Patent Publication Number: US-2021162639-A1

Title: Method for manufacturing rotary power transmission member

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation application of PCT/JP 2018/038537, filed on Oct. 16, 2018, which claims the benefit of priority to Japanese Patent Application No. 2018-167947, filed with the Japan Patent Office on Sep. 7, 2018, the disclosures of all of which are hereby incorporated by reference in their entireties. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a method for manufacturing a rotary power transmission member. 
     BACKGROUND OF THE INVENTION 
     As shown in a patent literature below, a rotary power transmission member such as a worm wheel, which is made of a resin material, has been developed in recent years. The rotary power transmission member disclosed in the Patent literature includes a main body part made of a fiber-reinforced resin (low melting point resin), and a gear part made of a resin (high melting point resin) having a melting point higher than that of the fiber-reinforced resin. Further, a method for manufacturing a rotary power transmission member disclosed in the Patent Literature below includes firstly molding the main body part with the low melting point resin by injection molding, and then, injecting the high melting point resin in a mold, where the main body part has been inserted, to integrally mold the gear part and the main body part. 
     CITATION LIST 
     Patent Literature 
     Patent Literature: Japanese Patent Application Publication No. 2004-52791 
     SUMMARY OF THE INVENTION 
     However, with the method for manufacturing described above, a mold, in which the gear part is molded, has a ring-shaped cavity, and the main body part is inserted on the inner circumference side of the cavity. Therefore, a runner and a gate cannot be arranged on the inner circumference side of the cavity. For this reason, the runner and gate are arranged on the side surface of the cavity (side surface of the gear part) (see FIGS. 3 and 4 of the Patent Literature above), and a volume or a surface area of the runner and gate is increased. As a result, the resin injected from an injection machine is easily cooled in the runner and gate, which may lead to instability in quality of a molded product. Further, a large amount of resin remains in the runner and gate, and improvement in a yield rate is desired. 
     The present invention is to solve problems as described above, and provides a method for manufacturing a rotary power transmission member, to prevent a resin injected at the time of molding a gear part from being easily cooled and to improve a yield rate. 
     To solve the problem described above, a method for manufacturing a rotary power transmission member of the present invention including a ring-shaped main body part fixed to a rotating shaft, and a ring-shaped gear part formed on an outer circumference of the main body part to transmit a rotational force of the rotating shaft to other members, the method including: gear part molding for injecting a high melting point resin into a first mold to mold the gear part, and main body part molding for injecting a low melting point resin to mold the main body part while the gear part being inserted into a second mold, wherein the first mold includes a runner defined on an inner circumference side of a ring-shaped cavity for molding the gear part, and a gate which communicates the runner with the cavity to supply a resin into the cavity. 
     The present invention causes a volume or a surface area of a runner and a gate of a first mold to be reduced. Accordingly, a resin injected from an injection machine is hard to be cooled in the runner and gate so that quality of a molded product (gear part) is stabilized and a yield rate is improved. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a plan view of a worm wheel of a first embodiment as viewed from an axial direction; 
         FIG. 2  is a cross-sectional view taken along a line II-II in  FIG. 1 ; 
         FIG. 3  is a partially enlarged plan view of a gear part, picked out of the worm wheel, as viewed from the axial direction; 
         FIG. 4  is a cross-sectional view taken along a line IV-IV in  FIG. 1 ; 
         FIG. 5  is a cross-sectional view of a first mold used for a gear part molding step, taken along the axial direction; 
         FIG. 6A  is a plan view of a resin (gear part) and a first radially outer mold after being hardened, as viewed from the axial direction; 
         FIG. 6B  is a plan view showing a state in which split cores of the first radially outer mold are separated, as viewed from the axial direction; 
         FIG. 7  is a cross-sectional view of a second mold used for a main body part molding step, taken along the axial direction; 
         FIG. 8  is a cross-sectional view of the worm wheel of a second embodiment, taken along the diameter thereof; and 
         FIG. 9  is a cross-sectional view taken along a line IX-IX shown in  FIG. 8 . 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Hereinafter, a description is given of embodiments of the present invention. Note that technical elements common to embodiments are denoted by the same reference numerals, and descriptions thereof are omitted. Further, in the embodiments, worm wheels  1 ,  1 A are described as a rotary power transmission member, but the present invention is not limited thereto. 
     First Embodiment 
     As shown in  FIG. 1 , the worm wheel  1  is a component made of resin having a substantially disk shape about an axis O and has a circular through hole  11  at the center thereof. The through hole  11  is a part into which a rotating shaft (not shown) is fittingly inserted. Further, a plurality of teeth  21  are formed on the outer circumference surface of the worm wheel  1 , and each of the teeth  21  is twisted so as to be displaced circumferentially toward the axis O direction. Note that, the teeth  21  of the present embodiment are twisted circumferentially, but the shape of the teeth  21  is not limited thereto in the present invention, as long as the teeth  21  mesh with teeth of a worm gear (not shown) to transmit power. 
     Further, a recess  21   a , which is recessed radially inward, is formed at the center in the axial direction of each tooth  21  (see  FIG. 2 ), which causes the tooth  21  less likely to interfere (contact) with a shaft of the worm gear (not shown). Note that each tooth  21  of the present embodiment has the recess  21   a , but the tooth of the present invention may not have any recess. 
     The worm wheel  1  includes a ring-shaped main body part  10  having the through hole  11  formed therein, and a ring-shaped gear part  20  formed around the outer circumference of the main body part  10  and having the plurality of teeth  21  formed on the outer circumference surface thereof. 
     As shown in  FIG. 2 , the main body part  10  and the gear part  20  are joined to each other by an engaged part  12  formed in the outer circumference of the main body part  10  and an engaging part  22  formed in the inner circumference of the gear part  20 . 
     Hereinafter, the engaging part  22  and the engaged part  12  are described in this order. Further, in the following description, the axis O direction may be referred to as a thickness direction. 
     The engaging part  22  includes an extending part  23 , which extends radially inward from the center in the thickness direction of the inner circumference surface of the gear part  20 , and a plurality of recessed parts  24  formed with the extending part  23  partially recessed. 
     As shown in  FIG. 3 , the extending part  23  extends circumferentially along the inner circumference surface of the gear part  20  to have a ring shape. Further, the extending part  23  includes one side surface  23   a  which has a ring shape on one side in the thickness direction and the other side surface  23   b  (see  FIG. 2 ) which has a ring shape on the other side in the thickness direction. 
     The recessed part  24  is a rectangular recess as viewed from the thickness direction and is formed at a position radially outward away from an inner circumference edge  23   c  of the extending part  23 . Note that, in the present invention, the shape of the recessed part  24  as viewed from the thickness direction is not limited to rectangular, and may be circular, elliptical, or polygonal (including triangular). 
     As shown in  FIGS. 3 and 4 , each recessed part  24  includes a first recessed part  24   a  formed on the one side surface  23   a  and a second recessed part  24   b  formed on the other side surface  23   b  of the extending part  23 . As shown in  FIG. 4 , the first recessed parts  24   a  and second recessed parts  24   b  are circumferentially formed alternately so as not to align with each other in the thickness direction. 
     As shown in  FIG. 2 , the engaged part  12  includes a peripheral groove  13 , which is recessed radially inward from the center in the thickness direction of the outer circumference surface of the main body part  10 , and a plurality of protruding parts  14 , which protrude in the thickness direction from the inner surface of the peripheral groove  13 . 
     The peripheral groove  13  extends circumferentially into which the extending part  23  of the gear part  20  is inserted. Therefore, the gear part  20  engages with the main body part  10  in the thickness direction so that the main body part  10  and the gear part  20  do not move relative to (separate from) each other in the thickness direction. 
     Further, as shown in  FIG. 4 , each protruding part  14  includes a first protruding part  14   a , which is inserted in the first recessed part  24   a  to engage therewith, and a second protruding part  14   b , which is inserted in the second recessed part  24   b  to engage therewith. 
     Therefore, the protruding parts  14  and the recessed parts  24  are engaged with each other circumferentially and radially so that the main body part  10  and the gear part  20  are not rotated relative to each other. Further, the main body part  10  and the gear part  20  are not radially separated from each other. 
     Next, a description is given of resin materials forming the main body part  10  and the gear part  20 . 
     The main body part  10  and the gear part  20  are formed by injection molding to be described later, and are made of resin materials each having thermoplasticity. 
     The main body part  10  occupies a most area of the worm wheel  1 , and the rigidity of the main body part  10  is almost equivalent to the rigidity of the worm wheel  1 . Therefore, a resin material satisfying predetermined rigidity is selected as a resin material of the main body part  10 . 
     Meanwhile, as a resin material forming the gear part  20 , a resin material (referred to as a high melting point resin hereinafter), which melts at a higher temperature than the resin material forming the main body part  10 , in other words, a hard resin material is selected. From the above description, the gear part  20  has abrasion resistance while the rigidity of the worm wheel  1  (main body part  10 ) is maintained. 
     Specifically, examples of thermoplastic resin materials, which can be used for the main body part  10  and the gear part  20 , include polyamide (hereinafter referred to as “PA”) 6 (melting point is at 220° C.), PA12 (melting point is at 176° C.), PA11 (melting point is at 187° C.), PA612 (melting point is at 215° C.), PA610 (melting point is at 215° C.), PA410 (melting point is at 250° C.), PA66 (melting point is at 265° C.), and PA46 (melting point is at 290° C.) Note that, as described above, resin materials are selected among the resin materials described above, such that the resin material forming the gear part  20  has a higher melting point than that forming the main body part  10 . 
     Further, in the present invention, the resin material (hereinafter, referred to as a low melting point resin) forming the main body part  10  may be a fiber-reinforced resin containing reinforced fibers such as glass fibers and carbon fibers in the resin material described above, to ensure predetermined rigidity. 
     Next, a description is given of a method for manufacturing the worm wheel  1 . 
     The method for manufacturing the worm wheel  1  includes: a gear part molding step of molding the gear part  20 ; and a main body part molding step of molding the main body part  10  after the completion of the gear part molding step. 
     In the gear part molding step, a melted high melting point resin is injected in a cavity (space) S 1  defined in a first mold  30 , and the resin is cooled to mold the gear part  20 . 
     As shown in  FIG. 5 , the cavity S 1  of the first mold  30  is formed to have the same shape as the gear part  20 . The first mold  30  includes a first upper mold  31  defining the upper side of the cavity S 1 , a first lower mold  32  defining the lower side of the cavity S 1 , and a first radially outer mold  33  defining the outer circumference of the cavity S 1 . Thus, the teeth  21  of the gear part  20  are molded by the first radially outer mold  33 . 
     As shown in  FIG. 6A , the first radially outer mold  33  includes a plurality of split cores  34  which are split circumferentially. Each split core  34  includes a trapezoidal first protrusion  34   a  as viewed from the axis O direction to form a tooth trace. Further, each split core  34  is slidable radially outward. Still further, as shown in  FIG. 5 , each split core  34  is formed with a second protrusion  34   b , with which the recess  21   a  (see  FIG. 2 ) of the tooth  21  is formed. Note that the second protrusions  34   b  are not shown in  FIGS. 6A and 6B . 
     As shown in  FIG. 5 , the first mold  30  includes a first sprue  35 , a first runner  36 , and a first gate  37  through which the resin injected from an injection machine (not shown) flows into the cavity S 1 . 
     The first sprue  35  is a through hole which penetrates the first upper mold  31  vertically, and is on the axis O. The first runner  36  and the first gate  37  are defined by a recess in the lower surface of the first upper mold  31  and a recess in the upper surface of the first lower mold  32 . 
     The first runner  36  is continuous with the lower end of the first sprue  35 , has a circular shape as viewed from the axis O direction, and is defined on the inner circumference side of the cavity S 1 . 
     The first gate  37  forms a ring-shaped flow path, which communicates the outer circumference side of the first runner  36  with the inner circumference side of the cavity S 1 , in other words, a disk gate. Further, the first gate  37  is continuous with the inner circumference side of the cavity S 1 , more specifically, at the position where the extending part  23  of the gear part  20  is formed, which corresponds to the center of the inner circumference surface of the extending part  23 . 
     A description is given of the procedure of the gear part molding step. The high melting point resin is injected into the first sprue  35  from the injection machine (not shown). Accordingly, the high melting point resin flows through the first runner  36  and the first gate  37  into the cavity S 1 . Once the cavity S 1  is filled up with the high melting point resin, the injection by the injection machine is stopped and the resin is cooled. When the high melting point resin is hardened, the first upper mold  31  is moved upward, and as shown in  FIG. 6B , the split cores  34  are slid radially outward. Accordingly, the gear part  20  is formed and the gear part molding step is completed. 
     In the main body part molding step, the gear part  20  formed in the previous step is inserted into a second mold  40 , and the low melting point resin is injected into a cavity S 2  of the second mold  40  to mold the main body part  10 . 
     As shown in  FIG. 7 , the cavity S 2  of the second mold  40  is defined to have the same shape as the worm wheel  1 . The second mold  40  includes a second upper mold  41  defining the upper side of the cavity S 2 , a second lower mold  42  defining the lower side of the cavity S 2 , and a second radially outer mold  43  defining the outer circumference of the cavity S 2 . Note that the second radially outer mold  43  includes a plurality of split cores  44  which are slidable radially outward, as with the first radially outer mold  33 . 
     The second mold  40  includes a second sprue  45 , a second runner  46 , and a second gate  47  through which the resin injected from the injection machine (not shown) flows into the cavity S 2 . 
     The second sprue  45  is a through hole which penetrates the second upper mold  41  vertically, and is on the axis O. The second runner  46  and the second gate  47  are defined by a combination of a recess in the lower surface of the second upper mold  41  and a recess in the upper surface of the second lower mold  42 . 
     The second runner  46  is a circular space in planar view, which extends radially outward from the lower end of the second sprue  45  and is defined on the inner circumference side of the cavity S 2  (main body part  10 ). The second gate  47  is a ring-shaped flow path (disk gate), which communicates the second runner  46  with the cavity S 2 . Further, the second gate  47  is communicated with the inner circumference of the cavity S 2  at the center in the thickness direction of the inner circumference surface of the main body part  10 . 
     A description is given of the procedure of the main body part molding step. The gear part  20  is inserted on the outer circumference side of the cavity S 2  in the second mold  40 . Then, the low melting point resin is injected into the second sprue  45  from the injection machine (not shown). Accordingly, the low melting point resin flows through the second runner  46  and the second gate  47  into the cavity S 2 . Once the cavity S 2  is filled up with the low melting point resin, the injection by the injection machine is stopped and the resin is cooled. When the low melting point resin is hardened, the second upper mold  41  is moved upward, and the split cores  44  are slid radially outward. Thus, the main body part  10 , which is integrally molded with the gear part  20 , is molded. Finally, the worm wheel  1  is formed and the main body part molding step is completed. 
     Next, a description is given of advantageous effects of the first embodiment. 
     In the conventional art, when the gear part is molded, the main body part is inserted on the inner circumference side of the cavity. Therefore, the runner and gate are arranged on the side (in the axis O direction) of the gear part, and the runner and gate each have a bottomed cylindrical shape (see a two-dot chain line L in  FIG. 5 ). 
     Meanwhile, according to the present embodiment, the first runner  36  and first gate  37  of the first mold  30  to be used for molding the gear part  20  are defined on the inner circumference side of the cavity S 1 , and the first runner  36  and first gate  37  each have a substantially disk shape. Accordingly, the first runner  36  and first gate  37  of the present embodiment are reduced in volume and surface area than the runner and gate in the conventional art. 
     That is, according to the present embodiment, the resin injected from the injection machine is hard to be cooled in the first runner  36  and first gate  37  so that the quality of the molded product (gear part  20 ) is stabilized. Further, the resin remaining in the first runner  36  and first gate  37  is reduced so that a yield rate is improved. 
     Further, according to the present embodiment, the gear part  20  inserted into the second mold  40  is made of a high melting point resin. Therefore, even when the low melting point resin is injected into the cavity S 2  to mold the main body part  10  integrally with the gear part  20 , the gear part  20  is hard to be deformed. 
     Still further, when the teeth  21  of the gear part  20  are molded by the first upper mold  31  or the like which is moved vertically (along the axis O), the teeth  21  (tooth traces) may be twisted or the recess  21   a  may be formed in the teeth  21 . Therefore, the first upper mold  31  or the like may come into contact with the teeth  21  and the teeth  21  may be partly broken or deformed. Meanwhile, the split cores  34  of the first radially outer mold  33  and the split cores  44  of the second radially outer mold  43  in the present embodiment are separated radially outward, so that the first protrusions  34   a  and second protrusions  34   b  do not come into contact with the teeth  21  at the time of being separated. Accordingly, breakage or deformation of the teeth  21  can be avoided. 
     Yet further, according to the present embodiment, the main body part  10  and the gear part  20  are joined by the engaged part  12  and the engaging part  22 , which increases joining strength so as to be used for a long period of time. 
     Second Embodiment 
     Next, a description is given of a second embodiment. 
     As shown in  FIGS. 8 and 9 , a worm wheel  1 A of the second embodiment differs from the worm wheel  1  of the first embodiment in that the gear part  20  includes protruding parts  25  protruding from the extending part  23 , in place of the plurality of recessed parts  24  (see  FIGS. 2 to 4 ) formed to have the extending part  23  partially recessed. Further, the worm wheel  1 A of the second embodiment differs from the worm wheel  1  of the first embodiment in that the main body part  10  includes recessed parts  15  formed to have the inner surface of the peripheral groove  13  recessed in the thickness direction, in place of the plurality of protruding parts  14  (see  FIGS. 2 to 4 ) protruding from the inner surface of the peripheral groove  13 . Hereinafter, the worm wheel  1 A of the second embodiment is described, focusing on the differences from the first embodiment. 
     The protruding part  25  includes a first protruding part  25   a  protruding from the one side surface  23   a  and a second protruding part  25   b  protruding from the other side surface  23   b  of the extending part  23 . Further, the first protruding part  25   a  and the second protruding part  25   b  are arranged so as not to align with each other in the thickness direction. 
     The recessed part  15  includes a first recessed part  15   a  into which the first protruding part  25   a  is inserted to engage therewith, and a second recessed part  15   b  into which the second protruding part  25   b  is inserted to engage therewith. 
     Even with the configuration described above, the main body part  10  and the gear part  20  are integrally molded to avoid them from being rotated relative to each other. Further, the main body part  10  and the gear part  20  are not separated from each other in the radial direction. 
     Thus, according to the worm wheel  1 A of the second embodiment, the coupling strength between the main body part  10  and the gear part  20  is high so that the worm wheel  1 A can be used for a long period of time. 
     Third Embodiment 
     Next, a description is given of a third embodiment. 
     The third embodiment differs from the first mold of the first embodiment in that the first mold used in the gear part molding step is a spoke-shaped mold. Note that the first mold  30  of the first embodiment includes the first runner  36  and the first gate  37  having a circular shape as viewed from the axis O and is a so-called disk shape. Hereinafter, a description is given of details of the first mold used in the gear part molding step of the third embodiment. 
     Though not particularly shown, the first mold of the third embodiment includes a first sprue, first runners, and first gates as flow paths, through which a resin injected from the injection machine flows into the cavity. 
     The first sprue is a hole vertically penetrating a first upper mold, and the lower end thereof is defined on the inner circumference side of the cavity. Note that the first sprue is the same as the first sprue  35  of the first embodiment. 
     The first runners extend radially outward from the lower end of the first sprue. Further, a plurality of first runners are defined circumferentially about the lower end of the first sprue. 
     The first gates serve as flow paths to communicate the outer circumference of the first runners with the inner circumference of the cavity and are formed as many as the first runners. 
     From the description above, the first runners and first gates of the third embodiment form radial flow paths about the lower end of the first sprue, to supply the resin from the inner circumference side of the cavity. Therefore, the spoke-shaped first mold of the third embodiment has a smaller volume and surface area than the runner and gate of a conventional art. The resin injected from the injection machine is hard to be cooled in the first runners and first gates, and the quality of the molded product (gear part  20 ) is stabilized. Further, the amount of resin remaining in the first runners and first gates is reduced so that the yield rate is improved. 
     The embodiments have been described above, but the present invention is not limited thereto. 
     The present invention is intended to achieve that the resin injected at the time of molding the gear part  20  is hard to be cooled and the yield rate is improved. The present invention is not limited to examples shown in the embodiments, to have the configuration of the main body part  10  and the flow path of the second mold  40  for the resin. That is, the present invention may include the main body part  10  having a configuration in which a press-fitting collar to be press-fitted onto a shaft is provided on the radially innermost part. To avoid the press-fitting collar, the flow paths of the second mold  40  for the resin may have a pin gate shape, in place of a disk shape or a spoke shape as described above, and a plurality of gates may be formed above the cavity to supply the resin from above the cavity. 
     Further, the split core  34  shown in  FIGS. 6A and 6B  includes one first protrusion  34   a  to form a tooth trace. Therefore, the number of split cores  34  is the same as that of teeth  21  of the worm wheel  1 , but the split core  34  may be modified to include two first protrusions  34   a , for example, to reduce the number of the split cores  34 . 
     Still further, the first mold  30  of the embodiments includes the first radially outer mold  33  having a plurality of split cores  34 , and the second mold  40  includes the second radially outer mold  43  having a plurality of split cores  44 , but the present invention is not limited thereto. That is, if the teeth  21  of the rotary power transmission member (worm wheel  1 ) are not twisted circumferentially and the recess  21   a  is not formed in the tooth  21 , a mold may be used, which includes an upper mold (first upper mold  31 , second upper mold  41 ) and a lower mold (first lower mold  32 , second lower mold  42 ). 
     REFERENCE NUMERALS 
       1 ,  1 A: worm wheel,  10 : main body part,  12 : engaged part,  13 : peripheral groove,  14 : protruding part,  15 : recessed part,  20 : gear part,  21 : tooth,  22 : engaging part,  23 : extending part,  24 : recessed part,  25 : protruding part,  30 : first mold,  33 : first radially outer mold,  34 : split core,  35 : first sprue,  36 : first runner,  37 : first gate,  40 : second mold,  43 : second radially outer mold,  44 : split core,  45 : second sprue,  46 : second runner,  47 : second gate, S 1 : cavity, S 2 : cavity