INTRODUCTI

ON TO GEAR’S:

Gear, toothed wheel or cylinder used to transmit rotary or reciprocating

motion from one part of a machine to another. Two or more gears, transmitting

motion from one shaft to another, constitute a gear train. At one time various

mechanisms were collectively called gearing. Now, however, the word gearing is used

only to describe systems of wheels or cylinders with meshing teeth. Gearing is chiefly

used to transmit rotating motion, but can, with suitably designed gears and flat-

toothed sectors, be employed to transform reciprocating motion into rotating motion,

and vice versa.

THERE ARE BASICALLY TWO TYPES OF GEARS IN GENERAL

USAGE.

Simple Gears

The simplest gear is the spur gear, a wheel with teeth cut across its edge

parallel to the axis. Spur gears transmit rotating motion between two shafts or other

parts with parallel axes. In simple spur gearing, the driven shaft revolves in the

opposite direction to the driving shaft. If rotation in the same direction is desired, an

idler gear is placed between the driving gear and the driven gear. The idler revolves in

the opposite direction to the driving gear and therefore turns the driven gear in the

same direction as the driving gear. In any form of gearing the speed of the driven

shaft depends on the number of teeth in each gear. A gear with 10 teeth driving a gear

with 20 teeth will revolve twice as fast as the gear it is driving, and a 20-tooth gear

driving a 10-tooth gear will revolve at half the speed. By using a train of several

gears, the ratio of driving to driven speed may be varied within wide limits. Internal,

or annular, gears are variations of the spur gear in which the teeth are cut on the inside

of a ring or flanged wheel rather than on the outside. Internal gears usually drive or

are driven by a pinion, a small gear with few teeth. A rack, a flat, toothed bar that

moves in a straight line, operates like a gear wheel with an infinite radius and can be 1

used to transform the rotation of a pinion to reciprocating motion, or vice versa. Bevel

gears are employed to transmit rotation between shafts that do not have parallel axes.

These gears have cone-shaped bodies and straight teeth. When the angle between the

rotating shafts is 90°, the bevel gears used are called miter gears.

Helical Gears

These have teeth that are not parallel to the axis of the shaft but are

spiraled around the shaft in the form of a helix. Such gears are suitable for heavy

loads because the gear teeth come together at an acute angle rather than at 90° as in

spur gearing. Simple helical gearing has the disadvantage of producing a thrust that

tends to move the gears along their respective shafts. This thrust can be avoided by

using double helical, or herringbone, gears, which have V-shaped teeth composed of

half a right-handed helical tooth and half a left-handed helical tooth. Hypoid gears are

helical bevel gears employed when the axes of the two shafts are perpendicular but do

not intersect. One of the most common uses of hypoid gearing is to connect the drive

shaft and the rear axle in automobiles. Helical gearing used to transmit rotation

between shafts that are not parallel is often incorrectly called spiral gearing.

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Another variation of helical gearing is provided by the worm gear, also

called the screw gear. A worm gear is a long, thin cylinder that has one or more

continuous helical teeth that mesh with a helical gear. Worm gears differ from helical

gears in that the teeth of the worm slide across the teeth of the driven gear instead of

exerting a direct rolling pressure. Worm gears are used chiefly to transmit rotation,

with a large reduction in speed, from one shaft to another at a 90° angle.

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GEAR NOMENCLATURE:

Pitch circle-It is an imaginary circle which by pure rolling action, would give the

same motion as the actual gear.

Pitch circle diameter-It is the diameter o the pitch circle. The size of the gear is

usually specified by the pitch circle diameter. It is also called the pitch diameter.

Pitch point-It is common point of contact between two pitch circles.

Pitch surface-It is the surface of the rolling discs which the meshing gears have

replaced at the pitch circle.

Pressure angle or angle of obliquity-It is the angle between the common normal to

two gear teeth at the point of contact and the common tangent to the pitch point. It is

usually denoted by . The standard pressure angles are 141/2ɸ O and 20O.

Addendum-It is the radial distance of a tooth from the pitch circle to the top of the

tooth.

Dedendum-It is the radial distance of a tooth from the pitch circle to the bottom of

the tooth.

Addendum circle-It is the circle drawn through the top of the teeth and is concentric

with the pitch circle.

Dedendum circle-It is the circle drawn through the bottom of the teeth. It is also

called root circle.

Circular pitch-It is the distance measured on the circumference of the pitch circle

from a point of one tooth to the corresponding point on the next tooth. It is usually

denoted by pc.

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FLOWCHART:

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BLANK PREPARATION

FOR TOOTH HOBBING

TOOTHHOBBING

GEAR MACHINING

CASE CABURISING

STABILIZING TEMPERING

HARDENINGFINAL

GRINDING OF BEARING

SETTING DIAINSPECTION

BLANK PREPARATION

MATERIAL TO BE USED

The material used for blank preparation should have following properties

1) The material should have high strength to weight ratio.

2) High yield strength and hardness

3) Simple, uniform section

4) Smoothness

5) Ductility

6) Fatigue

7) The material should have low cost.

A variety of cast iron, powder-metallurgy materials, nonferrous alloys, nonmetallic

materials are used in gears.

The blank is prepared by general metal forming process. The processes are

1) Forging

2) Rolling

3) Metal Extrusion Process

FORGING

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Forging, process of shaping iron and other malleable metals by hammering or

pressing them after making them plastic by application of heat. Forging techniques

are useful in the working of metal because the metal can be given the desired form,

and the process improves the structure of the metal, particularly by refining the grain

size of the metal.

Forged metal is stronger and more ductile than cast metal and exhibits greater

resistance to fatigue and impact.

The metal is heated above recrystallization temperature but below melting point.

Then it is forged by using any of the three types of hammer . After preparation of

blank stress relieving tempering is done. `The last stage of bank preparation is

inspection .

As after forging the surface finish is not up to the mark so the forged blank has

to be machined well. So that the required surface finish is obtained.

ROLLING

Cylindrical blanks are prepared by roll forming. In this type three rollers are arranged

and material is place in between them. So that a blank of required surface finish is

obtained .

EXTRUSION

Extrusion is a process of forcing substances, especially metals or thermoplastics,

through a die to produce various shapes of uniform cross section widely used in

industry and constructions. Hot extrusion being more common than cold extrusion.

The manufacturing of cylindrical hollow blanks are done by such extrusion process.

GEAR CUTTING AND TOOTH HOBBING

Three methods of cutting gear are

• Forming

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• Form cutting

• Generation

BASIC CHARECTERISTICS OF GEAR CUTTING MACHINES

Any gear cutting machine which works by any one of these methods must have

o A cutting tool of suitable shape, hardness and sharpness

o A means for properly holding the work piece and correctly aligning it with the

cutting tool, and

o A precision indexing device for accurately spacing the gear teethes.

o An accurately controlled means of providing relative role between the work and the

tool during cutting.

1) FORMING

In the forming method the gear tooth takes its shape directly from the shape of

the cutting tool. It is the oldest of the gear cutting methods yet is still widely and

efficiently used. Many spur gears-especially those with involute teeth profiles are cut

with brown and sharp forming milling cutters, which first were patented in 1864.

The accuracy of the form tooth profile depends upon the accuracy with which

the cutter was made is of the finish cutting edges must be identical to every other

The Gleason FORMATE of helix form gear finishing methods are forming

methods

2) FORM CUTTING

The Gleason straight bevel gear planner, introduced in the year 1875, used the

form cutting method. The reciprocating planning tool cut with its rounded point only,

as the tool slide arm traveled along a master former to trace out the desired tool

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profile curvature. The accuracy of the profile depends up on the skill and accuracy

with which the master template was made.

By today’s standard form cutting is the slow and cumbersome method. In 1875,

however, it represented tremendous improvement in the speed and quality of

production over the existing practice of hand file finishing cast gear teeth. Form

cutting is still used today for some larger group of 1 DP an coarser, although no new

Gleason machine of this type have been built since about 1940.

The surface of form cut teeth are composed of adjacent groups and ridges left by

the grounded point of the cutting tool. These surfaces are not as smooth as generated

surfaces, which are composed of adjacent flats. Only straight bevel gears are formed

by form cutting.

3) GENERATION

The generation of the gear tooth profile requires accurately controlled relative

role between the utter and the work. In the Gleason machine, relative role is provided

by ratio of roll gears. A generated profile shape does not depend on the shape of the

generating tool’s cutting edges. Since this is so, the cutting edges of any shape and it

is most convenient and least expensive to make them straight.

A generated tooth surface is composed of a series of adjacent flats, which can be

placed as close together as desired to control the finish of the surface. Naturally, the

closer and narrower the flats are, the smoother the finish will be.

TOOTH HOBBING

A gear-cutting worm made into a gear generating tool by a series of

longitudinal slots or gashes machined into it to form the cutting teeth

Available with one thread, two thread, or more

It can produce a variety of gears at high rate with good dimensional accuracy

Extensively used in the industry

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Hobbing is perhaps the most widely used method of cutting spur and helical gears.

The hob is a cutting tool, cylindrical is shape, with the teeth wrapped in a spiral

around the cylinder. The hob looks like a worm gear with axial slots to provide

cutting surfaces and with the teeth relieved back from the cutting edges to provide

cutting clearance. An axial section of a hob can be thought of as a rack.

Because the involute curve decrease as the gear size increases, the involute curve on a

rack has decreased to zero and the hob tooth, along he pressure angle, is essentially a

straight line. A gear to be cut is rotated about its own axis and therefore the hob tooth

will produce the proper involute regardless of the size of the gear to be cut. The hob is

rotated about its own axis in a timed relationship to the work piece so that, for a single

lead hob, one rotation of the hob equals a moment of one pitch of the gear.

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Fig. Gear hobbing device and hobbing mechanism

CASE CARBURISING

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Case hardening is the change of surface composition under the

combined action of Chemical and heat thermal treatments. E.g. carburization,

nitriding, cyaniding, carbunitiding etc.

Carburization of steel involves a heat treatment o the metallic surface

using a source of carbon. Early carburization used a direct application of a charcoal

packed into the metal (initially referred to as case hardening), but modern techniques

apply carbon bearing gases or plasma (such as carbon dioxide or methane). The

process depends primarily upon ambient gas composition and furnace temperature,

which must be carefully controlled, as the heat may also impact the microstructure of

the rest of the material. The application where great control of over gas composition is

desired carburization may take place in a very low pressure in a vacuum chamber.

The carbon content of case hardening steel is low, usually about 0.15

to 0.20 %. Case hardening steel also contains Ni, Cr, Mo, Mn, etc. It is suitable for

carburizing and quenching.

For such hardening the process followed is as follows

CARBURIZINGQUENCHINGCLEANING

TEMPERINGSHOT BLASTINSPECTION.

Case hardened steel is usually formed by diffusion of carbon

(carburizing) into the outer layer of the steel at high temperature. And putting carbon

into the surface of steel makes it high-carbon steel like S45C, which can be hardened

by heat treatment.

Surface hardness is about 55~60 HRC.

Depth of surface hardening is about 1.0 mm.

Carburizing and quenching produces a hard, wear resistant

surface over a strong tough core. Some special purpose steel gears are

case hardened by either carbo-nitriding or nitriding. Other special purpose

gears, such as those used in chemical or food processing equipment, are

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made of stainless steel or nickel based alloy or carburized because of their

corrosion resistance, their ability to satisfy sanitary standards or both.

WORK MATERIAL PROPERTIESEFFECTS OF CARBURIZING

Mechanical 1)Increased surface hardness. 2)Increased

wear resistance. 3)Increased

fatigue/tensile strength.

Physical 1)Grain growth may occur 2)Change in

volume may occur

Chemical 1)Increased surface carbon content.

CONVENTIONAL MACHINING

TURNING

This operation involves grabbing or holding the job or work piece on the lathe

or turning machine and rotating it at a very high speed and removing material from its

surface (may it be internal or exterior) using a tool. This is a very extensive method

used rigorously during gear preparation. Starting from blank preparation to thread

formation etc. this method is used.

MILLING

Milling is a form-cutting process limited to making single gears for prototype or very

small batches of gears as it is a very slow and uneconomical method of production.

An involute form-milling cutter, which has the profile of the space between the gears,

is used to remove the material between the teeth from the gear blank on a horizontal

milling machine. The depth of cut into the gear blank depends on the cutter strength,

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set-up rigidity and machineability of the gear blank material.

GEAR PLANING OR RACK GENERATION

This is used for mid volume production. A rack, which may be considered to

be a gear of infinite radius, is used as the cutter. It is constructed of hardened steel

with cutting edges round the teeth boundaries. The rack which is given a reciprocal

lateral motion equal to the pitch line velocity of the gear is slowly fed to the slowly

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rotating gear blank. In this way, the material between the teeth is removed and the

involute teeth are generated.

GEAR SHAPING

The cutter is a circular pinion-shaped cutter with the necessary rake angles

to cut as shown. Both the gear blank and cutter are set in a vertical plane and rotated

such as that the two are like gears in mesh. Gear shaping is faster than gear planing

because the cutting process is continuous and the cutter does not have to be stepped

back.

Shaping is practically the only method available for cutting teeth close to a

shoulder and is the only available method for generating internal spur gears. The

fellows process has been widely accepted since it development in 1900 and still

commonly used despite recently increased competition from hobbing, shaving and

grinding techniques. The Fellows gear shaper can be specially equipped to cut helical

gears, helical or spur racks and shaft ends with splined or solid keys

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Fig-Gear shaping

CYLINDRICAL GRINDING

The cylindrical grinder is a type of grinding machine used to shape the

outside of an object. The cylindrical grinder can work on a variety of shapes; however

the object must have a central axis of rotation. This includes but is not limited to such

shapes as a cylinder, an ellipse, a cam, or a crankshaft.

Basically 5 kinds of cylindrical grinding operations are carried out during

gear machining.

Outside diameter grinding-OD grinding is grinding occurring on

external surface of an object between the centers. The centers are end units

with a point that allow the object to be rotated. The grinding wheel is also

being rotated in the same direction when it comes in contact with the object.

This effectively means the two surfaces will be moving opposite directions

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when contact is made which allows for a smoother operation and less chance

of a jam up.

Inside diameter grinding-ID grinding is grinding occurring on the

inside of an object. The grinding wheel is always smaller than the width of the

object. The object is held in place by a collet, which also rotates the object in

place. Just as with OD grinding, the grinding wheel and the object rotated in

opposite directions giving reversed direction contact of the two surfaces where

the grinding occurs

Plunge grinding-A form of OD grinding, however the major difference is

that the grinding wheel makes continuous contact with a single point of the

object instead of traversing the object

Creep feed grinding-Creep Feed is a form of grinding where a full

depth of cut is removed in a single pass of the wheel. Successful operation of

this technique can reduce manufacturing time by 50%, but often the grinding

machine being used must be designed specifically for this purpose. This form

occurs in both cylindrical and surface grinding.

Centerless grinding-It is a form of grinding where there is no collet or

pair of centers holding the object in place. Instead, there is a regulating wheel

positioned on the opposite side of the object to the grinding wheel. A work

rest keeps the object at the appropriate height but has no bearing on its rotary

speed. The work blade is angled slightly towards the regulating wheel, with

the work piece centerline above the centerlines of the regulating and grinding

wheel; this means that high spots do not tend to generate corresponding

opposite low spots, and hence the roundness of parts can be improved.

Centerless grinding is much easier to combine with automatic loading

procedures than centered grinding; through feed grinding, where the regulating

wheel is held at a slight angle to the part so that there is a force feeding the

part through the grinder, is particularly efficient.

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Fig. Cylindrical grinding process and machine.

HARDENING

Martensite is a very hard phase(VHNFe3C=800, VHNMart= 880). It can be produced

only if the transformation of austenite to mixture of ferrite and carbide is avoided. In

most of the cases it is possible by faster cooling (quenching) of the steel.

Hardening consists of heating the steel to proper austenitising temperature, soaking

of this temperature to get fine–grained and homogenous austenite, and then cooling

the steel at a rate faster than its critical cooling rate. Such cooling is called quenching.

Normally, carbon steels are quenched in water, alloy steels in oil (as critical cooling

rate of alloy steels is much less), etc.

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OBJECTIVE OF HARDENING

Hardening is done to all tools, heavy – duty carbon steel machine parts and almost all

machine parts made of alloy steels.

1. Main aim of hardening tools is to induce high hardness. The cutting property

of the tool is directly proportional to the hardness of the steel.

2. Many machine parts and all tools are also hardened to achieve high wear

resistance. Higher is the hardness, higher is the wear and abrasion resistance.

For example, spindles, gears, shafts, cams, etc.

3. Develop high yield strength with good toughness and ductility, so that higher

working stresses are allowed.

DEFECTS IN HARDENED PRODUCTS

1. Mechanical properties not up to the specification

2. Soft spot

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3. Quench crack

4. Distortion and warpage

5. Change in dimensions

6. Oxidation and decarburization

7. Overheating

GRINDING OF BEARING SETTING DIAMETER

The gear has its primary purpose to transmit power from one source to another.

During this process the gears are required to be held by a bearing which holds it to be

allowed to be rotated.

The accuracy required here is very high because the entire mechanism of transmission

of rotation rests on the bearing. Hence it is maintained in microns. The bearing setting

is formed by the method of grinding.

TEMPERING

DEFINITION

Heating the steel to a tempering up to lower critical, soaking followed by slow

cooling. Temperature of tempering is decided by final required hardness and type of

steel.

ADVANTAGE OVER QUENCHING

As quenched steels have very limited applications due to the following reasons:

1. Martensite, generally a hard phase but very brittle

2. Possesses high internal stresses, relieve of internal stresses during use may

develop distortion and cracking

3. Martensite and retained austenite both unstable, decompose in to stable

phases, dimensional instability20

Tempering is done to address these limitations of as quenched steels.

OBJECTIVE

1. To relieve internal stresses

2. To restore ductility and toughness, however there is loss of strength

3. To stabilize dimension

4. To improve magnetic properties, as austenite is non-magnetic

STRUCTURE OF AS QUENCHED AND TEMPERED

STEEL

As quenched: Martensite (lath or twinned), Retained Austenite, Cementite and

undissolved alloy carbides

Tempered steel: Carbides (iron and alloy) embedded in a matrix of ferrite.

STAGES OF TEMPERING

Tempering involves heating. This allows diffusion of carbon and other elements as a

result changes in structure. This occurs in four district but overlapping stages:

1. First stage (up to 200C)- C atoms diffuse out from Mart., loss of tetragonality.

Precipitation of ε-carbide (Fe2.4C)

2. Second stage (200-300C)- Transformation of R Austenite (loss of C) to

Bainite/ Martensite

3. Third stage(200-350C)- Complete loss of tetragonality of Martensite,

Dissolution of ε-carbide, formation of rods or plate of Martensite.

4. Fourth stage (350-700C) - Sherardizing and coarsening of cementite and

recrystallisation of ferrite.

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5. Secondary Hardening or fifth stage of tempering. Tempering involves heating.

This allows diffusion of carbon and other elements as a result changes in

structure. This occurs in four district but overlapping stages:

1. First stage (up to 200C)- C atoms diffuse out from Mart., loss of tetragonality.

Precipitation of ε-carbide (Fe2.4C)

2. Second stage (200-300C)- Transformation of R Austenite (loss of C) to

Bainite/ Martensite

3. Third stage(200-350C)- Complete loss of tetragonality of Martensite,

Dissolution of ε-carbide, formation of rods or plate of Martensite.

4. Fourth stage (350-700C)- Sherardizing and coarsening of cementite and

recrystallisation of ferrite.

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Fig. Effect of tempering on 0.45% carbon and 0.1%

carbon steel

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INSPECTION:

Inspection is a very crucial and mandatory procedure carried out during gear

manufacturing. It is done at almost all levels or steps of gear production to ensure the

quality of gear is up to the mark. Along with the final gear inspection, there are

inspections or checking carried out at every alternate step like case carburizing,

hardening etc.

There are various ways of inspections done.

Magnetic inspections- Here the job is magnetically charged and powdered

metal is poured over it. The powdered metal flows over the surface of the

product and penetrates over the crack. Thereby the cracks are identified. Suck

type of methods are useful in identifying cracks at a laminar or surface profile.

Dye checking- It is a very important method for understanding the points of

gear engagement and pressure which is applied to the teethes. Here we have a

gear for testing which is covered in a dye. The product to be tested in engaged

to gear in a condition similar to that of the working condition. The engagement

of teethes shows the points or area of contact and the pressure distribution. If

any fault is found during contact or the colour is not transmitted then the error

is found.

Visual- It is one of the basic ways of inspecting the product. Simply by visual

checking of the gear the major discrepancies of the gear can be sought out and

relieved. It is done after every process to ensure the process is going in a right

direction.

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CONLUSION:

Gears are a very vital part in an aerospace engine as well as for others

products. And the understanding of its manufacturing process gives us an intricate

idea about the care taken in producing a different gear. It is though a production of a

small part, but it still involves a great deal of process, a lot of care and detailed

accuracy. To be able to sustain the adverse conditions is a very challenging task, and

the gear has to be made to face it.

This project makes the reader to be able to differentiate between an

automobile and an different manufacturing process. The various steps involved in

each gear making helps to understand the features of such gears. If the reader finds

this report to be relevant for understanding the manufacturing process of gears then

the report can be deemed to be successful.

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