1、09FTM11AGMA Technical PaperUnique DesignConstraints for MoldedPlastic Transmissionsby R. Kleiss and E. Wiita, KleissGears, Inc.Unique Design Constraints for Molded Plastic TransmissionsRoderick Kleiss and Eric Wiita, Kleiss Gears, Inc.The statements and opinions contained herein are those of the aut
2、hor and should not be construed as anofficial action or opinion of the American Gear Manufacturers Association.AbstractAny transmission design requires meticulous attention to detail including gear engineering, requiredperformance, and manufacturing methodology. Plastic geared transmissions are no d
3、ifferent in many waysthan metal; however they do require a unique knowledge of the materials, the molding process, and specialattributes that affect manufacture, performance, and life of the resulting transmission. In this paper we willdiscussthedesignandmanufactureofaparticularplasticmoldedtransmis
4、sionwiththepurposeofdescribingdesign intent, manufacturing reality, inspection and testing results, and lessons learned. The selectedtransmission is a molded plastic gear pump.Copyright 2009American Gear Manufacturers Association500 Montgomery Street, Suite 350Alexandria, Virginia, 22314September 20
5、09ISBN: 978-1-55589-964-63Unique Design Constraints for Molded Plastic TransmissionsRoderick Kleiss and Eric Wiita, Kleiss Gears, Inc.BackgroundA customers pump failure prompted the need for anew design. The customer intended to build aplastic gear pump with commercially purchasedpump gears and an i
6、n-house molded pump well.They chose the gears based on the manufacturersrecommendation. The resulting gears (see Figure1) were 9 tooth, 24DP, 5 helical gears cut fromPolyetheretherketone (PEEK). The pump housingwas molded in Polycarbonate (PC) and thenmachinedtothefinalinternaldimensions(Figure2).As
7、aresult,thecustomerspumpswerenoisy,ineffi-cient,andworeoutquickly. Thetaskathandwastoimprove the performance of the pump.Figure 1. Original gearsFigure 2. Pump housingInitial reviewThe intended application of this gear pump is tocirculate water at a flow rate of up to 1 liter per min-ute with at lea
8、st 50 psi under varying temperatureconditions. The gear pumps application as adisposable product is short-term, but must be ableto maintain its performance characteristics for 72hourswithminimalnoiseandefficiencychange. Aninitial review of the design history revealed a fewproblems.Material concerns
9、were immediately apparent.PEEK is an excellent semi-crystalline thermoplas-tic gear material with high strength, good wearcharacteristics, and non-hygroscopic properties(doesnotabsorbwater andswell). Typically, duetothe material properties and price, PEEK is consid-ered for more demanding applicatio
10、ns with highloads and temperatures. Polycarbonate is anamorphous resin which is non-hygroscopic, but isnot particularly good in wear conditions. These twomaterials are not a good match for this application.With a goal of minimizing development time, it wasdecidedtocontinuewiththesematerialsandmerely
11、optimize the current design. This decision resultedin the need for inserted bearings and bushings tominimize wear.Besides material considerations, the overall designof the pump resulted in stacked tolerances anddifficult manufacturing techniques. The polycarbo-nate pump well was glued inside another
12、 case, withitscoverthengluedandscrewedtothecase. Asep-arate molded seal was included to ensure againstleakage (Figure 3). This resulted in high tolerancestack-ups and glued interfaces.Inspection of the purchased gears proved them tobe of inferior quality due to cutting errors duringmanufacturing. Be
13、cause of the inferior manufac-ture of the existing gears, it was decided to improvethem first, then decide on further modificationsbased on the results.4Figure 3. Exploded pump assemblySince gear mold tooling is quite expensive, the firstsamples were cut. Dimensions of the cut proto-types were deter
14、mined by reverse engineering theactual size and pitch of the gears and fitting thehousing dimensions for minimal clearance.Prototype machined gearsOne of the most profound compromises in plastictransmission development is the decision tomachine first generation parts. Injection moldingentails inject
15、ing molten plastic under high pressureinto a precision mold and then cooling that materialin a controlled fashion until it can be safely ejected.In this process, the designer tries to keep wallsections relatively constant so there are no thickplastic areas which will result in slow and unevenshrinka
16、ge out of the mold. The flow of the materialinto the mold also causes alignment of melt in thedirection of flow, which is especially pronouncedwhen fillers are included in the plastic. Conversely,a machinedpart iscut froma solidblank resultinginseveralkeydifferenceswithits moldedcounterpart.Orientat
17、ion of the filler material different from themoldedpart. Additionally,removingmaterialtosim-ulatetheconstantwallsectionofmoldedpartscouldresult in poor cutting results for the cut parts.Molded gear tooth geometry can be modified tooptimize the tooth profile for molded gears and it ismore difficult t
18、o achieve these shapes with cutgears. Generally, hobbed gears have excellentbores and truly cylindrical outside diameters withminimal runout. Molded gears will quite often havetapered, barrel-shaped, or hourglass diameters asshown in Figure 4. A cut gear should only be usedasapreliminarystepindeterm
19、iningthesuitabilityofa particular design.Hobbed prototype and molded gearsThe hobbed prototype gears were machined with amaximum profile error of less than 0.02 mm. Theperformance of the pump improved significantlywiththehobbedprototypegears,anditwasdecidedtobuildmoldtooling. PEEK requires hightempe
20、ra-ture mold tooling the mold is heated above 175Candthematerialisinjectedatapproximately370C.It is important to inject as quickly as possible tocompletelyfillthemoldbeforethematerialbeginstosolidify so that the material can be packed underpressure while still in the molten state. For thisreason, we
21、 chose three gates at one end of thegear.The first molded gears were hour-glassed, andcontrol of the injection process could not removethis feature. Since the gear teeth act as a seal toprevent backflow in a gear pump, hour-glassingre-duces that seal decreasing efficiency and outputpressure. To corr
22、ect for this effect, the mold cavitywassplitintotwosectionsso thatit tookon theformof the barrel shape in Figure 4. The actual cavityplates are shown in Figure 5.Figure 4. Cylindrical distortions possible in molding5Figure 5. Split gear cavityThe gears molded with these split cavities had aless desi
23、rable profile than the hobbed gears, butto-talprofileerrorwaslessthan0.03mmandwasade-quate to provide the required performance atmolded part prices.2nd generation gear pumpUp to this point, it had been proven that moldedgears could provide the level of performance need-ed for the application, but ov
24、erall production costswere still too high. Assembly of the gear pumpsincluded pressing molded gears onto precisionstainless shafts, which were then mounted in smallbushingswithinthepolycarbonatepumpwell. Oncethe drive shaft was connected to the motor with aprecision coupling, the lid was then glued
25、to thepumpwell. The necessaryassembly tolerancesre-sulted in statistically unacceptable performancevariability.The challenge now was to improve performanceand affordability. The materials issue was attackedfirst. Hygroscopic materials were unacceptablebecause the moisture expansion of the totally im
26、-mersed gear would be greater than the partiallywetted pump well and cover, resulting in binding.We chose a relatively new material to us, a lowmoisture absorbing9T nylon. Thismaterial isavail-able only in a glass fiber filled version, but webelieved we could mold it accurately enough to useit for t
27、he gears, pump well, and lid.For the gear, we reduced the number of teeth to 7and modified the profile to provide higher flow. Toimprove molded accuracy, we removed the helix infavor of a simple spur gear. Bosses were added onbothendsofthegearssothatnoadditionalbearingsor bushings were required due
28、to the good wearproperties of the nylon when run against itself. Weclosedoffonefaceofeachgearsothatwecouldputa single gate in the center of that face to achievemore uniform filling of the mold. A single flat wasplaced on the inside diameter of the gear to matewith the drive shaft. The drive and the
29、driven gearswere the same except that the drive gear had theshaft inserted into the inner diameter. As a resultofthe redesign and material change, no bearings orbushings were required. The lid was glued to thebody for this version, but now couldbe convertedtoa snap fit at a later date. The new desig
30、n alsoreduced the height to diameter ratio of the gears,which was changed to help improve molded gearaccuracy. An exploded picture of the 2ndgeneration pump is shown in Figure 6.Figure 6. 2nd generation gear pumpMolding resultsMolded shrinkage for gear materials is not uniform,and one can only intel
31、ligently guess at expectedshrinkage for specific features. Shrinkage isinfluencedbytooldesign,moldmachinedesign,theactual molding process, and by variability in the6material. The approach here was to estimateshrinkage, measure the molded parts, and then ad-just the cavities toachieve thefinal requir
32、edsize. Agear pump is especially critical in this regard sinceevery surface is a sealing surface and deviationswill cause either binding or loss of efficiency.Centers, diameters and tooth geometry werescanned at three different heights to assure propermeshing clearance. Bearing fits and clearanceswe
33、re inspected. The flatness of the pump floor andlidweremeasuredandre-sculpted. Thegearfaceswere also not flat (Figure 7), but no attempt wasmade to correct for this feature on the first try.Figure 7. Dishing of tooth faceTested resultsQuite often, plastic geared transmissions arequalified because th
34、ey work. It can be difficult toqualify a transmission through testing. One prob-lem is the size of the transmission and the need forvery small driving torque motors and driven loads.Also, the behavior of plastic materials can changeradically with increased speeds or torques, soaccelerated testing ca
35、n result in failures that wouldnever be seen in the field.Ingeneral,theculpritisheat. Asplasticheatsup,itsmodulus of elasticity is reduced, load capacitydrops, frictionincreases, andoverall functiondeter-iorates. Thisdoesnothappenlinearlyandtheeffectof material property changes for gears cannot bede
36、termined by data sheets providedby thematerialsuppliers. The developed stress-strain curvespresented for most plastics are the result of slowlyapplied forces to certain specimens. Gear teethgenerally receive the equivalent of short termimpactloading. Inourexperience,theplasticsheatdeflection tempera
37、ture is also where significantoperating degradation occurs.This gear pump is an ideal application for testing.Thepumpcirculateswaterwhichlimitshightemper-atures. We built a tester using the product drivemotor, a torque transducer, and pressure sensorsbefore and after the calibrated pressure reducing
38、orifice and a flow meter (Figure 8).Figure 8. Gear pump tester7With this tester, we were able to run continuoustests with constant torque or current and measurethe resultant parameters. The most importantmeasured quantity for us was output pressure. Anexample of this graph is presented in Figure 9.I
39、t can be seen that pressure increasedapproximately 20 psi during this test. It was subse-quently confirmed that the gears wore into a moreproper fit with the housing walls as the testcontinued, whichis tobe expectedwith filledmater-ial. It was also found that the pressure increasestopped at this lev
40、el with very little wear occurringafter this initial settling period.Results and conclusionsThe second generation gear pumps performed tospecification. While the wear-in of the gears wasnot desirable, it had been expected with filledmaterial. The hydrodynamic bearings worked verywell, and after the
41、initial wear-in, there was littlesubsequent wear. Gluing the lid tothe bodyis stilladifficult and potentially troubling assembly aspect,but plans for a snap fit lid are in place.After initial design and testing, it was discoveredthat an unfilled version of the 9T nylon is available.Preliminary testi
42、ng shows promising results that itwill have minimal wear and the lower modulus willallow a better seal. 9T nylon has proven itself as avery low moisture absorbing material. Continuedmaterials testing in conjunction with the knowledgegained from this project has the potential tominimize wear-in of the components whilemaintaining customer requirements.Figure 9. Pressure change over time
