Brake noise is a very elusive problem that has plagued the industry since asbestos materials were banned decades ago. Until that time, noise had not been a problem. A friction material specialist explained that asbestos is hygroscopic, meaning it has the affinity
Brake noise is a very elusive problem that has plagued the industry since asbestos materials were banned decades ago. Until that time, noise had not been a problem. A friction material specialist explained that asbestos is hygroscopic, meaning it has the affinity to attract water molecules. In braking applications, it is thought that the moisture in the asbestos keeps the surfaces cooler during the evaporation process and lubricates the mating surfaces. But, people working with brakes and friction materials need a safe environment separated from the dangerous affects of breathing asbestos particles.
Sound is created by vibration. With dry-friction applications, two components are rubbing against each other; in this case the friction pad, and the cast-iron disc surface. The resulting vibration from these two bodies may result in frequencies above or below the audible human range. We run into trouble when it is within the audible human range. A professor at the University of Maine once compared a friction type brake pad and brake disc to a violin. As the players run the bow across the string, both vibrate and the string in engineered to resonate at a particular audible pitch. The design of the violin “box” amplifies the sound. With the brake system, contributing sources of vibration and the right environmental conditions lead to an audible “noise” and it is possible the disc amplifies these sound vibrations.
Montalvo has spent a great deal of time and energy over the years discovering causes and finding solutions to brake noise. There are two approaches to solving this problem. (1) eliminate the vibrations causing the audible noise or (2) accept a certain degree of vibration in the system and attempt to dampen it to levels not audible by the human ear. Dampening features can be added to any component of the brake system and into the friction material itself. The dampening characteristics of the pad, however, are affected by operating conditions and the environment. The feasibility of such dampening methods must also be considered.
We’ve heard about customers claiming to have resolved brake noise problems with “WD-40” and “Coca-Cola”, or a pile of shop rags stuffed between the discs. These are only temporary fixes and can be dangerous, especially where “WD-40” is flammable and the rags can get caught up in the brake. They also make it more difficult to diagnose since these “fixes” alter the system we are trying to troubleshoot.
Friction pads normally take the blame for brake noise. However, there are many other elements that can contribute to noise problems and therefore the brake system, environment, and nearby machine components must be examined. When brake noise occurs in the converting/paper/packaging industries, it is heightened by the fact that such brakes operate in a continuous slip manner (rather than a start-stop manner, such as with brakes on vehicles). These brakes are located inside a closed factory environment where people are subjected to the noise. It becomes a very sensitive issue that we take very seriously and approach with caution and diligence.
Each and every application is unique and can only be handled through careful analysis and experimentation. Again, it can be an elusive problem. An application can run great for many years and then, suddenly, it generates noise. Some customer’s only have squealing problems in the winter, some only during a particular process, and others can’t get rid of the noise no matter what the condition. To troubleshoot, we have to collect as much information as possible. In all but the rarest of cases the problem can be solved, but it may require some patience.
Residual Friction Pad Material
It is inevitable that in any dry-friction application, dust will be generated as a byproduct and will collect along the inside of the brake cover, around the friction pads, and on the brake modules. Excessive residual buildup may increase the propensity for noise, especially if it builds up around the working surface of the friction pads and discs. The residual no longer possesses the properties of the original friction material, and this may introduce audible vibrations, similar to a pebble or sand getting caught in your vehicle’s disc brakes. This residual material should be removed using a damp cloth or a vacuum cleaner, or compressed air.
Glazing can be identified as a hard, glassy surface on the friction pad. There are two common types of glazing. The first is “mechanical” which occurs typically at low temperatures and operating pressures. The second is “thermal” which occurs due to excessive operating temperatures; those exceeding 350° F.
Thermal glazing is typically more severe and occurs when the excessive heat in the pad begins to draw the resins to the surface and literally “cooks” the pad. When the pad cools, its surface crystallizes, changing the pad’s properties. Typically, the entire pad becomes very hard if subjected to disc temperatures of about 400° F for extended periods of time. Brake disc temperature should never exceed 350° F. If the pad has not been exposed to prolonged high temperatures, it is most likely that only its surface has been altered. Once the pad is cooled down, it is possible to break the glaze on the pad’s surface by rubbing it with medium to heavy grit sandpaper or emery cloth in a circular motion. If the cause of the glazing is due to overheated brakes, please review your application specifications with a Montalvo Applications Specialist to ensure proper brake sizing.
Mechanical glazing is essentially like polishing a surface with a mild abrasive cleaner. This normally occurs when the piston is at an air pressure below 20 psi and/or disc temperature