Brake Disc
Abstract
Safety is a key consideration in any engineering design. In motor vehicles, the most vital part is the brake system. The design of motor vehicle brakes requires keen attention. In the brake system, the most critical component is the brake disc. The brake disc should be designed in such a way that it withstands the heat produced due to friction between the brake pads and the disc. Without proper selection of the material used for the brake disc, the entire brake system might fail since the disc forms the largest part of the brake system. This might put the life of the passengers at risk. The CES Edupack software is normally used to select the most suitable material for the brake disc. Several factors are considered when choosing a material for brake disc. These include the specific gravity of the material, thermal conductivity, wear resistance and the material’s cost per unit volume. This research work focuses on the design of the brake disc and the entire brake assembly, the most commonly used material in the design and the calculations that are involved in the brake system. This research is supposed to sufficiently inform the King Swan Engineering Firm on the brake disc. The Company may, therefore, use the knowledge derived from this report to improve on the current existing brake disc.
Introduction
The brake disc is a metallic wheel in a brake system against which the brake pad rub. As the pad is pushed against the disc, heat is generated. The disc should there have the capacity to withstand the heat due to friction. The function of the disc is to stop the vehicle when it comes into contact with the brake pads. The brake disc is connected to the vehicles tire by means of bolts and nuts. The brake pads are normally pushed against the disc via the brake calipers. The brake discs are commonly made of grey cast irons. The cast iron, however, has some shortcomings such as high specific gravity and high heat conductivity. Application of brake discs is traced back to early 1890’s. The disc was first discovered by a scientist known as Fredrick William. The brake discs have better performance as compared to wheel drums. This is attributed to the fact that brake discs come with ventilation holes through which air circulates to the entire disc. The brake disc is composed of a brake disc bolted to the axle or hub of the wheel hubs and the housing called calipers. The calipers. The calipers are connected to the axle casting in two parts. Each of the parts has a piston. In between the piston and the disc, there is a friction pad held firmly with retaining plates or spring plate. The calipers has holes drilled so as to enable the free circulation of fluid in and out of the brake system. The diagram below shows a schematic of the brake system with the disc fitted.
Working Principles of the Brake Disc and the entire Assembly
As shown in the figure 2, when the driver applies a force on the brake pedal, the volume of the fluid in the master cylinder reduces. The pressure in the cylinder, therefore, increase due to increased particles collisions. This creates a pressure gradient. The fluid, therefore, moves from a region of high pressure to a region of low pressure as shown in the diagram. Given that the brake fluid is incompressible, the pressure is transmitted evenly throughout the fluid. The pressure is transmitted to the calipers as shown in the figure. The regions are at low pressure. The pressure is therefore transmitted from the master cylinder to the calipers. As a result of this, there is a pressure build up around the calipers, making the pistons move outwards. The pistons apply the forces according to the pressure on the brake pads. The force produced is directly proportional to the applied pressure. This force pushes the brake pads outwards towards the brake disc. This creates a frictional force between the brake pad and the brake disc, thus lowering or stopping the movement of the speed of the disc speed.
One the desired speed has been realized; the driver release the pressure on the brake pedals. Due to the released pressure, the regions in the master cylinder become regions of less pressure. The pressure gradient is thus created in the opposite direction. The fluid, thus moves in the opposite direction. The pistons are then moved back and the brake pads move off the brake disc.
The diagrams below shows the Brake Disc at work.
Materials Selection for Brake Disc
Material selection is the most vital part of the design phase. Different engineering application requires different materials. The factors, which are considered when selecting a material for brake system include the material’s ultimate strength, cost, maximum operating temperature, specific gravity, etc. Grey cast t iron is the most commonly used material for brake discs. However, this material comes with a number of problems. The most common one being the specific gravity of the iron. Grey cast iron’s specific gravity is relatively higher as compared to other candidate materials. The heavier the vehicle, the less efficient it becomes in terms of fuel consumption. The possible materials for brake disc include Aluminum alloys and Titanium alloys.
Currently, the use of Aluminum-based composites is on the rise. This is due to a low specific gravity of Aluminum alloys. The Aluminum-based composites are normally made by reinforcing Aluminum alloys with the ceramic matrix. A part from low specific gravity, Aluminum- Ceramic composites also have low thermal conductivity. They can also perform under various conditions such as high load, velocity, and temperature conditions. The use of Aluminum-based materials reduces the weight the vehicle by around 55%. The requirements for brake disc include reliability and ability to withstand high friction high temperature generated during Operation CES is normally used for effective selection of materials for brake disc
Advantages of Aluminum-based brake discs are as highlighted below
55% weight reduction
Relatively better gas mileage of up to 10 percent
Less friction between the disc and the pad
Shorter retarding distance
Quicker heat dissipation, hence lower braking temperatures
High resistance to corrosion
Faster vehicle acceleration
Relatively higher durability
The diagrams shown in the Appendix section shows CES materials selections based on certain parameters.
Maintenance Services
The most damages experienced on the disc include cracking, warping, vibration, noises, scaring, and excessive rusting for the case of cast irons. The services done may sometimes include replacing the entire disc or repairing the damaged part. Replacing the whole disc is mainly done during circumstances where the cost of acquiring the new disc is relatively lower as compared to the cost of repairing the damaged part. However, it is always advisable that a disc should only be replaced when its thickness has gone below the manufacturer’s recommended minimum thickness. The disc becomes dangerous when its thickness goes below the minimum thickness. The vehicle becomes more prone to an accident in such a case. Disc turning is recommended by the leading Companies to be used in disc maintenance so as to avoid subjecting the vehicle to an unsafe operational condition.
Calculations in Brake Disc.
Due to contact between the brake disc and the pads, forces are produced in the outer and inner surface of the disc as shown in the figure below.
Taking Arbitrary Parameters
Brake Disc Diameter = 240 mm.
Material of the Brake Disc = Carbon 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= 340 ºC
………………………………………..
Taking standard parameters,
Material of the Brake Disc = Aluminum Ceramic
Diameter of the brake disc = 0.240m
Brake pad area = 2000 mm2
Brake pad material = Asbestos
Coefficient of friction (Dry) = 0.3-0.5
Coefficient of the friction (Wet) = 0.07-0.13
Operation Temperature= 250 ºC
Maximum Allowable Pressure = 1.06MPa
Tangential force (inner face)
T(x) = µT(y)
T(y) = x Pad area
Tangential forces in the outer and inner faces of the disc are equal.
Retardation Distance, x
Work done before the vehicle comes to rest = x. T(y)…………….. (a)
Change in kinetic energy, K.E, equal to the work dork done by the brake (Law of conservation of energy)
Conclusion
The most critical part of the automobile assembly is the brake system. For smooth operation of the vehicle, proper design of the brake system should be ensured. The largest component of the brake system is brake disc. Careful material selection, with the use of the relevant software, should be done so as to ensure safety and durability of the brake system. Machining of the brake disc is normally done on special lathe machines. The lathes produce discs with high dimensional precision. Properly machined discs ensure maximized service duration. Disc brakes depend on friction to stop the vehicle. Hydraulic brakes are the most commonly used in modern vehicles. In this type of disc brakes, pressure is exerted on the brake pads. The brake pad is connected to the piston as shown above. The piston then squeezes the fluid in the master cylinder, resulting into pressure build up in the cylinder. As a result of this, the fluid moves from a region of high pressure to regions of lower pressure. The pressure is transmitted equally to the brake calipers. Due to increased pressure in the brake calipers, the brake pads are pushed against the disc, thus, lowering the speed of the vehicle. As the vehicle comes to rest, all the kinetic energy are converted to heat energy. The disc should, therefore, be able to withstand the cyclic thermal stresses. The brake pads should be regularly changed to avoid scarring, which may damage the wheel. In addition, proper maintenance should be regularly done on the disc so as to ensure service efficiency. However, before a firm can settle on a particular design, it is proper conduct a market research in order to ensure that the product is sellable.
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