Research concerning thermal stress of bandages on block braking in case of braking stop.

Dungan, Luisa Izabel ; Ionel, Ioana ; Dungan, Mircea Adrian 等

1. INTRODUCTION

Being in fierce competition, continuous and long-term with other means of transport, modern rail transport has an important share in most countries worldwide with an increasing trend in traffic speeds, hauled tonnages and offered competitive costs.

Continuous growth of velocity on the railway imposed special security issues regarding guiding rail safety in general.

One of the problems in the operation of railway vehicles equipped with wheels with bandages and brake blocks is the appearance of bandages rotating on disk and disk without the axle without axial displacements and axial movement without hammering strange sounds (Stoica, 1998).

To determine the causes leading to these spins there was examined the aspect of the tire and wheel heating through thermal calculation for the bandages in case of stopping braking.

2. BRAKING WITH BLOCKS FOR RAIL VEHICLES

2.1 General information

Braking facility holds the most important role in ensuring the movement of railway vehicles in safety conditions for the traffic. It is necessary for:

* Stopping the train (rail vehicle) within the limits of braking space;

* Partial reduction of speed;

* Lowering maintenance train on the slopes;

* Train (rail vehicle) immobilization after stopping it.

In the first two cases, inside the braking process was dissipated the kinetic energy stored in the train's speed. In the third case, the potential energy was dissipated, which is stored on boarding ramps and in the latter case the role of the brake is related with preventing movements that could be caused by external factors.

To ensure necessary braking space becomes a problem more difficult with increasing speed of movement, which is explained by the fact that with the axle speed increase, the coefficient of friction of cast iron brake blocks suddenly shrinks and pressure on the blocks is limited by the potential wheel lock. Also, on increasing the normal load force, its increase the wear degree and the danger of turning blocks of bandages on the centre of the wheel will become bigger (Cartigny, Dufrenoy & Desmet, 2004).

2.2 Establishing the computational relations for brake blocks for braking stop

At brake stopping, from a thermal point of view, heat transmission occurs in non-stationary process, the amount of heat changing over time. In this case, heat transmission is calculating using Fourier's differential equation (Talamba & Stoica, 2005) and for simplicity it was considered that the heat is transmitted only in a direction perpendicular to the running surface:

v([DELTA]v)/[v.sub.t] = a x [v.sup.2]([DELTA]v)/v x [x.sup.2]. (1)

Starting from this relationship, for stop braking it was obtained, for the raising of temperature [DELTA]v, the relationship:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (2)

where: [lambda] (coefficient of heat transmission), [rho] (specific mass in kg / m), c (specific heat in J / kg degree C) and a (temperature index) are quantities that depend on the material which were made pad and bandage (STAS'99, 1999), (STAS 112/1-80, 1980);

[t.sub.b]--breaking time;

x--distance from the braking surface to the interior of the bandage.

By integrating equation (2) as a function of t and x its result relation (3) for determining the temperature increase, for braking stop:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (3)

3. TEMPERATURE VARIATION CALCULATION INSIDE THE BANDAGE FOR BRAKING STOP

One of the cases frequently encountered in practice is the braking stop and the most disadvantageous type of braking is emergency braking from maximum speed to zero. Braking deceleration on the railway range varies with speed because of friction shoe-binding and binding-track.

To simplify calculations it's considered that the deceleration is uniform throughout the brake. In the calculations were admitted the following:

* for deceleration: 1.3 m/[s.sup.2];

* for wheel diameter: 1.250 m (new bandage) and 1.170 m (maximum used bandage);

* environment temperature: 20 [degrees]C;

* braking time: from 0 to 60 sec.;

* circulating speed : 120 km/h;

* mass factor: 1.2;

* wheel load: 10 MPa.

Temperature variation in bandage, brake off, was calculated in the following situations:

--new bandage, used block, deceleration of 1.3 m/[s.sup.2];

--used bandage, used block, deceleration of 1.3 m/[s.sup.2].

Given that all of the heat is distributed between the wheel and block, the amount of heat will received at the wheel will be:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (4)

3.1 The case: new bandage, used block, deceleration of 1.3 m/[s.sup.2]

The necessary preliminary values in relation (3) are given in Table 1.

Variation of temperature during braking bandage according to the distance from the braking surface toward bandage interior is given in Figures 1 and 2.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

3.2 The case: maximum used bandage, used block, deceleration of 1.3 m/[s.sup.2]

Preliminary values needed in equation (3) are all those from Table 1, only the thermal load has a value of [q.sub.a] = 561 736 kJ/[m.sup.2]h.

Temperature variation inside the bandage as a function of the duration of braking and distance from the braking surface toward bandage interior are given in Figures 3 and 4.

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

4. CONCLUSIONS

Due to the short time of acting braking power, temperature fluctuations occur at the beginning of the tread brake. Further, as approaching the stopping of the movement, braking power is reduced and, accordingly, variations in temperature are lowered.

In case of braking tread off at the bandage surface is obtained a maximum temperature of 244 [degrees]C for case 2 (worst case). This temperature creates a compression effort to tread of 48.5 daN/[mm.sup.2]. Value of effort is far below the prescribed breaking effort for material bandages. Braking feature allows binding material to tread absolute temperature of 370--415 [degrees]C without the appearance of defects on the tread.

5. REFERENCES

Cartigny, F., Dufrenoy, P., Desmet, B., (2004). A termal andlisis of a new railway brake concept using liquid cooling, Proceeding of the Institution of Mechanical Engeeners, Part F: Journal of Rail and Rapad Transit, Vol. 218, nr.2, ISSN: 0954-4097

Stoica, M. (1998). Frdnarea trenurilor, Ed. Feroviara, ISBN: 973-97334-4-1, Bucuresti

Talamba, R, Stoica, M. (2005). Osia montatd, Editura ASAB, ISBN: 973-7725-04-2, Bucuresti

*** STAS 112/1-80 (1980). Aparate de rulare pentru vehicule de cale feratd cu ecartament normal. Bandaje pentru rofi. Condifii tehnice generale de calitate

*** STAS'99 (1999). Vehicule de cale feratd cu ecartament normal. Sabofi de frdnd. Condifii tehnice generale de calitate.--SR 6289:1999, Bucuresti, Ed. Tehnica

Tab.1. Preliminary values

Crt.
no. Name Notation Calculating relation

1 Braking time [t.sub.b] [sec] [t.sub.b] = - v/v *

2 Wheel braking [F.sub.OR] [F.sub.OR] = [pi]
 surface [[m.sup.2]] [d.sub.R][[delta]
 .sub.k]

3 Thermal loading [q.sub.a] [kJ/ [q.sub.a] = [[xi].
 [m.sup.2]h] sub.M][G.sub.R]
 [v.sup.2]/q
 [F.sub.OR][t.sub.b]

4 Fourier coefficient [F.sub.OK] [F.sub.OK] =
 for the block [at.sub.b]/
 [b.sup.2]

5 Correction factor [f.sub.FOK] --

Crt.
no. Name Value

1 Braking time 25.6

2 Wheel braking 0.31415
 surface

3 Thermal loading 5962972

4 Fourier coefficient 3.4
 for the block
 1.6
5 Correction factor