Disc brakes were most popular on sports cars when they were first introduced, since these vehicles are more demanding about brake performance. Discs have now become the more common form in most passenger vehicles, although many use drum brakes on the rear wheels to keep costs and weight down as well as to simplify the provisions for a parking brake. As the front brakes perform most of the braking effort, this can be a reasonable compromise.
Disc brakes offer better stopping performance than comparable drum brakes, including resistance to “brake fade” caused by the overheating of brake components, and are able to recover quickly from immersion (wet brakes are less effective). Unlike a drum brake, the disc brake has no self-servo effect and the braking force is always proportional to the pressure placed on the braking pedal or lever.
As shown in fig a disc brake consists of a cast iron disc bolted to the wheel hub and a stationary housing called caliper. The caliper is connected to some stationary part of the vehicle, like the axle casing or the stub axle and is cast in two parts, each part containing a piston. In between each piston and disc there is a friction pad held in position by retaining pins, spring plates etc., passages are drilled in the caliper for the fluid to enter or leave each of the housing.
Two types brake discs are generally used the solid type and the ventilated type. But the ventilated type are thicker and heavier than solid type, they are liable to wrap at severe braking conditions, the dirt accumulates in the vents which affects cooling and apart produces wheel imbalance.
The discs of the brakes are made of pearlite gray cast iron. The material is cheap and has good anti-wear properties. Cast steel discs have also been employed in certain cases, which wear still less and provide higher coefficient of friction. Their main drawback is the non uniform frictional behavior.
On non-driving wheels, the centre of the brake disc or hub contains the wheel bearings. The hub can be part of the brake disc or a separate assembly between the wheel and hub with nuts or bolts. On driving wheels, the disc is mounted onto the driving axle and may be held in place by the wheel. On front wheel drive vehicles, it can be mounted on the front hub and wheel bearing assembly.
MANUFACTURING OF CERAMIC BRAKE DISCS
In the earlier days, the brake discs were made from the conventional brittle ceramic materials would have disintegrated into a thousand pieces under slightest pressure. The research division of Daimler Chrysler has developed techniques to make carbon fiber reinforced brake discs which avoid the brittleness problem. In the earlier days, long carbon fibers were used. Later the use of short carbon fibers increased the efficiency.
The composites for producing fiber reinforced ceramic brake discs are short carbon fibers, carbon powder, and resin mix. The process involves first compressing the carbon fibers, carbon powder and the resin mix together and then sintering at 1000 degree Celsius. In the furnace a stable carbon frame work created. This consists of carbon fibers in a carbon matrix. Once cooled this material can be ground like wood and the brake disc obtains its final shape.
Together with silicon the ground break disk blank is then inserted into the furnace a second time. The pores in the carbon framework absorb the silicon melt like a sponge; the fibers themselves remain unaffected by this process. The ceramic material is created when the matrix carbon combines with liquid silicon. This fiber reinforced ceramic material cools over night and the gleaming dark grey brake disc is ready.
Resin is a binder, which holds the different constituents together. Resins are of two types:
1. Thermosetting resins
2. Thermoplastic resins
COMPARISON OF CERAMIC BRAKES AND CONVENTIONAL BRAKE DISC
Using a ceramic composite takes advantage of a material with outstanding hardness (and potentially long life) and an ability to retain its strength and shape at temperatures that would melt conventional iron brake material into a glowing puddle.
Simple single-ingredient ceramics tend to be brittle like dinnerware, though some types work well in turbochargers or as bearings for jet engines. To make ceramics that are tough enough for a brake disc, the material is manufactured as a composite: strands of carbon fiber, which are highly resistant to stretching, are embedded in the material
Until now brake discs have been made up of grey cast iron, but these are heavy which reduces acceleration, uses more fuel and has a high gyroscopic effect.
Ceramic disc brake weigh less than carbon/carbon discs but have the same frictional values with more initial bite and cost a fraction of price. Carbon /carbon discs are used only in Formula 1 racing cars etc, because it is so expensive. More over ceramic brake discs are good even in wet conditions which carbon / carbon disc notoriously fails to do.
But comparing their weight, you will see right away that we are looking at two different worlds, with ceramic brake discs more than 61 per cent lighter than conventional cast iron discs. In practice this reduces the weight of the car, depending on the size of the brake discs, by up to 20 kg. And apart from saving fuel, resulting in better and lower emission for the same mileage, this also means a reduction in unsprung masses with a further improvement of shock absorber response and behavior. Another is the manufacturer can add more safety features without adding to current weight.
It was first introduced in Formula One, but applying to road cars seems impractical (F1 cars have warm up lap to bring the discs into appropriate working temperature), although the short-lived French sports car specialists Venturi made history by applying it to its road cars in the mid-90s
The new 911 GT2 comes with the most effective braking system ever featured on a production Porsche: the Porsche Ceramic Composite Brake (PCCB) – a powerful new technology designed to cope with even the most extreme conditions on racetrack and road?
Mercedes-Benz’s the futuristic Vision GST concept car features 22 inch wheels, ‘butterfly’ doors, three-dimensional instruments, a 360 horsepower 5.5 litre V8 engine, and carbon-fiber reinforced ceramic disc brakes. Mercedes Benz SLR IS ALSO available with ceramic disc brakes.
Sachin is a B-TECH graduate in Mechanical Engineering from a reputed Engineering college. Currently, he is working in the sheet metal industry as a designer. Additionally, he has interested in Product Design, Animation, and Project design. He also likes to write articles related to the mechanical engineering field and tries to motivate other mechanical engineering students by his innovative project ideas, design, models and videos.