The application of ceramic sliding bearing in high speed motor
The use of fluid floating and sliding bearing in high precision spindles has attracted wide attention in recent years. Because of the viscostic nature of lubricating oil causinghigh heat dissipation in high-speed rotation, thermal stability of friction pair and hence the adherent spindle becomes hard to be maintained. A a result, traditional oil lubricated sliding bearings are not suitable for high precision applications. Theoretically, water has lower viscosity than most lubricating oil. Heating problem of friction pair will be smaller when using water in the same working conditions. In addition, the inherent cleanliness and flame retardant performance of water is also good to meet the special application requirements of high-speed motor spindle.
The material of traditional sliding bearing is mostly metal. Metal in general has high density, high thermal conductivity and high thermal expansion coefficient. As a result, high rotation performance will be greatly affected if metal is used in precision spindle system. In addition, metal is also not suitable for being used as a bearing material using water lubrication because of the corrosion of metal by water. Ceramic materials has wear resistance, chemical corrosion resistance, small thermal expansion coefficient and other excellent characteristics, so it can be used for the boundary friction and dry friction in water lubrication without corrosion. Therefore, ceramic sliding bearing has the potential to achieve high speed and high precision index, extensive and rapid development.
1. Choice of lubricant
Mineral oil is the most common lubricant for sliding bearings. Water has a higher specific calorific than oil so that the rise of temperature of the friction pair is smaller. The water-lubricated spindle has better thermal stability when the spindle speed is high. In addition, water has no flash point, and water itself is a natural extinguishing agent, so that the safety is higher when using water-lubricated bearings.
2. Selection of ceramic materials
Compared with traditional metal bearing materials, ceramic bearing materials have many excellent properties, mainly including:
(1) The ceramic material has good wear resistance, so it can maintain good working condition in boundary lubrication or even short dry friction;
(2) Excellent performance of corrosion resistance, insulation and high temperature resistance;
(3) The high rigidity of ceramic materials makes ceramic sliding bearings can reduce mechanical vibration, so that improving the accuracy;
(4) The high hardness, high compressive strength and low friction coefficient of ceramic materials make ceramic sliding bearings have longer usable life;
(5) The low thermal expansion characteristics of ceramic materials enable ceramic sliding bearings to be used in the environment with large temperature difference.
Al2O3, ZrO2, SiC and Si3N4 are the commonly used engineering ceramic materials. The friction and wear mechanisms of those materials are different under dry friction and water lubrication conditions. When dry friction occurs, the friction performance of ceramic friction pair will be more critical. When the water film in the bearing is complete, the fluid friction effect of the ceramic friction pair will be obvious.
Under the condition of dry friction the toughened zirconia will occur plastic deformation, abrasive wear, and brittle fracture when the speed is high. Alumina will occur brittle spalling and abrasive wear under that condition. In terms of non-oxide ceramics, Si3N4 will occur micro-cracks, plastic deformation and adhesion under different working conditions, which is prone to friction layer migration. Although the wear mechanism of SiC is similar to Si3N4, but the reaction may occur during the process of friction,: SiC + 2O2 →SiO2 + CO2. The SiO2 attached to the surface of SiC can effectively reduce the amount of wear and reduce the friction coefficient. SiC is hence more suitable as a bearing material just considering the dry friction.
Under the condition of water lubrication, the influence of fluid lubrication on the oxide ceramics Al2O3 and ZrO2 is more obvious. Al(OH)3 can be produced by fluid friction of Al2O3, which can improve the wear resistance. Non-oxide ceramics Si3N4 and SiC are not only affected by fluid friction, but also have obvious hydration reaction under the condition of water lubrication, which can result in SiO2 that obviously improve the wear condition. The possible reactions of non-oxide ceramics under water lubrication are as follows:
Si3N4 + 6H2O→3SiO2 + 4NH3
SiC +O2 + H2O→SiO2 + CO + H2
SiC + 2H2O→SiO2 + CH4
SiO2 + 2H2O→Si (OH) 4
In fact, the Si (OH) 4 is easy to hydrolyzecreating gelatinous SiO2 attached on the surface of the friction pair in mostly cases, which makes the surface of friction smoother and the complete water film easier to form. Having better features to guarantee the stiffness and accuracy, the silicon ceramic bearing is more suitable to be used under the condition of the water lubrication.
3. Application of ceramic sliding bearings
Considering the advantages of ceramic sliding bearing, it has been tried to use in more occasions, such as Liquid-cooled cooling impeller, CPU cooling fan, pump body and high-speed motor, which has got good feedback. With the continuous progress of processing technology, the increasing improvement of the process level, the constantly falling of cost, the ceramic sliding bearing can be used in various industrial fields, which only apply to high-end industry a few years ago!
