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美国机械工程师手册_26663_ym5.pdf

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2372FLEXIBLEBELTSANDSHEAVES FLEXIBLEBELTSANDSHEAVES Flexiblebeltdrivesareusedinindustrialpowertransmissionapplications,especially whenthespeedsofthedriveranddrivenshaftsmustbedifferentorwhenshaftsmustbe widelyseparated.Thetrendtowardhigherspeedprimemoversandtheneedtoachievea slower,usefuldrivenspeedareadditionalfactorsfavoringtheuseofbelts.Beltshave numerousadvantagesoverothermeansofpowertransmission;theseadvantagesinclude overalleconomy,cleanliness,noneedforlubrication,lowermaintenancecosts,easy installation,dampeningofshockloads,andtheabilitiestobeusedforclutchingandvari- ablespeedpowertransmissionbetweenwidelyspacedshafts. PowerTransmittedByBelts.—Withbeltdrives,theforcethatproducesworkactsonthe rimofapulleyorsheaveandcausesittorotate.Sinceabeltonadrivemustbetightenough topreventslip,thereisabeltpullonbothsidesofadrivenwheel.Whenadriveisstationary oroperatingwithnopowertransmitted,thepullsonbothsidesofthedrivenwheelare equal.Whenthedriveistransmittingpower,however,thepullsarenotthesame.Thereis atightsidetensionTTandaslacksidetension,TS.Thedifferencebetweenthesetwopulls − (TTTS)iscalledeffectivepullornetpull.Thiseffectivepullisappliedattherimofthe pulleyandistheforcethatproduceswork. Netpullequalshorsepower(HP)×33,000÷beltspeed(fpm).Beltspeedinfpmcanbe setbychangingthepulley,sprocket,orsheavediameter.Theshaftspeedsremainthe same.Beltspeedisdirectlyrelatedtopulleydiameter.Doublethediameterandthetotal beltpulliscutinhalf,reducingtheloadontheshaftsandbearings. Abeltexperiencesthreetypesoftensionasitrotatesaroundapulley:workingtension (tightside−slackside),bendingtension,andcentrifugaltension. Thetensionratio(R)equalstightsidedividedbyslacksidetension(measuredin pounds).ThelargerRis,thecloseraV-beltistoslipping—thebeltistooloose.(Synchro- nousbeltsdonotslip,becausetheydependonthetoothgripprinciple.) Inadditiontoworkingtension(tightside−slackside),twoothertensionsaredeveloped inabeltwhenitisoperatingonadrive.BendingtensionTBoccurswhenthebeltbends aroundthepulley.O