An alternative to

bevel gealS uses

a cylindrical'

plaia.n and a face

gear ,for powe.r

transmission ,in

BVBrietyof

.app'iications ..

,26 ClEAR TECHNOLOGY

Cylkro" Gears: AnAlternarive in Mechanical

P T .."~,QW1er ' ,rans,mlS,S,lon

IntroductionBevel gears have been the standard for several

decades in iruaticns where power transmissionhas to occur between shafts mounted at a givenangle. Now a new approach has been developedthat challenges the bevel gear' de facto monop-oly in such applications. The concept is based onthe principle of the crown gear; i.e.. a cylindricalpinion mates with a face gear. Crown Gear B.V. inEnschede, Holland. is tile developer of these spe-dally gear teeth, which are marketed under thetrade name Cylkro®.

Features of the Cylkro GearThe Cylkro transmission is an angular gear

pair con isting of a cylindrical pinion with aninvolute tooth profile mating with a Cylkro gear.The upper gear pair in Fig. I is a normal cylindri-cal gear pair. Shown beneath it are three Cyl.krogears using '[hearne pinion with differen! shanangles and gear ratios.

The geometry of a Cylkro gear is determinedby the following factors: the geometry of the

mating pinions, the relative positioning of thepinion axi . in relation to the Cylkro gear axis andthe pair's gear ratio. The pinion's geometryremains constant along its face width, The geom-etry of tile Cylkm gear can be regarded as a rack;however. the pressure angle varies over the facewidth (See Fig. 2).

Devel.opmentThe general principles of face geargeometry

and the necessary cutting techniques were readilyavailable in the 1930s. but specific knowledge ofthe preci e geometry and load distribution char-acteristics of face gears were lacking. These top-ics were the first to be researched and applied tothe development of Cylkro gears. Advanced com-puter programs were used to develop and opti-mize the geometry. Many production methodswere evaluated, and the generating hob procewas eho en as the:rno t efficient.

The next step in the development proce wasthe de ign of a new generation of HSS and bardmetal generating hob. Initially the milling cut-ter profile were pinion-shaped. During ubse-quem development round , the cutter hapeswere graduany adapted for the production ofCylkro gear on commercially available CNCbobbing machines.

The power packing optimization abilities(minimal power transmission-volume ratios) ofthe first production batches were tested exten-sively. Testing and measurement methodologiesand instrumentation were developed along withthe production proce s. DIN 3962 and 3965 tan-dard were applied. The manufactured geometrycan. now be compared exactly with the theoreticalgeometry. allowing a quality-controlled manufac-turing process of the face gears.

It is now possible to manufacture Cyllo:o gearssuitable for power-intensive Iran missions. In thiarticle the rno t frequently u ed form of Cylkrogear transmissions, i.e., 900 shalt angles usingspur gear teeth, win serve as the basis for all

Fig..1 - Cylkro gears meshing at various shan angll$. examples. unless otherwise indicated.

TechnologyGeometry. The geometry of the pimon, its

position in relation to the axis of rotation of theCylkro gear and the transmission ratio determinethe geometry of the teeth. Formulas have beenapplied (0 calculate the geometry of the teeth in

every possible application and for every shaftangle between D° and I J D°. with or without an off-set pillion, and with spur or helical pillions.

Contact pat/l lines. Atransverse cross sectionof a Cylk:ro gear can be treated as a rack and pin- Jilig.2 - The vlI'ri~blegeometry oflhe Cylkro gear.

ion pair. The rack's pressure angle isa variablevalue along the entire face width of 'the gear. Thevariable pressure angle ensures that pinion andgear teeth keep mooth contact path lines betweenthem (see Fig. 3).

Since contact path lines are skewed, the char-acteristics of Cylkro gear pairs are similar to thoseof helical cylindrical gear pairs, with the samehelix angle. This is true for the gear's rotationcharacteristics. its acoustic performance, its powertransmission capabilities and its overlap ratios.

Transmission power packing density. Thestrength of material. calculations for bendingstrength and pitting resistance meet DIN 3990and ISO 6336 standards. Cylkro-speci£1c factorsin these randards were incorporated, and thenewly emerging standards were verified and lFig. 3 - LInes .of contact at the pillion and th Cylkro gearRll!lk.proven correct by means of finite element analy-sis methods (FEAfFEM) and with back-to-baeklife tests (see Fig. 4). All factors commonly usedin strength of material calculations. such as theKa factor, the dynamic Kv factor. etc., were alsoused with these gears.

These developmenrefforts, especially geartooth optimization. have resulted in transmissionpower packing densities equaling or exceedingthose of bevel gears. depending on transmissionratios and shaft and helix angles.

ManUfacturing. Cylkro gears are manufac-tured with standard CNC-contro.l1ed bobbingmachines. This guarantees aniefficient and contin-1I0US work flow. allowing high precision dimen-sioning. The production steps are lathing theblank, cutting the Leeth in the non-hardened basematerial. hardening to HRc 6~± I, machining thelocating faces (datum planes), finish-cutting theteeth flanks wilh hard metal milling cutters orgrinder and honing a fine-fiai hing profileadjustment if necessary.

The new generation milling cutters developedby Crown Gear ,can be sharpened without run-ning the risk of changing the cutting geometry.The geometry of the millingcutter is determinedby the pinion's gear teeth characteristics, uch asits module, number ofl.eeLh, helix angle and

I~l~ruonCylkroGear

Fig. 4 - FEM stresses on a CyII.-I)otooth.

addendum modification. Cylkro gears can bemanufactured with these milling cutters regard-less of axis angles, transmission ratios or axiscenterline offset.

Cylk:ro gear verification uses 3-D, CNC mea-surement banks. The gear's calculated standardprofile parameters are stored in the 3-D measure-ment bank and compared with the actual measured3-D values. The deviation patterns are obtained inthe fonn of a computer output wilh micrometerorders of magnitude (Il range), Thepatterns aredirectly related 10 the quality of production.

H. F. Gil'endelis sales managerwilh Crollif!' Oear B. V.Enschede. TireNemeriands.

MAY/JUNE lillie 27

fig. 5 -'ing'le-sta,ge driven turntabl (i = fOil.

Fig, 6, - Multi-power design for II pinion _-.S lin integra led p.art of a shaft.

Fig. 7 - Centmlly driven shaftwitb e eral dislri!:mll'on potnts,

28, GEAR TECHNOLOGY

Proper,ties. Cylkro gear systems have the fol-lowing characteristics:

• The pinion is a normal spur gear whose axialposit jon ha no influence on the me hing zone,

• Assembly time is reduced because only theaxial position of the face gear needs to be set,

• Lapping is unnecessary because all gearsirrespective of their tooth numbers or shaf anglesmay be interchangeably meshed with the com-mon pinion upon which the particular generatinghob geometry is based.

'. Axial load ona pinioa wi!h. stJraighl spurteeth is non-exi stent,

• Meshing is smooth thanks to the system'soblique contact lines and high contact ratio,

• The system offer ihigh transmission ratio inone stage,

• Cost-effective manufacture on conventional.CNC gear hobbing machines is possible.

• Using straddle-mounted pinions is po ible.• Zero backlash high-accuracy applications

may easily be realized.• High- trength teeth 3Ad good contact. geom-

etry give high torque capability.Tee1mical and economic benefits ...The unique

feall1res of Cylkro gears bringcertain economicand teehnicaj benefits that would not normally bepo ible with bevel gears or that would be possi-ble only at great cost.

The ab ence of the need for any axial adjust-ment of the pinion shaft during mounting, repairor maintenance isa 'lime saver. especially withhigh-tech assemblies. The can tructlon ofthe pin-ion bearing can be greatly simplifiedlhanks lO!heab ence of axial loads. The use of bilateral pinionbearing is pos ible when the assembly requiresextremely rigid construction. The ylikro gearconstrucrion is characterized by a high degree ofefficiency as a result of the slidingeffect of gen-eranng tooth flanks while ill motion.

Other economic benefits lie in the simplicityof tile cylindrical pinion, which has obvious man-ufacturing cost advantages over conical pinions.Furthermore. Cylkro gears can be produced withmuch more economical tooling and machinerythan bevel gears. The arne machine tool can beused for coarse and fine machining, eliminatingspecial machinery for finishing pas es. Th lap.ping proee s maybe eliminated altogether thanks10 the high tolerance fini hing pas gears receivein the standard production proce s.

Applications. The unique properties of theCylkro gear allow new approaches to design andmanufacturing. Cylkro gear sets can be made:in awide range of gear ratios. with axial freedom ofthe pinion and a free choice of shafl angle ..

I A wide range of gear ratios. 'Theoretically theCylkro transmission is suited for an possible gearratios (i -> 00). However, the effective face widthof the Cylkro teeth. in the area of the mner diame-ter will be reduced as a result of root undercuttingwhen u ing man gear ratios (i -c- 1.5} fora haftangle of 90°. In these instances, the mechanicalpower transmission capability of Cylk:ro gears issmaller than ill bevel gears of comparable size.Cylk:ro gears may definitely be used as powertransmission gears when gear ratios equal orexceed i = 1.5.

No technical limitations exist regarding theproduction proce s for large gear ratios. A gear •ratio of i = ~2, for in tance, would normally be !Idesigned in several stages, reducing the physical Iweight and, consequently, the material cost of the igear train. For some designs, however, it is advis-able to design a single, large stage transmission.

The largest gear ratio achievable with bevelgears (i = 6 - 8) is determined by the productionprocess itself. Cylkro gears, however, can han-dle large gear ratio in one single stage withoutany difficulties.

A typical application can. be found in environ-ments requ,iring a high degree of constant angularvelocity. For example, Fig. 5 shows an applicationwhere a gear ratio oLi = 10 is combined with tightangular velocity precision for the 'turning table. Itis obviou that the total cumulative pitclt error ofthis Cylkro gear is averaged 'Over the largest pos-sible gear diameter. With a diameter half the size'Orwith a double-staged gear, the error deviationwould approximately be twice as large. TheCylkro gear design combines both large gearratio andpreci ion constant angular velocity.Cylkro gear dimen ions play a secondary role.Different gear ratios can be achieved imply bychanging the number of teeth on the Cylkro gear.For some appficanons one gearbox can be usedfor several gear ratios with the same housing,bearings, pinion(s),etc.

Axial freedom of the pinion. Cylindrical gearsmay be offsei freely in both axial and radialdirections wiUlin reasonable limits without neg-ative impact on the lines of action of the teeth.This degree of freedom in either direction doesnot exist in bevel gears. Bevel gears individual-:Iy require precise installation and tuning toachieve a good bearing contact and the rightbacklash tolerance,

The Cylkro transmission oombinesboth typesof gears. It requires adjustment amy in the axial,direction to obtain a good bearing contact (thepinion teeth's ba e tangent lengths do not affectthis). Axial pinion motion is perfectly possible. fig. n - Dual-feed screwfor an iQjeclioo molding machlae,

Fig. 9 - Coaxial ,oounler-rotating propellers.

Fig. 10 -inety degrees (90°) dual-speed gearcbox.

Adjustmentsare greatly simplified; moreover, theengagement of several pinioas on one Cylkro gearor one pinion with two Cylkro gears simultane-ously is made easy.

Another good example of the axial freedom isthe pinion as an integrated part of a motor shaft.The motor shaft mates directly with the CyUcrogear; there is no requirement for any axial adjust-ment. The motor's own axial movement does notinfluence the bearing contact. Fig. 6 shows a multi-power design, which is used in situations where

!Fig. n- Conical mixer ..

/

Fig,. 13 - l\I1illing machine heads,

,30 GEAR TECHNOLOGY

one Cyllcro shaft drives several rapidly turningpinion shafts or in situations where more than onemotor drives one Cylkro shaft.

Fig. 7 shows a common driving shaft ontowhich a set of pillions i. mounted, each driving aCylkro gear. This conseuction is commonplace infarming machinery. Axial dilations of the pinionshaft: because of temperature changes, for exam-ple, have no .influence on the lines of action of theteeth, nor on the rotation ail symmetry of the out-going shafts ..

The rolling mill drum design shown in Fig. 8is an even better demonstration of the axial free-dom of the pinion. Thanks to the possibility ofthe axial displacement freedom offered by theCylkro gear design. the rolling mill drum's cen-terlines can be adjusted to accommodate millingmaterial plate thicknesses. It is also possible tomount mill drums with other diameters. Thisdoes not affect the bearing contact or any rei a-rive motion of one drum againstthe other. Whennecessary, both drums may have different diam-eters. Of course, this would result in a similarrequirement for different diameters of therespective Cylkro gears.

Fig. 9 shows a construction for distributingmechanical power to two counter-rotating pro-peller blades, where the propellers are mountedon coaxial hafts. Counter-rotating propellers areused in sllipbuilding to get higher propulsionyields. Half of the available mechanical power istransmitted directly on 'the inner shaft, and theother half is transmitted to the outer shaft througha planetary etup of pinions mating with the sec-ond Cylkro gear. The design allows differentrotating speeds on each shaft if required.

Fig. W shows 3. dual-speed perpendiculargearbox. Two Cylkro gears, each with differentnumbers of teeth, driven by a common pinionarecoupled to a shaft. through a clutch mechanism.The dutch ensures tbat the outgoing shaft staysconnected to one Cylkro gear at a time.

Another application. that takes advantage ofthe axial freedom of the pinion is the assemblythat drives the pressure cylinder in a printingpress. The position of the circumference of theprinting press cylinder relative to the feed cylin-der must be adjusted very precisely ..A helicalpinion with an adjustable (tunable) axial posi-tion on the driving shaft causes a proportionalrotational offset of the Cylkro gear for a givenaxial pinion shift.

Shaft angle s freedom of choice. The freedomto select almost any shaft angle, as well as allother features and benefits of the Cyllcro gear, iscrucial to making optimal use ofthis design. The

angular Cylkro gearbox does not affect !he 'bear-ing contactin any way.

A typical application in gearboxes with mallshaft angles is u ed for driving a ship propeller.Normally the motor i mounted horizontally,while the propeller shaft itsmounted at an angleof 7° to 20°.

Fig. l l shows a dri ve mechanism for dual-feedcrews working under all angle of 15° to 20°, as

commonly found in extrusion and injectionmachines for the plastics transformation industry.

The dual-stage drive mechanism shown in Fig. 12is part of a conical mixer with a still- tandingreservoir. The drive mechanism could be executedina different way. It could, for instance, bemourned on the bottom side of the reservoir. I

The drive mechani m for the tool flange of .the milling machine in Fig. 13 bas a connectingshaft equipped with a pinion and a Cylkro gear.The milling machine's head can rotate freelyaround irs drive shaft. The Cylk:ro gear 011 oneend of the connecting shaft can be freely adjust-edalong its axial position; therefore, the pinion

on the other end of the shaft will interfere withthe tooth's contact path.

Currently several kinds of Cylkro gears are

being tested in helicopter gear assemblies. A sim-pie demonstration of the ease of adjustment foreach Cylkro set is hown in Figs. [4 and 15.where tile use of power distribution is shown.Note that the shaft. angles differ from the classi-cal 900

• Fig, 14 demonstrares power distributionin the fir t stage, and Fig. 15 does so in the ec-ami stage. 0

References.:1. Chen. Y,"J,D. "Kinematic Study for Face Gear."Prepared for the ASME 1990 Design AutomationConference. February 15, 1990, Mesa. AZ, Paper No.9O-DAC-95.2. Overdij]; D. "De Driedimen ionale Construetievan Releaux, een Wiskundige Methode Toegepast opde Kroonwieloverbrenging." Eindhoven, 1988.Doctoral Thesis.3. Litvin, E L.et al. HDe ign and Geometry of FaceGear Drives," Transactions of the ASME, Vol. !14,December 1992, p. 642-647.4. Litvin.F. L et aI. "FaceGear Drives:Design.Analysisand Testing for HelicopterTransmissionApplicatiollS."AGMATecnnicalPaper 92-FTM-2, 1992.5. Basstein, A. F. H. "Cylkro-Getriebe, eine NeueHerausforderung," Antriebstechnik No. 11. Nov-ember 1994, p. 24-30.6. Basstein, A. F. H. .& A. L. Sijtstra. "NewDevelopments in Design, Manufacturing andApplications of Cylkro (Face) Gears," AGMATechnical Paper 93-fTM-7. 1.993.

i ! For more information about Crown Gear B. V" please.• I circle 219.

7. Basstein, A. F. H. .& A. L. Sijtstra. "Neue II 1 --11

F,ig.~4 - Helleepter gearasscmbllcs. (Patented by Lucas W;~st'Cf'l!l. Ine.)

Fig. 15 - Helicopter gear assemblles. (Patented by Dar SP.)

Entwicklungen bei Auslegung und Fertigung vonKronenradern," Antriebstechnik No. 11, November,1993, p. 53-60.8. Bak. D. J. "Face Gears Gel a Face Lift." GlobalDesign' News, Sept/Oct, 1994, p. 68-69.9. Harvey, D. S. "Helicopter Transmissions: ]s ItTime for a New Approach?" Rotor and WingInternational, Band 26, Magazine 6, [992.

Tell Us Whet You Think-.If you found this article of interest and/or useful,please circle 218.

t,e,,"V/JUtjE an 31