Braking efficiency is characterized by reduced braking time and distance, and therefore passenger safety depends on the design of the braking system. During the braking of a vehicle, the braking system must dissipate the kinetic energy by transforming it into heat energy. A too high temperature can lead to an almost total loss of braking efficiency. An excessive rise in brake temperature can also cause surface cracks extending to the outside edge of the drum friction surface. Heat transfer and temperature gradient, not to forget the vehicle's travel environment (high speed, heavy load, and steeply sloping road conditions), must thus be the essential criteria for any brake system design. The aim of the present investigation is to analyze the thermal behavior of different brake drum designs during the single emergency braking of a heavy-duty vehicle on a steeply sloping road. The calculation of the temperature field is performed in transient mode using a three-dimensional finite element model assuming a constant coefficient of friction. In this study, the influence of geometrical brake drum configurations on the thermal behavior of brake drums with two different materials in grey cast iron FG200 and aluminum alloy 356.0 reinforced with silicon carbide (SiC) particles is analyzed under extreme vehicle braking conditions. The numerical simulation results obtained using FE software ANSYS are qualitatively compared with the results already published in the literature.

The thermal behavior phenomena have been studied widely in different fields, as presented in References [

The dynamic behavior of the braking system is modelled in order to be able to determine the braking power developed in a drum brake. The braking forces on the front and rear wheels resulting from the friction forces of the brake drums are in opposite directions to the vehicle movement, as shown in

Kinetic energy

The work of the resistant forces acting on the vehicle is equal to:

From the above equations, it follows:

By deriving

Taking into account the brake power distribution of the vehicle, expressed by

When the wheel closes to lock the braking condition, the tires will have a certain amount of the slip rate (s); a part of the friction heat will be converted to friction heat between the tire and the road. The optimal value of the slip rate is between 0.05 and 0.20 [

During stopping braking, the drums absorb about 95% of the heat, and the brake shoes 5%. The relation

Another important thermal factor is the efficiency with which the braking system converts the movement of the brake drum into heat and, therefore, the dissipated heat rapidity by the brake. Note that disc brakes are entirely exposed to the surrounding atmosphere, and drum brakes are completely enclosed within the brake assembly. This can result in a relatively higher temperature compared to the disc brake system under the same braking conditions. The high temperature of the drum brake shoes can cause the brake to fade and eventually lose its effectiveness. The discoloration is the result of too much heat accumulation in the drum [

During the braking process, the friction heat released in the drum brake is dissipated in two different ways: in both bodies in contact by conduction and accumulation in the materials and in the environment by convection and radiation. Thermal conduction exists between the drum and the brake linings because the friction interface generates heat. As already mentioned above, the distribution of the heat generated on contact between the two different friction materials is unequal because the thermal properties of the two components are different. This heat distribution is described by the heat flow distribution coefficient α, which is determined for the temperatures achieved on the two contact surfaces. It defines the proportion of heat flow absorbed by the brake drum. The relations

Thermal radiation is related to the temperatures of the drum's outer surface; the higher the temperature, the greater the heat dissipation by thermal radiation. In general, the heat dissipated by thermal radiation is about 5%–10% of the total heat dissipated. Thermal radiation only has an important role at high temperature and low velocities. The radiation heat dissipation is defined by:

^{−8} W/(m^{2}⋅K^{4}).

The heat flow balance in the brake drum can be written as follows:

A complete three-dimensional structure of the drum brake has to be modelled. Three-dimensional models of the brake drums were built using the commercial simulation software ANSYS. The axisymmetric condition has been considered for the geometrical modelling and numerical simulation of the thermomechanical behavior of the brake drums. In order to evaluate and optimize the thermomechanical behavior of the brake drums and the heat release efficiency, four models are selected with the exact main dimensions, a basic brake drum, a brake drum with a groove on the front face, a drum with a circumferential fin and a drum with longitudinal fins, see

A three-dimensional structure of each drum brake model, composed of the drum, head pad and tail pad, must be modeled in 3D finite-element. For solid modeling, three-dimensional isoparametric tetrahedral elements with 10 nodes are used, which are appropriate for the analysis of the transient thermal flow in a circular axisymmetric structure. It is assumed that the connection between the lining and the shoe is uniform. This condition also applies to the contact areas between linings and drum. Fixed bolts are modeled using cylindrical support elements. The brake shoes can then rotate around the fixed bolts see

The chosen simulation conditions of the thermal behavior analysis of a truck brake drum are presented in

Mass of the vehicle laden | 9320 kg | Drum external diameter | 455 mm |

Rolling tire coefficient | 0.0150 | Drum internal radius | 412 mm |

Rolling radius of tire | 533 mm | Drum overall width | 346 mm |

Downhill slope | 10% | Hole number | 10 |

Initial vehicle velocity | 100 km/h | Bore diameter | 23 mm |

Vehicle deceleration | 4 m/s² | Shoe width | 180 mm |

Braking time | 7.0 s | Lining angle | 96° |

Slip rate | 0.08 | Friction coefficient | 0.23 |

Initial drum temperature | 40°C | Ambient temperature | 25°C |

Designation | Nodes | Elements |
---|---|---|

Drum | 4.128.907 | 2.693.060 |

Lining | 160.085 | 31.536 |

Shoe | 909.537 | 589.779 |

The brake drums as brake discs must be manufactured with a material that favors their thermal resistance with a high friction coefficient to generate the required friction force [

Mechanical properties | Drum | Lining | Shoe | |
---|---|---|---|---|

Material | FG200 | Al-MMC356.0 SiC | Semi metallic composite | Steel |

Density (kg/m^{3}) |
7100 | 2600 | 1034 | 7850 |

Thermal conductivity (W/m^{2} K) |
54 | 60.5 | 1.01 | 60.5 |

Specific heat (J/kg K) | 586 | 874 | 1034 | 434 |

In the thermal modeling of the four selected brake drum models, the convection coefficient h = h (A, t) of each heat exchange area of the drum is first computed using the ANSYS CFX software. ANSYS CFX Preprocessor imports the mesh generated of the domains and defines the flow physics, the border states and the parameters of the SOLVER module. All problem specifications produced in the ANSYS CFX-Pre-module are solved by ANSYS CFXSolver. To compute the heat transfer coefficient of each convective surface and at each time, the solver uses the following relation [

For thermal modelling, we consider the drum lining and clamps assembly, although we only want to determine the drum's temperature field. Thus, an indirect coupling of the thermal and mechanical model will be carried out, which implies that the brake shoes must be considered in the geometry to be able to take into account the pressure applied on the drum. The drum rotates around cylindrical support with a decreasing variable rotational velocity. Each brake shoe is fixed to a cylindrical bolt having one degree of freedom to transmit the force applied at the other end. A time-varying heat flow will be introduced to each drum-to-lining contact area. ANSYS-software will first solve the thermal problem and then the mechanical problem by exploiting the thermal results. The same mesh as in the CFX simulation will be used. For the determination of the temperature field, a transient thermal simulation is performed in ANSYS-Workbench. The parameters of this simulation are the total simulation time (^{2}⋅K^{4}), the average heat transfer coefficient during the braking process was imported from the CFD simulation. The inlet heat flux as a function of braking time was introduced in tabular form. The computing simulation results in

The brake drum temperature evolution over time of the four models is shown in

The temperature field in the brake drum has been computed for two different materials, namely FG200 grey cast iron and 356.0 SiC aluminum alloy. 2.

This study showed the positive effect of some practical measures on improving the heat dissipation of a heavy vehicle brake drum. The thermal energy stored in the reference brake drum model is the main cause of brake drum thermal problems. It should be noted that extreme braking conditions such as initial braking speed, vehicle load, braking on a steep slope result in very high drum temperatures that exceed the allowable temperatures. These high temperatures cause brake fading and therefore an undesired loss of brake performance and a significant increase in braking distance. In addition, excessive heat transferred to the brake fluid can even lead to its evaporation. The addition of longitudinal or circumferential fins on the modified brake drums has contributed significantly to the improvement of heat dissipation and structural strength without changing the initial main geometric dimensions of the original brake drum model. The simulation results obtained with the modified brake drum models can be used to orient automotive engineers in the development of other, more efficient brake drum models. The results obtained from the numerical simulation of the different brake drum models are similar to those of the previous comparable models studied. The absolute maximum drum temperature was reached at mid-point of the braking time, regardless of the model variant studied. The introduction of ventilation fins has favored heat transfer by convection and, therefore, the drum brake cooling. The temperature difference between a basis drum and a modified drum with longitudinal ventilation fins reached a value of 43°C at intermediate braking time, a decrease of 7%. This temperature difference is even higher with a circumferential finned drum in the order of 55°C, a reduction of 9.5%. This theoretical study also showed that the choice of brake drum material significantly influences the thermal behavior of the brake drum. Al MMC brake drums have a better thermal braking behavior than those made of grey cast iron. As a general conclusion, it is always possible to improve the brake drums’ thermal behavior and thereby avoid brake fading by modifying the aerodynamic design and the size of the original model and choosing the appropriate drum material.

The authors acknowledge the financial support of University Center Salhi Ahmed Naama, Algeria. The authors wish to express their gratitude to Van Lang University, Vietnam for financial support for this research.