CHAPTER 7 THERMAL ANALYSIS

Transcription

1 102 CHAPTER 7 THERMAL ANALYSIS 7.1 INTRODUCTION While applying brake, the major part of the heat generated is dissipated through the brake drum while the rest of it goes into the brake shoe. The thermal energy diffuses through conduction within the brake drum and dissipates by convection and radiation from the outer surface of the brake drum. Theoretical and finite element techniques have been used to determine and compare the temperature rise during braking. 7.2 THEORETICAL ANALYSIS Temperature Rise During Braking With a high deceleration level, resulting high heat generation in a single stop, the braking time may be less than the time required for the heat to penetrate through the drum and shoe which will lead to temperature rise at the interface. The maximum surface temperature rise T max in a single stop without ambient cooling may be expressed as (Limpert 1999). T max 1/ 2 t 1/ 2 5 q" (7.1) 18 max s c k 1/ 2 d d d

2 FINITE ELEMENT ANALYSIS The temperature rise in the cast iron and the MMC brake drum are determined using the finite element analysis. The brake drum has been modeled in Pro-E and the analysis is carried out using Ansys. The maximum laden weight of the light passenger vehicle is considered for the analysis to simulate the worst possible condition. The material properties for the brake drums are given in Table Finite Element Model The model of the brake drum used for the analysis is shown in Figure 6.1. The finite element analysis software version 8.0 has been used in the present work. The model of the brake drum is divided into number of ten nodded isoparametric elements. The finite element model of the brake drum is shown in Figure Boundary Conditions The brake drum is abutting against hub at its outer face. So, the brake drum hub interface is restrained axially. The relative motion between hub bolt and the brake drum mounting hole is simulated using radial restraints on 180 sector of brake drum mounting hole. The bolt head seating area on inner side of the drum is fully restrained to simulate mounting bolt restraints. Ten nodded isoparametric element has been used for the anlysis.

3 Assumptions analysis. The following are the assumptions made for the finite element Heat transfer co-efficient remains constant. All unexposed surfaces are treated as insulated. Heat transfer due to radiation was not considered. Distribution of heat flux at lining interface was considered to be uniform. Variation of material properties with temperature was not considered Clock Mechanism A clock mechanism has been developed (Ramachandra rao 1993) to take into account of heating and cooling in each revolution of the brake drum. Heat is generated for the time interval during which the brake drum moves from one end of the lining shoe with its varying velocity (Figure 7.1). In actual brake drum assembly of passenger car, the brake drum is rotating and the shoe is stationary. For the finite element method, the brake drum is considered as stationary and the shoe is assumed as moving. The heat generated due to friction is converted as heat flux (Equation 3.8) and applied at the inside of the brake drum. The heat flux is assumed to be moving along the inner surface of the brake drum. The maximum heat flux at the inside of the brake drum is given by P q max max A (7.1)

4 105 Figure 7.1 Clock mechanism 7.4 RESULTS AND DISCUSSIONS The temperature distribution in the cast iron and the MMC brake drum is shown in Figure 7.2 Figure 7.3 respectively. The maximum temperature rise in the cast iron and in the MMC brake drum has been observed as 208ºC and 193ºC respectively. The temperature distribution in the cast iron and the MMC brake drum along their thickness are shown in Figure 7.4 and Figure 7.5 respectively. The temperature gradient is observed more for the cast iron. The comparison of temperature distribution in cast iron and MMC brake drum is shown in Figure 7.6. It is observed that the temperature gradient in the MMC brake drum is less. It indicates the capability of MMC brake drum to transfer heat at faster rate. The temperature rise at the surface of the cast iron and MMC brake drum is shown in Figure 7.7. In MMC, the temperature rise is observed to be lower because of the rapid dissipation of heat along the thickness of the brake drum.

5 106 Figure 7.2 Temperature distribution in cast iron brake drum Figure 7.3 Temperature distribution in Al MMC brake drum

6 107 Figure 7.4 Temperature distribution in the cast iron brake drum Figure 7.5 Temperature distribution in the MMC brake drum

7 108 Figure 7.6 Comparison of temperature distribution in the brake drums 506 Surface Temperature, K CI Al-MMC Braking Time, sec Figure 7.7 Comparison of surface temperature in the brake drums

8 CONCLUSIONS The theoretical and the finite element analysis have been performed for both cast iron and the MMC brake drum and the results are shown in Table 7.1. Table 7.1 Temperature rise in brake drums Brake drum Theoretical Analysis ( o C) Finite Element Analysis ( o C) Cast iron brake drum MMC brake drum The following conclusions can be made from these analyses. From theoretical analysis the temperature rise in MMC brake drum is found to be 21 o C less than the cast iron brake drum. The heat dissipation rate in the MMC brake drum is also higher than cast iron brake drum. So, a rapid reduction in temperature is observed. The temperature rise in brake drum is computed for both cast iron and the MMC brake drum using clock mechanism for the same braking conditions. In this analysis also, the temperature rise in MMC brake drum is found to be 15 o C less than the cast iron brake drum.

9 110 The temperature distribution along the thickness of the brake drum is computed for the cast iron and MMC brake drum at different time intervals. It is observed that the temperature gradient of MMC brake drum is less than the temperature gradient in cast iron brake drum. From this, it can be concluded that the MMC brake drum is capable of transferring heat at faster rate.