Brake pads are the friction material fixed on the brake drum or disc that rotates with the wheel, in which the friction lining and the friction lining block are subjected to external pressure to produce friction to achieve the purpose of vehicle deceleration.
The friction block is the friction material that is pushed by the clamp piston and squeezed on the brake disc, due to the friction effect, the friction block will be worn gradually, generally speaking, the lower the cost of the brake pads wear faster. The friction block is divided into two parts: the friction material and the base plate. After the friction material is worn out, the base plate will be in direct contact with the brake disc, which will eventually lose the braking effect and damage the brake disc, and the repair cost of the brake disc is very expensive.
In general, the basic requirements for brake pads are mainly wear resistance, large coefficient of friction, and excellent thermal insulation properties.
According to the different braking methods brake pads can be divided into: drum brake pads and disc brake pads, according to the different materials brake pads can be generally divided into asbestos type, semi-metallic type, NAO type (i.e. non-asbestos organic material type) brake pads and other three.
With the rapid development of modern technology, like other brake system components, the brake pads themselves have been evolving and changing in recent years.
In the traditional manufacturing process, the friction material used in brake pads is a mixture of various adhesives or additives, to which fibers are added to improve their strength and act as reinforcement. Brake pad manufacturers tend to keep their mouths shut when it comes to the announcement of materials used, especially new formulations. The final effect of brake pad braking, wear resistance, temperature resistance and other properties will depend on the relative proportions of the different components. The following is a brief discussion of several different brake pad materials.
Asbestos type brake pads
Asbestos has been used as a reinforcement material for brake pads since the beginning. Asbestos fibers have high strength and high temperature resistance, so they can meet the requirements of brake pads and clutch discs and linings. The fibers have a high tensile strength, even matching that of high-grade steel, and can withstand temperatures up to 316°C. More importantly, asbestos is relatively inexpensive and is extracted from amphibole ore, which is found in large quantities in many countries.
Asbestos has been medically proven to be a carcinogenic substance. Its needle-like fibers can easily enter the lungs and stay there, causing irritation and eventually leading to lung cancer, but the latent period of this disease can be as long as 15-30 years, so people often do not recognize the harm caused by asbestos.
As long as asbestos fibers are fixed by the friction material itself will not cause health hazards to workers, but when asbestos fibers are released along with brake friction to form brake dust, it can become a series of health effects.
According to tests conducted by the American Occupational Safety and Health Association (OSHA), every time a routine friction test is conducted, the brake pads will produce millions of asbestos fibers emitted into the air, and the fibers are much smaller than a human hair, which is not observable to the naked eye, so a breath may absorb thousands of asbestos fibers without people being aware of it. Similarly, if the brake drum or brake parts in the brake dust blown away with an air hose, can also be countless asbestos fibers into the air, and these dust, not only will affect the health of the work mechanic, the same will also cause health damage to any other personnel present. Even some extremely simple operations such as hitting the brake drum with a hammer to loosen it up and let the internal brake dust out, can also produce a lot of asbestos fibers floating into the air. What’s even more worrisome is that once the fibers are floating in the air they will last for hours and then they will stick to clothing, tables, tools, and every other surface you can think of. Any time they encounter stirring (such as cleaning, walking, using pneumatic tools to generate air flow), they will float back into the air again. Often, once this material enters the work environment, it will remain there for months or even years, causing potential health effects to the people working there and even to the customers.
The American Occupational Safety and Health Association (OSHA) also states that it is only safe for people to work in an environment that contains no more than 0.2 asbestos fibers per square meter, and that asbestos dust from routine brake repair work should be minimized and work that could cause the release of dust (such as tapping brake pads, etc.) should be avoided as much as possible.
But in addition to the health hazard aspect, there is another important problem with asbestos-based brake pads. Since asbestos is adiabatic, its thermal conductivity is particularly poor, and repeated use of the brake will usually cause heat to build up in the brake pad. If the brake pads reach a certain level of heat, the brakes will fail.
When vehicle manufacturers and brake material suppliers decided to develop new and safer alternatives to asbestos, new friction materials were created almost simultaneously. These are the “semi-metallic” blends and the non-asbestos organic (NAO) brake pads discussed below.
“Semi-metallic” hybrid brake pads
“Semi-met” mixture brake pads are mainly made of coarse steel wool as a reinforcing fiber and an important mixture. From the appearance (fine fibers and particles) it is easy to distinguish asbestos type from non-asbestos organic type (NAO) brake pads, and they are also magnetic in nature.
The high strength and thermal conductivity of steel fleece makes “semi-metallic” blended brake pads have different braking characteristics than traditional asbestos pads. The high metal content also changes the friction characteristics of the brake pad, which usually means that the “semi-metallic” brake pad requires higher braking pressure to achieve the same braking effect. High metal content, especially in cold temperatures, also means that the pads will cause greater surface wear on the discs or drums, as well as producing more noise.
The main advantage of “Semi-metal” brake pads is their temperature control capability and higher braking temperature, compared to the poor heat transfer performance of asbestos type and the poor cooling capability of brake discs and drums. The heat is transferred to the caliper and its components. Of course, if this heat is not handled properly it can also cause problems. The brake fluid temperature will rise when it is heated, and if the temperature reaches a certain level it will cause the brake to shrink and the brake fluid to boil. This heat also has an effect on the caliper, piston seal and return spring, which will speed up the aging of these components, which is the reason for reassembling the caliper and replacing the metal parts during brake repair.
Non-asbestos organic braking materials (NAO)
Non-asbestos organic brake materials mainly use glass fiber, aromatic polycool fiber or other fibers (carbon, ceramic, etc.) as reinforcement materials, whose performance depends mainly on the type of fiber and other added mixtures.
Non-asbestos organic brake materials were mainly developed as an alternative to asbestos crystals for brake drums or brake shoes, but recently they are also being tried as a replacement for front disc brake pads. In terms of performance, NAO type brake pads are closer to asbestos brake pads than to semi-metallic brake pads. It does not have the same good thermal conductivity and good high temperature controllability as semi-metallic pads.
How does the new NAO raw material compare to asbestos brake pads? Typical asbestos-based friction materials contain five to seven base blends, which include asbestos fibers for reinforcement, a variety of additive materials, and binders such as linseed oil, resins, benzene sound awakening, and resins. In comparison, NAO friction materials contain approximately seventeen different stick compounds, because removing asbestos is not the same as simply replacing it with a substitute, but rather requires a large mixture to ensure braking performance that equals or exceeds the braking effectiveness of asbestos friction blocks.
Post time: Mar-23-2022