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Systems Operation
37-MT, 41-MT, and 42-MT Series Starting Motors
Media Number -SENR3581-04 Publication Date -01/10/2008 Date Updated -24/10/2008
i03334642
General Information
SMCS - 1453
Illustration 1 g00717738
42-MT Starting Motor
The heavy-duty starting motors are offered in 24Vand 12V versions. The 37-MT and 41-MT are
similar in operation and appearance. The significant differences between the 37-MT, 41-MT, and 42-
MT starting motors are noted. A list of starting motors that are covered in this manual is located in the
Specifications section.
The starting motor rotates the engine flywheel. The engine will run when the engine flywheel has
reached an adequate speed. The starting motor has a solenoid. Electricity will cause the solenoid to
move the pinion drive toward the flywheel ring gear of the engine when the key start switch is
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activated on the 37-MT and 41-MT starting motors. The electrical contacts in the solenoid close the
circuit between the battery and the starting motor before the pinion engages the ring gear. This will
cause the starting motor to rotate. This is a positive shift starting motor. Electricity will cause the
solenoid to move the pinion drive in order to engage the flywheel ring gear of the engine when the
key start switch is activated on the 42-MT starting motors. The electrical contacts in the solenoid
close the circuit between the battery and the starting motor. This causes the starting motor to rotate.
This is a positive engagement starting motor.
Damage to the armature can be caused by excessive speeds. The overrunning clutch portion of the
pinion drive prevents damage to the armature when the engine begins to run. This is done by breaking
the mechanical connection. The pinion will stay meshed with the ring gear however, until the key start
switch is released. A return spring in the overrunning clutch returns the clutch assembly to the rest
position.
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Systems Operation
i01362348
Component Description
SMCS - 1453
(1) Rear Housing
(2) Solenoid
(3) Plunger
(4) Shift Lever Housing
(5) Shift Lever
(6) Drive Housing
(7) Pinion Drive
(8) Brushes
(9) Brush Holder
(10) Pole shoes
(11) Field Winding (Coil)
(12) Armature
(13) Starting Motor Housing
The major components of the starting motor are the following items:
• Starting motor housing (13)
• Field winding (coil) (11)
• Pole shoes (10)
• Armature (12)
• Brushes (8)
• Brush holder (9)
• Pinion drive (7)
• Shift lever (5)
• Plunger (3)
• Solenoid (2)
• Housings (1), (4), and (6)
Field Winding and Starting Motor Housing
(10) Pole Shoes
(11) Field Winding (Coil)
(13) Starting Motor Housing
The starting motor housing (13) supports the field winding (coil) (11), the pole shoes (10), the brush holder
(9), and the brushes (8). These parts are removable. The field winding (11) and the pole shoes (10) provide
the magnetic field for the armature. The field winding also contains a shunt coil on 37-MT starting motors.
The armature rotational speed is limited by the shunt coil during engagement. This prevents damage to the
pinion drive.
Armature
(12) Armature
Armature (12) consists of a laminated iron core which is assembled on the armature shaft. The armature also
has a commutator on one end. The commutator is made of copper segments (bars) insulated from each other
and insulated from the armature shaft. The armature windings are wound in the slots of the core. The
armature windings are also connected to the commutator segments.
The armature is positioned in the field winding and the pole shoes. The armature is supported by the
brushings in the drive housing (6), the shift lever housing (4), and the rear housing (1) .
The armature and the field winding are connected in series. The same current flows through each
component. This generates high torque primarily when the motor is first started.
Brush Holder
(9) Brush Holder
(14) Brush Springs
The brush holder (9) is mounted over the commutator to the starting motor housing. Four brushes are held
against the commutator. Brush springs (14) keep pressure on the brushes so the brushes stay in contact with
the commutator. Two brush holders are connected to the field winding positive (+) leads. The two negative
brush holders are not insulated from the brush holder assembly onthe 37-MT and 41-MT starting motors.
The two negative brush holders are connected to the negative terminal on the 42-MT starting motors.
Brushes
(8) Brush
The brushes (8) are made of carbon. The brushes (8) conduct electricity. The brushes transmit current from
the field winding to the commutator. There are four brushes. Two brushes are positive and two brushes are
negative.
37-MT Pinion Drive
(7) Pinion Drive
(15) Pinion Gear
41-MT and 42-MT Pinion Drive
(7) Pinion Drive
(15) Pinion Gear
As the engine starts to run, the speed eventually exceeds the starting motor speed. The overrunning clutch
portion of the pinion drive (7) protects the starting motor from this overspeed condition. The mechanical
connection between the engine flywheel and the starting motor is disengaged.
The pinion drive (7) has a pinion gear (15) that engages the flywheel ring gear. The pinion gear will stop in
case of butt engagement on the 37-MT starting motors. The pinion drive spring is compressed by the shift
lever until the solenoid contacts meet the solenoid terminals as a result. The starting motor is now turned on.
The starting motor turns the pinion until the pinion meshes with the ring gear. The pinion gear will stop in
case of butt engagement on the 41-MT and 42-MT starting motors. The clutch assembly continues to rotate.
This rotation will allow the pinion gear to rotate and the pinion gear will engage onto the flywheel ring gear.
Solenoid
(2) Solenoid
(3) Plunger
(5) Shift Lever
(16) Windings
(17) Solenoid Contacts
(18) Return Spring
Solenoid assembly (2) contains the following components: windings (16), plunger (3) and solenoid contacts
(17). When the key start switch is closed, the pull-in winding and the hold-in winding are energized. The
energized windings (16) will pull the plunger (3) to the left. The plunger movement pulls the shift lever
assembly (5) which will allow the pinion to engage. When the solenoid contacts (17) close, full battery
current is sent to the motor and the engine begins to crank.
When the engine starts, the pinion drive protects the armature from excessive speed until the key start switch
is released. After the switch is released, the return spring (18) moves shift lever (5) to the right which will
disengage the pinion.
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Systems Operation
i01362622
Normal Operation
SMCS - 1453
General Information
Rest Position
(1) Return Spring
(2) Solenoid
(3) Pull-In Winding
(4) Start (S) Terminal
(5) Hold-In Winding
(6) Battery (Bat) Terminal
(7) Key Start Switch
(9) Motor (Mtr) Terminal
(10) Shift Lever
(11) Field Winding
(12) Pinion Drive
(13) Pinion
(14) Armature
(15) Battery
A DC series wound motor is used as the starting motor. The starting motor has field winding (11) and
armature (14). The excitation and armature windings are connected in series. The pinion end of the
armature shaft has splines for pinion drive (12) .
The solenoid is an integral part of the starting motor. The solenoid pushes the pinion drive (12) with
the pinion (13) forward through the shift lever (10) and the plunger. The end of the plunger is a spool
which is connected to the shift lever. Return spring (1) helps return pinion drive (12) and shift lever
(10) to the rest position after key start switch (7) is released and solenoid contacts (8) have been
opened. A semi-solid link mechanism prevents welding of the solenoid contacts.
The starting motor has a two-stage mechanical pinion drive (12) that protects armature (14) from an
overspeed condition. The pinion drive also allows a smooth engagement of pinion (13) into the
flywheel ring gear. The pinion drive is held on the splines of the armature shaft. The pinion drive is
connected to the pinion (13) through the teeth of the integral overrunning clutch. Shift lever (10)
moves the pinion drive (12) axially in the direction of the ring gear.
37-MT And 41-MT Operation
Partial Engagement
(1) Return Spring
(2) Solenoid
(3) Pull-In Winding
(5) Hold-In Winding
(10) Shift Lever
(11) Field Winding
(12) Pinion Drive
(13) Pinion
(14) Armature
(15) Battery
When the key start switch (7) is closed, the following components are energized: start (S)terminal (4),
pull-in winding (3) and hold-in winding (5). The solenoid plunger pulls the shift lever (10) against the
force of the spring (1). The shift lever pushes the pinion drive (12) toward the engine flywheel. The
armature (14) has not yet turned. The solenoid contacts (8) have not closed. This sends starting motor
current to the excitation windings and the armature windings. If the pinion (13) can immediately
engage the flywheel ring gear, the pinion moves forward. When the pinion reaches the end of the
drive pinion shaft and the solenoid contacts (8) close. The starting motor is now in partial
engagement.
Blocked Engagement
(1) Return Spring
(2) Solenoid
(3) Pull-In Winding
(5) Hold-In Winding
(10) Shift Lever
(11) Field Winding
(12) Pinion Drive
(13) Pinion
(14) Armature
(15) Battery
If a pinion tooth meets a ring gear tooth, the pinion cannot immediately mesh with the ring gear. The
solenoid contacts (8) close as the meshing spring is compressed through the shift lever (10). The
starting motor is now turned on. The operation turns the pinion (13). The pinion initially meshes with
the teeth of the ring gear. The pressure from the meshing spring causes the pinion (13) to fully mesh.
The pressure from the meshing spring causes the ring gear to fully mesh.
Full Engagement
(1) Return Spring
(2) Solenoid
(3) Pull-In Winding
(5) Hold-In Winding
(10) Shift Lever
(11) Field Winding
(12) Pinion Drive
(13) Pinion
(14) Armature
(15) Battery
At the end of solenoid armature travel, solenoid contacts (8) close. This allows full battery voltage to
be applied to battery (Bat) terminal (6) and motor (Mtr) terminal (9). The starting motor current
energizes the field winding (11). The starting motor current powers the starting motor. Armature (14)
now begins to rotate and the helical spline forces the pinion farther into the ring gear until the pinion
contacts the stop ring of the armature shaft.
The pull-in winding (3) is turned off when the starting circuit is closed. Hold-in winding (5) remains
energized. The hold-in winding magnetic force is enough to hold the solenoid plunger in the pull-in
position until the engine is started.
When the engine starts and the ring gear turns pinion (13) faster than the starting motor, the clutch
section of the pinion drive (12) breaks the connection between the pinion and the armature shaft. This
prevents damage to the armature from being rotated too fast. The pinion remains meshed with the ring
gear while the shift lever (10) is held in the engaged position. The shift lever assembly and pinion
drive are returned to the rest position by the return spring (1) only when the key start switch is
released. The return spring also keeps the pinion in the rest position until the starting motor is again
operated.
42-MT Operation
Partial Engagement
(1) Return Spring
(2) Solenoid
(3) Pull-In Winding
(5) Hold-In Winding
(10) Shift Lever
(11) Field Winding
(12) Pinion Drive
(13) Pinion
(14) Armature
(15) Battery
When the key start switch (7) is closed, the following components are energized: start terminal (4),
pull-in winding (3) and hold-in winding (5). The activation of the pull-in winding (3) and the hold-in
winding (5) produces a magnetic force. The magnetic force will pull the plunger to the right. The
plunger will pull the shift lever (10) which will compress the return spring (1). The shift lever will
push the pinion assembly along the armature shaft toward the flywheel ring gear.
Blocked Engagement
(1) Return Spring
(2) Solenoid
(3) Pull-In Winding
(5) Hold-In Winding
(10) Shift Lever
(11) Field Winding
(12) Pinion Drive
(13) Pinion
(14) Armature
(15) Battery
The pinion gear (13) may meet a tooth of the flywheel ring gear as the pinion gear tries to move
forward. The other drive components will continue to be pushed forward. The helical spline of the
overrunning clutch portion of the pinion drive assembly (12) rotates the pinion gear in the cranking
direction. The spring of the pinion drive assembly is being compressed. The pinion tooth will slide
past the tooth on the flywheel ring gear until the next gap is found. The pinion gear will then engage
onto the flywheel due to the pressure that is built up in the compressed spring. At the same time, the
overrunning clutch rotates in the overspeed direction.
The pinion gear may also meet a damaged ring gear tooth or a notched ring gear tooth. While the
pinion drive assembly (12) is being pushed forward, the armature assembly (14) is being rotated in the
opposite direction. This is due to the helical spline. The spring of the pinion drive assembly is also
becoming compressed. Due to the forces that are being applied to the drive components onto the
damaged tooth, the starting motor current may not have force enough to turn the armature. The
starting attempt must be stopped. When the key start switch is released, the pinion drive assembly will
release the compression of the spring. This will cause the pinion gear to rotate. The pinion gear is now
ready to mesh with the flywheel ring gear. The starting attempt can now be retried.
Full Engagement
(1) Return Spring
(2) Solenoid
(3) Pull-In Winding
(5) Hold-In Winding
(10) Shift Lever
(11) Field Winding
(12) Pinion Drive
(13) Pinion
(14) Armature
(15) Battery
When the pinion gear (13) meshes completely with the flywheel ring gear, solenoid contacts (8) close.
Full battery voltage is then applied to the battery terminal "Bat" (6) and the motor terminal "Mtr" (9).
Starting motor current will then energize the field winding (11) and the armature (14). The pull-in
winding (3) will become de-energized. The hold-in winding (5) will still have enough force to hold in
the plunger assembly until the solenoid contacts (8) are open. The starting motor starts to crank the
engine with full torque.
As soon as the engine starts, the flywheel ring gear turns the pinion gear (13) faster than the starting
motor's normal speed. The overrunning clutch breaks the mechanical connection between the two
gears. The pinion gear turns on the helical spline which will compress the spring in the pinion drive
assembly. The pinion gear will then be pulled from the flywheel gear. Flyweights also help to break
the mechanical connection between the two gears. This process must take place in order to prevent the
starting motor from being rotated at speeds higher than maximum permissible speed. When the key
start switch (7) is released, the shift lever assembly and pinion drive assembly will return to the rest
position. The solenoid contacts open and the starting motor turns off. The rotating armature is quickly
stopped by a mechanical brake disc.

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Media Search - SENR3581 - 37-MT, 41-MT, and 42-MT Series Starting Motors Página 1 de 2

Product: NO EQUIPMENT SELECTED

42-MT Starting Motor

Product: NO EQUIPMENT SELECTED

(1) Rear Housing

(4) Shift Lever Housing

(5) Shift Lever

(6) Drive Housing

(7) Pinion Drive

(9) Brush Holder

(10) Pole shoes

(11) Field Winding (Coil)

(13) Starting Motor Housing

• Starting motor housing (13)

• Field winding (coil) (11)

• Pole shoes (10)

• Brush holder (9)

• Pinion drive (7)

• Shift lever (5)

• Housings (1), (4), and (6)

Field Winding and Starting Motor Housing

(10) Pole Shoes

(11) Field Winding (Coil)

(13) Starting Motor Housing

(9) Brush Holder

(14) Brush Springs

37-MT Pinion Drive

(7) Pinion Drive

(15) Pinion Gear

41-MT and 42-MT Pinion Drive

(7) Pinion Drive

(15) Pinion Gear

(5) Shift Lever

(17) Solenoid Contacts

(18) Return Spring

Product: NO EQUIPMENT SELECTED

(1) Return Spring

(3) Pull-In Winding

(4) Start (S) Terminal

(5) Hold-In Winding

(6) Battery (Bat) Terminal

(7) Key Start Switch

(8) Solenoid Contacts

(9) Motor (Mtr) Terminal

(10) Shift Lever

(11) Field Winding

(12) Pinion Drive

37-MT And 41-MT Operation

(1) Return Spring

(3) Pull-In Winding

(4) Start (S) Terminal

(5) Hold-In Winding

(6) Battery (Bat) Terminal

(7) Key Start Switch

(8) Solenoid Contacts

(9) Motor (Mtr) Terminal

(10) Shift Lever

(11) Field Winding

(12) Pinion Drive

(1) Return Spring