Abstract:
The invention relates to a worm wheel for use in a reduction gear, for example, and to a machine and a method for forming the same. The worm wheel includes a tooth having a tooth crest. At least the teeth are formed by injection molding a synthetic resin. The tooth crest includes a trace formed along a parting line as a result of the injection molding process. The trace does not exist on a surface of action of the tooth. The invention is suitable for applications demanding high contact gear accuracies.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
         [0001]    This application claims the benefit under 35 U.S.C. § 119 of Japanese Patent Application Nos.2000-398469 and 2001-349162, the abstracts of disclosure of which are incorporated herein by reference.  
         BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to a worm wheel for use in reduction gears and the like as well as to a machine and a method for forming the same.  
           [0004]    2. Description of Related Arts  
           [0005]    In general, it is very difficult to form a worm wheel from a synthetic resin because the worm wheel, which is assembled with a worm shaft in a worm gear mechanism, has a lead angle at its teeth and is formed with recessed surfaces at bottom lands.  
           [0006]    Hence, it is a general practice to use a hobbing or milling machine for shaping the worm wheel.  
           [0007]    The fabrication of the worm wheel using a machine tool is costly because it involves a very difficult operation for transforming a gear shape into a shape to be hobbed. In addition, the hobbed piece requires deburring.  
           [0008]    If the worm wheel is formed from the synthetic resin, there may be another problem that the presence of a parting line on a surface of action of the tooth will affect contact gear accuracies.  
           [0009]    It is an object of the invention to provide a worm wheel formed from the synthetic resin at low costs and featuring high contact gear accuracies as well as to provide a method and machine for forming the same.  
         SUMMARY OF THE INVENTION  
         [0010]    In one preferred aspect of the invention for achieving the above object, there is provided a worm wheel comprising a tooth having a tooth crest, and a bottom land formed at a bottom of a tooth space between adjoining teeth, the bottom land including a recessed surface, wherein at least the teeth are formed by injection molding a synthetic resin, and wherein the tooth crest includes a trace formed along a parting line as a result of the injection molding process.  
           [0011]    The invention achieves high contact gear accuracies because the trace of the parting line does not exist on the surface of action of the tooth. The invention accomplishes a lower material cost than the method of forming the worm wheel by cutting and also negates the need for deburring which is necessary in the cutting process. This leads to the reduction of the manufacturing costs. The invention further offers an advantage of increasing the wear resistance of the worm wheel by virtue of a close-grained skin layer formed on the surface of the formed teeth.  
           [0012]    In another preferred aspect of the invention, there is provided a forming machine comprising a stationary mold, a movable mold movably provided relative to the stationary mold, a stationary mold plate disposed at the stationary mold, a movable mold plate disposed at the movable mold, recesses formed in opposite surfaces of the stationary mold plate and the movable mold plate, a core disposed in the recess of the movable mold plate, a plurality of slide cores allowed to move radially of the core as arranged radially about the core, diameter decreasing means for simultaneously moving the slide cores radially inwardly relative to the core at closure of the movable mold, and diameter increasing means for simultaneously moving the slide cores radially outwardly relative to the core at opening of the movable mold. Each slide core includes a block shaped like a fan in plan, the block including a tooth crest forming face and a tooth-space forming projection formed centrally of a width of the tooth crest forming face. During the closure of the mold, adjoining slide cores are in intimate contact at a respective lateral side of their blocks while defining the parting line between their tooth crest forming faces.  
           [0013]    According to this embodiment, the forming step is carried out with the mold closed in a manner that the plural slide cores intimately contact each other at the lateral sides of their blocks. Hence, the parting line formed in correspondence to a clearance between adjoining slide cores exists on the tooth crest of the tooth of the formed worm wheel but not on the surface of action of the tooth. Therefore, the contact gear accuracies are not decreased. The worm wheel featuring high contact gear accuracies can be formed from the synthetic resin at low manufacturing costs. In addition, the formed article need not be forcibly removed because the diameter defined by the slide cores is increased when the mold is opened. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    [0014]FIG. 1 is a vertically-sectioned front view of a forming machine according to the invention;  
         [0015]    [0015]FIG. 2 is a vertically-sectioned side view of the machine of FIG. 1;  
         [0016]    [0016]FIG. 3 is a plan view of a movable mold shown in FIG. 1;  
         [0017]    [0017]FIG. 4 is an enlarged plan view of slide cores shown in FIG. 3;  
         [0018]    [0018]FIG. 5 is a sectional view showing the slide cores moved away from a worm wheel staying on the movable mold;  
         [0019]    [0019]FIG. 6 is a sectional view showing the worm wheel being ejected from the mold;  
         [0020]    [0020]FIG. 7A is a perspective view of the slide core whereas  
         [0021]    [0021]FIG. 7B is a front view of the slide core;  
         [0022]    [0022]FIG. 8 is a schematic perspective view of the formed worm wheel; and  
         [0023]    [0023]FIG. 9A is a front view of a tooth crest whereas FIG. 9B is an enlarged schematic sectional view taken on the line  9 B- 9 B in FIG. 9A. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0024]    A preferred embodiment of the invention will hereinbelow be described with reference to the accompanying drawings. FIGS.  1  to  4  illustrates a worm wheel forming machine according to one embodiment of the invention whereas FIGS. 5 and 6 shows an opened mold.  
         [0025]    Now referring to FIGS. 5 and 6, a worm wheel  40  according to the embodiment comprises a core  41 , and a synthetic resin member  42  with the core  41  molded therein. The core  41  possesses a boss  43  in an inside diameter portion thereof, and an annular flange  44  at an outside diameter portion thereof.  
         [0026]    As shown in FIG. 8, the synthetic resin member  42  defining an outer periphery of the worm wheel  40  is formed with a plurality of teeth  45  in its outer periphery, the tooth having a lead angle. As shown in FIG. 6, a bottom land  47  defining a bottom of a tooth space between the teeth  45 ,  45  includes a recessed surface  47   a . Now referring to FIGS. 8 and 9A, there exists a trace  51  of a parting line  49  on a tooth crest  48  of each tooth  45 , the trace  51  extending longitudinally of the tooth crest  48 . Indicated at  50  is a projection formed in a sprue  5 , which will be described hereinlater. The trace  51  may be exemplified by a projection of a chevron-shaped section as shown in FIG. 9A or a linearly extended step.  
         [0027]    As shown in FIGS. 1 and 2, a forming machine comprises a stationary mold  1  and a movable mold  10  adapted to move relative to the stationary mold  1 . The stationary mold  1  possesses a stationary-side mounting plate  2 , to which a stationary mold plate  3  is fixed.  
         [0028]    A sprue bush  4  is fixed to place centrally of the stationary mold plate  3 , whereas the sprue  5  is formed in the sprue bush  4 . The stationary mold plate  3  is formed with a circular recess  6  in a bottom surface thereof, whereas a diameter decreasing taper ring  7  is fixed to place in the recess  6 . The taper ring  7  has a tapered surface  7   a  defined by its inside surface.  
         [0029]    The movable mold  10  possesses a movable-side mounting plate  11 , which is provided with a pair of spacer blocks  12  at laterally opposite ends thereof, the spacer blocks  12  supporting a movable mold plate  13 . The spacer blocks  12  and the movable mold plate  13  are combined together by tightening bolts  14 . The movable mold plate  13  rests on a plurality of supporters  15  at a center of its bottom surface, the supporters  15  fixed to an upper surface of the movable-side mounting plate  11 .  
         [0030]    The movable mold plate  13  is formed with a circular recess  16  centrally of an upper surface thereof, whereas a core  17  is fixed to place centrally of a bottom surface of the recess  16 . The core  17  is divided into an upper part and a lower part. An upper core  17   a  has a positioning core  17   b  seated thereon for retaining the core  41  of the worm wheel  40  in a positioned state as shown in FIG. 6.  
         [0031]    When the movable mold  10  is closed, the core  17  defines a space between itself and a bottom surface of the sprue bush  4  of the stationary mold  1 , the space corresponding to a face width of the worm wheel  40  shown in FIG. 8.  
         [0032]    A guide plate  18  formed of an annular plate is disposed on the bottom surface of the recess  16  formed in the movable mold plate  13 . A plurality of slide cores  19  rest on the guide plate  18  as arranged radially about the core  17 .  
         [0033]    As shown in FIGS. 4 and 7, the slide core  19  includes a forming portion  20  at an upper part thereof and a guide portion  21  at a lower part thereof. The guide portion is formed in a uniform thickness along its length. The guide portion  21  is adapted to move radially of the recess  16  as carried on the guide plate  18 .  
         [0034]    It is noted that the guide plate  18  shown in FIG. 1 may be formed with a guide groove in its upper surface for slidably supporting the guided portions  21  of the slide cores  19 .  
         [0035]    The forming portion  20  includes a projection  22  at an inside upper part thereof, the projection resting on an upper surface of the upper core  17   a . The projection  22  is formed with a tooth crest forming face  23  at its distal end, whereas a tooth-space forming projection  24  projects centrally of a width of the tooth crest forming face  23 .  
         [0036]    The forming portion  20  is inclined toward one lateral side relative to the guided portion  21  therebelow at an angle B which is equal to a lead angle A of the tooth  45  of the worm wheel  40  shown in FIG. 8.  
         [0037]    The forming portion  20  has a fan-like shape in plan and is formed with a tapered surface  25  at a rear upper part thereof. The tapered surface  25  has a taper angle equal to that of the tapered surface  7   a  of the taper ring  7  mounted in the stationary mold plate  3 .  
         [0038]    When the movable mold  10  is closed, the slide core  19  contacts the tapered surface  7   a  of the taper ring  7  at the tapered surface  25  formed on its outside upper part, so as to be moved inwardly. The inward movement of the slide core brings its lateral sides  26  into intimate contact with those  26  of the adjoining slide cores  19  while a cavity  27  for forming the worm wheel  40  is formed between the bottom surface of the recess  6  of the stationary mold plate  3  and the upper surface of the upper core  17   a , as shown in FIG. 1.  
         [0039]    As seen in FIGS. 1 and 2, a slide holder  28  is fixed to an aperture of the recess  16  of the movable mold plate  13 . The slide holder  28  has an annular shape and presents its inner periphery to the upper surface of the guide portion  21  of the slide core  19  thereby preventing the slide core  19  from disengaging from the recess  16 .  
         [0040]    The slide cores  19  are biased outwardly by means of a spring  29  disposed between itself and the upper core  17   a.    
         [0041]    A diameter increasing taper ring  30  is fitted about the core  17  fixed to the movable mold plate  13 . The taper ring  30  is formed with a tapered surface  30   a  on its outer periphery, whereas the slide core  19  is formed with a tapered surface  31  at an inside lower part thereof in conformity with the tapered surface  30   a.    
         [0042]    The taper ring  30  is connected with upper ends of plural rods  32 . Each rod  32  slidably extends through the movable mold plate  13  and has its lower end coupled to a push plate  33  interposed between the pair of spacer blocks  12 . The push plate  33  is connected with a push-out jig  34 , which is connected to a cylinder not shown. Thus, activating the cylinder causes the diameter increasing taper ring  30  to move up or down. When the taper ring  30  moves up, the tapered surface  30   a  thereof presses against the tapered surfaces  31  of the slide cores  19 .  
         [0043]    Disposed above the push plate  33  is an ejector plate  35 , to which a plurality of ejector pins  36  are fixed at their lower ends. The ejector pins  36  slidably extend through the movable mold plate  13  and the core  17 .  
         [0044]    The ejector plate  35  assumes a lower position at which the ejector plate rests on a step  12   a  formed on an inner side of each spacer block  12 . The ejector plate  35  is adapted to move up as pushed by the rising push plate  33 .  
         [0045]    Referring to FIG. 2, a guide pin  37  vertically movably supports the push plate  33  and the ejector plate  35 . Indicated at  38  is a shock-absorbing spring for preventing the ejector plate  35  from impactively bumping against the steps  12   a  or the movable mold plate  13 .  
         [0046]    The forming machine according to the embodiment is constructed as described above. When the worm wheel  40  of FIG. 8 is formed, the core  41  of FIG. 6 is set on the positioning core  17   b  of the movable mold  10  and thereafter the movable mold  10  is closed.  
         [0047]    When the movable mold  10  is closed, the outside tapered surfaces  25  of the plural slide cores  19  contact the tapered surface  7   a  of the diameter decreasing taper ring  7  disposed in the stationary mold  1 , thereby being simultaneously moved inwardly. Thus, the slide cores bring their lateral sides  26  into intimate contact with those of the adjoining slide cores, as shown in FIG. 4. On the other hand, the inward movement of the slide cores  19  results in the formation of the cavity  27  between the sprue bush  4  of the stationary mold  1  and the core  17  of the movable mold  10 .  
         [0048]    Then, the worm wheel  40  shown in FIG. 8 may be formed by pressurizing a molten resin into the cavity  27  through the sprue  5  formed through the sprue bush  4 .  
         [0049]    In this process, the tooth crest forming face  23  of one slide core  19  cooperates with the tooth crest forming face  23  of the adjoining slide core  19  to form the tooth crest  48  of the tooth  45  of the worm wheel  40  shown in FIG. 8. This results in the formation of the parting line  49  of FIG. 1 along a joint between the tooth crest forming faces  23  of the adjoining slide cores  19 . On the other hand, the tooth-space forming projection  24  forms the overall inside surface of a tooth space  46 .  
         [0050]    When the movable mold  10  is opened after the forming of the worm wheel  40 , the formed worm wheel  40  remains on the movable mold  10 .  
         [0051]    When the push-out jig  34  is raised after the opening of the movable mold  10 , the diameter increasing taper ring  30  is raised along with the push plate  33  so that the tapered surface  30   a  on the outer periphery of the tapering  30  presses against the tapered surfaces  31  at the inside lower parts of the slide cores  19 . The pressure causes the plural slide cores  19  to move outwardly at a time, so that the projections  22  at the distal ends of the slide cores  19  are separated from the outer periphery of the worm wheel  40 , as shown in FIG. 5. At this time, the slide cores  19  move diametrally outwardly of the worm wheel  40  so that the tooth-space forming projections  24  are smoothly moved away from the tooth spaces  46  of the worm wheel  40 .  
         [0052]    When further raised, the push plate  33  presses against the ejector plate  35  which, in turn, is raised by the pressure. Hence, the ejector pins  36  connected with the ejector plate  35  push up the worm wheel  40 , thereby removing the worm wheel  40  from the movable mold  40 , as shown in FIG. 6.  
         [0053]    According to the embodiment of the invention, the trace  51  of the parting line  49  exists on the tooth crest  48  of the tooth  45  but not on the surface of action thereof and hence, the contact gear accuracies are increased.  
         [0054]    Although the conventional method of hobbing the gear requires the machining allowance, the method of molding a resin material into the gear shape does not require such an allowance. Accordingly, the material cost is decreased. On the other hand, the cutting process must be followed by a deburring step. However, the method of the embodiment negates the need for the deburring step. This results in a notable reduction of manufacturing costs.  
         [0055]    The hob machining process involves quite a cumbersome fabrication of a hob. According to the embodiment of the invention wherein the gear shape is obtained by molding, the mold can be readily fabricated by directly inputting three-dimensional CAD data on the gear shape into a machine such as a machining center. This not only results in an easy and less costly fabrication of the worm wheel  40  having high precisions, but also provides a greater freedom of configuring the gear shape.  
         [0056]    Furthermore, the method of molding the worm wheel achieves an increased wear resistance of the worm wheel because a close-grained skin layer is formed on the surface of the formed teeth  45 .  
         [0057]    Particularly where the gear shape is formed by cutting an article formed from a resin material containing a reinforcing fiber, a fear exists that the reinforcing fiber may project from the tooth surface, decreasing the strength or fatigue strength of the worm wheel. According to the embodiment of the invention wherein the skin layer is formed as described above, the strength or fatigue strength of the worm wheel is increased. The embodiment of the invention further increases the strength or fatigue strength of the worm wheel because the reinforcing fiber is oriented along the surface of the teeth.  
         [0058]    Although the embodiment of the invention has been described by way of example of the worm wheel  40  comprising the synthetic resin member  42  with the core  41  molded therein, the core  41  may be dispensed with. In this case, the positioning core  17   b  is not required whereas the upper surface of the upper core  17   a  has a shape in conformity with a shape of a side surface of the worm wheel  40  to be formed.  
         [0059]    While the invention has been described in detail with respect to specific embodiments thereof, it will be appreciated that those skilled in the art, upon attaining an understanding of the foregoing, may readily conceive of alterations to, variations of, and equivalents to these embodiments. Accordingly, the scope of the present invention should be assessed as that of the appended claims and any equivalents thereto.