1、90 FTM 4Different Types of Wear-How to Classify ?by: Louis Faure, CETIMAmerican Gear Manufacturers AssociationTECHNICAL PAPERDifferent Types of Wear - How to Classify ?Louis Faure, CETIMThe Statements and opinions contained herein are those of the author and should not be construed as anofficial act
2、ion or opinion of the American Gear Manufacturers Association.ABSTRACT:In the first part, this document describes all the types and aspects of wear which can occur on the gearteeth in operation, with causes and explanations concerning their appearance.For each type of wear, the possible evolution an
3、d the limits which should not be overpassed to avoid afailure or severe damage to the teeth, thus reducing the life of the gear, are clearly specified.Aspects of wear types which can be considered as normal, medium or progressive are also described andsolutions are presented as a guide for wear stab
4、ilizing.This document is illustrated with numerous pictures of all types of wear.The other types of gear failures such as surface fatigue, corrosion, plastic flow and breakage are notincluded in this paper which is only dealing with wear.Copyright 1990American Gear Manufacturers Association1500 King
5、 Street, Suite 201Alexandria, Virginia, 22314October, 1990ISBN: 1-55589-556-5DIFFERENT TYPES OF WEAR,HOW TO CLASSIFY ?L.FAUREHEAD OF GEAR DEPARTMENTCETIM SENLIS - FRANCETABLE OF CONTENTS 1 INTRODUCTIONSection Title The phenomena of deterioration of surfaces are generally verycomplex and depend on nu
6、merous conditions which include :1. INTRODUCTION 1 - the operating conditions2. CURRENT WEAR (two bodies abrasion) 2 - the type of load applied- the relative speeds of surfaces in contact3. SCORING - STREAKS 3 - the temperature- the lubrication4. ADHESIVE WEAR 3 - the surface hardness- the surface r
7、oughness4.1 Polishing 3 - the compatibility and nature of materials in presence4.2 Adhesions or adhesures 4 The wear is a general term covering the local phenomenon describingthe removal of some material and occuring when two surfaces slide4.3 Hot scuffing 5 onto one another.4.4 Cold Scuffing 7 This
8、 term alsoapplies to the removal of material resulting from thepresence of impurities in the lubricant.5. WEAR WITH THREE BODIES 8 The other types of gear failures such as surface fatigue, corrosion,plastic flow and breakage are not covered by this paper which is5.1 Scratches, Grooves (Scratching) 8
9、 only dealing with wear.5.2 Abrasive wear 8 To separate the different types of wear, we will use two distinctclassifications :6. INTERFERENCE WEAR 9One, qualitative will be based on the action modes of7. NORMALWEAR 10 different wearphenomenaon gear-teethand will allow tocharacterize the different su
10、rface aspects which we can8. MODERATEWEAR 12 encounter.9. EXCESSIVE WEAR 13 The other will take into account the intensity of the wearphenomena and will define the dements allowing toD 10. CONCLUSION 15 measuretheirevolution.The qualitative analysis of wear phenomena will bnng us to definesuccessive
11、ly- abrasive wear with two bodies- streaks and scoring- polishing- scuffing (cold or hot)- abrasive wear with three bodies- scratches or grooves- interference wear.During the quantitative analysis we will define successively :-normal wear-moderate wear-excessive wearA synthes_s of these two classifi
12、cations has been made under a chartform which is placed at the end of this paper.2 CURRENT WEAR (called two bodies abrasion)The current wear is revealed very early in the life of the gearing andis shown in the removal of micro-particles of metal on the gear-teethsurface. This phenomenon ts due to co
13、ntact and metal-to-metalsliding which occurs through the oil film.The distinctive machining marks of the cutting procedure or fmish Figure2 Traces of wear recorded on a gear wheel in operation(for example, the facets resulting from hobbing, the streaks left bya gear cutter or by a rack cutter, the s
14、urface pattern from certain We will note the presence of a dull zone below the pitch, where thegrinding operations) are diminished or erased by wear. wear is greater, and :This wear brings about a progressive reduction of gear-teeththickness along with a more or less marked distorsion of the profile
15、 - of a lustrous area on each side of the pitch and slightlyin the heavy sliding zones but without noticeable degradation of the higher,surfaceroughness. - of transversalscoring slightlymarkednear the toothtip,As indicated on figure 1, the wear, which is almost nul in the pitchzone where the sliding
16、 speeds are lower or nul, becomes more andmore important as we move away from this zone. This zone is - of machining streaks still visible near the pitch.maximum at the tip circle and at the active dedendum circle where The aspect of this wear as well as its developing speed vary greatlytheslidingsp
17、eedsare the highest, accordingto :- the pressure level between the contact surfacesThe aspect of the wear zone is generally grey colored and slightly - the hardness of the materialsdull, with sometimes lustrous areas and the presence of scoring - the roughness of surfaces(el.figure 2) - the thicknes
18、s of lubricant film.wear on tooth tip For example :a) In the easeof slightly loaded gears operating with an oil._“ of relatively high viscosity at medium speeds, we will have;_i_? an oil film sufficiently thick to avoid metal-to-metalcontact.f_-_._._ This will not generate wear except during gear st
19、arting or_ _- pitch circle stopping._I- - -gj,._- -t_ The original machining traces will still be intact on the.1_._ gear-teeth after long periods of operation.b) In practice it is not always possible to have a continuouslubricant film between the gear-teeth according to the loadtransmitted. There i
20、s then contact between the top of theasperities made during machining and there is a tendencytooth root wear to polish or score the surfaces in contact (Cf.descriptionhereafter of “polishing, scoring or streaks“).c) In the ease of surface hardened gears, due to the highFigure 1 Distinctive aspect of
21、 distorsion of gear-teeth profile under hardness of flanks in contact, we encounter only slight wearwearaction, whichis oftendifficultto appreciate.3 SCORING - STREAKSThis type of wear presents under the form of fine grooves or linesthat are in the sliding direction. These steaks or scores are forme
22、dprogressively in the zones characterized by a high sliding speed, atthe tooth tip and near the root of the gear-teeth.They are often developped on portions of pinion-teeth and wheelteeth that are in contact at the very beginning of meshing of matingprofiles. The bottom of the strias is smooth compa
23、red to that foundin traces of scuffing.CAUSESThe presence of these-scores or streaks reveals the existence ofrelatively high pressures affecting locally the gear-teeth flanks.Under high load action, the asperities due to the roughness of themating flanks as well as foreign particles of small dimensi
24、ons thatare there and imprint into the gear-teeth flanks, cause, with theslippage effect, cavities which present under the form of streakswhich in operation produce other streaks on the mating flanks duringgear engagement.The formation of scoring can be considered as a preliminary stagepreceeding th
25、e scuffing. These streaks generally lead to the localwear of gear-teeth more important in the zones where there is a Figure4 Scoring by circular streaks ascertained on a worm-gearhigher pressure. It occurs frequently that in time we note an evolutionstabilisation of these streaks when the wear level
26、 of these scoredsurfaces has been sufficient to generate a better distribution of the 4 ADHESIVE WEARload, which as a consequence diminishes the maximum pressure onthe gear-teeth. Adhesivewear appearson two surfaces sliding one onto anotherwhen the pressure between the asperities in contact is suffi
27、cient toIn this case the black color base of streaks or scoring only will cause localized plastic deformations, miero-weldings or localremain. This stabilisation phenomem can be accelerated by adhesions. When there is generation or plastic deformation there isincreasing the oil viscosity and by impr
28、oving filtration, energy absorption which leads to overheating due to friction.The slightest indication of adhesive wear is the formation of“polishing“ on the active flanks of the gear-teeth. When the loadconditions and friction become more intense and when the tempe-rature at contact level increase
29、s we may witness the apparence of :- localized metal adhesures on the teeth- hot scuffing- cold scuffing (for low speeds and heavy loads)4.1 POLISHINGIt is a type of very slowly progressive wear in which theasperities of the mating flanks are progressively distortedand laminated and then appear on t
30、he gear-teeth verysmooth and polished surfaces which take on amirror aspect.Such wear conditions are caused by metal-to-metal contactwhich occurs during operation. Generally the polishingappears for applications at low speeds ( 20 m/see) whenthe lubricant oil film in elasto-hydrodynamic yield rate i
31、snot sufficiently thick and when we happen to be near thelimit of the lubricant performances.Generally this type of wear does not cause considerablevariations in the shape and in the dimensions of gear-teeth.However, when examining with a microscope, the structurejust below the contact surface, we c
32、an ascertain the presenceof platic flows in the material under slippage effect andFigure 3 Scoring or streaks ascertained at the beginning of a gear sometimes encounter very loealized overheating traces nearoperation the surface.The polishing encourages establishing gear-teeth contactThis type of we
33、ar is often encountered in worm-gears as pattern in service and allows to obtain a good conformityindicated on the cliche hereafter, of surfaces in contact.This type of wear is not a damage and can be tolerated inservice unless the material initial specifications forbid it.If the load increases or i
34、f the lubrication conditions becomeinsufficient, this type of gear-teeth aspect can developbecause the temperature between surfaces in contact willincrease and render them superficially less hard and more _1_sensitive to localized micro welding formations.The appearance of adhesion or scuffing trace
35、s on thegear-teeth flanks can then be ascertained (Cf. followingparagraphs)If we want an absolute security in service to be protectedfrom such evolutions we can, after the appearance ofpolishing, increase slightly the oil viscosity to obtain, innormal working condition, a thicker hydrodynamic film.W
36、e can also, when possible, reduce slightly the load to betransmitted but in all cases assure that in service there isnot risk of producing important overlaods (no matter howshort they may be) which would damage greatly thegear-teeth surface.Generally the polishing appears quickly enough on theloaded
37、 gears madewith good precision and most oftenwith Figure6 Aspect of polishing noted on a spiral bevel gearsurface hardened. The most noticeable examples are foundin automotive gears (satellite and planet-gear, gear-box 4.2 ADHESIONS OR ADHESURESpinions and spiral-bevel gears).This type of phenomenon
38、 which appears on some gearThe two figures hereafter provide two examples : mating flanks is always very localized and generally is onlypresent on a small surface of a few teeth of the gear.For each metal pull-off or adherence, we can almost alwaysdistinguish a zone where a brutal welding has happen
39、ed Ibetween the profiles in contact and which was immediatelysheared. On one of the flanks, we may have pull-off of a_ _ “ “ metallic particle which is often found fixed by adhesiononto a flank of the other gear.The irregularities on the two surfaces after separation have_“ then generated streaks or
40、 scratches which are oriented inthe separation direction of the profiles, starting in the zone_“ where the adherence or metal pull-off occured.In the welding zone, the profile is generally altered in depth_ whereasit is more superficialnear the scratcheswhichgenerally are less and less important as
41、we go further fromthe initially damaged zone.This type of damage on is encountered on gear-teeth flanksin the high slippage zones and where the contact surface ispreponderant or localized because of the presence, for. “ example of :A slight excess of material due to a machining defect or aprofile wi
42、th excessive crowning.- A slight alignment defect of gear-teeth due to the machining_“ operation of the gear or due to distorsion under load.- A local distorsion along the flanks caused by a temperaturegradient generated by an irregular flow of heat duringmeshing.This contact surface localization ca
43、uses such an increase inload that it can no longer be supported by the lubricant filmwhich leads to metal-to-metal contact and the formation ofFigure5 Aspect of polishing noted on a gear-box satellite micro-welding by friction.4The development of this type of damage is encouraged by When we are face
44、d with such degradation, the remediesa state of “polishing“ surface of flanks in contact (general are generally simple. After checking, the gear teethcase for surface hardened teeth) and by the presence of precision and that eventual profile and helix modificationsimportant undulations or poor teeth
45、 surface roughness(in fullfill their function, one must :general rough machine-cut teeth and steel normalized), a) Assure that the lubricant and lubrication device are alwaysThe formation of localized metal pull-off can also be caused capable of accomplishing their functions.by the passage of a fore
46、ign metallic particle between the b) That there is no risk of intimely accasional overloadsprofiles. In fact, after itslaminationbetweenthegear-teeth applied to the gear (torsional vibrations, high torqueit will come and weld itself onto the surface of one of the variations, pumping effect, etc.)pro
47、files under the action of slippage thus creating alocalized excrescence. For more safety the lubricant viscosity can be inceased ifThis latter risks to generate during the meshing an overload necessary).which may cause a sudden rupture of the oil film thus The following figure gives an example of me
48、tal adhesionscausing the formation of a metal pull-off on the mating encountered on several teeth of a spiral-bevel pinion.profile.When the numbers of gear-teeth on the pinion and on thewheel are in an integer ratio, the adhesion after it occurson one tooth, for example, may damage later on, byscrat
49、ches or adhesion a limited number of teeth and thephenomanon has all the chances to stabilize by itselfafterwards.If the numbers of teeth are prime between themselves, wewill have a general sweep of all the teeth by those from thepinion and wheel which have been affected by the initialmetal pull-off.From the start the deterioration will thus progress to allthe teeth flanks and there is a risk of evolution towards hotscuffing (high speeds) or cold scuffing (low speeds) of thegear (see following paragraphs). Quite often when thisevolution occurs, it is very q
