document: CROSS REFERENCE TO RELATED APPLICATIONS 
         [0001]    This non-provisional patent application claims priority under 35 U.S.C. §119(a) from Patent Application No. 200810142381.1 filed in The People&#39;s Republic of China on Aug. 15, 2008. 
         FIELD OF THE INVENTION 
         [0002]    This invention relates to a motor assembly, and in particular, to a motor assembly having a force transmission structure. 
         BACKGROUND OF THE INVENTION 
         [0003]    Usually, a window lift system for a vehicle window comprises a driving motor, a lift device for moving up or down the glass of the window, and a force transmission structure for transmitting rotation of the output shaft of the motor to the lift device. The transmission structure comprises a drive plate and a shaft coupled to the drive plate. The drive plate is connected to the output shaft of the motor via a gear train. The shaft is connected to the lift device via a pinion attached to an end of the shaft and meshed with a gear of the lift device. In operation, the motor drives the drive plate to rotate. The drive plate drivingly rotates the shaft to thereby cause the lift device to move the glass of the window up or down. 
           [0004]    Conventionally, the shaft is coupled to the drive plate via a cylindrical coupling end with two flat surfaces at opposite sides thereof fittingly received in a waist-shaped hole of the drive plate. Two opposite flat interfaces are formed between the coupling end of the shaft and the hole of the drive plate. In operation, two reverse forces are exerted on the two flat surfaces of the coupling end of the shaft, which will generate impact on the shaft and the drive plate to thereby generate vibration and noise. 
           [0005]    As such, there is a desire for an improved transition structure which can solve the above-mentioned problems. 
         SUMMARY OF THE INVENTION 
         [0006]    Accordingly, in one aspect thereof, the present invention provides a force transmission structure comprising: a drive plate having a mounting hole and a shaft fitted to the mounting hole for rotation with the drive plate, wherein the mounting hole has at least three sections interconnected with one another at a common area, the shaft has a toothed portion with at least three teeth fittingly received in the sections of the mounting hole such that the shaft is fixed to rotate with the drive plate. 
           [0007]    Preferably, the drive plate comprises a body and a coupling formed at the center of the body, the coupling is deeper than the body in the axial direction of the body, the mounting hole being formed in the coupling. 
           [0008]    Preferably, the coupling has buffer holes respectively located between adjacent sections. 
           [0009]    Preferably, the drive plate has a plurality of protrusions formed on a first side of the body and configured to engage with a driving member such that the driving member is able to drive the drive plate, the shaft further comprises a pinion configured to drive a driven member. 
           [0010]    Preferably, the drive plate further comprises a plurality of ribs formed at an opposite second side of the body. 
           [0011]    Preferably, the mounting hole and the toothed portion are Y-shaped. 
           [0012]    Preferably, the drive plate is made of a plastics material. 
           [0013]    According to a second aspect, the present invention provides a motor assembly comprising: a motor; a force transmission structure comprising a drive plate and a shaft; and a gear train connecting the motor to the drive plate for driving the drive plate; wherein the drive plate has a mounting hole with at least three sections interconnected with one another at a common area, the shaft has a toothed portion with at least three teeth fittingly received in the sections of the mounting hole of the drive plate such that the shaft is fixed to rotate with the drive plate. 
           [0014]    Preferably, the gear train comprises a worm driven by the motor, a worm gear meshed with the worm, and a damper attached to and rotatable with the worm gear, the drive plate being driven by the worm gear through the damper. 
           [0015]    Preferably, the worm comprises an inner ring, an outer ring, and a plurality of ribs extending from the inner ring to the outer ring, the damper being received in a space formed between the inner ring and the outer ring and having a plurality of first slots for fittingly receiving the ribs respectively. 
           [0016]    Preferably, the drive plate comprises a body and a plurality of protrusions formed at one side of the body, and the damper has a plurality of second slots engaging with the protrusions of the drive plate. 
           [0017]    Preferably, the protrusions are V-shaped, the width of the protrusions increasing gradually from the inner most portion towards the outer most portion in a radial direction of the body. 
           [0018]    Preferably, the drive plate further comprises a coupling formed at the center of the body, the coupling having a greater axial depth than the body, and the mounting hole being formed in the coupling. 
           [0019]    Preferably, the coupling has buffer holes respectively located between adjacent sections of the mounting hole. 
           [0020]    Preferably, the mounting hole and the toothed portion of the shaft are Y-shaped. 
           [0021]    Preferably, the drive plate is made of a plastics material, and the damper is made of rubber. 
           [0022]    Preferably, the shaft further comprises a pinion for driving a gear of a window lift system. 
           [0023]    Preferably, the shaft is held captive within the mounting hole by a circlip located within a groove in the distal end of the toothed portion. 
       
      
       
         BRIEF DESCRIPTION OF THE DRAWINGS 
           [0024]    A preferred embodiment of the invention will now be described, by way of example only, with reference to figures of the accompanying drawings. In the figures, identical structures, elements or parts that appear in more than one figure are generally labelled with a same reference numeral in all the figures in which they appear. Dimensions of components and features shown in the figures are generally chosen for convenience and clarity of presentation and are not necessarily shown to scale. The figures are listed below. 
             [0025]      FIG. 1  is a partial cross sectional view of a motor assembly in accordance with an embodiment of the present invention; 
             [0026]      FIG. 2  is an exploded view of the motor assembly of  FIG. 1 ; 
             [0027]      FIG. 3  is a plan view of a drive plate of the motor assembly of  FIG. 1 ; 
             [0028]      FIG. 4  is an isometric view of a shaft of the motor assembly of  FIG. 1 ; 
             [0029]      FIG. 5  is an assembled view of the drive plate of  FIG. 3  and the shaft of  FIG. 4 ; and 
             [0030]      FIGS. 6A and 6B  are schematic diagrams showing forces acting between the drive plate and the shaft. 
         
      
      
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
         [0031]      FIG. 1  shows a partial cross sectional view of a motor assembly in accordance with the preferred embodiment of the present invention. The motor assembly comprises a motor  10  and a gear train driven by the motor  10 . The gear train includes a force transmission structure. The gear train is contained in a gear housing  14  and a capstan  16 , which is a part of a window lift mechanism, is visible at the back. The capstan is driven through gears (not shown) by the motor assembly. 
           [0032]      FIG. 2  is an exploded view of the gear train, with the gear housing removed, to show the various components. The gear train comprises a worm  20  fitted to a motor shaft  12  driven by the motor  10 , a worm gear  30  which meshes with the worm  20 , a damper  40 , a drive plate  50  and a shaft  60 . The force transmission structure comprises the drive plate  50  and the shaft  60 . The worm  20  may be press fitted to the motor shaft  12 . Alternatively, the worm  20  may be formed integral with the motor shaft  12 . The worm gear  30  comprises an inner ring  31 , an outer ring  33 , and a plurality of ribs  32  radially extending from the inner ring to the outer ring. Teeth are formed at the outer circumferential surface of the outer ring  33 , for meshing with the worm  20 . The damper  40  is made of rubber material, has a through opening at the center thereof and has a plurality of first slots  41  and second slots  42  extending radially thereof. The slots  41 ,  42  are arranged alternately in the circumferential direction. 
           [0033]    Referring also to  FIG. 3 , the drive plate  50 , which may be made of an engineering plastics material, comprises a round body  52 , a coupling  54  formed at the center of the body  52 , a plurality of V-shaped protrusions  56  formed on one side of the body  52 , and a plurality of ribs  58  formed on the opposite side of the body  52 . The coupling  54  extends beyond the body  52  in opposite axial directions of the body  52  and therefore the coupling  54  has a greater depth or thickness than the body  52 . The coupling  54  has a Y-shaped mounting hole  55  at the center thereof, that is, the mounting hole  55  comprises three sections interconnected at the center thereof. Preferably, the coupling  54  further has a plurality of buffer holes  57 . In the embodiment, the buffer holes  57  are three blind holes which do not pass completely through the coupling  54  axially, and are evenly distributed in the circumferential direction, each one being located between adjacent sections of the Y-shaped hole  55 . In this embodiment, the protrusions  56  comprise three protrusions  56  evenly distributed in the circumferential direction, the width of the protrusions increasing gradually from the inner most portion towards the outer most portion in the radial direction of the body  52 . The central line of each protrusion  56  extends radially through the center of the body  52 . The protrusions  56  are shaped and sized to fit the second slots  42  of the damper  40 . 
           [0034]    Referring to  FIG. 4 , the shaft  60 , which is the output shaft of the gearbox in the preferred embodiment, comprises a round portion  61 , a toothed portion  62  formed at one end of the round portion, and a pinion  64  formed at the other end of the round portion. The toothed portion  62  has a Y-shaped cross section and comprises three teeth evenly distributed in a circumferential direction of the shaft  60 . The shape and size of the teeth of the toothed portion  62  conform to that of the mounting hole  55  of the drive plate  50 . Preferably, the shaft  60  is made of low alloy steel. Alternatively, the shaft  60  may be made of other metal material. The pinion  64  is configured to couple with a gear, such as a gear train of a lift mechanism of a window lift system. 
           [0035]    Referring to  FIGS. 1 and 5 , when assembled, the damper  40  is located in a spaced formed between the inner ring  31  and outer ring  33  of the worm  30  and the ribs  32  of the worm  30  are received in the first slots  41  of the damper  40 . The protrusions  56  of the drive plate  50  are respectively, interferentially and fittingly received in the second slots  42  of the damper  40 . Thus, the drive plate  50  is rotated by the damper  40  and the worm gear  30  when the worm  20  drives the worm gear  30 . 
           [0036]    The Y-shaped toothed portion  62  of the shaft  60  extends through the inner ring  31  of the worm gear  30  to be fitted in the Y-shaped mounting hole  55  of the drive plate  50 . The free end of the toothed portion  62  of the shaft  60  extends beyond the coupling  54 . A circlip  70  is fitted in a slot  66  formed at the free end of the toothed portion  62  to prevent the toothed portion  62  escaping from the mounting hole  55 . In operation, the motor  10  rotates the motor shaft  12 , which rotates the worm  20 , which drives the worm gear  30 , which rotates the drive plate  50  via the damper  40 , and thus rotates the shaft  60 . The drive plate  50  drives the shaft  60  to rotate by the Y-shaped mounting hole  55  of the drive plate mating with the Y-shaped toothed portion  62  of the shaft  60 . Consequently, the pinion  64  drives the capstan  16  via one or more gears (not shown) of the window lift system to thereby raise up or lower down the glass of the window. The window lift system may have a wire which is wound about the capstan to raise or lower the glass 
           [0037]    Referring to  FIGS. 6A and 6B , in the embodiment of the present invention, when the shaft  60  is rotated by the drive plate  50 , three equal forces A, B, C from the coupling  54  are exerted on the three teeth of the toothed portion  62  of the shaft  60  respectively. These three forces A, B, C exerting on the three teeth of the toothed portion  62  constitute a triangle as shown in  FIG. 6B . Therefore, the shaft  60  is rotated stably to thereby move up and/or down the glass of the window lift system quietly. Furthermore, the contact area between the teeth of the shaft  60  and the coupling  54  of the drive plate  50  is greater than that in the traditional design, which results in the connection between coupler and the shaft being able to withstand a greater torque. Moreover, the drive plate  50  is ideally made of an engineering plastics material which has good strength and resistance to impact and can absorb vibration, which is helpful to reduce the noise generated by the gear train as well. The buffer holes  57  aid molding of the drive plate by providing relief when the plastics material is cooling in the mould to reduce distortion of the mounting hole  55 . 
           [0038]    In the description and claims of the present application, each of the verbs “comprise”, “include”, “contain” and “have”, and variations thereof, are used in an inclusive sense, to specify the presence of the stated item but not to exclude the presence of additional items. 
           [0039]    Although the invention is described with reference to one or more preferred embodiments, it should be appreciated by those skilled in the art that various modifications are possible. Therefore, the scope of the invention is to be determined by reference to the claims that follow.
summary: A motor assembly includes a motor, a force transmission structure comprising a drive plate and a shaft, and a gear train connecting the motor to the drive plate for transmitting rotation of the motor to the drive plate. The drive plate has a mounting hole with at least three sections interconnected with one another at a common area, the shaft has a toothed portion with at least three teeth fittingly received in the sections of the mounting hole of the drive plate such that the shaft is rotated with the drive plate.