1. Starter motor drive mechanism
by
Mr. P. Karthik selvan, AP

2. • The starting motor is located near the flywheel .The drive gear on
the starter is arranged so that it can mesh with the teeth on the
flywheel (or the ring gear) when the starting switch is closed.
• The drive mechanism has two functions:
(1) To transmit the turning force to the engine when the starting motor runs
and to disconnect the starting motor from the engine immediately after the
engine has started and
(2) To provide a gear reduction ratio between the starting motor and the engine.
(The gear ratio between the driven pinion and the flywheel is usually about 15
to 1. This means that the starting motor rotates 15 times as fast as the engine, or
at 1500 rpm to turn the engine at a speed of 100 rpm.)

3. • The drive mechanism must disengage the pinion from the flywheel
immediately after the engine starts.
• After the engine starts, the engine speed may increase rapidly to
approximately 1500 rpm.
• If the drive pinion were to remain meshed with the flywheel and
locked with the shaft of the starting motor, at a normal engine speed
(1500 rpm), the shaft would spin at a rapid rate of speed (between
22,500 and 30,000 rpm). At such a rate of speed, the starting motor
would be badly damaged.

4. Types of starter drives
The common kinds of starter drives used are:
1. Bendix drive (Inertia drive)
2. Improved Inertia type
3. Bendix folo-thru type
4. Pre-engagement drive
5. Sliding armature with overrunning type (Axial type)
6. Permanent magnet type.
All of these starter drives use a one-way clutch (overrunning
clutch) to release the starter drive pinion from the flywheel after
the engine has started

5. Bendix drive (Inertia drive)
• A Bendix drive is a type of engagement mechanism used in a starter
motors of internal combustion engines
• Inertia type drive works on the principle of inertia of unbalanced weight.
In this type, pinion is mounted on threaded sleeve
• Engage- When ignition switch pressed, the starter motor begins turning,
the inertia of the drive pinion assembly causes it to wound the spring,
forcing the length of the spring to change and engage with the ring gear
• Disengage- When the engine starts, back drive from the ring gear causes
the drive pinion to exceed the rotation speed of the starter, at which point
the drive pinion is forced back and out of the mesh with the ring gear
• Spring – To pull back the assembly, To avoid hitting of ring gear against
the casing

7. • Disadvantage
• Lots of moving parts cause rapid wearing of teeth
• Engagement of pinion is done after starter motor starts
To overcome this, pinion must be engaged before
starter motor rotating. As a result, pre-engage drive is
developed.
• Application- old vehicles

8. Pre Engagement type
• This has a solenoid or magnetic switch which is used to shift the
drive pinion magnetically into mesh with the engine flywheel
complete the circuit between the battery and self starter
• In this arrangement the drive pinion on the starter motor shaft is
pushed axially into mesh with the flywheel ring gear before the
starter motor is switched ON
• When the pinion meshes with flywheel ring gear, the starter motor
is switched ON by engagement relay
• A solenoid is usually employed to move the lever and close the
switch contacts. This method is quite simple and having the
advantage not to damage gear teeth

9. • Engage- When we turn the key to start solenoid switch ON, the
starter motor shifts the pinion with the help of lever for the
engagement with ring gear
• As the engine is started, it is the time to disengage the pinion form
the flywheel
• Disengage- This is done by the overrunning clutch which is
attached to a pinion overrunning clutch make the pinion free from
the starter motor shaft as the engine starts. And motor is saved
from the high speed damage
Advantage - * A smaller pinion can be utilized for higher reduction
* Pinion drive is not so highly stressed
* Prolonged cranking life
Application – Car and medium size vehicles

12. Axial or sliding armature type
• The axial starter works on the principle of a sliding armature
mechanism
• It is similar like pre-engagement drive, In pre engagement type
ring gear only moves forward and backward but in this axial
armature type whole armature unit will moves forward and
backward to produced high starting torque
• Here, when the starter switch is operated a solenoid coil is
energized.
• This completed the circuit of a shunt winding and also of an
auxiliary, series field winding
• It consists of two winding- one for energizing and another one for
moving the armature or sliding

13. • The armature is drawn into a central position in addition to
rotating because of the magnetic field
• As the pinion is attached to the armature shaft, it is moved
axially and gets engaged with the flywheel ring gear
• After the complete engagement of the pinion with the
flywheel ring gear, a catch on the switch is tripped, thereby
emerging the main series winding and marking the motor to
operate on its full power current supply
• A slipping clutch is provided between the pinion and the
armature to avoid over speed of the motor

15. OVERRUNNING CLUTCH
• The overrunning clutch is roller-type clutch that transmits
torque in one direction only and freewheels in the other
direction
• This allows the starter motor to transmit torque to the ring
gear but prevents the ring gear from transferring torque to the
starter motor
• In a typical overrunning type clutch, the clutch housing is
internally splined to the starter armature shaft
• The drive pinion turns freely on the armature shaft within the
clutch housing

16. • When torque is transmitted through the armature to the clutch
housing, the spring loaded rollers are forced into the small ends of
their tapered slots
• They are then wedged tightly against the pinion barrel
• The pinion barrel and clutch housing are now locked together,
torque is transferred through the starter motor to the ring gear and
engine
• This unloads the clutch rollers and releases the pinion gear to rotate
freely around the armature shaft

18. SHUNT MOTOR SERIES MOTOR
Field winding connected parallel with
armature
Filed winding connected series with
armature
Shunt motor are parallel and hence low
current
Series motor are series so high armature
current
Shunt DC Motor is used in constant
torque application
Series DC Motor is used for high starting
torque application