Abstract:
A drive pinion assembly  10  for a starter motor has a drive plate  12  adapted to be driven by a shaft of the motor. The drive plate has axially extending projections which co-operate with corresponding projections on a pinion  14.  The projections are separated circumferentially by fingers of a resiliently compressible member to reduce impact loading on the pinion  14  as it engages teeth of a ring gear.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field  
           [0002]    This invention relates to electric starter motors and in particular, to the pinion assembly of the starter motor.  
           [0003]    2. Prior Art  
           [0004]    A starter motor is an electric motor which is used to turn an internal combustion engine to start it. The starter motor has a pinion arranged to engage a ring gear fitted to the engine and fitted to its drive shaft in such a way that when the starter motor is turned off, the pinion is disengaged from the ring gear. When the starter motor is turned on, the pinion moves along the shaft and engages the ring gear to turn the engine. The pinion may be moved by a solenoid or by helical splines on the shaft effected by inertia.  
           [0005]    In either arrangement, when the pinion strikes the ring gear, there is considerable impact. This impact causes wear on the ring gear and on the pinion eventually leading to the failure of the starter motor. Hence, there is a need to reduce the impact loading on the pinion so as to increase the life expectancy of the starter motor.  
         SUMMARY OF THE INVENTION  
         [0006]    The present invention reduces the impact loading on the pinion by providing a cushioning between the pinion and the drive plate.  
           [0007]    Accordingly, the present invention provides a cushioned pinion for use with a starter motor, the pinion comprising:  
           [0008]    a first part adapted for being driven in connection with a drive shaft of the starter motor,  
           [0009]    a second part having gear teeth adapted to- engage a ring gear of an internal combustion engine, and  
           [0010]    a third part comprising energy absorbing material disposed between the first and second parts,  
           [0011]    wherein the first and second parts have axially extending projections which are circumferentially offset and engage the energy absorbing material therebetween so as to allow only slight relative motion therebetween about a common axis.  
           [0012]    Preferably, at least one of the first and second parts has a radially inner barrier for containing radially inward movement of the third part during compression.  
           [0013]    Preferably, at least one of the first and second parts has a radially outer barrier for containing radially outward movement of the third part during compression.  
           [0014]    Preferably, the third part is a disc with cutouts receiving the projections of the first and second parts.  
           [0015]    Alternatively, the third part is an annular ring with radially extending fingers located between and separating the projections of the first part and the projections of the second part.  
           [0016]    Preferably, the third part substantially fills the gaps between the projections of the first and second parts.  
           [0017]    Preferably, the energy absorbing material is rubber.  
           [0018]    The present invention also provides a starter motor for an internal combustion engine comprising: an electric motor having a housing supporting a stator, an output shaft, a wound rotor fitted to the shaft and located to interact with the stator, a pinion assembly mounted on the shaft, engagement means for moving the pinion assembly along the shaft between an engaged position where in use, the pinion assembly engages a ring gear of the engine and a disengaged position, wherein the pinion assembly has a first part coupled to the shaft to be driven thereby a second part having pinion teeth for engaging the ring gear and a third part of compressible material disposed between the first and second parts, wherein the first and second parts have axially extending projections which are axially aligned and circumferentially spaced whereby the second part is driven by the first part through the third part. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]    Two preferred examples of a cushioned pinion drive will now be described by way of example only, in which:  
         [0020]    [0020]FIG. 1 is a side view of a pinion drive according to a first embodiment;  
         [0021]    [0021]FIG. 2 is an exploded view of the pinion of FIG. 1; and  
         [0022]    [0022]FIG. 3 is an exploded view of a pinion according to a second embodiment. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0023]    Starter motors are well known and will not be described in detail here for the sake of brevity. The pinion drive shown in FIGS. 1 and 2 is designed for use with a clutchless inertia drive type starter motor in which the pinion drive is adapted to directly engage helical splines on the starter motor shaft and is moved along the shaft against the force of a return spring due to inertia when the starter motor is turned on.  
         [0024]    The drive pinion  10  shown in FIGS. 1 and 2 comprises three parts, a first part being the drive plate  12 , a second part being the pinion  14  and a third part being the energy absorbing member  16 .  
         [0025]    The drive plate  12  has three main sections, a collar  18 , a flange  20  formed at one end of the collar and projections  22  extending axially from a face of the flange remote from the collar. In the embodiment of FIG. 1 and  2 , the drive plate  12  has only two projections  22 . The collar  18  has a central bore  24  with an inner surface arranged to engage a helical spline of the motor shaft.  
         [0026]    The pinion  14  is of a similar construction to the drive plate  12 . It has a collar  26  with a central opening  28 . The inner surface of the collar  26  is adapted to slide along the shaft and does not engage the splines on the shaft. The radially outer surface of the collar  26  has a plurality of gear teeth  30  adapted to engage gear teeth of a ring gear or flywheel of the engine. A flange  32  is formed at one end of the collar  26  and projections  34  extend axially from a face of the flange  32  remote from the collar  26 . In this example, there are two projections  34 .  
         [0027]    The energy absorbing member  16  is a rubber block or disc in the form of an annular ring with four equally spaced radially extending fingers  36 . The thickness of the rubber disc is greater than the length of the projections  22 ,  34  on the drive plate  12  and the pinion  14 . The fingers  36  are interposed between and separate the projections  22 ,  34 . Thus, the rubber disc allows cushioned or restrained relative rotational movement between the drive plate  12  and pinion  14 . It also allows the two parts to be pressed axially together.  
         [0028]    In use, the pinion drive assembly is mounted on the shaft of the starter motor and held together by a return spring, the drive plate being engaged with helical splines on the shaft. When the starter motor is turned on, the shaft rotates but due to the inertia of the drive pinion assembly  10 , the assembly does not initially rotate with the shaft but is moved axially along the shaft by the helical splines. The pinion drive assembly  10 , by this axial movement engages the teeth of the ring gear causing the splines to push the pinion drive assembly  10  axially to the fully engaged position where the motor now has to rotate the pinion with the shaft, causing the engine to rotate for starting.  
         [0029]    As can be expected, there is considerable force applied axially and rotationally to the drive plate  12  and these forces are transmitted to the pinion  14  by the rubber member  16 . The axial force results in the rubber member  16  being squashed axially allowing some absorption of the axial force applied to the pinion  14  to reduce the impact loading on the pinion and ring gear. Torque is transmitted from the drive plate  12  to the pinion  14  by squeezing the fingers  36  of the rubber member  16  between the projections  22  on the drive plate  12  and the projections  34  on the pinion  14 . This compression of the fingers  36  reduces the impact force between the teeth  30  of the pinion  14  and the teeth of the ring gear. The compression also allows a slight rotation between the drive plate  12  and the pinion  14  allowing for better engagement before full torque is applied between the pinion and the ring gear.  
         [0030]    [0030]FIG. 3 illustrates a second embodiment shown in exploded form. This embodiment has three projections  22  on the drive plate flange  20  and three projections  34  on the pinion flange  32  and the energy absorbing member  16  has six fingers  36 . The drive plate flange  20  also has an inner tubular extension forming an inner skirt  38  and the pinion flange  32  has an outer tubular projection forming an outer skirt  40 . The two skirts  38 ,  40  form compression barriers for limiting the deformation of the energy absorbing member  16 . The outer skirt  40  also provides a degree of sealing preventing contaminants from entering the space between the drive plate  12  and the pinion  14 .  
         [0031]    Although a simple starter motor has been used as the basis for this example, the pinion device could be used with more complicated designs including positive engagement, inertia assisted positive engagement arrangements and with models incorporating an overrunning clutch assembly. When used with a clutch assembly, the first part is connected to the splines of the motor shaft via the clutch assembly.