Patent Publication Number: US-11035439-B2

Title: Injection molded shaft

Description:
TECHNICAL FIELD 
     The present invention relates to an injection molded shaft formed into a desired shape by injection molding, and that can absorb rotation energy by undergoing twisting deformation. 
     BACKGROUND OF THE INVENTION 
     Many shafts used as power transmission components for automobiles or the like are made of metal and are created by a cutting operation, so the product cost is high and the weight is heavy. 
     An injection molded shaft  100  made of synthetic resin in  FIG. 10  was proposed to solve such problems. In the injection molded shaft  100  illustrated in  FIG. 10 , a gear  101  and an inward flange  102  are formed integrally with one end of a cylindrical shaft body  103  and a rotary torque is transferred to another rotary component (not illustrated) via the gear  101  (see JP-A-2003-33947). 
     However, since the twisting stiffness of the cylindrical shaft body  103  is large in the injection molded shaft  100  illustrated in  FIG. 10 , even when a rotary torque is applied impulsively, the shaft body  103  cannot undergo sufficient twisting deformation and the shock caused by sudden changes in torque could not be absorbed by twisting deformation of the shaft body  103 . Therefore, in the injection molded shaft  100  illustrated in  FIG. 10 , rotation transmission components such as the gear  101  receive a shock caused by sudden changes in torque, possibly breaking rotation transmission components such as the gear  101 . 
     Therefore, the invention provides an injection molded shaft that can absorb a shock caused by sudden changes in torque by undergoing twisting deformation of a shaft body when sudden changes in torque are applied. 
     SUMMARY OF THE INVENTION 
     As illustrated in  FIG. 9 , the invention relates to an injection molded shaft  1  including a first torque applied part  3  formed at one end in an axial direction, a second torque applied part  4  formed at another end in the axial direction, and a shaft body  2  connecting the first torque applied part  3  to the second torque applied part  4  along a direction of a shaft core (longitudinal center axis)  17 . In the invention, the shaft body  2  includes a first connecting part  15  formed integrally with the first torque applied part  3 , a second connecting part  16  formed integrally with the second torque applied part  4 , a core part  18  extending from the first connecting part  15  to the second connecting part  16  along the shaft core (longitudinal center axis)  17 , the core part  18  having a crisscross cross section orthogonal to the axial direction, and a diagonal-brace-shaped framework part  60  disposed in a portion partitioned by the first connecting part  15 , the second connecting part  16 , and the core part  18 , the diagonal-brace-shaped framework part  60  extending across the first connecting part  15 , the second connecting part  16 , and the core part  18  like a diagonal brace. 
     In addition, as illustrated in  FIGS. 1 to 9 , the invention relates to an injection molded shaft  1  including a first torque applied part  3  formed at one end in an axial direction, a second torque applied part  4  formed at another end in the axial direction, and a shaft body  2  connecting the first torque applied part  3  to the second torque applied part  4  along a direction of a shaft core (i.e., longitudinal center axis)  17 . In the invention, the shaft body  2  includes at least one framework unit  61 . In addition, the framework unit  61  includes a core part  18  extending along the shaft core (longitudinal center axis)  17 , and the core part  18  having a crisscross cross section orthogonal to the axial direction. A pair of discoid framework parts  62  and  62  are disposed at one end and another end along the shaft core (center axis)  17  of the core part  18  so as to face each other, the pair of discoid framework parts  62  and  62  having discoid cross sections orthogonal to the axial direction, and a diagonal-brace-shaped framework part  60  disposed in a portion partitioned by the pair of discoid framework parts  62  and  62  and the core part  18 . The diagonal-brace-shaped framework part  60  extending across the pair of discoid framework parts  62  and  62  and the core part  18  like a diagonal brace. 
     In addition, as illustrated in  FIGS. 1 to 8 , the invention relates to an injection molded shaft  1  including a first torque applied part  3  formed at one end in an axial direction, a second torque applied part  4  formed at another end in the axial direction, and a shaft body  2  connecting the first torque applied part  3  to the second torque applied part  4  along a direction of a shaft core. In the invention, the shaft body  2  includes
         a first connecting part  15  formed integrally with the first torque applied part  3 ,   a second connecting part  16  formed integrally with the second torque applied part  4 ,   a core part  18  extending from the first connecting part  15  to the second connecting part  16  along the shaft core (center axis)  17 , the core part  18  having a crisscross cross section orthogonal to the axial direction,   a plurality of first framework parts  21  formed at regular intervals along the direction of the shaft core (center axis)  17  in the core part  18  between the first connecting part  15  and the second connecting part  16 , each of the first framework parts  21  having a discoid cross section orthogonal to the axial direction,   a second framework part  22  disposed in a portion partitioned by the first connecting part  15 , the first framework part  21  adjacent to the first connecting part  15 , and the core part  18 , the second framework part  22  extending across the first connecting part  15 , the first framework part  21 , and the core part  18  like a diagonal brace,   a third framework part  23  disposed in a portion partitioned by a pair of the first framework parts  21  and  21  adjacent to each other and the core part  18 , the third framework part  23  extending across the pair of first framework parts  21  and  21  adjacent to each other and the core part  18  like a diagonal brace, and   a fourth framework part  24  disposed in a portion partitioned by the second connecting part  16 , the first framework part  21  adjacent to the second connecting part  16 , and the core part  18 , the fourth framework part  24  extending across the second connecting part  16 , the first framework part  21 , and the core part  18  like a diagonal brace.       

     Advantageous Effects of Invention 
     Even when sudden changes in torque are applied, the injection molded shaft according to the invention can absorb the energy caused by sudden changes in torque by undergoing twisting deformation of the shaft body and reduce the shock caused by sudden changes in torque using the shaft body. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram illustrating the use state of an injection molded shaft according to a first embodiment of the invention. 
         FIG. 2A  is a front view (seen from an X-axis direction) illustrating the injection molded shaft according to the first embodiment of the invention,  FIG. 2B  is a left side view illustrating the injection molded shaft seen from the direction of an arrow B 1  in  FIG. 2A ,  FIG. 2C  is a right side view illustrating the injection molded shaft seen from the direction of an arrow B 2  in  FIG. 2A ,  FIG. 2D  is a cross sectional view illustrating the injection molded shaft taken along a line A 1 -A 1  in  FIG. 2A ,  FIG. 2E  is a cross sectional view illustrating the injection molded shaft taken along a line A 2 -A 2  in  FIG. 2A , and  FIG. 2F  is a cross sectional view illustrating the injection molded shaft taken along a line A 3 -A 3  in  FIG. 2A . 
         FIG. 3A  is a plan view (seen from a Y-axis direction) illustrating the injection molded shaft according to the first embodiment of the invention,  FIG. 3B  is a cross sectional view illustrating the injection molded shaft taken along the line A 1 -A 1  in  FIG. 3A ,  FIG. 3C  is a cross sectional view illustrating the injection molded shaft taken along the line A 2 -A 2  in  FIG. 3A , and  FIG. 3D  is a cross sectional view illustrating the injection molded shaft taken along the line A 3 -A 3  in  FIG. 3A . 
         FIG. 4  is an enlarged view illustrating a part of the injection molded shaft in  FIG. 2A . 
         FIG. 5A  is a diagram illustrating an injection molded shaft die taken along a Y-Z coordinate plane and  FIG. 5B  is a diagram illustrating the injection molded shaft die taken along an X-Z coordinate plane. 
         FIG. 6A  is a diagram illustrating an injection molded shaft according to a first modification of the first embodiment of the invention and an enlarged view (corresponding to  FIG. 4 ) illustrating a part of a shaft body. In addition,  FIG. 6B  is a diagram illustrating an injection molded shaft according to a second modification of the first embodiment of the invention and an enlarged view (corresponding to  FIG. 4 ) illustrating a part of a shaft body. 
         FIG. 7  is a diagram illustrating an injection molded shaft according to a third modification of the first embodiment of the invention and the diagram illustrates one end in an axial direction of the injection molded shaft. 
         FIGS. 8A-8F  are diagrams illustrating an injection molded shaft according to a fourth modification of the first embodiment of the invention and the diagrams correspond to  FIGS. 2A-2F . 
         FIG. 9  is a diagram illustrating an injection molded shaft according to a fifth modification of the first embodiment of the invention and the diagram corresponds to  FIG. 2A . 
         FIG. 10  is a vertical cross sectional view illustrating a conventional injection molded shaft. 
         FIG. 11  is a diagram illustrating an injection molded shaft according to a modification of the first embodiment of the invention illustrating a second end in an axial direction of the injection molded shaft. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments of the invention will be described in detail below with reference to the drawings. 
     First Embodiment 
       FIGS. 1 to 4  illustrate the injection molded shaft  1  according to the first embodiment of the invention.  FIG. 1  is a diagram illustrating the use state of the injection molded shaft  1 . In addition,  FIG. 2A  is a front view (seen from the X-axis direction) illustrating the injection molded shaft  1 ,  FIG. 2B  is a left side view illustrating the injection molded shaft  1  seen from the direction of the arrow B 1  in  FIG. 2A ,  FIG. 2C  is a right side view illustrating the injection molded shaft  1  seen from the direction of the arrow B 2  in  FIG. 2A ,  FIG. 2D  is a cross sectional view illustrating the injection molded shaft  1  taken along the line A 1 -A 1  in  FIG. 2A ,  FIG. 2E  is a cross sectional view illustrating the injection molded shaft  1  taken along the line A 2 -A 2  in  FIG. 2A , and  FIG. 2F  is a cross sectional view illustrating the injection molded shaft  1  taken along the line A 3 -A 3  in  FIG. 2A . In addition,  FIG. 3A  is a plan view (seen from the Y-axis direction) illustrating the injection molded shaft  1 ,  FIG. 3B  is a cross sectional view illustrating the injection molded shaft  1  taken along the line A 1 -A 1  in  FIG. 3A ,  FIG. 3C  is a cross sectional view illustrating the injection molded shaft  1  taken along the line A 2 -A 2  in  FIG. 3A , and  FIG. 3D  is a cross sectional view illustrating the injection molded shaft  1  taken along the line A 3 -A 3  in  FIG. 3A . In addition,  FIG. 4  is an enlarged view illustrating a part of the injection molded shaft  1  (particularly the shaft body  2 ) in  FIG. 2A . 
     (Structure of Injection Molded Shaft) 
     As illustrated in  FIGS. 1 to 3 , the injection molded shaft  1  includes a helical gear  3  as the first torque applied part formed at a first end in the axial direction, a worm  4  as the second torque applied part formed at a second end in the axial direction, and the shaft body  2  connecting the helical gear  3  to the worm  4  integrally along the shaft core direction. In addition, the injection molded shaft  1  has a rod-shaped first boss  5  formed integrally at the rotation center of a side  3   a  of the helical gear  3  and a rod-shaped second boss  6  formed integrally at the rotation center of a side  4   a  of the worm  4  (see  FIGS. 2A to 2C ). The injection molded shaft  1  in such a structure is integrally formed by injecting molten resin such as POM (polyacetal) or PA (polyamide) into a cavity  8  of a die  7  as described in detail later. 
     As illustrated in  FIG. 1 , in the injection molded shaft  1 , the helical gear  3  at the first end in the axial direction is engaged with another first helical gear  10  to form a screw gear  11 , and the worm  4  at the second end in the axial direction is engaged with another second helical gear  12  to form a worm gear  13 . In the injection molded shaft  1  as described above, when, for example, the rotation of a motor (not illustrated) or the like is transferred via the screw gear  11 , the rotation is transferred to the second helical gear  12  via the worm  4  formed integrally with the shaft body  2 . At this time, the shaft body  2  of the injection molded shaft  1  undergoes twisting deformation by the rotation torque applied via the helical gear  3  at the one end in the axial direction and the rotation torque applied via the worm  4  at the other end in the axial direction. 
     As illustrated in  FIGS. 2 to 4 , in the shaft body  2  of the injection molded shaft  1 , the discoid first connecting part  15  positioned at the first end in the axial direction is formed integrally with the helical gear  3  and the discoid second connecting part  16  positioned at the other end in the axial direction is formed integrally with the worm  4 . In the shaft body  2 , the first connecting part  15  is connected to the second connecting part  16  by the core part  18  extending along the shaft core (center axis)  17 . The core part  18  has a crisscross cross section orthogonal to the axial direction and is positioned so that the center of the intersecting portion of the cross is aligned with the shaft core (center axis)  17 . In the core part  18  between the first connecting part  15  and the second connecting part  16 , the plurality of first framework parts  21  are formed at regular intervals along the direction in which the shaft core (center axis)  17  extends. The first framework parts  21  are formed into the core part  18  so as to have discoid cross sections orthogonal to the axial direction. In the shaft body  2 , in the portion partitioned by the first connecting part  15 , the first framework part  21  adjacent to the first connecting part  15 , and the core part  18 , the second framework part  22  is disposed. The second framework part  22  extends across the first connecting part  15 , the first framework part  21 , and the core part  18  like a diagonal brace and includes a pair of diagonal brace members  22   a  and  22   b  intersecting with each other like an X shape. In the shaft body  2 , the third framework part  23  is disposed in the portion partitioned by the pair of first framework parts  21  and  21  adjacent to each other and the core part  18 . The third framework part  23  extends across the pair of first framework parts  21  and  21  and the core part  18  like a diagonal brace and includes the pair of diagonal brace members  23   a  and  23   b  intersecting with each other like an X shape as in the second framework part  22 . In the shaft body  2 , the fourth framework part  24  is disposed in the portion partitioned by the second connecting part  16 , the first framework part  21  adjacent to the second connecting part  16 , and the core part  18 . The fourth framework part  24  extends across the second connecting part  16 , the first framework part  21 , and the core part  18  like a diagonal brace and includes a pair of diagonal brace members  24   a  and  24   b  intersecting with each other like an X shape as in the second and third framework parts  22  and  23 . 
     As illustrated in  FIGS. 2D and 2E ,  FIGS. 3B and 3C , and  FIG. 4 , in the shaft body  2  of the injection molded shaft  1 , a first core portion  18   a  of the core part  18  extending along the Y-axis has a wall thickness of W and a second core portion  18   b  of the core part  18  extending along the X-axis has a wall thickness of W, which is the same as the wall thickness W of the first core portion  18   a . The wall thickness of the second core portion  18   b  is measured at the connection portion with the first core portion  18   a . In  FIGS. 2 to 4 , the X-axis matches the direction in which a first movable mold  33  for molding the shaft body  2  is separated (see  FIG. 5B ) and the second core portion  18   b  has a disconnection gradient for facilitating separation from the first movable mold  33 . As a result, the wall thickness of the second core portion  18   b  becomes small with distance from the first core portion  18   a  along the X-axis direction. In addition, in the first to fourth framework parts  21  to  24 , a wall thickness of a connection portion with the first core portion  18   a  is W, which is the same as the wall thickness W of the first core portion  18   a . The first to fourth framework parts  21  to  24  have a disconnection gradient as in the second core portion  18   b  and the wall thickness becomes small with distance from the first core portion  18   a  along the X-axis direction. In addition, the shaft body  2  of the injection molded shaft  1  is formed so as to have the same outer dimension (D) from the one end in the axial direction to the other end in the axial direction. In the shaft body  2  of the injection molded shaft  1  described above, the wall thicknesses W of the core part  18  and the first to fourth framework parts  21  to  24  are determined so as to enable flexible twisting deformation as compared with a conventional cylindrical shaft (see  FIG. 10 ). 
     As illustrated in  FIGS. 2A and 3A , the shaft body  2  of the injection molded shaft  1  is line-symmetric with respect to the shaft core  17  along the Z-axis direction and a plurality of constant shapes formed by the first to fourth framework parts  21  to  24  and the like are formed at regular intervals along the shaft core (center axis)  17 . In addition, as illustrated in  FIGS. 2D to 2F and 3B to 3D , the cross section orthogonal to the axial direction of the shaft body  2  of the injection molded shaft  1  is line-symmetric with respect to a center line CL 1  along the X-axis and a center line CL 2  along the Y-axis. Accordingly, the injection molded shaft  1  according to the embodiment is accurately injection-molded since the core part  18  and the first to fourth framework parts  21  to  24  of the shaft body  2  are formed to have the same wall thickness W in addition to the above reason. 
     As illustrated in  FIG. 4 , in the shaft body  2  of the injection molded shaft  1 , the pairs of diagonal brace members  22   a  to  24   a  and  22   b  to  24   b  constituting the second to fourth framework parts  22  to  24  intersect with the shaft core (center axis)  17  of the injection molded shaft  1  at an angle θ. The angle θ is appropriately set depending on the spacing between the first connecting part  15  and the first framework part  21  adjacent to the first connecting part  15 , the spacing between the pair of first framework parts  21  and  21  adjacent to each other, the spacing between the second connecting part  16  and the first framework part  21  adjacent to the second connecting part  16 , the outer dimension D of the shaft body  2 , and the like. 
     (Injection Molding Die) 
       FIG. 5  is a diagram schematically illustrating the injection molding die  7  for the injection molded shaft  1  according to the embodiment.  FIG. 5A  is a cross sectional view illustrating the injection molding die  7  taken along a Y-Z coordinate plane of the rectangular coordinate system and  FIG. 5B  is a cross sectional view illustrating the injection molding die  7  taken along an X-Z coordinate plane of the rectangular coordinate system. 
     As illustrated in  FIG. 5 , the injection molding die  7  includes a fixed mold  25  and a movable mold  26 . The fixed mold  25  includes a first fixed mold  28  having a first cavity  27  for shaping the first boss  5  at the first end in the axial direction of the injection molded shaft  1  and a second fixed mold  31  having a second cavity  30  for shaping the helical gear  3  of the injection molded shaft  1 . The movable mold  26  includes the first movable mold (shaft body formation portion of the injection molding die  7 )  33  in which a third cavity  32  for shaping the shaft body  2  of the injection molded shaft  1  is formed, a second movable mold  35  in which a fourth cavity  34  for shaping the worm  4  of the injection molded shaft  1  is formed, and a third movable mold  37  in which a fifth cavity  36  for shaping the second boss  6  at the other end in the axial direction of the injection molded shaft  1  is formed. The first movable mold  33  is split into two so as to be opened along the X-axis direction from the position of the shaft core (center axis)  17  of the third cavity  32  (see  FIG. 5B ). In addition, a gate  38  is provided in the first fixed mold  28  of the injection molding die  7  so as to be opened toward the inside of the first cavity  27 . In addition, the first to fifth cavities  27 ,  30 ,  32 ,  34 , and  36  constitute the cavity  8  for shaping the injection molded shaft  1 . 
     As illustrated in  FIG. 5 , in the state in which the fixed mold  25  and the movable mold  26  are closed, molten synthetic resin is injected into the first cavity  27  from the gate  28  and the molten synthetic resin injected into the first cavity  27  is supplied to the second to fifth cavities  30 ,  32 ,  34 , and  36 . As a result, the entire injection molded shaft integrally formed therein and described above has a unitary, one-piece construction formed of the injected resin material. When the injection molding die  7  is configured by changing the fixed mold  25  in  FIG. 5  to a movable mold and the movable mold  26  in  FIG. 5  to a fixed mold, the gate  28  is provided so as to be opened toward the fifth cavity  36  to be positioned on the fixed mold side. 
     In the state illustrated in  FIG. 5 , after the synthetic resin injected into the cavity  8  of the injection molding die  7  is cooled and solidified (after the injection molded shaft  1  is formed), the movable mold  26  is separated from the fixed mold  25  while being rotated (moved in the Z-axis direction). This causes the injection molded shaft  1  to be separated from the fixed mold  25  while being held by the movable mold  26 . Next, the first movable mold  33  is opened (split into two) along the X-axis direction, the first boss  5 , the helical gear  3 , and the shaft body  2  are exposed from the second movable mold  35 , the worm  4  is housed in the second movable mold  35 , and the second boss  6  is housed in the third movable mold  37 . Next, an eject pin  40  housed in the third movable mold  37  in a slidable state continuously presses the second boss  6  of the injection molded shaft  1 , the worm  4  moves in the second movable mold  35  while being rotated, the worm  4  is pushed out of the second movable mold  35 , and the injection molded shaft  1  is removed from the injection molding die  7 . Note that the first movable mold  33  of the injection molding die  7  is opened (split into two) along the Y-axis direction in the coordinate axes (the up-down direction in  FIG. 2D  is the X-axis direction and the left-right direction in  FIG. 3B  is the X-axis direction) obtained by rotating the cross sections orthogonal to the axial direction illustrated in  FIGS. 2D and 3B  counterclockwise (left rotation direction) by 90 degrees. 
     (Effect of Present Embodiment) 
     The injection molded shaft  1  according to the embodiment as described above can absorb the energy caused by sudden changes in torque by undergoing the flexible twisting deformation of the shaft body  2  even when sudden changes in torque are applied, and can reduce the shock caused by sudden changes in torque by undergoing the twisting deformation of the shaft body  2 . As a result, the injection molded shaft  1  according to the embodiment can prevent the helical gear  3  formed at the one end in the axial direction and the worm  4  formed at the other end in the axial direction from receiving an excess load, prevent the teeth of the helical gear  3  at the one end in the axial direction and the teeth of another first helical gear  10  engaged with the helical gear  3  from being broken, and prevent the teeth of the worm  4  formed at the other end in the axial direction and the teeth of another second helical gear  12  engaged with the worm  4  from being broken. 
     In addition, since the core part  18  and the first to fourth framework parts  21  to  24  of the shaft body  2  in the injection molded shaft  1  according to the embodiment have the same wall thickness (W), the shaft body  2  can be injection-molded accurately without occurrence of a molding failure due to variations in the shrinkage ratio. 
     In addition, in the injection molded shaft  1  according to the embodiment, many hollowed concave parts  41  to  43  are formed between the first connecting part  15 , the second framework part  22 , the first framework part  21 , and the core part  18 . In addition, in the injection molded shaft  1 , many hollowed concave parts  41  to  43  are formed between the first framework parts  21  and  21  adjacent to each other, the third framework part  23 , and the core part  18 . In addition, in the injection molded shaft  1 , many hollowed concave parts  41  to  43  are formed between the second connecting part  16 , the fourth framework part  24 , and the core part  18 . Accordingly, in the injection molded shaft  1  according to the embodiment, as compared with the case in which the injection molded shaft  1  is shaped in a rod, since the amount of synthetic resin material can be reduced and the cooling time after injection into the cavity  8  of the injection molding die  7  can be shortened, the injection molding cycle can be shortened, the production efficiency can be improved, and the total weight can be reduced. 
     In addition, since the injection molded shaft  1  according to the embodiment can reduce a shock caused by sudden changes in torque by undergoing the twisting deformation of the shaft body  2 , vibrations caused by sudden changes in torque can be reduced and the generation of noise caused by sudden changes in torque can be suppressed. Accordingly, the injection molded shaft  1  according to the embodiment reduces operation noise during power transmission. 
     In addition, in the injection molded shaft  1  according to the embodiment, since the plurality of hollowed concave parts  43  are formed at regular intervals along the axial direction at the end of the core part  18  (the first core portion  18   a  and the second core portion  18   b ) of the shaft body  2 , voids (air bubbles) are not easily generated, thereby efficiently preventing occurrence of a molding failure caused by voids. 
     (First Modification and Second Modification of First Embodiment) 
       FIG. 6A  is a diagram illustrating an injection molded shaft  1  according to a first modification of the first embodiment of the invention and an enlarged view (corresponding to  FIG. 4 ) illustrating a part of the shaft body  2 . In addition,  FIG. 6B  is a diagram illustrating an injection molded shaft  1  according to a second modification of the first embodiment of the invention and an enlarged view (corresponding to  FIG. 4 ) illustrating a part of the shaft body  2 . 
     In the injection molded shaft  1  according to the first embodiment of the invention, the second to fourth framework parts  22  to  24  include the pairs of diagonal brace members  22   a  to  24   a  and  22   b  to  24   b  intersecting with each other like an X shape. However, the invention is not limited to the injection molded shaft  1  according to the first embodiment and each of the second to fourth framework parts  22  to  24  may be configured by one diagonal brace member, which is one of  22   a  to  24   a  (or  22   b  to  24   b ), and the twisting stiffness of the injection molded shaft  1  may be reduced according to the use condition and the like. In the injection molded shaft  1  illustrated in  FIGS. 6A and 6B , the same structural portions as in the injection molded shaft  1  illustrated in  FIG. 4  are given the same reference numerals and duplicate descriptions as in the injection molded shaft  1  according to the first embodiment are omitted. 
     (Third Modification of First Embodiment) 
       FIG. 7  is a diagram illustrating an injection molded shaft  1  according to a third modification of the first embodiment of the invention and a diagram illustrating one end in the axial direction of the injection molded shaft  1 . 
     As illustrated in  FIG. 7 , in the injection molded shaft  1  according to the first embodiment of the invention, when the outer dimension of the first boss  5  close to the helical gear  3  is large, a hollowed hole (through-hole)  44  is desirably formed along the center axis  17  from an end face  5   a  of the first boss  5  to the first connecting part  15  of the shaft body  2 . In the injection molded shaft  1  in which this hollowed hole  44  is formed, occurrence of a molding failure due to shrinkage or voids can be prevented and the cooling time and the injection molding cycle can be shortened. As shown in  FIG. 11 , in the injection molded shaft  1  according to the first embodiment, when the outer dimension of the second boss  6  close to the worm  4  is large, to obtain the same effect of the hollowed hole (first through-hole)  44  in the first boss  5 , a hollowed hole (second through-hole)  49  is desirably formed along the center axis  17  from the end face  6   a  of the second boss  6  to the second connecting part  16  of the shaft body  2 . 
     (Fourth Modification of First Embodiment) 
       FIGS. 8A-8F  are diagrams illustrating an injection molded shaft  1  according to a fourth modification of the first embodiment of the invention and the diagram corresponds to  FIG. 2 . In the injection molded shaft  1  illustrated in  FIGS. 8A-8F , the same structural portions as in the injection molded shaft  1  illustrated in  FIG. 2  are given the same reference numerals and duplicate descriptions as in the injection molded shaft  1  according to the first embodiment are omitted. 
     As illustrated in  FIGS. 8A-8F , the injection molded shaft  1  according to the modification has a semicylindrical hollowed concave part  46  at an intersecting portion (first intersecting portion)  45  at which the second framework part  22  (diagonal brace members  22   b ) positioned at the one end in the axial direction of the shaft body  2  intersects with the core part  18 . When seen from the direction along the X-axis, the chord of this semicylindrical hollowed concave part  46  is positioned along the outer periphery of the border between the first connecting part  15  and the intersecting portion  45 . In addition, the injection molded shaft  1  has a cylindrical hollowed concave part  48  at an intersecting portion (second intersecting portion)  47  between the first framework part  21 , the second framework part  22  (diagonal brace members  22   a ), the third framework part  23  (diagonal brace members  23   b ), and the core part  18 . In addition, the injection molded shaft  1  has a cylindrical hollowed concave part  51  at an intersecting portion (third intersecting portion)  50  between the first framework part  21 , the third framework part  23  (diagonal brace members  23   a  and  23   b ), and the core part  18 . In addition, the injection molded shaft  1  has a cylindrical hollowed concave part  53  at an intersecting portion (fourth intersecting portion)  52  between the first framework part  21 , the third framework part  23  (diagonal brace members  23   a ), the fourth framework part  24  (diagonal brace members  24   b ), and the core part  18 . In addition, the injection molded shaft  1  has a semicylindrical hollowed concave part  55  at an intersecting portion (fifth intersecting portion)  54  between the fourth framework part  24  (diagonal brace members  24   a ) and the core part  18 . When seen from the direction along the X-axis, the chord of the semicylindrical hollowed concave part  55  is positioned along the outer periphery of the border between the second connecting part  16  and the intersecting portion  54 . 
     The above hollowed concave parts  46 ,  48 ,  51 ,  53 , and  55  are line-symmetric with respect to the center line CL 2  along the Y-axis (see  FIGS. 8A and 8F ). In addition, each of the hollowed concave parts  46 ,  48 ,  51 ,  53 , and  55  has a disconnection gradient for facilitating separation from the injection molding die  7  and the wall thickness between these hollowed concave parts and the adjacent hollowed concave parts  41  and  42  is substantially the same as the wall thickness W of the connection portion between the first to fourth framework parts  21  to  24  and the first core portion  18   a . In addition, the hollowed concave parts  46 ,  48 ,  51 ,  53 , and  55  have a depth that reaches the first core portion  18   a  (see  FIGS. 8A and 8F ). The invention is not limited to the case in which the wall thickness between the hollowed concave parts  46 ,  48 ,  51 ,  53 , and  55  and the adjacent hollowed concave parts  41  and  42  is substantially the same as the wall thickness W of the connection portion between the first to fourth framework parts  21  to  24  and the first core portion  18   a  and the wall thickness may be changed depending on the size of the shaft body  2 , the magnitude of transfer torque, and the like. In addition, the depths of the hollowed concave parts  46 ,  48 ,  51 ,  53 , and  55  are not limited to the depths that reach the first core portion  18   a  and the depths may be changed depending on the size of the shaft body  2 , the magnitude of transfer torque, and the like. 
     In addition, the hollowed concave parts  46 ,  48 ,  51 ,  53 , and  55  are formed orthogonally to the Y-Z coordinate plane and opened along the open direction of the injection molding die  7  (see  FIG. 5 ). 
     In the injection molded shaft  1  according to the modification as described above, since the hollowed concave parts  46 ,  48 ,  51 ,  53 , and  55  are formed, the number of portions having substantially the same wall thickness is larger than in the injection molded shaft  1  according to the first embodiment. Accordingly, the accuracy of the shape after injection molding is higher than in the injection molded shaft  1  according to the first embodiment. 
     (Fifth Modification of First Embodiment) 
       FIG. 9  is a front view of an injection molded shaft  1  according to the modification and the front view corresponds to  FIG. 2A . As illustrated in  FIG. 9 , in the injection molded shaft  1  according to the modification, the shaft body  2  is shorter than the shaft body  2  of the injection molded shaft  1  according to the first embodiment. That is, in the modification, the shaft body  2  of the injection molded shaft  1  includes the discoid first connecting part  15  formed integrally with the first torque applied part  3 , the discoid second connecting part  16  formed integrally with the second torque applied part  4 , the core part  18 , extending from the first connecting part  15  to the second connecting part  16  along the shaft core  17 , that has a crisscross cross section orthogonal to the axial direction, and a diagonal-brace-shaped framework part  60 , disposed in the portion partitioned by the first connecting part  15 , the second connecting part  16 , and the core part  18 , that extends across the first connecting part  15 , the second connecting part  16 , and the core part  18  like a diagonal brace. The diagonal-brace-shaped framework part  60  includes the pair of diagonal brace members  22   a  and  22   b  intersecting like an X shape or the pair of diagonal brace members  24   a  and  24   b  intersecting like an X shape as described in detail in the first embodiment. In the injection molded shaft  1  illustrated in  FIG. 9 , the same structural portions as in the injection molded shaft  1  illustrated in  FIG. 2A  are given the same reference numerals and duplicate descriptions as in the first embodiment are omitted. 
     It can be considered that the shaft body  2  of the injection molded shaft  1  as described above is configured by one framework unit  61 . That is, the framework unit  61  includes the core part  18 , extending along the shaft core  17 , that has a crisscross cross section orthogonal to the axial direction, a pair of discoid framework parts  62  and  62  (the first connecting part  15  and the second connecting part  16 ), disposed at one end and the other end along the shaft core  17  of the core part  18  so as to face each other, that have discoid cross sections orthogonal to the axial direction, the diagonal-brace-shaped framework parts  60  and  60 , disposed in the portion partitioned by the pair of discoid framework parts  62  and  62  and the core part  18 , that extend across the pair of discoid framework parts  62  and  62  and the core part  18  like a diagonal brace. 
     In the injection molded shaft  1  according to the modification as described above, even when sudden changes in torque are applied, the energy caused by sudden changes in torque can be absorbed by flexible twisting deformation of the shaft body  2  and the shock caused by sudden changes in torque can be reduced by twisting deformation of the shaft body  2 . 
     When the injection molded shaft  1  according to the first embodiment is considered from the viewpoint of configuring the shaft body  2  using the framework unit  61  like the injection molded shaft  1  according to the modification, the shaft body  2  of the injection molded shaft  1  according to the first embodiment can be considered to have a plurality of (six) framework units  61 . In the shaft body  2  of the injection molded shaft  1  according to the first embodiment, the first connecting part  15 , the second connecting part  16 , and the first framework part  21  are equivalent to the discoid framework parts  62 . In addition, in the shaft body  2  of the injection molded shaft  1  according to the first embodiment, the pair of diagonal brace members  22   a  and  22   b  intersecting like an X shape, the pair of diagonal brace members  23   a  and  23   b  intersecting like an X shape, and the pair of diagonal brace members  24   a  and  24   b  are equivalent to the diagonal-brace-shaped framework part  60 . In addition, in the injection molded shaft  1 , the shaft body  2  may be configured by two or more pairs of framework units  61 . 
     In addition, in the injection molded shaft  1  according to the modification, the diagonal-brace-shaped framework part  60  is configured by the pair of diagonal brace members  22   a  and  22   b  intersecting like an X shape or the pair of diagonal brace members  24   a  and  24   b  intersecting like an X shape. However, the invention is not limited to this modification and the diagonal-brace-shaped framework part  60  may be configured by one diagonal brace member  22   a  ( 24   a ) or one diagonal brace member  22   b  ( 24   b ). 
     (Other Modifications) 
     The injection molded shaft  1  according to the invention is not limited to the first embodiment described above and the first torque applied part may be a gear other than a helical gear, such as a spur gear or bevel gear and the second torque applied part may be a gear other than a worm, such as a spur gear or bevel gear. In addition, in the injection molded shaft  1  according to the invention, the first torque applied part and the second torque applied part only need to be portions to which a rotation torque is applied and may be, for example, a spline formation part, a key groove formation part, or the like for fixing a helical gear or the like. 
     REFERENCE SIGNS LIST 
       1 : injection molded shaft 
       2 : shaft body 
       3 : helical gear (first torque applied part) 
       4 : worm (second torque applied part) 
       15 : first connecting part 
       16 : second connecting part 
       17 : shaft core (longitudinal center axis) 
       18 : core part 
       21 : first framework part 
       22 : second framework part 
       23 : third framework part 
       24 : fourth framework part 
       33 : first movable mold (shaft body formation portion) 
       60 : diagonal-brace-shaped framework part 
       61 : framework unit 
       62 : discoid framework part