Ngày tải lên :
17/12/2013, 11:15
... Inc.
Thekineticenergiesmaybefoundfrom
n
14
¼
1
16
n
24
¼
1
3:2
n
34
¼
1
3:2
m
1
¼1278m
2
¼276m
3
¼721m
4
¼231
r
1g
¼0:81r
2g
¼0:17r
3g
¼0:32r
4g
¼0:09
I
g1
¼m
1
r
2
1g
I
g2
¼m
2
r
2
2g
I
g3
¼m
3
r
2
3g
I
g4
¼m
4
r
2
4g
I
d
¼m
d
r
2
dg
ke
d
¼
1
2
I
d
n
2
14
ke
g1
¼
1
2
I
g1
n
2
14
ke
g2
¼
1
2
I
g2
n
2
24
ke
g3
¼
1
2
I
g3
n
2
34
ke
g4
¼
1
2
I
g4
wherem
1
throughm
4
arethemassesofgears1through4,respectively,andr
1g
throughr
4g
aretheirrespectiveradiiofgyration.Thus,
KE
d
¼ke
d
N
2
m
¼185:5N-mKE
L
¼
1
2
m
L
v
2
c
¼957:3N-m
KE
r
¼
1
2
m
r
v
2
c
¼49:2N-mKE
g1
¼ke
g1
N
2
m
¼4224:5N-m
KE
g2
¼ke
g2
N
2
m
¼1004:7N-mKE
g3
¼ke
g3
N
2
m
¼9299:2N-m
KE
g4
¼ke
g4
N
2
m
¼2413:3N-m
Additionofthesegives
KE¼ðke
g1
þke
g2
þke
g3
þke
g4
þke
d
ÞN
2
m
þKE
L
þKE
r
¼18;133:6N-m
Souponaddingthistothetotalpotentialenergyof
PE¼23;202:3N-m
thetorquerequiredmaybefoundfrom
T
o
¼
PEþKE
ðN
m
=2Þt
¼813:9N-mð8-6Þ
inwhichtheaveragemotorspeedduringbrakingwastakentobeN
m
/2.
Thebrakingrequirementof813.9N-mmaybemetbyusingavarietyof
brakes, suchasband,externallinear,annularcaliper,andannulardisk
brakes. Tochooseamongthese,recallequation(1-10)fromChapter1,equa-
tion(2-1)fromChapter4,andequations(1-7 )and( 3-5)fromChapter5
corresponding ... Inc.
theannularcaliperandannulardiskbegivenbyequation.Accordingly,eval-
uatetheformulas
T¼p
max
wr
2
o
ð1Àe
ÀAf
Þð8-7Þ
forabandbrake,
T¼2Ap
max
wr
2
o
sin
f
o
2
ð8-8Þ
foreithertwoopposinginternalorexternallinearlyactingbrakeshoes,
T¼2Ap
max
r
3
o
3
ffiffiffi
3
p
f
o
ð8-9Þ
fortwoopposingdiscbrakepads,eachsubtendingangleB
o
,and
T¼4kAp
max
r
3
o
3
ffiffiffi
3
p
ð8-10Þ
fortwocompleteannularpadsinwhichB
o
=2kinequation(8-9).
Weshallalsoconsideranexternalpivoteddrumbrakewithaleading
andtrailingshoethatmaybeevaluatedbyinvokingtheprogramusedin
Chapter3.Inallofthesecalculationsassumeafrictioncoefficientof0.3 ,and
setthewidthfortheband,thelinearlyactingdrumbrake,andtheexternally
pivotedbraketo5cm.Limitthemaximumliningpressureforthebandbrake
andfortheexternallypivotedbraketo2.0MPa,andlimitthepressureforthe
otherliningsto3.0MPa,whichmaybeeitherformedorsolid.Liningpressure
fortheexternallypivotedbrakewastakentobe2.0MPa,merelyfor
comparisonwiththebandbrake.
Figure11(a)showsthetorquecapacityinnewton-metersasafunction
ofangleBsubtendedbyeachshoeforadrumdiameterof300mm,andFigure
11(b)showsthetorquecapacityinnewton-metersforband,linearlyacting,
caliper,andannularbrakesasafunctionofthedrumordiscdiameterin
millimeters.
Althoughthelinearlyactingdrumbrakeisclearlymoreeffectivethan
theotherbrakesshowninFigure11(b),itandalloftheotherthreebrakesin
thatfigurerequiremorehardwarethandoesthebandbrake.Therefore,select
thebandbrake,becauseitcanprovidethenecessarytorquecapabilitywith
mechanicalsimplicity.
Externaldual-shoedrumbrakesarethenextsimplest.Increasingthe
maximumliningpressureto3.0MPaforanexternallypivoteddual-shoe
brakeallowsthedrumdiametertobereducedto170mmandtheradial
distancetotheshoepivottobereducedto100mm,fromthe150mmas-
sociatedwithFigure11(a),togetatorquevs.anglecurvesimilarinshape
andmagnitudetothatinFigure11(a).Thus,eitheranexternallypivoted
Chapter8174
Copyright ... !
¼6:344lb-ftð6-6Þ
¼76:130lb-in:
wheren
i
=1/5.488fortheendrollersandn
i
=1/1.254fortheintermediate
rollers.
Ifthebrakehadbeenmountedoneitheroftheendrollershafts,
equation(6-6)wouldhavebeenreplacedby
T¼
4:114
0:1553
"
2ð0:0759Þþ20ð0:000556Þð4:375Þ
2
þ
230
32:2
4:375
12
2
#
¼34:811lb-ftð6-7Þ
andthebrakingtorquerequirementwouldhavebeenn=5.488timeslarger
thanthatfoundbyequation(6-6).Thiscomparisonisanexampleofthegen-
eralrulethatthebrakeshouldusuallybeplacedinthefastershaft.
VII.EXAMPLE3:ROTARYKILN
ThecurvesinFigure6clearlyimplythatefficientuseofaclutchbyreducing
thepowerlossduetoheatgeneration,alongwithwear,requiresthatthe
speedsofitsinputandoutputshaftsshouldbenearlyequal.Accordingly,
dependinguponthepowersource(electricorhydraulicmotor,turbine,or
internalcombustionengine),aclutchmaybeusedtochangegearratios,to
changefromonepowersourcetoanotherwhenthespeedsarenearlyequal,or
todisconnectthepowersourcebeforebraking.
Thisexamplewillconsideraloadthatisessentiallyrotationalinorderto
concentrateonclutchandbrakeselectionwhendynamictorqueandbrake
heatingcurvesareavailable.Bothclutchandbrakeanalyseswilldisplaysome
ofthecalculationinvolvedwhenthespeedsoftheinputandoutputshaftsare
notalmostequal.
Arotarykilnistobedrivenbya15-hpthree-phasemotoroperatingat
870rpmandratedtodeliveratorqueof240lb-ftwithaKfactor(overload
factorforstarting)of2.64.Themotor,clutch,geartrainwitha28.4speed-
reductionratio,androtarykilnarearrangedasshowninFigure7.Theoverall
damping...