Minor Projec Report Final

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AMinor Project Report

OnOVERDRIVE MECHANISM

Submitted to

Rajasthan Technical University, KotaIn Partial fulfillment for the award of the degree

Bachelor of TechnologyIn

Automobile Engineering

Session 2009-2010

SUBMITTED BY: GUIDED BY: SUBMITTED TO:Devashish Mazumdar Mr. Arjun Yadav Mr. Arun Kumar AryaJasdeep Singh Sahota Mr. Mukesh Batra (Head of Department)Gaurav Singh Panwar Automobile EngineeringDanish Ali

DEPARTMENT OF AUTOMOBILE ENGINEERINGARYA COLLEGE OF ENGINEERING AND I.T.

SP-40/42, Kukas Industrial area (RIICO), Delhi Road, Kukas, Jaipur

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ACKNOWLEDGEMENT

We feel great pleasure in expressing our esteemed regards to our teacher and

guide Mr. Arjun Yadav and Mr. Mukesh Batra for their skillful guidance and

unparallel supervision throughout the session. We have enormously benefited by

working under their guidance and will always be greatly indebted to them for their

perennial source of help. Everlasting inspirations and innumerable discussion as a

consequence of which present study reached fruition.

We would specially wish to thank Er. Arun Kumar Arya (H.O.D., Automobile

Engineering) for his kind help and valuable suggestions towards the successful

completion of the project.

We also thankful to all the faculty members of our department for their kind help

and encouragement throughout the tenure of our project.

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PREFACE

Before completion of our project on OVERDRIVE MECHANISM we only had anidea of how it will work?, but after completion of our project we understood all the

aspects of overdrive mechanism.

The project is the door which opens lots and lots of opportunities and pushes

ourselves to think as an engineer.

This report does discuss about the parts, mechanism, working and technologies which

are used in OVERDRIVE MECHANISM. It is much more fulfilling experience

than any report will be able to express

We thank everyone who has contributed to make this experience truly complete and

stimulating.

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DEPARTMENT OF AUTOMOBILE ENGINEERING

Certificate

This is to certify that the Minor project entitled OVERDRIVE MECHANISM

has been submitted to Rajasthan Technical University, Kota by the below mentioned

students in partial fulfillment for the award of degree, Bachelor of Technology in

Automobile engineering.

Devashish Mazumdar. (06EARAE016)

Jasdeep Singh Sahota. (06EARAE025)

Gaurav Singh Panwar. (06EARAE019)

Danish Ali . (06EARAE014)

Mr. Arun Kr. Arya Mr. Arjun Yadav Mr. Mukesh Batra(HOD) (Guide) (Guide)

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TABLE OF CONTENS

TOPIC PAGE NO.

(1) HISTORY OF OVERDRIVE MECHANISM 6

(2) INTRODUCTION 7

(3) OVERVIEW 8

(4) WORKING 9

(5) ADVANTAGES 16

(6) CONCLUSION 17

(7) BIBLIOGRAPHY 18

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HISTORY OF OVERDRIVE MECHANISM

Early manual automotive transmissions were limited to three or sometimes four speeds, with onlythe most sophisticated being five speeds. This left an unfulfilled need for a higher gear ratio for highway cruising, which was filled by the addition of separate overdrive units (auxiliary or bolt-on),in order to simulate an extra gear. Today however, automotive transmissions manufactured sincethe 1980s tend to include overdrive within the transmission. In the aftermarket early non-overdrivevehicles and 3/4 ton or heavier diesel trucks (where fuel economy and more gears are always indemand) are the main use of bolt-on auxiliary overdrives.

The vast majority of overdrives in European cars were manufactured by an English company calledLaycock de Normanville (later GKN Laycock), at its Little London Road site in Sheffield, which isnow demolished and remanufactured in the UK by an ex-Laycock de Normanville employee tradingas Overdrive Spares. The system was devised by an Englishman, named Captain Edgar J de

Normanville (1884 - 1968)[1], through a chance meeting with a Laycock Products Engineer. De Normanville overdrives were found in vehicles manufactured by Ford, British Leyland, Jaguar,Rootes Group and Volvo to name but a few. Another British company, the former aircraft builder Fairey, built a successful all-mechanical unit for the Land Rover, which is still in production inAmerica today.

The first production vehicle to feature the Laycock system was the 1948 Standard VanguardSaloon. The first unit to be created was the A-type overdrive, this was fitted to many sports cars

during the 1950s. Several famous marques used A-type overdrives, including Jaguar, Aston Martin,Ferrari, Austin Healey, Jensen, Bristol, AC and Armstrong Siddeley. Later the A-type overdrivewas to be fitted to the entire range of TR cars.In 1959, the Laycock Engineering Companyintroduced the D-type overdrive, which was fitted to a variety of motor cars including Volvo 120and 1800s, Sunbeam Alpines and Rapiers, Triumph Spitfires, and also early 3-synchro MGBs.From1967 the LH-type overdrive was introduced, and this featured in a variety of models, including 4-synchro MGBs, the Ford Zephyr, early Reliant Scimitars, TVRs, and Gilberns.

In the days before automatic transmissions were common, especially in the 1950s, many rear-wheeldrive American cars were available with an overdrive option. Borg Warner provided the box that

was factory-installed between the transmission and a foreshortened driveshaft. Since the overdrivefunction, if enabled, could be shifted by simply easing up on the accelerator without depressing theclutch pedal, the action was much like a semi-automatic. Also, an electrically operated solenoidwould deactivate the unit via a switch under the accelerator pedal providing the equivalent of thekickdown of the automatic. A knob connected to a bowden cable, similar to some emergency brakeapplications, was also provided to lock out the unit mechanically.

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INTRODUCTION

An overdrive is sometimes a separate unit that fits into the back of a gearboxt. A plate warns to onlyengage the unit in third and fourth gears. Overdrive can refer to two different things. An overdriveis a device which was commonly used in automobiles to allow the choice of an extra-high overallgear ratio for high speed cruising, thus saving fuel, at the cost of less torque. Usually the final or topgear is called overdrive. Non direct drive gears increase torque multiplication at the expense of higher engine rpm and thus, decreased fuel economy.

It also refers to a combination of gearing inside a transmission or transaxle which results in theoutput speed being greater than the input speed. In the latter case, "overdrive" does not refer to a

physically separate identifiable part/assembly. Generally speaking, overdrive (OD) is the highestgear in the transmission. Overdrive allows the engine to operate at a lower rpm for a given roadspeed. This allows the vehicle to achieve better fuel efficiency, and often quieter operation on thehighway. When it is switched on, an automatic transmission can shift into overdrive mode after acertain speed is reached (usually 70+ km/h [43+ mph] depending on the load). When it is off, theautomatic transmission shifting is limited to the lower gears. For an automatic transmission, it isalmost always best to select overdrive and allow the transmission to control engagement of theoverdrive. (It may be necessary to switch it off if the vehicle is being operated in a mountainousarea or when towing a trailer). With a manual transmission, overdrive should usually be selectedwhen the average speed is above 70 km/h (approx. 43 mph).

The automatic transmission automatically shifts from OD to direct drive when more load is present.When less load is present, it shifts back to OD. Under certain conditions, for example drivinguphill, or towing a trailer, the transmission may "hunt" between OD and the third gear, shifting back and forth. In this case, switching it off can help the transmission to 'decide'. It may also beadvantageous to switch it off if engine braking is desired, for example when driving downhill. Thevehicle's owner's manual will often contain information and suitable procedures regarding suchsituations, for each given vehicle.

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OVERVIEW-OVERDRIVE MECHANISM

The overdrive consists of an electrically or hydraulically operated epicyclic gear train bolted behindthe transmission unit. It can either couple the input driveshaft directly to the output shaft (or

propeller shaft) (1:1), or increase the output speed so that it turns faster than the input shaft (1:1 +n). Thus the output shaft may be "overdriven" relative to the input shaft. In newer transmissions, theoverdrive speed(s) are typically as a result of combinations of planetary/epicyclic gearsets whichare integrated in the transmission. In these cases, there is no separately identifiable "overdrive" unit.A number of such transmissions and transaxles are manufactured by Aisin, for use in vehicles

produced by many different manufacturers. In older vehicles, it is sometimes actuated by a knob or button, often incorporated into the gearshift knob, and does not require operation of the clutch. Newer vehicles have electronic overdrive in which the computer automatically adjusts to theconditions of power need and load.

Using overdrive gearing, the car's engine RPM drops, reducing engine wear and normally savingfuel. Since 1981 U.S. CAFE (corporate average fuel economy) legislation virtually all domesticvehicles have included overdrive to save fuel. One should refer to the car's owner's manual for the

proper speed to run at overdrive. All engines have a range of peak efficiency and it is possible for the use of overdrive to keep the engine out of this range for all or part of the time of its use if usedat inappropriate speeds, thus cutting into any fuel savings from the lower engine speed. There issome debate on the overall efficiency of overdrive gearing, as it requires more moving parts thandirect 1:1 drive, but most will agree that within the transmission, this effect is minimal. The other difficulty can be in the drive shaft rotation speed. Given below are pictures of epicyclic gear trains.

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WORKING OF OVERDRIVE SYSTEM

An overdrive mechanism is basically an epicyclic or in simpler words a planetary gear arrangementwhich is used generally in high performance sports cars to increase their top speed. Planetary gear arrangement used with various combination of locks can provide an increase in speed or increase intorque ,in case of cars it is used for highway cruising and increasing top speed but in case of tractors it is used for providing more toque where it is also known as reduction gear.

The working of an overdrive system is very simple ,it is placed right after the gear box and iscontrolled by either an electrical device or hydraulic. When it is not in use the transmission is sentstraight away to the wheels from the gear box but when the overdrive is engaged the torque isreduced and the speed gets increased. It can either couple the input driveshaft directly to the outputshaft (or propeller shaft) (1:1), or increase the output speed so that it turns faster than the input shaft(1:1 + n). Thus the output shaft may be "overdriven" relative to the input shaft. In newer transmissions, the overdrive speed(s) are typically as a result of combinations of planetary/epicyclicgearsets which are integrated in the transmission. In these cases, there is no separately identifiable"overdrive" unit.The main technology responsible for the success of overdrive mechanism is theEpiclic Gear train ,which is illustrated below in detail.Any planetary or Epicyclic gearset has threemain components:

The sun gear The planet gears and the planet gears' carrier The ring gear

Each of these three components can be the input, the output or can be held stationary. Choosingwhich piece plays which role determines the gear ratio for the gearsetOne of the planetary gearsetsfrom our transmission has a ring gear with 72 teeth and a sun gear with 30 teeth. We can get lots of different gear ratios out of this gearset.

Input Output Stationary CalculationGear Ratio

A Sun ( S )PlanetCarrier ( C)

Ring ( R) 1 + R/S 3.4:1

BPlanetCarrier ( C)

Ring ( R) Sun ( S ) 1 / (1 + S/R) 0.71:1

C Sun ( S ) Ring ( R)Planet Carrier (C)

-R/S -2.4:1

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Also, locking any two of the three components together will lock up the whole device at a 1:1 gear reduction. Notice that the first gear ratio listed above is a reduction -- the output speed is slower than the input speed. The second is an overdrive -- the output speed is faster than the input speed.The last is a reduction again, but the output direction is reversed. There are several other ratios thatcan be gotten out of this planetary gear set, but these are the ones that are relevant to our automatictransmission.

Given below is a diagram of the parts of the overdrive mechanism

Epicyclic gearing is used here to increase output speed. The planet gear carrier (green) is driven byan input torque. The sun gear (yellow) provides the output torque, while the ring gear (red) is fixed.

Note the red marks both before and after the input drive is rotated 45 clockwise.

Epicyclic gearing or planetary gearing is a gear system that consists of one or more outer gears, or planet gears, revolving about a central, or sun gear. Typically, the planet gears are mounted on amovable arm or carrier which itself may rotate relative to the sun gear. Epicyclic gearing systems

may also incorporate the use of an outer ring gear or annulus, which meshes with the planet gears.

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The carrier (green) is held stationary while the sun gear (yellow) is used as input. The planet gears(blue) turn in a ratio determined by the number of teeth in each gear. Here, the ratio is -24/16, or -3/2; each planet gear turns at 3/2 the rate of the sun gear, in the opposite direction.

In many epicyclic gearing systems, one of these three basic components is held stationary; one of

the two remaining components is an input, providing power to the system, while the last componentis an output, receiving power from the system. The ratio of input rotation to output rotation isdependent upon the number of teeth in each gear, and upon which component is held stationary.

One situation is when the planetary carrier is held stationary, and the sun gear is used as input. Inthis case, the planetary gears simply rotate about their own axes at a rate determined by the number of teeth in each gear. If the sun gear has S teeth, and each planet gear has P teeth, then the ratio isequal to -S/P. For instance, if the sun gear has 24 teeth, and each planet has 16 teeth, then the ratiois -24/16, or -3/2; this means that one clockwise turn of the sun gear produces 1.5 counterclockwiseturns of the planet gears.

This rotation of the planet gears can in turn drive the annulus, in a corresponding ratio. If theannulus has A teeth, then the annulus will rotate by P/A turns for each turn of the planet gears. For instance, if the annulus has 64 teeth, and the planets 16, one clockwise turn of a planet gear resultsin 16/64, or 1/4 clockwise turns of the annulus. Extending this case from the one above:

One turn of the sun gear results in S / P turns of the planets

One turn of a planet gear results in P / A turns of the annulus

So, with the planetary carrier locked, one turn of the sun gear results in S / A turns of the annulus.

The annulus may also be held fixed, with input provided to the planetary gear carrier; outputrotation is then produced from the sun gear. This configuration will produce an increase in gear ratio, equal to 1+A/S.

These are all described by the equation:

(2 + n)a + ns 2(1 + n)c = 0

where n is the form factor of the planetary gear, defined by:

If the annulus is held stationary and the sun gear is used as the input, the planet carrier will be theoutput. The gear ratio in this case will be 1/(1+A/S). This is the lowest gear ratio attainable with anepicyclic gear train. This type of gearing is sometimes used in tractors and construction equipmentto provide high torque to the drive wheels.In bicycle hub gears, the sun is usually stationary, beingkeyed to the axle or even machined directly onto it. In this case the gear ratio is simply given by(S+A)/A where S is the number of teeth on the sun and A is the number of teeth on the annulus.

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PROJECT OVERVIEW

In the project we have successfully arranged gears to demonstrate overdrive mechanism byfacilitating increase in the velocity of the output shaft. In the project we have kept the ring gear stationary while the input drive is given by the planet carrier and the output is taken by the sun gear which provides considerable amount of increase in the speed of transmission. The figures below

will illustrate further the purpose and the geometry of the project .

THE SUN GEAR

(With Support Bearing)

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The Ring Gear With the Planet Carrier And Planet Gears

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The Whole Arrangement Of The EPICYCLIC GEAR TRAIN

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EPICYCLIC GEAR TRAIN with RING LOCK FRAME And HANDLE

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ADVANTAGES AND DISADVANTAGESOF OVERDRIVE MECHANISM

Using overdrive gearing, the car's engine RPM drops, reducing engine wear and normally savingfuel. Since 1981 U.S. CAFE (corporate average fuel economy) legislation virtually all domesticvehicles have included overdrive to save fuel. One should refer to the car's owner's manual for the

proper speed to run at overdrive. All engines have a range of peak efficiency and it is possible for the use of overdrive to keep the engine out of this range for all or part of the time of its use if usedat inappropriate speeds, thus cutting into any fuel savings from the lower engine speed.

There is some debate on the overall efficiency of overdrive gearing, as it requires more moving parts than direct 1:1 drive, but most will agree that within the transmission, this effect is minimal.The other difficulty can be in the drive shaft rotation speed.

Overall drivetrain reduction comes down to three basic factors: transmission gearing (includingoverdrive), differential gearing (in the axle), and tire size. The rotation speed problem comes intoeffect when the differential gearing is a high ratio and an overdrive is used to compensate. This maycreate unpleasant vibrations at high speeds and possible destruction of the driveshaft due to thecentripetal forces or uneven balance.

The driveshaft is usually a hollow metal tube that requires balancing to reduce vibration and

contains no internal bracing.

The higher speeds on the driveshaft and related parts can cause heat and wear problems if anoverdrive and high differential gearing (or even very small tires) are combined, and createunnecessary friction. This is especially important because the differential gears are bathed in heavyoil and seldom provided with any cooling besides air blowing over the housing.

The impetus is to minimize overdrive use and provide a higher ratio first gear, which means moregears between the first and the last to keep the engine at its most efficient speed. This is part of thereason that modern automobiles tend to have larger numbers of gears in their transmissions. It is

also why more than one overdrive gear is seldom seen in a vehicle except in special circumstancesie. where high (numerical) diff gear is required to get the vehicle moving as in trucks or performance cars though double overdrive transmissions are common in other vehicles, often with asmall number on the axle gear reduction, but usually only engage at speeds exceeding 100 km/h or about 60mph.

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CONCLUSION

As the minor project comes to complete here we would like to share our experience. First of all ithas been a pleasant suffering for us. Everything has happened successfully . All the work we havedone before was totally theoretical now we realized the importance of practical application and

projects because we feel without practical whatever we have achieves is only the faith knowledge

we would like to share conclusion of our project :using overdrive gearing, the car's engine RPM drops, reducing engine wear and normally saving fuel.

All engines have a range of peak efficiency and it is possible for the use of overdrive to keep the engine outof this range for all or part of the time of its use if used at inappropriate speeds, thus cutting into any fuelsavings from the lower engine speed.

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BIBLIOGRAPHY

www.wikipedia.com www.howstuffworks.com Automotive Technology by N.K. GIRI AUTOMOTIVE ENGINEERING by WILLIAM CROUSE

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