Abstract:
A starter motor for an internal combustion engine has an inertia type pinion mechanism and an axial solenoid 34 which is arranged to prevent pump out of the pinion 48 during start up of the engine.

Description:
This application is a divisional of application No. 09/676,509, filed on Oct. 2, 2000, Pat. No. 6,466,116 the entire contents of which are hereby incorporated by reference and for which priority is claimed under 35 U.S.C. §120. 

   BACKGROUND OF THE INVENTION 
   The present invention relates to an inertia drive type starter motor for an internal combustion engine. 
   Inertia drive type starter motors rely on inertia of the pinion or clutch mechanism to move the pinion from a rest position to an engaged position against a spring force when the motor is switched on. Such motor drives have been used successfully but do suffer from false starts whereby the pinion is disengaged prematurely by sudden rotation of the engine being started which occurs not only when the motor starts but also when the engine misfires or fires but does not start. These false starts disengage the starter motor pinion requiring the starting sequence to be re-initiated. They can also suffer from bounce out or pump out which is a condition where the pinion oscillates along the shaft while engaging the engine ring gear and is a condition that can result in complete disengagement. 
   Thus a positive engagement mechanism for an inertia drive is desirable. Two such type drives are shown in U.S. Pat. Nos. 2,923,162 and 4,502,429. U.S. Pat. No. 4,502,429 shows a device which is very complex while U.S. Pat. No. 2,923,162 shows a device wherein the inertia drive is not assisted by the holding mechanism. 
   SUMMARY OF THE INVENTION 
   According to one aspect thereof, the present invention provides an electric starter for an internal combustion engine comprising: an electric motor having a housing and a rotatable armature shaft extending therethrough, the shaft having a helical spline portion; a pinion gear mounted for selectively engaging a ring gear of the engine; a clutch assembly for transmitting torque between the shaft and the pinion gear, the clutch assembly having a driving part and a driven part, the driving part having an internal helical spline portion engaging the helical spline portion of the shaft whereby relative rotary movement between the shaft and the driving part creates axial movement of the clutch assembly along the shaft, and the pinion gear being fixed for rotation with the driven part; and a solenoid for holding the pinion gear in engagement with the ring gear wherein the solenoid has a toroidal coil and a tubular plunger located about the shaft between the motor housing and clutch assembly, the tubular plunger having a radially extending flange at a first end which is arranged to be attracted to the radial housing wall toward the coil. 
   According to a second aspect, the present invention provides a solenoid comprising a housing; a cap fitted to the housing and defining an internal void, the housing and the cap each having a through hole defining therebetween a through passage having an axis; a toroidal coil fitted to the housing about the through passage; a bearing fitted to the through hole in the housing and having a through hole aligned coaxially with the through passage; and a plunger having a tubular body extending axially along the through passage and slidably retained in the through hole of the bearing, the plunger having a radially extending flange at a first end of the tubular body. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A preferred embodiment will now be described by way of example only with reference to the accompanying drawings, in which: 
       FIG. 1  depicts a starter motor according to a preferred embodiment of the present invention; 
       FIG. 2  is a sectional view of the motor of  FIG. 1 ; 
       FIG. 3  is an enlarged sectional view of a drive mechanism of  FIG. 2 ; 
       FIG. 4  is a view similar to  FIG. 3  with the drive mechanism in an alternate engaged position; and 
       FIG. 5  is an exploded view of a solenoid forming a part of the drive mechanism. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1  shows a starter for an internal combustion engine. The starter comprises an electric motor  12  having a driving shaft  14 , and a pinion mechanism. The pinion mechanism has a solenoid  34  that is mounted on an end plate  22  of the motor and a pinion  48  that is movable along the shaft  14 . 
     FIG. 2  is a longitudinal sectional view of the starter of FIG.  1 . The motor  12  is of the DC permanent magnet type. The motor  12  has a housing  18  supporting permanent magnets  20 . End plates  22  support bearings  24  in which the motor shaft  14  is journaled. The shaft supports a wound armature  26  and a commutator  28  fed by four conducting brushes  30 . Two brushes are connected to the single motor terminal  32  and the other two are connected to the housing  18  which acts as a ground terminal. 
   On the output end of the shaft  14 , outside the motor housing, is the pinion mechanism which is more clearly shown in  FIGS. 3 and 4 . The pinion mechanism comprises the pinion  48 , an overrunning clutch  40  and the solenoid  34 . The pinion  48  is moveable along the shaft  14  between a disengaged position as shown in FIG.  3  and an engaged position as shown in FIG.  4 . In the engaged position, the pinion engages the teeth of a ring gear for starting an internal combustion engine (not shown). 
   Disposed between the pinion  48  and the solenoid  34  is an overrunning clutch, ORC  40 , which is fitted to a helical spline  42  on the shaft  14 . The ORC has a driving part  44  which engages the spline  42  and a driven part  46  which is integral with the pinion  48 . The driving part and the driven part are connected together by a one way clutch mechanism  50  which allows the driven part  46  to turn with respect to the driving part  44  in one direction only. 
   The solenoid  34  is shown in exploded form in FIG.  5 . The solenoid  34  has a cap  60 , a plunger  38 , a coil  36 , a bearing  66  and a housing  68 . The housing  68  accommodates the coil  36  and has a slot  70  for a lead wire  72  of the coil. Lead wire  72  is directly connected to the motor terminal ( 32 ,  FIG. 2 ) so that the solenoid is energized with the motor. A rubber grommet  74  guides the lead wire  72  through the slot  70  and also seals the slot  72  against water and dust ingress. The other end of the coil (not shown) is soldered directly to the solenoid housing. The coil  36  is located about the bearing  66  and may be pressed onto the bearing  66  for support. One end of the bearing  66  is fitted to an axial hole passing through the solenoid housing  68 . The other end of the bearing  66  has a flange for supporting the coil  36  against axial movement. The plunger  38  has an axially extending tube portion  76  which slides in the bearing  66  and locates about the shaft  14 . A flange portion  78  extends radially from one end of the tube portion  76 . The cap  60  covers the space about the plunger  38  between the housing  68  and the end plate  22  of the motor. The cap is crimped over the housing to seal the solenoid. The solenoid is fixed to the motor by two screws passing through motor end plate  22  and screwed into the cover  60 . 
   When the solenoid is actuated, the magnetic field attracts the flange portion  78  to the radial wall of housing  68  toward coil  36 . In the disengaged position, the force on the plunger may not be very strong but in the engaged position, the flange  78  is adjacent the coil  36  and is held very strongly which is where the strength is needed. The plunger butts against the driving part  44  of the ORC allowing the ORC to rotate about the shaft with respect to the plunger. Alternatively, the plunger could be coupled or fixed to the ORC so that the plunger does rotate with the ORC, if desired. 
   Returning to  FIGS. 3 and 4 , a nut  52  is threaded onto the end of the shaft  14 . An anti-drift spring  54  extends between the pinion  48  and the nut  52  to bias the pinion  48  into the disengaged position. A washer  56  is provided between the spring  54  and the nut  52  to provide a seat for the spring  54 . At the other end of the spring, a sleeve or spacer  58  forms a seat and retainer for the spring  54  allowing the pinion  48  to rotate about the shaft  14  while compressing the spring  54  axially without significant torsional stress which may otherwise cause the spring  54  to bind on the shaft  14  or to become unwound affecting its spring properties. 
   When the motor  12  is turned on, the shaft  14  starts to rotate. Due to the inertia of the ORC  40 , it does not rotate initially as fast as the shaft  14  and is thus moved axially to the right by the helical splines  42  as the shaft  14  turns relative to the ORC  40 , against the urgings of the anti-drift spring  54 . At the end of travel, the ORC  40  has moved towards the end of the shaft  14  to the engaged position, as shown in  FIG. 4 , where the pinion  48  is, in use, engaged with teeth of a ring gear fitted to a flywheel of the engine being started (not shown). The anti-drift spring  54  is now compressed. As the motor is switched on, power is also supplied to the solenoid  34 , causing the plunger  38  to move to the right, axially with respect to the shaft, pressing against the ORC  40 , helping the inertia movement and resisting pump out or disengagement of the pinion  48  from the ring gear, thereby providing positive retention of the pinion  48  in the engaged position until the power to the starter is switched off. 
   Once the power is switched off, the solenoid  34  releases the plunger  38  allowing the ORC  40  to return to the disengaged position. Assuming that the engine has started at this time, then the pinion  48  which is engaged with the ring gear will be rotating faster than the motor shaft because of the ORC  40 . The ORC can now move axially under the influence of the anti-drift spring  54  by rotating about the shaft  14  on the helical splines  42 . 
   If the engine has not started, once the starter motor has stopped rotating, the pinion  48  will slide freely out of engagement with the ring gear under the influence of the anti-drift spring  54 . Thus the ORC  40  and pinion  48  return to the disengaged position, ready to try again. 
   While only the preferred embodiment has been described, various modifications will be apparent to persons skilled in the art and it is intended that all such modifications and variations form part of the invention as defined by the appended claims.