20-09-2014, 02:32 PM
[b]DISC BRAKES[/b]
DISC BRAKES.docx (Size: 220.83 KB / Downloads: 12)
The disc brake is a device for slowing or stopping the rotation of a wheel while it is in motion.
A brake disc is usually made of cast iron, but may in some cases be made of composites such as reinforced carbon- carbon or ceramic matrix composites. This is connected to the wheel and/or the axle. To stop the wheel, friction material in the form of brake pads is forced mechanically, hydraulically, pneumatically or electromagnetically against both sides of the disc. Friction causes the disc and attached wheel to slow or stop. Brakes convert motion to heat, and if the brakes get too hot, they become less effective, a phenomenon known as brake fade.
DIFFERENT PARTS
BRAKE PADS
Brake pads are designed for high friction with brake pad material embedded in the disc in the process of bedding while wearing evenly.
Friction can be divided into two parts: Adhesive and abrasive.
Depending on the properties of the material of both the pad and the disc and the configuration and the usage, pad and disc wear rates will vary considerably. The properties that determine material wear involve trade-offs between performance and longevity.
The friction coefficient for most standard pads will be in the region of .40 when used with cast iron discs. Racing pads with high iron content designed for use with cast iron brake discs reach .55 to .60 which gives a very significant increase in braking power and high temperature performance. High iron content racing pads wear down discs very quickly and usually when the pads are worn out so are the discs.
The brake pads must usually be replaced regularly, and some are equipped with a mechanism that alerts drivers that replacement is needed, such as a thin piece of soft metal that rubs against the disc when the pads are too thin causing the brakes to squeal, a soft metal tab embedded in the pad material that closes an electric circuit and lights a warning light when the brake pad gets thin, or an electronic sensor.
Early brake pads contained asbestos, producing dust which should not be inhaled. Although newer pads can be made of ceramics, kevlar and other plastics, inhalation of brake dust should still be avoided regardless of material.
BRAKE SQUEAL
Sometimes a loud noise or high pitched squeal occurs when the brakes are applied. Most brake squeal is produced by vibration of the brake components, especially the pads and discs. This type of squeal should not negatively affect brake stopping performance. Simple techniques like adding chamfers to linings, greasing or gluing the contact between caliper and the pads, bonding insulators to pad backplate, inclusion of a brake shim between the brake pad and back plate, etc. may help to reduce squeal. Cold weather combined with high early morning humidity often makes brake-squeal worse, although the squeal stops when the lining reaches regular operating temperatures. Dust on the brakes may also cause squeal; there are many commercial brake cleaning products that can be used to remove dust and contaminants. Finally, some lining wear indicators,located either as a semi-metallic layer within the brake pad material or with an external squealer sensor, are also designed to squeal when the lining is due for replacement.
BRAKE JUDDER
Brake judder is usually perceived by the driver as minor to severe vibrations transferred through the chassis during braking. The judder phenomenon can be classified into two distinct subgroups: hot (or thermal), or cold judder.
Hot judder is usually produced as a result of longer, more moderate braking from high speed where the vehicle does not come to a complete stop. It commonly occurs when a motorist decelerates from speeds of around 120 km/h to about 60 km/h which results in severe vibrations being transmitted to the driver. These vibrations are the result of uneven thermal distributions, or hot spots. Hot spots are classified as concentrated thermal regions that alternate between both sides of a disc that distort it in such a way that produces a sinusoidal waviness around its edges.
BRAKE DUST
When braking force is applied, the act of abrasive friction between the brake pad and the rotor wears both the rotor and pad away. The brake dust that is seen deposited on wheels, calipers and other braking system components consists mostly of rotor material. Brake dust can damage the finish of most wheels if not washed off. Generally brake pad that aggressively abrades more rotor material away, such as metallic pads, will create more brake dust.
Properties:
1. Cast iron tends to be brittle, except for malleable cast irons
2. Relatively low melting point
3. Good fluidity, castability
4. Excellentmachinability
5. Resistance to deformation and wear resistance
6. Resistant to destruction and weakening by oxidation
Advantages and Disadvantages
Gray iron is a common engineering alloy because of its relatively low cost and good machinability, which results from the graphite lubricating the cut and breaking up the chips. It also has good gallingandwear resistance because the graphite flakes self -lubricate.
The graphite also gives gray iron an excellent damping capacity because it absorbs the energy. It also experiences less solidification shrinkage than other cast irons that do not form a graphite microstructure. The silicon promotes good corrosion resistance and increase fluidity when casting.
Gray iron is generally considered easy to weld. Compared to the more modern iron alloys, gray iron has a low tensile strength and ductility; therefore, its impact and shock resistance is almost non-existent.
COPPER
Copper has been regarded as one of the indispensable ingredients in the brake friction materials since it provides high thermal diffusivity at the sliding interface. However, the recent regulations against environmentally hazardous ingredients limit the use of copper in the commercial friction material and much effort has been made for the alternatives. In this work, the role of the cuprous ingredients such as copper fiber, copper powder, cupric oxide (CuO), and copper sulfide (CuS) are studied using the friction materials based on commercial formulations. The investigation was performed using a full inertial brake dynamometer and 1/5 scale dynamometer for brake performance and wear test. Results showed that the cuprous ingredients played a crucial role in maintaining the stable friction film at the friction interface, resulting in improved friction stability and reduced aggressiveness against counter disk. In particular, the addition of copper fiber or copper sulfide contributed to improved friction stability during effectiveness test.
FRICTION MODIFIERS
Friction modifiers are added to lubricants to reduce the surface friction of the lubricated parts. Typically these are polar chemical compounds having high affinity for metal surfaces and possessing long alkyl chains.
Glycerol mono-oleate is a common example of a friction modifier. Whale oil was formerly used as a friction modifier in automatic transmission fluid.
This additive is needed in limited slip differentials.
In case of Limited Slip Differential (LSD), the friction modifiers are designed to modify the friction characteristics of the fluid such that clutch engagement occurs when you need it for the rear differential to allow the rear wheel to rotate at differential speeds.
For example- If a vehicle is on an icy road and one wheel is on ice and slipping with no traction, in a normal differential the other wheel would be left with no torque because the one wheel s spinning. In a vehicle with LSD, even though one wheel is spinning, that spinning action causes the clutch to engage the gear which produces the torque required to power the non-spinning wheel, enabling the car to move.