PINION DRIVE ASSEMBLYThe pinion
gear, pinion drive mechanism, and solenoid.
FIELD FRAMEThe center housing that holds
the field coils and pole shoes.
DRIVE END FRAMEThe end housing
around the pinion gear, which has a bushing for
the armature shaft.
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
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
PINION DRIVE ASSEMBLY.The
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 designsBendix 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.