Finite element analysis of the thermal behaviour of single-disc clutches during repeated engagements

Most of the failures in the sliding systems occur due to the high thermal stresses, which generated at the interface between the contacting surfaces due to sliding between parts, such as friction clutches and brakes. In this paper, the thermal behaviour of a single-disc clutch is investigated. The surface temperatures of the friction clutch disc will be increased during repeated * System Technologies and Mechanical Design Methodology, Hamburg University of Technology, Germany. ** Department of Mechatronics, AL-Khwarizmi college of Engineering, University of Baghdad, Iraq. *** Department of Mechanical Engineering, College of Engineering, University of Baghdad, Iraq. T R I B O L O G I A 2-2016 10 engagements, in some cases, will lead to premature failure of the clutch disc. In order to avoid this kind of failure, it the surface temperature should be calculated with high accuracy to know the maximum working temperature of the friction system. In this work, the temperature distributions are computed during four repeated engagements at regular intervals (5 s) for the same energy dissipation. Three-dimensional finite element models are used to simulate the typical friction clutch disc. List of Symbols and Abbreviations Symbol Description Pmech. input The mechanical energy input to the clutch system Pmech. output The mechanical energy output from the clutch system Ptherm. output Thermal energy losses t Time ts Sliding time Ti The initial temperature of the clutch system Tmax The maximum temperature of the clutch system tmax The time corresponding to the maximum temperature Tmax Tf Final temperature of the clutch system at the end of clutch cycle t t c gen Q ∆ + . , The energy generated on the clutch disc t t c conv Q ∆ + . The heat transfer by convection of clutch disc ∆QInt. energ.c The change in the internal energy of a clutch disc V The volume c The specific heat ρ Density of body QI. int. energ. Initial internal energy of the clutch disc [C] The heat capacity matrix [K] The heat conductivity matrix {F} Thermal load Vele Volume of element Tele. (t) Temperature of element at time ( t) Tele. (t+∆t) Temperature of element at time ( t) INTRODUCTION Friction clutches and brakes are considered to be the most common type used in automotive application. Two or more surfaces are pressed together by a normal force to create a friction torque. The friction surfaces could be flat and . 2-2016 T R I B O L O G I A 11 perpendicular to the axis of rotation. Figure 1 shows the parts of typical singledisc clutch system. In the design of friction clutches, it is very important to calculate the temperature distribution on the contact surfaces accurately during the slipping period when the clutch starts to engage. High thermal stresses in the contact surfaces produce several disadvantages, such as surface cracks and permanent distortions, and they are likely to lead the friction facings to failure, before the expected lifetime of the clutch. For these reasons, investigations of the effects of the repeated engagements on the thermal behaviour (maximum temperature) of the friction clutches are necessary. Abdullah and Schlattmann [L. 1–8] investigated the temperature field and the energy dissipated of a dry friction clutch during the beginning of engagement, assuming uniform pressure and uniform wear conditions. They also studied the effect of contact pressure between the contact surface when varying with time on the temperature field and the internal energy of the clutch disc using two approaches: the heat partition ratio approach to compute the heat generated of each part individually, and the second approach that applies the total heat generated of the whole model using the contact model. Furthermore, they studied the effect of engagement time, sliding speed function, thermal load, and dimensionless disc radius (inner disc radius/outer disc radius) on the thermal behaviour of the friction clutch in the beginning of the engagement. Yevtushenko et al [L. 9] investigated a transient thermal problem of threeelements (disc/pad/calliper) with time-dependencies. The effects of Biot number and the duration when the pressure is increasing (from zero at the initial moment of time to the nominal value at the moment of a stop) on the values of the temperature of the cast iron disc/metal ceramic and pad/steel calliper have been studied. The research results showed that the effect of Biot number will reduce the heat transfer through the contact surface. Seo et al. [L. 10] suggested a thermal model to estimate the temperature distribution of a wet clutch in 4WD coupling to avoid the thermal failure of the clutch plate during the operating condition of the vehicle. The results of this model are validated with the experimental results of the actual 4WD vehicle under the limitation of torque 900 [N.m]. The theoretical results have shown acceptable agreement with the experimental results. Adamowicz and Grzes [L. 11] studied the influence of convective heat transfer on the transient temperature distribution of a real disc brake. They used the finite element method to investigate an impact of the heat transfer coefficient on the amount of heat dissipation from the solid disc rotor of a disk brake. The results showed that, in terms of a single braking process for the specified dimensions and thermo-physical properties of materials, the amount of convection heat transfer does not allow significantly reducing the temperatures of the rotor. However, in the following release period, after the . T R I B O L O G I A 2-2016 12 braking action when the velocity of the vehicle remains on the same level, a considerable decrease in temperature values has been observed. Yevtushenko and Kuciej [L. 12] solved the transient heat conduction problem analytically and numerically. They computed the surface temperature of a tribosystem consisting of the pad sliding with time-dependent velocity on a disc surface. The two-dimensional finite element model was used to study the thermal stresses during the braking process and the effect of boundary conditions on the temperatures and thermal stresses of the tribosystem. They concluded that the value of thermal stresses decreases with a significant increase of braking time. This paper highlights the importance of the effect of the repeated engagements on the thermal behaviour (the magnitude and distribution of surface temperature) of the friction facing of clutches, assuming the pressure is uniform between the contact surfaces.