Jtam-A4.dvi JOURNAL OF THEORETICAL AND APPLIED MECHANICS 55, 2, pp. 621-634, Warsaw 2017 DOI: 10.15632/jtam-pl.55.2.621 A STUDY ON TRANSIENT WEAR BEHAVIOR OF NEW FREIGHT WHEEL PROFILES DUE TO TWO POINTS CONTACT IN CURVE NEGOTIATION Morad Shadfar, Habibollah Molatefi School of Railway Engineering, Iran University of Science and Technology, Tehran, Iran e-mail: morad.shadfar@gmail.com; molatefi@iust.ac.ir Systematic examinations on wear behavior of stick/slip contact around metal on metal have shown that the dissipated energy and contact forces are two important parameters of wear of wheels and rails. Nevertheless, an accurate estimation of these parameters is still a great challenge. Recent developments of non-linear dynamical models and simulation of operational conditions have tried to find a solution of this challenge. These results are used as the input to calculations of wear propagation. Though, the dynamic model should be able to predict wheel-rail interaction with high accuracy. In addition, wheel-rail wear is a function of several other parameters whose their integrated influence becomes more than themain discussed ones. In this study, with the help of multi-body dynamics (MBD), an open wagon equipped with three pieces bogies, considering non-linear effects of friction wedges and structural clearances is modeled in Universal Mechanism. Tangent and curved sections of the track considering random vertical and lateral irregularities are simulated. The simulation results are used to calculate wear of both left and right wheels separately. Specht’s wear model based on Archard’s wear model is used. The studied parameters are the rail side coefficient of friction, track quality, track curvature, velocity and rail side wear. Finally, the effects of mentioned parameters are studied on wear depth andwear pattern of newwheel profiles under incompatible contact (which occurs in Iran railway network). The results show different wear volume and wear pattern compared to compatible contact. Keywords: three pieces bogie, Specht wear theory, wear depth, incompatible contact, rail side wear 1. Introduction Scientific investigations on the rolling contact problemwere begun in early of the 20’th century. The primary results showed the dependency of motion state on the wheel-rail contact forces. A practical progress was made in the late sixties and early seventies. In 1967, the computer based theory ofKalkerwas knownby railway experts and as a consequence, this theory foundpractical applications in railway industries. The first investigation on wear of railway wheel profiles was based on computer simulations with simpleandsteady state considerations, like constantvelocity on ideal tangent track (Zobory, 1997). Sherrat andPearce (1991) presented a very simplemodel. In theirmodel after calculation of contact forces and creepages, the volume of removal mass was calculated with a wear index. They also considered one S track followed by a tangent track (Braghin et al., 2006). In some works, there is an emphasis on the relation between themaximumcontact pressure and removal mass which sometimes considered coefficients as the effects of energy (Zobory, 1997; Telliskiv and Olofsson, 2004). Nevertheless, most assumptions in wear are used in the correspondence of dissipated energy in the slip area and special removedmass per unit distance ([5], Jendel, 2002; Enblom and Berg, 2005; Pombo et al., 2011; Jin et al., 2011; [13]). Most of the researchers consider that the wear phenomenon occurs only in the wheel, not in the rail. Also in themodels, a linear relation between wear and friction work is usually assumed 622 M. Shadfar, H. Molatefi (Zakharov andZharov, 2002). Examination of changes in the contact point could lead to predict catastrophic wear which has great importance for increasing velocity (Telliskiv and Olofsson, 2004). Zobory used two different wear regimes: mild wear on thewheel tread and severe wear in the flange (Braghin et al., 2006). Recent investigations give us an ability to predict wear of the wheel-rail system under spe- cified operation with reasonable accuracy. As a consequence, one can model tangent and curve sections of the track and simulate passage frequency in numerical analysis of railway operation. This process could be done on tracks with random irregularities. With the help of simulation results, thewear volume of the rail andwheel can be calculated ondifferent sections of the track. It is common to use contact codes like CONTACT, FASTSIMor other innovative codes in such works (Braghin et al., 2006). For example, Iwnicki and Xie (2008) considered a 3D wheel-rail system for calculation of the rail headwear in the presence of short pitch irregularity, considering non-Hertzian and non-steady contact based on Kalker’s method (Xie and Iwnicki, 2008a,b). In this study, with the use of a 3D non-linear dynamic model in the presence of random irregularities, sensitivity analysis of the wear pattern for different parameters is performed. The innovation in this paper is the examination of wear in presence of two point contact due to incompatible contact. As it is seen later, the effects of operational parameters would be different compared to compatible, one point contact. 2. Iran railway network Iran geographic location in theMiddle East caused freight mass transit development in compa- rison to passenger transportating. In the recent years, with respect to an increase in the transit volume and vehicle ages, variant wheel defects are reported byNational RailwayAdministration [1]. These defects are different from one vehicle type and age to another, butmost of thewheels show 1mm hollow tread at early passages. Although wheels show little hollow tread on early service life, but these hollows do not fall into repair regulations. With the use of these wheels in service, finally thin flanges would cause the wheels to reject. By considering harmful effects of the hollow tread especially in lubricated curves [5], there is a necessity for a comprehensive study around dynamic performance and energy consumption of vehicles. Figure 1 shows a 1mm hollow tread after 10000km passage. Table 1 shows total repaired wheel defects in a range of 14 months [1]. Fig. 1. Tread defects after 10000km passage Table 1.Total repaired wheel defects (April 2010 – June 2011) [1] Defect type No. of recorded wheels Hollow tread 104 Un-conical wheel 58 Sharp flange 847 Thin flange 2085 Total wheel defects 5612 A study on transient wear behavior of new freight wheel profiles... 623 3. Wear According to Zakharov’s theory, wear of the wheel and rail are generally proportional to the energy used to overcome the rolling resistance of thewheels and rails [5].Wear of thewheels and rails is definedwith the stressP and relative slip in contact area.Wear is also dependent on the third layer properties which depend, in turn, on lubrication, environment conditions and sand. On the basis of laboratory tests under un-lubricated conditions, three different wear regimes are defined:mild, severe and catastrophic. Figure 2 shows a shakedowndiagram. It determines areas of normal and un-normal performance. P is the maximum contact pressure and λ is creepage. The curve Pλ=40 determines changes in the wear regime frommild to severe while Pλ=120 is the change between severe to catastrophic wear [5]. Fig. 2. Shakedown diagram for steel wheels and rails: normal area (1) and abnormal area (2) 3.1. Mathematical description of wear For mild and severe modes of wear, the wear rate can be defined as a linear function of friction work [5]. The friction work can be defined as A= t ∫ 0 v(t)(Fxζx+Fyζy) dt (3.1) whereA is the frictionwork, v stands for velocity,Fx andFy are longitudinal and lateral contact forces, and ζx and ζy are creepages which are defined ξ= rω−V V (3.2) where r is wheel radius and ω is angular velocity of the wheel. All dimensions are in SI. One of the most applicable wear theories was presented by Archard (Jendel, 2002). He considered a linear relation between the wear volume and friction work. Acordingly I =KvA (3.3) where I is in m3 andKv is the wear volume coefficient [m 3/J]. For the use of this model, it is necessary to determine the coefficient Kv at any instan- ce. Specht suggested a jumping factor α for every wear regime, therefore Kv can be assumed constant. By implementing the jumping factor into Archard’s equation, it can be rewritten as follows I = { KvA for w