An introduction to rail wear and rail lubrication problems

WEAR

AN INTRODUCTION

TO RArL WEAR AND RAIL

LUBRICATION

PROBLEMS* K. R. KILBURN Quebec North Shore and Labrador Railway. Office ofthe (Canada) (Received

July

2,

chief Etigineer, Sept-lks, Quebec

1963)

SUMMARY It is anticipated that the future trend in railroad’s traffic is towards long, heavily loaded, bulkmaterials carrying trains Experience in the effects of this type of traffic on rail life has been studied closely on the Q.N.S. $ L. Rly., over the past 8 years of its ore-haul operations. A survey is made of the types of wear which have been observed: plastic metal flow, cold working, surface and subsurface metal rupture and metal fatigue developments. Analysis of these development indicate that they arise from an intimate inter-relation of the magnitude of the wheel-loading to the load carrying capacity of the rail with respect to both rolling contact and structural support, the dynamic effects of a complex wheel to rail relationship which is aggravated by damaging impact loading, the metallurgy of the rail steel and the configuration of both the components and the assembly of the track structure. Certain remedial measures are suggested which include improved track and roadway structure, improved rail metallurgy, rail surface rehabilitation by grinding and inhibiting rail wear by improved lubrication. The latter is dealt with at some length, a current rail lubrication research program being described in detail. Of principal interest is the investigation of the behaviour of lubricants at sub-zero temperatures and the employment of special techniques such as radio-isotope tracing of the dispersal of rail lubricants along the track by rail traffic.

Les chemins de fer, en particulier ceux qui font le transport de matieres premieres, sont appeles a utiliser des trains de plus en plus longs et de plus lourds. Ceci s’applique a la compagnie Quebec North Shore and Labrador Railway qui fait le transport de minerai de fer. Depuis les huit dern&es an&es, ce chemin de fer a observe le comportement du rail vis-a-vis de telles conditions de service. On a remarque le developpement de divers types d’usure tels que la deformation plastique et l’ecrouissage et le developpement de fissures prenant naissance soit a la surface ou a I’interieur du rail, Ie developpement de ces fissures &ant quelques fois relic au phenomene de fatigue. 11 a =&e Btabli que ces types d’usure dependent: I) de la valeur de la charge appliquee par rapport 8- la resistance du rail, tenant compte de la structure de la voie fern% ainsi que de l’effet de roulement; 2) des effets de la relation complexe rail-roue, aggraves par le caractere dynamique de l’application de la charge; 3) de la metallurgic du rail; 4) de la forme de chacune des parties composantes ainsi que de l’ensemble de la voie ferree. On a suggere certains remedes pour prevenir le developpement de ces differentes sortes d’usurc. En premier lieu, il appert d’ameliorer la structure de la voie ferree ainsi que la metallurgic du rail. La vie du rail peut etre Cgalement prolongee par meulages periodiques et en reduisant l’usure par une lubrification adequate. L’article dccrit un programme de recherches portant sur la lubrification du rail, On a particulierement etudie le comportement de divers lubrifisnts a basse temperature ainsi que l’utilite de techniques specialisees faisant appel & l’utilisation de substances radioactives pour suivre le deplacement des lubrifiants. * Contribution to a Conference on Some Aspects ofFriction and Wear in Meclannical Elaginee+g [R~~l~~y Pmblems), held at Queen’s University, Kingston, Ontario, June 20th and arst, 1g6.2. Permission given by the National Research Council of Canada to publish this work from the Proceedings of I.&eConferenc& is gratefully acknowledged. Wear, 7 (1964) 255-269

If the, railways arc to su~cc~ssfully compete> \vith other forms of transportation, thcl!, will have to streamline their operations cxtrnsi\rclly in or&r to offei- an attracti\-c, price for their services. ‘fhcrc is plenty of justification under these circumstancc3 f01 their objections to the op<,ration of uneconomical services but mart> significant is tllc, fact that their method of operation will chnng~. This will not be so muc~h an innovation but the accclcration of a long established trend towards longer ant1 IIIOTC hcavil\* loaded trains and greater traffic densitics. This is the typ’ of traffic that the (J.S.S.

8~ I,., in company with arestricted

numl)~~r

of other railroads, ha\-c pioneered in o\.er the past eight years of their ore-haul operations: trains consist of one hundred and twenty-five y-ton capacity cars drawn by foun

1,7501i.p. diesel locomoti\~es at 30 m.p.11. and traffic densities art’ in (tx~ess of 120,000 gross tons per da!,. Such a low cost per revenue ton operation has brought with it new problems in thcl field of rail wear. It is currently the subject of intensive research by a diversity of agencies including the railways themselves, their engineering associations, supplier industries from steel to lubricating oil and independent agencies from universities to research foundations. 17ct the problems of rail wear and rail lubrication what ill-defined and still to a large extent unsolved.

On the Q.Y.S.

& L. Kailwa!;

three

specific

are still some-

types of rail wear have been identified.

The classification into three types is somewhat arbitrary: it refers to location rather than wear mechanism since any type on analysis is found to involve a number of mechanisms

and in some cases to progress from one wear phenomena

to another.

Thrl

three types (Figs. 2, 3 and 7) have been identified as follows: (a) Continuous abrasive wear of the inside edge of the high rail on curved track. (1,) Plastic metal flow and crushing of the low rail of sharp, super-elevated curv(‘b. (c) Shelling: a progressive fatigue type wear that is an incipient rail failure which develops at a decreased rate in tangent, and at an accelerated rate in curved track. The continuous abrasive type wear is due to wheel flange contact (Fig. I). This is almost entirely attributable to the nosing action of the leading outside wheel of the cars’ trucks when negotiating a curve. Primarily because of the incipient danger of the wheel flange mounting the rail and rtsulting in derailment this rail nosing action is restricted by four counter measures: restricted train speed on sharp curves, spc~d adjusted supercle\ration of the outside rail compared to the inside rail on curved track. coning of the wheel tread and inward canting of the rail. These measures are conductivcl to maintaining the truck centrally located between the rails. Nevertheless because the trailing axle of a truck invariably rides radial to the curve and, further, because of the 5 ft. 6 in. longitudinal spacing of the trucks’ wheels, nosing action of the lead wheels is always present. This is further increased bv chance factors ranging from restrictedrotationof the truck about its cc>ntre plate to &cillation of the truck initiated by irregularities of the, track gauge, surface ilntl alignment. Plastic metal flow and crushing of the low rail on sharp curves is due primarily to heavy wheel loading such that contact stresses exceed the yields strength of the rail metal. Since it OCCUI-scsclusively outside of tangent track one must suppose that thr

RAIL

WEAR

AND

LUBRICATIOK

257

cp- 9 Locomotive

Tie plate 7-3/4" x14"

Fig. I. Rail and wheel profiles.

.-.-‘-.-._*

b. \

‘\

i i .i

II/

GAUGE

Fig. 2. Profile of typical curve worn rail. (a) High rail. (b) Low rail. Wear,

7 (19%) 255-269

Fig.

3,

Crustied

railhead.

.S~.~l~~~g is a progressive type of wear which. starts with cold working of the gauge corner of the rail snd follows a sequence through cracking of the surface metal to metal fatigue pitting or spalling, concurrent with development of sub-surface metal separation, which progresses to both extensive metal breakouts and internal fatigue failure mostly in the horizontal plane but sometimes catastrophically in the transverse plane. It is due to combined loading in excess of the rail metal’s yield strength and stress fluctuation ~~~i~otin~ fatigue. The exterior forces responsibie are suspected to be complex and are believed to arise from an unfortunate combination of those same chance factors that accelerate the abrasive and metal flow types of we~ mentioned above.

RAIL WEAR ANDLUBRICATION

259

ANALYSISOF THE TYPES OF WEAR Wear may be defined as the change in dimensions or in the nature of the lubricant or in the nature of the mating surfaces in service resulting from the pressure exerted or the relative motion between the contacting bodies. We have attempted in the foregoing section to enumerate the types of rail wear experienced and the in-service factors which promote them. Let us now attempt to identify these three characteristic types of wear with the classical variety of mechanisms forremoval of material from a surface. For want of any better we will adopt Barwell’s classification: (I) continuous wear, (z) scuffing, (3) pitting, (4) abrasion, (5) fretting, (6) surface flow and, adding one other (7) metal fatigue. Continuozts abrasive wear The continuous abrasive type wear of the wheel flange contact surface of the rail meets all BARWELL’S criteria for continuous wear and does not significantly diverge from them. “The main feature used to distinguish this type of wear is that material (is removed) in small particles . . without resulting in any gross surface damage. Indeed this form of wear may frequently result in (the) surface becoming smoother, rather thanrougher. The following processes . . . contribute to removal of the particles : (a) the mechanical interlocking of asperities pressed together on the rubbing surfaces in the manner postulated by 1 Ming Feng, (b) localized adhesion of the mating surface in the manner discussed by Bowden and Tabor, (cf the abrasion of surfaces by hard particles or by adventitious matter, and (d) the dislodgement from the surface of wholly or partly oxidized particles by corrosive action of the lubricant or erosion caused by cavitation”. There is even support, in our experience, of BARWELL’S hypothesis that “microstructure of a steel may be more important than its bulk hardness from the point of view of (continuous) wear resistance”. For in our field experiments comparing service life of Heat Treated vs. Standard Rail we have observed that the flange wear resistance of 350 BHN (Brine11 Hardness Number) heat treated rail is little better than that of 2.60 BHN standard rail. Fortunately this type of wear, because it is due mainly to sliding and under comparatively light loading and because of its location outside of the traction running surface, lends itself to be inhibited by rail lubrication. Plastic metal flow and crushimg

Wear of the low rail which is characterized by plastic metal flow and crushing is more complex. It is believed to arise from a combination of high load rolling contact and some degree of sliding in, perhaps, several directions. From the evidence of extensive surface metal flow and progressive cold working to depths of 314 in. below the surface it appears that the high load rolling contact is more critical. Some estimate of the damaging effect to rail, of this type of wear, is apparent when one applies the Hertz theory “On the contact of elastic solids”. This shows that the maximum static wheel loading of a 36 in. diam. wheel on the IO in. radius of new 132 lb. R.E. rail is 5,400 Ibs. Loading in excess of this will produce a maximum stress difference in excess of the yield strength of the rail metal. Beyond this point plastic WfXr. 7 (1964) 255-269

Nevertheless,

the Hertzian

thcor\- does provide some uwful indications

of the mcs-

chanisms involsecl in this t!y of we;tr if one is prepared to accept tilat the strw distribution of plastic deformation is not entirely dissimilar to the elastic. For example under e&tic contact the rn~x~rnu~n stress, -+~hit:his compressi\-c, o~c‘urs at the contact surface but the critical nl~ximurn stress difftwnw occ’urs below thtb iurf:tw. A possiblt* interpretation is, therefore, that cold working is initiated below the surface ;tnd thcb the metal subsequent formation of these internal seyavations, which would w&en internally, could foresecably account for the extensive depth of cold worked occurring wherc~v~~rthis typr of plastic flow war is found.

The progressive

type of wear development,

termed shelling,

Fig. 4. Head checks.

metd

must be analIysed ste1)

by step throu& involves

;t

its ~ff~~e~t

stages of de~~~~~prn~nt, It is s~s~cted

tkat each, in t.urn,

differerit me&a&m or cornbinaGsn of wear mechanisms. Herzd dzecks, as shuwn in Fig. 4, originate with pla&ic flaw of meta1 which buitds up as a lip on the gauge corner of the high rail tog&xx with hair line cracks appearing, initially, at the gauge corner and developing into the running surf ace area of the rail head. This would appear to be due to relatively light rolling load contact ~hcn compared with the plsutic flow phenomena of the low rail.

of any surface imperfection (which may act as :I. stress raiser) c)r climrrlsional inaccuratics of the rolling elements (which may lead to a localized increase in stress) will have the effect af reducing the life considerably. Thus operational experience of a ~idr scatter in iife may rest 04 some sound explanation”. The reference to dimensional inac32nracies of the rolling &merits is cogent because this may w&l be the point of divergence of the high rail development from the low rail’s plastic wear: the fit of the wheels to the high rail being poorer than their fit to the low rail. Further : the presence

RAIL WEAR

AND

LUBRfCATIO?J

263

of head checking and flaking of the high rail acting as stress raisers would promote this fatigue type of wear. Shellilzg, as shown in Fig. 7, is usually considered the most critical stage for two reasons: it is the incipient stage to rail failure and initiates a marked acceleration of the fatigue failure development. The stage is evidenced by actual or imminent removal of often extensive and invariably thick portions of the surface metal at the gauge corner of the rail, Shelling is conclusively identified by the depth at which separation occurs beneath the surface of the metal: Thisisnoticeab~~deeper than forflakingor

LAROE SIIELL CYIOEWCED 07 OLACX SPOT

Fig. 7. Shelling.

spalling. The depth at which they occur is believed to be greater than predicted by Hertzian contact theory: they must, therefore, be propagated to this depth by fatigue, or the separations are themselves developed fatigue cracks. A variety of internal defects developing in the rail head mark the fifth stage of shelling wear development. These are mainly cracks of up to I in. in length which when restricted to the horizontal and vertical planes (see Figs. Q and IO) of the rail section are not always an immediate cause of ultimate failure of the rail. If allowed to progress Wear, 7 (‘964) z55-zOg

to the boundaries of the rail head large sections of the metal will split out ~- henct~ they are referred to as horizontal and vertical split heads respectively. The danger of this type of defect is that it may at any time propagate into the transverse plane of the rail and initiate a transverse defect. ?‘runs~~evsedtfccts, 9s shown in Fig. 8, mark the final stages of shelling dcvelopnient. It is imminent to ultimate failure because the transverse plane (see Fig. r r), is normal to both the vertical

and side-thrust

loading of wheel traffic

and, therefore,

fatigut~

Fig. 8. Detail of feature development. Cross-section through a shell, which has not broken out, showing rapid development of a transverse defect.

development is rapid. It is because of this rapid rate of development that transvcrsr defects are most obviously identifiable as fatigue failures. It should be noted that all the above six stages of development do not always occur progressively, and, for this reason, it is suspected that several of the forms of wear identified occur simultaneously. This hypothesis indicates that a complexity of phenomena is involved which does not lend itself readily to analysis.

RAIL WEAR AND LUBRICATION

265 VERTICAL

PLANE

Figs. g, 10 and II ii&&rate the relative position of planes through a rail. Defects lying in the horizontal or vertical plane are classed as longitudinal. In the transverse plane, defectsare classed as transverse. MEASURES APPLIED TO REDUCE RAIL WEAK

incidence of rail wear is, for the most part, infrequent. This reaffirms that the wheel loading is not generally excessive but is sporadically increased by impact and change factors. The impact is kept to a minimum, by the elasticity of the track structure, and its maintenance in such condition that sources which amplify dynamic impact, and contribute to erratic loading, are kept to a minimum. l~~ntenance of Way forces are charged with keeping irregularities of track surface, alignment and gauge to a ~nimum. This involves constant servicing of all parts of the track structure: rehabilitation of the rail surface by grinding, repair of battered rail ends by welding, tightening of rail fastenings, surfacing, lining and gauging of the track. There are practical and economic limits to the degree which the track can be maintained under constant impact laading and high traffic density. The reduction of rail wear is therefore also sought in improvement of the configuration and components of the track structure - principally to find means of improving its capacity to transmit dynamic loading from the wheel-to-load contact to the road bed. The current track structure is designed to accomplish this by the relative elasticity and the loose jointing of its component parts. Any radical change in this principal, for example, adoption of the French double-elastic fastening and use of concrete ties, is restricted by the imposition of retiring a heavy capital investment in conventional track components. Therefore improvements to the track structure have to date - in ‘l’he

Wear, 7 (x964) 255-269

Sorth America -- been made by increasing the weight and quality of the ~~onl~or~el~ts: heavier and improved design rail sections, tic plates and joint bars, heavier, improved quality cross ties and more sciective roadway ballast, The comparatively recent introduction of continuous welded rail has undoubtedly made the greatest con.tribution 10 improved track structure b!- reducing the number of rail joints, which are the principal source of impact. The adoption of continuous rail has at the same time focused attention on rail wc~ because of the difficulty of making spot replacements of worn rail. This has added emphasis to the need for wear resistant rail. Rail with certain wear resistant qualities has been produced in comparatively limit-ed quantities bv the addition of alloying elements --- manganese, silicon, chrome ant1 vanadium --’ and h\- full heat treatment and local heat treatment (P.R., flame harden ing), To date all these have only been partiallv successful. Either wear resistance has been obtained at the expense of some other essential rail steel characteristic or the rail has proved resistant to onl!. one form of wear. For example, one type of heat treated rail shows resistance to high load rolling contact but no appreciable resistance to wheel flange contact wear. The main drawback, to the development of wear resistant rail, has been the absence of a suitable specification of its physical properties and tlris is clue, directly, to the failure to fully analyze the wheel-to-rail relationship. Each of the measures applied to reduce rail wear make a significant contribution to the reduction of excessive dynamic loading yet none offer any more than an expedient and partial solution to the rail wear problem. The app,rientl\r marginally high static loading effect, once impact loading has been reduced to a rni~in~u*~, can only he borne providing suitable wear resistant rail and effective

rail lubrication

are utilized. THE

RAIL

LUBRI(‘XI‘ION

PROBLEM

On the Q.N.S. & I,. Railway, rail lubrication has been applied in highly curved tr-ack territory since the beginning of ore-haul. Its effectiveness in reducing rail wear has resulted in it being extended to provide lubrication (Jf the entire length of the railroad in subsequent years. A recently instituted research program, in anticipation of yearround ore-haul, has indicated certain limitations in established practice. It is these limitations that constitute the current rail lubrication problem. The most significant limitation is lack of a comprehensive rail lubricant specification. This apparently startling oversight is axiomatic with the mixture of casualness and controversy that the majority of railroads, on one hand, and a very limited number of interested railroadson the other hand, practice rail lubrication. To the majority of railroads, rail lubrication is still viewed, in, its original context, as curve lubrication. Their interest is invariably directly proportional to their amount and sharpness of curved track. This is because their wheel loading is still sufficiently low to restrict rail wear, if any, to sharp curves. The fact that where wheel loading is marginally high “curve wear” will even occur in tangent track -- which because of the limits of line of sight is actually a series of low curvature swings -- is not generally appreciated. Even the interested railroads are limited in drawing up a lubricant specification because of their ignorance of what they want. This is in part due to indecision as to how to apply the lubricant: whether, for instance, to use locomotive mounted oiler devices, track-side grease dispensers or solid stick lubricators directly mounted over

RAIL

WEAR

267

AND LUBRICATION

each wheel. But mainly, it is due to ignorance of the wear mechanisms which they seek to counteract. It is apparent from the foregoing section on Analysis of Rail Wear that the wear mechanisms involved are almost so complex as to defydefinition. A further limitation to specifying desirable physical properties is posed by the environment : for example a lubricant must be pumpable over the extended range of climatic temperatures, it must adhere to the rail under all conditions of heat, cold, rain and snow yet, at the same time, be free to wide and unvarying distribution along the track by wheel traffic. Cognizance must also be taken of the difficulty of relating any, let alone all, of the service characteristics of a desirable lubricant to any of its physical properties. This may well be an insurmountable difficulty as in the case of hypoid lubricants which can only be satisfactorily evaluated by service testing in a laboratory test rig of hypoid gears. The evaluation of a suitable rail lubricant therefore by full scale in-service testing may seem ludicrous, but is not to be ignored. It must be remembered that a lubricating oil, for instance, is not a manufactured or synthesized product but a natural mixture of a large number of different hydrocarbons, further it varies widely with origin : it is not surprising therefore that there is no easy way of defining its properties. Consider, then, a special lubricant containing, in addition to hydrocarbons, metallic soaps, extreme pressure additives and perhaps special long chain polymers : here we may have a Newtonian fluid (the oil), a Bingham plastic, a colloidal suspension and a miscible solution each obeying different physical laws and undoubtedly modifying each other. The specification of such a lubricant by a simple statement of physical properties appears impossible. Yet the problem of specifying - and this is synonymous with developing - a suitable rail lubricant must be solved if rail wear is to be appreciably reduced by lubrication. RAIL LUBRICATION

RESEARCH

The foregoing section though attempting to state the rail lubrication problem does so only in general terms. The specific limitations of established practice were derived in the course of the current rail lubrication research program of the Q.N.S. & L. Railway. As intimated earlier, this program was initiated to determine the most economical and practical means of providing effective lubrication for year-round ore-haul on the subject road. In describing the progress of this program to date a fair idea of the depth and specific details of the rail lubrication problem is brought to light. Originally the problem appeared to be one of cold temperature operation. To determine what significant effect ambient temperatures down to minus 50°C would have on (a) the pumpability of existing rail lubricants (b) operation of existing track lubricators. It was suspected that normal summer grade, or all season rail greases, would be too viscous at these temperatures to be dispensed by either mechanical positive displacement pumps or air-motor pumps. Further that interference from snow and ice would preclude the feasibility of mechanical operation of the lubricant pump by physical contact of wheels of the passing train. The possibility of providing electrical heating to modify environment temperatures wasassessed and found uneconomic. A major consideration became the number of lubricators that would be required to provide adequate lubricant coverage of the track at extremely low temperatures. This could only be assessed by quantitatively measuring the carry of lubricant along the Wear, 7

(1964)

255-269

track by train traffic at various temperatures. Up to this time this carry had only IXY~I~ measured qualitatively by visual observation of either the grease on the rail or 1))~ smears taken off the gauge corner of the rail. The techniyue was then developed I,? Q.N.S. c(: L. and Atomic Energy of Canada of dispensing, in the normal manner. lubricant samples tagged with known concentrations of radio-isotopes. The dispel.sion of the tagged lubricant could then be measured quantitatively by radiation particle counting. Field tests showed that concentrations of lubricant down to ten micrograms per square inch could be traced. Included in this program was an investigation of two methods of dispensing rarl lubricants. The established method is by a track side lubricator: the wheels of a passing train depress a ramp mounted on the outside of the rail: by means of a linkage a positive displacement pump is actuated in agrease filled reservoir and the grease is delivered, in blobs, at the gauge edge of the rail where it is picked up upon contact of the passing wheel flanges. Since the type of lubricator in current use is always located on tangent track, only a limited number of wheel flanges are thus lubricated directl!.. The remaining wheels being lubricated indirectly by grease deposited on the rail tth the already lubricated wheels. The second t.ype of lubricator tested was a locomotives mounted wheel flange, mist application lubricator, with this a special high load carrving capacity, asphaltic base lubricating oil containing a volatile solvent was discharged air-entrained from nozzles located close to the wheel flanges. This type of lubricator was originally developed in Switzerland where, it is claimed, that the successive passages of a limited number of locomotives, equipped with this device, will deposit sufficient lubricant on the gauge corner of the rail to provide for effective lubrication of all the cars in the train. A number of different rail greases have been field tested as part of this research program to evaluate their effectiveness under winter conditions. The greases tested included both competitive and experimental types. A continuing part of the program is the development of further experimental greases, with extreme pressure additives for instance, to inhibit other types of wear besides that due directly to wheel flange contact. One of the most significant findings of this program to date is the highly adver-se effect of certain snow conditions on the deposit and carry of lubricants along the track by wheel traffic. The isotope tracer technique has established that the carry may be reduced as much as 90%, for certain lubricants under particularly adverse snow conditions. Quantitative measurement of grease deposits indicates that the carry characteristic may not be directly related to any of the lubricant’s physical properties. This rail lubrication research program is continuing. To date it has given little in tk way of tangible results other than to indicate the direction that future rail lubricatioll research must take and the complexity of the problem being pursued.

CONCLUSION

This article has attempted to introduce the general aspects of rail wear and rail lubrication in sufficient detail that the inference of their ‘respective complexities will stimulate research. The areas of research which must be exploredif theserelatedproblems are to be solved are so extensive that their rigorous development will undoubtedly make a significant contribution to the understanding of wear and lubrication in

RAIL WEAR

269

AND LUBRICATION

general. In the final analysis, the problems discussed are of fund~ental not to a particular type of railroad operation, nor to railroadsingeneral, whole field of mechanical engineering.

importance but to the

REFERENCES 1 1’. T. BARWELL, Lubrication of Bearings, Butterworths Sci. Publ., London, Igy5. 2 H. HERTZ, Gesammelte Werke, Vol. I, Leipzig, 1895. English translation in Miscellaneous papers, H. Hertz, 1896. 3 H. R. THOMAS AND V. A. HOERSCH, Stresses Due to the Pressure of One Elastic Solid Upon Another, University of Illinois, 1958, Bull. No. 2x2. &‘eUr, 7 (1964) 255-269