(96957601) Gear Manufacturing

8/13/2019 (96957601) Gear Manufacturing

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Module7

Screw threadsand gear

manufacturingVersion 2 ME, IIT Kharagpur


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methods

Version 2 ME, IIT Kharagpur


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Lesson32

Manufacturing of

Gears.Version 2 ME, IIT Kharagpur


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Instructional objectives

At the end of this lesson, the students will be able to

(i !tate the basic purposes of use of gears(ii "ite the general applications of gears(iii "lassif# the t#pes of gears of co$$on use(iv !pecif# gears(v %escribe the different $ethods of $anufacturing various t#pes of gears

(a &refor$ing(b &roducing gear teeth b# $achining(c 'inishing gear teeth

(i asic &urpose )f *se )f +ears

+ears are widel# used in various $echanis$s and devices to trans$it powerand $otion positivel# (without slip between parallel, intersecting ( ais ornon-intersecting non parallel shafts,

without change in the direction of rotation

with change in the direction of rotation

without change of speed (of rotation

with change in speed at an# desired ratio)ften so$e gearing s#ste$ (rac. / and / pinion is also used to transfor$

rotar# $otion into linear $otion and vice-versa0

Fig. 7.2.1 Features of spur gears and involute tooth profile



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+ears are basicall# wheels having, on its peripher#, e1uispaced teeth whichare so designed that those wheels trans$it, without slip, rotar# $otions$oothl# and unifor$l# with $ini$u$ friction and wear at the $ating tooth /profiles0 To achieve those favourable conditions, $ost of the gears have theirtooth for$ based on involute curve, which can si$pl# be defined as ocus of a

point on a straight line which is rolled on the peripher# of a circle or ocus ofthe end point of a stretched string while its unwinding over a c#linder asindicated in 'ig0 30204

(ii +eneral Applications )f +ears

+ears of various t#pe, si5e and $aterial are widel# used in several $achinesand s#ste$s re1uiring positive and stepped drive0 The $ajor applications are 6

!peed gear bo, feed gear bo and so$e other .ine$atic units of$achine tools

!peed drives in tetile, jute and si$ilar $achineries

+ear boes of auto$obiles

!peed and 7 or feed drives of several $etal for$ing $achines

Machineries for $ining, tea processing etc0 arge and heav# dut# gear boes used in ce$ent industries, sugar

industries, cranes, conve#ors etc0

&recision e1uip$ents, cloc.s and watches

Industrial robots and to#s0(iii T#pes )f +ears And Their "haracteristics

+ears are broadl# classified

(a) According to configuration ('ig0 30202

Eternal gear Internal gear



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(a)

(b)

Fig. 7.2.2 Configuration of (a) external and (b) internal gears



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(b) According to axes of transmission

Spur gears / trans$itting rotation between parallel shafts asshown in 'ig0 30208o !traight toothedo 9elical toothed

!ingle helical

double helical (herringbone

Fig. 7.2.3 (a) Straight toothed (b)Helical and (c) Double helical gears

"o$pared to straight toothed gears, helical toothed gears run $ores$oothl# and can trans$it larger tor1ue0 %ouble helical gears are of large si5eand used for heav# tor1ue trans$ission0

evel gears / trans$itting $otion between intersecting shafts(aes ('ig0 3020:

o !traight toothedo 9elical toothed !piral bevel gear 9#poid gear

9elical toothed bevel gears are used for s$oother and larger tor1ue

trans$ission0

+ears trans$itting $otion and power between non-parallel non-intersecting shafts ('ig0 3020;o


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(c) According to pattern of motion

=otation to rotation 6 - ('ig0 3020> wheel t#pe gears

=otation to translation or vice versa / e0g0 rac. and piniono !traight toothed

o 9elical toothed

+

(a)

+

+

(b) (c)

Fig. 7.2.4 Bevel gears; (a) straight toothed, (b) spiral and hypoid gears


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(a)(b) (c)

Fig. 7.2.5 ears trans!itting po"er bet"een non#parallel non intersectingshafts$ (a) "or! and "or! "heel, (b) hypoid gear and (c) spiral gears$


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(a) (b)

Fig. 7.2.6 earing syste!s transfor!ing (a) rotation to rotation and(b) rotation to translation

(iv !pecification )f +ears

+ears are generall# specified b# their T#pe? e0g0 spur, bevel, spiral etc0

Material

!i5e or di$ensions

+eo$etr#

!pecial features, if an#

Type of gears/ has alread# been discussed in the previous section (iii

Gear materialsThe $aterials of $ost of the gears used for trans$itting reasonable tor1ue

and speed $ainl# need to be $echanicall# strong in shear and bending,sufficientl# tough and resistant to wear, fatigue and che$ical degradation09owever, the $aterial for an# gear is selected based on,

The wor.ing condition ie, power, speed and tor1ue to betrans$itted


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@on ferrous $etals / for light load

Alu$iniu$, bron5e and brass are used for $a.ing gears havingfine teeth and wor.ing at ver# light load / e0g0, in e1uip$ents,

to#s etc0 or against hard steel $ating gears

Alu$iniu$ allo#s li.e alu$iniu$ bron5e, inc / Al0 allo# etc0 @on-$etals / widel# used for light load, non-precision and noiseless

operation0 &ol#$ers (plastics 6 both the$oplastic and ther$osetting

t#pe and various co$posites ($etals, graphite, wood dust or cera$icpowders dispersed in ther$osetting plastics

Size or major dimensions

The di$ensional features that are used to epress or specif# the gears are ?

'or spur gears and wor$ wheels

nu$ber of teeth, 5 $odule, $ heli angle, if an# ( width (b

'or ea$ple, pitch circle dia$eter (&"% B $7cos

'or wor$ (single or double toothed gears nu$ber of start $odule heli angle length

Gear geometry!o$e geo$etrical features also need to be $entioned while specif#ing gears,

such as, &ressure angle

Addendu$ and dedendu$

Special featuresIf there be an# special feature, that also has to be included with gear

specification, such as Tooth bevelling for safe handling Tooth crowning for unifor$ wear and long service life Tooth rounding for eas# engage$ent

as indicated in 'ig0 30203


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(a) (b)

Fig. 7.2.7 %odification of spur gear tooth; (a) cro"ning and (b) tooth rounding

(V Manufacture )f +ears

Manufacture of gears needs several processing operations in se1uential stagesdepending upon the $aterial and t#pe of the gears and 1ualit# desired0 Thosestages generall# are 6&refor$ing the blan. without or with teethAnnealing of the blan., if re1uired, as in case of forged or cast steels&reparation of the gear blan. to the re1uired di$ensions b# $achining

&roducing teeth or finishing the prefor$ed teeth b# $achining'ull or surface hardening of the $achined gear (teeth, if re1uired'inishing teeth, if re1uired, b# shaving, grinding etc0Inspection of the finished gears0

In this section, perfor$ing, producing teeth b# $achining and gear teeth

finishing have been discussed in detail0

&refor$ing +ear lan.s

Casting

+ear blan.s and even gears along with teeth re1uiring substantial to little

$achining or finishing are produced b# various casting processes0

Sand castingThe blan.s of large cast iron gears, if re1uired to be $ade one or few

pieces, are produced b# sand casting0 Then the blan. is prepared toappropriate di$ensions and the teeth are produced b# $achining thatcast prefor$0 "o$plete gears with teeth can also be directl# produced b#such casting and used at low speed in $achineries li.e far$


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$achiner# and hand operated devices where gear accurac# and finishare not that $uch re1uired0

Metal mould castingMediu$ si5e steel gears with li$ited accurac# and finish are often

$ade in single or few pieces b# $etal $ould casting0 !uch unfinishedgears are used in several agro-industries0 'or general and precision usethe cast prefor$s are properl# $achined0

ie castingarge lot or $ass production of s$all gears of low $elting point allo#s

of Al, n, "u, Mg etc0 are done $ainl# b# die casting0 !uch reasonabl#accurate gears are directl# or after little further finishing are used underlight load and $oderate speeds, for ea$ple in instru$ents, ca$era,to#s0

!n"estment castingThis near-net-shape $ethod is used for producing s$all to $ediu$

si5e gears of eotic $aterials with high accurac# and surface finish hardl#re1uiring further finishing0 These relativel# costl# gears are generall#used under heav# loads and stresses0

S#ell mould casting!$all gears in batches are also often produced b# this process0 The

1ualit# provided b# this process lies in between that of sand casting andinvest$ent casting0

Centrifugal castingThe solid blan.s or the outer ri$s (without teeth of wor$ wheels $ade

of cast iron, phosphor bron5e or even steel are preferabl# prefor$ed b#centrifugal casting0 The perfor$s are $achined to for$ the gear blan. ofproper si5e0 Then the teeth are developed b# $achining0

Manufacture of gears by

rollingThe straight and helical teeth of disc or rod t#pe eternal steel gears of s$all

to $ediu$ dia$eter and $odule are generated b# cold rolling b# either flat diesor circular dies as shown in 'ig0 3020C0 !uch rolling i$parts high accurac# andsurface integrit# of the teeth which are for$ed b# $aterial flow unli.e cutting0+ear rolling is reasonabl# e$plo#ed for high productivit# and high 1ualit# thoughinitial $achiner# costs are relativel# high0 arger si5e gears are for$ed b# hotrolling and then finished b# $achining


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gear blan$

flat die (rac$ type)

Fig. 7.2.8 &roduction of teeth of spur gears by rolling$

%o&d

er

metall

urgy!$all si5e high 1ualit# eternal or internal spur, bevel orspiral gears are also

produced b# powder $etallurg# process0 arge si5e gears

are rolled after bri1uetting and sintering for $ore strengthand life0 &owder $etallurgicall# produced gears hardl#re1uire an# further finishing wor.0

'lan$ing

in %ress

tool


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Massproductionofs

$allandthin$etallicgearsre1uiringlessaccurac#andfinishareoftendone

b#bla

n.ing fro$ sheets b# suitabl# designed die and punch0!uch gears are used for cloc.s, watches, $eters, to#s etc09owever, 1ualit# gears can also be produced b# slightfinishing (shaving after blan.ing0

%las

tic

mou

ldin

g

!$all to $ediu$ si5e plastic gears with or without$etal core are

$anufactured in large 1uantit# b# injection $oulding0!uch $oderatel# accurate and less nois# gears, both

eternal and internal t#pes, are used under light loadssuch as e1uip$ents, to#s, $eters etc0

xtru

sion

proce

ss

9igh 1ualit# s$all $etallic or non $etallic eternal gearsare often produced

in large 1uantit# b# etrusion0 @u$ber of gears of desired

width are obtained b# parting fro$ the etruded rod ofgear / section0


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ire M+eo$etricall# accurate but $oderatel# finished straight toothed $etallic spur

gears, both eternal and internal t#pe, can be produced b# wire t#pe Electro- dischargeMachining (E%M as shown in 'ig0 3020D

Fig. 7.2.9 &roduction of teeth of external and internal spur gears by 'ire#lectrodischarge !achining (D%)

%roduction of Gear Teet# by Mac#ining

It appears fro$ the previous section that gears are $anufactured in several routes ? The prefor$ed blan.s of approi$ate shape and irregular

surface are $achined to desired di$ensions and finish and then

the teeth are produced generall# b# $achining and occasionall# b# rolling0 'ull gears with teeth are $ade b# different processes and then

finished b# further $achining and 7 or grinding

Accurate gears in finished for$ are directl# produced b# near /net / shape process li.e rolling, plastic $oulding, powder

$etallurg# etc0 re1uiring slight or no further finishing0

The $ost co$$onl# practiced $ethod is prefor$ing the blan. b# casting, forging etc0followed b# pre-$achining to prepare the gear blan. to desired di$ensions and then

production of the teeth b# $achining and further finishing b# grinding if necessar#0+ear teeth are produced b# $achining based on

*orming/ where the profile of the teeth are obtained as the replica ofthe for$ of the cutting tool (edge? e0g0, $illing, broaching etc0

Generation/ where the co$plicated tooth profile are provided b#$uch si$pler for$ cutting tool (edges through rolling

t#pe, tool / wor. $otions, e0g0, hobbing, gear shaping etc0


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Met#ods of production of gear teet# by mac#ining on *ormingprinciple

S#aping planing and slotting

'ig0 30204 sche$aticall# shows how teeth of straight toothed spur gear canbe produced in shaping $achine, if necessar#0 oth productivit# and product 1ualit# arever# low in this process which therefore, is used, if at all, for $a.ing one or few teeth onone or two pieces of gears as and when re1uired for repair and $aintenance purpose0In principle planning and slotting $achines wor. on the sa$e principle0 &laning$achine is used, if re1uired at all, for $a.ing teeth of large gears whereas slotting,generall#, for internal gears0

Milling+ear teeth can be produced b# both disc and end $ill t#pe for$ $illing cutter

as shown in 'ig0 302044&roduction of gear teeth b# for$ $illing are characterised b# 6

use of 9!! for$ $illing cutters

use of ordinar# $illing $achines

low production rate for need of indeing after $achining each tooth gap slow speed and feed low accurac# and surface finish inventor# proble$ / due to need of a set of eight cutters for each

$odule / pressure angle co$bination0

End $ill t#pe cutters are used for teeth of large gears and 7 or$odule0

Fig. 7.2.10 ear teeth cutting in ordinary shaping !achine$


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(a) (b) (c)

Fig. 7.2.11 &roducing external teeth by for! !illing cutters (a) disc type and end !illtype for (b) single helical and (c) double helical teeth

*ast production of teet# of spur gears

%arallel multiple teet# s#apingIn principle, it is si$ilar to ordinar# shaping but all the tooth gaps are $ade

si$ultaneousl#, without re1uiring indeing, b# a set of radiall# infeeding single pointfor$ tools as indicated in 'ig0 302042(a0 This old process was highl# productive butbeca$e al$ost obsolete for ver# high initial and running costs0cutting tools

gear

(a)

(b)

Cutting stro$e

Fig. 7.2.12 High production of straight teeth of external spur gears by (a)

parallel shaping (for!ing) and (b) broaching


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'roac#ingTeeth of s$all internal and eternal spur gears? straight or single helical, of

relativel# softer $aterials are produced in large 1uantit# b# this process0 'ig0302042 (b sche$aticall# shows how eternal teeth are produced b# a broaching in one

pass0 This $ethod leads to ver# high productivit# and 1ualit# but cost of $achine andbroach are ver# high0

%roduction of gear teet# by mac#ining on Generation principle

+eneration $ethod is characterised b# auto$atic indeing and abilit# of a

single cutter to cover the entire range of nu$ber of teeth for a given co$bination of$odule and pressure angle and hence provides high productivit# and econo$#0

Sunderland met#od using rac$ type cutter

'ig0 302048 sche$aticall# shows the principle of this generation process wherethe rac. t#pe 9!! cutter (having ra.e and clearance angles reciprocates to acco$plishthe $achining (cutting action while rolling t#pe interaction with the gear blan. li.e a pairof rac. and pinion0 The favourable and essential applications of this $ethod (and$achine include 6

$oderate si5e straight and helical toothed eternal spur gears withhigh accurac# and finish

cutting the teeth of double helical or herringbone gears with a centralrecess (groove

cutting teeth of straight or helical fluted cluster gears9owever this $ethod needs, though auto$atic, few indeing operations0

Fig. 7.2.13 xternal gear teeth generation by rac type cutter(Sunderland !ethod)

Gear s#aping

In principle, gear shaping is si$ilar to the rac. t#pe cutting process, ecepting

that, the linear t#pe rac. cutter is replaced b# a circular cutter as indicated in 'ig030204:, where both the cutter and the blan. rotate as a pair of spur gears in addition to

the reciprocation of the cutter0


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+eneration $ethod is characterised b# auto$atic indeing and abilit# of a single cutterto cover the entire range of nu$ber of teeth for a given co$bination of $odule andpressure angle and hence provides high productivit# and econo$#0The gear t#pe cutter is $ade of 9!! and possesses proper ra.e and clearanceangles0

The additional advantages of gear shaping over rac. t#pe cutting are 6 separate indeing is not re1uired at all

straight or helical teeth of both eternal and internal spur gears canbe produced with high accurac# and finish

productivit# is also higher0

(a)(b)

Fig. 7.2.14 ear teeth generation by gear shaping (a) external and

(b) internal spur gear

,obbingThe tool-wor. configuration and $otions in hobbing are shown in 'ig0 30204;,

where the 9!! or carbide cutter having teeth li.e gear $illing cutter and the gear blan.apparentl# interact li.e a pair of wor$ and wor$ wheel0 The hob (cutter loo.s andbehaves li.e a single or $ultiple start wor$0 9aving lesser nu$ber (onl# three of tool /wor. $otions, hobbing $achines are $uch $ore rigid, strong and productive than gearshaping $achine0 ut hobbing provides lesser accurac# and finish and is used onl# forcutting straight or helical teeth (single of eternal spur gears and wor$ wheels0

(a) (b) (c)

Fig. 7.2.15 eneration of external gear teeth by Hobbing *(a straight tooth (b helical tooth and (c wor$ wheel


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Manufacture of &ormThe screw li.e single or $ulti-start wor$s (gears $ade of steel are generall#

$ade b# $achining li.e long thread $illing or b# cold rolling li.e thread rolling followedb# heat treat$ent for surface hardening and finishing b# grinding0

Manufacture of be"el gears

In $anufacture of bevel gears, first the blan.s are prefor$ed b# casting or

forging followed b# $achining to desired di$ensions in lathes or special purpose$achine0Then the teeth are produced in the blan. b# $achining0 The wa# of $achining and$achine tool are chosen based on the for$ of teeth and volu$e of production asfollows 6

!traight toothed bevel gearo 'or$ing b# $illing cutter / low productivit# and 1ualit# hence

e$plo#ed for production re1uiring less volu$e and precisiono +eneration / high accurac# and finish, hence applied for batch to $ass

production0

'ig0 30204> sche$aticall# shows the principle of for$ing and generation of teeth ofstraight toothed bevel gear0 In generation process, the inner flan.s of two adjacent teethare developed with involute profile b# the straight teeth of the cutters under rollingaction0

Teeth of spiral and h#poid bevel gears are produced b# al$ost thesa$e generation principle but the cutter rese$bles face $illing cutter

as shown in 'ig0 3020430

(a)(b)

Fig. 7.2.16 &roduction of teeth of straight toothed spur gear by (a) for!ing and (b)generation


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Fig. 7.2.17 eneration of teeth of spiral and hypoid bevel gear$

*inis#ing of Gear Teet#

'or s$ooth running, good perfor$ance and long service life, the gears need to be accurate in di$ensions and for$s

to have high surface finish and

to be hard and wear resistive at their tooth flan.swhich are achieved b# so$e gear teeth finishing wor. after near accurate

prefor$ing and $achining0 !$all gears $ade b# cold rolling generall# do not re1uirefurther finishing0 If a rolled gear needs further surface hardening onl# then little finishingb# grinding and 7 or lapping is done after hardening0+ears produced to near-net-shape b# die casting, powder $etallurg#, etrusion,blan.ing etc0 need little finishing0ut $achined and hardened gear teeth are essentiall# finished for accurac# andsurface finish0

"o$$on $ethods of gear teeth finishing

+ear teeth, after prefor$ing and $achining, are finished generall# b#? for soft and unhardened gears

gear shaving

gear rolling or burnishing

for hard and hardened gears

grinding

lapping

for soft but precision gears

shaving followed b# surface hardening and then lapping


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Gear s#a"ingThe teeth of straight or helical toothed eternal spur gears and wor$ wheels

of $oderate si5e and $ade of soft $aterials li.e alu$iniu$ allo#, brass, bron5e, castiron etc0 and unhardened steels are $ostl# finished b# shaving process0 'ig0 30204C

shows the different t#pes of shaving cutters which while their finishing action wor.apparentl# as a spur gear, rac. or wor$ in $esh with the conjugate gears to befinished0 All those gear, rac. or wor$ t#pe shaving cutters are of hard steel or 9!! andtheir teeth are unifor$l# serrated as shown in 'ig0 30204D(a to generate sharp cuttingedges0


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(a) (b)

Fig. 7.2.19 Cutting teeth of gear shaving (a) cutter and its (b) action

Gear teet# grinding

+rinding is a ver# accurate $ethod and is, though relativel# epensive, $ore

widel# used for finishing teeth of different t#pe and si5e of gears of hard $aterialor hardened surfaces0 The properl# for$ed and dressed wheel finishes the gearteeth flan.s b# fine $achining or abrading action of the fine abrasives0i.e gear $illing, gear grinding is also done on two principles

'or$ing

+eneration, which is $ore productive and accurate

Gear teet# grinding on forming principleThis is ver# si$ilar to $achining gear teeth b# a single disc t#pe for$ $illing

cutter as indicated in 'ig0 30202 where the grinding wheel is dressed to the for$that is eactl# re1uired on the gear0 @eed of indeing $a.es the process slowand less accurate0 The wheel or dressing has to be changed with changein $odule, pressure angle and even nu$ber of teeth0 'or$ grinding $a# beused for finishing straight or single helical spur gears, straight toothed bevelgears as well as wor$ and wor$ wheels0

Gear teet# grinding on generation principle

'ig0 302024 sche$aticall# shows the $ethods of finishing spur gear teeth b#

grinding on generation principle0The si$plest and $ost widel# used $ethod is ver# si$ilar to spur gear teethgeneration b# one or $ulti-toothed rac. cutter0 The single or $ulti-ribbedrotating grinding wheel is reciprocated along the gear teeth as shown0 )ther tool


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/ wor. $otions re$ain sa$e as in gear teeth generation b# rac. t#pe cutter asindicated in 'ig0 3020480 'or finishing large gear teeth a pair of thin dish t#pegrinding wheels are used as shown in 'ig0 302024 (c0


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eel

gear

Fig. 7.2.20 ear teeth finishing by for! grinding

"


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(a) (b) (c)

Fig. 7.2.21 ear teeth grinding on generation principle$

Gear teet# finis#ing by lapping

The lapping process onl# corrects $inute deviations fro$ the desired gear

tooth profiles0 The gear to be finished after $achining and heat treat$ent and evenafter grinding is run in $esh with a gear shaped lapping tool or another $ating gearof cast iron0 An abrasive lapping co$pound is used in between the$0 The gear tooth

contact substantiall# i$proves b# such lapping0

(96957601) Gear Manufacturing

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