TRANSMISSION SYSTEM: GEAR DRIVETRANSMISSION SYSTEM: GEAR DRIVE

GEARBOXGEARBOXFUNDAMENTAL, OPERATION FUNDAMENTAL, OPERATION

& & MAINTENANCEMAINTENANCE

Presentation byPresentation by

Nirjhar ChakravortiNirjhar Chakravorti

Features…Features…► IntroductionIntroduction►Why gears are used?Why gears are used?► Fundamentals of gearingFundamentals of gearing► Classification of gearsClassification of gears► Gear Making processGear Making process► Type of gear boxesType of gear boxes►Material for gear boxesMaterial for gear boxes► Gear lubrication and coolingGear lubrication and cooling► Gear failuresGear failures► Trouble shootingTrouble shooting►Market products and specificationMarket products and specification

IntroductionIntroduction

The term gear is defined as a machine The term gear is defined as a machine element used to transmit motion and element used to transmit motion and power between rotating shafts and by power between rotating shafts and by means of progressive engagement of means of progressive engagement of projections called teeth.projections called teeth.

Toothed gearing system evolved gradually Toothed gearing system evolved gradually from the primitive forms where wooden pins from the primitive forms where wooden pins were arranged on the periphery of simple were arranged on the periphery of simple wooden wheels to drive opposite member of wooden wheels to drive opposite member of the pair. These wheels served the purpose of the pair. These wheels served the purpose of gears those days. Of course the operation was gears those days. Of course the operation was neither smooth nor quite and obviously speeds neither smooth nor quite and obviously speeds were very slow. The motive power to turn were very slow. The motive power to turn these systems was generally provided by the these systems was generally provided by the trade mills, which were operated by men, trade mills, which were operated by men, animals and windmills.animals and windmills.

Archimedes knew primitive forms of Archimedes knew primitive forms of gear before the Christian era. Leonardo gear before the Christian era. Leonardo de Vinci used the concept of gear de Vinci used the concept of gear system in many of his proposed system in many of his proposed machines.machines.

In recent times, however, the In recent times, however, the technologists and engineers have technologists and engineers have concentrated their efforts to device concentrated their efforts to device curves for gear teeth, which would curves for gear teeth, which would provide constant relative velocity of the provide constant relative velocity of the gear tooth faces. With the invention of gear tooth faces. With the invention of various geometric curves (such as various geometric curves (such as involutes, cycloids) the gearing system involutes, cycloids) the gearing system has become very attractive, quite and has become very attractive, quite and continuous.continuous.

For the evolution of modern day gearing For the evolution of modern day gearing technology we must acknowledge the technology we must acknowledge the contribution of scientists like Louise, contribution of scientists like Louise, Buckingham, Euler. Buckingham, Euler.

Why gears are used?Why gears are used?► To transmit torque/ power from one shaft to To transmit torque/ power from one shaft to

another shaft.another shaft.Other commonly used power transmitting elements:Other commonly used power transmitting elements: Belt driveBelt drive

major limitation: major limitation: SlippageSlippage Rope drive Rope drive Chain drive major limitation: Chain drive major limitation: Discontinuous driveDiscontinuous drive

To avoid slippage and discontinuity during power transmission, the best To avoid slippage and discontinuity during power transmission, the best solution is gear drive.solution is gear drive.

► To get desired rotational speedTo get desired rotational speed► To change direction of rotationTo change direction of rotation► Where distance between drive shaft and driven Where distance between drive shaft and driven

shaft is very smallshaft is very small

Disadvantages of gear driveDisadvantages of gear drive

Disadvantages:Disadvantages:• Gear drive is costlier than other drivesGear drive is costlier than other drives• Error during cutting teeth causes Error during cutting teeth causes

vibration & noise while operationvibration & noise while operation• Gear drive requires proper lubrication Gear drive requires proper lubrication

for smooth runningfor smooth runningSo as usual, designers has to be logical So as usual, designers has to be logical and optimal during selection of drive.and optimal during selection of drive.

Fundamental of GearingFundamental of GearingHow gears transmit power and motion: Concept of friction How gears transmit power and motion: Concept of friction

wheelwheelThe motion and power transmitted by gear is kinematically equivalent to that transmitted by friction wheel.Consider two circular wheels A & B mounted on two parallel shafts and the wheels are pressed against each other. Shafts are mounted on anti-friction bearings.When wheel A is rotated by shaft, it will rotate wheel B in opposite direction.

A BT

F=µRN

RN

Force on the system:• RN= Normal reaction between two rubbing wheels

(Separation force)• T= Tangential force• F= Frictional Force (F= µRN),

where µ= co-efficient of frictionWhen T≤ µRN wheel B will rotated by wheel AWhen T> µRN slippage will take place

How to prevent this slippage?How to prevent this slippage?►Simply by increasing the frictional resistance Simply by increasing the frictional resistance

between the two surfaces. To increase this between the two surfaces. To increase this frictional resistance, a number of projections frictional resistance, a number of projections (called teeth) are made on the periphery of (called teeth) are made on the periphery of both the wheels.both the wheels.

►A friction wheel with teeth cut on it is called A friction wheel with teeth cut on it is called gear.gear.

Gear Tooth TerminologyGear Tooth Terminology

Important definitionsImportant definitions

► Pitch circle: It is the imaginary circle that rolls without slip with a pitch circle of a mating gear.

► Pitch Point:It is the point of tangency of two pitch circles and is on the line-of-centers. The pitch point of a tooth profile is at its intersection with the pitch circle.

► Circular pitch:Circular pitch:

It is the distance measured on the It is the distance measured on the circumference of the pitch circle from a circumference of the pitch circle from a point on the tooth to the corresponding point on the tooth to the corresponding point on the next tooth. Mathematically,point on the next tooth. Mathematically,circular pitch, pc=circular pitch, pc=∏∏D/T,D/T,where, where, D= P.C.D.D= P.C.D.T= No. of teethT= No. of teeth

► Module:Module:

It is the ratio of the pitch circle diameter in It is the ratio of the pitch circle diameter in millimeters to the number of teeth.millimeters to the number of teeth.Mathematically, Module, m=D/TMathematically, Module, m=D/T

The recommended series of the modules in The recommended series of the modules in Indian standard areIndian standard are1, 1.25, 1.5, 2, 2.5, 3, 4, 5, 6, 8, 10, 12, 16, 1, 1.25, 1.5, 2, 2.5, 3, 4, 5, 6, 8, 10, 12, 16, 20, 25, 32, 40, 50.20, 25, 32, 40, 50.

► Backlash:Backlash:It is the distance between the tooth space and the tooth It is the distance between the tooth space and the tooth thickness of mating gear as measured on the pitch circle.thickness of mating gear as measured on the pitch circle.Purpose of backlash:Purpose of backlash: prevent gears from jamming togetherprevent gears from jamming together making contact on both sides of their teeth making contact on both sides of their teeth simultaneously.simultaneously. Lack of backlash may cause noise, overloading, Lack of backlash may cause noise, overloading, overheating of the gears and bearings, and even seizing and overheating of the gears and bearings, and even seizing and failure.failure.Excessive backlash is objectionable, particularly if the drive is Excessive backlash is objectionable, particularly if the drive is frequently reversing, or if there is an overrunning load as in frequently reversing, or if there is an overrunning load as in cam drives. Excess backlash causes noisy operation.cam drives. Excess backlash causes noisy operation.Backlash does not affect involute action.Backlash does not affect involute action.

► Pressure Angle:Pressure Angle:If a tangent is drawn to the involute profile If a tangent is drawn to the involute profile of a tooth at any point on the curve, and if a of a tooth at any point on the curve, and if a radius line is drawn through this point of radius line is drawn through this point of tangency connecting this point with the tangency connecting this point with the centre of the gear, then the acute angle centre of the gear, then the acute angle included between this tangent and radial included between this tangent and radial line is defined as pressure angle at that line is defined as pressure angle at that point.point.In gear design, the pressure angle is In gear design, the pressure angle is measured at the pitch point.measured at the pitch point.

Principle of transmission&conjugate Principle of transmission&conjugateactionaction

When a pair of gear teeth act together When a pair of gear teeth act together against each other, rotary motion is against each other, rotary motion is produced which is transmitted from the produced which is transmitted from the driver to the driven gear. If such a pair of driver to the driven gear. If such a pair of tooth profiles which are so designed that a tooth profiles which are so designed that a constant angular velocity ratio is produced constant angular velocity ratio is produced and maintained during meshing, the two and maintained during meshing, the two gears are said to have conjugate action and gears are said to have conjugate action and the tooth profiles are said to have conjugate the tooth profiles are said to have conjugate curves.curves.

Conjugate actionConjugate action

In other words, conjugate action is assumed In other words, conjugate action is assumed if, if, ωω11/ / ωω22=constant, where,=constant, where,ωω11= Angular velocity of the driver (generally called pinion)= Angular velocity of the driver (generally called pinion)

ωω22= Angular velocity of the driven (generally called gear)= Angular velocity of the driven (generally called gear)

In case of conjugate action,In case of conjugate action,ωω11/ / ωω22= D= D22/D/D11

DD11= PCD of pinion= PCD of pinion

DD22= PCD of gear= PCD of gear

Gear teeth profileGear teeth profile

To produce conjugate action, following To produce conjugate action, following tooth profiles are used:tooth profiles are used:

► Involute teethInvolute teeth► Cycloidal teethCycloidal teeth► Epi-cycloidal teethEpi-cycloidal teeth► Hypo cycloidal teeth Hypo cycloidal teeth

Involute curveInvolute curve

The involute curve is most easily understood as the trace of a point at the end of a taut string that unwinds from a cylinder. It is imagined that a point on a string, which is pulled taut in a fixed direction, projects its trace onto a plane that rotates with the base circle. The base cylinder, or base circle as referred to in gear literature, fully defines the form of the involute and in a gear it is an inherent parameter, though invisible.

Involute curveInvolute curve

►Involute teeth is most commonly used Involute teeth is most commonly used profile for industrial gears. profile for industrial gears.

Cycloidal curveCycloidal curve► Consider a point, “P” on the perphery of a Consider a point, “P” on the perphery of a

circle. Now if the circle rolls on a fixed line, circle. Now if the circle rolls on a fixed line, then the contour made by the point “P”, is then the contour made by the point “P”, is known as cycloidal curve.known as cycloidal curve.

► If the circle rolls on the outer periphery of If the circle rolls on the outer periphery of another circle (which is fixed), then the contour another circle (which is fixed), then the contour made by the point “P”, is known as epi-cycloidal made by the point “P”, is known as epi-cycloidal curve.curve.

► If the circle rolls on the inner periphery of If the circle rolls on the inner periphery of another circle (which is fixed), then the contour another circle (which is fixed), then the contour made by the point “P”, is known as hypo-made by the point “P”, is known as hypo-cycloidal curve.cycloidal curve.

Cycloidal curveCycloidal curveP (Trajectory followed by point “P” is called Cycloidal Curve)

Fixed circle

P (Trajectory followed by point “P” is called Epi-cycloidal Curve)

P (Trajectory followed by point “P” is called Hypo-cycloidal Curve)

Fixed circle

Why involute curve is widely used?Why involute curve is widely used?

The involute curve is almost exclusively used in The involute curve is almost exclusively used in modern gearing system. The advantage of involute modern gearing system. The advantage of involute profile over the cycloidal profile is given below:profile over the cycloidal profile is given below:

►The form of the basic rack tooth is straight-sided, and therefore is relatively simple and can be accurately made; as a generating toolift imparts high accuracy to the cut gear tooth. A hob cutter for the cycloidal gear is not as easily made.

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Why involute curve is widely used?Why involute curve is widely used?► In cycloidal gears, for a mating pair, there is only one In cycloidal gears, for a mating pair, there is only one

theoretical correct centre distance for which this will theoretical correct centre distance for which this will transmit motion maintaining a constant angular velocity transmit motion maintaining a constant angular velocity ratio.ratio.in case of involute gearing system, the centre distance in case of involute gearing system, the centre distance can be changed without affecting the angular velocity can be changed without affecting the angular velocity ratio.ratio.This advantage of involute system is of prime This advantage of involute system is of prime importance as most of the modern gears are corrected importance as most of the modern gears are corrected ones having changed centre.ones having changed centre.Also, in case of gearing systems having standard centre Also, in case of gearing systems having standard centre distance, it is not possible to accurately maintain that distance, it is not possible to accurately maintain that distance due to mounting inaccuracies, misalignments distance due to mounting inaccuracies, misalignments and a number of other diverse factors.and a number of other diverse factors.

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Why involute curve is widely used?Why involute curve is widely used?

►While the cycloidal tooth profile has double While the cycloidal tooth profile has double curvature, an involute tooth has single curvature, an involute tooth has single curvature, which facilitates ease of curvature, which facilitates ease of manufacturing.manufacturing.

► In involute profile as the path of contact is In involute profile as the path of contact is straight line and the pressure angle is straight line and the pressure angle is constant, there is constant force acting on the constant, there is constant force acting on the axes. In cycloidal gear, the pressure angle axes. In cycloidal gear, the pressure angle continuously changes. This results in continuously changes. This results in separating forces of variable magnitude, separating forces of variable magnitude, which in turn gives rise to unquiet operation.which in turn gives rise to unquiet operation.

Disadvantages of involute curve over Disadvantages of involute curve over cycloidal curve:cycloidal curve:

►In case of cycloidal gear, minimum number In case of cycloidal gear, minimum number of teeth can be as low as 6 or 7; but in case of teeth can be as low as 6 or 7; but in case of involute gears, minimum number of teeth of involute gears, minimum number of teeth is 17.is 17.

►The cycloidal teeth is stronger than the The cycloidal teeth is stronger than the involute teeth. There is more material at the involute teeth. There is more material at the root portion of the cycloidal tooth as root portion of the cycloidal tooth as compared to an involute tooth.compared to an involute tooth.

►Involute teeth has the problem of Involute teeth has the problem of interference, but the cycloidal gears do not interference, but the cycloidal gears do not have interference and the problem thereof.have interference and the problem thereof.

Interference & UndercuttingInterference & Undercutting

► The maximum length of the line-of-contact is The maximum length of the line-of-contact is limited to the length of the common tangent.limited to the length of the common tangent.

► Any tooth addendum that extends beyond the Any tooth addendum that extends beyond the tangent points (T and T') is not only useless, but tangent points (T and T') is not only useless, but interferes with the root fillet area of the mating interferes with the root fillet area of the mating tooth. This results in the typical undercut tooth. tooth. This results in the typical undercut tooth. The undercut not only weakens the tooth with a The undercut not only weakens the tooth with a wasp-like waist, but also removes some of the wasp-like waist, but also removes some of the useful involute adjacent to the base circle.useful involute adjacent to the base circle.

► From the geometry of the limiting length-of-From the geometry of the limiting length-of-contact (T-T') it is evident that interference is first contact (T-T') it is evident that interference is first encountered by the addenda of the gear teeth encountered by the addenda of the gear teeth digging into the mating-pinion tooth flanks.digging into the mating-pinion tooth flanks.

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Interference & UndercuttingInterference & Undercutting

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Interference & UndercuttingInterference & Undercutting

Geometry of proper teeth meshing

Tooth correction: One of the method of avoiding undercut, is tooth correction or tooth enlargement. This is achieved by pinion enlargement (or correction as often termed), wherein the pinion teeth, still generated with a standard cutter, are shifted radially ourward to form a full involute tooth free of undercut The tooth is enlarged both radially and circumferentially.

Interference & UndercuttingInterference & Undercutting► Interference condition becomes more severe as the Interference condition becomes more severe as the

number of teeth on the gear increases. The limit is number of teeth on the gear increases. The limit is reached when the gear becomes a rack. This is a reached when the gear becomes a rack. This is a realistic case since the hob is a rack-type cutter. The realistic case since the hob is a rack-type cutter. The result is that standard gears with tooth numbers result is that standard gears with tooth numbers below a critical value are automatically undercut in below a critical value are automatically undercut in the generating process. The limiting number of teeth the generating process. The limiting number of teeth in a gearin a gear

►meshing with a rack is given by the expression:meshing with a rack is given by the expression:► NNcc = 2/sin² = 2/sin²αα► This indicates the minimum number of teeth free of This indicates the minimum number of teeth free of

undercutting decreases with increasing Pressure undercutting decreases with increasing Pressure angle.angle.

► For 14½º the value of NFor 14½º the value of Ncc is 32, and for 20° it is 17. is 32, and for 20° it is 17.

System of gear teethSystem of gear teeth

Following four system of gear teeth is Following four system of gear teeth is commonly used:commonly used:

►14 ½ 14 ½ ° composite° composite►14 ½ 14 ½ ° full depth involute° full depth involute►20 20 ° full depth involute° full depth involute►20 20 ° full stub involute° full stub involute

Classification of gearsClassification of gearsAccording to the position of shafts:According to the position of shafts:

► Parallel shaft:Parallel shaft:(i) (i) Spur gearSpur gear: gear teeth are straight along the length and : gear teeth are straight along the length and are parallel to the axes.are parallel to the axes.(ii) (ii) Helical gearHelical gear::

(a) (a) Single helical gearSingle helical gear: the teeth are helices and these helices : the teeth are helices and these helices may be left handed or right handed.may be left handed or right handed.(b) (b) Double helical gear (herringbone gear)Double helical gear (herringbone gear): these gears are : these gears are actually two helical gears of hands, placed side by side and actually two helical gears of hands, placed side by side and cut on the same blank to obtain composite unit.cut on the same blank to obtain composite unit.

► Intersecting shaft:Intersecting shaft:(i) (i) Bevel gearBevel gear::(a) (a) Straight bevel gearStraight bevel gear: the gear blank is cone on which teeth : the gear blank is cone on which teeth are generated. The teeth are straight.are generated. The teeth are straight.(b) (b) Spiral bevel gearSpiral bevel gear: the elements are curved in the shape of : the elements are curved in the shape of spiral, so that contact between intermeshing teeth become spiral, so that contact between intermeshing teeth become gradual and continuous.gradual and continuous.

► Non-intersecting and non-parallel shaft:Non-intersecting and non-parallel shaft:(i) (i) Hypoid gearHypoid gear: these are similar to spiral bevel gears, but : these are similar to spiral bevel gears, but have non-intersecting axes, ie. the axis of the pinion is offset have non-intersecting axes, ie. the axis of the pinion is offset relative to the gear axis.relative to the gear axis.(ii) (ii) Crossed helical gearCrossed helical gear: these are cylindrical helical gears, but : these are cylindrical helical gears, but their axes are at am angle when in mesh and do not intersect.their axes are at am angle when in mesh and do not intersect.(iii) (iii) Worm gearWorm gear: the axes are normally at right angles to each : the axes are normally at right angles to each other. The tooth elements of both the components are helices.other. The tooth elements of both the components are helices.

Classification of gears

Single Helical GearSpur Gear Double Helical Gear

Straight Bevel Gear Spiral Bevel Gear Worm Gear

Gear Making processGear Making process

Modern methods of producing gear teeth cover a Modern methods of producing gear teeth cover a wide variety:wide variety:

►MachiningMachining► GrindingGrinding► CastingCasting►MoldingMolding► Forming (drawing, extruding, rolling)Forming (drawing, extruding, rolling)► StampingStamping

Each method offers special characteristics relating Each method offers special characteristics relating to quality, production quantity, cost, material and to quality, production quantity, cost, material and application.application.

MachiningMachining

Machining constitutes the most important Machining constitutes the most important method of generating gear teeth. It is method of generating gear teeth. It is suitable for high precision gears in both suitable for high precision gears in both small and large quantities.small and large quantities.Various methods of machining is described Various methods of machining is described below:below:

►Teeth formation processTeeth formation process►Tooth finishing processTooth finishing process

Teeth formation processTeeth formation process

Gear Milling:Gear Milling:

Basic Principle: After each tooth is milled, Basic Principle: After each tooth is milled, gear blank is rotated to the next cutting.gear blank is rotated to the next cutting.

►Disadvantage: Disadvantage:

(1)Time consuming (1)Time consuming (2) Involves heavy stock removal operation (2) Involves heavy stock removal operation which require high accuracy of indexing which require high accuracy of indexing mechanism mechanism

Hob generation:Hob generation:► This is the most widely used method of cutting gear This is the most widely used method of cutting gear

teeth.teeth.► Basic Principle: In this process the work piece rotates in Basic Principle: In this process the work piece rotates in

constant relationship with the cutting tool ie. The gear constant relationship with the cutting tool ie. The gear hob. The rack is in the form of a worm.hob. The rack is in the form of a worm.

► The central section of the hob is identical to that of the The central section of the hob is identical to that of the worm and gear. The differences are that the thread of worm and gear. The differences are that the thread of the hob is axially gashed or fluted in several places so as the hob is axially gashed or fluted in several places so as to form cutting edges, while the sides and top of these to form cutting edges, while the sides and top of these teeth are relieved behind the gash surface to permit teeth are relieved behind the gash surface to permit proper cutting action. This arrangement, in eftect, gives proper cutting action. This arrangement, in eftect, gives an infinitely long rack so that cutting is both steady and an infinitely long rack so that cutting is both steady and continuous. To generate the full Width of the gear, the continuous. To generate the full Width of the gear, the hob slowly traverses the face of the gear as it rotates. hob slowly traverses the face of the gear as it rotates. Thus, the hob has a basic rotary motion and a Thus, the hob has a basic rotary motion and a unidirectional traverse at right angles.unidirectional traverse at right angles.

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Hob generation:Hob generation:

► Both movements are relatively simple to effect, Both movements are relatively simple to effect, resulting in a very accurate process. resulting in a very accurate process.

► A further advantage of hobbing is that the hob can A further advantage of hobbing is that the hob can be swiveled relative to the blank axis. This permits be swiveled relative to the blank axis. This permits cutting helical gears of all angles with the same cutting helical gears of all angles with the same tooling. tooling.

►With regard to accuracy, hobbing is superior to the With regard to accuracy, hobbing is superior to the other cutting processes. Gears can be directly other cutting processes. Gears can be directly hobbed to ultra-precisionhobbed to ultra-precision

► tolerances without resorting to any secondary tolerances without resorting to any secondary refining processes.refining processes.

Gear shaping:Gear shaping:►It’s a continuous generating process.It’s a continuous generating process.►Cutter is a gear with cutting edges. Profile is Cutter is a gear with cutting edges. Profile is

generated with number of enveloping cuts generated with number of enveloping cuts produced by the cutter teeth.produced by the cutter teeth.

►It is time consuming as time is lost during It is time consuming as time is lost during return stroke of cutter and half of the total return stroke of cutter and half of the total machining time is spent in metal removal.machining time is spent in metal removal.

Gear grinding:Gear grinding:

► Although grinding is often associated with quantity Although grinding is often associated with quantity fabrication of high quality gears as a secondary refining fabrication of high quality gears as a secondary refining operation, it is also a basic process for producing operation, it is also a basic process for producing hardened gears. In addition, many high-precision fine-hardened gears. In addition, many high-precision fine-pitch gears have their teeth entirely ground from the pitch gears have their teeth entirely ground from the blank state.blank state.

► There are a number of distinct advantages to ground There are a number of distinct advantages to ground gears. These are listed as follows:gears. These are listed as follows:1. Achievement of high precision is possible because 1. Achievement of high precision is possible because the process can remove very little material in the final the process can remove very little material in the final pass.pass.2. Grinding results in a much finer surface finish than 2. Grinding results in a much finer surface finish than any machining process.any machining process.3. Hardened steel alloys can be ground.3. Hardened steel alloys can be ground.4. Residual surface stresses are minimal.4. Residual surface stresses are minimal.

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Gear grinding:Gear grinding:► Ground gears’ superior load carrying capacity is not only due to Ground gears’ superior load carrying capacity is not only due to

the hardened alloys higher mechanical properties, but also the hardened alloys higher mechanical properties, but also because of the finer surface finish. A fine finish enables because of the finer surface finish. A fine finish enables maintenance of a good continuous oil film versus boundary maintenance of a good continuous oil film versus boundary lubrication and breakthrough. The result is higher load capactry lubrication and breakthrough. The result is higher load capactry along with reduced wear and longer useful life.along with reduced wear and longer useful life.

► Although there are distinct advantages for ground gears, there Although there are distinct advantages for ground gears, there are some limitations and disadvantages. These are:are some limitations and disadvantages. These are:1. Grinding is limited to ferrous materials.1. Grinding is limited to ferrous materials.2. Hard metals grind better than soft ones.2. Hard metals grind better than soft ones.3. Grinding of helicals and worms has limitations that possibly 3. Grinding of helicals and worms has limitations that possibly involve profile deviations and removal.involve profile deviations and removal.4. Pro-grind hobbing requires special protruberance hobs to 4. Pro-grind hobbing requires special protruberance hobs to provide grind wheel clearance at the root.provide grind wheel clearance at the root.5. Gear grinding machinery is scarcer than hobbing machines.5. Gear grinding machinery is scarcer than hobbing machines.6. Grinding is a secondary operation which increases total gear 6. Grinding is a secondary operation which increases total gear costcost

► Despite involving higher cost and other limitations, ground gears Despite involving higher cost and other limitations, ground gears are always superior in precision and strength.are always superior in precision and strength.

Tooth finishing processTooth finishing process

Lapping:Lapping:► Gear finishing process, used after heat treatment.Gear finishing process, used after heat treatment.► It improves surface finish, correct distortions and It improves surface finish, correct distortions and

improved wear life.improved wear life.► Lapping also contribute to have proper backlash, Lapping also contribute to have proper backlash,

between meshing teeth.between meshing teeth.

Other tooth finishing operations are shaving, Other tooth finishing operations are shaving, honning, etc.honning, etc.

Type of gearboxType of gearbox

Generally following gearboxes are used in Generally following gearboxes are used in industries:industries:

►Helical gearboxHelical gearbox►Bevel-helical gearboxBevel-helical gearbox►Worm gearboxWorm gearbox►Heli-worm gearboxHeli-worm gearbox►Geared motorGeared motor

MaterialMaterial

Gear steels may be divided into two general Gear steels may be divided into two general classes:classes:

► the plain carbonthe plain carbon► the alloy steels.the alloy steels.

Alloy steels are used in the industrial field, Alloy steels are used in the industrial field, but heat-treated plain carbon steels are also but heat-treated plain carbon steels are also common.common.

The The advantagesadvantages obtained from using obtained from using heat-treated alloy heat-treated alloy steelssteels in place of heat-treated plain carbon steels are in place of heat-treated plain carbon steels are as follows:as follows:1) Increased surface hardness and depth of hardness 1) Increased surface hardness and depth of hardness penetration for the same carbon content and quench.penetration for the same carbon content and quench.2) Ability to obtain the same surface hardness with a 2) Ability to obtain the same surface hardness with a less drastic quench and, in the case of some of the less drastic quench and, in the case of some of the alloys, a lower quenching temperature, thus giving less alloys, a lower quenching temperature, thus giving less distortion.distortion.3) Increased toughness, as indicated by the higher 3) Increased toughness, as indicated by the higher values of yield point, elongation, and reduction of area.values of yield point, elongation, and reduction of area.4) Finer grain size, with the resulting higher impact 4) Finer grain size, with the resulting higher impact toughness and increased wear resistance.toughness and increased wear resistance.5) In the case of some of the alloys, better machining 5) In the case of some of the alloys, better machining qualities or the possibility of machining at higher qualities or the possibility of machining at higher hardness.hardness.

There are generally two types of heat There are generally two types of heat treated alloy steels:treated alloy steels:

►Volume hardened steel (or through Volume hardened steel (or through hardened steel)hardened steel)

►Case hardened steelCase hardened steel

Volume hardened steel (or through hardened steel)Volume hardened steel (or through hardened steel)

► Thru-hardening steels are used when great strength, high Thru-hardening steels are used when great strength, high endurance limit, toughness, and resistance to shock are endurance limit, toughness, and resistance to shock are required. These qualities are governed by the kind of steel and required. These qualities are governed by the kind of steel and treatment used. Fairly high surface hardnesses are obtainable in treatment used. Fairly high surface hardnesses are obtainable in this group, though not so high as those of the casehardening this group, though not so high as those of the casehardening steels. For that reason, the resistance to wear is not so great as steels. For that reason, the resistance to wear is not so great as might be obtained, but when wear resistance combined with might be obtained, but when wear resistance combined with great strength and toughness is required, this type of steel is great strength and toughness is required, this type of steel is superior to the others. Thru hardening steels become distorted superior to the others. Thru hardening steels become distorted to some extent when hardened, the amount depending upon the to some extent when hardened, the amount depending upon the steel and quenching medium used.steel and quenching medium used.

► When the grinding of gear teeth is not practicable and a high When the grinding of gear teeth is not practicable and a high degree of accuracy is required, hardened steels may be drawn or degree of accuracy is required, hardened steels may be drawn or tempered to a hardness that will permit the cutting of the teeth. tempered to a hardness that will permit the cutting of the teeth. This treatment gives a highly refined structure, great toughness, This treatment gives a highly refined structure, great toughness, and, in spite of the low hardness, excellent wearing qualities.and, in spite of the low hardness, excellent wearing qualities.

Case hardened steelCase hardened steel

► Casehardening steels with their extremely hard, fine-Casehardening steels with their extremely hard, fine-grained (when properly treated) case and grained (when properly treated) case and comparatively soft and ductile core are generally used comparatively soft and ductile core are generally used when resistance to wear is desired. Casehardening when resistance to wear is desired. Casehardening alloy steels have a fairly tough core, but not as tough alloy steels have a fairly tough core, but not as tough as that of the full-hardening steels.as that of the full-hardening steels.

► But wear resistance combined with toughness, these But wear resistance combined with toughness, these steels are more effective, and a same size of properly steels are more effective, and a same size of properly case-hardened gear combination can transmit more case-hardened gear combination can transmit more torque, compared to a gear pair made of volume torque, compared to a gear pair made of volume hardened steel.hardened steel.

Material of gear/ gearbox generally used in Indian industriesMaterial of gear/ gearbox generally used in Indian industries► Helical gears:Helical gears:

(a) Volume hardened steel (or through hardened steel)(a) Volume hardened steel (or through hardened steel)Pinion: EN 19 (Hardened & Tempered), Hardness: 280-320 BHNPinion: EN 19 (Hardened & Tempered), Hardness: 280-320 BHNGear: EN 24 (Hardened & Tempered), Hardness: 230-280 BHNGear: EN 24 (Hardened & Tempered), Hardness: 230-280 BHN(b) Case hardened steel(b) Case hardened steelPinion & Gear: SAE 8620 or 17CrNiMo6 or EN 356, Case Hardness: 55 Rc, Pinion & Gear: SAE 8620 or 17CrNiMo6 or EN 356, Case Hardness: 55 Rc, Core Hardness: 180-210 BHNCore Hardness: 180-210 BHN

► Bevel Gears:Bevel Gears:Same as helical gearsSame as helical gears

► Worm Gears:Worm Gears:Worm shaft: C45Worm shaft: C45Worm wheel: Phosphor BronzeWorm wheel: Phosphor Bronze

► Housing: CI FG 400 IS 210 or Fabricated (IS 2062)Housing: CI FG 400 IS 210 or Fabricated (IS 2062)

► Modern gear technologists are trying to develop new alloying materials for gears, Modern gear technologists are trying to develop new alloying materials for gears, to increase gear strength, with compact and reduced size.to increase gear strength, with compact and reduced size.

► Apart from steels, other materials also used for gears (depending upon Apart from steels, other materials also used for gears (depending upon requirement), ie:requirement), ie:Die Cast Alloys, Sintered Powder Metal, Plastics, etc.Die Cast Alloys, Sintered Powder Metal, Plastics, etc.

Gear box lubricationGear box lubrication► Gearbox lubrication:Gearbox lubrication:► All gears are delivered from manufacturer without oil.All gears are delivered from manufacturer without oil.► Before start up, the gear should be filled with oil Before start up, the gear should be filled with oil

recommended.recommended.► Oil should be filled to oil level indicator.Oil should be filled to oil level indicator.► The oil level should be checked regularly. The checking The oil level should be checked regularly. The checking

must be made while the drive is stationary.must be made while the drive is stationary.► Some times, bearings are normally oil lubricated on Some times, bearings are normally oil lubricated on

primary side and grease lubricated on secondary side.primary side and grease lubricated on secondary side.

Lubrication continued to next page…….

►First oil change should take place after First oil change should take place after running-in period. Drain oil immediately running-in period. Drain oil immediately after stop while oil is still worm. Second oil after stop while oil is still worm. Second oil change should take place after 200 working change should take place after 200 working hours. Afterwards, oil change is hours. Afterwards, oil change is recommended after period of 2000-4000 recommended after period of 2000-4000 working hours depending on working working hours depending on working conditions.conditions.

Lubrication continued to next page…….

► The oil in the sump carries out lubrication of the The oil in the sump carries out lubrication of the gears and bearings automatically. Units are fitted gears and bearings automatically. Units are fitted with filler, oil level indicator and breather for easy with filler, oil level indicator and breather for easy routine inspection and maintenance. For low speed routine inspection and maintenance. For low speed application, grease or forced feed lubrication may application, grease or forced feed lubrication may be necessary. Maximum heat dissipation is be necessary. Maximum heat dissipation is achieved by means of a radial fan for directing achieved by means of a radial fan for directing high velocity air over the cooling ribs of gear case. high velocity air over the cooling ribs of gear case. For higher capacities, the units are provided with For higher capacities, the units are provided with cooling coils in the lower part of the housing for cooling coils in the lower part of the housing for allowing cool water passing through a suitable port allowing cool water passing through a suitable port entry. With pressure lubrication, oil cooler can be entry. With pressure lubrication, oil cooler can be mounted in the oil circuit to achieve more efficient mounted in the oil circuit to achieve more efficient and effective cooling.and effective cooling.

Gear failureGear failureCauses for gear failure:Causes for gear failure:

► Surface fatigue:Surface fatigue:(a) Case exfoliation(a) Case exfoliation(b) Initial pitting(b) Initial pitting(c) Progressive pitting(c) Progressive pitting

► Scuffing:Scuffing:(a) Light scuffing(a) Light scuffing(b) Heavy scuffing(b) Heavy scuffing

► Abrasive wear:Abrasive wear:(a) Foreign matter in the lubricant (scoring)(a) Foreign matter in the lubricant (scoring)

► Tooth breakage:Tooth breakage:(a) Brittle fracture resulting from high shock load(a) Brittle fracture resulting from high shock load(b) Excessive loading causing fatigue fracture(b) Excessive loading causing fatigue fracture

► Plastic deformationPlastic deformation► Smooth chemical wearSmooth chemical wear

Case exfoliationCase exfoliation

►It happens in case of case hardened gears. It happens in case of case hardened gears. Causes:Causes:

►Improper case hardening process, causes Improper case hardening process, causes uneven case depth. When the gear is uneven case depth. When the gear is subjected to running conditions, the layer of subjected to running conditions, the layer of hardened case comes out from the soft hardened case comes out from the soft core, causing case exfoliation.core, causing case exfoliation.

Remedies:Remedies:► Proper case hardening procedureProper case hardening procedure► Use of proper lubricationUse of proper lubrication

Tooth PittingTooth Pitting

► Pits are formed when material is flaked off the Pits are formed when material is flaked off the tooth surface near or below the pitch line due to tooth surface near or below the pitch line due to heavy contact pressure.heavy contact pressure.

► This is the removal of small bits of metal from the This is the removal of small bits of metal from the surface, due to fatigue, thereby leaving small surface, due to fatigue, thereby leaving small holes or pits. This is caused by high tooth loads holes or pits. This is caused by high tooth loads leading to excessive surface stress, a high local leading to excessive surface stress, a high local temperature due to high rubbing speeds, or temperature due to high rubbing speeds, or inadequate lubrication. Minute cracking of the inadequate lubrication. Minute cracking of the surface develops, spreads and ultimately results in surface develops, spreads and ultimately results in small bits breaking out of the tooth surface.small bits breaking out of the tooth surface.

Tooth PittingTooth Pitting

Remedies:• Avoid high load concentration• Increase surface strength• Use a higher viscosity oil

Tooth ScuffingTooth Scuffing► These are due to constantly repeated breakage These are due to constantly repeated breakage

of the oil film. It happens due to localised of the oil film. It happens due to localised welding between two meshing teeth.welding between two meshing teeth.

Remedies:• Use recommended Use recommended viscosity oilviscosity oil• Provide better cooling Provide better cooling by effective spray by effective spray lubricationlubrication• Provide good surface Provide good surface finishfinish

Tooth ScoringTooth Scoring

► This is a heavy scratch pattern extending from This is a heavy scratch pattern extending from tooth root to tip. It appears as if a heavily-loaded tooth root to tip. It appears as if a heavily-loaded tooth pair has dragged foreign matter between tooth pair has dragged foreign matter between sliding teeth. It can be caused by lubricant failure, sliding teeth. It can be caused by lubricant failure, incompatible materials and overload. incompatible materials and overload.

Remedies:• Avoid high load concentration• Increase surface strength• Use proper lubricant

Tooth breakageTooth breakageTooth breakage occurs due to: Tooth breakage occurs due to:

►Repeated overloadingRepeated overloading►Impact load during meshingImpact load during meshing►Uneven distribution of load due to Uneven distribution of load due to

misalignment of axesmisalignment of axes

Remedies:► Increase tooth bending strengthIncrease tooth bending strength► Proper design of the gear teethProper design of the gear teeth► Use large chamfer at the end of the toothUse large chamfer at the end of the tooth► Proper assembly of the drive system to avoid Proper assembly of the drive system to avoid

misalignment.misalignment.

Plastic deformationPlastic deformation

►Sometimes, due to over loading, uneven Sometimes, due to over loading, uneven stress distribution and misalignment, gear stress distribution and misalignment, gear teeth does not break, but it got distorted, teeth does not break, but it got distorted, and permanent plastic deformation takes and permanent plastic deformation takes place.place.

Remedies:► Proper hardness of the gear teethProper hardness of the gear teeth► Proper design of the gear teethProper design of the gear teeth► Proper assembly of the drive system to avoid Proper assembly of the drive system to avoid

misalignment.misalignment.

Smooth chemical wearSmooth chemical wear

Cause: Cause: ►Presence of mineral impurities in the Presence of mineral impurities in the

lubricantlubricant

Remedies:Remedies:► Case harden the teethCase harden the teeth► Use proper and pure lubricant Use proper and pure lubricant ► Good surface finishGood surface finish

Trouble shootingTrouble shooting

Common defects in gearbox:Common defects in gearbox:►Gear heating upGear heating up►Gearbox leakageGearbox leakage►Gearbox noiseGearbox noise►Gearbox vibrationGearbox vibration►Oil seal whistleOil seal whistle

Gearbox heating upGearbox heating up► High surrounding temperature may cause heating upHigh surrounding temperature may cause heating up► Over loading: If gear box is heating considerably right from Over loading: If gear box is heating considerably right from

the beginning of its installation, immediate attention the beginning of its installation, immediate attention should be paid in the fact whether gearbox was properly should be paid in the fact whether gearbox was properly selected, taking into account all the factors under which it selected, taking into account all the factors under which it is required to operate.is required to operate.

► Use of wrong lubricant: Depending upon the conditions of Use of wrong lubricant: Depending upon the conditions of working, proper lubricant must be used.working, proper lubricant must be used.

► Excessive churning: High viscosity of oil or over filling of Excessive churning: High viscosity of oil or over filling of the gearbox will produce excessive churning losses the gearbox will produce excessive churning losses consequently heating up the gearbox.consequently heating up the gearbox.

► Bearing clearance not proper: Improper bearing clearance Bearing clearance not proper: Improper bearing clearance may allow the axial floating of shaft and bearing damage.may allow the axial floating of shaft and bearing damage.

► Misalignment: Misalignment between the shaft of gearbox Misalignment: Misalignment between the shaft of gearbox and the prime mover and or the driven load causes non and the prime mover and or the driven load causes non uniform load on bearing and shafts. Whenever possible, uniform load on bearing and shafts. Whenever possible, use common base plate for motor and gearbox or else use common base plate for motor and gearbox or else proper alignment should be done.proper alignment should be done.

Gearbox leakageGearbox leakage

► Due to over filling, the oil level may rise above the Due to over filling, the oil level may rise above the mark and upto the level of oil seal, and there fore mark and upto the level of oil seal, and there fore leakage could start.leakage could start.

► Leakage may very rarely take place through joint Leakage may very rarely take place through joint faces between top and bottom half or end caps in faces between top and bottom half or end caps in which case tightening of bolts and screws will stop which case tightening of bolts and screws will stop leakage, if not then a fresh layer of good jointing leakage, if not then a fresh layer of good jointing compound will have to be applied to the joint compound will have to be applied to the joint faces.faces.

► Any slight damage to the lips of the oil seals due Any slight damage to the lips of the oil seals due to wear or misuse could start leakage, in which to wear or misuse could start leakage, in which case they need to be replaced.case they need to be replaced.

Gearbox noiseGearbox noise

Causes of noisy gearbox operation:Causes of noisy gearbox operation:►Bearing worn outBearing worn out►Worn out gearsWorn out gears►Foreign particle in the gearbox, may be Foreign particle in the gearbox, may be

mixed with lubricantmixed with lubricant►Excessive backlashExcessive backlash►Tooth contact not properTooth contact not proper►Eccentric running of shaftEccentric running of shaft

► Proper action to be taken so that, these errors can be Proper action to be taken so that, these errors can be eliminated.eliminated.

Gearbox vibrationGearbox vibration

►Vibration is another area for concern. Solid Vibration is another area for concern. Solid foundation and perfect alignment is required foundation and perfect alignment is required for the gearbox. Couplings should be well for the gearbox. Couplings should be well balanced to avoid vibration.balanced to avoid vibration.

Oil seal whistleOil seal whistle

►A dry condition of oil seal lips, which A dry condition of oil seal lips, which allowed to continue, will char and ruin oil allowed to continue, will char and ruin oil seal lips. Smear the oil seal lips with seal lips. Smear the oil seal lips with lubricant and do not let it remain dry even lubricant and do not let it remain dry even when stored for a long time.when stored for a long time.

Market products and Market products and specificationspecification

Specification:Specification:►Type of gearboxType of gearbox►Transmitted powerTransmitted power►Ratio=Input RPM/Output RPMRatio=Input RPM/Output RPM►Duty conditionDuty condition►Type of loading- applicationType of loading- application►Type of prime moverType of prime mover

Name of some gearbox manufacturing company Name of some gearbox manufacturing company used in Indian Industries:used in Indian Industries:

►New Allenberry WorksNew Allenberry Works►GreavesGreaves►EleconElecon►ShanthiShanthi►FlenderFlender►SEWSEW►BonfiglioliBonfiglioli

Designation of gearboxDesignation of gearbox

Example:Example:►Helical/ Bevel-helical Gearbox:Helical/ Bevel-helical Gearbox:►Make: New Allenberry WorksMake: New Allenberry Works►Unit brand name: HelimaxUnit brand name: Helimax

CC SS CC HH 110110Type of gearboxType of gearbox Number of stage Number of stage Special case Centre distance Special case Centre distance

CS-HelicalCS-Helical A-Single stageA-Single stage H-Foot mounted (last stage) H-Foot mounted (last stage)CK-BevelCK-Bevel B-Double stageB-Double stage hollow output shaft hollow output shaft

C-Triple stageC-Triple stage A-Shaft mounted A-Shaft mountedD-Quadruple stage hollow output shaftD-Quadruple stage hollow output shaft

V-Vertical output shaftV-Vertical output shaft

Example:Example:►Geared motor:Geared motor:►Make: New Allenberry WorksMake: New Allenberry Works►Unit brand name: OptimaxUnit brand name: Optimax

Gear unitGear unit Motor unit Motor unit R.R. 93.93. 52 V 52 V DCDC 90S90S -- 44 Type Size SizeType Size Size Type Type Size Size No. of poles No. of poles

(only 2 (only 2

gear casing)gear casing)

No. of stage No. of stage No. of stage No. of stage

Type of gear unit (for geared motor, New Allenberry Type of gear unit (for geared motor, New Allenberry make):make):

► R- foot mounted with co-axial or offset output shaft & with R- foot mounted with co-axial or offset output shaft & with one or two gear casingone or two gear casing

► RF- flanged type with co-axial or offset output shaft & with RF- flanged type with co-axial or offset output shaft & with one or two gear casingone or two gear casing

► R..F-foot cum flanged type with co-axial or offset output R..F-foot cum flanged type with co-axial or offset output shaft with one or two gear casingshaft with one or two gear casing

► RUF-flanged type with extended hub and shaft and with RUF-flanged type with extended hub and shaft and with co-axial or offset output shaft with one or two gear casing. co-axial or offset output shaft with one or two gear casing.

Type of motor:Type of motor:► DC-three phase with squirrel cage face mounting motorDC-three phase with squirrel cage face mounting motor► DF-three phase with squirrel cage flange mounting motorDF-three phase with squirrel cage flange mounting motor

Example:Example:►Worm Gearbox:Worm Gearbox:►Make: New Allenberry WorksMake: New Allenberry Works► Unit brand name: AllroydUnit brand name: Allroyd

SS UU MM 33Type of gearboxType of gearbox Metric unit Metric unit Centre distance (in inches)Centre distance (in inches)SU-UnderdrivenSU-Underdriven SO-OverdrivenSO-OverdrivenSV-Vertical output shaftSV-Vertical output shaft

For different manufacturer, specification changes; so example given here is only indicative.For different manufacturer, specification changes; so example given here is only indicative.