Materials and Manufacturing Process Selection: Brake Disc Design - Case Study Example

BRAKE DISC By: + Brake Disc Design Each and every vehicle system is carefully developed to so as to meet the safety requirements. The most critical component of a vehicle system is the brake system. Keen attention needs to be paid during the analysis of a vehicle brake system. Without a properly designed brake system, the passengers will be in a very dangerous position. Therefore, for the purposes of safety of the passengers and the vehicle, all the vehicles must be equipped with properly designed brake systems. This research paper aims at designing the vehicle brake disc, the major materials used in manufacturing of the brake discs; the main factors considered when choosing the materials for brake discs and the general working principles of the brake discs. The paper will also look at general calculations involving operations of the brake discs. Parameters such as shear forces, normal forces, and piston forces will be calculated based on different materials suitable for brake discs. The main objective of this research is to study the possible improvements that can be undertaken on the existing brake discs. The paper is supposed to inform the King Swan Multinational Engineering Firm on such possible improvements. Introduction The brake disc is a wheel in a vehicle braking system. The main function of the brake disc is to slow down or stop a vehicle on a motion. It achieves this function by slowing down the rotation of the vehicle’s wheel through frictional force between the brake pad and the wheel. The brake pads are pushed towards the brake disc with the help of calipers. The brake disc, also known as rotors in American English, is normally made of cast iron. In some cases, composites materials such as ceramics matrix or reinforced carbon– carbon composites. The disc is then connected to the vehicles wheel and the axle. In order to stop the wheel, frictional materials known as brake pads are mounted onto another component known as brake calipers. The use of disc brakes was discovered in England in the early 1890s. (Baker, 2012). The first brake calipers were discovered by William Fredrick in his Birmingham, United Kingdom firm. This caliper was first used on Lanchester cars. The disc brakes relatively provide better stopping performances as compared to drum wheels. This is because brake discs can easily be cooled. The brake discs are composed of cast-iron discs, which are bolted onto the hub of the wheel and housings known as calipers. The calipers are normally connected to a stationary part of a car such as the stub axle or the axle casting, normally in two parts. Each part contains a piston. There is a friction pad, which is normally held firmly in between the discs and the pistons. The friction pads are held in position with the use of either spring plates or retaining plates. There are holes drilled through the calipers so as to allow the fluid to flow in and out of each system. Each cylinder normally contains rubber sealing ring in between the piston and the cylinder. A schematic diagram in the next page is a schematic of a brake disc showing brake disc. Figure 1: Schematic of Disc Brake showing Brake Disc Working Principles of the Brake Disc and the entire Assembly The diagrams below shows the Brake Disc at work. Figure 2: Operation of the Brake Disc and the entire Brake System The working principle can be explained stepwise as shown below: 1. As shown in figure 2, the driver applies force on the brake pedal. The force applied reduces the volume in the master cylinder. As a result of reduced volume of the master cylinder, pressure is created in the cylinder. With the assumption that the brake fluid is incompressible, the pressure is transmitted equally throughout the fluid. The pressure is transmitted to the regions of low-pressure concentration. 2. The brake fluid transmits the pressure to the calipers as shown above. The regions around the calipers have low-pressure concentration. 3. The brake fluid is forced through into the calipers causing a pressure build-up around the calipers. As a result of this, the pistons are pushed outwards. 4. The Pistons, in turn, applies a force corresponding to the pressure applied. This pushes the brake pads towards the brake disc. 5. Friction is then created due to the rubbing between the brakes pads and the disc. 6. Due to increased friction between the brake disc and the pads, the movement of the disc is slowed down. Since the car tires are connected to the disc, the motion of the car is in turn slowed down. 7. The brake pedal is then released once the desired speed has been achieved. The release of the brake pedals increases the volume of the fluid in the master cylinder. The fluid will then travel back to the master cylinder. As a result of this, the pistons move backwards and the brake pads move away from the disc as shown in figure 3 (Baker, 2012). Figure 3: The working Principle of a Disc Brake Materials Selection for Brake Disc The most commonly used brake disc material is the grey cast iron. The used cast iron, however, comes with some challenges. The grey cast iron has a relatively high specific gravity. This, therefore, makes the vehicle consume a lot of fuels. This section aims at developing optimum materials selection with a view to finding the most suitable material to replace the grey cast iron. Apart from the material’s specific gravity, there are also other factors considered when selecting a material for a specific engineering application. These factors include cost per unit property, the economic importance of the product to be designed, and the materials availability, among other factors. The mechanical properties of the materials commonly considered include wear resistance, the coefficient of friction, compressive strength, and thermal conductivity. These are the key parameters used when selecting materials for engineering applications. Other than cast iron, the other candidate materials for use in brake disc include Titanium Alloys and Aluminum Matrix Composite. (Dike, 2006). In order to achieve a reduced fuel consumption as well as the vehicle’s weight, the automobile industry has intensely adopted the use of Aluminum –based composites reinforced with ceramics. (Dike, 2006). These materials have a lower specific gravity and low thermal conductivity as compared to grey cast iron and Titanium The use of Aluminum-based composites reduces the weight of an automobile by around 50-60 percent. Also, these materials have the ability to perform in extreme service conditions, such as higher load, higher speed, and higher temperature. In order effectively select the most suitable material for brake disc, CES Edupack Material selector is normally used. Figure 4 shows the material selection procedure using CES. (Limpert, 2011). Figure 4: CES Procedure for Material’s Selection Services done on the Disc Brake discs are commonly made of grey cast irons. Disc are, therefore, mainly damaged through excessive rusting, cracking, scaring, or warping. The services done on the disc are mainly in response to ant disc issues by changing the entire disc. This is mainly done where the cost of acquiring a new disc is relatively lower than the cost of repairing the existing disc. Mechanically, this is always not necessary. This should be applied in situations where the disc thickness has gone below the minimum manufacturers specification. When the disc thickness goes below the minimum required thickness, it becomes unsafe to use. Most of the leading car manufacturers normally recommend brake disc skimming (or turning as it known in the United States) as a workable solutions for vibration issues, lateral run out, brake noises. (Limpert, 2011). Calculations involved in Brake Disc The forces acting on the outer and inner disc faces as a result of the disc contact with the brake pads are as shown in the figure below Figure 3: Forces acting on the Brake Disc Using Standard Brake Parameters. Brake Disc Diameter = 240 mm. Material of the Brake Disc = Aluminum Ceramic Area of the Pad Brake = 2000 mm2 Material of the Brake Pad = Asbestos Coefficient of Friction (Wet) = 0.08-0.14 Coefficient of friction (Dry) = 0.3-0.6 Maximum allowable Operation Temperature= 240 ºC Maximum Allowable Pressure =.1.0MPa Tangential force between the brake disc and the pad (inner face), FTR1 FTRI = µ1.FRI FTRI is the normal force between the disc and the pad. µ1 is the coefficient of friction = 0. 6 FRI = x Therefore, FTRI = µ1.FRI FTRI = 0.6 x x FTRI = 600 N. Tangential force between the Brake disc and the brake pad (outer face), FTRO. Here, the FTRO is equal to FTRI since the material and the normal force are the same. Brake Torque Assuming that the coefficient of friction is the same and normal forces FR on the outer and inner faces are equal, Brake Torque = R x FT Where FT = Total normal forces on the brake disc Total Normal forces= FTRI + FTRO => FT = 600+ 600 = 1200N R = Diameter of the disc => TB = 1200x 120 x 10-3 TB = 120 N.m The Brake Distance, x Given the tangential force, we can calculate the work done by the brake. Work is done by the brake= FT. x.................. (1) => FT = FTRI + FTRO x is known as the retarding distance before the vehicle finally comes to rest. The work is done by brake = change in kinetic energy Kinetic Energy, K.E, =................ (2) Where, m= mass of the vehicle v= change in velocity For us to determine x, we equate the two equations = FT. x x= x can, therefore, be calculated for any value of m and v. Heat Generated on the disc, Q. Q = M .Cр. ΔT J/S Heat Flux q = Q/A W/m² Thermal Gradient (K) = q / k K / m For Aluminum-Ceramic Composite, Heat generated is given by Q= m x cp x ∆T Taking the following parameters, Taking the mass of the disc to be 0.6 kg Specific Heat Capacity = 742 J/kgoC Time is taken to stop the vehicle = 4sec Temperature variation = 15K Q = 0.6x 742x 15= 6678J Area of the disc rea = Π (R2 – r2) = Π (0.1202 – 0.0552) = 0.03574m2 Brake Disc Development Cycle include the following: 1. Do market research 2. Develop a prototype of the design 3. Perform a test on the prototype in the field 4. Perform an evaluation on the design based on the field tests 5. Do a modification on the design based on the field results. (Baker, 2012). Conclusion The brake disc is a vital component of a brake system. The materials used must have reliable and stable wear and frictional properties under various conditions of speed, load, environment temperature and high durability. The most significant consideration made when selecting a brake disc material is the ability of the material to withstand high frictional force without wearing out. Another consideration is the materials ability to withstand high temperatures that are normally associated with high friction. (Dike, 2006). The disc must be able to possess high thermal energy so as to minimize distortion cracking or distortion, which arises as a result of thermal stresses. The disc machining process is conducted in a lathe machine, which produces discs with high precisions. Proper machining of the brake wheel will ensure maximized mileage on the vehicle before the disc wears out. Braking systems normally depend on frictional force to stop the vehicle. Most of the modern vehicles use hydraulic brake systems. In the hydraulic brake system, hydraulic pressure is applied on the brake pads. (Dike, 2006). The brake pads are in turn pushed against the brake disc pushes the brake disc against the drum. As the car is decelerated, the load is normally transferred to the front wheels. It, therefore, means most of the work is done by the front wheels .Scarring may occur as a result of failure to promptly change the brake pads when they reach the end of their lifecycle. Cracking is mostly limited to discs with drilled holes. The holes may create tiny cracks around the disc edges. This is normally due to uneven expansion of the brake pads. Given that the brake disc are commonly made from grey cast iron rusting is a common phenomenon. At times, pitched squeal or brake noise occurs when the forces are applied on the brake. References Alley, W. and Billiet, W. (2007). Disc and drum brake service. Chicago: American Technical Society. Baker, A. (2012). Industrial brake and clutch design. London: Pentech Press. Budinski, K. (20011). Engineering materials. Reston, Va.: Reston Pub. Co. Chen, F., Tan, C. and Quaglia, R. (2006). Disc brake squeal. Warrendale, Pa.: SAE International. Dike, G. (2006). On disc brake design with special reference to temperature, stresses, and cracks. Lund. Limpert, R. (2011). Brake design and safety. Warrendale, PA: SAE International. Mott, R. (n.d.). Machine elements in mechanical design. Storer, J. and Haynes, J. (2009). Ford Focus automotive repair manual. Sparkford, Nr Yeovil, Somerset, England: Haynes Pub. Group. . APPENDICES: Materials properties Appendix A: Strength vs. Density Appendix B: Young Modulus vs. Strength Appendix C: Thermal Conductivity vs. Thermal diffusivity Appendix D: Strength vs. Maximum Service Temperature Appendix E: Strength vs. Relative Cost per unit volume Read More