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The Materials And Characteristics Of Disc Brakes Engineering Essay

Disc brakes are commonly used in modern cars. It is usually used on all the four wheels or only on the front wheels with drum brakes on the rear. The main components of the disc brakes are:

Braking system is defined as a mechanism which helps a vehicle to reduce its acceleration drastically by converting its kinetic energy into a different form of energy (Heat) and further leads the vehicle to become stationary. When the brake pedals are depressed, the master cylinder converts the force into the hydraulic pressure which is transmitted to the brake pads (piston) via hydraulic hoses. (Automotive-online., familycar, 2010, ‘22 October2010.’)

2.0 Component Function:

Discs: The discs are usually fixed to the wheel hub and frictional force created between the brake pads and discs causing the wheels to stop. There are simple solid discs and others are ventilated. These ventilated discs are mainly used for heat dissipation properties. (Singh 1999).

Braking system.jpg

Fig.1 (2carpros, 2010, ‘28 th October’)

Brake Pads: Brake pads are considered to be a vital part of the car’s braking system. It creates the frictional force which is necessary to stop the car. Usually in most passenger cars, two brake pads are contained in the callipers with their frictional surface facing the discs. When the brakes are applied the hydraulic forces pushes the piston with brake pads inside the callipers to come in contact with the disc, the friction between the two creates the braking force. If a bake pad becomes overheated it can delay the brake operation until the system is cooled. Brake pads are usually designed in a manner to withstand large amount of pressure provided by the master cylinder. There are various classifications of the brake composite like organic, semi metallic and ceramic to name a few. (Singh 1999).

1.jpg

Fig.2 (cartuningcentral, 2010 ,‘ 28 th October.’)

Brake Callipers: The brake calliper is the assembly which contains brake pads and pistons. There are two types of callipers: floating and fixed. Floating callipers also known as sliding callipers are the most widely used type. In these callipers type the hydraulic fluid pushes one friction pad directly onto the disc, and pulls the calliper so that the other pad is pressed and pressure is applied to the both sides of the disc. (Singh 1999).

generic_image.jpg

Fig.3 (audi,2010, ‘ 28 th October.’)

2.1 In-service Conditions:

The frictional force which is necessary for breaking the vehicle is generated at the brake pads and the disc. The co-efficient of friction between brake pads and disc decides the necessary pedal force required to obtain deceleration. This plays a vital role in the stability of the vehicle while braking. The temperatures of pad and disc may reach up to 700°C. Since there is a large amount of heat, it needs to dissipate them quickly. The optimal solution for this is internally – ventilated discs. The air flow through the internal cooling channels promotes rapid cooling. (Becker et al. 1994). The major portion of the heat generated is dissipated into the air while the rest stored in the discs. The disc also undergoes thermal strain i.e. elastic strain that results from expansion and contraction due to increase and decrease in temperatures respectively. Due to the excess build up of heat, the disc is softened and allows it to disfigure which might results in wheel shimmy. When the friction pad is worn out, the disc will be pressed against the piston inside the disc, scratching the discs. Hwang and Xu (2010).

In a survey conducted there was 1/436 involved in an accident .The major contribution of 3% road accidents are due to brake defects. When Fluid samples were taken for analysis, they were contaminated with mineral oil and dirt. Faulty adjustment is the most common defect found in brake assemblies’. Hobbs (1994:228).

2.2 Operational requirements:

The major portion of the heat generated is dissipated into the air while the rest stored in the discs, so the convective heat transfer co-efficient of the disc is the major requirement while choosing the material. The co-efficient of friction between the discs and the pad plays the vital role in braking efficiency of the system. So it is vital to choose the pads material which has good frictional property, less wear rates, dissipate heat, has constant coefficient of friction and don’t lose efficiency at high temperature. (Harper1998).

In discs, shape and weight needs to be given a vital importance and should be maintained as per standards to withstand serious braking and avoid the vehicle from drifting. The material composition involved in discs should be calculative to maintain the co-efficient of friction. (Harper1998).

The callipers need to be properly designed and machined during the manufacture in order to avoid breakage. The pistons are given a huge importance since they need to withstand wear resistance and prevent ingress of dirt. (Harper1998).

When disc pads are considered the key things to be taken care is the frictional material used in the backing plate. The pads are usually machined and held together with synthetic resin adhesive to prevent the worn out of sides and backing plate of the pads. (Harper1998).

3.0 Material Characteristics:

There are conditions upon which materials are chosen such as Tensile strength, Modulus of elasticity, machinability, annealing, Hardness, Impact strength, Fatigue resistance, Wear and Hardness and Corrosion and Abrasion. Cast iron which is widely employed in all major groups of engineering is most widely used material for disc brakes. When cast iron are considered they contain a rich mixture of nickel, copper, chromium, molybdenum and vanadium to withstand wear, corrosion and heat resistance for braking components. So the brakes are generally coated with an extra composition of nickel and austenitic cast iron to withstand the corrosion resistance to acids, water etc. The strength of cast iron 309 N/mm² is machined under high temperatures of 350- 537.77°C to get the desired outputs. The abrasion resistance of a surface is required for the hardening of the materials. The cast iron with 30 % of chromium and 14-18% of silicon iron has hardness of 320-420 DPN. The abrasive and wear resistance is great when metal hardness is high which cast iron posses. The actual tensile strength of cast iron does not exceed 124-154 N/mm². Considering the factor of safety, the discs need to be machined properly to withstand high pressure forces involved in braking. (Angus1976). , (Harper1998). When callipers are introduced in olden days they were machined heavily as a single unit. Later they were made in two separate units and bolted together. Early days, asbestos was used as the lining materials. To reduce the health effects of using asbestos, pads are made using materials like ceramics, kevlar and other. Steel pistons are chromium plated to decrease the wear resistance. Brake pads were formulated in late 1950’s and improving the features of the brake pads is still is an ongoing process. (Ashby1998). Brake pads are usually replaced before the thickness of the remaining friction on backing plate reaches to 1/8 to 3/16. So replacement of pads is done when they are worn out and pad material matches the same thickness of the backing plate to improve the performance. (Gilles2005).

4.0 Summary:

Considering the braking there is a constant need for increase in performance. Factors such as Fatigue resistance, Wear, Hardness and Corrosion and Abrasion needs to be concentrated to improve the performance in braking system. The future technology involved in vehicles is “Regenerative braking“, where 80 % of the input energy will be converted into mechanical energy and the valuable kinetic energy would also be saved. (Isaacs2010).Usage of ceramic materials in disc brakes helps in improving braking efficiency .Ceramic-coated disc brakes are considered to be future technology in braking system, since they will give high output. (Kang et al. 2002).

References:-

[online]<http:automotive-online.com>[22 October2010]

[online]<http://www.familycar.com>[22 October2010]

Singh,k(1999) Automobile Engineering Vol.1.Delhi :Standard Publishers Distributors

[online] <http://www.2carpros.com> 22 October2010]

[online] <http://www.cartuningcentral.com>28 October2010]

[online] <http://www.audi.co.uk >28 October2010]

(Becker et al. 1994).Brake systems for passenger cars, edition 95/96. Stuttgart: Robert Bosch GmbH

(Hwang,P ,Wu,X . 2010)Investigation of temperature and thermal stress in ventilated disc brake based on 3D thermo-mechanical coupling model 24,81-84[online],<books.google.co.uk>[26 October 2010]

Hobbs, C.A (1977)Braking Of Road vehicles 1 Mech E Conference 1976-5, The institution of Mechanical Engineers. Berkshire: The Lavenham Press Limited

Harper,G.A. Brakes and Friction Materials. Suffolk: Mechanical Engineering Publications Limited

Angus, H.T(1998)Cast Iron: Physical and Engineering Properties, Butler & Tanner Ltd,'2nd edn.',:London.Butlerworth & Co (Publishers )Ltd

Gilles,T(2005)Automotivechassis(Brakes,steering&Suspension[online],<books.google.co.uk>[27 October 2010]

Ashby,M.F(2005)Materials Selection in Mechanical Design ,’3rd edn.’[online],<books.google.co.uk>: [27 October 2010]

Issacs,D(2010)Electric Cars - the Future Is Now![online ],<books.google.co.uk>.[27 October 2010]

Kang,B.B , Choe,K.Y , Kim,H.J(2002) International Railway Journal [online] <findarticles.com> [27 October 2010 ]

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