Keywords

1 Introduction

Safety of the vehicle strongly depends on stable operation of the braking system [1, 2]. The temperature in the friction elements contact has a significant effect on braking efficiency. Temperature in the contact zone considerably increases during operation of brake elements. In case of drag (downhill) braking, this occurs wear increasing and early crack forming. The tests carried out in laboratory conditions by using the friction test stand and the test bench [3,4,5] showed that the initial contact temperature rise results in increasing of the friction coefficient. The critical temperature depends on the contacting bodies’ materials and structure, the ambient air temperature and other factors. When it is reached, a severe decrease of friction coefficient occurs. This adversely affects braking properties of the rolling stock. The designers and operators face the challenge of developing new construction of brake elements that will allow controlling the temperature in the contact, keeping the optimum friction coefficient and providing higher wear resistance of friction surfaces.

2 Analysis of the Friction Elements Temperature Stabilization Methods

One of the factors which significantly affects safety of railway vehicles operations is the interaction of elements in the brake system, which ensures the braking process implementation reliability [6,7,8]. In rail transport, a significant part of the accidents is caused by the structural defects of the rolling stock, occurred as a result of problems with its brake equipment [9, 10]. Nowadays, the most common construction to ensure stable operation of brake elements are using ventilated discs. The cooling of the discs is intensified by means of ventilation channels of a different geometry, as shown in Fig. 1. It is necessary for the channel geometry to allow the flow of such an amount of air to provide the required cooling power of the brake elements. The main advantage of this design is that disc cooling surface works when braking up to certain speed. But the significant disadvantage is that train movement resistance is increased because of air pumping effect also when discs does not need to be cooled. According to the preliminary calculations and studies [10] dealing with disc brake resistance, the train power may be reduced by 2.3–4.2% depending on the speed and length of the rolling stock as well as by number of disks situated on the wheelset. Therefore in case of use such disk brake structure, it may be helpful to use construction elements to prevent the air flow in the brake discs ventilation channels while vehicle running.

Fig. 1.
figure 1

Brake discs geometry: (a) disc without channels; (b), (c), (d) disc with radial vanes; (e), (f) disc with ventilation bars; (g), (h) disc with tangential vanes; (i) disk with ventilation bars and protrusion system on the inner wall of the channels.

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In operational practices as well as in scientific research works of leading scientists dealing with new construction of brake elements, ways for cooling and friction contact temperature stabilizing this options are investigated:

  • using ventilated discs [11];

  • forced air flow supply in the friction elements contact [9];

  • forced air supply, the temperature of which is regulated depending on conditions and modes of operation;

  • using of pads with porophore inserts with forced air supply, the temperature of air is regulated [12];

  • using of pads with cooling ribs [13];

  • application of brake pads with outer surface cowered with heat dissipating material;

  • supply of friction activators in the friction elements contact zone.

3 Innovative Proposals for Stabilizing the Coefficient of Friction (by Temperature) in Tribocontacts “Brake Disk – Brake Pad”

Authors proposed several innovative methods with the aim to create the optimum temperature regime while braking. To do this, new and modified elements in the brake assembly constructions were used. Materials with phase-transitions were used. For the proposed solutions, patent application have been placed. Three of them are described in the following.

3.1 Increasing of Heat Removing Effect from Brake Friction Elements by Brake Pads

By authors proposed method for creating the optimum temperature regime during braking, patent application number a201712235 from 2017-12-11 is the use of additional materials with first kind phase transition in the design of brake elements. The phase state is understood as the thermodynamic equilibrium state of the substance that differs in physical properties from other possible equilibrium states of the same substance. Within one aggregate state, the substance can be in several phase states, differing in their properties, composition and structure. The transition of a substance from one phase state to another - a phase transition - is always associated with qualitative changes in the properties of the substance (changes in the aggregate state of the substance or transitions associated with changes in the composition, structure, and properties of the substance). The first-order phase transition (thermal phase transition) is accompanied by heat absorption and is characterized by a constant temperature, changes in entropy and volume. The heat supplied to the body goes not to heating the body, but to breaking the interatomic bonds.

Structurally, the brake pad with increased energy capacity contains cavities for placing materials with thermal phase transitions of the first kind. For the reason that depending on the volume of the material and the temperature of the thermal phase transition, the absorbing capacity of the heat from the pad varies, subsequently the design provides for the presence of several types of materials with phase transitions. For example, material n1 with a thermal phase transition temperature of 150–200 °C, n2-nn is above 200 °C. Such solution will allow to stabilize temperatures and friction coefficient over a wide range of values.

When friction heating of the pad base material and it reach the temperature T1 of the ζ1 material thermal phase transition, a transition occurs from one phase state to another, which is accompanied by heat absorption Q1, Q2, …, Qn (Fig. 2) and temperature stabilization of the friction pair. The transition time is fixed (t1, t2, …, tn) and depends on the ζ1 material volume. Similar processes occur with other materials of ζn inserts of the brake pad when reaching higher temperatures as shown in Fig. 2. When the train stops, the generation of heat from the friction pair also stops and a reverse phase transition occurs. By cooling the base material of the pad, the aggregate state returns to its original state.

Fig. 2.
figure 2

Changing the brake pad temperature with increased power consumption during braking.

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It can be seen from the Fig. 2 that the critical temperature Tcr of the pad material, which leads to deterioration of the pad friction parameters and to increased wear, is not achieved when using elements with a phase thermal transition, which allows to stabilize the temperature and coefficient of friction in the contact, to improve the braking efficiency, to reduce wear and rolling noise level.

3.2 Increasing of Heat Removing Effect from Brake Disc by Adding New Elements

The bigger amount of air pass through the disc’s ventilation channels, the strongest heat removal effect from the friction surface can be reached. It is proposed to increase the amount of caught air for brake disc cooling by additional elements. The authors proposed two ways to do this (patents application a201712176 from 2017-12-11, a201712177 from 2017-12-11).

  1. (A)

    Using of two additional elements installed on the brake disc centre

    A significant increase in temperature on the brake elements surface dramatically reduces the friction coefficient. This negatively affects the braking performance, leads to wear increasing, cracks formation and to the failure of entire brake system [14, 15]. This way, vehicle safety can be threatened. While braking, it is necessary to stabilize the temperature of the brake friction elements. In this case, by the ventilation channels 3 (Fig. 3) and routing air into them.

    Fig. 3.
    figure 3

    Brake disc with half-bell shaped elements.

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Air routing into the ventilation channels 3 of the brake disc 2 is provided by additional half-bell shaped elements 5. They are located in a vertical section of the brake disc 2 on both sides of its axial gaps 4 and are concentric with the axis of the wheelset 1. The half-bell shaped element 5 is faced to the axial gap 4 of the brake disc 2 (Fig. 3).

While vehicle moving (braking), the half-bell shaped elements 5 are catching air and routing it into the axial gaps 4 of the brake disc 2 (Fig. 4). The speed and pressure of the air flowing through the ventilation channels 3 increases. Air-pumping resistance is reduced by increasing the pressure and speed of air flow through the ventilation channels 3. The pressure and speed of the air in cannels 3 are higher than the ones on the ambient air on brake disc perimeter. This fact leads to the reduction of the air-pumping resistance.

Fig. 4.
figure 4

Air circulation visualisation.

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The authors have modelled the process in the SolidWorks Flow Simulation. According to the simulations, using of the proposed method can reduce the air pumping resistance and increase the volume of air flowing through the disc cooling channels. This will be helpful for increase the heat removal effect from the brake disc. Proposed method appears to be perspective for high-speed rolling stock. Improved cooling system can help to increase vehicle operation life and ride safety.

Using the proposed design allows:

  • increase the speed and volume of air that flows through the ventilation channels;

  • reduce the brake disc air pumping resistance due to the fact that the speed and pressure of air in the ventilation ducts is higher than the ones of the ambient air;

  • increase wear-resistance of friction elements by increasing the heat outflow from them, thereby extending their service life and driving safety;

  • increase disk cooling, respectively stabilize the temperature of the brake disk, improve braking efficiency.

  1. (B)

    Using of an additional element installed on the brake disc perimeter

    To improve the efficiency of the friction brake elements operation, it is proposed to limit incoming ambient air flow to the brake disk 2 and so increase the air flow to the ventilation channels 3 of the brake disk 2 (Figs. 5 and 6). For this purpose, a half-ring shaped limiter 6 was proposed. It is installed on the brake disc perimeter, with small clearance between disc and the limiter. The location of the limiter 6 need to be changed depending on the direction of vehicle ride.

    Fig. 5.
    figure 5

    Brake disc with ambient air limiter.

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    Fig. 6.
    figure 6

    Air flow through brake disc-section view.

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When braking, to stabilize the temperature of the brake disc 2 installed on the wheelset axis 1, some air is caught by the gaps 4 of the brake disc 2 (Fig. 6). Next, the air enters the ventilation channels 3 and exits between the vanes on the perimeter 5 of the brake disc 2. The air leaving the ventilation channels 3 from the front side (in the vehicle ride direction) of the brake disc 2, enters the inner side of the limiter 6, follows it to its lower part and there is blown off to the ambient. At the same time, the oncoming air flow will not affect the movement of air that escapes from the brake disc. This will increase the amount of air that passes through the ventilation channels and so improves the cooling effect.

The authors have modelled this process using SolidWorks Flow Simulation. The simulation have shown that the limitation of the ambient air in the openings 4 of the ventilation ducts 3 which are located on the brake disk 2 perimeter 5 allows an increase in the volume of air passing through the ventilation ducts 3, air pumping resistance can be significantly reduced.

Using the proposed technical solution in operation can results in:

  • eliminate the pressure of the ambient air on the blown off air by installing a limiter;

  • increase the volume of air that flows through the cooling channels;

  • increase brake elements wear resistance by increasing the heat outflow from them, thereby extending their service life and improving ride safety.

4 Conclusions

A significant increase in temperature on the brake elements surface dramatically reduces the friction coefficient. This negatively affects the braking performance, leads to increasing of wear, cracks formation and to the failure of the entire brake system. This way, vehicle safety can be threatened. While braking, it is necessary to stabilize the temperature of the brake friction elements.

Using of the proposed methods can stabilize the temperature by thermal phase transitions in brake pads, reduce the air pumping resistance and increase the volume of air flowing through the disc cooling channels. This will be helpful for increase the heat removal effect from the brake disc. Proposed methods appears to be perspective for high-speed rolling stock. Improved cooling system can help to increase vehicle operation life and ride safety.