Share The Technical Requirements Of Friction Materials

What technical requirements do brake pad manufacturers share for friction materials? Friction materials are the key safety parts in the clutch assembly and brake of vehicles and machinery. In most cases, friction materials are friction with various metal pairs. Therefore, in the process of transmission and braking, the following technical requirements should be met.
1、 Suitable and stable friction coefficient
Friction factor is a very important performance index for evaluating any kind of friction material, which is related to the performance of the friction plate in transmission and braking. It is not a constant, but a coefficient that changes due to the influence of temperature, pressure, friction speed or surface state and surrounding media factors.} An ideal friction factor should have an ideal cold friction factor and a controllable heat decay.
Temperature is a very important factor affecting friction coefficient. During the friction process of friction materials, due to the rapid rise of temperature, -When the temperature reaches 200 "After C, the friction coefficient begins to decrease. If the temperature reaches the decomposition temperature range of resin and rubber, the friction coefficient will suddenly decrease. This phenomenon is called heat decay. Severe heat decay will lead to poor and worse braking efficiency. In practical applications, it will reduce friction, that is, reduce the braking effect. This is very dangerous and must be avoided. Adding high-temperature friction modifier filler into friction materials is to reduce and overcome heat decay An effective means of withdrawal.
After heat decay, the friction coefficient of the friction plate will gradually recover to the original normal condition when the temperature gradually decreases, but sometimes the friction coefficient will recover too much when it is higher than the original normal friction coefficient. This phenomenon of excessive recovery of the friction coefficient is called over-recovery. Friction coefficient usually decreases with the increase of speed, but excessive reduction cannot be ignored. The brake torque speed stability is required in the bench test standard of automobile brake linings in China. Therefore, when the vehicle speed is increased, the brake efficiency should be prevented from decreasing. When the surface of the friction material is stained with water, the friction coefficient will also decrease. When the water film on the surface is removed and restored to the dry state, the friction coefficient will return to normal. When the surface of the water recovery friction material is stained with oil, the friction coefficient will significantly decrease, but a certain friction force should be maintained to make it still have a certain braking effect.
2、 Good wear resistance
The wear resistance of friction materials is the reflection of their service life, and is also an important technical index to measure the durability of friction materials. The better the wear resistance, the longer its service life. However, the wear of friction materials in the working process is mainly caused by the shear force generated by the friction contact surface.
Working temperature is an important factor affecting the wear amount. When the surface temperature of the material reaches the thermal decomposition temperature range of the organic binder, rubber and resin will produce decomposition, carbonization and weightlessness. With the increase of temperature, this phenomenon will intensify, the bonding effect will decline, and the wear amount will increase sharply, which is called thermal wear. Selecting proper antifriction fillers and resin and rubber with good heat resistance can effectively reduce the working wear of materials, especially thermal wear, and prolong their service life.
There are many ways to express the wear resistance index of friction materials. The wear index specified in GB 5763-2008 Brake Linings for Vehicles is to measure the wear rate of material samples on a constant speed friction tester at a temperature of 100 to 350 "C (50" C is the first gear). The wear rate is the volume wear amount when the unit friction work is performed during the relative sliding process between the sample and the dual surface, which can be calculated from the sliding distance of the measured friction force and the reduction of the thickness of the sample due to wear.
In the process of friction performance test, the tested sample will have different degrees of thermal expansion under the influence of high temperature, which covers the thickness wear of the sample, and sometimes even has a negative value, that is, the thickness of the sample after high temperature wear increases instead, which cannot truly and correctly reflect the actual wear. Therefore, some manufacturers also need to measure the mass wear rate of the sample in addition to the volume wear of the sample.
Some domestic automobile manufacturers have specified requirements for the wear rate of the matched brake pads. In the constant speed friction test of the test sample, the total wear rate at the temperature of 100 ° C, 150 ° C, 200 ° C, 250 ° C and 300 "C should not exceed the limit value (generally, it is 2.5X10-7cm2/(N.m) or 2.0X10 'cm2/(N.m)).
3、 Certain mechanical strength and physical properties
Before assembly and use, some friction material products need to be drilled, riveted, assembled and other mechanical processing before they can be made into brake pad assembly or clutch assembly. In the process of friction, the friction material should not only bear high temperature, but also bear large pressure and shear force. Therefore, it is required that the friction material must have sufficient mechanical strength to ensure that there is no damage and fragmentation during processing or use. For example, for brake pads, certain impact strength, riveting stress and compressive strength are required; For bonded brake pads (such as disc brake pads), it is necessary to have sufficient bonding strength at room temperature and high temperature (200~250C) to ensure that the brake pads and steel back are firmly bonded, and will not separate from each other when subjected to high shear force during the braking process of disc brake pads, which will cause serious consequences of brake failure; For the clutch plate, it is required to have sufficient impact strength, static bending strength, very large strain value and rotational damage strength, which is to ensure that the clutch plate will not be damaged during transportation, riveting and processing, and also to ensure that the clutch plate will not break under the working conditions of high-speed rotation.
4、 Low brake noise
Brake noise is related to the comfort of the vehicle when driving, and whether it will cause noise pollution to the surrounding environment, especially to the urban environment. For cars and city buses, braking noise is an important performance requirement. The relevant departments have put forward standard regulations: the noise generated by braking of general vehicles should not exceed 85dB. There are many factors that cause brake noise, because the brake pad is only a component of the brake assembly. When braking, the brake pad and the brake drum (or disc) rub strongly under the relative motion of high speed and high pressure ratio, and vibrate with each other, resulting in different levels of noise. As far as friction materials are concerned, the factors that cause braking noise are generally as follows:
(1) The higher the friction coefficient of the friction material, the easier it is to generate noise. When the friction coefficient reaches 0.45~0.5 or higher, it is easy to generate noise.
(2) The product has high hardness and is easy to generate noise.
(3) When the amount of high hardness filler is large, it is easy to generate noise.
(4) After high temperature braking, the working surface of the brake pad will form a bright and hard carbonization film (also called enamel layer), which will generate high-frequency vibration and corresponding noise during braking friction.
It can be seen from this that proper control of the friction factor, make it not too high, reduce the hardness of the product, reduce the amount of hard filler, avoid the formation of carbonization film on the working surface, and use the damping pad or coating film to reduce the vibration frequency, are conducive to reducing and overcoming noise. Because the causes of brake noise are quite complex and not fully understood at present, it is an important issue to solve the noise problem in the braking process of friction materials.
5、 The wear of the dual surface is small
The transmission or braking function of friction material products should be realized by friction with the counterpart, namely friction disc or brake drum (or disc). In this friction process, this pair of friction coupling parts will wear each other, which is a normal phenomenon. However, friction material products as consumable materials, in addition to their own wear as small as possible, the wear of the dual parts should also be small, even if the service life of the dual parts should be relatively long, which fully shows that they have good friction performance characteristics. At the same time, the surface of the friction disc or brake drum (or disc) should not be abraded with heavy scratches, scratches, grooves and other excessive wear during the friction process