Creating a Suitable Braking Assembly
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To understand the advanced material and processing technologies specifically to study the interaction between different materials and the design process to create a suitable braking assembly consisting of disc, caliper and brake pads.
Disc - A disc brake or rotor is used to stop or slow down a moving vehicle. There are two types of disc brakes namely solid and ventilated disc .The rotating disc is connected to the axle and rotates with the wheel. When brake is applies the kinetic energy is converted to thermal energy. The temperature range of a disc braking under braking is 400C-600C. The weight if the disc also matters. For better braking and longer disc life. The disc should have grooves to disperse water to give maximum performance under wet conditions. Disc which are likely to corrosion are coated with anti rust paint. It should be resistant to wear and tear. The reliability of the disc increases when it responses fast and gives safety in all situations.
Caliper - The main function of the caliper is to hold the brake pads and to hub the brake fluid. The caliper is the actuator in braking and is attached to the steering knuckle. The pistons inside the calipers push against the brake pads which rub against the disc. There are two types of caliper-floating and fixed calipers. Fixed type does not rotate and are fixed. Floating type rotates in relation with the disc on an axis. The caliper should be clean and dirt free for maximum performance. It should also be corrosion resistant. Since calipers are actuators, they have to be made for maximum safety. Need to check if all the pistons are working or if they are engaged. Calipers should also dissipative heat produced. The calipers should provide complete safety in all conditions.
Brake pads - Brake pads are friction materials mounted on the caliper. They have high friction and help stop the vehicle. They wear out and have to be replaced for better braking. Brake pads or friction pads are the most important part in a brake assembly as this is the part that stops the vehicle. The most important property of the pad is the have a constant high coefficient of friction in a wide range of temperature. The pads should have enough surface area. Heat produces during braking should be dissipated or absorbed. It should have high melting point or the pads could face a phenomenon called brake fade. The brake pads must not wear out rapidly neither should it wear the disc. All the braking action should be quiet, even and stable.
Disc - Since the disc is thin, the material should be high compressive strength and should not deform under pressure or stress. Therefore the disc rotors need to have high melting point, high dynamic and thermal stability. Rotors should also be wear and rust resistant. The material should also have high dampening to absorb all the vibrations and noise when braking. It should also high have high thermal conductivity and should dissipate heat quickly. It should be able to minimize thermal expansion and should also be hard enough to withstand cracking. Finally the material should have superior machineability and castability.
Caliper - Material selection for caliper is crucial as it takes in high pressure and stress. It should be light and should have high tensile strength. The pistons should be made of high strength materials to transfer the force from the hydraulic fluid to the brake pads for effective braking. The material should not deform under high stress. Caliper design can get complicated so it should be easy to machine. The material should help in absorption and dissipation of heat and vibrations. Moreover it ought to be rust and dirt resistant. It should also exhibit high dynamic and thermal stability.
Brake Pads - The material selection for brake pads must have high coefficient of friction for a wide range of temperature and high melting point. Moreover it should withstand heat and dissipate quickly. Another desirable quality of the material is low dust and also low NVH which gives brakes a quiet operation. The material should provide a fade free operation and help eliminate rotor squeal. Pads should have long life but should be rotor friendly also. Since the pads are replaced frequently the pads should be easily machineable and the materials should be easily available.
Disc and Caliper - The material selected for Disc rotors and caliper is Cast Iron. Cast iron is an alloy of iron and carbon, where the carbon presence is about 3%-4.5% by weight. It is popular because of its low cost and ability to take complicated shapes and ease of machineability. The mechanical properties can be divided into the following
Tensile strength-Cast Iron's have comparatively less tensile strength but it serves the purpose in disc brakes. The strength is of the magnitude of 7 tonnes per square inch. Hence making the disc brake safe under tensile loading.
Compressive strength-Cast iron is popular for its high compressive strength and can take the high compression from the friction material under braking. Consequently brake caliper can take in high load and resist bending or crack.
High melting point-The melting ranges from 1140C to 1200C, which is more than sufficient for disc rotors. This helps in removing the disc fade phenomenon and helps braking more efficient at high temperature.
Resistances to deformation-Cast irons resist any form of deformation and give a very rigid frame. Hence any form of cracking or breaking is eliminated by careful manufacturing methods.
Resistance to Corrosion-Cast iron provides resistance to rust as it inhibits the oxidation of iron. But any part subjected to rust is coated with paint for extra protection.
Wear and tear - The chosen material is known for its hardness. Wearing out or eroding cast iron is incredibly difficult. Cast iron has proved its excellent abrasion resistance for years. For this reason, cast iron has been selected for disc rotors and caliper for excellent braking performance.
Durability and Environment Issues - Cast Iron are known to have high durability against fuels where the disc and caliper are often in attendance. The hydraulic fluids used in calipers cause no harm to the metal and hence provide good braking. Cast irons are environmentally friendly material which can be recycled and also doesn't require high energy in machining which reduces cost and pollution.
Therefore Cast Iron proves to be the suitable material for disc rotors and brake caliper as it serves the entire design requirement. Adding to the qualities is the dampening ability which helps in NVH free running. It even has an extremely low coefficient of expansion .moreover it also has low sound emission level and also dissipates heat quickly. All these qualities add up to make the cast iron the suitable material.
Pads - The material selected for brake pad is Asbestos because of its high coefficient of friction. It mainly found in silicate minerals and usually found as long, thin fibrous crystals. Asbestos became popular with its physical properties and came to be used widely in spite of its health hazards. The physical properties are divided below.
Tensile Strength - Asbestos is known to have very high tensile strength which makes it perfect for use as brake pads. As the brakes are applied at high speed, the pads are rubbed against the disc at high speed producing high tensile force. Therefore asbestos provides the require strength to counter it.
High Melting point - Asbestos has melting point in the order of 1200C-1500C.This helps in removing the fear of brake fade giving high performance even at high temperatures.
Resistance to deformation - Asbestos is found to be flexible material. The flexibility of the material helps it to change angle and help increase the braking as the surface of the pads will be evenly run over the disc generating excellent braking. Adding to flexibility is its high tensile strength.
Wear and tear - Asbestos has high coefficient of friction which makes it perfect but since asbestos are fibers, they tend to wear out but on a reduced or controlled rate according to the manufacturing method. They are designed to wear out eventually. But asbestos are durable material resistant to a number of elements.
Environmental issues - Asbestos are not environment friendly material neither is it safe for humans. Its fibers often cause health risk which makes it quiet dangerous to use.
Asbestos has a number of outstanding abilities such as heat, fire, chemical, friction, electrical, acoustical resistance. All these qualities help in providing a smooth braking feat. It insulates heat and helps dissipate heat quickly. The acoustical properties help absorb the noise and sound and give a quieter operation. Since it's chemically inert, it helps in operation in any conditions. Moreover it is insoluble in water and corrosion has no effect. All these qualities add up to prove that asbestos is a good material for brake pads.
Disc - Disc brake rotors are made using a simple process of casting. Casting is a manufacturing process where the selected material is poured into a mold which has the shape and geometry of the design and then allowed to solidify. Once the metal is solidified, the finished product is taken out by ejection or breakage of mold. Casting is also helpful in mixing two or more materials.The current manufacturing route for the disc brake rotor is casting as cast iron has excellent cast ability and machineability. Since casting has been used for a very long time, it has been developed over the years too. It's economical because of the low equipment cost and tooling cost which helps keep cost down. Disc rotors are made using sand mold. The disc shape is made in a sand cavity and molten cast iron is poured into the cavity. The liquid metal flows to all the space and empty crease in the cavity and takes the shape of the mold. Once the mold shape is taken, it is allowed to cool. After the mold has been cooled to a temp where it can be taken out of the mold, the product can be ejected or the mold can be broken to take the product. Once it is casted, the shape of the disc rotor has been created. It then goes for machining where it is trimmed to final dimensions and level the surface for critical surfaces/dimensions. After machining, the disc is taken over to surface abrasive section where all the rough edges and surface are made clean and smoothen to increase smoothness. After all the work has been done it might be given a coat of anti rust or left over for packaging. Many disc manufacturers drill holes and mill the disc rotors for better air flow and heat dissipation.
Casting is a low cost process but often it come with flaws. The rotors would have sand inclusions, bubbles, porosities and usually end up in one piece which can induce really high stress. This can cause uneven cooling or heating and produce cracks in the rotor. Once a crack is formed in a disc, it can lead to the destruction of the whole disc. Therefore the cracks should be taken care of beforehand. To counter this problem, many companies resort to what is called ASTM high temperature cast iron. They use vacuum assist degassed to eliminate the pososite to help stop cracking.
Caliper - The selected material for caliper is cast iron. Since cast iron have good castability, calipers are also made from casting. But since calipers take a very complex shape and have minute detailing, a high pressure casting is used. High pressure casting is similar to sand casting except that very high pressure of the order 14-140MPa is used to force the liquid to the mould giving good surface finish and also production of very thin sections. The process involves the cast iron melted to be flowing liquid. A mold of the caliper is made and the liquid is injected to the die using a plunger. The liquid fills the die cavity and since high pressure is used, the liquid goes to the smallest creases and takes the whole of the die space leaving less room for inclusions or porosity. Once the liquid is cooled, the die is opened and the product is taken out for further processing. Since the surface finish is good, there is less need to machining. It may be treated with chemicals and all the complicated shapes are cleaned. The caliper is given a protective paint coating for anti rust property. The major advantage is of the high pressure casting is the high production rate compared to sand cast. This is the current route of the manufacturing of calipers using cast irons.
Pads - Pads are manufactured rather traditionally. Basic process involves cutting of 2 metals one with a friction lining and then sticking them together to make the brake pad. The process starts with cutting two sheets metal. One metal will have the friction material and the other metal will act as a carrier. Once the sheet metals are cut, the two metals are bonded together by a damping layer. The damping layer is an adhesive that sticks both the metals together. It's a layer is a peroxide-cross linked polyacrylate adhesive of a thickness of about 75 μm or a peroxide-cross linked silicone adhesive of a thickness of about 100 μm. First the damping layer is applied onto one material and then the metal is pressed to the other metal. Few individual supports are punched out from the metal bonded together, providing support on one side and friction on the other. Thus the brake pad is manufactured and the process is very simple.
d) Alternate Material
Disc - The alternate material that can be used is Carbon Ceramic rotors. Carbon ceramics rotors were into application in high performance cars because it called in for high performance.CC rotors are very heat resistant and much more durable than cast iron brake.
Advantages of Ceramic Brakes over Cast iron
Carbon Ceramic s rotors have more frictional force which helps in stopping the car quickly.
They dissipate heat quickly which helps reduce brake fade which is the loss of friction at high temperature
Since the heat buildup is lesser, the wear and tear is also reduced
The overall weight of the rotors are reduced which help in reducing the weight of the car especially the unsprung mass which helps in handling.
CC rotors can withstand 2.5 times the heat steel can withstand which enables it to work in extreme heat.
Ceramics have found their place in most of the high performance cars. Ceramics are still under the development stage.
Disadvantages of Ceramic Brakes over Cast Iron
CC rotors are limited to open wheel configuration because of the high temperature generated.
The major disadvantage is the low coefficient of friction in wet or damp condition.
CC rotors phenomenally expensive
They cannot be repaired if cracked or damaged
The production process is very complicated too.
Ceramic Rotors are perfect selection for sports car and high performance cars but they seem to be inefficient for normal passenger cars. There are many researches going on in ceramics as they seem to give the best of all the worlds. The potential for ceramics are immense in automotive industry. They are widely used in aerospace now. The manufacturing process is also being refined which are moderating the cost which enables ceramics in a wider driving spectrum.CC rotors help reduce the weight of the vehicle as it is 4 times lighter than cast iron which increases fuel efficiency and help reduce the unsprung mass of the vehicles which gives better handling.CC rotors are made from a carbon reinforcement impregnated with a carbon matrix which is subjected to long and expensive process. There is new ceramics under development and this field is expanding.
Caliper - The alternate material used in caliper is aluminum. Aluminum is a very light material and can be cast without much expense. Aluminum is used because it is light weight, stiff and non corrosive.
Light weight material comparing to cast iron which helps reduce the upsprung mass of car
It can be manufactured easily and tooling is inexpensive.
It can be anodized for better look and protection
Stiff material and gives sufficient strength
Non corrosive and is not affected by water under most conditions.
Disadvantages over cast iron
Material not strong enough and have lesser compressive and tensile strength.
Aluminum has low damping and doesn't absorb vibrations
More expensive than cast iron, more manufacturing process
Poor ability to transfer heat, aluminum is a poor thermal insulator.
Aluminum is widely used to make calipers in modern cars due to its properties and charactertics. More research is being carried out to make aluminum stiffer and harder. Many new aluminum alloys are being produced and tested. Aluminum calipers are larger in size when compared to cast iron calipers; this is due to the fact that the required stiffness is reached only within a certain size. The pistons used in the caliper vary from steel to titanium. The caliper needs good surfaces finish and should also take high stress and shape. Aluminum delivers all the required design specification and making aluminum the best alternate material to be used in calipers.
Pads - As the increasing rate of technology, brake pads have improved over the years. Asbestos were replaced by semi metallic and now replaced by ceramic pads. Asbestos pad caused health issues and semi metallic pad were noisy and eroded the rotors. Ceramic brakes have been selected as the alternate material for brake pads.
The major advantage is low dust formed when braking.
Improved braking over semi metallic and asbestos
Stable and even friction available at all temperature range
Extremely quiet performance and provide comfort to the passenger
The pad life has also been extended also the rotor life as the pads don't erode the rotors.
The only disadvantage till now is the rather expensiveness of the brake pads, but with constant manufacturing up gradation, the prices will come down.
Ceramic brake pads are found to be an ultimate product, but research is continuing in the process of making it more efficient and cheaper. The dusts produced by ceramics are minimal and are not noticeable making the disc more cleanly in appearance. The ceramic pads are made up of a carbon-ceramic mostly with carbon fibers within silicon carbide matrix(C/SiC).This gives all the qualities to the brake pads according to the design requirement.
e) Manufacturing Route for Alternate material
Disc - The manufacturing process of a ceramic disc rotor is complex and involves a number of steps. Rotors are expensive because of the expensive manufacturing process. The process starts with the preparation and arrangement of multiple layers of fiber fabric. Forming a material in with all the properties involves complex manufacturing process and the properties depend on the selection and densification process. The fibers are taken and heated to a point where they don't melt or burn. Fiber arrangement range from random chopped mat to woven fabric. Then the fiber material is formed in mould to produce the disc shape and then heated to eliminate non carbon elements. After this a process known as chemical vapor deposition (CVD) is used. In CVD porous product is heated in vacuum and then hydrocarbon gas is supplied in to diffuse all the cracks. In isothermal CVD, densification occurs at uniform temperature. Once the specific density and porosity is obtained, the disc is taken out and machined, drilled and treated with an antioxidant or special paint for protection. The process is a very length one and takes about 30 to 40 days. The manufacturing process increases the cost of the brakes. Research is being done to reduce the process while maintaining the property of the ceramic disc rotors.
Caliper - The manufacturing process used to make calipers out aluminum is Gravity Die Casting (GDC).This is different from High pressure die casting used for cast iron. This involves pouring the liquid gently into a reusable metal die. In this process the cores make the difference, the cavity formed due to the cores help the movement of liquid through the core. It includes two cores where one is moveable. The liquid is poured into the die and due to gravity it flows to the cavity and fills the gaps. Once it is solid and set, the die is removed and the finished is removed. After that it is machined to give it a surface finish. The main advantage of gravity die casting over sand casting is the production speed. The same die can be used for hundreds of casting. This process also gives a dimensional accuracy and surface finish. This process is widely used in cast aluminum alloy calipers.
Pad - Ceramic brake pads are manufactured similar to the cast iron brake pads. The ceramic material is cut into the shape and another metal piece is taken and the both are cold pressed into green compact. Sintering is carried out in vacuum where the organic binding agent concentrations get pyrolized and the metal particles bonded by reaction to carbon. Several binding agents are suited for pyrolysis.eg silicon carbide. One side of the pad will have the friction material and the other will have the support. Thus the manufacturing process for brake pad for alternate material uses expensive materials and process making it more expensive than conventional brake pads.
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