Worm Worm Gears
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Transcript of Worm Worm Gears
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1.Introduction:
Generally internal combustion engine must be rotated before it will run under its
own power. For this starting system is required which is designed to convert electrical
energy into mechanical energy. To accomplish this conversion, a starter or cranking
motor is used, which includes Battery, Ignition switch, olenoid, tarter motor
!"rmature#, Flywheel ring gear, tarting safety switch.
The starting system has two separate but related circuits. $ne is the low%current
control circuit. The other is the high%current motor circuit. Both operate on battery
voltage. &hen the driver turns the ignition key to T"'T, the control circuit causes
heavy contacts to close in a starter relay or solenoid switch. (igh current then flows from
the battery insulated cable through these contacts to the starting motor. " gear on thestarting%motor shaft moves into mesh with the ring gear around the engine. But in order
to increase the torque for starting of the engine gear reduction should be done. This is
possible by means of the epicyclic gear arrangement. But to increase more torque this is
possible only by Worm-Worm gear arrangement. Fig shows the simple diagram of
electrical circuit of the starting system.
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2.Cranking motor designs:
The most common type of starter motor used today incorporates the overrunning
clutch starter drive instead of the old inertia%engagement bendi) drive. There are four
basic groups of starter motors*
+.irect drive
-.Gear reduction
./ositive%engagement !movable pole#
0./ermanent magnet
2.1 Direct drive starters:
The most common type of starter motor is the solenoid%operated direct drive unit.
"lthough there are construction differences between applications, the operating principles
are the same for all solenoid%shifted starter motors.
&hen the ignition switch is placed in the T"'T position, the control circuit
energi1es the pull%in and hold%in windings of the solenoid. The solenoid plunger moves
and pivots the shift lever, which in turn locates the drive pinion gear into mesh with the
engine flywheel.
&hen the solenoid plunger is moved all the way, the contact disc closes the circuit
from the battery to the starter motor. 2urrent now flows through the field coils and the
armature. This develops the magnetic fields that cause the armature to rotate, thus turning
the engine.
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2.2 Gear reduction starters:
It provides increased torque. The gear reduction starter differs from most other
designs in that the armature does not drive the pinion gear directly. In this design, the
armature drives a small gear that is in constant mesh with the large gear. epending on
the application, the ratio between these two gears is between -*+ and .3*+.The additional
reduction allows for a small motor to turn at higher speeds and greater torque with less
current draw. "nother characteristic of most gear reduction starters is that the
commutator and brushes are located in the center of the motor.
The solenoid operation is similar to that of the solenoid 4shifted direct drive
starter in that the solenoid moves the plunger, which engages the starter drive.
+ rive shaft - top ring /inion 0 'oller%type overrunning clutch 3 5eshing spring 6
/inion%engaging lever 7 olenoid switch 8 (old%in winding 9 /ull%in winding +: 'eturn
spring ++ witch contact +- witch contact + ;lectrical connection +0 2ommutator end
shield +3 2ommutator +6 Brush holder +7 "rmature +8 5agnet +9 tator housing -:
/lanetary gear
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"s soon as the starter drive pinion gear contacts the ring gear, a contact is on the
pole shoe opens a set of normally closed grounding contacts. &ith the return to ground
circuit opened, all the starter current flows through the remaining three field coils and
through the brushes to the armature. The starter motor then begins to rotate to prevent the
starter drive from disengaging from the ring gear if battery voltage drops while cranking,
the movable pole shoe is held down by a holding coil. The holding coil is a smaller coil
inside the mail drive coil and is strong enough to hold the starter pinion gear engaged.
2.4 Permanent magnet starters:
The permanent magnet gear reduction !/5G'# starter design provides for less
weight, simpler construction, and less heat generation as compared to conventional field
coil starters. The /5G' starter uses four permanent magnet field assemblies in place of
field coils. Because there are no field coils, current is delivered directly to the armature
though the commutator and bushes.
The permanent magnet starters also use gear reduction through the planetary gear set. The
planetary gear train transmits power between the armature and the pinion shaft. This
allows the armature to rotate at greater sped and increased torque. The planetary gear
assembly consists of a sun gear on the end of the armature, and three planetary carrier
gears inside a ring gear. The ring gear is held stationary, when the armature is rotated, the
sun gear causes the carrier gears to rotate about the internal teeth of the ring gear. The
planetary carrier is attached to the output shaft. The gear reduction provided for by this
gear arrangement is 0.3*+.By providing for this additional gear reduction, the demand for
high current is lessened.
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The electrical operation between the conventional field coil and /5G' starters
remains basically the same.
Instead of using the epicyclic arrangement in gear reduction starter motor, usage of
the worm gear arrangement in the starter motor is the new type of concept.
Below fig shows the permanent magnet starter motor*
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+ rive shaft - top ring /inion 0 'oller%type overrunning clutch 3 /inion%engaging
lever 6 olenoid switch 7 ;lectrical connection 8 2ommutator bearing 9 Brush holder
+: 2ommutator ++ "rmature +- 5agnet + tator housing +0 5eshing spring.
Parts of the starter motor:
It 5ainly 2onsists $f Three /arts*
+. 5otor "ssembly
-. olenoid
. Gear "ssembly
Su !arts of the starter motor:
+. 5otor ub /arts*
+. "rmature
-. 2ommutator
. Brushes
0.
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For Gear reduction starter motors the drive parts are epicyclic gear arrangement,
overrunning clutch, drive shaft, pinion and stop ring.
"rmature:
The armature is constructed with a laminated core made of several thin iron
stampings that are placed ne)t to each other. =aminated construction is used because in a
solid iron core the magnetic fields would generate counter voltages in the core called
eddy currents. By using laminated construction, eddy currents in the core are minimi1ed.
;ddy currents cause heat to build up in the core and waste energy.
The slots in the laminations hold the armature windings. The windings loop
around the core and are connected to the commutator. ;ach commutator segment is
insulated from the ad>acent segments. " typical armature can have more than :
commutator segments. " steel shaft is fitted into the center hole of the core laminations.
The commutator is insulated from the shaft.
Two basic winding patterns are used in the armature* lap winding and wave
winding. In the lap winding, the two ends of the winding are connected to ad>acent
commutator segments. In this pattern, the wires passing under a pole field have their
current flowing in the same direction.
In the wave%winding pattern, each end of the winding connects to commutatorsegments that are 9: or +8: degrees apart. In this pattern design some windings will have
no current flow at certain positions of armature rotation. This occurs because the segment
ends of the winding loop are in contact with brushes that have the same polarity. The
wave wound pattern is the most common used due to its lower resistance.
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Commutator:
The main purpose of the commutator is to collect the current from the battery and
it gives to the armature and causes the armature shaft to rotate. The commutator is made
of copper segments insulated from each other by mica sheet and mounted on the shaft of
the machine.
The coppers segments are connected to the loops of heavy wire that from the
armature windings. 2urrent enters and e)its the armature through a set of brushes made
up of copper and carbon. (ere the brushes slide over the commutators sections.
Brushes:
Brushes are used to make electrical contact between armatures the armature,
which rotates and the battery, which is stationary. Brushes are made from a high
percentage of copper and carbon. This material minimi1es electrical losses use to
overheating. #$IG%shows an e)ample of the brush held in place on the starter end plate.
The main purpose of the springs is to ad>ust the pressure. Because if the brush
pressure is very large, the friction produces heats the commutator and the brushes. If the
brush pressure is too weak, the imperfect contact with the commutator may produce
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plunger so that contact takes place between the battery terminal and the starter terminal.
o the current flows to the armature through the 2ommutator and the magnetic flu) is
created and causes the armature shaft to rotate. "t the same time the plunger, which is
connected to the lever, pushes the pinion to move forwards and causes the engagement
with the ring gear of the flywheel. This is how solenoid switch works.
$=;?$I
efinition of a solenoid
" solenoid is an electromechanical device that switches electrical circuits on and
off. !IG 7%+9 39 /G# shows the operation of a solenoid. &hen current flows through
the electrical coil, a magnetic field is created inside the coil. If soft iron core of metal is
placed near the center of the coil, the metal core tends to center itself inside the coil. If a
spring is used to hold the metal core outside the center, it again tries to center itself. Thus,
when current passes through the coil, the metal core will move. This process, then,
converts electrical energy into mechanical energy. The movement of the metal core is
used to open and close the electrical circuits. Therefore, a solenoid is considered an
electromechanical switch.
$=;?$I $/;'"TI$?
!FIG 7%-: 39 /G# shows the internal operation of a solenoid used on certain vehicles.
The solenoid is used to start and stop the heavy current that flows to the starter motor
during cranking.
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The electrical winding is placed around the iron core. This core is called the armature or
plunger. $n the bottom of the armature is a set of moving contacts. $n the bottom of the
solenoid is asset of fi)ed contacts connected to the heavy current terminals. &hen the
solenoid windings are energi1ed, the center armature is forced downward magnetically.
This causes an electrical connection between the two fi)ed contact points. &hen the
current is stopped in the windings, the armature returns to its original position. ome
older armatures can also be pushed manually to accomplish the same result.
2=$I?G "? ($= I? 2$I=.
5any solenoids contain two coils. $ne is called the closing or pull%in coil. $ne is called
the hold%in coil. This is shown in the electrical circuit in !IG 7%-+ 39/G#.
The heavy gauge winding is called the closing coil, or pull%in winding. It has low
resistance. The finer or thinner coil is the hold%in winding. It has higher resistance. The
closing coil has less resistance. This means that there will be more current and more force
in the coil. This force is needed along with the hold%in coil to move the center plunger.
$nce the plunger movement has been completed, much less magnetism is needed to hold
the plunger in. &hen the contact disc touches the terminals, the pull%in winding is shorted
out, thus no current flows through the winding. This reduces the current draw on the
battery during cranking.
Drive assem(+:
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Gear drive assembly is mainly useful to reduce the speed of the pinion gear and to
increase the torque during starting of the engine. It consists of a planetary gear
arrangement !;picyclic arrangement#, overrunning clutch and a pinion. ;picyclic gear
train arrangement includes planetary gears, annulus, sun gear, planetary gear carrier .The
sun gear on the armature shaft and the planetary gears on the output shaft rotate with
annulus fi)ed for transmitting the power from armature to pinion to crank the engine i.e.
as the sun gear rotates then the planetary gears rotate inside the annulus which is fi)ed
and it causes the planetary gear carrier to rotate which is attached to the output shaft.
Then output shaft turns the overrunning clutch. Below fig shows the drive assembly for
the gear reduction starter motor.
In the case of direct drive starter motors the drive assembly is shown below*
Starter Drives:
The starter drive is the part of the starter motor that engages the armature to the
engine flywheel ring gear !fig%9.+0#. It includes a pinion gear set that meshes with the
flywheel ring gear on the engine@s cranck shaft. To prevent damage to the pinio gear or
the ring gear, the pinion gear must mesh with the ring gear before the starter motor
rotates. To help assure smooth engagement, the ends of the pinion gear teeth are tapered.
"lso, the action of the armature must also be from the motor to the engine. The engine
must not be allowed to spin the armature. The ratio of the number of teeth on the ring
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gear and the starter drive pinion gear is usually between +3*+ and -:*+. The ratio of the
starter drive is determined by dividing the number of teeth on the drive gear!pinion gear#
into the number of teeth on the driven gear!flywheel#.This means starter motor is rotating
+3 to -: times faster than the engine. ?ormal cranking speed for the engine is about -::
rpm. If the starter drive had a ratio of +8*+, the starter would be rotating at a speed of
,6:: rpm. This would destroy the starter motor if it was not disengaged from the engine.
,-DI/ I-0I" D0I-:
The bendi) drive depends on inertia to provide meshing of the drive pinion with the ring
gear !$IG 1& 14& PG%Inertia is the tendency of an ob>ect that is at rest to stay at rest,
and an ob>ect that is in motion to stay in motion. The screw shaft threads are apart of the
armature, and will turn at armature speed.
"t the end of the pinion and barrel is the pinion gear that will mesh with the ring gear.
The pinion and barrel have internal threads that match those of the screw shaft. &hen
current flows through the starter motor, the armature will begin to spin. Torque from the
armature is transmitted via a shock absorbing drive spring and drive head to the screw
shaft. This causes the screw shaft to rotate. (owever the barrel does not rotate. The barrel
has a weight on one side to increase its inertial effect. The barrel tends to stay at rest, and
the screw shaft rotates inside the barrel. "s a result, the barrel is threaded down the length
of the screw shaft to the end. "t the end of the screwshaft, the pinion gear engages the
ring gear. (ere the pinion gear locks to the screw shaft and transfers torque from the
armature to the ring gear and engine. $nce the engine starts and is running under its own
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power, it will rotate faster than the armature. This causes the barrel to screw back down
the screw shaft and bring the pinion gear out of engagement with the ring gear.
&.5 6verrunning c(utch drive:
The most common type of starter drive is the overrunning clutch. The
overrunning clutch is a 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 atypical overrunning 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. &hen torque is transmitted through the armature to the cutch housing, the
spring%loaded rollers are forced into the small ends of their tapered slots. They are then
wedged tightly giant against the pinion barrel. The pinion barrel and clutch housing are
now locked togetherA torque is transferred through the starter motor to the ring gear and
engine.
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&hen the engine starts and is running under its own power, the ring gear attempts to
drive the pinion gear faster than the starter motor. This unloads the clutch rollers and
releases the pinion gear to rotate freely around the armature shaft.