Tài liệu Clutches and brakes design and selection P11 ppt

11
FluidClutchesandBrakes
Fluidclutchesandbrakesmaybedividedintotwogroups:thosecontaininga
fluidonlyandthosecontainingamixtureoffluidsandsolids.Thosecon-
tainingonlyafluidrelyprimarilyuponthemassofthefluidandsecondarily
uponitsviscositytotransmittorque.Unitscontainingbothafluidandasolid
inaparticulateformrelyuponthesuspendedsolidstoprovidethemajor
bondbetweenthecomponentsthateithertransmitorresisttorquewhenun-
dertheinfluenceofanexternalelectromagneticfield.
Theadvantageoffluidclutchesandbrakesisthatthereisnoliningto
wearandreplace.This,however,isobtainedattheexpenseofsomepowerloss
inthetransmissionoftorqueandthedistinctneedforsomesortoffluid
coolingforbothfluidclutchesandfluidbrakes.Moreover,occasionalfluid
sealreplacementmayalsoberequired.
I.FLUIDCOUPLINGSASCLUTCHES
Fluidcouplingsmayserveassoftstartclutchesandastorquelimiting
clutches.Atypicalfluidcouplingconsistsofaninputshaftattachedtoan
impellerandanoutputshaftattachedtoarunner,withbothencasedwithina
closedhousingandorientedasshowninFigure1.Animpellermaydiffer
fromarunnerintheshapeoftheradialvanesofthesortshowninFigure2
andmaybeattachedto,androtatewith,thehousingthatcontainsboththe
impellerandtherunner.AsindicatedinFigure1,theshaftsaresupportedby
bearingsatthehousingandbybearingsatthefarendsofeachshaftthatin
turnaresupportedbyanenclosure,asshowninFigure3.Eachimpellerand
Copyright © 2004 Marcel Dekker, Inc.
runnerconsistsofhalfofatorus,asshownincrosssectioninFigure1,thatis
fittedwithradialvanesthatextendradiallyinwardacrossthetorus,asis
evidentinFigure2.Thelocationoftheimpellerandrunnerinafluidcoupling
isalsoshownontheright-handsideofFigure3foracommerciallyavailable
couplingthatrestsuponitsoilreservoir,whichisalsoknownasasump.
Aninternallydrivenpumplocatedontheright-handsideoftheouter

housingistopumpfluidfromthereservoirintotheinnerchamberthat
enclosestheimpellerandrunnertoprovideasoftstartoveranintervalof
approximatelyfive(5)seconds.Fluidfromthereservoirmustbecirculated
throughapumpingandcoolingsystemprovidedbytheuser.Standard
coolingsystemsaregenerallynotprovidedbythefluidcouplingmanufacturer
becauseoftheextensivevarietyofserviceconditionsinwhichthesecoupling
maybeused.
Typicallytheheattobedissipatedisapproximatelythreepercent(3%)
oftheinputpower.Conversionbetweenthepowerdissipated,ineitherwatts
orhorsepower,andheatproducedperunittime,asexpressedineitherlarge
caloriesorBtu,isgivenby
1Btu=sec¼1:41391hp
1kilocalorie=min¼69:7333W
TransmittedpowerPisrelatedtotheinputrpm(revolutionsper
minute)naccordingtotherelation
P¼P
0
ðn=n
0
Þ
a
ð1-1Þ
F
IGURE
1Crosssectionofasemitoroidalimpellerandrunnerandtheirenclosure,
or housing. (Courtesy TRI Transmission & Bearing Corp., Lionville, PA.)
Chapter 11258
Copyright © 2004 Marcel Dekker, Inc.
inwhichP
0

isareferencepowerandn
0
isareferencerpm.Bothofthem,along
withexponenta,aredependentuponthefluiddriveinvolved.Relation(1.1)
maybedisplayedonlog-logpaper,asinFigure4,foreaseofselectingan
appropriatefluidcouplingwithouttheuseofpocketcalculatororacomputer
toevaluateequation(1-1).
UseofFigure4isstraightforward.Forexample,toselectacouplingto
bedrivenbyanmotorturningat1160rpmthatistotransmit150hp,merely
enterthegraphat1160rpmandreadupto150hp.Asaguidetoreadingthe
F
IGURE
2Runnerandshaftinafixtureusedfordynamicbalancing.Notallofthe
balancing equipment is shown. (Courtesy TRI Transmission & Bearing Corp., Lion-
ville, PA.)
Fluid Clutches and Brakes 259
Copyright © 2004 Marcel Dekker, Inc.
logarithmicscaleforpower,noticethatonlytheunlabeled200-hpgridlinelies
betweenthelabeled100-hpand250-hpgridlines.Hence,thepointwhose
coordinatesare1160rpmand150hplieswithintheregionofthemodel230
coupling.
Theseandsimilarfluidcouplingsaresuitableforusewithcrushersand
chippers,withconveyorsandsimilarmaterialshandlingequipment,aswellas
withportableequipment.Theymayalsobeusedinserieswithmarinedrives
toofferpropellerprotection.
Notallfluidcouplingscontroltheirtorquelimitsbyadjustingthe
amountoffluidintheimpellerchamber.Onecouplingmanufactureproduces
asmallcoupling,showninFigure5,thatisfilledwithfluidatalltimes;no
pumporreservoirisneeded.Thehousingsrotatewiththeinputshaftsinboth
clutchandbrakeapplications,soinbothusestheattachedcoolingfinsrotate

todissipatetheheatgeneratedbyfluidlosses.
Averageheatlossdropsfrom240%for0.125-hpcontinuousdutyat600
rpmto30%for5.0-hpcontinuousdutyat3600rpm.Simplicitygainedby
pumpandreservoiromissionhasbeenexchangedfortheselosses.
Typicalapplicationsincludeexercisemachines,amusementrides,bak-
ingovens,valveoperations,cranetrolleys,reversingcarriages,andwinding
andunwindingequipment.
F
IGURE
3Fluidcouplingdesignedforasheavetobeboltedtothefaceplateon
the left. Dextron ATF, automatic transmission fluid, is the recommended fluid.
(Courtesy TRI Transmission & Bearing Corp., Lionville, PA.)
Chapter 11260
Copyright © 2004 Marcel Dekker, Inc.
F
IGURE
4 Output power as a function of input revolutions per minute. (Courtesy
TRI Transmission & Bearing Corp., Lionville, PA.)
F
IGURE
5 Photograph of a fluid clutch with input from an electric motor and a belt
drive using the sheave that is a part of the right-hand side of the housing, shown in
cross section. (Courtesy Fluid Drive Engineering Co., Inc., Burlingame, CA.)
Fluid Clutches and Brakes 261
Copyright © 2004 Marcel Dekker, Inc.