Wear 191 (1996) 107-112

WEAR

The transition from the wear to the stress regime

H.M. Tournay, J.M. Mulder Spoornet Mechanical Engineering, PO Box 30824, Braamfontein, Transvaal 2017, South Africa

Received 9 December 1994; accepted 18 May 1995

Abstract

This paper summarises the conclusions drawn from experiences gained on Spoomet with rail-wheel interaction. The reduction in wear between rail and wheel can result in the formation of higher contact stresses under worn conditions. A strategy is proposed combining design and maintenance practices on both vehicle and track to counter the so-called stress regime.

Keywords: Rail-wheel interaction; Combating wear; Contact stresses; Maintenance procedures

1. Introduction

In South Africa we have been observing over the past few decades, particularly on our heavy haul lines, an increase in surface defects in the form of head checks on the gauge corner of the rail and in spalling where contact is made to the field side of the crown of the rail. In particular instances this has led to the occurrence of these defects in a cyclic pattern on the track with a wavelength of 15.4 m, in a similar manner to that reported on the Mt Newman railroad in the late 1970s.

In other instances it occurs randomly, but in association with lateral track discontinuities. This phenomenon has been associated with vehicle dynamic characteristics, Our inves- tigations over the past few years have led us to believe, how- ever, that it is primarily a function of the improved tracking characteristics of modern rail vehicles together with straighter track, rail grinding procedures concentrating contact on the rail crown, and tighter gauge control which have led us from the wear regime to the stress regime. This paper warns of the need for better wheel-rail management if a railroad moves towards improved wear standards as degradation models move from pure linear wear models towards those fatigue models which may not be linear and which are certainly more difficult to measure.

2. The mechanism of the wear and stress regime

Wheel and rail profiles are designed to meet certain desired properties of conicity, gravitational suspension stiffness and resultant contact stresses. The wheel and rail then enter serv- ice and change shape over time. The nature of this shape

0043-1648/96/$15.00 0 1996 Elsevier Science S.A. All rights reserved LTDI0043-1648(95)06693-4

change is a function of the wear and material flow caused by various contact conditions between the two bodies. These contact conditions depend inter alia on track curvature, vehi- cle alignment, axle load, vehicle speed, vehicle type, traction and braking.

The wear regime is characterised by high flange wear rates. The phenomenon of flange wear implies that a wide band of the wheel tread is being utilised for carrying the vertical load between wheel and rail both from the taping line to the flange as well as an equivalent distance on the opposing wheel of the wheelset from the taping line to the field side of the wheel tread. Flange wear increases the width of contact between wheel and rail over the life of the wheel as does loose gauge control and gauge widening in curves. Typical wheel profiles with worn flanges have treads that are worn flat with little hollow wear.

Improvements in the tracking performance of the vehicle generally reduce, or can eliminate wheel flange wear, con- centrating what wear does occur over the tread of the profile typically as shown in Fig. 1 and Fig. 2.

This wear distribution is either a result of contact being more concentrated around the taping line of the profile as a

I Fig. 1.

108 H.M. Tournay. J.M. Mulder/ Wear 191 (1996) 107-112

Fig. 2

Fig. 3.

Fig. 4.

Fig. 5

result of better tracking ability of the vehicle or as a result of improved wear mechanisms (lubrication) over a broader con- tact band on the wheel tread, or a combination of both of these effects. Typical contact distributions are shown in Fig. 3.

Notwithstanding the more concentrated contact band or the presence of lubrication, certain lateral excursions of the wheelset still occur outside the hollow wear band as shown in Fig. 4 and Fig. 5.

These contacts result in the formation of extremely small contact areas and often result in high longitudinal creepages as a result of the large radius differential generated on the wheelsets. This can cause high longitudinal material flow in the rail and generate surface shelling and head checks.

This is particularly true if wear limits are based on assumed conventional wear patterns; in this regard hollow wear limits of 6 mm are typically the norm, but can be extremely dam- aging if the wear patterns change.

Under high hollow wear conditions the typical compres- sive Hertzian contact stresses calculated are 7000 MPa and 8000 MPa, under axle loads of 26 tons, together with conic- ities in excess of 0.4, as shown in Fig. 6 and Fig. 7.

The result of this contact is crushing of the gauge corner as well as the reaction of the vehicle to the high conicities present, causing damage to the railcrown at further points down the rail in a cyclic manner. This oscillation causes a broadening of the contact distribution on the wheel tread, which is further enhanced by hollow wear formation on the wheel tread and is associated with further propagation of surface fatigue defects (head checks and shelling) down the line. The effect of increasing hollow wear on the contact distribution on the wheel tread is shown in Fig. 8, where the increased conicity of the wheelsets causes larger lateral excur- sions of the wheelset causing the wheelset to ride-up on the convex portions of the wheel tread as shown in Figs. 4-6.

The distributions shown in Fig. 8 have been established experimentally by measuring the lateral displacement of the wheel relative to the rail.

3. Strategies to counter the stress regime

The basic counter move to the concentration of the hollow wear described in Section 2 is to force contact over a broader band on the wheel tread. This can only be done on straight track by either physically varying the gauge of the track and/ or reprofiling the crown of the rail. Attempts to do this in curved track may adversely alter the curving performance of the vehicle or be ineffective as the vehicle steers using existing contact bands on the tread which produce the required radius differential on the wheelset. This strategy has been promoted by inter alia Kalousek [ 1 ] and has been termed pummelling. We are introducing a strategy of pum-

Fig. 6

H.M. Tournay. J.M. Mulder/ Wear 191 (1996) 107-112 109

0.6 I I -

I, I c .a__.

0.1 ;;I 0.2 j-=5=-

-15 -10 -5 0 5 10 15 ._ ._ Lotard dl&c.mmt

Fig. 7. Typical conicities.

hmm HOL3W

Fig. 8

melling on our 26 t axle load heavy haul lines together with wheel profile redesign and the alteration of wear limits. This section will discuss our strategy and the reasons behind the decisions taken.

3.1. Restricting wear limits on hollow wheels

The changing of track gauge under wheelsets running with the existing hollow wear limits would have resulted in con- tact, with the wheelset centrally placed on the track, as shown in Fig. 9.

This would result in higher incidences of high stresses at least initially in the pummelling exercise. It would also increase the risk of producing vehicle instabilities over parts of the line subjected to pummelling or contact towards the flange of the wheel and gauge corner of the rail as a result of higher conicities being realised.

A study was made of the contact between differing worn wheels and the rail, The results of which are summarised in Fig. 10.

In essence it was shown that stresses are appreciably lower in new and 2 mm hollow worn wheels. It was thus decided to impose a limit on the hollow wear on the wheels of 2 mm.

With an average rate of hollow wear on wheels in the fleet of 225 000 km mm- this would initially reduce the life of wheel treads between machining from 900 000 km to 450 000 km. The advantages of this measure were that exist-

Fig. 9.

ing parts of the line suffering from high stress contact would be relieved in the short term and that sections of track where pummelling was introduced would be subjected to lower con- tact stresses and lower variation in conicity.

We thus, in limiting the amount of hollow wear, decrease the contact stresses, bringing the load factor in Fig. 11 down from 28 to 6, as well as reducing the traction [ 2,3], shifting our operating conditions towards the left-hand side of Fig. 11. The load factor is however still high, and will become lower if pummelling is introduced, flattening the convex portion of the wear pattern on the wheel tread.

3.2. Pummelling

Pummelling is to be introduced to hopefully restore the wheel tread life which was lost owing to the restriction in hollow wear limits. It is to be done by:

reprofiling of the rail crown, shifting the contact point when the wheelset is centrally placed on the track to either side of the taping line on the wheel tread. This action will be done whilst restoring rail damaged as a result of high stresses and longitudinal creepage. purchasing asymmetrical rail pads on existing sleepered track as part of an existing pad replacement programme. This will be done firstly on sections of track having less damaged rail crowns. The magnitude of pummelling is to be equivalent to

increasing and decreasing the gauge by 5 mm. This was done after extensive measurement of the lateral excursions of the wheelset relative to the track in order to determine the resul- tant contact distributions which might be attained on the wheel tread as discussed in Section 4 and after re-evaluation of the wheel profile as discussed in Section 3.3.

3.3. Wheel projile

Existing wheel profiles are machined with a slight convex shape towards the field side. This does little to enhance further contact to the field side of the profile. The wheel and rail must contact where each body has a common tangent; if the rail crown stands at 1:20 and because of its profile, parts of the crown lie at flatter slopes towards the field side, then contact cannot be made if the profile of the wheel becomes steeper towards the field side. (See Fig. 12.).

In order to enhance contact towards the field side and promote wider contact on the tread, a new profile has been introduced which is flatter towards the field side as shown in Fig. 13.

110 H.M. Tournay, J.M. Mulder/ Wear I91 (1996) 107-112

Badly damaged back

6mm Hollow Wear

I 4mm Hollow Wear

2mm Hollow Mar

1 712 mPa

Fig. 10.

4. Contact distribution

Measurements of the lateral excursion of the wheel relative to the rail and resultant determination of contact points (determined geometrically) showed contact distribution on the wheel as depicted for various wheel wear conditions in Fig. 8 and for various track conditions in Fig. 14.

A knowledge of track conditions over the line enabled us to estimate the existing contact distributions on the tread of the present wheels as shown by the dotted line in Fig. 15. Our estimate of the resultant effect of pummelling on a newly machined wheel (tread) to the latest profile is superimposed as a solid line in Fig. 15.

An estimate of improved tread wear was deduced as shown in Fig. 16.

5. Conclusion

Tighter gauge tolerances, rail crown grinding producing tighter contact bands and improved vehicle tracking proper-

ties have concentrated contact on the wheel profile enhancing hollow wear. This gives rise to the generation of high contact stresses and conicities. Tighter profile wear limits together with the introduction of pummelling on straight track provide a long term solution to this problem.

Appendix A. Questions and answers

Question (A. Kapoor): Some of your slides show contact pressures of about 7000 MPa, well above the actual hardness of the rail material. How do you calculate these? The elastic Hertzian analysis would not be expected to apply in this regime.

Answer (H.M. Tournay): We estimate stresses from wheel and rail profiles taken in practice. If their stresses were calculated as if the system was elastic, plastic flow must occur. The point is however, that their stresses are unaccept- able and must be lowered.

Question (A.J. Reinschmilt): Are some of the hollow worn wheel problems you mentioned related to the improved

H.M. Tournay, J.M. Mulder/Wear 191 (1996) 107-112

AS R&SULT OF HOLLOW WEAR LlMlTATION

PUMMELLING 7

0 1 I 1 I 1 I

0.1 03 03 0.4 03 0.6 TRAClIONCOJSWCJENT TN

Fig. 11. Shakedown limits in point contact.

14c3 R4Qo

Fig. 12.

14c4

Fig. 13.

steering of the Scheffel bogies-and is this one of the down- sides of these bogies?

Question (0. Orringer): In your discussion of excur- sions to very high contact pressure, you associated these events with track anomalies and estimated that any given wheel would be subjected to excursions for only a few percent of the revolutions in its life. What about the other side of the interface? Would the high contact pressures occur frequently upon the short lengths of rail near the track anomalies?

Answer (H.M. Tournay): What I have described is the Answer (H.M. Tournay): The rail sees repeated high downside of any vehicle-track system, where either because stress contacts at, and after the discontinuities as a result of of very straight track (e.g. Mt. Newman) or better tracking relatively consistent vehicle dynamic characteristics. The ability, little flange contact is made. The hollow wear is par- more varied the vehicle the less do particular points on the ticularly pronounced if the track gauge is very consistent with track suffer from there high stresses.

Fig. 14

Fig. 15.

0 I 10 II *cl 25 ID

KTrmkP-llcd

Fig. 16. Anticipated benefit of pummelling.

a tight tolerance (of say f 2 mm) (e.g. Vancouver Skytrain). The answer to this situation would seem to be pummeling.

112 H.M. Tournay. J.M. Mulder/ Wear 191(1996) 107-112

Question (M. Roney): Have you considered use of profile rail grinding to add pummeling effect?

Answer (H.M. Tournay): Yes. We will be introducing pummeling by means of grinding on that part of the track suffering from rail crown damage.

References

[ l] R.E. Smith and J. Kalousek, A design methodology for wheel and rail profiles on steered railway vehicles, Proc. 3rd In?. Symp. on Contact Mechanics and Wear of Rail-Wheel Systems. Cambridge, UK, July 1990, Elsevier, Amsterdam, 1990, pp. 334-338.

[2] Rail-Wheel Interaction and Metallurgy Course, 1993, Under the Auspices of the University of Pretorias Department of Civil Engineering, Spoomet and S.A. Institution of Civil Engineers, Railway and Harbour Engineering Division, Chapter 4.3.

[3] A.F. Bower and K.L. Johnson, Plastic flow and shakedown of the rail surface in repeated wheel-rail contact, Proc. 3rd Int. Symp. on Contact Mechanics and Wear of Rail-Wheel Systems, Cambridge, UK, July 1990, Elsevier, Amsterdam, 1990, pp. l-18.

Biographies

H.M. Tournay: is Senior Manager, Rolling Stock, Spoor- net. He is a mechanical engineer, specializing in railway rolling stock, engineering and manufacture. He has broad experience of rolling stock with extensive experience of sus- pension design, the structural design of suspension compo-

nents and of vehicle bodies. His career has involved broad experience of the head office engineering function of Spoor- net with particular emphasis on design and development of rolling stock. This function includes the following:

Consulting on vehicle suspension design and vehicle- track interaction. Advise on vehicle suspension and wheel and rail profile maintenance strategies to extend rail life on heavy haul lines. Responsible for assessing vehicle and rail condition, developing vehicle performance monitoring methods, and vehicle and rail maintenance and restoring limits. Advice on planned maintenance programme on bogies, suspensions and brake systems on locomotives and wag- ons on the heavy haul lines. The development of a range of wagon designs.

J.M. Mulder: is Manager, Rolling Stock, Spoornet. He is a mechanical engineer, specializing in railway rolling stock, engineering and manufacture. His present involvement includes the following:

Development of new wheel and rail profiles and the per- formance evaluation of these profiles. Development and implementation of measurement techoniques with respect to bogie performance. Production of maintenance specifications and manuals, and the development of degradation models for different types of bogies and bogie components. Involvement with wheel-rail lubrication.