Asbestos was added as an common ingredient to Brake Pads post-WWI, as car speeds began to increase, because research showed that its properties allowed it to absorb the heat (which can reach 500 °F) while still providing the friction necessary to stop a vehicle. However, as the serious health-related hazards of asbestos eventually started to become apparent, other materials had to be found. Asbestos brake pads have largely been replaced by non-asbestos organic (NAO) materials in first world countries. Today, brake pad materials are classified into one of four principal categories, as follows:
Non-metallic materials - these are made from a combination of various synthetic substances bonded into a composite, principally in the form of cellulose, aramid, PAN, and sintered glass. They are gentle on rotors, but produce a fair amount of dust, thus having a short service life.
Semi-metallic materials - synthetics mixed with varying proportions of flaked metals. These are harder than non-metallic pads, more fade-resistant and longer lasting, but at the cost of increased wear to the rotor/drum which then must be replaced sooner. They also require more actuating force than non-metallic pads in order to generate braking torque.
Fully metallic materials - these pads are used only in racing vehicles, and are composed of sintered steel without any synthetic additives. They are very long-lasting, but require more force to slow a vehicle while wearing off the rotors faster. They also tend to be very loud.
Ceramic materials - Composed of clay and porcelain bonded to copper flakes and filaments, these are a good compromise between the durability of the metal pads, grip and fade resistance of the synthetic variety. Their principal drawback, however, is that unlike the previous three types, despite the presence of the copper (which has a high thermal conductivity), ceramic pads generally do not dissipate heat well, which can eventually cause the pads or other components of the braking system to warp.However, because the ceramic materials causes the braking sound to be elevated beyond that of human hearing, they are exceptionally quiet.
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The treatment method refers to the spur gear forming process data [1-3] at home and abroad, and through experimental comparison, the extrusion forming step of the spur gear as shown is determined. On the 2000kN hydraulic universal material testing machine, four sets of spur cylindrical gear closed extrusion forming process with LD2 material were completed by using the mold device with floating mold frame. The experimental speed was 1520mmmin-1, and the appropriate amount of animal oil was applied as lubricant on the surface of the blank. The force stroke curve was recorded using a displacement sensor, a load cell, and an XY recorder.
Experimental Results The actual picture of the gear obtained from the experiment is as it is. When the gear is formed by extrusion process, the fullness of the tooth cavity, the life of the mold and the stability of the accuracy are the three most important factors affecting the quality of the closed-end extruded spur gear, especially the first two factors directly Forming loads are closely related. Therefore, it is important to analyze the relationship between load and tooth shape parameters during forming.
The influence of the modulus and the number of teeth on the upper limit force The upper limit force of the radial extrusion spur gear is related to the structural parameters such as the modulus and the number of teeth of the gear. It is found through experiments that when the other parameters are constant, the increase of the modulus will cause the unit upper limit force to decrease, and the increase of the number of teeth will increase the unit upper limit force, which is especially obvious at the end of the forming stage, and the upper limit force changes abruptly, so the tooth is also caused. The cavity is difficult to fill and the accuracy of the gear is low.
The influence of the modulus and the number of teeth on the frictional power According to the plastic forming theory, the magnitude of the friction factor is different from the change of the unit upper limit force with the reduction amount when the number of teeth is different when the number of teeth is different. Large, especially at the end of the formation, it is easy to produce upsetting. Similarly, the influence of frictional power on the formation of cold extrusion teeth is relatively large. It is known from experiments that the frictional power of cold extrusion gears always increases with the increase of modulus and number of teeth, especially at the end of deformation. Fast, when the modulus is m3, the friction power increases sharply, so when the modulus is large, the forming is greatly affected by the friction, as shown in 6.
The change of the total upper limit power during the forming process is calculated by the upper limit analysis of the extrusion of the spur gear [4, 5], and experimentally verified, in the total upper limit power of the radially extruded spur gear, when the deformation At the end of the friction power accounted for the total deformation power percentage close to the fixed value (about 10), the total shear power accounted for a small proportion, but the plastic forming power accounted for more than 90 (in the final full phase multiplied).
Analysis and Discussion The closed-type cold extrusion forming process of spur gear is a high-efficiency, time-saving and no-cutting method, but its deformation resistance is large, filling is difficult, and the life of the mold is low, which becomes a difficult problem of plastic forming. Through the above experimental data It can be seen from the analysis that the effect of extrusion is closely related to the material of the tooth blank, the parameters of the tooth profile, the friction factor, and the forming device [6, 7].
When the total upper limit power in the gear blank material extrusion molding is large, the forming effect is easily affected, and the plastic forming power accounts for 90 in the total upper limit power, and the plastic deformation power is directly affected by the metal material characteristics (strength, yield limit, etc.). Low-carbon, low-alloy steels have good plasticity and low deformation resistance, so it is effective to use low-carbon, low-alloy steel or quenched and tempered materials to form gears by precision forging.
The tooth profile parameter and the number of teeth are the most important structural parameters of the gear. The effect of the spur gear extrusion is directly related to the number of teeth and the modulus. In the case where the pitch of the gear is constant, the cold-squeezing cylindrical gear is selected to have a large modulus and a small number of teeth, which is advantageous for forming. When the number of teeth and the modulus are relatively large, it is necessary to reduce the friction factor as much as possible, and effective lubrication is necessary.
The forming device experiment found that the deformation resistance increased sharply at the end of the extrusion deformation, which seriously affected the forming effect. According to the plastic forming principle and through experiments, it can be proved that the floating mold frame can be used when designing the mold, the fixed end flow is restricted to increase the fullness of the tooth cavity or the shunt method is used to reduce the deformation resistance.
Conclusion The total upper limit power during cold forging extrusion of spur gears has a great relationship with the modulus of the gear to be formed, the number of teeth, the degree of lubrication in forming and the material properties of the blank. The frictional power of cold precision forging has a significant increase with the increase of the modulus and the number of teeth. At the end of the deformation, the deformation resistance increases sharply, especially the plastic deformation resistance increases rapidly. The friction factor is also an important factor affecting the deformation load.