ABOUT BRAKES
BRAKE
A brake is a device that decelerates a moving object such as a machine or vehicle by converting its kinetic energy into another form of energy. In simple, brake is a device which prevents an object from accelerating.
CONVERSION OF ENERGY
In conventional braking, kinetic energy is converted into heat this is done by brakes using friction. But in regenerative braking the kinetic energy is converted instead into useful electrical energy or potential energy (in the form of pressurized air).
NEED OF BRAKES
To stop the vehicle.To control speed, when and where required quickly and efficiently.To control the vehicle while descending along a slope.To keep the vehicle in the required place after bringing it to rest even when operator is not present.
TYPES OF BRAKES
By Application
Feed brake or Service brake.Hand brake or Parking brake.Pneumatic
By Method of Power
MechanicalHydraulicVacuumElectricalAir
By Method of Operation
ManualServo SystemPower operation
By Construction
Drum brakeDisc brake.
DRUM
Material Cast steel or alloy of chrome nickel made by centrifugal castings.
REQUIREMENTS
It should have surface with good anti-wear quality.It should allow optimum rate of heat transfer.
BRAKE SHOES
Brake shoes are radial plates having linings of friction materials at their backs.
Material Organic or metallic, metallic=sintered metal and finely powdered granite. Brake linings either riveted or cemented on the bearing surface.
2.2 GROWTH OF BRAKES
All brakes work by friction. Friction causes heat which is part of the kinetic energy conversion process.
Bicycle wheel brakes
The construction is very simple and out-in-the-open. A pair of rubber blocks is attached to a pair of calipers which are pivoted on the frame. When you pull the brake cable, the pads are pressed against the side or inner edge of the bicycle wheel rim. The rubber creates friction, which creates heat, which is the transfer of kinetic energy that slows you down. There are only really two types of bicycle brake - those on which each brake shoe shares the same pivot point, and those with two pivot points.
bicycle brakes
Drum brakes - single leading edge
The next, more complicated type of brake is a drum brake. The concept here is simple. Two semicircular brake shoes sit inside a spinning drum which is attached to the wheel. When you apply the brakes, the shoes are expanded outwards to press against the inside of the drum. This creates friction, which creates heat, which transfers kinetic energy, which slows you down. The example below shows a simple model. The actuator in this case is the blue elliptical object. As that is twisted, it forces against the brake shoes and in turn forces them to expand outwards. The return spring is what pulls the shoes back away from the surface of the brake drum when the brakes are released.
single drum brake
The "single leading edge" refers to the number of parts of the brake shoe which actually contact the spinning drum. Because the brake shoe pivots at one end, simple geometry means that the entire brake pad cannot contact the brake drum. The leading edge is the term given to the part of the brake pad which does contact the drum, and in the case of a single leading edge system, it's the part of the pad closest to the actuator. The diagram below shows what happens as the brakes are applied. The shoes are pressed outwards and the part of the brake pad which first contacts the drum is the leading edge. The action of the drum spinning actually helps to draw the brake pad outwards because of friction, which causes the brakes to "bite". The trailing edge of the brake shoe makes virtually no contact with the drum at all. This simple geometry explains why it's really difficult to stop a vehicle rolling backwards if it's equipped only with single leading edge drum brakes. As the drum spins backwards, the leading edge of the shoe becomes the trailing edge and thus doesn't bite. drum brake leading shoe
Drum brakes - double leading edge
The drawbacks of the single leading edge style of drum brake can be eliminated by adding a second return spring and turning the pivot point into a second actuator. Now when the brakes are applied, the shoes are pressed outwards at two points. So each brake pad now has one leading and one trailing edge. Because there are two brake shoes, there are two brake pads, which mean there are two leading edges. Hence the name double leading edge.
double drum brake
Disc brakes
Disc brakes are an order of magnitude better at stopping vehicles than drum brakes. Disc brakes are two-part system. Instead of the drum, you have a disc or rotor, and instead of the brake shoes, you now have brake caliper assemblies. The caliper assemblies contain one or more hydraulic pistons which push against the back of the brake pads, clamping them together around the spinning rotor. The harder they clamp together, the more friction is generated, which means more heat, which means more kinetic energy transfer, which slows down the vehicle. Standard disc brakes have one or two cylinders in them - also known as one or two-pot calipers. Where more force is required, three, or more cylinders can be used. Sports bikes have 4- or 6-pot calipers arranged in pairs. The disadvantage of disc brakes is that they are extremely intolerant of faulty workmanship or bad machining.
basic disc brake
FLOATING ROTOR
Standard brake rotors are cast in a single piece which bolts directly to the wheel or drive plate. If the mounting surface of your wheel or drive plate isn't perfectly flat, you'll get vibration at speed. Floating rotors are typically cast in two pieces - the rotor and the carrier. The carrier is bolted to the wheel and the rotor is attached to the carrier using float buttons. The other method of floating a brake rotor is to have the rotor bolted directly to the wheel itself without a carrier, but the bolts have float buttons built into them. These buttons allow the brake rotor some freedom to move laterally, but restrict the angular and rotational movement as if they were bolted directly to the wheel. This slight lateral motion which can be less than 0.03mm is just enough to prevent vibration in the brake system. Because the calipers are mounted solidly, and warping or misalignment in the wheel or brake rotor mounting face can be compensated for because the rotor will "float" laterally on the float buttons.
floating disc brake
This side-to-side vibration is separated from the carrier by the float buttons themselves, so none of the resulting motion is transferred into the suspension or steering. The rendering below shows an extreme close-up of the brake disc shown above.
floating disc brake button
Full-contact Disc brakes
NewTech full contact disc brakes
NewTech designed a disc brake system called "full contact disc brakes". They looked at traditional pad and rotor design and figured that the pads only contact about 15% of the rotor surface at any one time. With a change of design, NewTech were been able to add 5 more pads to the system so that 75% of the brake rotor is in contact with the pads at any one time. With traditional pads and rotors, the brake rotor is clamped between the pads. With the NewTech design, the brake rotor itself becomes a floating rotor, similar to those found on motorbikes. It is covered with a 'spider' (the red structure in my renderings below) and the spider has 6 brake pads on the inside of it. The hydraulic system acts on fully circular elastomeric composite diaphragm behind the brake disc, mounted in the black structure in the renderings. This had 6 pads on it which push the entire disc out against the 6 pads inside the spider. This provides and even force across the entire disc to push it out and the disc gets an even contact with all 12 pads. To ensure the brakes remain cool, the system is covered in cooling fins connected to the outer pads to dissipate heat. The inner pads are fitted with a molded thermal barrier made of a composite material. Special inserts made of a variety of frictional materials are distributed evenly on the entire surface of the pad. The range of materials is used to ensure performance under diverse conditions. NewTech believed that the system had considerable advantages over conventional brakes with better cooling, higher strength and reduced noise and vibration.
POWER BRAKES AND MASTER CYLINDERS
Power brakes (also known as power assisted brakes) are designed to use the power of the engine and/or battery to enhance your braking power. Whilst you can generate a fair amount of force using your foot, using systems from elsewhere in the car to help you apply even more force means that you get more powerful brakes as a result. The four most common types of power brakes are: vacuum suspended; air suspended; hydraulic booster, and electro hydraulic booster. Most cars use vacuum suspended units (vacuum boosters). In this type of system, when you press the brake pedal, the push rod to the master cylinder opens a vacuum control valve. This allows vacuum pressure (normally from the intake manifold) to "suck" on a diaphragm inside the vacuum assist unit. This extra vacuum suction helps you to produce more force at the pedal end of the brake system. Hydraulic booster systems usually utilize pressure from the power steering system to augment pressure on the master brake cylinder. Electro hydraulic booster systems use an electric motor to pressurize the hydraulic system downwind of the brake pedal which has the effect of amplifying the internal pressure in the whole system. The advantage to this system is that as long as you have battery power, you have power brakes even if the engine fails. With vacuum-assist brakes, no engine means no assistance.
Components of master cylinder
Brake master cylinders are complicated affairs involving finely manufactured parts, minute tolerances, springs, o-rings and rubber seals. The diagram below is a simplified representation of a dual-circuit master brake cylinder. When you step on the brake, it’s connected to the main plunger. As this is pushed into the master cylinder it acts on the components inside. The rear plunger (in blue) is the first one to start moving. As it moves forward, brake fluid from the reservoir is sucked in through the fluid intake and return port. At the same time, fluid is sucked in through the equalization port. As the second circuit rear seal passes the intake and return port (about 1.5mm after the plunger starts moving), it creates a fixed volume of fluid between the rear and front plungers. The more you step on the brake pedal, the more this fluid is now forced out into the second brake circuit to apply those brakes. At the same time, the pressure building up in this area overcomes the strength of the first circuit return spring and the front plunger (red) begins to move too. As with the rear plunger, it too sucks fluid from the reservoir until the first circuit rear seal passes the fluid intake and return port (again about 1.5mm), trapping fluid between it and the front of the master cylinder. This fluid is then forced out into the first brake circuit, applying those brakes.
When you take your foot off the brakes, the return springs push the plungers back into their neutral position. Fluid returns to the brake fluid reservoir and the system goes back to an unpressurised state.
brake master cylinder
Anti lock Braking Systems - ABS
The system is typically comprised of 4 ABS rings, 4 sensors, an ABS computer and a number of pressure-management circuits in the brake lines. The ABS rings are attached either to the wheels, or more often, to the brake discs. They look like a notched ring. The sensors are magnetic field sensors which are held very close to the ABS rings and can detect the slight change in magnetic field as the teeth on the ring pass them. The pulsing field tells the ABS computer that the wheels are spinning, and how fast they're spinning. When you brake, the wheel rotation starts to slow down. The ABS computer "listens" to the input from the sensors and can detect if one wheel is slowing down much quicker than the others - the precursor to the wheel locking up. (This all happens in milliseconds, by the way). When the computer detects this condition, a pressure regulator in the brake circuit interrupts the pressure in the brake lines by momentarily reducing it so that the brakes release just enough to give the wheels a chance to keep spinning rather than locking up. The computer then instructs the regulator to re-apply full pressure and again measures the wheel rotation. This on/off/measure cycle happens around 15 to 30 times a second. If the ABS kicks in, you'll feel it through the brake pedal as a vibration because the pulsing in the brake circuit affects all the components.
Newer generation ABS systems
As technology marches on, so does the control/feedback system used in ABS. It used to be the case that any single wheel approaching lockup would cause the ABS system to pulse the brake pressure for all the wheels. With the latest vehicles, the ABS computer is connected to 4 pressure regulators instead of just the one. This means it can selectively apply pulsed braking only to the wheel(s) that need it. So if three of the tyres are gripping well, but the front-left is beginning to skid, the ABS can unlock the front-left brake and pulse it to try to regain grip. It's called three- or four-circuit ABS. When hooked up to the traction control system, this type of multi-circuit ABS can also be used to influence the overall traction of a car in extreme maneuvers, such as helping to prevent rollover and inside-wheel-lifting.
COMPONENTS OF BRAKING SYSTEM
MASTER CYLINDER
DESCRIPTION
The purpose of the master cylinder is to displace fluid for application of the hydraulic applied caliper swing brake.
In the normal off position, fluid movement through the bypass port compensates the closed hydraulic system for temperature expansion or contraction and seepage.
When a brake application is made, the cylinder piston moves in. As the piston moves in, the primary cup closes off the bypass port and fluid is displaced from the master cylinder until sufficient pressure is developed to apply the caliper brake.
Upon release, the piston is returned against its stop by a spring. Some reservoir fluid is allowed to flow through the passages in the piston face and around the primary cup lips to relieve the vacuum caused by the slower return of the displaced fluid from the system. Finally, the piston clears the bypass port opening it so that excess fluid can return to the reservoir.
AIR COMPRESSOR
The function of the air compressor is to build up and maintain the air pressure required to operate air powered devices in the air system.
The compressor runs continuously while the engine is running but actual compression of air is controlled by the governor which stops or starts the compression of air by loading or unloading the compressor in conjunction when the air pressure in the system reaches the desired maximum or minimum pressures.
SAFETY VALVE
The safety valve protects the air system against excessive air pressure above 150psi. Should reservoir pressure, below the ball valve, rise to a point above the setting of the safety valve, the force developed will overcome the force of the regulating spring holding the ball on its seat, and the ball will lift. This permits air to flow into the spring cage and exhaust to atmosphere through the exhaust port. As soon as this exhaust process reduces the pressure to the setting of the safety valve, the regulating spring forces the ball back on its seat, stopping the exhaust.
The safety valve is screwed into the tee on the reservoir at the left side of the carrier.
BRAKE VALVE
The brake valve is the control unit of the air brake system. It provides the driver with an easily operated and graduated means of applying and releasing the vehicle brakes.
RELAY VALVE
The relay valve functions as a relay station to speed up the application, modulation, and release of the vehicle brakes. This valve can be considered a remote mounted, air controlled (pilot), brake valve which delivers or releases air to air/hydraulic intensifiers in response to the signals received from the brake valve in the upper.
One of the relay valve supply port is used to supply air to the parking brake and outrigger safe lock solenoid valves.
AIR DRYER
The air dryer collects and remove moisture and contaminants from the air before the air is delivered to the first reservoir. It is distinctly different than a reservoir drain or alcohol evaporator in that it provides “dry air” for the air system.
AUTOMATIC DRAIN VALVE
An automatic drain valve is mounted on the supply reservoir at the right side of the carrier. The purpose of the drain valve is to automatically eject moisture and contaminants from the wet tank when activated. The automatic drain valve is activated whenever system pressure drops 6psi.
The drain valve is protected from freezing by a heater unit inside the valve. This heater unit is protected by an 8amp fuse.
Wear Alert
Brakes and tires - Braking - utmost important - worn off brake lining - brake associated parts such as hub, disk, shaft, etc. - The current spring steel alert system - Drawbacks - only activated while the car moves and secondly, when the car is on move, rarely the alerting sound reaches the driver. - a micro sensor is embedded into the pad - The evaluation of thickness, hardness, layer properties and critical wear region has enabled the spotting of exact sensor location.
The braking system is composed of many parts, including friction pads on each wheel, a master cylinder, wheel cylinders, and a hydraulic control system. - The trailing end of a pad is always hotter than the leading end, - wears more rapidly - excessive taper wear - excessive flex
low brake pedal. - aimed to determine the capability of using micro sensor to detect the wear imit of a brake pad lining. - If the linings are used without replacement after the lining exceeds its safety wear limit, the backing plate of the pad will damage the disk rotor which would cause the newly replaced lining to wear at faster rate due to the disk’s rough surface. - mechanical noise which produced by mechanical touch sensing technique using spring steel when the safety wear limit is reached.
development of asbestos free brake pad. - experiment focuses on - brake pad surface topography and the occurrence of squeals. - experimental result - small contact plateaus have a larger tendency to generate squeal than pads with a few large plateaus. - the size of the contact plateaus increases rapidly with brake pressure - Friction layers and friction films - investigated by - Focused Ion Beam (FIB) technique, - The experiment was mainly to show that the FIB technique provides additional information which in combination with the more conventional techniques LM, SEM and TEM increases the knowledge on the role of third body formation and superficial layers on brake pads – result - friction performance - dependent on the base metallic matrix composition and formation of a friction layer on the brake pad surface - The effect of metal fibers on the friction performance of automotive brake friction materials were investigated – results - when gray cast iron was used as a counter disk at low temperatures, the friction materials containing copper or steel fibers showed high speed sensitivity. - failure has been studied on small cracks on the disc brake. - cracks were mainly located nearby the holes placed on flange to ventilate and refresh pads.- lifespan - depends strictly on the geometry , the material properties at high temperatures and operating conditions. - investigated about third body formation on brake and rotors. - The Focused Ion Beam technique (FIB) was used to reveal tribologically induced surface films and for cross-sectional preparation of superficial layers. - investigations on the nanometre scale are essential for understanding the frictional behaviours of such contacts.
This observation is to confirm whether the distance of pad from the master pump gives any effect on the lining wear. - the pad on the piston side and calliper side were also critically analyzed to see if the effect of floating brake system affects - lining wear is inverse proportional to the distance from the master pump, - nearest to master pump wears slightly more that the further ones - the left hand and right hand side of the rotor disk also witnessed some differences. - reason for this - because the piston on wheel reacts and pushes the piston side pad first before pulling the other side pad to the disk. - the wear pattern is noticed to be almost experiencing same trend - tested for hardness at four different depths of lining using hardness testing machine - study the hardness region change in pad over the depth. - result observed, as the depth (wear) increases the hardness of the brake pad also found to be higher. - pad surface is harder, it will reduce the braking efficiency and together, will damage the disc brake
Based on the criteria of size, durability, simplicity and costing, micro switch has been selected for sensing the lining safety limit.