1、92 FTM1Experimental Characterizationof Surface Durability ofMaterials for Worm Gearsby: Michel Octrue and Mich_le GuingandCETIM, FranceAmerican Gear Manufacturers AssociationTECHNICAL PAPERExperimental Characterization of Surface Durability ofMaterials for Worm GearsMichel Octrue and Mich_ie Guingan
2、d, CET1M, FranceThe statementsand opinionscontainedherein arethoseof the authorand shouldnotbe construedas an official action oropinion of the American Gear Manufacturers Association.ABSTRACT:Thispaperpresentsthemethodology usedfor testing materialswithaworm gearset-up andwith a disc-roller machine.
3、Several experimental results are discussed and analysed.A correlation between metallurgical analyses of the structure of bronzes and experimental observations of subsurfacecracks is given.On the subject of wear, a specific method of measurement is explained and results are described.Copyright 1992Am
4、erican Gear Manufacturers Association1500 King Street, Suite 201Alexandria, Virginia, 22314October, 1992ISBN: 1-55589-581-61 INTRODUCTION The geometry and test conditions have been determinedso as to obtain pitting before 200 hours of testing on rollersThe admissible values for the same combination
5、of two submitted to average loads. No wear has been observedmaterials, such as Bronze/Steel, varies according to the on rollers: the rolling track is formed very quickly whenauthor ofthe document (see references 2, 4, 6). Also the disc contacts it and there is no width modificationthe method and pre
6、cise conditions of the tests differs during test. Controls were undertaken before the tests tobetween them and in some cases is unknown. CETIM has measure thecrown and the external diameter of each roller.thus manufactured three disc/roller simulators for testing After the tests the width of the rol
7、ling track was alsocombinations of materials used in worm gears, determined accurately.The fatigue curves obtained will be introduced later in theanalytical calculation methods for load capacities 3, 8, The appearance, to the naked eye, of the first visible pitting9. For each combination of material
8、s tested it was decided was recorded whenever possible. In fact during tests withto metallurgically analyse the structure of the different heavy loads, pitting evolves very quickly towards a largebronze types. Also the cracks within simulated test rollers and unique scaling. However with rollers sub
9、mitted towere observed to try and explain the fatigue phenomenon lower loads, very small size pittings appear on theof contact pressure on the bronze surface, periphery of the roller and evolve slowly. With the lattercase a criteria to stop the test had to be set and was definedas the time when sign
10、ificant pitting of a least 2 mm in2 FATIGUE TESTS diameter appeared.2.1 Test descriptionThe fatigue tests have been performed on a disc/roller 2.3 Examination of testssimulator which reproduces the kinematic conditionsparticular to worm gears: a bronze roller represents the Contact pressures are cal
11、culated in accordance with theworm wheel and rolls and slides on a steel disc which Hertz model applied to a cylindrical type contact. In factrepresents the worm. if the contact is at a point at the beginning of the test (dueto the crowning), it evolves very quickly towards aThe two mobile axles are
12、 orthogonal and independently cylindrical contact, the roller track being formed m a shortdriven. The roller can be positionned anywhere on the time to the detriment of the crowning disappearance. Thesurface of the disc. It is thus possible to simulate any length of the cylinder in contact is taken
13、account in thecontact segment with a choice of the applied load and of width of the track.the slipping speed (in magnitude and direction ).D,_c 2.4 Material analysisDisc: case hardened tempered: 35NCD 16L_ Hardness: H_,jz5 = 750Rollers: Three types are available, all of the same grade:Fig. 1: Disc/r
14、oller contact UEI2P, but obtained by different methods of manufacture.The hardness is given in the following chart:Over 20000 hours of tests have been completed. The finalcurves concerning the first bronze type (chilled castUE12P) have been drawn up. A few results have also been SAND CAST 103,33 Hbo
15、btained for a second type : centrifugal UEI2P. CHILLED CAST 108,07 HbCENTRIFUGAL 110,44 Hb2.2 Test methodThe geometry of the bronze rollers and the test conditionsused are as follows: 2.5 Results obtainedGEOMETRY OF ROLLERSExternal radius: 49 mm 2.5.1 UEI2P chill east bronzeRadius of crown: 250 mmWi
16、dth: 18 or 25 mm 14.000 hours of testing on 35 rollers have been requiredto set out the fatigue curve for this grade (figure 2). TheTEST CONDITIONS longest test lasted 41 million cycles while the roller wasRoller speed: 420 rpm submitted to a pressure of 344MPa. The roller with theDisc speed: 850 rp
17、m heaviest load (671 MPa) produced scaling after 0, 8 millionRoller position: x=50 y= 34mm cycles. The envelope curves give an off-set of 150 MPa._)tllm2.5.3 Verification on actual gear-boxes“950“.,. In order to validate the analytical method for the loadcapacity calculation, it is should be noted t
18、hat tests on twot “:. worm gears reducers were undertaken in parallel. For both1.50 ,t_ttma - the center distance was fixed to 125ram with a gear ratioof 1/40 for one and 1/31 for the other._o “. The materials chosen were identical to those tested in thedisc/roller simulator. The endurance of these
19、gears allowedconfimation of the results obtained for rollers.t“/_1.em,t _t 3 METALLURGICAL ANALYSISLt t Lt6 I7 LtaFig. 2 : UEI2P chili cast bronze 3.1 IntroductionThe lower curve in figure 3 represents the appearance of The analysis made of the two rollers tested on the simulatorthe first visible pi
20、tting, the upper curve corresponds to (chill casting and centrifugal) had two aims. The first wasthe appearence of the scaling. It was only possible to to observe the structure of the three different types ofrecord the first pitting after 10 million cycles. Below this bronze to try and predict their
21、 behavior to fatigue and thevalue, the evolution of the pit is too quick (see the end of second was to interpret the structural effects of fatigue dueto the surface pressure. These different analysis were madeparagraph 2.2) at the ECAM Metallurgy Laboratory under the supervisiona“m, of Mr P. COUVRAT
22、.3.2 Structural observations of the different types ofbronzestm g SAND CASTf it st pit t _ng_ In this case the cooling of the bronze during its solidifi-cation is slow. The process of solidificationstarts by thea_m _. formation of SSct “germs“ followed by their growth. Aheterogeneous structure forme
23、d of dentntes is thenobtained. Between these dentrites, the remaining liquid, r , _r:1- provides the eutectoid component._ l.L7 IXa The first tests made on this grade did not produceFig. 3: UEI2P chili cast fatigue curve conclusive results. In fact the rolling track did not stabilizeand had a tenden
24、cy to occupy the whole width of the rollerin a very short time, even when the roller was submittedto light loads.2.5.2 UEI2P centrifugal bronzeAt CETIM the new test benches, that permit the applicationof heavierloads, have allowedresults concerningthis germsgrade to be supplyed. An off-set of 150 MP
25、a in relationto the scaling curve of UEI2P chill cast bronze is observed.All the pits obtained were singular and large.7_“-“ .:x _ 5S.kI , 4t , etlttectolcrwP,h “_%centrifuoar,-* “r i/7“_ : _ ,“_.y _,! Fig. 5: Structure of the bronze, c h iI1 e a st a UEI2P CHILL CAST BRONZEf ,: . , ,_:;: ;:/ Castin
26、gsin metallic moulds have a quicker cooling processT: . .,ii those in sand casting with germination and dendrite , . _ formation. The grains obtained are of greater dimension“ - = “ and have little homogeneous structure. On the other handoT_x- a greater quantity of eutectoid et - 5 is observed betwe
27、enFig. 4: Fatigue curves for UEI2P centrifugal bronze the SSct grains. It is the hard and fragile eutectoid thatgives the mechanical characteristics of the material. The The presence of cracks and added ladle effect due to rollerfatigue tests have not caused any special problems for this friction we
28、re noted and gave a very particular shape to thegrade, scaleinformation.UEI2P CENTRIFUGAL CAST BRONZE All the detachments were parallel to the rotating directionThe casting by centrifugation allows an even quicker and corresponded to crack progression in an orthogonalcooling rate than the two previo
29、us techniques. The quantity plane to the friction track. They had an average dimensionof eutectoid a -/5 is thus greater. The grain obtained is of 2,5 microns: they are fatigue striations. The propa-more compact and much smaller. Due to these observations gation of cracks in this plane is thus crack
30、ing by fatigue.we can consider that this type of bronze has good Numerous cracks were observed on the roller surfacemechanical characteristics. This type of bronze has the around the pit. The starting points of these cracks cot-greatest hardness of SSct grain (see the hardness chart in responded to
31、the maximum sheafing points; presentedparagraph 2.4). later is a verification of this phenomenon with Hertz stresscalculations.3.3 Analysis of two deteriorated rollers Our aim is to establish the history of the pit, it wasnecessary to study the transverse cuts of the pit to find3.3.1 Introduction L_
32、git_tmai pl_._ the direction of crack propagation. In addition, theinclusions of lead seem to have had an influence on theCut view in roiling band cracking propagation. We observed multiple shallow(longitudinal plane) Fncti_ z_e detachments near the lead inclusions.Friction zone I _ Transversal 3.3.
33、3 Slip planes45 cut view planeThe grains of solid solution ct have a face centred cubicTransversalplane t_a systemwith 12 possible slip systems (4 planes and 3Fig 6: Cut view studied on a piece of a roller, directions). The plastic distortsion is the result of dislo-cations occuring inside the cryst
34、al under action of theThe analysis undertaken was on two rollers, each showinga large scaling: one roller was manufactured by chill applied stresses.casting and the other by centifugation. The parts were cut _“Ainto pieces. The observations with an electron microscopewere made on three planes: an or
35、thogonal face to thefriction zone (transverse plane), a cut parallel to the rollingtrack (longitudinal plane), and a cut at 45 to the rollingtrack.The whole of the observations concerning the chill castroller were presented in reference 10. So here the resultsof the centrifugal roller are described.
36、 Fig. 8: Active componentActivation of slippage planes:3.3.2 General study of pit Under an applied stress 6, the active componant -ccausesthe activation of a slip-system. The planes concernedhng Direction become the preferred direction for the propagation ofcracks.Change of direction:so_t slant * If
37、 the direction of the principle stresses remains constant _ _ _ Brutal metal_. p,ck-up the orientation change of the crack comes from the changein slip-plane direction in the SSc_grains. Generally, thecracks evolves in a particular direction.I M_._ RollerCrack 55_Fig. 7: Ladle effect in the longitud
38、inal planeIn both cases only one pit was observed formed solely bycrack mechanics, even when it became highly enlarged in general dlrec_lonolpropagatJonthe rolling band zone away from the scaling. The scalinghas the rough form of a crater and is composed of asuccession of metal pick-ups in a “step s
39、hape. Figure 9: Propagation of cracks* If the stress direction changes, then direction of theactive shearingcomponentchangestoo. Thus there is _ /activation of the other slip-planes which causes a changein the general crack direction. ZONE 11,5* In both cases defects and osbtacles in the material ca
40、use _ ZONEWHERE_r_ NE lJchanges in direction: cavities, lead inclusions, eutectoid _ CRACKSORIG- 6. _ArE.Ot3.3.4 Study of transverse plane a,_OBSERVATIONS t|Several types of observations were made with a lightmicroscope: Fig. 10: Main cracks in the longitudinal plane.* There were no cracks far from
41、the pit* Around the pit, cracks were located inside the Main cracksroiling track. They were practically parallel to thecrevice surface and propagated perpendicularly to the Zone I: The cracks follows a slight curve, which allowrolling track in “steps“. them to come back to the surface of the roller.
42、* The cracks propagated to the maximum depth of Zone II: Between the zones I and II we observe a change1,2ram. in thecurvewithanpointofinflexion.Thecurveisgreater* We believe that the cracks evolve first along the in the zone II and we ascertain an orientation towards theorthogonal planes to the rol
43、ling surface. It is thus center of the roller.necessary to determine the propagation mode of thecracks in a plane parallel to the roiling surface. Zone III: The crack goes almost vertically towards thecenter of the roller, then stops at a depth of about 2mmCRACKING BY FATIGUE from the surface.The av
44、erage width of the fatigue striations was determinedfrom measurements made directly onto pictures: the Secondary cracksfatigue striations have an average width of 2,5 microns.The “steps“ were observed with enlargement of localised These start from the main crack at right angles to it andcracks in tr
45、ansverse planes. The striations were clearly then come back towards the surface. The change ofapparent in the cracks, directionby 90 occurs at the level of the cavitiesor of the eutectoid.The cracks propagated in a linear manner near the edge of Rolhngdarect,onthe part. The phenomenon of cracking wa
46、s more evident Metalpick-up :for depths between 0,6 and 1 mm (zones of maximumshearing stress). /Thus it was concluded that the cracks viewed in the Laddle _/transverse plane propagate through fatigue towards the shape 0,9to I mmedge of the part from the bottom of the crack. In this zonewe ascertain
47、 the fast fracture of the cracks and the _propagation of micro-cracks.In addition, it has been observed that the cavities seem toattract the cracks that propagate nearby: these cavities givean important stress concentration and also form an idealcondition for propagation of cracks.DIRECTION OF PROPA
48、GATIONThe propagation of cracks is most likely with a stressconcentration at the head of the crack. At very highenlargement lines of metal slippage near the crack were Fig. 11: Secondary cracks in the longitudinal planeobserved. It was noticed that the cracks did not alwaysfollow the slip lines. The
49、se changes in direction are due Secondary cracks are inexistant below 1.lrrma.to the fact that the crack splits into two directions aroundthegrains. Inseparabilityof mainand secondarycracksThe two types of cracks show evidence of the phenomenon3.3.5 Study of longitudinal plane of ladle which causes material pick-up.CRACKING THROUGH FATIGUEGENERAL SHAPE OF THE CRACKIn the longitudinal plane all the cracks observed had the Unlike the transverse plane, fatigue cannot explain thesame characteristic shape. We studied the m
