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1、 3ODVWLF JHDUVWKH KHDUW RIWKH PDWWHUIncorrect shrinkage, not materialinadequacy, is the root of mostplastic gear failures. Now theres something you cando about it.S qÃFyrv ÃFyrv ÃBrh ÃD p EhpxÃFyrv Fyrv à tv rr v t icrosoft may be bestknown for its software,but one of i

2、ts mostinnovative products isactually a toy. It s ananimated Barney doll,and it almost didn t make it out ofdevelopment.MS Barney, a mechatronic marvel, wasdesigned to walk and talk and interactdynamically with its namesake program onPBS. Put it in front of a TV tuned to the"Barney" show,

3、and the purple dinosaursings, marches, and plays along, waving itshands and nodding its head on cue, inresponse to broadcast commands.Early on, however, Barney had someUurà r Ã7h r Ãq yyÃs à s h rÃtvh ÃHvp s Ãv Ãpu pxÃs yyà sÃryrp vpÃ

4、v h q à uh r r vhyy à ryr Ãvsà u rrà yh vpÃtrh à h v v Ãq à xà vtu ÃWr h vyrÃh qÃv r r v r yh vpÃtrh à hxrÃhyyà à sà hpuv r à viyrà qh à y pxv tÃqr vt Ãp rh v v Ãyvxrà r

5、r irs rwhat of a hitch in his giddyup. Ironically,the elaborate electronic control systemworked just fine, but the plastic geartransmission, a relatively simplecomponent, kept binding, stopping the dollin its tracks and nearly scuttling the project.Had the engineering team blamedBarney s problem on

6、plastic gears the storywould end right here. But someone hadenough insight to search for the real causeof the doll s halting gait, which turned outto be incorrect plastic shrinkage thatoccurred during the gear-making process.In other words, the final dimensions andprofile of the gears in the transmi

7、ssionweren t as designed.Rather than throw in the towel, thedevelopers redesigned the transmission,more closely profiling the gears as well asthe mold cavities to establish the trueshrinkage. They not only solved the bindingproblem but, by switching to a softermaterial for the high-speed motor pinio

8、n,made the gears, and Barney s walk,smoother and quieter.8 qrà yrSuch misunderstandings, if not resolved,can quickly derail an engineering project;if not caught, they can sink even the mostwell-intentioned product, possiblydragging the manufacturer down alongwith it. Engineers have to learn to

9、dealwith it, however, because the trend towardplastic gears isn t going to go away. On thei i ki23T h r s ur h à yh vpÃtrh Ãqr vt Ãh qÃhÃyv yrÃv tr v uh rÃp rà tr ur Ãv à urÃqr ry r à sÃhà yh pr vp à iv v tÃq v r &#

10、195;Uuv à h vp yh Ãtrh à r Ãhpuvr r ÃhÃ# rq p v Ãh qÃr r à u tuÃv Ãuh à Ãtyh Ãsvyyv t Ãv Ãph Ãp v y Ãqryv r v tÃ" ÃDi v à sà à r Uurà Ãv à hxr à ssà ur

11、5; iiyv tÃr r hyÃtrh steam as designers and OEMs learn lessonafter painful lesson through the school ofhard knocks.Most of the confusion surroundingplastic gears centers on the shrinkage issue.When you design a plastic gear mold youhave to allow for non-uniform andnon-linear shrinkage, rea

12、lizing that theIt doesn t help that the designer or end user is usually isolated from the people who turn raw materials into finished parts. And what a gaggle of people it is. You don t just buy a molded gear, you buy into a convoluted and fragmented process.It s not uncommon for a molder, for examp

13、le, to subcontract tooling to a toolmaker who s never molded a gear. Although the molder may offer some guidance, it will typically have less to do with the function of the gear and more to do with such things as mold maintenance and faster cycle times. The bottom line is that you, the designer, wil

14、l have many decisions made for you without your knowledge and perhaps without your best interests in mind. P rà qà à yh vpDespite the complexity of the procurement process, there are many reasons to consider plastic gears. Whether you re working on a medical system, semiconductor equi

15、pment, handheld tool, office product, or a heavy duty industrial machine, plastics have several things to offer that you re unlikely to achieve with metal.Tph v tÃp qv h rà rh r r à hpuv r Ãh rÃvqrhyÃs svyv tà rpv v Ãtrh ÃUurÃ8HHà u Ãs 

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22、ROOLQJ PRWRU Cost is obviously one of them. The costof a molded plastic gear is significantlyless than that of a cut steel gear.Design flexibility is another. Just aboutanything is possible with molded plastics,from orbiting compound differential gearsets to asymmetric designs. With plastic,internal

23、 gears are no harder to make thanexternal gears and they can carry moreload.3ODVWLF WUDQVPLVVLRQ GHVLJQ$QDOVLV GHWHUPLQH LI SODVWLFVFDQ EH HIIHFWLYH LQ WKHDSSOLFDWLRQ'HVLJQ ² FUHDWH JHDUV RSWLPLHGIRU HIIHFWLYHQHVV,QVSHFWLRQ YHULI WKDW DOOGLPHQVLRQV KDYH EHHQ PROGHGDQG VKUXQN FRUUHFWO7HVWLQJ

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25、195;Uurà yh vpà h v v r Ãv Ãih rqà Ãh à iv v tÃtrh v uà à v tà h Ãh qÃhà rq p v h rà sÃhy Ã$ Another advantage derives from thephysical properties of plastics themselves.Such attributes as chemical resistance,lubric

26、ity, low weight, and elasticity add awhole new dimension to gears. Because ofplastic s elasticity, for example, a gear caneven serve as a vibration damping element.Plastic gears can also be molded in allsorts of colors. If a gear transmission isgoing to be visible, it might as well lookgood. Who kno

27、ws, one day we may see atrend toward small appliances and handheldtools with translucent or even transparenthousings in the image of the stylish newApple computers.Don t forget too that plastics are still arelatively young technology. Materialscientists are continually developing newgrades, material

28、s, additives, and fillersThey re also improving consistency. Inaddition, third-party companies thatcombine resins from different suppliershave begun to issue custom blends tailoredspecifically for gears. Some "blenders,"such as LNP, even test their materials asgears, providing invaluable d

29、ata to users aswell as manufacturers.Manufacturing-wise the case for plasticgears is stronger now than ever, with recentimprovements in precision tooling,high-tolerance molding machines, andinspection equipment like scanning CMMs.Computers have had an impact tooparticularly in the area of testing. T

30、oday s automated test systems offer new insightsinto the function of gear transmissions,pushing them to the brink of catastrophicfailure to obtain information never beforepossible.9 ÃirÃs yrqTo the unsuspecting designer, theadvantages of plastic, unfortunately, canquickly become disadvanta

31、ges - startingwith its versatility and the fact that there arealmost no bounds to what you can do withit. Too often the person who gets a brilliantidea has no idea how to design it, andneither do the molders or toolmakers. Oncein a while someone will crack open a book,but standard gear design as def

32、ined in mosttexts doesn t work for the typical plasticapplication.Then there s the overload associated withchoice. Today there s such a vast number ofpolymers and blends, each with its ownunique properties and3UHGLFWLQJ ZHDU,Q SODVWLF JHDUV $ JRRG HDPSOH RI VRPH RI WKH QHZWHVW HTXLSPHQW PDGH SRVVLEO

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37、 rh Ãv à rh rqÃh ÃhÃs p v à sihpxyh u à vqv tÃhà rÃhpp h rv qvph v à sÃu à urà h r vhyà vyyÃhp hyy r s Ãv ÃhÃtrhshrink ages, that the material selectionprocess usually turns into a wild goosechase. At best

38、 - and this is where a lot ofdesigners get into trouble - someone willlook at a spec sheet and attempt to makeassessments and selections based onproperties taken out of context. Materialproperties in gears manifest themselves in acomplex manner, however, requiringa more thoughtful approach.Another p

39、itfall has to do with tooling.Although there s no shortage of opinionsabout gear tooling, very little isdocumented. In fact, what s presumablyknown about tooling and shrinkage is keptsecret. There s also a shortage of toolmakerswith sophisticated inspection equipment.When you re "cooking"

40、with plastic, theingredients, the temperature, and even thesamples you choose for inspection have tobe just right.Although everyone claims to inspectgears for size and shrinkage, what theyreally mean is that they employ gear-rolltesters which are great for proving that thegear shape stays constant,

41、but nearlyuseless for telling you what the shapeactually is. Molders can and do inspectsimple dimensions, but there s usuallynothing simple about the dimensions of amolded gear.The best advice, since there are nostandards in the plastics industry for geardesign, inspection, shrinkage, andtolerances,

42、 is to proceed with caution. If asupplier seems to be maintaining secretsShv Ã7v qÃtrh rqà h à v xyr Ãp hv à rà sà ur Ãv vph rÃtrh Ãv à urÃv q ÃT rÃh rà Ãivttr uh Ãhà yh vpà ryyr à v uà yr h

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