Understanding of friction coefficient performance of brake pads (1)
The quality of brake pads is directly related to the reliability of vehicle braking. In addition to 100% safety, car owners also expect the best braking comfort, responsiveness, noiselessness and smoothness at all times. For brake pads, the most important thing is the choice of friction material, which basically determines the braking performance of the brake pads. Friction materials can be divided into four types based on basic materials: asbestos, NAO type (that is, asbestos-free organic type), semi-metal and special fiber. In 1999, my country banned the use of asbestos in friction plates. At present, semi-metallic friction materials are generally used in China, mainly steel or copper materials.
The performance of the friction plate should have four basic requirements:
1. Appropriate and stable friction coefficient. Friction coefficient is one of the most important technical indicators of friction materials, which determines the basic braking torque of brake pads. Usually it is not a constant, it is changed by temperature, pressure, speed or surface state, friction environment. When affected by these factors, the friction coefficient changes less, which is the ideal friction coefficient. The national standard strictly requires manufacturers to indicate the coefficient of friction on the packaging. The coefficient of friction must be appropriate, if it is too high, it will cause wheel lock, direction loss of control and burnt discs during braking, if it is too low, the braking distance will be too long. For example, the coefficient of friction of Bosch safety brake pads is 0.39.
Second, reliable security. The brake pads will generate instantaneous high temperature when braking, especially when driving at high speed or emergency braking. At high temperature, the coefficient of friction of the friction plate will decrease, which is called thermal recession. The thermally decaying high temperature determines the safety of the high temperature state and emergency braking. The friction plate maintains its stability and recovery performance under various working conditions, that is, it requires good heat resistance, and it can still maintain stable performance at high temperatures. It is less sensitive in different humidity and after immersion in water, and can recover as soon as possible after being exposed to water. performance. At the same time, it must have good physical and mechanical properties, which can not only meet the requirements in the processing process, but also maintain good performance.
Three, satisfactory comfort. Comfort is a direct reflection of friction performance, including braking feeling, noise, dust, smoke, and peculiar smell. With the increase of private cars and the improvement of people's living standards, under the basic premise of satisfying safety, comfort has become an important indicator of friction plates. Among the comfort indicators, car owners are often most concerned about the noise of brake pads. In fact, noise is also the most difficult problem for all friction material manufacturers to solve. Noise is generated from the abnormal friction between the friction plate and the friction disc. The reasons for the noise are very complicated. The braking force, the temperature of the brake disc, the speed of the vehicle and the weather conditions may all be the cause of the noise. In addition, the causes of noise are different in the three different stages of braking initiation, braking implementation and braking release. If the noise frequency is between 0-500Hz, the car will not feel it, but if it exceeds 1500Hz, the owner can obviously feel the braking noise.
4. Good wear resistance, reliability and durability. This is an important index to measure the service life of friction materials, and it is also a product index that users generally pay attention to. Maintaining sufficient mechanical strength at room temperature and high temperature is an important factor affecting the reliability of the friction lining, but at the same time it is required not to scratch the brake disc or brake drum. Its wear resistance is the main factor affecting durability. The main factors affecting wear in use are pressure, speed and temperature, among which temperature is the first. The front brake pads can usually guarantee the service life of 30,000-50,000 kilometers, and the rear brake pads can usually guarantee the service life of 60,000-80,000 kilometers. The large-grained graphite and brass used in the brake pad materials also reduce wear on the brake discs. In particular, the noise during use is reduced to the lowest level, ensuring no pollution to the environment. There is no noise when braking, and no odor at high temperatures.
