AGMA-01FTM9-2001.pdf
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1、01FTM9 New Opportunities with Molded Gears by: R.E. Kleiss, A.L. Kapelevich and N.J. Kleiss Jr., Kleiss Gears, Inc. TECHNICAL PAPER American Gear Manufacturers Association Copyright American Gear Manufacturers Association Provided by IHS under license with AGMA Licensee=IHS Employees/1111111001, Use
2、r=Wing, Bernie Not for Resale, 04/18/2007 12:08:32 MDTNo reproduction or networking permitted without license from IHS -,-,- New Opportunities with Molded Gears Roderick E. Kleiss, Alexander L. Kapelevich and N. Jack Kleiss Jr., Kleiss Gears, Inc. Thestatementsandopinionscontainedhereinarethoseofthe
3、authorandshouldnotbeconstruedasanofficialactionor opinion of the American Gear Manufacturers Association. Abstract Molded gearing includes plastic and powder metal injection molded gears as well as powder metal sintered gears. Near- -net forged gears may also share some unique similarities and oppor
4、tunities as well. This type of manufacturing offerssomeparticularlyintriguingopportunitiesforthegeardesigner,andalsosomechallengesnotusuallyencountered with cut gears. The challenges are often related to the mechanical properties of the material. Proper steel, cut and hardened correctly, is hard to
5、beat for strength. Ordinary attempts to replace steel gears with the molded variety are usually doomed to failure. On the other hand, molded gears can offer some material properties not achievable with cut gears, including unique advantages in weight, noise, modulus, self- -lubrication, magnetism, c
6、hemical resistance, and most appealing low cost. Thechallengeistomakethemsurvivethedemandsputuponthem.Forinstance,thermoplasticgears,whenplacedunder continuoushighload,willmelt.Thisisaphenomenonnotsharedwithanymetalcounterpart.Thegoalmustbetomakea weakermaterialappearstronger.Theuniquetoolsthatareav
7、ailabletothemoldedgeardesignerareconcentratedinthe method of manufacture. When the proper mold is constructed and combined with the optimized molding process, a remarkablyconsistentanduniformgearcanbecontinuouslymanufactured.Theconstructionofthismoldedtoolingcan bealmostcompletelyCADbased.Traditiona
8、lgearcuttingprocessesarealmostneverusedtodevelopthemoldcavities. Uniquetoothgeometrythatmightbedifficultorevenimpossibletoachievewithcutgearscanbeappliedtomoldedgears matter- -of- -factly.Thispaperwillinvestigatetwotypesofgearsthatwehavedesigned,molded,andtestedinplastic.The first is an asymmetric m
9、esh, the second is an orbiting transmission. The asymmetric gears have dissimilar 20_ and 48_ pressureangleswhiletheorbitinggearsetworkswitha65_pressureangle.Bothtransmissionshavehigherloadpotential than traditional design approaches. Copyright ? 2001 American Gear Manufacturers Association 1500 Kin
10、g Street, Suite 201 Alexandria, Virginia, 22314 October, 2001 ISBN: 1- -55589- -788- -6 Copyright American Gear Manufacturers Association Provided by IHS under license with AGMA Licensee=IHS Employees/1111111001, User=Wing, Bernie Not for Resale, 04/18/2007 12:08:32 MDTNo reproduction or networking
11、permitted without license from IHS -,-,- 1 New Opportunities with Molded gears Rod Kleiss, President, Kleiss Gears Alex Kapelevich, Principal Engineer, Kleiss Gears N. Jack Kleiss Jr., Consultant, Kleiss Engineering Molded gears share some very basic similarities with cut metal gears, principally th
12、e involute gear shape and a need for precise design, manufacturing, and inspection. They also diverge from cut metal gears in some very significant ways. The design of the gears and mold tool construction usually does not require or employ any traditional gear cutting techniques. Spur gear molds are
13、 almost invariably made utilizing a wire Electrical Discharge Machine (EDM) which is capable of producing any 2-dimensional and even some slightly 3-dimensional shapes with surface accuracy on the order of a single micron. Helical cavities are cut using electrode EDMs. These electrodes can be made w
14、ith end mills having gear profiles wire EDMd into their cutting edges, or by using high-speed surface generation on a 4- axis CNC mill. The difficulty of using traditional gear cutting techniques is most often due to the required physical geometry of the cavity. Since most molded gears will shrink f
15、rom the mold, the mold cavity must be adjusted for enlarged base pitch, tooth thickness, major, and minor diameters. Form grinding electrodes is an option, but standard hob generation is almost never suitable. Additionally, very few electrodes will be needed to generate the mold cavities. Gear house
16、s tend to shy away from such small jobs that will only infrequently be repeated. Still, the options left open to the molded gear designer are quite extensive To achieve an effective transmission design, every available tool and resource is usually required. First of all is the design of the gears th
17、emselves. As with many design constraints, space is usually at a premium with large expected loads. The danger with molded plastic gears especially is heat. Plastic gears melt, and as temperature increases, their modulus of elasticity decreases and they get even weaker. Constant duty cycles under he
18、avy loading is one of the most difficult designs to achieve successfully with plastic molded gears. Using exotic materials with higher heat capacity brings along its own set of problems. Quite often these materials will be brittle, or difficult to mold accurately, or just too expensive to be cost co
19、mpetitive with high speed gear cutting. Most successful plastic gear designs will be molded with basic engineering thermoplastics such as nylon or acetal. Molded Gear Design One of the biggest opportunities for the molded gear designer is the design of the gears themselves. Since the spur tooling ca
20、n be generated with wire EDM, any 2- dimensional shape that can be drawn, can usually be produced, and even adjusted mathematically for shrinkage before being cut. There are only 8 variables required to completely describe a symmetrical spur gear mesh design. Figure 1 is a screen dump of our design
21、approach to this task. Copyright American Gear Manufacturers Association Provided by IHS under license with AGMA Licensee=IHS Employees/1111111001, User=Wing, Bernie Not for Resale, 04/18/2007 12:08:32 MDTNo reproduction or networking permitted without license from IHS -,-,- 2 Figure 1 Typical spur
22、gear design The input data field in the upper left-hand corner of Figure 1 shows the required information to complete this design. For symmetric gears the operating pressure angle will be the same in both directions. The numbers of teeth in each gear is followed by the tooth thickness of one of the
23、gears. In this case tooth thickness is defined as a non-dimensional ratio to the base pitch of the drive pinion. The outside diameter of both gears is required as well as the wire diameter of the EDM that will cut the cavities. This will cause the tips of each gear to be rounded, which will affect b
24、oth the contact ratio and the formation of the mating gears root geometry. Finally, either center distance of the mesh or the base pitch is required to physically size the gears. With these minimum inputs the rest of the gear features are produced by generation from the principal features. In this c
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