1、05FTM01Fine Pitch, Plastic Face Gears: Designand Manufactureby: I. Laskin, Consultant and E. Reiter, Web Gear Services, Ltd.TECHNICAL PAPERAmerican Gear Manufacturers AssociationFine Pitch, Plastic Face Gears: Design andManufactureIrving Laskin, Consultant and Ernie Reiter, Web Gear Services, Ltd.Th
2、e statements and opinions contained herein are those of the author and should not be construed as anofficial action or opinion of the American Gear Manufacturers Association.AbstractFace gear technology has attracted attention. Products benefiting include those which use molded plasticgears. More ap
3、plications could benefit, justifying the need for more information on the special features of facegears, their design and manufacture, in comparison to other non-parallel-shaft gears. A description ofmanufacturing methods, particularly in plastic molding is given with inter-related design and gearpe
4、rformance issues. New methods of graphic modeling are included with descriptions of face gearconfigurations and applications.Copyright 2005American Gear Manufacturers Association500 Montgomery Street, Suite 350Alexandria, Virginia, 22314October, 2005ISBN: 1-55589-849-11Fine Pitch, Plastic Face Gears
5、: Design and ManufactureIrving Laskin, Consultant, and Ernie Reiter, Web Gear Services, Ltd.IntroductionFace gear technology is not widely recognized. Ifmentioned at all in gear related educational docu-ments, it is described as merely one of a number ofunusual gear geometries. Its beneficial applic
6、ationsare largely overlooked not only for coarse pitch, highpower applications where the gears are made fromhardened steel, but also in fine pitch limited powerapplications where the gears are made from materi-als such as molded plastic. The informationpresented in this paper counters such oversight
7、,particularly in the molded plastic applications wheremodeling of such gears is critical. An example of afine pitch plastic face gear that is used in a powertool application is shown in figure 1.Figure 2 shows a face gear and pinion which is usedas a main drive gear in a commercial power swingdoor a
8、pplication. In this case, a cut steel pinion wasused, although it would not be uncommon to usepowder metal or plastic pinions as well as shown infigure 3.Subjects to be coveredTo increase familiarity with this face gear technolo-gy, it will be necessary to consider a number ofsubjects.Figure 1. 28 D
9、iametral Pitch - 44:15 Ratio Face Gear 90 Degree Shaft AnglePower Tool Application2Figure 2. 20 Diametral Pitch 56:16 Ratio Face Gear 90 Degree Shaft Angle Wheelchair AccessDoor Opener ApplicationFigure 3. 0.8 Module - 62:12 Ratio Face Gear Set 90 Degree Shaft Angle3These will start with a descripti
10、on of typical facegears and their combinations with mating gears.Since these combinations are always non-parallelshaft drives, comparisons to other such drives willfollow. Any discussion of gears would be incom-plete without some consideration of their manufac-ture, which will be the next subject. T
11、his will leadinto a discussion of design issues. Present daywork in gear technology, both in manufacture anddesign, has moved into graphic modeling. Thetreatment of the topic of graphic modeling for facegears may represent the first such treatment in pub-lished gear literature. The final subject, to
12、 begin toconvey the flexibility of face gears, will consist of abrief description of face gear configurations that gobeyond the typical, and simplest, versions de-scribed earlier.Many of these descriptions will apply equally well toface gears made from materials other than moldedplastic. This is esp
13、ecially true for general face geargeometry and many operating conditions. The dis-cussions naturally become specific to plastic facegears when manufacture is covered. There is alsosome consideration of certain operating conditionstypically encountered in products containing plasticgears of any type.
14、Description of face gears and meshingactionIn conventional gears, the gear teeth project radiallyfrom the outside rim of the gear blank. In commonface gears, the teeth project axially from one of thefaces of the gear blank as can be seen in the CADmodel of figure 4.The radial limits on these teeth a
15、re defined by insideand outside circles. Conditions which impose re-strictions on the diameters of these circles then de-termine the net face width, or radial length, of eachtooth. The tooth tips, or top lands, lie in a plane per-pendicular to the gear axis. The tooth cross-sec-tion changes with its
16、 radial location as can be betterseen in figure 5. Part of this change is reflected inthe changing top land which tapers to a reducedwidth with increasing radius.Figure 4. Mating Face Gear and Pinion4INNERSURFACEOUTERSURFACETOPLANDTOOTHCROSS SECTIONCHANGES WITHRADIAL POSITIONFigure 5. Changes in Too
17、th Cross Section with Radial PositionA real example of this changing cross section canbe clearly seen in the photograph in figure 6. Notethe change in the top land and the tooth form be-tween the inside surface and the outside surface.Figure 6. 32 Diametral Pitch - 21:14 RatioFace Gear Set 90 Degree
18、 Shaft Angle Auto-motive Transfer Case Actuator ApplicationThe face gear is mated with a pinion, as shown inthe cross sectional view of figure 7. In the mostcommon type of face gear drive, the pinion is a spurpinion. (Less common versions of face gears andmating pinions are described later in this p
19、aper.)The pinion may be of conventional design or itstooth proportions may be modified to optimize theperformance of the face gear drive. To take full ad-vantage of what may be the limited face width of theface gear teeth, the face width of the pinion is madelarge enough and so positioned as to stra
20、ddle theface gear teeth.Contact between pinion and face gear tooth sur-faces is ideally along a line extending the full width ofthe face gear tooth. These ideal lines are not exact-ly straight or parallel to the pinion axis as seen in fig-ure 8. This line contact depends not only on gearand shaft an
21、gle accuracy but also on axial positionof the face gear. Deviation from the ideal axial posi-tion changes contact from a line to a nominal point.This contact will be either at the inner or outer end ofthe face gear tooth, depending on whether the axialposition is either too much in the tight or loos
22、e direc-tion. Despite such a shift in contact location, in aproperly designed gear set, conjugate, or smoothmeshing, action is maintained. Contact ratio is gen-erally similar to that of spur gears of similar propor-tions, even when contact is localized at one end ofthe tooth.5PINIONAXISFACEGEARFACEG
23、EAR AXISINVOLUTESPUR PINIONMOUNTINGDISTANCEFigure 7. Cross Sectional View of Mounted Face Gear and PinionTIPROOTINSIDECONTACTLINESOUTSIDEFigure 8. Lines of Contact6Mating tooth action is essentially a combination ofrolling and sliding as in spur gears. Due to the rota-tion of the face gear, there is
24、 some axial sliding onthe pinion tooth with corresponding radial sliding onthe face gear tooth. This sliding will be greater forgear sets of lower gear ratio, but for all gear ratios,sliding adds very little to the overall friction losses.As a result, face gear drives will have similar effi-ciency t
25、o spur gear drives, excluding possible differ-ences in bearing losses.The most common mounting has the axes of the twogears intersecting, and at right angles. In principle,this arrangement should permit designs with thesupporting bearings for each gear straddling thegear. However, practical design c
26、onsiderationslead to one or both gears being supported in anoverhung arrangement.Comparisons with other types ofnon- parallel shaft gearingSuch comparisons are best made with some de-scription of typical operating conditions for moldedplastic gears. They are rarely made to ideal levels ofaccuracy, r
27、igidly supported in low clearance ballbearings, and precisely positioned in ideally accu-rate housings. Instead, some eccentricity, out-of-round, and out-of-flat conditions are to be expectedin the molded face gear. Supporting shafts may lackfull rigidity and may be guided in journal bearingswith ge
28、nerous clearances. The housings are mostoften of molded plastic, with distortions and otherbearing location issues. Taken together, the gears,of whatever type, are expected to perform underless than ideal conditions. All this should enter intoevaluating the comparisons.The first comparison is to bev
29、el gears, most oftenstraight bevel gears. Face gears are commonlyconsidered as substitutes for such gears. Bevelgears require careful, almost precise, positioning toavoid a rough mesh similar to mating gears of slight-ly miss-matched module. With face gears, axialpositioning of the pinion is not a f
30、actor in gear me-shing. Axial position of the face gear remains a fac-tor, but not in a way that is much more demandingthan for a set of spur gears. Backlash requirementsmust be met with some extra care to achieve somedegree of control over tooth length contact. As withbevel gears, this can be helpe
31、d somewhat by theintroduction of crowning, or adding of slightamounts of material on the tooth flanks at the pre-ferred contact locations.Bevel gears of very high gear ratios are often re-stricted by mechanical angle limitations on the gearcutting machines to be used in cutting the gears orthe elect
32、rodes that will be needed to make the gearmolds. Face gears do not have such a restriction.However, face gears have a gear ratio restriction onthe other end. It is very difficult to design a face gearfor a gear ratio under 2.0, or 1.5 at best (as was thecase for the part in figure 6), thereby disqua
33、lifyingface gears as miter gears, with ratios of 1.0.The major possible limitation of face gears in com-parisons to bevel gears will be in load capacity. Thisresults not so much in the selection of module, forwhich gears of similar size and numbers of teethmay have similar module values. Instead it
34、is theface width of the load carrying teeth. In face gears,this width may be only 15 or 20 percent of the facegear outside radius. In bevel gears, it may be 25 to30 percent, or even greater for low ratio gear sets.Of course, this advantage in bevel gears is lost if thegears cannot be positioned to i
35、nsure contact alongthe entire tooth width.Face gears may also be compared to cross-axeshelical gears, especially the common version inwhich the driving member is recognized as a wormand the driven member as the helical gear. Suchgears are best able to adjust to variations inmounted position. Axial p
36、ositioning is not a restric-tion as long as each gear has adequate length. Vari-ations in shaft angle often have little effect on thegear mesh. Control over center distance is no moredemanding than for parallel-shaft gearing.These gears can accommodate the biggest rangeof gear ratios, well beyond fa
37、ce gears for the veryhigh ratios. This flexibility extends to low gear ra-tios, in which the driving member no longer re-sembles a worm, although this is rarely exploited.In other respects, these cross-axes helical gearsets are clearly limited in comparison to face gears.The contact between the flan
38、ks of the two gears isnominally a point, which leads to high local contactpressures. The result is excessive wear unless theloads are severely restricted. Furthermore, thegear meshing action introduces considerable axial7sliding. The friction associated with this sliding ma-terially reduces the gear
39、 set efficiency, placing it wellbelow face gear efficiency.Manufacture of face gearsManufacture of the mating pinion does not requirespecial attention. The pinion may be made of a dif-ferent material than the face gear. It is not unusualfor a sintered powder metal pinion or machined met-al pinion to
40、 run against a plastic face gear. If ma-chined, a pinion of optimized design may requirespecial cutting tools.The plastic face gear generally has machining in itshistory. Often prototype parts are machined fromplastic materials for design evaluation before moldsare built for production parts. Machin
41、ing is generallyrequired for preparations of face gear shaped elec-trodes to be used for EDM, electrical discharge ma-chining, of mold cavities.Except as described later, this machining is per-formed on special gear shaping machines. In gearshapers, a cutter in the shape of a gear with cuttingedges
42、on one face (figure 9) is reciprocating alongthe width of the machined gear tooth as can be seenin figure 10 for shaping a face gear. The cutter andthe gear are rotated between or during strokes withthe gear ratio needed to give the require number ofmachined teeth. In conventional machines, say fors
43、pur gears, the cutting stroke is parallel to the spurgear axis. For face gears, the cutting stroke is ra-dial, from the outside diameter to the inside, in a di-rection determined by the orientation of the pinionaxis to the face gear axis in the mounted assembly.Figure 9. 16 Tooth Shaper Cutters for
44、CuttingFace GearsRECIPROCATINGCUTROTATIONBETWEENRECIPROCATINGCUTSFigure 10. Machining of Face Gears8In conventional machining of spur gears, the num-ber of teeth in the cutter is selected for convenienceand is not tied to the number of teeth in the gear tomate with the machined spur gear. In face ge
45、ar ma-chining, however, there is a design connection tothe pinion mate to the face gear. The cutter designis commonly of special proportions to make this de-sign connection, although the pinion is sometimesdesigned to match an available cutter with an ac-ceptable number of teeth.The cutter must be o
46、f hardened and ground steelwhen many face gears of tough materials are to bemachined. For a limited number of machined proto-type plastic face gears or the few machined fromelectrode materials, the cutter is often made by ma-chining from medium hard steel.The mold cavities, as noted above, are commo
47、nlymade from electrodes machined on gear shapers.A number of electrodes are needed for each cavity,some designed for more rapid roughing burns andothers for finishing. A more recent method usesCNC machining from precise graphical models, inwhich the cutter is a tiny ball-shaped end mill. Thismachini
48、ng may be applied to electrodes or, in somecases, to the mold cavity itself. In another metal re-moving process, the end mill machining is replacedby the application of laser technology.The mold cavity design must include allowances forshrinkage of the plastic material. These allowancesmay need to b
49、e adjusted to any non-uniform sizechange in the molding process. The electrode re-quires a further allowance for the over burn, thesmall gap between the electrode and mold cavitysurfaces.Design issuesThe general objectives in design start with the spe-cified gear ratio. The design must also conform tothe specified size and space limitations. It must becompatible with anticipated manufacturing varia-tions in gear dimensions and mounting locations.Tooth proportions and material selection must pro-vide load and life capacity. There are likely to be fur-ther re
