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TERM PAPER OF 4TH SEMESTER 2010
MECHANICAL SCIENCES
MEC101
TOPIC: DIFFERENT TYPES OF GEARS AND THEIR APPLICATION IN
AUTOMOBILES
DOA: 13th March 2010
DOS: 11th May 2010
Submitted to: Submitted by:
Mr.Gagandeep Singh Mr. William Anthony
Deptt. Of Mechanical Science Rollno:RD1803B30
Regd no.:10805460
B.Tech M.Tech(Cse)
ACKNOWLEDGEMENT
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I would like to express my gratitude for the many helpful comment and
suggestions .I have received over the last few days regarding the expository and
critical expects of my term work and especially for those comments which bear
directly or may various argument for the center thesis of term work.
Most importantly I would like to thank my HOD (head of department) and my
teacherMr.Gagandeep Singh for his days of supervision. His critical commentary
on my work has played a major role in both the content and presentation of our
discussion and arguments.
I have extend my appreciation to the several sources which provided various kinds
of knowledge base support for me during this period.
Last and not the least my God and my parent helped me to overcome the problem
while preparing this project.
WILLIAM ANTHONY
TABLE OF CONTENTS
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HISTORY OF GEARS
INTRODUCTION TO GEARS
COMPARISION WITH OTHER DRIVE MECHANISMS
GEAR DESIGN
GEAR MANUFACTURE
TYPES OF GEARS
APPLICATION OF GEARS IN AUTOMOBILES
GEAR MODEL IN MODERN PHYSICS
HISTORY OF GEARS
Gears have existed since the invention of rotating machinery. Because of their force-multiplying
properties, early engineers used them for hoisting heavy loads such as building materials. The
mechanical advantage of gears was also used for ship anchor hoists and catapult pre-tensioning.
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Early gears were made from wood with cylindrical pegs for cogs and were often lubricated with
animal fat grease. Gears were also used in wind and water wheel machinery for decreasing or
increasing the provided rotational speed for application to pumps and other powered machines.An early gear arrangement used to power textile machinery is illustrated in the following figure.
The rotational speed of a water or horse drawn wheel was typically too slow to use, so a set of
wooden gears needed to be used to increase the speed to a usable level.
An 18th Century Application of Gears for Powering Textile Machinery
The industrial revolution in Britain in the eighteenth century saw an explosion in the use of metal
gearing. A science of gear design and manufacture rapidly developed through the nineteenth
century.
Today, the most significant new gear developments are in the area of materials. Modern
metallurgy has greatly increased the useful life of industrial and automotive gears, and consumer
electronics has driven plastic gearing to new levels of lubricant-free reliability and quietoperation.
INTRODUCTION TO GEARS
A gear is a rotatingmachine part having cut teeth, or cogs, which mesh with another toothed part
in order to transmit torque. Two or more gears working in tandem are called a transmission andcan produce a mechanical advantage through a gear ratio and thus may be considered a simple
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machine. Geared devices can change the speed, magnitude, and direction of apower source. The
most common situation is for a gear to mesh with another gear, however a gear can also mesh a
non-rotating toothed part, called a rack, thereby producing translation instead of rotation.
The gears in a transmission are analogous to the wheels in a pulley. An advantage of gears is that
the teeth of a gear prevent slipping.
When two gears of unequal number of teeth are combined a mechanical advantage is produced,
with both the rotational speeds and the torques of the two gears differing in a simple relationship.
In transmissions which offer multiple gear ratios, such as bicycles and cars, the term gear, as infirst gear, refers to a gear ratio rather than an actual physical gear. The term is used to describe
similar devices even when gear ratio is continuous rather than discrete, or when the device does
not actually contain any gears, as in a continuously variable transmission.
The earliest known reference to gears was circa 50 A.D. by Hero of Alexandria, but they can be
traced back to the Greekmechanics of the Alexandrian school in the 3rd century BC and weregreatly developed by the GreekpolymathArchimedes (287-212 BC).
A gear is a rotatingmachine part having cut teeth, or cogs, which mesh with another toothed part
in order to transmit torque. Two or more gears working in tandem are called a transmission andcan produce a mechanical advantage through agear ratioand thus may be considered a simple
machine. Geared devices can change the speed, magnitude, and direction of apower source. The
most common situation is for a gear to mesh with another gear, however a gear can also mesh anon-rotating toothed part, called a rack, thereby producing translation instead of rotation.
The gears in a transmission are analogous to thewheels in apulley. An advantage of gears is that
the teeth of a gear prevent slipping.
When two gears of unequal number of teeth are combined a mechanical advantage is produced,with both the rotational speedsand the torques of the two gears differing in a simple relationship.
In transmissions which offer multiple gear ratios, such as bicycles and cars, the term gear, as in
first gear, refers to a gear ratio rather than an actual physical gear. The term is used to describe
similar devices even when gear ratio iscontinuousrather than discrete, or when the device doesnot actually contain any gears, as in a continuously variable transmission.
COMPARISON WITH OTHER DRIVE MECHANISMS
The definite velocity ratio which results from having teeth gives gears an advantage over otherdrives (such as traction drives andV-belts) in precision machines such as watches that depend
upon an exact velocity ratio. In cases where driver and follower are in close proximity gears also
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have an advantage over other drives in the reduced number of parts required; the downside is that
gears are more expensive to manufacture and their lubrication requirements may impose a higher
operating cost.
The automobiletransmissionallows selection between gears to give various mechanical
advantages.
GEAR DESIGN
Gears have been around for hundreds of years and are as old as almost any machinery ever
invented by mankind. Gears were first used in various construction jobs, water raising devicesand for weapons like catapults.
Nowadays gears are used on a daily basis and can be found in most peoples everyday life fromclocks to cars rolling mills to marine engines. Gears are the most common means of transmitting
power in mechanical engineering.
Gears are used in almost all mechanical devices and they do several important jobs, but most
important, they provide a gear reduction. This is vital to ensure that even though there is enough
power there is also enough torque(is a movement of force).
GEAR MANUFACTURE
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The materials that are used for gear manufacturing depend massively on the conditions that the
gears will be operating under, conditions like wear and noise etc. Gear manufacturers use
metallic or non-metallic materials.
The Metallic materials used in gear manufacture are normally available in cast iron, steel and
bronze. Steel is used when there is a need for a high strength design. But cast iron is mainly usedbecause of its good wearing properties, excellent machineability and the ease that complicated
shapes can be created due to the casting method.Castings are created by pouring molten metal
into a mould and once the metal as cooled it takes the shape of the mould.
The non metallic, materials used are generally wood, rawhide, compressed paper and synthetic
resins like nylon. These types of materials are generally used when there is a need for the
reduction in noise from the gears
TYPES OF GEARS
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External vs. internal gears
Internal gear
An external gear is one with the teeth formed on the outer surface of a cylinder or cone.Conversely, an internal gear is one with the teeth formed on the inner surface of a cylinder or
cone. For bevel gears, an internal gear is one with the pitch angle exceeding 90 degrees. Internal
gears do not cause direction reversal.
Spur
Spur gear
Spur gears or straight-cut gears are the simplest type of gear. They consist of a cylinder or disk,and with the teeth projecting radially, and although they are not straight-sided in form, the edge
of each tooth thus is straight and aligned parallel to the axis of rotation. These gears can be
meshed together correctly only if they are fitted to parallel axles.
Helical
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Helical gears
Top: parallel configuration
Bottom: crossed configuration
Helical gears offer a refinement over spur gears. The leading edges of the teeth are not parallel tothe axis of rotation, but are set at an angle. Since the gear is curved, this angling causes the tooth
shape to be a segment of ahelix. Helical gears can be meshed in a parallel or crossed
orientations. The former refers to when the shafts are parallel to each other; this is the most
common orientation. In the latter, the shafts are non-parallel.
The angled teeth engage more gradually than do spur gear teeth causing them to run more
smoothly and quietly. With parallel helical gears, each pair of teeth first make contact at a single
point at one side of the gear wheel; a moving curve of contact then grows gradually across thetooth face to a maximum then recedes until the teeth break contact at a single point on the
opposite side. In spur gears teeth suddenly meet at a line contact across their entire width causing
stress and noise. Spur gears make a characteristic whine at high speeds and can not take as much
torque as helical gears. Whereas spur gears are used for low speed applications and thosesituations where noise control is not a problem, the use of helical gears is indicated when the
application involves high speeds, large power transmission, or where noise abatement is
important. The speed is considered to be high when the pitch line velocity exceeds 25 m/s.[5]
A disadvantage of helical gears is a resultant thrust along the axis of the gear, which needs to beaccommodated by appropriate thrust bearings, and a greater degree ofsliding frictionbetween
the meshing teeth, often addressed with additives in the lubricant.
Double helical
Double helical gears
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Double helical gears, or herringbone gears, overcome the problem of axial thrust presented by
"single" helical gears by having two sets of teeth that are set in a V shape. Each gear in a doublehelical gear can be thought of as two standard mirror image helical gears stacked. This cancels
out the thrust since each half of the gear thrusts in the opposite direction. Double helical gearsare more difficult to manufacture due to their more complicated shape.
Bevel
A bevel gear is shaped like a right circular cone with most of its tip cut off. When two bevelgears mesh their imaginary vertexes must occupy the same point. Their shaft axes also intersectat this point, forming an arbitrary non-straight angle between the shafts. The angle between the
shafts can be anything except zero or 180 degrees. Bevel gears with equal numbers of teeth and
shaft axes at 90 degrees are called miter gears.
The teeth of a bevel gear may be straight-cut as with spur gears, or they may be cut in a variety
of other shapes. Spiral bevel gears have teeth that are both curved along their (the tooth's) length;
and set at an angle, analogously to the way helical gear teeth are set at an angle compared to spur
gear teeth. Zerol bevel gears have teeth which are curved along their length, but not angled.Spiral bevel gears have the same advantages and disadvantages relative to their straight-cut
cousins as helical gears do to spur gears. Straight bevel gears are generally used only at speedsbelow 5 m/s (1000 ft/min), or, for small gears, 1000 r.p.m.
Hypoid
Hypoid gear
Hypoid gears resemble spiral bevel gears except the shaft axes do not intersect. The pitch
surfaces appear conical but, to compensate for the offset shaft, are in facthyperboloids of
revolution. Hypoid gears are almost always designed to operate with shafts at 90 degrees.Depending on which side the shaft is offset to, relative to the angling of the teeth, contact
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between hypoid gear teeth may be even smoother and more gradual than with spiral bevel gear
teeth. Also, the pinion can be designed with fewer teeth than a spiral bevel pinion, with the result
that gear ratios of 60:1 and higher are feasible using a single set of hypoid gears. This style ofgear is most commonly found in mechanical differentials.
Crown
Crown gears or contrate gears are a particular form of bevel gear whose teeth project at rightangles to the plane of the wheel; in their orientation the teeth resemble the points on a crown. A
crown gear can only mesh accurately with another bevel gear, although crown gears aresometimes seen meshing with spur gears. A crown gear is also sometimes meshed with an
escapement such as found in mechanical clocks.
Worm
Worm gear
Worm gears resemble screws. A worm gear is usually meshed with an ordinary looking, disk-
shaped gear, which is called the gear, wheel, or worm wheel.
Worm gears can be considered a species of helical gear, but its helix angle is usually somewhatlarge (close to 90 degrees) and its body is usually fairly long in the axial direction; and it is these
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attributes which give it its screw like qualities. The distinction between a worm and a helical
gear is made when at least one tooth persists for a full rotation around the helix. If this occurs, it
is a 'worm'; if not, it is a 'helical gear'. A worm may have as few as one tooth. If that toothpersists for several turns around the helix, the worm will appear, superficially, to have more than
one tooth, but what one in fact sees is the same tooth reappearing at intervals along the length of
the worm. The usual screw nomenclature applies: a one-toothed worm is called single thread orsingle start; a worm with more than one tooth is called multiple thread or multiple start. The
helix angle of a worm is not usually specified. Instead, the lead angle, which is equal to 90
degrees minus the helix angle, is given.
In a worm-and-gear set, the worm can always drive the gear. However, if the gear attempts todrive the worm, it may or may not succeed. Particularly if the lead angle is small, the gear's teeth
may simply lock against the worm's teeth, because the force component circumferential to the
worm is not sufficient to overcome friction. Worm-and-gear sets that do lock are called selflocking, which can be used to advantage, as for instance when it is desired to set the position of a
mechanism by turning the worm and then have the mechanism hold that position. An example is
the machine head found on some types ofstringed instruments.
Non-circular
Non-circular gears
Non-circular gears are designed for special purposes. While a regular gear is optimized to
transmit torque to another engaged member with minimum noise and wear and maximum
efficiency, a non-circular gear's main objective might be ratio variations, axle displacement
oscillations and more. Common applications include textile machines,potentiometersandcontinuously variable transmissions.
Rack and pinion
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Rack and pinion gearing
A rack is a toothed bar or rod that can be thought of as a sector gear with an infinitely largeradius of curvature. Torque can be converted to linear force by meshing a rack with a pinion: the
pinion turns; the rack moves in a straight line. Such a mechanism is used in automobiles to
convert the rotation of the steering wheel into the left-to-right motion of the tie rod(s). Racks alsofeature in the theory of gear geometry, where, for instance, the tooth shape of an interchangeable
set of gears may be specified for the rack (infinite radius), and the tooth shapes for gears ofparticular actual radii then derived from that. The rack and pinion gear type is employed in a rackrailway.
Epicyclic
In epicyclic gearing one or more of the gearaxes moves. Examples are sun and planet gearing(see below) and mechanical differentials.
Sun and planet
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Sun and planet gearing was a method of convertingreciprocal motion into rotary motion insteam engines. It played an important role in theIndustrial Revolution. The Sun is yellow, the
planet red, the reciprocatingcrankis blue, theflywheel is green and the driveshaftis grey.
Harmonic drive
A harmonic drive is a specializedproprietary gearing mechanism.
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APPLICATIONS OF GEARS IN AUTOMOBILES
Gears are now used for a wide range of industrial applications. There are gears weighing from
15grams to 15 tons. Lots of industries use gears for transmitting power or rotational force from
one component to another. Gears have varied application starting from textile looms to aviation
industries. Undoubtedly the demand for gears in industries is estimated at a whopping 125 billiondollars worldwide.Cement plants use girth gears for crushing raw materials. These girth gears
have high strength and highly durable that they can be used in any operating environment. Forrotating equipments in the cement industry power transmission is essential and that is providedby various open gear drives. Other types of gears which are used in cement industries are
reduction gears, pinions, Sprockets, pinion shafts and helical gears.Coal plants also most
commonly use girth gears in their mining equipments. Heavy gearboxes are also being used inheavy machineries. There are two basic gearing principles which are:
- Craggy reliable gearing
- Worm gearing
These are used in many mining machines such special cutting machines, axle for forming tunnelsfor locomotives, etc. Gears are also used in compressors, cages, trolleys and diesel locomotives
that are used for coal mining.
Medical industry uses gears in a huge way. They are being used in the surgical tables, patient
beds, gear pumps for medical dosing applications, and also in medical diagnostic machine.
Micro assembly uses micro gears which enable us to have miniature components built with a
very compact size. Use of micro gears is really advantageous as they provide zero backlash, long
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Automobile Differential Complete Schematic
Without the "square" set of four gears in the middle of the above diagram which yields to thefigure below, both wheels turn at the same angular velocity. This leads to problems when the car
negotiates a turn.
Automobile Wheel Drive without Differential
Now imagine the differential "square" alone, as illustrated in the following figure. It should be
apparent that turning one wheel results in the opposite wheel turning in the opposite direction at
the same rate.
Automobile Differential Alone
This is how the automobile differential works. It only comes into play when one wheel needs to
rotate differentially with respect to its counterpart. When the car is moving in a straight line, thedifferential gears do not rotate with respect to their axes. When the car negotiates a turn,
however, the differential allows the two wheels to rotate differentially with respect to each other.
One problem with an automotive differential is that if one wheel is held stationary, the
counterpart wheel turns at twice its normal speed as can be seen by examining the completescheme of automobile differential. This can be problematic when one wheel does not have
enough traction, such as when it is in snow or mud. The wheel without traction will spin without
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providing traction and the opposite wheel will stay still so that the car does not move. This is the
reason for a device known as a "limited slip differential" or "traction control".
Differential gear train on a turning car
The gear train in an automobile differential is a common application of gears, but oftenmisunderstood by the lay public. Here we present a simplified explanation of how and why an
automobile differential works.
The car is turning about a circle with nominal radius rn. (For this discussion, we assume that the
axis of the wheel axle for the driven (rear) wheels passes through the turn circle center. This istypically true only for a fairly large radius of turn.)
The outer wheel traverses an arc with radius ro and the inner wheel traverses an arc with radius ri.
As illustrated, the lengths of the arcs traversed are so, sn, and si. The outer arc so is obviouslylarger than the inner arc si for a given traversed angle theta. Some way of ensuring that the outer
wheel is able to turn slightly faster than the inner wheel must be ensured in order to prevent
binding and slippage of the tires on the road. For non-driven wheels which simply rotate freelyindependently of other machinery, this is not a problem. Driven wheels connected to the engine
via the driveshaft, however, must both be turned by gearing and this gear train must allow for
differential movement of the left wheel with respect to the right wheel. This is a difficult
problem since for every turning circle the differential rotation of the left and right wheels isdifferent. Fortunately, the automobile differential solves this problem with only one transmission
and one drive shaft for both driven wheels.
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Gear model in modern physics
Modern physics adopted the gear model in different ways. In nineteenth centuryJames Clerk
Maxwell developed a model of electromagnetism in which magnetic field lines were rotating
tubes of incompressible fluid. Maxwell used gear wheel and called it idle wheel to explain theelectrical current as a rotation of particles in opposite direction to that of the rotating field lines.
A new consideration of gear in quantum physicsis regarded as a quantum gears. A group of
gears can serve as a model for several different systems such as an artificially constructed
nanomechanical device or a group of ring molecules.
http://en.wikipedia.org/wiki/Modern_physicshttp://en.wikipedia.org/wiki/James_Clerk_Maxwellhttp://en.wikipedia.org/wiki/James_Clerk_Maxwellhttp://en.wikipedia.org/wiki/James_Clerk_Maxwellhttp://en.wikipedia.org/wiki/Quantum_physicshttp://en.wikipedia.org/wiki/Quantum_physicshttp://en.wikipedia.org/wiki/Modern_physicshttp://en.wikipedia.org/wiki/James_Clerk_Maxwellhttp://en.wikipedia.org/wiki/James_Clerk_Maxwellhttp://en.wikipedia.org/wiki/Quantum_physics8/8/2019 TermPaper MEC101
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BIBLIOGRAPHY
www.articlesbase.com/automotive-articles/applications-of-gears-
2028719.html
www.efunda.com/designstandards/gears/gears_auto_diff.cfm
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http://www.articlesbase.com/automotive-articles/applications-of-gears-2028719.htmlhttp://www.articlesbase.com/automotive-articles/applications-of-gears-2028719.htmlhttp://www.efunda.com/designstandards/gears/gears_auto_diff.cfmhttp://www.articlesbase.com/automotive-articles/applications-of-gears-2028719.htmlhttp://www.articlesbase.com/automotive-articles/applications-of-gears-2028719.htmlhttp://www.efunda.com/designstandards/gears/gears_auto_diff.cfmRecommended