width of each of the above-described plurality of hinge thick portions  52   a  in the right and left direction means the width of each of the hinge thick portions  52   a  which does not include the width of each of the protrusions  52   d ,  52   e  in the right and left direction. Specifically, the width of each of the plurality of hinge thick portions  52   a  in the right and left direction can be considered to mean a distance (width) between the left end and the right end of the hinge thick portion  52   a  in the right and left direction in, each of the plurality of hinges  52  in a state in which the position of the left end of the left hinge thin portion  52   b  is aligned with the position of the left end of the hinge thick portion  52   a , and the position of the right end of the right hinge thin portion  52   c  is aligned with the position of the right end of the hinge thick portion  52   a  in each of the plurality of hinges  52 . In other words, the width of each of the plurality of hinge thick portions  52   a  in the right and left direction can also be considered to be a distance (width) in the right and left direction between the left end of the left hinge thin portion  52   b  connected to the hinge thick portion  52   a  and the right end of the right hinge thin portion  52   c  connected to the hinge thick portion  52   a.    
     Likewise, the center position of each of the above-described plurality of hinges  52  in the right and left direction means the center position of each of the hinge thick portions  52   a  not including the protrusions  52   d ,  52   e  in the right and left direction. Specifically, the center position of each of the plurality of hinge thick portions  52   a  in the right and left direction can be considered to mean a center position between the left end and the right end of the hinge thick portion  52   a  in the right and left direction in each of the plurality of hinges  52  in the state in which the position of the left end of the left hinge thin portion  52   b  is aligned with the position of the left end of the hinge thick portion  52   a , and the position of the right end of the right hinge thin portion  52   c  is aligned with the position of the right end of the hinge thick portion  52   a  in each of the plurality of hinges  52 . In other words, the center position of each of the plurality of hinge thick portions  52   a  in the right and left direction can also be considered to be a center position in the right and left direction between the left end of the left hinge thin portion  52   b  connected to the hinge thick portion  52   a  and the right end of the right hinge thin portion  52   c  connected to the hinge thick portion  52   a.    
     In the above-described embodiment, the common mounting portion  53  of the third key unit  50  including the black keys  51  is disposed at the uppermost position among the common mounting portions  33 ,  43 ,  53 , and the protrusions  52   d ,  52   e  are provided on the right and left ends of the hinge thick portion  52   a , whereby the hinge thick portions  32   a ,  42   a  of the respective first and second key units  30 ,  40  including the white keys  31 ,  41  are not easily viewed from an upper side thereof. In this construction, the hinge thick portions  32   a ,  42   a  are white like the white keys  31 ,  41 , and the hinge thick portions  52   a  and the protrusions  52   d ,  52   e  are black like the black keys  51 . Since the lower components are thus hidden by the black components, the hinge thick portions  32   a ,  42   a  as the lower components are not easily viewed, and the lower components can be hidden effectively when compared with the case where the common mounting portion  33  of the first key unit  30  or the common mounting portion  43  of the second key unit  40  is disposed at the uppermost position among the common mounting portions  33 ,  43 ,  53 , and white protrusions each having the same shape as that of a corresponding one of the protrusions  52   d ,  52   e  are provided on the right and left ends of the hinge thick portion  32   a  or the binge thick portion  42   a.    
     In the above-described embodiment, the length of each of the protrusions  52   d ,  52   e  in the uppermost third key unit  50  in the front and rear direction is less than that of the hinge thick portion  52   a  in the front and rear direction, and the rear surfaces of the respective protrusions  52   d ,  52   e  are located in front of the rear surface of the hinge thick portion  52   a . The space is formed, between the pair of the hinge thick portions  52   a ,  52   a  of each adjacent two of the black keys  51 , over the portion of the upper surface of the rear portion of the corresponding one of the hinge thick portions  32   a ,  42   a  of the respective first and second key units  30 ,  40 . Furthermore, the gate corresponding portions  32   a   1 ,  42   a   1  used for molding the respective first and second key units  30 ,  40  are formed in the upper surfaces of the rear portions of the respective hinge thick portions  32   a ,  42   a . With this construction, when each of the first and second key units  30 ,  40  is molded in one piece by injecting resin into a corresponding one of the gate corresponding portions  32   a   1 ,  42   a   1 , the lower surfaces of the respective protrusions  52   d ,  52   e  is never brought into contact with the gate marks (e.g., protrusions) remaining on the gate corresponding portions  32   a   1 ,  42   a   1 . In molding of the first and second key units  30 ,  40 , resin can be evenly and smoothly poured into the white keys  31 ,  41  and the common mounting portions  33 ,  43 , whereby each of the first and second key units  30 ,  40  can be well molded in one piece without weld lines, for example. 
     In particular, each of the rear surfaces  52   d   1 ,  52   e   1  of the respective protrusions  52   d ,  52   e  is inclined so as to be lower at its front portion than at its rear portion, and accordingly even when a rear end of the upper surface of each of the protrusions  52   d ,  52   e  is disposed at a relatively rear position, the rear end of the lower surface of each of the protrusions  52   d ,  52   e  is not brought into contact with the gate mark during depression and release of each black key  51 . This construction allows increase in the lengths of the upper surfaces of the respective protrusions  52   d ,  52   e  in the front and rear direction, enabling the protrusions  52   d ,  52   e  to hide the lower components more effectively. 
     Each of the rear surfaces  52   d   1 ,  52   e   1  is inclined so as to be lower at its front portion than at its rear portion as described above, but the rear surfaces  52   d   1 ,  52   e   1  may be formed such that a rear end of a lower end of each of the rear surfaces  52   d   1 ,  52   e   1  is located in front of a rear end of an upper end of each of the rear surfaces  52   d   1 ,  52   e   1 . In this case, each of the rear surfaces  52   d   1 ,  52   e   1  may be inclined and may be formed so as to have a step. Also, each of the rear surfaces  52   d   1 ,  52   e   1  may be curved so as to be recessed in the front direction. 
     It is to be understood that the disclosure is not limited to the details of the illustrated embodiment, but may be embodied with various changes and modifications, which may occur to those skilled in the art, without departing from the spirit and scope of the disclosure. 
     In the above-described embodiment, each pair of the hinge thin portions  32   b ,  32   c ,  42   b ,  42   c ,  52   b ,  52   c  are connected to the rear surfaces of the respective opposite outer end portions (i.e., right and left outer end portions) of the corresponding one of the hinge thick portions  32   a ,  42   a ,  52   a . Instead of the hinge thin portions  32   b ,  32   c ,  42   b ,  42   c ,  52   b ,  52   c , however, a single hinge thin portion may be provided for each of the keys  31 ,  41 ,  51  and connected to the rear surface of the corresponding one of the hinge thick portions  32   a ,  42   a ,  52   a.    
     In the above-described embodiment, the protrusions  52   d ,  52   e  are provided on the right and left ends of the hinge thick portion  52   a  for all the black keys  51 . Instead of this construction, however, the protrusion  52   d  or the protrusion  52   e  may be provided on only one of the right and left ends of the hinge thick portion  52   a  for each of the black keys  51  such that a small space is formed between the hinge thick portion  52   a  and the protrusion  52   d  or the protrusion  52   e  in the right and left direction in adjacent two of the black keys  51 , in order to make the lower hinge thick portions  32   a ,  42   a  inconspicuous when viewed from an upper side thereof. 
     The clearances are not large between the hinge thick portions  52   a ,  52   a  of the black keys  51 ,  51  respectively corresponding to C# and D#, between the hinge thick portions  52   a ,  52   a  of the black keys  51 ,  51  respectively corresponding to F# and G#, and between the hinge thick portions  52   a ,  52   a  of the black keys  51 ,  51  respectively corresponding to G# and A#. Accordingly, the protrusion  52   e  located to the right of the black key  51  corresponding to C#, the protrusion  52   d  located to the left of the black key  51  corresponding to D#, the protrusion  52   e  located to the right of the black key  51  corresponding to F#, the protrusion  52   d  located to the left of the black key  51  corresponding to G#, the protrusion  52   e  located to the right of the black key  51  corresponding to G#, and the protrusion  52   d  located to the left of the black key  51  corresponding to A# may be omitted, and this omission is not problematic. In this construction, this keyboard device is preferably constructed such that the length of the hinge thick portion  52   a  in the right and left direction is made slightly greater, and the clearance between the hinge thick portions  52   a ,  52   a  in the right and left direction is made as small as possible. 
     In the above-described embodiment, the protrusions  52   d ,  52   e  and the hinge thick portion  52   a  have the same thickness in the up and down direction. Instead of this construction, however, the thickness of the protrusions  52   d ,  52   e  in the up and down direction may be less than that of the hinge thick portion  52   a  in the up and down direction. For example, the thickness of the protrusions  52   d ,  52   e  in the up and down direction may be substantially equal to that of the hinge thin portions  52   b ,  52   c  in the up and down direction. 
     In the above-described embodiment, the length of each of the protrusions  52   d ,  52   e  in the front and rear direction is less than the length of the hinge thick portion  52   a  in the front and rear direction, whereby the protrusions  52   d ,  52   e  do not overlap portions of the upper surfaces of the hinge thick portions  32   a ,  42   a  of the respective first and second key units  30 ,  40 , and the gate corresponding portions  32   a   1 ,  42   a   1  are formed in the portions of the respective upper surfaces. However, the length of each of the protrusions  52   d ,  52   e  in the front and rear direction in the third key unit  50  having the uppermost common mounting portion  53  may be equal to the length of the hinge thick portion  52   a  in the front and rear direction as long as the problem of contact between the lower surfaces of the respective protrusions  52   d ,  52   e  and the gate marks (e.g., protrusions) remaining in the case where the first and second key units  30 ,  40  are molded by resin can be solved by another method, e.g., a method of molding the first and second key units  30 ,  40  by forming gate corresponding portions in other inconspicuous portions in terms of external appearance. 
     In the above-described embodiment, the shapes and the dimensional relationships of the hinge thick portions  32   a ,  42   a ,  52   a  and the hinge thin portions  32   b ,  32   c ,  42   b ,  42   c ,  52   b ,  52   c  of the first through third key units  30 ,  40 ,  50  are determined in detail. For example, the length of the hinge thick portion  52   a  of the third key unit  50  in the front and rear direction is extremely slightly less than the length of each of the hinge thick portions  32   a ,  42   a  of the respective first and second key units  30 ,  40  in the front and rear direction, and the length of the hinge thick portion  52   a  in the right and left direction is slightly greater than the length of each of the hinge thick portions  32   a ,  42   a  in the right and left direction. Also, the length of each of the hinge thin portions  52   b ,  52   c  of the third key unit  50  in the front and rear direction is substantially equal to the length of each of the hinge thin portions  32   b ,  32   c ,  42   b ,  42   c  of the first and second key units  30 ,  40  in the front and rear direction, and the length of each of the hinge thin portions  52   b ,  52   c  in the right and left direction is slightly less than the length of each of the hinge thin portions  32   b ,  32   c ,  42   b ,  42   c  in the right and left direction. However, the shapes and the dimensional relationships may be changed in some degree as needed. 
     In the above-described embodiment, the common mounting portion  53  of the third key unit  50  including the black keys  51  is placed on the common mounting portions  33 ,  43  of the respective first and second key units  30 ,  40  including the white keys  31 ,  41 , such that the common mounting portion  53  is located at the uppermost position among the common mounting portions  33 ,  43 ,  53 . However, the common mounting portion  33  (or the common mounting portion  43 ) of the first key unit  30  (or the second key unit  40 ) including the white keys  31  (or the white keys  41 ) is located at the uppermost position among the common mounting portions  33 ,  43 ,  53 . In this construction, protrusions similar to the protrusions  52   d ,  52   e  are provided on right and left outer ends of the hinge thick portion  32   a  (or the hinge thick portion  42   a ) of the first key unit  30  (or the second key unit  40 ). With this construction, these protrusions allow the hinge thin portions  32   b ,  32   c  (or the hinge thin portions  42   b ,  42   c ) and the hinge thick portion  32   a  (or the hinge thick portion  42   a ) to be arranged at desired positions with respect to the white keys  31  (or the white keys  41 ) in the right and left direction while ensuring good external appearance.