Abstract

At present, Nadal's formula is used for prediction of derailment that contains a limited number of parameters. Besides, insufficient study of laws of variation of the noted parameters and ignorance of the influence of other parameters on the derailment complicate solution of the problem. The sliding distance and the relative sliding velocity are the most sensitive factors contributing to the destruction of the third body. Moreover, increased friction coefficient between the steering surfaces of the wheel and rail promotes climbing of a wheel on the rail and derailment. Dependences of the main parameters, influencing the destruction of the third body, the sliding distance and the relative sliding velocity on the rail track curvature, and difference of diameters of wheels of the wheelset and the non-roundness of one of the wheels of the wheelset are shown in the work. The methods for estimation of the third body destruction degree and consideration in Nadal's formula of the additional criterion of impossibility of the wheel rolling on the contact point of the wheel and rail steering surfaces, containing a value of this contact point advancing, which in turn depends on the angle of attack, are proposed.

Keywords: tribological parameters, derailment, sliding distance, wear rate, third body

### 1. Introduction

The correlation of lateral and vertical forces, angle of inclination of the wheel flange, and coefficient of friction between the wheel flange and rail gauge are considered as main parameters acting on derailment [1]. Avoidance of derailment and ensuring durability of the wheelsets, rails, brake shoes, etc. are vital for railways for both safety and economic reasons [2]. The prevalent case of derailments is climbing of a wheel on the rail that is influenced by such main parameters as the flange angle, vertical and lateral forces, angle of attack, friction factors, etc. There are many works devoted to these phenomena [1, 3–13] that indicate urgency of the problem.

The climbing of the wheel on the rail is stipulated by the tribological, geometric, and dynamical parameters of the wheel-rail interaction. For the solution of the problem, qualitative and quantitative estimations of influences of these parameters are necessary.

The well-known Nadal's criterion (1896) of the wheel climb derailments uses the lateral-to-vertical force limit (L/V limit) of a single wheel [8] depending on the angle of inclination of the wheel flange and friction coefficient. However, the latter changes in the wide range and laws of this variation are not sufficiently studied. Besides, the wheel climb derailments generally occur in situations where the climbing wheel experiences a high lateral force at great angle of attack, which is not considered in Nadal's formula. The number of experimental researches confirms the insufficient reliability of Nadal's criterion [14–16].

To prevent the aforementioned undesirable phenomena, it is important to provide the third body with due properties in the contact zone, control of the friction factor, and protection of the third body from destruction. However, until recently, despite considerable quantity of works, devoted to the study of dependences between wheel/rail and wheel/brake shoe friction forces and their durability,

Influence of Tribological Parameters on the Railway Wheel Derailment

Our attention in the paper is mainly focused on the parameters that promote destruction of the third body. Some geometric features of the wheel and rail interaction and their influence on the friction path (sliding distance) and relative sliding velocity are shown. A corrected criterion of the wheel derailment is developed.

2. Dependence of the friction coefficient on the degree of destruction

The phenomenon of seizure is typical for interacting surfaces. This may occur when the third body is destructed and the surfaces are juvenile (free from dirty, oxide films and adsorbed layers) and are approached sufficiently. Seizure of the interacting surfaces leads to the most dangerous and dominating kind of deteriora-

For prevention of this phenomenon, they try to improve the tribological characteristics of the contact zone (improve properties of contacting surfaces and their ambient by applying the friction modifiers), stabilize the boundary layers, minimize a sliding distance and relative sliding velocity, etc. As it is noted in [23], the variation of the friction coefficient is mainly caused by changing a composition of the interfacial layer (the "third body") between interacting surfaces. Our experimental researches have shown that for the given friction modifier, the variation of the friction coefficient mainly depends on the degree of destruction of the third body. An increase of the relative sliding velocity leads to an increase of the friction power and the contact temperature and decrease of the lubricant viscosity, film thickness, and friction force (friction coefficient). It corresponds to the "negative

Worsening of the working conditions is caused by the partial, unit seizures and nonprogressive damage of the third body in the separate unit places (Figure 3) that corresponds to the separate small impulses of the friction moment. In Figure 3 are shown the stages of damage of the interacting surfaces due to seizures and scuffing

friction" in Figure 2, where a friction/creep relationship is shown [24].

expected results are not obtained yet.

DOI: http://dx.doi.org/10.5772/intechopen.89135

of the third body

tion—scuffing.

of the surfaces.

Figure 2.

207

Friction/creep relationship.

In Figure 1 is shown a rail with a trace left on it after the wheel climbing [17]. The trace starts on the rail lateral surface and then passes on the rail tread surface.

The mechanism of generation and development of this trace is not studied sufficiently yet and needs additional researches [18]. Besides, according to this paper, friction coefficient in the contact zone of the wheel and rail reaches 0.5 and more at derailments.

The wheel climbing on the rail is also promoted by decreasing the rail radius of curvature and deviation of the axle of symmetry of the wheelset from radial position (increased angle of attack) that causes advancement of the wheel flange and railhead lateral surface contact point.

As it is known, a vertical axis of symmetry of the rail is inclined by 20° according to the standard. Deflection of the rail in the opposite direction that decreases the angle of inclination of the wheel flange is especially dangerous for the wheel climbing on the rail.

A creep is typical for the wheel and rail interaction. Different parts of interacting surfaces of the wheels and rails need to have different properties. Friction factor for the wheel flange and rail gauge face should be as low as possible—less than 0.1. Excessively high friction of the tread surfaces causes severe wear, plastic flow, and fatigue, and low friction can cause poor traction and braking. For tread surfaces of the wheel and rail, friction factor should not be less than 0.25 and greater than 0.4. Optimal value of the fiction factor for these surfaces is 0.35 [12].

At common operational conditions, interacting surfaces are covered by various types of boundary layers—products of interaction of the surfaces and the environment, friction modifiers, etc.—that prevent a direct contact of the rubbing surfaces. Depending on the friction conditions, properties of the environment and surfaces, these layers may have various tribological properties that will have a great influence on the boundary friction [19–23]. This is confirmed by the results of the experimental researches in the inert gas environment and vacuum that excludes the possibility of interaction with the environment [2]. Under such conditions, unhindered seizure and intensive wear rate are observed.

Figure 1. The trace of the wheel climbing on the rail.

Influence of Tribological Parameters on the Railway Wheel Derailment DOI: http://dx.doi.org/10.5772/intechopen.89135

The well-known Nadal's criterion (1896) of the wheel climb derailments uses the lateral-to-vertical force limit (L/V limit) of a single wheel [8] depending on the angle of inclination of the wheel flange and friction coefficient. However, the latter changes in the wide range and laws of this variation are not sufficiently studied. Besides, the wheel climb derailments generally occur in situations where the climbing wheel experiences a high lateral force at great angle of attack, which is not considered in Nadal's formula. The number of experimental researches confirms the

In Figure 1 is shown a rail with a trace left on it after the wheel climbing [17]. The trace starts on the rail lateral surface and then passes on the rail tread surface. The mechanism of generation and development of this trace is not studied sufficiently yet and needs additional researches [18]. Besides, according to this paper, friction coefficient in the contact zone of the wheel and rail reaches 0.5 and

The wheel climbing on the rail is also promoted by decreasing the rail radius of curvature and deviation of the axle of symmetry of the wheelset from radial position (increased angle of attack) that causes advancement of the wheel flange and

As it is known, a vertical axis of symmetry of the rail is inclined by 20° according to the standard. Deflection of the rail in the opposite direction that decreases the angle of inclination of the wheel flange is especially dangerous for the wheel

A creep is typical for the wheel and rail interaction. Different parts of interacting surfaces of the wheels and rails need to have different properties. Friction factor for the wheel flange and rail gauge face should be as low as possible—less than 0.1. Excessively high friction of the tread surfaces causes severe wear, plastic flow, and fatigue, and low friction can cause poor traction and braking. For tread surfaces of the wheel and rail, friction factor should not be less than 0.25 and greater than 0.4.

At common operational conditions, interacting surfaces are covered by various types of boundary layers—products of interaction of the surfaces and the environment, friction modifiers, etc.—that prevent a direct contact of the rubbing surfaces. Depending on the friction conditions, properties of the environment and surfaces, these layers may have various tribological properties that will have a great influence on the boundary friction [19–23]. This is confirmed by the results of the experimental researches in the inert gas environment and vacuum that excludes the possibility of interaction with the environment [2]. Under such conditions,

Optimal value of the fiction factor for these surfaces is 0.35 [12].

unhindered seizure and intensive wear rate are observed.

insufficient reliability of Nadal's criterion [14–16].

Transportation Systems Analysis and Assessment

more at derailments.

climbing on the rail.

Figure 1.

206

The trace of the wheel climbing on the rail.

railhead lateral surface contact point.

To prevent the aforementioned undesirable phenomena, it is important to provide the third body with due properties in the contact zone, control of the friction factor, and protection of the third body from destruction. However, until recently, despite considerable quantity of works, devoted to the study of dependences between wheel/rail and wheel/brake shoe friction forces and their durability, expected results are not obtained yet.

Our attention in the paper is mainly focused on the parameters that promote destruction of the third body. Some geometric features of the wheel and rail interaction and their influence on the friction path (sliding distance) and relative sliding velocity are shown. A corrected criterion of the wheel derailment is developed.
