17-07-2014, 02:34 PM
TRADITIONAL CLASSIFICATION
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INTRODUCTION.
In the low frequency domain, there are two fundamental different types of structural vibrations, namely judder and groan, and their associated airborne noise called hums and moan respectively. Even though the frequency of judder is normally lower than that of groan and groan is lower than squeal, the frequency ranges overlap in the region 400-500Hz.Any how, brake judder is easily recognized, as its frequency is proportional to the vehicle speed. The frequency of brake squeal is independent of the speed of the vehicle.
While judder is a forced vibration mostly due to geometrical deviations of the brake disc, groan is an instability phenomenon occurring as a result of specific types of friction-velocity characteristics. Squeal, on the other hand, involves bending mode of the brake components.
TRADITIONAL CLASSIFICATION
Traditionally, brake noise and vibrations are classified according to their dominant frequencies, with those below a certain limit being called judder or hum. Above this limit the vibrations are regarded as high frequency noise including squeal. The main disadvantage with this classification is that one physical phenomenon can be split into two different classes and, at the same time, fundamentally different phenomena will be included in the same term. However the driver and passengers relate the classification to the way in which the phenomena are experienced by the driver and passengers.
Thermal or hot judder is caused by
Thermal deformation, for e.g. coning and waving of a disc.
Uneven thermal expansion.
Phase transformation of disc material.
Cold judder is caused by geometrical irregularities due to machining, mounting, uneven wear, uneven corrosion or uneven friction film generation. Normally judder consists of a combination of hot and cold judder and hence the terms ‘hot’ and ‘cold’ are somewhat misleading.
PHYSICAL EFFECTS CAUSING RAKE TORQUE VARIATIONS AND BRAKE PRESSURE VARIATIONS.
There are several physical effects causing brake torque variation and brake pressure variation and hence judders the ones discussed are geometrical irregularities uneven wear of brake rotor, uneven friction film between rotor and lining, uneven heating, uneven pressure distribution, friction characteristics and friction level and external forces. The different effects or sources are generally not independent of each other. There is a strong coupling between geometrical irregularities, uneven wear and uneven heating.
UNEVEN WEAR.
Off-brake wear also known as cold erosion is coupled to DTV and can lead to cold judder. Brake discs can develop DTV because the brake pads in the off brake mode lightly touch the rotors in some sectors but not at all in others, resulting in non-uniform wear .DTV created by pad contact has two phases: -generation and cleaning. Ratio between generation and cleaning is influenced by the selection of friction material. Friction materials which generate DTV in the off brake mode will usually have the ability to eliminate DTV by a higher disc wear in the on-brake mode.
Careful drivers may experiences brake judder more quickly than sporty drivers since slow application of the brake at low brake pad pressure. Prevents cleaning of the disc. Discs causing BTV and hence judder can be cured by making several hard stops. During long brake applications wear, especially pad wear, becomes substantial. Generally wear results in a more even temperature distribution and lower temperature maximal. However a high wear will move the contact surface, which might contribute to thermal fatigue of discs.
UNEVEN HEATING
Thermal deformation of a disc consists of:
Waving or warping of the disc.
Coning of the disc. This leads to high run out.
Phase transformation.
Deposition of heated pad material on the disc
Uneven thermal expansion: A temperature difference of 250C, which is not unusual, causes DTV amplitude of 10 m. Even if no visible hot spots occur during braking this phenomenon is important for brake judder occurrence by causing temporary growth of geometrical disturbances.
The thicker areas of a brake disc become hotter than the thinner ones, which cause thermal deformation of the disc and also uneven thermal expansion that is thermal DTV. The process can become unstable and is then defined as a thermo elastic instability (TEI). TEI leads to a more and more localized brake pressure and temperature field as the braking continues. In extreme cases the disc will crack.
It is the temperature gradients that cause the increase in judder level during long or repeated braking. These cause temporary DTV owing to uneven thermal expansion of disc material .the DTV level (hence BTV) increases, often more or less linearly, with time while braking, hence brake application time and how often the braking is repeated are important factors for brake judder development.
Also a higher revolution speed of the disc was found to increase the hot spot generation. In addition the localization process (& thermal DTV) is pronounced at high speeds, above 100 kmph. This might be explained by the fact that TEI process demands a minimal critical speed to develop. As a consequence of this it is essential to test friction material at speeds above the limit 100kmph, which is usually standard.
APPROACH
The way of looking at and describing a problem is called d the problem approach. This determines which physical effect can be studied and also states the time and space scale .the chosen problem approach will have consequences for how to analyze the problem as well as for the type of solution that will be found. It also determines which type of analytical and experimental tool is the most appropriate.
While studying brake judder there are two fundamentally different approaches in use namely the cause approach and the effect approach. While the effect approach focuses on the source of the vibration in the form of amplitude and vibrational order of the BTV & BPV, the cause approach deals with number of physical effects such as wear and heating.
CONCLUSION
The following are possibilities for reducing judder:
Decrease BTV & BPV.
Increased damping.
Lighter braking.
Increase relative stator mass and inertia moment compared with the rotor.
A lighter vehicle and a smaller wheel radius decrease the judder problem. A reduction of vehicle mass will also decrease the thermal DTV and hotspots and reduce the risk of thermal cracking, provided that the thermal capacity of the disc remains the same. A lighter stator will tend to increase the judder problem\s, since amplification of BTV will become larger.