DESIGN CONSIDERATIONS OF CASTINGS

DESIGN CONSIDERATIONS OF CASTINGS: Complex parts, which are otherwise difficlt to machi!e, are made "# casti!$ processes si!$ sa!d mold% The cast compo!e!ts are sta"le, ri$id a!d stro!$ compared with respect to some other part or strctre with machi!ed or for$ed parts% E$:& machi!e tool "ed a!d strctre, c#li!der "loc' of i!ter!al com"stio! e!$i!e, pmp a!d $ear "ox hosi!$% The $e!eral pri!ciples for the desi$! of casti!$ are as follows: (% Alwa# Alwa#ss 'eep the stresse stressed d areas areas of the part part i! i! compressi compressio!% o!% Cast iro! has more compressi)e stre!$th tha! its te!sile stre!$th% *% Ro Ro!d !d all ext exter! er!al al cor cor!er !ers% s% It has two ad)a!ta$es:& •

It i!creases the e!dra!ce limit of the compo!e!t%

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It redces the formatio! of "rittle chilled ed$es%

+% her here) e)er er po possi ssi"l "le, e, th thee se sect ctio io! ! th thic ic'! '!es esss th thro ro$ $ho hot t sh sho old ld "e he held ld as ! !if ifor orm m as compati"le with o)erall desi$! co!sideratio!s%  A"rpt cha!$es i! the cross - sectio! reslt i! hi$h stress co!ce!tratio!% If the thic'!ess is to "e )aried at all, the cha!$e shold "e $radal% .% A)oid thic' thic'!ess !ess of of the the metals metals at the /!ct /!ctio!s% io!s% E)e! after the metal solidifies, the ce!tral portio! still remai!s i! the molte! sta$e, with thee re th res slt lt th that at a sh shri ri!' !'a$ a$ee ca ca)i )it# t# or "l "low owho hole le ma ma# # ap appe pear ar at th thee ce ce!t !tre re Th Thee co!ce!tratio! of metal ca! "e a)oided i! two wa#s:& •

O!e is to pro)ide cored ope!i!$ i! we"s a!d ri"s%

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Sta$$er the ri"s a!d we"s%

0% A) A)oi oid d )er# )er# thi! thi! sec secti tio! o!s% s% The mi!imm sectio! thic'!ess depe!ds po! the process of casti!$, sch as sa!d casti!$, perma!e!t mold casti!$ or die casti!$% The mi!imm thic'!ess for $re# cast iro! compo!e!t is a"ot 1 mm for parts pto 022 mm lo!$, which $radall# i!creases to *2 mm for lar$e a!d hea)# casti!$s% 3% Sho Shott "last "last the part partss where)e where)err possi"l possi"le% e% The shot "lasti!$ impro)es the e!dra!ce limit of the compo!e!t, particlarl# i! case of  thi! sectio!s%

DESIGN CONSIDERATIONS OF FORGINGS: For$ed compo!e!ts are widel# sed i! atomoti)e a!d aircraft i!dstries% The# are sall#  made of steels a!d !o! - ferros metals% The# ca! "e as small as the $d$eo! pi! a!d as lar$e as cra!'shaft% For$ed compo!e!ts are sed !der followi!$ circmsta!ces:& •

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4o)i!$ compo!e!ts re5iri!$ li$ht wei$ht to redce i!ertia forces% E$:& co!!ecti!$ rod of I%C% e!$i!e% Compo!e!ts s"/ected to excessi)e stresses% E$:& aircraft strctre% Small compo!e!ts that mst "e spported "# other strctre or part% E$:& ha!d tools a!d ha!dle% Compo!e!ts re5iri!$ pressre ti$ht!ess where the part mst "e free from i!ter!al crac's% E$:& )al)e "odies% Compo!e!ts whose failre wold case i!/r# a!d exte!si)e dama$e are for$ed for safet#%

I! order to o"tai! maximm "e!efit from for$ed compo!e!ts, followi!$ pri!ciple shold "e adopted:& (% hile desi$!i!$ a for$i!$, ad)a!ta$e shold "e ta'e! of the directio! of fi"re li!es% I! case of for$ed parts, the fi"re li!es are arra!$ed i! a fa)ora"le wa# to withsta!d stresses de to exter!al load% *% The for$ed compo!e!t shold "e pro)ided with a! ade5ate draft% +% A parti!$ li!e di)ides the for$i!$ i!to two hal)es which e!sres the mi!imm depth to  which the steel mst flow to fill the die impressio!s% .% The for$i!$ shold "e pro)ided with ade5ate fillet a!d cor!er radii% Small radis reslts i! folds o! the i!!er srface a!d crac's o! the oter srface% 6ar$e radis is !desira"le% Sharp cor!ers reslt i! i!creasi!$ difficlties ! filli!$ the material, excessi)e for$i!$ forces a!d poor die life% 0% Thi! sectio!s a!d ri"s shold "e a)oided i! for$ed compo!e!ts% A thi! sectio! cools at a faster rate i! the die ca)it# a!d re5ires excessi)e force for plastic deformatio!% It redces the die life a!d the remo)al of the compo!e!t from the die ca)ities "ecomes difficlt%  A properl# desi$!ed for$i!$ is !ot o!l# so!d with re$ard to stre!$th "t it also helps redce the for$i!$ forces, impro)es die life a!d simplifies die - desi$!%

DESIGN CONSIDERATIONS OF 4AC7INED 8ARTS: 4achi!ed compo!e!ts are widel# sed i! i!dstrial prodcts% The# are sall# made from ferros a!d !o! - ferros metals% The $e!eral pri!ciples for the desi$! of machi!e parts are as follows:& (% A)oid machi!i!$% 4achi!i!$ operatio!s i!crease the cost of the prodct% *% Specif# li"eral tolera!ces% Depe!di!$ o! the f!ctio!al re5ireme!t of the compo!e!t, the desi$!er shold specif#  most li"eral dime!sio!al a!d $eometric dime!sio!s% +% A)oid sharp cor!ers% The sharp cor!ers reslt i! stress co!ce!tratio!% .% 9se stoc' dime!sio!s% 9si!$ stoc' dime!sio!s elimi!ates machi!i!$ operatio!s% 0% Desi$! ri$id parts%  A!# machi!i!$ operatio!s sch as tr!i!$ or shapi!$ i!dce ctti!$ forces o! the compo!e!ts% The compo!e!t shold "e ri$id e!o$h to withsta!d sch forces% 3% A)oid sholders a!d !dercts% The# sall# i!)ol)e separate operatio!s a!d separate tools, which i!crease the cost of  machi!i!$% 1% A)oid hard materials% The# shold "e a)oided !less sch properties are esse!tial for the f!ctio!al re5ireme!t of the prodct%

DESIGN CONSIDERATIONS OF E6DED ASSE46IES:  eldi!$ is the most importa!t method of /oi!i!$ the parts i!to a complex assem"l#% The $e!eral pri!ciples i! desi$! of welded parts are as follows:& •

Select the material with hi$h welda"ilit#%  A low car"o! co!te!t steel is more easil# welded tha! a hi$h car"o! steel%

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9se mi!imm !m"er of welds%

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Do !ot shape the parts "ased o! casti!$ or for$i!

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9se sta!dard compo!e!ts%  A)oid straps, laps a!d stiffe!ers%

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Select proper locatio! for weld%

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8rescri"e correct se5e!ce of weldi!

%$TO6ERANCES: Tolera!ce is defi!ed as permissi"le )ariatio! i! the dime!sio!s of the compo!e!t% The "asic dime!sio! is called the !ormal or "asic si;e, while the differe!ce "etwee! the two limits is called permissi"le tolera!ce% The two limits are sometimes called the pper a!d lower de)iatio!s% There are two s#stems of specificatio!s for tolera!ces, !amel# !ilateral a!d "ilateral% •

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I! !ilateral s#stem, o!e tolera!ce is ;ero while the other ta'es care of all permissi"le  )ariatio! i! "asic si;e% The# are sed for shafts a!d holes% I! case of "ilateral tolera!ces, the )ariatio! are $i)e! i! "oth directio!s from !ormal si;e%

T<8ES OF FITS:  he! two parts are to "e assem"led, the relatio!ship reslti!$ from the differe!ce "etwee! their si;es "efore assem"l# is called a fit% Depe!di!$ po! the limits of the shaft a!d the hole, fits are "roadl# classified i!to three $rops:& (% Cleara!ce fit It is a fit which alwa#s pro)ides a positi)e cleara!ce "etwee! the hole a!d the shaft )er the e!tire ra!$e of tolera!ces% I! this case, the tolera!ce ;o!e of the hole is e!tirel# a"o)e that of the shaft% Hole

=ero li!e Shaft

Fi$%:& dia$ram of cleara!ce fit

*% I!terfere!ce fit It is a fit, which alwa#s pro)ides a !e$ati)e cleara!ce o)er the whole ra!$e of tolera!ces% I! this case, the tolera!ce ;o!e of the hole is completel# "elow that of the shaft%

Shaft

Hole

Fi$%:& dia$ram of i!terfere!ce fit +% Tra!sitio! fit It is a fit, which ma# pro)ide either a cleara!ce or i!terfere!ce, depe!di!$ po! the actal )ales of the i!di)idal tolera!ces of the mati!$ compo!e!ts% I! this case, the tolera!ce ;o!es of the hole a!d the shaft o)erlap%

Shaft

Shaft

Shaft

7ole

Fi$%:& dia$ram of tra!sitio! fit%

There are two "asic s#stems for $i)i!$ tolera!ces to the shaft a!d the hole:& (% 7ole "asis s#stem >7? The differe!t cleara!ces a!d i!terfere!ces are o"tai!ed "# associati!$ )arios shafts with a si!$le hole, whose lower de)iatio! is ;ero% The si;e of the hole is the "asic si;e% This s#stem has a! ad)a!ta$e o)er the shaft "asis s#stem, "ecase holes are machi!ed "# sta!dard drills or reamers ha)i!$ fixed dime!sio!s, while the shafts ca! "e tr!ed or $ro!d to a!# $i)e! dime!sio!% De to this reaso!, the hole "asis s#stem is widel# sed% *% Shaft "asis s#stem >h? The differe!t cleara!ces or i!terfere!ces are o"tai!ed "# associati!$ )arios holes  with a si!$le shaft, whose pper de)iatio! is ;ero% The si;e of the shaft is the "asic si;e, while the cleara!ce or i!terfere!ce is applied to the dime!sio!s of the hole%

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F!dame!tal de)iatio!% The f!dame!tal de)iatio! $i)es the locatio! of the tolera!ce ;o!e with respect to the ;ero li!e% It is i!dicated "# a! alpha"et - capital letters for holes a!d small letters for shafts%

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4a$!itde of tolera!ce% The ma$!itde of tolera!ce is desi$!ated "# a !m"er, called the $rade% The $rade of tolera!ce is defi!ed as a $rop of tolera!ces, which are co!sidered to ha)e the same le)el of accrac# for all "asic si;es% There are ei$htee! $rades of tolera!ces with desi$!atio!s - IT(, IT* %%% IT(@% IT - I!ter!atio!al tolera!ce% E$%:& the tolera!ce for a shaft of 02 mm diameter as the "asic si;e, with the f!dame!tal de)iatio! de!oted "# the alpha"et $B a!d the tolera!ce of $rade 1 is  writte! as 02$1%

The fit is i!dicated "# the "asic si;e commo! to "oth compo!e!ts followed "#  s#m"ols for tolera!ce of each compo!e!t% E$%:& 027@$1 or 027@ - $1%

SE6ECTIE ASSE46<: The selecti)e assem"l# is a process of sorti!$ the ma!factred compo!e!ts i!to differe!t $rops, accordi!$ to their si;es a!d the! assem"li!$ the compo!e!ts of o!e $rop to the correspo!di!$ compo!e!ts of a matchi!$ $rop% •

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It reslts i! closer cleara!ce or i!terfere!ce withot i!)ol)i!$ costl# machi!i!$ methods% It reslts i! fi!er cleara!ce or i!terfere!ce e)e! with lar$e ma!factri!$ tolera!ces%

The mai! draw"ac's of this method are:& •

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(22 i!spectio! is re5ired for compo!e!ts a!d there is additio!al operatio! of  sorti!$ the compo!e!ts% I!tercha!$ea"ilit# is affected a!d ser)ici!$ or replaceme!t of wor! ot compo!e!ts "ecomes difficlt% The method ca! "e sed o!l# whe! lar$e !m"er of compo!e!ts are ma!factred, otherwise some $rops ma# !ot co!tai! sfficie!t !m"er of  compo!e!ts%

It is particlarl# sefl i! the case of i!terfere!ce fits%

TER4INO6OG< RE6ATED TO FITS AND TO6ERANCES: Certai! termi!olo$# co!!ected with limits a!d tolera!ces is $i)e! "elow:& (% De)iatio! It is the al$e"raic differe!ce "etwee! a si;e > actal, maximm etc? a!d the correspo!di!$ "asic si;e% *% Actal de)iatio! It is the al$e"raic differe!ce "etwee! a! actal si;e a!d the correspo!di!$ "asic si;e%

+% 9pper de)iatio! It is the al$e"raic differe!ce "etwee! the maximm limit of si;e a!d the correspo!di!$ "asic si;e% .% 6ower de)iatio! It is the al$e"raic differe!ce "etwee! the mi!imm limit of si;e a!d the correspo!di!$ "asic si;e% 0% =ero li!e It is a strai$ht li!e to which the de)iatio!s are referred i! a $raphical represe!tatio! of limits a!d fits% 3% F!dame!tal de)iatio! It is o!e of the two de)iatio!s that is co!)e!tio!all# chose! to defi!e the positio! of the tolera!ce ;o!e i! relatio! to the ;ero li!e% 1% Tolera!ce ;o!e It is the ;o!e "o!ded "# two limits of si;e of a part i! the $raphical represe!tatio! of tolera!ce% It is defi!ed "# its ma$!itde a!d "# its positio! i! relatio! to the ;ero li!e%