Effects of Material on Braking Abilities

Abstract:

The most important utility in our lives as of today is automobiles, there is a lot of research and development is going in every section of the automobile field to give the safest possible vehicle to the world. One such is Brakes section; this is very important part in every vehicle, though we have most accurate and efficient brakes now-a-days, but they fail at the extreme conditions of heat , vibrations , high frequencies etc., this project is mainly concentrated on the aspects which are causing adverse affect to fail and create unusual  disturbances in the brakes. One such is brake squeal, an unwanted sound which is caused due to the vibration of the brake pads against the disc generating the high pitch noise. There are few other problems which leads into the high frequency problems, vibrations, tear in the brake disc etc.

Here in this project a detailed study of disc brake is taken into account by considering with two different materials cast iron and aluminium alloy. The performance of brake is studies using normal stress analysis, vibration analysis and thermal analysis. The main problem of squeal can be due to the combination of vibration in brake set-up from callipers to the brake discs. And can also due to the uneven surfaces of the brake disc which causes due to the improper heat dissipation in the brake disc, all this problems are kept into consideration and this analysis are carried out for brake disc with two different materials as mentioned. A simple change in the design of brake disc has also been done for the maximum heat dissipation and this design also studied in this extreme conditions. And these two type brake disc are compared in all its possibly calculated attributes and the best one is concluded.

Introduction:

An automobile is a creation of history when they started and now it has become a crucial part of the world. Automobile is a wheeled motor vehicle used for transporting passengers, good etc. There has been a lot of improvement in the automotive field in all ways from the past few decades. The main aim of the automotive department is to provide safest vehicle from its design to the material used for each component. Every single thing is decided on basis of the effect of it on the human lives. The vehicles we use are very efficient when compared with older generation vehicles, the safety increases with the development and technology. But there are few problems which are yet to be solved. One such is about brakes, as we know BRAKES are the most important part in the every moving body, we have seen very drastic improvement in the methods of braking system. In olden days the brakes were handled by hand and now we have disc brakes and hydraulic brakes.  But the whole concept of brakes is working on same principle of kinetic energy is converted into heat energy.

We find brakes very commonly used in cars, bikes, aeroplanes, automated machines etc. Brake is a device which decelerates the vehicle or machine by converting its kinetic energy into other form of energy, which will bring the vehicle down to the rest. We have disc brakes now which require very less amount of effort and time to stop the vehicle due to its accuracy and sensitivity; this are widely used in vehicles now-a-days. The drum brakes were used for very long time and we use even now, these were also very efficient brakes but due to the very common issues of changing the brake pads very often due to the short hand braking or improper usage, this has been replaced by disc or hydraulic in few machines. But we have found very common issues in disc brakes due to the brake rotor and brake pads. Due to the high friction while short braking we get unwanted noise brake squeal, though it doesn't have much effect on the performance of the vehicle, but it may fail the brake due to the tear of brake disc or brake pads. A detailed study of each problem will be studied in further report taking disc brake system.

History of Brakes:

Invention of brake has started with the automated machines; brakes came into light mainly when the motorised wheeled vehicles came into existence. Brakes were not only used in the vehicles, these are used in other machines as brake lever to stop the motion rotor and so on. It has been an essential part of the moving systems.

Brakes have developed a lot from first generation vehicles to present time. Early braking systems used steel rimmed wheels to stop the vehicle, which consists of curved wooden block designed to bear against the steel tire when operated by a single leverage system from the driver seat. But the brake shoe used was normal way of braking either in the horse drawn vehicle or steam locomotive. In 1895 Michelin Brothers move a step ahead to replace the steel rimmed wheels with the pneumatic rubber tire to come out with a new braking system as the brake shoes were no longer satisfactory.  A new method of braking system was required in those times as two early devices attempted to apply the friction force to the axle or to drum on axle or transmission shaft were not efficient as wanted. One method was use of wooden block inside a flexible contracting metal band which when pressed together would tighten the drum causing friction between the drum, which is connected to the wooden block and wheel, then the vehicle gets slow down. The other method was inner wheel or brake drum which will add an external contracting band to bear against the drum to bring the vehicle to rest.

In 1899 Daimler started cable operated braking system; a cable was wound around a drum and attached to the chassis, so that when the cable is tightened when car was moving forward, the rotation of drum will increase the tightness and grip of cable, so that it will reduce the amount of force required to pull the cable-lever on the pedal to stop the moving vehicle. This brake system worked well but still the braking efficiency was less, the added efficiency called servo assistance is still an important factor in drum brakes even today. The external brake was vulnerable to the atmospheric conditions like weathering and to un-even roads which caused a rapid loss of efficiency and wear of the brake shoe lining and on few times automatic brake use to happen due to the drum expansion. To overcome all these problems an internal shoe brake was developed were the expanding the brake shoes were placed the inside the brake drum, mainly to protect from the dust and weather.

Originally early brakes were operated by mechanically, i.e. the transformation by mechanical system was used to expand the brake shoes against drum by the driver's effort applied on the brake pedal. It works by pressing the brake pedal, which make cam to rotate by a lever connected to pedal, which forces the shoes to come on contact with the brake drum. And springs attached to both of the brake shoes to retain the original position when brakes are released. As the early brakes use to operate on lever supported cables and linkage system of fixed rods, equalising the same amount of brake pressure on the wheels has presented many problems , which were solved when hydraulic system was introduced, which used fluid to transfer the force applied to the brake pedal.

In hydraulic braking system the set-up of cylinders and pipe lines forms a closed system which is filled with fluid. The pressure generated in master cylinder will transmit equal force to the each wheel cylinder and then equal pressure is applied to all brake shoes. This hydraulic system is used with the disc brake as well. Disc brake is more efficient braking system which was used mostly sports car when introduced but now it has been a very common system in passenger vehicles. It has a disc and a calliper is attached to it which can be operated mechanically, hydraulically, pneumatically, electromagnetically.

Types of brakes:

The brakes are broadly explained as using friction, pumping or electromagnetism. But one brake system can also use more than one principle. As the project is based on the disc brake we will look forward only to discuss about the frictional brake.

Frictional brakes: These types of brakes are mostly rotating devices with a stationary pad attached with a rotating surface. These types of brakes are widely used even now in modern automotive braking system. Basically this are developed from band brakes to drum brakes then to the disc brakes.

Band brakes: A simple type of brake, works on a principle that a band is wrapped round the rotating drum. When a force is applied on the lever, the restraining torque is found from the difference in tension between the two ends of the belt. The principle of operation of the band brake is same as for belt drives.

This is external type braking system, as we see in picture the brake shoe are mounted on the drum.

Ref: http://school.mech.uwa.edu.au/~dwright/DANotes/brakes/intro/intro.html

Drum shoe brakes: This type of brake usually mean the brake shoe is mounted to press the inside surface of the drum. Two brake shoes are connected with a calliper set-up; it has lining on it which will create a frictional force when pressed towards the moving surface of the vehicle.

These types of drum brakes are still used in modern cars due to the some cost and engineering benefits. These are applied to the rear wheels of today cars.

Ref: http://school.mech.uwa.edu.au/~dwright/DANotes/brakes/intro/intro.html

Disc Brake: these most widely used brake of these days, A disc is mounted on the wheel or axle and brake pads are placed within the jaws of the calliper, this is give the necessary friction when it will grip on the brake disc.

Ref: http://static.howstuffworks.com/gif/disc-brake3.jpg

Characteristics of brakes:

Brakes are described on basis of several characteristics as

Peak force: The maximum force obtained to decelerate the vehicle is called peak force. Few times this can be more than the traction limit of tires, then the wheel gets skid

Continuous Power dissipation: As we know that the brakes get hot in use and get fail when the temperature gets too high, the maximum amount of power that dissipates through the brake without failure is the continuous power dissipation.

Fade: Due to the over heat of the brakes the surface of the brake pads or disc get fade which will cause decrease in the effectiveness of brakes. Few time's even cooling will have big effect on to fade.

Power: when a very less force leads to the braking force, which is higher than the other brakes in similar class then it's stated as powerful brake, which is due to the sensitivity of the brake system.

Durability: Friction brakes have the lining which has wear surfaces, these to be renewed periodically.  Like brake shoes, pads or brake disc. Even if wear surface due to brake shoe generates high peak force this will lead to wear quickly.

Weight: In some circumstances brakes are often mounted on wheels, this can disturb the traction significantly. Here weight means brake itself or an additional support structure

Noise: generally brakes create some minor noise when applied, but sometimes create a high pitch noise called squeal or grinding noise.

Principles of braking system:

Brakes work on simple principle to slow down the moving vehicle by applying an external force, it may use more than one component; directly or indirectly several components effect on the momentum of the moving vehicle. Various types of process are used for braking effect. Frictional brakes are most commonly used these days, these works on the simple mechanism of changing the kinetic energy into the heat energy, which is done by only factor friction. Friction is created when a stationary brake pad grips on the moving brake disc with a force applied.

This type of braking system is also used in the disc brakes; the stationary brake pads rub on the disc rotor which moves relatively in its own axis.

Basic laws of braking system:

Braking system can be explained by two simple Newton laws; which will explain the whole concept of the braking system

As we know that the Newton's second law "The net force on an object is equal to the its mass times its acceleration and points in the direction of the acceleration"

F = m* a

The law is used to find out the net force required by brake pads to bring the moving vehicle to rest, tough there are many other factors to be considered for calculation of brake force which will be discussed further.

It can also be explained through Newton first law "Every object remains at rest or in motion in a straight line at constant speed unless it is forced to change its state by an external force" if we check this law, an external force (brake force) is required to change the state of the vehicle (motion to rest).

As this law explained the evaluation of the brake force and system of braking, a further law explains how the brake force will stop the vehicle.

Law of thermodynamics: "Energy can neither be created nor be destroyed; it will be converted from one form of energy to other form."

This explained the frictional force (brake force) is converted into the heat energy.

Factors considered while Braking:

There are many factors to be considered for the high braking efficiency and performance; here we will see the few important factors which have a more effect on braking efficiency.

1. Condition of disc rotor: An uneven disc or ruffed disc rotor will decrease the braking effect; it has to be very well maintained as few a times due to dust and weather a extra layer is formed on the disc surface which will have a negative effect on the braking concept. Or wear of disc rotor due to overheat.

2. Pressure on brake rotor: A sufficient amount of brake pressure is to be maintained to get enough frictional force to stop the vehicle. If less pressure is applied it will directly affect the brake efficiency as force acting on the two rubbing surface will become less.

3. Contact surface: The area of contact between the brake pads and disc rotor should be more then the high friction is obtained. If lining of brake pads or surface of disc rotor is uneven then area of contact will be less, which will affect the brake efficiency.

4. Tyres: The design of tyre also have a normal affect on the brake efficiency, the more the contact of tyre to the road surface the good will be the braking system. Worn tyres will have less braking action and will not allow vehicle to stop the required point and may also skid, braking action is found high in new tyres due to its more contact with the road surface.

There are other factors which also play an crucial part in brake efficiency like aerodynamics, heat dissipation, weight of body etc.,

Causes of failure:

The most commonly found causes for the brake failure are

1. Oil or grease on the brakes will cause failure as it interferes with the friction. If we found oil in brakes mean the oil seal has failed and is why oil is leaking.

2. Overheat of brakes to great degree, which will develop a hot spot on the rotor and drums. This spots will resist the friction from brake shoe and pads, therefore braking power is lost and brake fails.

3. Brake squeal which indicates the tear in the brake pads, by the time the brakes start making a grinding sound, they would have worn out past the pads to rotors. This will require to change more than just brake pads for new one, which will increase replacement cost as well.

4. Improper wheel alignment or disturbance in the steering system will also lead into the failure of brakes. Even if the mass of vehicle will get out of its range due to extra load will cause a brake failure due to the uneven load distribution.

DISC BRAKE

Introduction: Disc brakes came into existence in 1890's in England. But due to the poor state of roads and dusty conditions the disc and pads use to get rust and get wear, so the system got non-viable at that time. But later it was fully adopted in 1950's with new innovations in the previous design. Many companies started using it in their cars as it was better when to compared with drum brakes, the main reason as it has overcome the brake fade problem by providing the resistance to the brake components and overall performance is better when compared with drum brakes.  Disc brakes become popular in sports cars due to its braking performance. Now it has been common in commercial vehicles as well, as it's used at front of the vehicles, as the front brakes perform most of the braking effort.

Components: A disc brake assembly consists of a

1. Cast-iron disc (disc rotor) that rotates with the wheel.

2. Calliper assembly attached to the steering knuckle (operated mechanically or hydraulically).

3. Friction materials (disc pads) that are mounted to the calliper assembly.

Operation: (Hydraulically powered)

Disc brakes prove to be efficient when compared with drum brakes even the working principle is same for both systems. The basic principle of braking system "The kinetic energy is converted into heat energy" 

When brake pedal is pressed, the hydraulic pressure is applied on the piston; it pushes the brake pad to get in contact with the disc. As the pressure increases the calliper pushes the outside pad to get in contact with the disc. Due the friction generated between pads and disc will create the braking force as the pads gets in contact fully with the disc rotor. But if we study the disc brakes widely, it shows that it does not use the much of the friction between the lining and rotor to increase the braking power as drum brakes does, they likely cause a pull between them.

Disc brakes have constrained self-energizing action, there has to be sufficient hydraulic pressure to get the required braking force. The braking force can be increased by changing the size of calliper piston. Even less heat dissipation occurs as the friction surface is exposed to the air, which also reduces the braking fade. 

Design of disc rotor: generally disc rotors are either solid or ventilated. The ventilated type disc rotor has cooling fins in the middle of the disc to ensure good cooling. Proper cooling ensures longer pad life and it also prevents fading. Some ventilated rotors have spiral fins which allow more air flow and better cooling. These fins are mounted on side of the vehicle and directional. Spiral fins are used in the front of the vehicles as front brakes take most of the braking action as more heat generates.

Ref: https://pontiworld.com.au

Design of calliper: The calliper are used as floating calliper or fixed calliper design, and these are mounted on the axle or attached to the wheel.

Floating calliper type: The calliper are used as floating calliper or fixed calliper design, and these are mounted on the axle or attached to the wheel.

this type of calliper requires less parts than the other type and it's also less in weight and economical. Based on requirement it consists of one or two pistons. The piston is either side of the calliper. Hydraulic pressure from the main cylinder is generated and the piston thus press the brake pad towards the disc, then instantly an equal hydraulic pressure is generated at the other side and right brake pad pushed towards disc rotor and vehicle stops due to the braking force.

Fixed calliper type: The calliper are used as floating calliper or fixed calliper design, and these are mounted on the axle or attached to the wheel.

This type of calliper design has piston on both sides of the calliper, which provides the equal force to each brake pad. These fixed calliper types can set-up either one or two pistons on each side. The two pistons can generate a more braking force and a compact design, as these absorb and dissipate more heat, due to its size and weight. This design is capable for greater number of hard stops of short brakes without the effect of brake fade.

Materials used for component:

The material used for disc rotor plays an important role in braking force. The material should be tuff enough to stand on high pressures and high friction forces. Generally cast iron is used for disc rotor commercially, but ceramic discs are used for high-performance vehicles and heavy automobiles. Recent study shows that aluminium alloys also do a great job as disc. Now we will discuss in detail about the commonly used materials cast iron and aluminium alloys.

Cast iron:

Grey cast iron alloy is used for the manufacturing of disc rotor; it has superior properties and various advantages over other materials, as it best suitable for manufacturing and machining when compared with other materials which lead to the graphite lubricating the cut and breaking the chips. It contains 2.5%-4% of carbon and greater than 2% of silicon.  This material has good wear resistance and galling which leads to self lubricating due to the graphite flakes. The graphite microstructure of grey cast iron allows less shrinkage. The silicon percentage in the grey cast iron makes the material corrosion resistance and increases its fluidity when casting which also makes material easy to weld.

Compared with other alloys of the cast iron, grey cast iron has low tensile strength , the good point about this material is the shock and impact resistance is almost doesn't exist.

Here we will see the properties of the grey cast iron for ASTM 60,

Tensile strength    62.5 Kpsi

Compressive strength     187.5 Kpsi

Shear modulus of rupture  88.5 Kpsi

Modulus of elasticity ( Tension )     20.4 - 23.5 Mpsi

Modulus of elasticity ( Torsion )      7.8 - 8.5 Mpsi

Endurance Limit     24.5 Kpsi

Brinerll Hardness        302 H_b

Specific heat      447 j/kg deg

   

Ref: http://www.engineershandbook.com

Characteristics of Grey cast iron:

Wear resistance: Grey cast iron works as excellent resistance to sliding friction wear, it is widely used for manufacturing of sliding components, most of the automobile components are made up of this item, it's all because of the low coefficient of friction, resistance to the galling it has which is due to the effect of graphite flakes . This will help the disc rotor to withstand for high friction forces.

Thermal conductivity: The important merit of grey cast iron is its high thermal conductivity, due to its flake graphite structure, heat dissipates occurs in material with good rate. This will increase the cooling of the disc rotor

Machinability: Due to the ferrite graphite structure, grey cast iron is simplest and easiest alloys for machining and casting. This will make easy to get the typical designs of the disc rotor with spiral and fins

Damping capacity: Grey cast iron has high ability to absorb vibration energy and also damping vibrations, which is due to the high percentage of the graphite flakes it has in it. It also has great property to resist high frequency vibrations. This will help disc rotor

Aluminium Alloys:

Aluminium is the important alloy of aluminium alloys, were the other metals are copper, silicon and magnesium. It has less tensile strength and low melting pointing. It has good casting characteristics due to its great levels of silicon (4-13%) in it. It is widely used as disc rotor due to its corrosion resistance.  But this is quite expensive when compared with cast iron. Though is in light in weight, has good electrical and thermal conductivity. It does also can be recyclable. All these properties brought a change in many industries to change their traditional materials i.e., aluminium alloys.

Characteristics of aluminium alloys:

Light weight: The best part of this material is its light in weight, actually its one third of steel in weight, due to its specific weight of 2.7 g/cm3. The more the energy consumed by aluminium the high will be the load capacity of the vehicle.  This will decrease the vibrations and also increases the brake efficiency due to its light weight.

Properties of the aluminium alloys:

Density     2600-2800 kg/m3

Melting point     660 deg

Elastic modulus     70-79 Gpa

Poisson's ratio      0.33

Tensile Strength   230-570 Mpa

Yield Strength     215-505 Mpa

Percent Elongation        10-25%

Thermal expansion coefficient      20.4 -25.0 10-6 /k

   

Thermal conductivity: it's used in most of the high conductivity line like power transmission, it has twice of the copper in the properties, and it's a good conductor of heat and electricity. This will increase the heat dissipation of the disc rotor.

Ductility and recyclability: Due to the lower density and melting point, the material is considered as ductile, this will help to the complicated design, as we need in the disc rotor with spiral fins and groves for the air dissipation. Recyclability is also one great factor of aluminium as it consumes only 5% of the energy to melt it, and it doesn't affect its original quality of aluminium.

Corrosion resistance: This is the main factor which made it to select for disc rotor manufacturing. It has a natural property of corrosion resistant as it surfaces form an oxide coating on it. While using as disc rotor further surface treatments like lacquering and anodising are done to increase its resistance towards corrosion.

Major problems with Disc Brakes:

The disc rotor of Disc Brakes is more exposed to the air, due to which there are very frequent chances of getting damaged. Usually this damage of disc rotor is explained in four common ways warping, cracking, rusting and scarring. Few a times the damage takes the disc into the unsafe area then the new disc is replaced with damaged ones. But sometimes using simple machining and other process this damages can be overcome. As scratches or damage on disc rotor can be removed by removing the thin layer on the disc on lathe machine if after that the thickness of the disc falls in the safe dimensions for the braking force.

Warping: This is caused due to the excessive heat of the friction area of the disc rotor, due to excessive shorthand braking, uneven cooling of disc may also causes warping. If braking pads get into contact of disc excessively this will lead to warping after certain times, this warping is most commonly found in the racing cars as brakes are used very frequently due to speed and used at high gear times. There are few methods which can avoid the warping, braking at lower gear which will help to reduce the braking load on the brakes, and less heat is generated. The more the braking load, the more the heat generated and more the chances of improper cooling to take place. Changing the disc design with more fins can few times avoids warping. Improper installation of disc rotor may also leads to the warping as only the overwhelmed disc will come in contact with the brake pads.

Cracking: This is found mainly in the drilled disc. Cracking occurs mostly around the edges and holes of the disc, due to the uneven expansion in severe environments. As the uneven expansion take place and the braking load varies every time the brake is applied which will lead to the cracking. And in several situations the disc will fail and no possible repair can be done. Even cracking can be overcome by maintaining proper the cooling of the disc and proper heat dissipation. If cracking is become severe then there might be a chance to replace the disc rotor.

Rusting: This is found in the vehicle which are not used for period of time, the regular use keeps the friction area clean, but when not in use the friction area and slots get rusted and it reduces the braking power, due to which the strength of the disc also falls down and at this conditions disc should replace with new one.

Scarring: Scarring occurs if something hard gets in contact with the disc while moving, which forms a hard spot on the disc, which reduces the braking power and weakens the brake. Generally this happens when the brake pad service life come to end and still its attached to the brake set-up then the steel support of the brake pad get in contact with the disc which creates a hard scratch. These scratches will form black slots when a vehicle come in use, and reduces the structure strength. This can be avoided by regular checking of brake pads checking life of the friction layer, machining the disc to remove the layer on surface of disc which removes the scratches.

There are few phenomenal problems which has been a challenging task in braking department like brake squeal, brake judder and brake dust. These problems can be dangerous when they get on excessively. Change of design, material, brake assembly setup all these have been checked extensively to find out the perfect solution for these problems, this project is undertaken based on this problems. Lets discuss in detail about these problems and what are the methods being simplified to overcome it.

Brake Squeal: squeal mean noise, this brake noise is a vehicle system problem due to the regularity and severity. The noise generated is mainly because of brake and suspension components together. This is not considered as the main problem to the vehicle were performance is the initial objective, but it's not so proper for the road use. This noise is mainly between the pad and disc during the braking. But squeal can also be combination of disc, calliper and brake pads. To avoid squeal under the braking condition is not so possible, if we are assuming brake to absorb very high energy inputs.

Actually this brake squeal came into account when front wheel drive and metallic brakes arrived. These metallic pads are harder than asbestos counterparts, and thus more squeal is observed if there are more irregularities and roughness on the rotor surface or if you found looseness between the pads and callipers. Few types of calliper designs are more noisier, if the pads of this callipers are not fitted so tightly and the calliper itself move around when brakes are applied. The more the brakes play the more are the chances of noise. Fixing the squeal problems in wrong way can lead to the squeal more worse.

Brake squeal also occurs due to vibration of brake pads towards the disc rotor when vehicle is moving at low speed, this may not affect the braking performance much but will lead to the replacement of brake pads, this is very common problem which most of the vehicles are facing now.  Lot of methods are used to reduce the level of noise,  simple methods like having chamfers to the linings, applying grease between the pads and calliper, adding the brake shim between  the brake back plate and brake pads etc., these may help in some way to reduce the noise.

Ref: http://www.aa1car.com/library/bsqueal.htm

While using the methods to reduce the brake noise, have to assume the vehicle suspension system which acts more on the braking system, but at the end the squeal is avoided by trial and error method. Although we see an efficient improvement in understanding the actual reason for the brake squeal, it has been difficult to solve it. It was easily verified through the design and study to overcome the squeal, but it was different case practically. The failure was mainly caused because of the dynamic behaviour of the braking system and due to the gap between braking components. The easiest way was using high temperature