The Brake System Of Moving Machines And Vehicles
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Brakes are one of the compulsory components in a moving machine. Any moving machine or vehicle needs a brake to decelerate and stop. As we want the car to go fast, we would also want the car to stop quickly and safely. Not just for a safety, the vehicle equipped with a good and consistent brake will also increase the driving pleasure.
In this paper we will analyse the principle of brakes. Furthermore, we will discuss current variation of brakes system, brakes type and actuators mechanism. The aim of this paper is to discuss and understand how different automobile braking system work. In addition, we will discuss the future development system and the sustainability of the system.
Brakes were first used on horse-drawn wagon, which was a tire brakes. History of brake is a part and integral with the automobile history. Later, when the cars has been develop, the need for a proper brake become crucial as the cars turned heavier and more powerful. Earlier engineer has tested and failed many design until they found the design that worked.
In 1902 few brake systems were introduced to the market. Frederick William Lanchester patented disk brake system in 1902. Because of the design create a terrible noise, the design doesn't accept in the market. In 1907, after Herbert Frood improved the pads with asbestos material and solved the noise problem it started to catch the market attention (Puhn, 1985). However, the disk brakes were not become standard in Europe until much later.
The drum brakes were first tested in 1902, by Ransom E. Olds, which was designed in New York. The design was the external drum brakes. It works by wrapping stainless steel band around the drum, which attached to the rear axle (Harris, 2009). The new design by Olds, has allowed the car to stop faster compare to the older design on horse-drawn wagon. It becomes popular as most of the car manufacturer during that time used the design. Later, the external drum brakes were found not entirely reliable especially on dirt road and wear out after 200 miles. The solution was the internal drum brake (Harris, 2009).
Malcolm Lougheed introduced the hydraulics brakes by using tubes to generate hydraulic power to brakes shoe in 1918. In 1921, model A Dusbenberg becomes the first car to use the hydraulic brakes. Most of the manufacturers used the hydraulic brake by 1931. Ford started to use the hydraulic brake by 1939 (Harris, 2009).
The early antilock brake system was invented for airplane in 1929 by Gabriel Voisin and been applied to automobile in 1950s. Even though the antilock brakes were proven reliable, it had not applied to the cars until Chrysler introduced its "Sure Brake" on the 1971 Impala model (Harris, 2009). The modern antilock brakes were produced by Mercedes-Benz and Teldix and were used in the 1978 Mercedes-Benz S-Class (Harris, 2009).
The basic purposes of the brake system in the motor vehicle are:
- To prevent unwanted movement of the resting vehicle with used of parking brakes.
- To prevent unwanted acceleration, i.e. while driving downhill with the continuous braking.
- To reduce the speed and if necessary to stop the vehicle with deceleration braking
Different types of the brake are arranged differently with a different method of forcing friction surfaces together. It will also create a different heat dissipating once its generated. As a system, brake consists of few components. The system includes pedals, levers, lines, brakes component and the actuating medium (mechanical, hydraulic, pneumatic and electric) (Breuer and Bill, 2008).
Brake Circuit Configuration
It is legally mandatory for the car manufacturer to provide the brakes system with a dual circuit transmission system for safety purpose, because the probability of failure is lesser compare to single circuit system. They are five available options for the circuit configuration.
In contrast to II and X distribution, HI, LL and HH distribution have a weakness in the event of overheating at one wheel, could cause a failure to a circuit. The X distribution is used on vehicle with forward weight bias, while the II distribution is a solution for vehicle with rear weight bias (Bosch, 2007).
BRAKE ACTUATING SYSTEM
Between driver's foot and wheel brakes, there are components that transmit force from driver's foot into friction force at the brake friction surface. Those components act as actuating system for the generated force. The system could be mechanical, hydraulic, pneumatic, electric or combination of these. All kind of system will give the same result. As the system operated, brakes applied. Brake system application always begins with driver operating the pedal. Brake pedal are design to determine the foot force required to stop the vehicle.
The simplest brake actuating system is a mechanical system. The brake pedal operates rods or cables that apply the brake when actuated. The system was widely used in earlier system and still used in modern system for parking brakes. Most of system in modern vehicle are used the hydraulic system. Instead of cables or rod used to actuate the brake, fluid filled line and hoses are used for actuation (Breuer and Bill, 2008). However, cable and linkage system directly transferred force generated at the pedal without any assistance.
Pneumatic brake actuator systems are generally used in large commercial vehicles and trucks. The only advantage of the pneumatic system compare to hydraulic system is leak safe, where small leak in the system could not cause loss in braking due to air constantly supplied by compressor and stored in large reservoir.
However, there are factors that hinder pneumatic brakes. First, air is compressible. Because of that nature, a relatively large amount of air required to flow from reservoir to brakes chamber before the air is compresses enough to built the pressure to decelerate the vehicle (Ewel, n.d.).
The second immutable fact is, air contain moisture. While passing through the pressure-regulating valve in pneumatic brake system, air expands and become cools. Subsequently, the cool air condenses. In cold environment, this condenses air can freeze and cause the brakes to fail. In order to overcome the problem, the system must be equipped with drain valve, air dryer and alcohol evaporators (Ewel, n.d.).
All of passenger cars used hydraulic brake system instead of pneumatic system because, hydraulic system are smaller and simpler compare to pneumatic system. In addition, pneumatic system requires bigger size and more components to be installed.
In contrast to pneumatic brakes system, hydraulic brake system require less preventive maintenance such as draining the reservoir and refilling the alcohol evaporator for pneumatic system. Unlike pneumatic system, hydraulic system offers more reliability in term of maintenance cost and downtime over the life of the system (Ewel, n.d.). With all the advantages of hydraulic brakes system, its make it suitable and reliable for used in passenger car.
Electro Hydraulic Brakes (EHB)
In conventional braking system, force from brake pedal transmitted mechanically via brake booster to the wheel cylinder. In contrast to the conventional system, electrohydraulic brakes transmit the brake request from pedal to wheel cylinder via sensors and ECU (Bosch, 2007).
Electrohydraulic brakes operate by actuating the actuator unit that detecting the pedal travel and pedal pressure, the information from the actuator unit then transmitted to ECU (Bosch, 2007). The ECU calculates the signal to be send to wheel pressure modulator, which converted the signal into brake pressures for individual wheels .The electrical driven hydraulic pump with pressure monitoring system provides the hydraulic pressure supply to the wheels (Bosch, 2007).
Since there are no mechanical actuating parts, no vibration felt through the brake pedal, which normally felt in other type of actuating system. Other significant advantage of the system is it will reduce space use in engine bay.
Antilock Brakes System (ABS)
The tendency of vehicle manufacturer to increase safety of vehicle has lead to the development of the vehicle intelligent assistance system such as ABS. Similar to other intelligent brake assistance system such as ESP, EBD and others, the system typically is to assist drivers with critical driving situation. In this section, we will only discuss the ABS system.
ABS brakes were designed to overcome the problem of tire lock up and uncontrolled spins. Since brakes are most effective at slowing the car at a point just before wheel lock up, a system that provides wheel braking while preventing wheel lock up such as ABS is very desirable. The features give ABS a significant advantage over conventional brakes system.
In contrast to conventional braking system, ABS can provide a shorter controlled braking distance on a slippery road condition. In addition, ABS will also enhance the steering ability, as it will allow the driver to safely manoeuvres the vehicle while cornering, as illustrated in figure 1.6.
With electrical interface, the ABS features could be activated using software and sensors, instead of additional hydraulic or mechanical components.
Figure 1.6 Comparison of ABS braking and Non-ABS braking (NAZA Wheels, 2010)
Most of production car equipped either with one or both type of braking assemblies. They are drum brakes and disc brakes. Each type offers inherent advantage over the other. However, the operation and function of those brakes will not be discussed, as they are beyond the scope of discussion in this paper.
Unlike disc brake, drum brake is more complicated. There are two types of drum brakes, single leading edge and double leading edge. Leading edge refers to part of brake shoe that actually contact with drum relative to drum rotation while braking. In contrast to single leading edge, double leading edge have two actuators instead of one. Therefore, it will give maximum point of drum contact.
Disc brakes already known as better in performance compare to its drum counterpart. As a result, they normally fitted to sports or high performance cars. Standard disc brakes normally have one or two cylinders in them, also know as one or two pot calliper. Those cars that required more braking ability will use three or more cylinders. However, major disadvantage of disc brake is they are intolerant of uneven disc surface, which will cause vibration during braking.
Part of disc brake system is rotor. Unlike older type of rotors, new design of brake rotors has come with many features to overcome the problem of heat dissipation as well as trapped gas between pads and rotors. Those design features produced several types of brake rotors such as grooved rotor, grooved drilled rotor, and dual vented rotor.
They are several types of material being used for friction material in brakes also known as brake pads. Most of pads used to use asbestos, but the use of asbestos become lesser as its dust is harmful to health and environment. Today, all kind of combinations of materials used for pads. They are several typical materials combination for pads.
Organic pads are the commons used in saloon car. These pads are suitable for street driving because they wear well, less noise, soft and less dust. This type of pads function well in cold condition, but less efficient at higher temperature condition (Car bible, 2010).
Semi metallic or sintered pads is a better choice for sportier car as its compromised between street and track use. However, the drawback in this type of pads compare to organic pads is they are noisier (Car bible, 2010).
While for metallic pads, they are typically used in racing or performance car. Even though the braking efficiency is very good at high range of temperature, metallic pads squeals and relatively produces lots of dust compare to other type of pads. Furthermore, the metallic compound is hard on rotor and will chew the rotor up.
Ceramic pads used about 15% of metal fiber in its compound. Copper fiber is used instead of steel and therefore causes less wear and better heat dissipation. Ceramic pads have many advantages over other type of pads. They don't easily fade, cool faster, longer lasting and effectively silent. In addition, they create a minimum dust (Car bible, 2010).
BRAKE PRINCIPLES AND ANALYSIS
Nowadays the structure of brake systems has become increasingly standardized. The most widely used configuration consist a pedal incorporated with vacuum assisted brake booster (Breuer and Bill, 2008). The brake pedal linked to push rod will create an input force into the booster. Then the force generated in the brake booster transmitted to brakes master cylinder. The pressure from master cylinder provides the hydraulic pressure that acts on the brake pistons of the wheel brake, which is transmitted via hydraulic line.
The pedal operation is based on basic principle of leverage, which covered by principle of moments (figure 1.7).
Clockwise Moment = Anticlockwise moments
F1 (N) . y (m) = F2 (N) . x (m)
Example 1.1: A footbrake lever is 350mm long from pivot to footpad center. The push rod attached 60mm below pivot point. If force applied to footbrake pad is 300N, how much force is applied to master cylinder.
F1 . y = F2 . x
\ 300N X 0.35m = F2 X 0.2m
F2 = 300 N X 0.35 m
F2 = 1750 N
The moment of force in brake pedal is defined as a product of force act at perpendicular distance from fulcrum to the line of push rod (Bonnick, 2008). Pedal ratio (fig. 1.8), which is equivalent to mechanical advantage, is the distance ratio from pivot of effort to load.
Pedal ratio = Velocity ratio = Mechanical advantage
The force transmitted to input rod of brake booster is force applied to the pedal (Fped) multiplied with the transmission ratio of pedal (iped).
Therefore; Force on brake booster
F = Fped . iped
Example 1.2: The force applied to the brake pedal is 450N. The movement of footbrake pad is 80mm. The pedal mechanical advantage is 3.15. If the machine efficiency is 90%, find the push rod travel and force applied on it.
MA = Pedal Travel
\ 3.15 (90%) = 0.08 m
x = 0.0282 m
Force on push rod;
F = Fped . iped
\ F = 450N X 3.15 (0.9)
F = 1275.75 N
In the vacuum booster, the push rod operates the valve of the booster to allow lows pressure flow into working chamber of the booster. The diaphragm force (Fdia) is created by diaphragm pressure (pdia) on the affective diaphragm area (Adia),
Fdia = Adia . pdia
Adia = Total diaphragm area - Power piston area,
pdia = Ambient pressure - Manifold pressure
The boost ratio of vacuum booster is the result from relationship of sensor piston to reaction disc. Thus, in the calculation of the output force (FoutB) of the booster, the force of return spring (Fsp)of the booster must be taken into consideration (Breuer and Bill, 2008). The output force of the brake booster appears as below:
FoutB = Fdia - Fsp
Fsp = (spring pre-tension + movement of spring (m) X spring rate)
Example 1.3: The diaphragm in the booster has a diameter of 300mm and the power piston diameter is 65mm. The diaphragm spring pre-tension is 180N. Spring rate is 30 N/mm and the ambient pressure is 110 Kpa. Engine manifold absolute pressure is 30 Kpa. Find the maximum assistance from vacuum booster if the power piston moves 5mm.
Fdia = Adia . pdia
Adia = 0.25p (0.3)ô - 0.25p (0.065) ô
= 70.68 X 10 Ã õ - 3.31 X 10 Ã õ
Adia = 67.36 X 10 Ã õ
Pdia = 110 Kpa - 30 Kpa
= 80 Kpa
\ Fdia = (67.36 X 10 Ã õ m ô) X (80 X 10 õ N/mô)
= 5388.8 N
Hence, booster output force;
FoutB = Fdia - Fsp
\ FoutB = 5388.8 N - (180N + 6mm X 30 N/mm)
= 5028.8 N
Master cylinder is component that transmits force created by brake booster on the master cylinder push rod to hydraulic pressure in the brake system. The hydraulic pressure (Phyd) is calculated from the master cylinder piston force (FoutMC) and the affective area of master cylinder (AMC) as,
Phyd = FoutMC / AMC
= FoutD - FSp.MC
Where, FSp.MC = (Piston spring pre-tension + movement of piston x spring rate)
Figure .9 Cross section view of tandem master cylinder
Example1.4: A master cylinder being pushed 7mm by 850N force from booster. Master cylinder diameter is 10mm and spring pre-tension is 12 N. If return spring rate is 3 N/mm, find the pressure crated in brake pipe.
Phyd = FoutMC / AMC
= FoutD - FSp.MC
\ Phyd = 850N - (12N + 7mm X 3 N/mm)
0.25p (0.01) ô
= 10.40 Mpa
The calculation of force transmitted to the brake unit is different, depending on brake construction and characteristic (e.g. disc brake, drum brake etc.). As for disc brake, regardless any arrangement of the brake calliper cylinders, the force (FoutC) developed on calliper piston basically is the hydraulic pressure (phyd) generated in the system acts on the total calliper piston area (ACP) (Breuer and Bill, 2008). The output force on callipers defines as below:
FoutC = p hyd . ACP
Example 1.5: The master cylinder develops a pressure of 10.40 Mpa to the brake line, which deliver pressure to calliper cylinder. It the calliper piston diameter is 50mm, find the force developed by calliper piston.
FoutC = p hyd . ACP
\ FoutC = (10.40 x 106 ) N/m2 X (0.25 p (0.050)2 ) m2
= 20.420 KN
Braking on the wheels occur when the friction surface (pads or lining) acts on disc or drum. The transmitted force (Ftrans) from calliper piston to the wheel is calculated based on perpendicular output forced from calliper piston (FoutC) acts on pads with a calculated coefficient of friction (m). The coefficient of friction (m) is determined with a ratio of frictional force over applied force (Breuer and Bill, 2008).
Coefficient of friction; m = Frictional force
Ftrans = FoutC . m . no. of pads
Example 1.6: The calliper piston applies a force of 20.42 KN on the two brake pads. The coefficient of friction of pars is 0.48. Find the transmitted force.
Ftrans = FoutC . m . no. of pads
\ Ftrans = 2042N X 0.48 X 2
= 1.96 KN
The braking torque is the force generate for vehicle to come to stop. The transmitted force from the brake exerts the frictional force on the wheels, which creates a torque on axles. The torque created, acts opposite direction of the vehicle movement. Thus, the vehicle will stop. The braking torque for the disc brake system is calculated as:
Braking torque (TB) = Ftrans . Effective radius (R)
Example 1.7: If the distance between the pad's center of pressure and the center of disc rotation is 0.12m and coefficient of friction between the rubbing faces is 0.35, determine the braking torque produced by clamping force of 1000N.
Braking torque (TB) = Ftrans . Effective radius (R)
TB = Ftrans . R
\ TB = (1000N X 0.35 X 2) X 0.12
= 84 Nm
For a drum brake system, the sample analysis examine the construction of simplex drum brake is shown in fig 1.11. Other types of brake drum (e.g. double anchor/double cylinder in fig 1.12, duo servo) are commonly used as a parking brake due to higher brake characteristic value (Breuer and Bill, 2008). Due to clamping force acts in the direction of rotation, self-energizing effect is achieved with drum brake but not in disc brake system.
Example 1.8: A brake drum has an internal diameter of 0.3m. The distance between the shoe pivot and the points of application of the forces actuating the shoe is 0.18m. Assuming the shoes are centrally positioned in the drum and the coefficient of friction between the linings and drum is 0.38. Find the braking torque on each shoes and total braking torque developed by both shoes, if the actuating force if 680N.
n = r = 0.15m
m = 0.18/2 = 0.09m
For leading shoe;
FA = me
\ FA = PA me
= 680N X 0.38 X 0.18m
(0.09m - 0.38 X 0.15)
= 1409.45 N
= F . r
= 1409.45 X 0.15m
= 211.4 Nm
For trailing shoe;
FB = me
\ FB = PB me
= 680N X 0.38 X 0.18m
(0.09m + 0.38 X 0.15)
= F . r
= 316.40 X 0.15m
= 47.46 Nm
\ Total braking Torque = (FA + FB) . r
= (1409.45 + 316.40) N X 0.15m
= 258.87 Nm
BRAKE OF THE FUTURE
There are many improvements has taken place in brakes development, from older drum brakes to disc brakes then to multiple callipers. Now, a technology developed by Siemens VDO called Electronic Wedge Brakes being introduced. The electronic wedge brakes totally replaced the conventional hydraulic system. The system is powered by an existing 12-volt power from the car. In addition the system has a faster reaction time, about a third quicker compare to conventional system. It only required 100 ms to reach full braking power compare to hydraulic brake's 170 ms (Tan, 2007).
The systems work by pressing the brake pad that connected to the wedge with wedge shaped calliper. The callipers are moved by electric motors. With used of wedge principle of application, the braking power is multiplied with minimal energy expenditure (about one tenth of conventional hydraulic brakes). Because of no mechanical connection at all between brake pedal and callipers, the system can be defined as brake by wire system. Furthermore, because of the system controlled by ECU, other features such as ABS, TCS and ESP can be integrated together with the system. Test result comparing Audi A6 fitted with EWB against another A6 with conventional brakes system shows the braking distance from 100 km/h to 0 km/h was reduced to half in A6 equipped with EWB (Tan, 2007).
For safety reason, the EWB is specified to be connected with two power supplies, the main and the backup with secondary battery.
Brakes system sustainability could be achieved in several ways, either to manufacture the friction material with the non-hazardous materials, through the lightweight component design or through the principle of regenerative braking. Through the lightweight component design, it could contribute towards reducing CO2, as the lighter the vehicle the less fuel will be consumed. One means of reducing the component weight is to replace the component material with lighter materials. However, it could also be achieved by developing the component with used of current material such as steel and iron, but with a difference manufacturing methods. For instance, Continental AG has developed a new process in manufacture the calliper piston. Instead of cast or turning process, it used the press form process. It was formed from 3.5 mm thick circular metal blank into a very thin, light and equally stable piston Subsequently, it resulted the piston weight 25% less than its cast predecessor (Continental AG, 2008).
The production of the brake pads using the asbestos material has stopped due to health and environmental issues, is has been replaced with other fibre material such as aramid fibres. Recently, they are brake pads being develop using natural fibre such as hemp and jute.
While, the sustainability approach in context of regenerative braking system could be achieved with energy recycling. Regenerative braking is actually a process of recapturing and storing the kinetic energy generated by braking for later use. The energy in form of heat that normally dissipated to the air can be stored mechanically in form of compressed air or flywheel, or electrically in capacitors or batteries.
The brake system has gone through many development eras since people started using it in moving vehicle. It seems that, the earlier developments has contributed towards the braking efficiency and reliability. However, later developments are moving towards sustainability and environmental friendly. There is no doubt that the brake technology has to keep developing as the vehicle technologies developed.
Together with the brakes technology, the brakes components also went trough great phase of development, in term of material and production processes. Component developments have produced lighter brakes components with the same capabilities as conventional component materials. It is apparent that, the automotive industries are actually moving towards the sustainability globally. Incorporating design features to facilitate end-of-life recycling and recovery is vital in component production. The trend will be towards lightweight materials and lesser parts in vehicle design.
Through lots of reading during preparing this paper, lots of knowledge has been gained and it appears that the brake system is a very vast topic to be discussed.
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