Brake technology, just like suspension technology and fuel-system technology, has come a long way in recent years. The introduction of components like carbon fibre, sintered metal and lightweight steel, along with the adoption of abs, have all contributed to reduced stopping distances and generally safer vehicles.
What began in the 60s as a serious attempt to provide adequate braking for performance cars has ended in an industry where brakes range from supremely adequate to downright phenomenal. One of the first steps taken to improve braking came in the early 70s when manufacturers, on a widespread scale, switched from drum to disc brakes. Since the majority of a vehicles stopping power is contained in the front wheels, only the front brakes were upgraded to disc during much of this period. Since then, many manufacturers have adopted four-wheel disc brakes on their high-end and performance models as well as their low-line economy cars.
Early automotive brake systems, after the era of hand levers of course, used a drum design at all four wheels. They were called drum brakes because the components were housed in a round drum that rotated along with the wheel. Inside was a set of shoes that, when the brake pedal was pressed, would force the shoes against the drum and slow the wheel. Fluid was used to transfer the movement of the brake pedal into the movement of the brake shoes, while the shoes themselves were made of a heat-resistant friction material similar to that used on clutch plates.
This basic design proved capable under most circumstances, but it had one major flaw. Under high braking conditions, like descending a steep hill with a heavy load or repeated high-speed slow downs, drum brakes would often fade and lose effectiveness. Usually this fading was the result of too much heat build-up within the drum. Remember that the principle of braking involves turning kinetic energy (wheel movement) into thermal energy (heat). For this reason, drum brakes can only operate as long as they can absorb the heat generated by slowing a vehicles wheels. Once the brake components themselves become saturated with heat, they lose the ability to halt a vehicle, which can be somewhat disturbing to the vehicles operator.
Though disc brakes rely on the same basic principles to slow a vehicle (friction and heat), their design is far superior to that of drum brakes. Instead of housing the major components within a metal drum, disc brakes use a slim rotor and small caliper to halt wheel movement. Within the caliper are two brake pads, one on each side of the rotor that clamp together when the brake pedal is pressed. Once again, fluid is used to transfer the movement of the brake pedal into the movement of the brake pads. The disc is made of pearlitic grey cast iron because of its wear resistance and its relatively low cost.
The advantage of a disc brake is that only the caliper covers a small area of the disc and, since the disc is rotating, it has time to cool down between revolutions helping against dangerous temperature build-up at the surface. The calipers are stationary and are mounted on the wheel axle. They can be placed in front of the wheel pin or to the rear of it (with respect to the front of the vehicle).
But unlike drum brakes, which allow heat to build up inside the drum during heavy braking, the rotor used in disc brakes is fully exposed to outside air. This exposure works to constantly cool the rotor, greatly reducing its tendency to overheat or cause fading. Not surprisingly, it was under racing circumstances that the weaknesses of drum brakes and the strengths of disc brakes were first illustrated. Racers with disc brake systems could carry their speed "deeper" into a corner and apply greater braking force at the last possible second without overheating the components. Eventually, as with so many other automotive advances, this technology Filtered driven by everyday people on public roads.
However every school child knows that carbon burns easily. So what about brake disks? Those used in Formula-1 race cars and certain top-of-the-range Porsche and Mercedes cars are made of silicon carbide reinforced with carbon fibres. Their braking performance is so exceptionally good that similar materials will soon find their way into mid-range family automobiles. But like their all-metal cousins, they tend to overheat if pushed to their limits over winding mountain roads.
Very much the same happens with trucks, wherein the kinetic energy of several tonnes of freight is converted to heat. To prevent the hot disks from losing material by way of erosion through contact with oxygen in the air, extensive tests must be performed to assess the properties of the materials: how do they behave at different temperatures, and what are the associated oxidation effects? Will it shrink or deform? Or burn up and become dust?
Highly specialised devices are needed and special techniques are needed for answering such questions. The brake material is stable at high temperatures hence the measurements have to be highly precise; otherwise wrong data will render the testing askew. Also, tight -fits is employed in such components and if precision is not maintained, undue friction will result. All such questions can be investigated using laboratory apparatus developed to maturity by scientists at the Fraunhofer Institute for Silicate Research ISC. " The first device, little TOMMI, was already able to observe sintering and melting processes without direct contact." reports business unit manager Dr Friedrich Raether. "That was a major step, because it is important not to interfere with the raw ceramic or glass part in the oven. The series of measurements to identify changes in the shape of the test object require great precision - down to an accuracy of two micrometers." One obvious inference from what the scientist tell is that the device can be used for a wide range of materials, apart from the carbon-silicon carbide composites, eg, silicates [glass type materials], ceramics, etc. This definitely gives a very huge spectrum of effectiveness, making the device and the process very versatile indeed.
Other parameters recorded include the creep behavior of the material and the mass of the object. Creep is the name given to the rather bothersome phenomenon that all metals suffer when they are subjected to non-stop working at high temperatures: it is the propagation of micro-cracks in such a silent and unobtrusive manner that the user gets no warning of an impending disaster; for example, catastrophic failures of metallic fixtures in furnaces, or crucial parts in petrochemical processing plants. Mass of the object refers to the actual weight of the component that may be huge in size and weigh very little as happens in porous sintered materials, or vice versa when wrought metal parts are employed.
TOMMIs younger brother, the bigger TOM II, enables the researchers to obtain a full set of Thermo-Optical Measurements on objects up to four centimeters in size. They can also measure heat transfer between the test object and the firing chamber, and vary the composition of the gas. This allows the exhaust gas from the burner to be set to imitate that in a real industrial furnace. The resulting data on the physical and technical properties of the material are input to simulation programs that the researchers use to optimise the process. There is a specific economic target for such mass-produced articles: Is it possible to discharge the furnace and reload it after just 80 per cent of the usual firing time, without any loss of product quality? This also represents a considerable energy saving, given that a furnace may be heated to a temperature of up to 2000 degree C.
|Posted : 8/12/2005|