Gearbox Ni Rj Har

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Presentation by

Nirjhar Chakravorti


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Nirjhar Chakravorti has completed Mechanical

Engineering in 2000 from Jalpaiguri GovernmentEngineering College, India.

Nirjhar has experience in design, development,project and plant engineering.

During professional life, Nirjhar has worked onGears also; its design, application, maintenanceand failure.


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Features IntroductionWhy gears are used? Fundamentals of gearingClassification of gears

Gear Making processType of gear boxesMaterial for gear boxesGear lubrication and cooling

Gear failuresTrouble shootingMarket products and specification


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Toothed gearing system evolved graduallyfrom the primitive forms where wooden pinswere arranged on the periphery of simplewooden wheels to drive opposite member of

the pair. These wheels served the purpose ofgears those days. Of course the operation wasneither smooth nor quite and obviously speedswere very slow. The motive power to turn

these systems was generally provided by thetrade mills, which were operated by men,animals and windmills.


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Archimedes knew primitive forms ofgear before the Christian era. Leonardode Vinci used the concept of gear

system in many of his proposedmachines.


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For the evolution of modern day gearingtechnology we must acknowledge thecontribution of scientists like Louise,

Buckingham, Euler.


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Why gears are used?

To transmit torque/ power from one shaft toanother shaft.Other commonly used power transmitting elements:

Belt drivemajor limitation: Slippage

Rope drive Chain drive major limitation: Discontinuous drive

To avoid slippage and discontinuity during power transmission, the best

solution is gear drive. To get desired rotational speed

To change direction of rotation Where distance between drive shaft and driven

shaft is very small


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Disadvantages of gear drive

Disadvantages:

Gear drive is costlier than other drives

Error during cutting teeth causesvibration & noise while operation

Gear drive requires proper lubrication

for smooth runningSo as usual, designers has to be logicaland optimal during selection of drive.


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Gear Tooth Terminology


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Circular pitch:

It is the distance measured on thecircumference of the pitch circle from apoint on the tooth to the correspondingpoint on the next tooth. Mathematically,

circular pitch, pc=D/T,

where,D= P.C.D.

T= No. of teeth


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Pressure Angle:

If a tangent is drawn to the involute profileof a tooth at any point on the curve, and if aradius line is drawn through this point oftangency connecting this point with the

centre of the gear, then the acute angleincluded between this tangent and radialline is defined as pressure angle at thatpoint.

In gear design, the pressure angle ismeasured at the pitch point.


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Principle of transmission &

conjugate action

When a pair of gear teeth act togetheragainst each other, rotary motion isproduced which is transmitted from the

driver to the driven gear. If such a pair oftooth profiles which are so designed that aconstant angular velocity ratio is producedand maintained during meshing, the two

gears are said to have conjugate action andthe tooth profiles are said to have conjugatecurves.


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Conjugate action

In other words, conjugate action is assumedif, 1/ 2=constant, where,

1= Angular velocity of the driver (generally called pinion)

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

In case of conjugate action,

1/ 2= D2/D1D1= PCD of pinion

D2= PCD of gear


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Gear teeth profile

To produce conjugate action, followingtooth profiles are used:

Involute teeth

Cycloidal teeth

Epi-cycloidal teeth

Hypo cycloidal teeth


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Cycloidal curve

P (Trajectory followed by point P iscalled Cycloidal Curve)

Fixed circle

P (Trajectory followed by point P iscalled Epi-cycloidal Curve)

P (Trajectory followed by point P iscalled Hypo-cycloidal Curve)

Fixed circle


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Why involute curve is widely used?

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

The form of the basic rack tooth is

straight-sided, and therefore is relativelysimple and can be accurately made; as agenerating toolift imparts high accuracy

to the cut gear tooth. A hob cutter forthe cycloidal gear is not as easily made.

Continued to next page.


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

theoretical correct centre distance for which this willtransmit motion maintaining a constant angular velocityratio.

in case of involute gearing system, the centre distance

can be changed without affecting the angular velocityratio.

This advantage of involute system is of primeimportance as most of the modern gears are correctedones having changed centre.

Also, in case of gearing systems having standard centredistance, it is not possible to accurately maintain thatdistance due to mounting inaccuracies, misalignmentsand a number of other diverse factors.

Continued to next page.


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Why involute curve is widely used?

While the cycloidal tooth profile has doublecurvature, an involute tooth has singlecurvature, which facilitates ease ofmanufacturing.

In involute profile as the path of contact isstraight line and the pressure angle isconstant, there is constant force acting on theaxes. In cycloidal gear, the pressure angle

continuously changes. This results inseparating forces of variable magnitude,which in turn gives rise to unquiet operation.


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Disadvantages of involute curve over

cycloidal curve:

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

The cycloidal teeth is stronger than theinvolute teeth. There is more material at theroot portion of the cycloidal tooth ascompared to an involute tooth.

Involute teeth has the problem ofinterference, but the cycloidal gears do nothave interference and the problem thereof.


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Interference & Undercutting

Geometry of proper teeth meshing

Tooth correction: One of themethod of avoiding undercut, is toothcorrection or tooth enlargement. This

is achieved by pinion enlargement (orcorrection as often termed), whereinthe pinion teeth, still generated with astandard cutter, are shifted radiallyourward to form a full involute toothfree of undercut The tooth is enlarged

both radially and circumferentially.


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Interference & Undercutting

Interference condition becomes more severe as thenumber of teeth on the gear increases. The limit isreached when the gear becomes a rack. This is arealistic case since the hob is a rack-type cutter. Theresult is that standard gears with tooth numbers

below a critical value are automatically undercut inthe generating process. The limiting number ofteeth in a gear

meshing with a rack is given by the expression:

Nc = 2/sin

This indicates the minimum number of teeth free of

undercutting decreases with increasing Pressureangle.

For 14 the value of Ncis 32, and for 20 it is 17.


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Classification of gears

According to the position of shafts:

Parallel shaft:

(i) Spur gear: gear teeth are straight along the length and

are parallel to the axes.

(ii) Helical gear:

(a) Single helical gear: the teeth are helices and these helicesmay be left handed or right handed.

(b) Double helical gear (herringbone gear): these gears areactually two helical gears of hands, placed side by side andcut on the same blank to obtain composite unit.


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Intersecting shaft:

(i) Bevel gear:

(a) Straight bevel gear: the gear blank is cone on which teethare generated. The teeth are straight.

(b) Spiral bevel gear: the elements are curved in the shape ofspiral, so that contact between intermeshing teeth becomegradual and continuous.

Non-intersecting and non-parallel shaft:

(i) Hypoid gear: these are similar to spiral bevel gears, buthave non-intersecting axes, ie. the axis of the pinion is offsetrelative to the gear axis.

(ii) Crossed helical gear: these are cylindrical helical gears, buttheir axes are at am angle when in mesh and do not intersect.

(iii) Worm gear: the axes are normally at right angles to eachother. The tooth elements of both the components are helices.


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Gear Making process

Modern methods of producing gear teeth cover awide variety:

MachiningGrindingCastingMolding Forming (drawing, extruding, rolling) Stamping

Each method offers special characteristics relatingto quality, production quantity, cost, material andapplication.


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Machining

Machining constitutes the most importantmethod of generating gear teeth. It issuitable for high precision gears in both

small and large quantities.

Various methods of machining is describedbelow:

Teeth formation process

Tooth finishing process


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Teeth formation process

Gear Milling:

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

Disadvantage:

(1)Time consuming

(2) Involves heavy stock removal operationwhich require high accuracy of indexingmechanism

b i


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

teeth.

Basic Principle: In this process the work piece rotates inconstant relationship with the cutting tool ie. The gearhob. The rack is in the form of a worm.

The central section of the hob is identical to that of theworm and gear. The differences are that the thread of

the hob is axially gashed or fluted in several places so asto form cutting edges, while the sides and top of theseteeth are relieved behind the gash surface to permitproper cutting action. This arrangement, in eftect, givesan infinitely long rack so that cutting is both steady and

continuous. To generate the full Width of the gear, thehob slowly traverses the face of the gear as it rotates.Thus, the hob has a basic rotary motion and aunidirectional traverse at right angles.

Hob generation continued to next page.


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Type of gearbox

Generally following gearboxes are used inindustries:

Helical gearbox

Bevel-helical gearbox

Worm gearbox

Heli-worm gearboxGeared motor


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Volume hardened steel (or through hardened steel)

Thru-hardening steels are used when great strength, highendurance limit, toughness, and resistance to shock arerequired. These qualities are governed by the kind of steel andtreatment used. Fairly high surface hardnesses are obtainable inthis group, though not so high as those of the casehardeningsteels. For that reason, the resistance to wear is not so great asmight be obtained, but when wear resistance combined withgreat strength and toughness is required, this type of steel issuperior to the others. Thru hardening steels become distortedto some extent when hardened, the amount depending uponthe steel and quenching medium used.

When the grinding of gear teeth is not practicable and a highdegree of accuracy is required, hardened steels may be drawnor tempered to a hardness that will permit the cutting of theteeth. This treatment gives a highly refined structure, greattoughness, and, in spite of the low hardness, excellent wearingqualities.


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Case hardened steel

Casehardening steels with their extremely hard, fine-grained (when properly treated) case andcomparatively soft and ductile core are generally usedwhen resistance to wear is desired. Casehardening

alloy steels have a fairly tough core, but not as toughas that of the full-hardening steels.

But wear resistance combined with toughness, thesesteels are more effective, and a same size of properly

case-hardened gear combination can transmit moretorque, compared to a gear pair made of volumehardened steel.

Material of gear/ gearbox generally used in Indian industries


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

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

Bevel Gears:

Same as helical gears Worm Gears:

Worm shaft: C45Worm wheel: Phosphor Bronze

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

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

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


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First oil change should take place afterrunning-in period. Drain oil immediatelyafter stop while oil is still worm. Second oil

change should take place after 200 workinghours. Afterwards, oil change isrecommended after period of 2000-4000

working hours depending on workingconditions.

Lubrication continued to next page.


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Case exfoliation


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Case exfoliation

It happens in case of case hardened gears.Causes:

Improper case hardening process, causes

uneven case depth. When the gear issubjected to running conditions, the layer ofhardened case comes out from the softcore, causing case exfoliation.

Remedies:

Proper case hardening procedure

Use of proper lubrication


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Tooth Pitting

Pits are formed when material is flaked off thetooth surface near or below the pitch line due toheavy contact pressure.

This is the removal of small bits of metal from thesurface, due to fatigue, thereby leaving smallholes or pits. This is caused by high tooth loadsleading to excessive surface stress, a high localtemperature due to high rubbing speeds, or

inadequate lubrication. Minute cracking of thesurface develops, spreads and ultimately results insmall bits breaking out of the tooth surface.


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Tooth Pitting

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


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Tooth Scuffing

These are due to constantly repeated breakageof the oil film. It happens due to localisedwelding between two meshing teeth.

Remedies: Use recommendedviscosity oil Provide better coolingby effective spray

lubrication Provide good surfacefinish


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Tooth breakage


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Tooth breakage

Tooth breakage occurs due to:

Repeated overloading

Impact load during meshing

Uneven distribution of load due to

misalignment of axes

Remedies:

Increase tooth bending strength Proper design of the gear teeth

Use large chamfer at the end of the tooth

Proper assembly of the drive system to avoid

misalignment.


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Smooth chemical wear

Cause:

Presence of mineral impurities in thelubricant

Remedies:

Case harden the teeth Use proper and pure lubricant

Good surface finish


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Trouble shooting

Common defects in gearbox:

Gear heating up

Gearbox leakage

Gearbox noise

Gearbox vibration

Oil seal whistle

Gearbox heating up


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

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

Use of wrong lubricant: Depending upon the conditions of

working, proper lubricant must be used. Excessive churning: High viscosity of oil or over filling of

the gearbox will produce excessive churning lossesconsequently heating up the gearbox.

Bearing clearance not proper: Improper bearing clearance

may allow the axial floating of shaft and bearing damage. Misalignment: Misalignment between the shaft of gearbox

and the prime mover and or the driven load causes nonuniform load on bearing and shafts. Whenever possible,use common base plate for motor and gearbox or elseproper alignment should be done.

Gearbox leakage


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Gearbox leakage

Due to over filling, the oil level may rise above the

mark and upto the level of oil seal, and there foreleakage could start.

Leakage may very rarely take place through jointfaces between top and bottom half or end caps in

which case tightening of bolts and screws will stopleakage, if not then a fresh layer of good jointingcompound will have to be applied to the jointfaces.

Any slight damage to the lips of the oil seals dueto wear or misuse could start leakage, in whichcase they need to be replaced.


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Gearbox vibration

Vibration is another area for concern. Solidfoundation and perfect alignment is requiredfor the gearbox. Couplings should be well

balanced to avoid vibration.


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Oil seal whistle

A dry condition of oil seal lips, whichallowed to continue, will char and ruin oilseal lips. Smear the oil seal lips with

lubricant and do not let it remain dry evenwhen stored for a long time.

Market products and


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Market products andspecification

Specification:

Type of gearbox

Transmitted powerRatio=Input RPM/Output RPM

Duty condition

Type of loading- applicationType of prime mover


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Name of some gearbox manufacturing companyused in Indian Industries:

New Allenberry Works

Greaves

Elecon

Shanthi

Flender

SEW

Bonfiglioli

Designation of gearbox


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Designation of gearbox

Example:

Helical/ Bevel-helical Gearbox:

Make: New Allenberry Works

Unit brand name: Helimax

C S C H 110

Type of gearbox Number of stage Special case Centre distanceCS-Helical A-Single stage H-Foot mounted (last stage)

CK-Bevel B-Double stage hollow output shaft

C-Triple stage A-Shaft mounted

D-Quadruple stage hollow output shaft

V-Vertical output shaft


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Example:

Geared motor:Make: New Allenberry Works

Unit brand name: Optimax

Gear unit Motor unitR. 93. 52 V DC 90S - 4

Type Size Size Type Size No. of poles

(only 2gear casing)

No. of stage No. of stage


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

R- foot mounted with co-axial or offset output shaft & withone or two gear casing

RF- flanged type with co-axial or offset output shaft & withone or two gear casing

R..F-foot cum flanged type with co-axial or offset output

shaft with one or two gear casing RUF-flanged type with extended hub and shaft and with

co-axial or offset output shaft with one or two gear casing.

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


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Example:Worm Gearbox:Make: New Allenberry WorksUnit brand name: Allroyd

S U M 3Type of gearbox Metric unit Centre distance (in inches)

SU-UnderdrivenSO-OverdrivenSV-Vertical output shaft

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


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Gearbox Ni Rj Har