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Thermal Investigation of Disc Brake Fade During Long Braking

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Thermal Investigation of Disc Brake Fade During Long Braking Journal of Interdisciplinary Cycle Research ISSN NO: 0022-1945 Thermal Investigation of Disc Brake Fade During Long Braking Anant W. Nemade1* , Samir A. Telang2 , Arvind L. Chel3 1Lecturer, Dept. of Mechanical Engg. Government Polytechnic, Aurangabad (M.S) India-431005 2Lecturer, Dept. of Automobile Engg. Government Polytechnic, Aurangabad (M.S) India 3Associate Professor, Dept. of Mechanical Engineering, MGM’s Jawaharlal Nehru Engg. College, Aurangabad (Maharashtra) India *Corresponding Author: [email protected] Abstract: Brakes are used to inhabit the motion of a vehicle. During braking kinetic energy of vehicle is converted in to heat energy due to friction between pads and rotor. There should be sufficient provision for heat dissipation otherwise heat accumulation will result in to rise in temperature. In case of disc brake, they are open to atmosphere therefore more convection takes place than in drum brake. Vehicle is in motion which forces air on the brake disc resulting in to fast cooling. To increase the surface area brake disc of light motor vehicles are made up of two plates with small joining ribs between them called ventilated disc. These types of brake disc braking system are presently used in all most all light motor vehicle and are working satisfactorily for small braking time. But in modern vehicles the speed of the vehicles is increasing as per the modern generations demands, braking to these vehicles from high speed requires more time therefore braking time increases, which results in to more friction and high heat generation sometimes may be above critical temperature. This paper investigates the heat generation in long braking time, its effect on friction pad material and braking performance of vehicle. Key Words: Disc Brake, Heat, Braking Time 1.0 Introduction that µ goes on decreasing [1]. This is termed as brake fade and vehicle will not stop at expected location. Braking is an active safety provided to every vehicle. Heat generation measurement in braking is a very The braking inhabits the motion of the vehicle with complicated process [2]. During braking vehicle is the help of friction between rotor and friction pads. In also moving with high velocity therefore convection light motor vehicles rotors are made up of cast iron rate is also high. Exact temperature rise for a and friction pads are made up of combination of particular instance is required to be noted in dynamic different metal powder. When friction takes place condition [3]. In this paper experimentation is carried between rotor and friction pads kinetic energy is out on the test rig specially designed for measuring converted in to heat energy. If this heat energy heat generated during braking with constant source of increases beyond the critical temperature then it forced air on ventilated brake disc. With constant affects on the different components of braking system braking pressure every time the effect of temperature such as friction pads, braking fluid, piston of calliper, is investigated on friction pads. calliper, braking disc/rotor etc. friction pads are manufactured from the composition of different powders and are compressed to form a friction liner of 2.0 Theoretical Kinematic Friction required thickness with predefined coefficient of Coefficient friction (µ). The heat generated during braking for a long time results in to weakening of bond in friction Coefficient of friction between brake disc and friction material and will loos the coefficient of friction of pads depends upon too many factors such as oil pads. Andrew Day in its book “Braking” investigated pressure, temperature generated, rotating speed of disc that up to certain extent of rise in temperature and other material compositional factors of disc and coefficient of friction goes on increasing but beyond pads [4]. Volume XII, Issue VIII, August/2020 Page No:1402 Journal of Interdisciplinary Cycle Research ISSN NO: 0022-1945 The friction force generated at contact surfaces of 2.2 Temperature Generated on Disc brake disc and pads can be determined as it depends Surface upon the coefficient of friction and the normal force acting on it [5], [6]. It can be calculated as, Temperature Generated on disc surface can be calculated by using thermal relation [9]. Ff = µFn Heat Generated Q = M x Cp x T J/s Fn Can be known from the oil pressure (P) gauge installed on the experimental setup M is mass of disc in KG, Cp is specific heat capacity J/kg0c , T is time taken to stop the vehicle Fn = P X(piston area) A And heat flux q = Q/A W/m2 Braking torque can be calculated from the equation A is the area of disc = (R2-r2) TB = Ff re And Thermal Gradient K = q/k K/m Where re = effective radius of friction pad. Above terms in Torque equation can be written as. 3.0 Experimental Work TB = µPAre For caring out experimentations to investigate brake fade due to heat generation in brake disc an For two pair of friction pads this equation can be experimental setup is designed. This setup enables to written as measure the temperature generated during braking by using rubbing thermostat [10]. The setup is fitted with TB = 2µPAre hydraulic braking system to apply brake, pressure And from setup the values of P, A, TB, re can be measuring gauge installed in line. Electric motor is determined. Therefore, coefficient of friction between used to drive the brake assembly with inertia load. brake disc and friction pads can be determined as Brake disc can easily be installed on setup. To create the realistic situation force air cooling is provided on µ = TB/2PAre brake disc [11]. 2.1 Thermal Stresses Induced in Brake Disc Disc and friction pad is a compact assembly, due to friction between them heat is generated at contact surfaces this heat induces thermal stresses in disc as there is less chance to expand the material due to continuous rotation of disc as vehicle is in motion. These thermal stresses mostly induced because of the variation of temperature [7] [8]. This can be obtained by equation σ = Whereσ is thermal expansion coefficient of brake disc, E is Young’s modulus, is poisons coefficient, is difference between local disc temperature and Fig 1: Experimental setup environmental temperature, depends upon thermal stress with temperature. For experimentation initial temperature of brake disc is considered at room temperature, and then with the help of electric motor brake disc is rotated at a 2540 Volume XII, Issue VIII, August/2020 Page No:1403 Journal of Interdisciplinary Cycle Research ISSN NO: 0022-1945 RPM measured by tachometer. The inertia weight is Table1. Chemical composition of brake pads 42 kg. After rotating the brake disc sufficient time is Sr. No. Composition Weight by % given to stabilize the system. Then brakes were 01 Mg 11.1 applied at regular interval with varying time interval 02 Si 6.3 [12]. Stopping time of disc is noted after each brake. 03 Al 9.8 The braking time is increased after each braking, 04 S 5.6 temperature and brake applied pressure is recorded. 05 Ca 5.2 Thermostat can be set at any position e.g. t1, t2 or t3 06 Fe 7.6 as shown in fig 2. 07 Cu 5.8 08 Zn 13.4 09 Cr 3.5 10 Zr 0.1 11 Sn 9.3 12 C 22.3 Brake disc made of gray cast iron has chemical composition as shown in table No. 2. Table2. Chemical composition of brake disc Sr. No. Composition Weight by % 01 C 3.40 02 Si 1.70 03 Mn 0.57 04 P 0.03 Fig 2: Rubbing K type Thermocouple and locations on brake disc 05 S 0.26 Experimental setup is designed for fitting different 06 Fe 93.6 friction pads. In this experimentation two types of friction pads are used [13]. Both the friction pads are During testing some assumptions are made that there manufactured from same material composition, only is no driver’s reaction time, no system response time, they differ in surface pattern which will be in contact no deceleration rise time, no release time, but in actual with brake disc. One is flat with uniform surface practice when brakes are applied these time zones are where as other is having linings on the surface as in existence as shown in fig 4 [15]. shown in figure [14]. Fig 4: Braking time zones Fig 3: friction pads used for experimentation, flat surface and lining When driver responds to situation and move his foot surface. to pedal the time zone is t0 to t1, next when he applies braking force and reaches to tyre t1 to t2, now time to Chemical composition of brake pads is as shown in reach peak deceleration is t2 to t3, braking time till the table No. 1. vehicle stops is t3 to t4, brake release time t4 to t5 Trials were taken on the friction pads of above description. The braking time is varied and noted readings of Volume XII, Issue VIII, August/2020 Page No:1404 Journal of Interdisciplinary Cycle Research ISSN NO: 0022-1945 temperature generated at the interface of disc and pad. Fig 5: Temperature analysis at different braking time Every time the braking pressure is kept constant at 27 bar with the help of adjustment rod and air flow on the disc is also kept constant. The recorded readings were analysed in the Ansys software for obtaining the results [16]. 4.0 Results and Discussion Using Finite Element thermal analysis method, steady state thermal analysis was performed with some boundary conditions on brake disc pad assembly [17] [18]. The heat flux of 57317.72 W/m2 was applied on the surface of disc where pads are in contact with disc.
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