In my literature review, I willl talk about all the thing that related to the objectives of my study. Performance of brake system in vehicle is very important to ensure the safety of the driver. Brake was invented to do onlyone thing that is to stop vehicle. It might sound easy, but based on Fred Puhn (1985), the main problem of brake system start when brakes must stop the vehicle from high speed in a short distance, and to do it over and over again. All the brake system nowadays using friction mechanism to stop the vehicles either drum brakes or disc brakes or the combination of the two.
Both disc brakes and drum brakes consist of two main part that is stator and rotor. Stator is the friction part while the rotor is the rotating part. Brake operate by pressing stator against the rotor. The rotating part for the drum brakes is the brake drum, while the friction part is the brake shoes. For disc brakes the rotating part is the disc, while the friction part is the brake pad. Brake system consist several part that is master cylinder, brake line, calliper, disc, pad and other.
Figure 2.1 shows typical vehicle braking system componentC:\Users\Luqman Hakim\Desktop\component brake.png
2.2 Type of Pad Material
Choosing the right material as the friction part of brake system gives much effect in term of performance such as
2.3 Temperature Performance
As we know, in term of temperature dissipation and cooling, disc brakes has much advantages as compared to drum brakes. Based on Arthur Stephen (2006), the rubbing surface of disc brakes is exposed to atmosphere, thus it will provide a better cooling and will reduce the thermal failure. On top of that, at elavated temperature drum will expand and can cause longer pedal travel while for disc brake, the increase of thickness of disc brakes at elavated temperature does not effect braking at all. Other advantages of disc brakes is it can withstand over 1000°C, while drum brakes maximum temperature is about 500°C-600°C, Limpert (1975) Moreover, when wear occur at the friction part of the brake, the drum brake needto adjust when shoe wear while for disc brake, the brake pad automatically adjusted, Arthur Stephen (2006)
Temperature dissipation is the most important aspect in determining the temperature performance of the brake system. This is because, overheating brakes gives much problem such as deterioration. The most common problem is brake fade. Brake fade is a temporary loss of braking power when paddle is pushes due to high temperature that reduce the coefficient of friction between material and the rotor. After temperature decrease, normal braking will return. The brake fade was designed to occur at lower temperature than the flame temperature of the material. This is due to avoid possibility of fire at a very high temperature. Overheating brakes also gives other problem such as thermally excited vibration or brake judder, excessive component wear and also rotor deterioration.
Brake judder can be defined as the low frequency vibration that may be felt by the driver that occur due to steering shake or body shake. There are two types of judder that is thermal judder and cold judder. Based on (Arthur Stephen), cold judder occur due to at uneven rotor thickness that leads to deviation in contact pressure of disc and pad. This will cause of uneven braking torque variation. The thermal judder occur at the elavated temperature and it is cause by the thermal deformation of the rotor. Cold disc thickness variation gives bad effect while braking, the thicker part of rotor will experienced much greater pressure than the thinner part, thus thicker part will become hotter and lead to uneven thermal expansion of rotor. This thickness variation of rotor may cause by the phased change from pearlite to martensite. When brake disc that made from cast iron cooled rapidly, the martensite will occupied lager volume than pearlite thus thickness variation of rotor formed. Thermal judder may caused by 'hotspot' on the rotor surface that cause by localised contact between pad and rotor in small area of very high temperature.
According to Limpert (1975), the heat conduction from the friction part of the brake system can lead to several problem such as damaged seals, brake fluid vaporisation and wheel bearing damaged. The heat radiated from the brake to the tyre can also damaged the tyre at temperature as low as 93°C. Brake fluid vaporization is cause by the extreme temperature during brake that exceed the boiling point of the hydraulic fluis. Thus fluid is vaporized. This lead to vapor lock in hydraulic circuit and since gas is more compressable than liquid, when paddle is pushed it does not actuate the brake efficiently due to much pressure used to compress the gas than the brake fluid.
In any braking operation, the rise of temperature is depend on certain factor that is mass of vehicle, rate of retardation and the duration of braking, Arthur Stephen (2006). Normally at short braking, the level of retardation is low. At this point, the rotor and the friction material absorb all the thermal energy generated. In extreme braking such as in in steep descents or repeated high speed brake application, the heat generated is equal to the heat dissipation at steady state condition. However if the heat generation exceeding the dissipation, brake failure may occur.
Heat dissipation from brake disc occur by three method of heat transfer that is via conduction of heat through brake assembly and hub, radiation of heat to a nearby component and convection of heat to atmosphere. Conduction is one of the most efficient type of heat transfer because it can gives bad effect to surrounding component such as damaged seal, brake fluid vaporization and wheel bearing damage. Radiation is very effective at higher temperature but it must be control to prevent beading of the tyre. From all of heat transfer, convection to the atmosphere from the rotor is the primary of heat dissipation, Arthur Stephen (2006). Based on Newton's law of cooling
Q=hAs(Ts-T∞) Equation 2.1
Q = the rate of heat transfer (Watts)
h = the convection heat transfer coefficient (W/m2k)
As = the surface area of rotor (m2)
Ts = surface temperature of the brake rotor
T∞ = ambient air temperature
From the above equation we can say that to maximise the heat transfer from the rotor is by icreasing the rate of heat transfer, Q by keep the surface temperature Ts, at minimum, increase the surface area, As and the best solution will be to make the convection heat transfer coefficient, h increases. This is because, according to Limpert (1975), improvement in cooling is best made by increasing the values of heat transfer coefficient due to the heat transfer coefficientis depend on the boundry layer that is influenced by surface geometry, the nature of fluid motion around rotor and thermodynamic and fluid transfer properties.
For the prediction of brake pad temperature
2.4 Surface Topography
The wear analysis of pad surface was also carried out in term of morphology characterization performed by appropriate observing equipment such as Scanning Electron Microscopy (SEM) and energy dispersive X-ray (EDX). The purpose of this qualitative assesment is to investigate the contact situation during sliding between pad and disc and to compare the surface condition at different braking regardless of the operating conditions. Brake pads are composites material which consists of various materials with different properties, such as hardness and strength, also with a wide spectrum of wear resistance which resulting in complex contact situation
2.5 Hard Particle
In this subtopic, I will explain about the effect of additing hard particle to the brake assembly line system of vehicle. As we know, modern driving of vehicle barking system is an open air cooling type. Since it is being exposed to the surrouding, it may allowed any contaminated particle such as dust and sand from the road.
2.6 Summary
