tailieunhanh - Bearing Design in Machinery Episode 2 Part 7

Tham khảo tài liệu 'bearing design in machinery episode 2 part 7', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | alumina oxide silicon carbide and silicon nitride decreased with increasing sliding velocities. The minimum friction coefficient of silicon carbide and silicon nitride is as low as in the presence of water. Experiments were conducted in oil-lubricated ceramic journal bearings. The experiments showed lower friction coefficient for silicon nitride journals in comparison to steel journals for bearings made of tin-coated Al-Si alloy forged steel and cast aluminium matrix composite with silicon carbide reinforcement cast MMC . All bearings were lubricated with SAE 10W-30 oil Wang et al. 1994 . Silicon Carbide and Silicon Nitride As discussed earlier these are the best-performing high-temperature high-strength ceramics. They have high-temperature-oxidation resistance and the highest strength in structures. Silicon nitride has higher strength than silicon carbide up to 2600 F but above this temperature silicon carbide is stronger. Silicon carbide and silicon nitride are inert to most chemicals and for most applications they exhibit similar corrosion resistance. An important design consideration is that they have the lowest thermal expansion coefficient in comparison to other ceramics. In addition they have the lowest density. They also have the highest compressive strengths. Silicon nitride is the only ceramic material used as a roller bearing material see Chap. 13 . Ceramic journal bearings are widely used in very corrosive environments where metals cannot be used. For example sealed pumps driven by magnetic induction are used for pumping corrosive chemicals. Most sealed pumps operate with ceramic sleeve bearings of silicon carbide. The ceramic sleeves are used because of their corrosion resistance and for their nonmagnetic properties. However the use of a silicon carbide sleeve in a sliding bearing was not successful in all cases. These bearings operate with the process fluid as lubricant. These fluids such as gas and water often have low viscosity. .