PINION  DRIVE  ASSEMBLY—The  pinion gear, pinion drive mechanism, and solenoid. FIELD FRAME—The center housing that holds the field coils and pole shoes. DRIVE   END   FRAME—The   end   housing around the pinion gear, which has a bushing for the armature shaft. ARMATURE   ASSEMBLY.—The armature assembly consists of an armature shaft, armature core, commutator,  and  armature  windings. The  armature  shaft  supports  the  armature assembly  as  it  spins  inside  the  starter  housing.  The armature  core  is  made  of  iron  and  holds  the  armature windings  in  place.  The  iron  increases  the  magnetic field strength of the windings. The  commutator  serves  as  a  sliding  electrical connection   between   the   motor   windings   and   the brushes  and  is  mounted  on  one  end  of  the  armature shaft.  The  commutator  has  many  segments  that  are insulated from each other. As the windings rotate away from  the  pole  shoe  (piece),  the  commutator  segments change  the  electrical  connection  between  the  brushes and  the  windings.  This  action  reverses  the  magnetic field   around   the   windings.   The   constant   changing electrical  connection  at  the  windings  keeps  the  motor spinning. C O M M U T A T O R    E N D    F R A M E .—The commutator  end  frame  houses  the  brushes,  the  brush springs, and the armature shaft bushing. The brushes ride on top of the commutator. They slide on the commutator to carry battery current to the spinning  windings.  The  springs  force  the  brushes  to maintain   contact   with   the   commutator   as   it   spins, thereby  no  power  interruptions  occurs.  The  armature shaft   bushing   supports   the   commutator   end   of   the armature  shaft. PINION  DRIVE  ASSEMBLY.—The pinion drive  assembly  includes  the  pinion  gear,  the  pinion drive  mechanism,  and  solenoid.  There  are  two  ways that   a   starting   motor   can   engage   the   pinion assembly—(1) with a moveable pole shoe that engages the pinion gear and (2) with a solenoid and shift fork that engages the pinion gear. The  pinion  gear  is  a  small  gear  on  the  armature shaft that engages the ring gear on the flywheel. Most starter pinion gears are made as part of a pinion drive mechanism.  The  pinion  drive  mechanism  slides  over one end of the starter armature shaft. The pinion drive mechanism  found  on  starting  motors  that  you  will encounter   are   of   three   designs—Bendix   drive, overrunning clutch, and Dyer drive. The  BENDIX  DRIVE  (fig.  2-38)  relies  on  the principle  of  inertia  to  cause  the  pinion  gear  to  mesh with  the  ring  gear.  When  the  starting  motor  is  not operating, the pinion gear is out of mesh and entirely away from the ring gear. When the ignition switch is engaged,  the  total  battery  voltage  is  applied  to  the starting motor, and the armature immediately starts to rotate at high speed. The pinion, being weighted on one side and having internal  screw  threads,  does  not  rotate  immediately with the shaft but because of inertia, runs forward on the revolving threaded sleeve until it engages with the ring gear. If the teeth of the pinion and ring gear do not engage,  the  drive  spring  allows  the  pinion  to  revolve and forces the pinion to mesh with the ring gear. When the pinion gear is engaged fully with the ring gear, the pinion   is   then   driven   by   the   starter   through   the compressed  drive  spring  and  cranks  the  engine.  The drive spring acts as a cushion while the engine is being cranked   against   compression.   It   also   breaks   the severity  of  the  shock  on  the  teeth  when  the  gears engage and when the engine kicks back due to ignition. When the engine starts and runs on its own power, the ring gear drives the pinion at a higher speed than does the starter. This action causes the pinion to turn in the opposite   direction   on   the   threaded   sleeve   and automatically   disengages   from   the   ring   gear.   This prevents the engine from driving the starter. The  OVERRUNNING  CLUTCH  (fig.  2-39) provides  positive  meshing  and  demeshing  of  the  starter motor pinion gear and the ring gear. The starting motor armature shaft drives the shell and sleeve assembly of the  clutch.  The  rotor  assembly  is  connected  to  the pinion  gear  which  meshes  with  the  engine  ring  gear. Spring-loaded  steel  rollers  are  located  in  tapered notches  between  the  shell  and  the  rotor.  The  springs Figure 2-38.—Starting motor with a Bendix drive. 2-26