Worm Worm Gears

download Worm Worm Gears

of 16

Transcript of Worm Worm Gears

  • 8/13/2019 Worm Worm Gears

    1/16

    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.

  • 8/13/2019 Worm Worm Gears

    2/16

    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.

  • 8/13/2019 Worm Worm Gears

    3/16

    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

  • 8/13/2019 Worm Worm Gears

    4/16

  • 8/13/2019 Worm Worm Gears

    5/16

    "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.

  • 8/13/2019 Worm Worm Gears

    6/16

    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*

  • 8/13/2019 Worm Worm Gears

    7/16

    + 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.

  • 8/13/2019 Worm Worm Gears

    8/16

    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.

  • 8/13/2019 Worm Worm Gears

    9/16

    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

  • 8/13/2019 Worm Worm Gears

    10/16

  • 8/13/2019 Worm Worm Gears

    11/16

    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.

  • 8/13/2019 Worm Worm Gears

    12/16

    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(+:

  • 8/13/2019 Worm Worm Gears

    13/16

    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

  • 8/13/2019 Worm Worm Gears

    14/16

    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

  • 8/13/2019 Worm Worm Gears

    15/16

    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.

  • 8/13/2019 Worm Worm Gears

    16/16

    &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.