mm207 project gear materials

8/7/2019 MM207 Project Gear Materials

http://slidepdf.com/reader/full/mm207-project-gear-materials 1/12

Index Problem Definition: Gears Introduction Common Modes Of Gear Failures Gear Design : Involute Spur Gear Identification of Desired Properties in Gear Material Candidate materials for gear Quantitative Analysis of physical properties of candidate materials Heat Treatment and Microstructure Cost considerations Conclusion Bibliography

Problem Definition Designing of a gear for light automobile vehicle (say motorcycle). The gear should be able to transfer power from the engine output to the differential

drives(eventually to the wheels). The engine’s maximum power would be around 11 bhp (at 4400 rpm) and maximum torque

20.2 Nm (at 2800 rpm) and reduction ratio of 3/4 ( first gear).

The noise of gear and friction losses should be minimized and the gear should be durable aswell.

Gears: IntroductionGears are machine elements that transmit rotary motion and power from one shaft to another by thesuccessive engagements of teeth on their periphery. They have been in use for more than threethousand years now, and they are an important element in all manner of machinery used in currenttimes. Particularly, use of specific types of gears drastically affects the quality of transmission inautomobiles. If appropriate gears are used considering the power and torque of engine, efficiency of automobiles can be increased, thereby, not only increasing the power of car but also, decreasing thefuel consumption.

Selection of Proper Gear Model:-

There is a wide variety of types of gears in existence. Broadly, they can be classified by therelationship of the shaft axes on which the gears are mounted. The shafts may be parallel, intersectingor nonintersecting and nonparallel. Some of the important types of gears are listed below:

Gears that operate on parallel shafts- Spur gears, Helical gears, Herringbone gears,Internal gears

Gears that operate on Intersecting shafts- Bevel gears, Face gears Gears that operate on nonparallel and nonintersecting shafts- Worm gear

sets, Crossed helical gears, Hypoid gears, Spiroid gears



The first step in designing a set of gears is to select the correct type. In automobiles, thegears are on parallel axes; hence, spur or helical gears are appropriate. External helical gearsare used when both high speeds and high horsepowers are involved. External helical gearshave been built to carry as much as 45,000 kW of power on a single pinion and gear.

While, spur gears are relatively simple in design and in the machinery used tomanufacture and check them. These can be used at almost any speed that can be handled byother types of gears but, it produces noise.

So, to keep the design simple and as we are looking for light automobile gears, spur gearsis good choice. After selecting the gear type, the next important concerns are: - gear design and gear material properties . But before we delve further into these topics, it is important toanalyze the common causes of gear failures.

Analysis of common gear failure modes:The three most common types of gear tooth failures are tooth breakage, surface pitting, and

scoring.

a) Tooth breakage may be caused by an unexpectedly heavy load being imposed on the teeth. Amore common type failure is due to bending fatigue, which results from the large number of repetitions of load imposed on the tooth as the gear rotates. A small value for the radius of thefillet may accentuate the bending fatigue effects.

b) Pitting is a surface fatigue phenomenon caused by stresses exceeding the endurance limit of the surface material. After a sufficient number of repetitions of the loading cycle, bits of metal

on the surface will fatigue and drop out. The process sometimes continues at an increasingrate since the remaining unpitted areas are less able to carry the load. Lubrication difficultiesnay contribute to pitting failures.

c) Scoring can occur under heavy loads and inadequate lubrication. The oil film breaks downand metal-to-metal contact occurs. High temperatures result and the high spots of the twosurfaces of the teeth undergo rapid wear. Gearboxes should be broken in preliminaryoperation at lower loads and smaller speeds until the tooth surfaces are highly polished.Misalignment of the shafts may shift the entire load to one edge of the tooth, with resultingexcessive stress and the diametric pitch should be at least 10 or greater. For coarse pitches,scoring is sometimes a greater hazard than breakage or pitting.

d) Abrasive wear is caused by the presence of the foreign particles in gears that are not enclosed,or in enclosed gears that were assembled with abrasive particles present, or in gears lubricatedby an oil supply with inadequate infiltration.

Design of Gears to Prevent Failures1) Scoring can often be prevented by directing adequate flow of appropriate lubricant. Surface

finish is also an important factor for scoring. Surface finish as fine as 0.5 um cla is desirableto avoid scoring

2) Pitting can be reduced by reducing contact stresses by reducing loads or by optimizing geargeometry; using clean steel, properly heat treated to hardness preferably by carburizing; usingsmooth tooth surfaces produced by grinding and honing; using an adequate amount cool,clean and dry lubricant of adequate viscosity.

3) Abrasive wear can be prevented by observing the following guidelines:



a. Built-in contaminations should be removed from the gearboxes by draining andflushing the lubricant

b. Minimize internally generated wear debris by using surface hardened gear teeth,smooth tooth surfaces, and high-viscosity lubricants.

GEAR NOISEThe gear noise arises due to several reasons. At the contact point due to error in the gear profile,surface roughness, impact of tooth and sliding and rolling friction; bearings, churning of thelubricant, and windage.The principal methods of combating noise are: improving the tooth finishing operations, changingover to helical gearing, modifying the profile by flanking, increasing the contact ratio, equalizingthe load along the face width of the tooth rim, using crowned gears, and improving the design of the covers and housings.

Design and Material selection for Spur GearNow having analyzed common modes of failure in gear, the next step is selection of design

and material for spur gear, which possess resistant qualities to the failure modes. We start with thedesign part.

Spur Gear Design

i Portion of Involute Spur Gear

The function of a gear is to work smoothly while transmitting motion or torque. For this theangular velocity ratio at all times should remain constant. Hence, the gear profile is designed toensure Constant Meshing. This kind of gear profile is also known as Involute gear profile.

Involute Gear Profile: - Involute is the path generated by the end of a thread as it unwinds from areel. Involute profiles have special properties. Imagine two involute teeth in contact as shown inFigure i. If a normal is drawn at the contact point to the involute profile, it will be tangent to thegenerating circles .



Advantages of Involute gear tooth profile: -

1. Variation in centre distance does not affect the velocity ratio.2. Pressure angle remains constant throughout the engagement, which results in smooth running.3. Straight teeth of basic rack for involute admit simple tools. Hence, manufacturing becomes

cheap and simple.

Identification of Properties desired in the material for gearThe steels selected for gear applications must satisfy two basic sets of requirements that are

not always compatible – those involving fabrication and processing and those involving service .

Fabrication and processing requirements include machinability, forgeability, and response toheat treatment as it affects fabrication and processing.

Service requirements are related to the ability of the gear to perform satisfactorily under theconditions of loading for which it was designed and thus encompass all mechanical-propertyrequirements, including fatigue strength, response to heat treatment, and resistance to wear.

There are some very critical matters that need to be given a consideration before selecting a gearmaterial. They are the following:

Allowable bending and hertz stress

Wear resistance Impact strength Water and corrosion resistance Manufacturing cost Size Weight Reliability Lubrication requirements No Moisture Absorption Dimensionally Stable Stress-Free structure Environmental and surface temperature

Material Quality As it is well known that material quality has a strong influence on factors like pitting resistance andbending strength. So for getting a high quality material, the following metallurgical variables shouldbe properly controlled.

Chemical composition Hardenability Material Toughness Hardness of Surface and core Micro structure of surface and core

Cleanliness and inclusions Surface defects like flanks and root fillets Structure and size of grain Residual stress pattern Internal defects



Decarburization

Materials Available for gears

Alloys for gears

Ferrous Alloys

Wrought GearSteels

Surface HardeningSteels

Carburizing

Nitriding

Carbonitriding

ThoroughHardening Steels

Cast Steels

Cast Carbon andalloy steels

Gray and ductilecast irons

P/M irons andsteels

Stainless Steels

Tool Steels

Maraging Steels

Non-Ferrous Alloys

Bronzes

other non ferrousalloys for low load



Wrought Gear Steels Surface Hardening Steels:

Carburized Steels: General Properties Harder than normal steels

More resistant to abrasive wear Improved fatigue properties compared with unaffected core

Some representative SAE-AISI carburizing steels used for gears include: PlainCarbon Steels ( 1015, 1018, 1020, 1022, 1025), Free Machining Steels(1117 and 1118), Alloy steels(4020, 4026, 4118, 4320, 4620, 4820, 5120, 8620, 8720 and 9310).

The nickel-bearing carburizing steels are used chiefly where exceptional coretoughness combined with the highest degree of wear resistance and greatestsurface compressive strength is required.

Nitriding Steels: General Properties Can be used where a hard, wear-resistant case, good fatigue strength, low

notch sensitivity, and some degree of corrosion resistance are desired. Relatively free from wear up to the load at which surface failure occurs, but

at this load they become badly crushed and pitted. Not suitable for applications where overloads are likely to be encountered.

Any of the SAE-AISI steels that contain nitride-forming elements, such aschromium, vanadium, or molybdenum, can be nitride. The steels most commonlynitride are 4140, 4340, 6140 and 8740. Both Nitralloy N and Nitralloy 135 M areoutstanding for heavy-duty gears that are highly stressed.

Thorough-Hardening Steels : Possess greater core strength than carburized gears due to higher carbon content They are not as ductile or as resistant to surface compressive stresses and wear as

case-hardened gears. Hardness of gear surfaces may vary from 300 to 575 HB Greater hardenability provides greater strength. Suitable for gears requiring medium-to-high wear resistance and high load-

carrying capacity.

Typical of the low-alloy, medium-to-deep hardening gear materials are (in order of increasing hardenability): 4042, 5140, 8640, 3140, 4140, 8740, 6145, 9840 and 4340.When selecting a thorough-hardening stel, it should be considered that a highercarbon and alloy content is accompanied by greater strength and hardness (but lowerductility) of the surface and the core. Fully hardened and tempered medium-carbonalloy steels possess an excellent combination of strength and toughness at roomtemperature and lower temperatures. Because of their good hardenability andimmunity to temper brittleness, molybdenum steels have been widely used for gearsrequiring good toughness at room and low temperatures.



Cast steels Gray Cast Iron : General Properties

Low in cost Can be easily cast into desired shape for the rim, web, and hub of a gear. Easily machinable Good resistance to wear and are often less sensitive to lubrication

inadequacies Good dampening qualities Low impact strength and should not be used where severe shock loads occur

Ductile cast Iron : General properties Hardness can range from less than 160 HB to more than 300 HB Austempered ductile irons have very high strength, some ductility and

toughness, and often an ability to work harden, which gives appreciablyhigher wear resistance.

Used in automobiles, trucks, and railroad and military vehicles P/M iron and steels : General properties

Sintered steels and powder forged nickel steels Have excellent tensile strength qualities

Other types of steels do not have properties required for automobile gears, hence we moveon to non-ferrous alloys available for gears.

Non-Ferrous Alloys

Tin Bronzes : General Properties These alloys are tough and have good corrosion resistance They possess excellent wear resistance

Most commonly used alloy in this group is C90700, or gear bronze, which contains89% Cu and 11% Sn.

Manganese Bronzes : General Properties High strength and hardness, toughest material in bronze family Do not possess the same degree of corrosion resistance, wearability, or

bearing quality as the tin bronzes or aluminum bronzes.

Aluminum Bronzes : General Properties Lighter in weight Can attain higher mechanical properties through heat treatment As the strength is increased, their ductility decreases Bearing characteristics are better than manganese bronzes but inferior to tin

bronzes.



Quantitative Study of Candidate MaterialsMaterial Tensile

Strength(inMPa)

Yield Strength(in MPa)

Elongation in 50mm, %

Hardness (inHB)

Carbon steel bar1015 385 325 18 1111020 420 350 15 1211025 440 370 18 1491045 625 530 12 1791117 475 400 15 1371118 495 420 15 143Low-alloy steels4130 1040 979 18.1 3024150 1310 1215 13.5 3754340 1207 1145 14.2 3525140 972 841 18.5 2938620 635 360 26.3 1838740 1225 1130 16 2699310 2169 2135 7.7 55.1 HRCBronzesC86100 655 345 20 180C87500 462 207 21 134C95200 552 186 35 125C95500 689 303 12 192

Hardenability characteristics of commonly used gear steelsCommon Alloy steel grades HardenabilityNoncarburizing grades1045 Low hardenability4130 Marginal hardenability4140 Fair hardenability4145 Medium hardenability8640 Medium hardenability4340 Good hardenability in heavy sections4150 Quench crack sensitive

Good hardenability4142 Used when 4140 exhibits marginal hardenability4350 Quench crack sensitive, excellent hardenability in heavy sectionsCarburizing grades1020 Very low hardenability4118 Fair core hardenability4620 Good case hardenability8620 Fair core hardenability4320 Good core hardenability8822 Good core hardenability in heavy sections3310 Excellent hardenability (in heavy sections)4820 Excellent hardenability (in heavy sections)

9310 Excellent hardenability (in heavy sections)



Machinability of commonly used gear materialsMaterial RemarksLow-carboncarburizing steelgrades1020 Good machinability, as rolled, as forged, or normalized4118462086208822

Good machinability, as rolled, or as forged. However, normalized ispreferred. Inadequate cooling during normalizing can result in gummymaterial, reduced tool life and poor surface finish. Quench and temper as aprior treatment can aid machinability. The economics of the pretreatmentsmust be considered

3310432048209310

Fair to good machinability if normalized and tempered, annealed orquenched and tempered. Normalizing without tempering results in reducedmachinability

Medium-carbonthorough-hardeningsteel grades104511411541

Good machinability if normalized.

413041404142

Good machinability if annealed, or normalized and tempered toapproximately 255 HB or quenched and tempered to approximately 321HB. Over 321 HB, machinability is fair. Above 363 HB, machinability ispoor. Inadequate (slack) quench with subsequent low temperingtemperature may produce a part which meets the specified hardness, but

produces a mixed microstructure which results in poor machinability.41454150434043454350

Remarks for medium carbon alloy steel (above) apply. However, the highercarbon results in lower machinability of these grades. 4340 machinability isgood up to 363 HB. The higher carbon level in 4145, 4150, 4345 and 4350makes them more difficult to machine and should be specified only forheavy sections. Inadequate (slack) quench can seriously affectmachinability in these steels.

Other gear materialsGray Irons Gray cast irons have good machinability. Higher strength gray cast irons

[above 345 MPa tensile strength] have reduced machinability.Ductile Irons Annealed or normalized ductile cast iron has good machinability. The “as

cast” (not heat treated) ductile iron has good machinability up to 285 HB

and fair machinability. Quenched and tempered ductile iron has goodmachinability up to 285 HB and fair machinability up to 352 HB. Above352 HB, machinability is poor.

Gear bronzes andbrasses

All gear bronzes and brass have good machinability. The very highstrength heat treated bronzes [above 760 MPa tensile strength] have fairmachinability.

Austenitic StainlessSteel

All austenitic steel grades only have fair machinability. Because of work hardening tendencies, feeds and speeds must be selected to minimize work hardening.

Coarse grain steels are more machinable than fine grain. However gear steels are generally usedin the fine grain condition since mechanical properties are improved, and distortion during heattreatment is reduced. Increasingly cleaner steels are now also being specified for gearing.However, if sulfur content is low, less than 0.015% machinability may decrease appreciably.



Heat TreatmentThrough-hardening is generally used for gears that do not require high surface hardening. Gear toothhardness ranges from 30 to 248 HRC. Gears have medium carbon content 0.3-0.6%. The higher thehardenability the deeper through hardening of gear tooth.

Carburizing is a process in which austenitized ferrous metal is brought into contact with anenvironment of sufficient carbon to cause its absorption at the surface, creating a concentrationgradient between the surface and interior. As a rough approximation, carburized depth of approx.0.03-0.05 inch on a six diametral pitch can be obtained in about four hour at 930 0C. The primaryobjective is to secure a hard case and a relatively tough core. For this purpose, low carbon steels(around 0.3% carbon) are normally used.

Nitriding is a surface hardening heat treatment that introduces nitrogen into the surface of steel at atemperature generally in the range of 500- 575 0C while it is in the ferritic condition. Steels that arenitrided are generally medium carbon steels that contains strong nitride forming elements such asaluminium, chromium, molybdenum, tungsten and vanadium. Prior to nitriding, the steels areaustenitized, quenched and tempered. Tempering is performed at temperature 540-750 0C, a rangeabove which nitriding is performed. Tempering above nitriding temperature provides a core structurethat is stable during nitriding.

Quenching and HardeningAfter carburizing, gears are quenched in a cooling medium for hardening. Quenching develops amartensitic or a bainitic case with core microstructures other than a mixture of proeutectoid ferrite andpearlite. The quenching should not be fast enough that the case cracks. Most often oil is used forquenching purpose as it is suitable for most carburizing grades of steel. For load distortion, high alloysteels are first cooled in air after carburizing and then reheated and quenched. Direct quenching has

gained wider acceptance primarily because of economy and simplicity of the procedure. A singledirect quench operation minimizes distortion by bringing about crystallographic phase changes duringonly one heating and one cooling cycle. In case the temperature of gear is reduced prior to quenchingto minimize thermal shock, carbon content near the surface must be held to below saturationotherwise carbides will precipitate. Reheating before quench is one way to avoid the development of acarbide network. In the case of severe quench required to obtain high core hardness, sometimes maylead to cracking. Cracking results due to difference in the rate of cooling of thick and thin sections.

Tempering of quenched and carburized gearsSurface hardness of quenched gears decreases as the tempering temperature increases. Higher

tempering temperature reduces both case hardness and case depth. Gears required to maintain highcompressive and bending strength, steels that are least affected by tempering temperature arepreferred. Tempering provides some benefits to resist cracking or chipping of gear under as loading.

Cold treatmentThe presence of retained austenite in a heat treated case can be source of dimensional instability,excessive residual stress or cracking. One way of reducing the amount of retained austenite is to coldtreat a gear following quenching, in which retained austenite in the case would transform tomartensite. Temperatures in the range -75 to -100 0C are routinely used in cold treating.

In general, we attain a tooth surface hardness around 60 HRC and a core hardness between 32

and 40 HRC after heat treatment.



Effect of common alloying elementAlloying elements commonly used in gear materials: -

1) Nickel: - the principle advantage lies in higher tensile strength. Nickel also lowers the criticaltemperature and hence lower heat treat temperature can be used. Nickel increases

hardenability and fatigue strength of steels.2) Chromium: - it is essentially a hardening element and frequently used frequently used with

nickel to improve strength and wear resistance and hardenabililty. Chromium has however,the disadvantage of being temper-brittle.

3) Molybdenum: - it is relatively added in small amount(0.13 – 0.3 %). Effects: -a. Greater ductility and toughnessb. Reduced temper embrittlenessc. Improved creep resistance at high temperaturesd. Greater hardenabililty when present with chromium

4) Vanadium: - When present with Ni, Cr, and Mo, it improves fatigue resistance, provides fine

grain structure, reduces grain growth tendency.5) Tungsten: - it forms a hard, stable carbide that imparts wear and abrasive resistance. In the

dissolved form, it increases hardenability. It decreases the tendency to form cracks in thecase-core boundary.

6) Cobalt : - cobalt improves high temperature strength characteristics and corrosion resistance.In addition, it imparts excellent wear resistance. But, the hardenability of steel is reduced withcobalt over 0.4%.

Cost consideration

To minimize the cost of designing and machinability, Spur gear design is preferred becauseof its simple design.

Manufacturing cost also includes machinability and heat treatment of the material. As for the cost of material, cast iron is cheapest and aluminium bronze alloys are the

costliest.

Cast Iron and Ductile Iron, < Low Alloy steels, <<< Bronze alloys

Cost of Low alloy steels depend primarily on the composition of the steel. Thus, despite bronze alloys possessing better machinability and other characteristics, it

is not preferred due to high costs. Low Alloy steels, with better properties than Cast iron are the most preferred choice.

Gear Material Selection1. Gears are commonly made of cast iron, steel, bronze.2. Cast iron is the least expensive. ASTM / AGMA grade 20 is widely used. Grades 30, 40, 50,

60 are progressively stronger and more expensive.3. Cast Iron gears have greater surface fatigue strength than bending fatigue strength. Better

damping properties enable them to run quietly than steel.4. Nodular cast iron gears have higher bending strength together with good surface durability.

These gears are nowadays used in automobile cam shafts. A good combination is often a steelpinion mated against cast iron gear. Steel finds many applications since it combines both highstrength and low cost. Plain carbon and alloy steel usage is quite common.



5. Through hardened plain carbon steel with 0.35 - 0.6% C are used when gears need hardnessmore than 250 to 350 Bhn. These gears need grinding to overcome heat treatment distortion.

6. When compactness, high impact strength and durability are needed as in automotive andmobile applications, alloy steels are used. These gears are surface or case-hardened by flamehardening, induction hardening, nitriding or case carburizing processes.

7. Steels such as En 353, En36, En24, 17CrNiMo6 widely used for gears

8. Bronzes are used when corrosion resistance, low friction and wear under high slidingvelocity is needed as in worm-gear applications. AGMA recommends Tin bronzescontaining small % of Ni, Pb or Zn. The hardness may range from 70 to 85Bhn.Microstructure

Conclusion After analyzing tensile strength, hardness, hardenability, wear resistance, corrosion resistance

etc, carburized low alloy steel 8620 matches most of the required properties for the spur

gear. Although it is costlier than Cast Iron, it has better durability, and in the long run, it willprove to be more economical than cast iron and other cheaper iron alloys. Bronze alloys havebetter strength and corrosion resistance, yet its low ductility and high cost makes it a lesspreferable choice than 8620.

Bibliography Gear Materials, Properties, And Manufacture, by American Society of Materials(ASM)

International Design of Machine Elements, 7 th Edition, M. F. Spotts and T. E. Shoup, Pearson Education

NPTEL course Machine Design II on website http://nptel.iitm.ac.in/courses/IIT-MADRAS/Machine_Design_II/index.php , by Prof. K. Gopinath and Prof. M. M. Mayuram(IIT Madras)

www.sciencedirect.com

Thank You!

Saurav Aryan

Roll No. 09010057

Second Year Undergraduate

Mechanical Department

Shashwat Gopal

Roll No. 09010064

Second Year Undergraduate

Mechanical Department

http://nptel.iitm.ac.in/courses/IIT-MADRAS/Machine_Design_II/index.php






mm207 project gear materials

Documents