FEA Analysis and Correlation of Thermo-Mechanical Deformations of a Disc Brake Rotor 2015-26-0206
Brake is a critical safety aspect of an automobile and hence its failsafe design is of prime importance. Modern vehicles, especially cars and SUVs, with high speed and high acceleration capacities require the braking system also to be equally robust and stable in all critical driving conditions. Typically, during braking, front brakes take up higher torque than rear brakes due to vehicle dynamics and associated load transfers. Generally, these are disc brakes using cast iron rotors. Rotor absorbs heat energy generated during braking and dissipates it into components in its vicinity and largely, into surrounding air. Due to stringent target of overall reduction in weight and cost of an automobile, rotors are fairly optimized on these aspects with a balance of needed braking performance. This optimization in weight of rotor limits the thermal absorption equally of rotor i.e heat dissipation. Also, a rotor designed too weak on thermal aspects can succumb to heat deformations and may crack in extreme conditions, thereby causing risk to the vehicle safety. Moreover, NVH attributes of brake systems are becoming increasingly important due to reduction in power train noise of IC engines, introduction of hybrid and battery operated vehicles with extremely low interior as well as exterior noises. NVH attributes for brakes are often judged more as a customer comfort and an implied need, unlike legislative requirement of friction performance. Typical brake NVH phenomena like squeal, moan, groan, hot judder are of major concerns depending upon the field usage pattern in a particular geographical area. One of the above concerns which are largely affected due to rotor heat dissipation is hot judder. Rotor at elevated temperatures tends to bend on one side of face along its axis of rotation due to thermo-mechanical stresses and associated deformations. This causes a lateral displacement fluctuation along rotation path of rotor braking face. As brake pads are rigidly in contact with rotor during braking, the displacement variations of rotor are transferred into linear displacement of pads. This in-turn creates pressure fluctuations within brake fluid path and can be felt as pulsations on pedal due to amplification. The phenomenon is called as hot judder. Limiting maximum lateral displacement of rotor thus becomes essential from strength as well as NVH perspective.
Taking above into consideration, this paper details a methodology of calculating thermal behavior of rotor including its temperature rise and associated thermal deformation using FEA (Finite Element Analysis) tools. FEA Calculations based on convective heat dissipation assumptions have been discussed for an indentified rotor and its correlation has been established with test data. In addition, above thermo-mechanical parameters are compared for an inboard and outboard neck connection type rotors using FEA only and observations are discussed.