1. A brief introduction of graphite bearings
Bearing is a kind of sliding parts commonly used in machinery industry. Graphite bearing is a carbon bearing developed on the basis of metal bearing with the performance requirements of mechanical equipment. Carbon bearings have different characteristics from metal bearings, such as self-lubrication, high temperature resistance, corrosion resistance, light weight and so on. With the development of science and technology, people have developed ceramic ball bearings, carbonized silicon bearings and so on. Graphite bearings have also been further expanded, and bronze graphite oil-bearing and Mosaic graphite bearings have been produced. Because graphite bearing has its unique aspects in manufacturing process and technical performance, it has certain expansibility in the future development. In order to get further development and get a broader market in application, further analysis and research are done here.
2. Lubrication basis, classification and performance analysis of graphite bearings
Analysis of lubricity of graphite materials from carbon atomic structure. The lubricating property of graphite material itself is determined by its crystal structure. In addition to the specific reason of the lattice has a congenital lubricating structure, the lubrication performance of graphite is the good lubrication effect of water and air on it. The presence of water and air adsorbed water and gas molecules on the graphite working surface and increased the distance between the graphite sliding cleavage planes, thus weakening the binding force between them. Bearings are divided into rolling bearings and sliding bearings, and the material is different from metal, non-metal and composite materials.
Graphite bearing with graphite material as the main base material, used in food, beverage, textile, chemical and other industrial sectors in the transport machine, dryer, textile machine, submersible pump motor bearings, such as these parts with grease lubricant will inevitably cause pollution, and the graphite bearing self-lubrication is strong, corrosion resistance, can not use lubricating oil and long-term operation. In order to improve the mechanical strength of products, especially in order to improve the impact toughness, some fusible metals with good wear resistance are commonly used for impregnation. The allowable working temperature of antimony graphite bearing can reach 500C, antimony and copper are not easy to stick when grinding, in the case of large load and fast work, its wear resistance can be improved by 2- 3 times. The strength of graphite material increases with the increase of temperature until about 2600C, which is 3 times stronger than room temperature at high temperature.
There are two forms of friction in sliding bearings, namely static friction and dynamic friction. The static friction of general carbon graphite bearings is smaller than that of metal bearings under the same circumstances. Under dry operation, the dynamic friction coefficient of ordinary carbon graphite bearings is between 0.08-0.3. Under wet operation, due to the existence of boundary lubrication, the dynamic friction coefficient is generally within the range of 0.01-0.1. In the friction between the carbon brush and the commutator, a layer of brown, dark purple, light blue film can be produced on the surface of the commutator, which can play a protective role in the lubrication process. The analysis shows that this film is mainly composed of two parts, one is combined with metal oxide film and metal hydroxide, called oxide film. The other is carbon film, which is composed of extremely fine carbon and graphite particles peeled off from the carbon block in the process of operation, impurity contained in the carbon block material, and floating dust in the air, which is absorbed by water and oxygen.
The manufacturing of inlaid graphite bearing is based on the metal material as the matrix, its mechanical strength basically depends on the mechanical properties of the metal material used, graphite as a solid lubrication material inlaid in the friction surface of the bearing on the metal matrix, for the bearing working state of lubrication material, but graphite also has the effect of oil. Graphite Mosaic can take two methods, one is molten metal casting method, the other is in the metal substrate in accordance with a certain arrangement of the processing of some holes, and then the solid lubricant embedded in it, Mosaic graphite bearing compressive strength, bending strength, thermal expansion coefficient generally depends on the matrix metal material. But the strength of inlaid graphite bearings is greatly improved and can operate under heavy loads.
3. Design and manufacture of graphite bearings
The structural diversity of carbon materials comes from the diversity of chemical bonds between carbon/carbon atoms, which also provides the designability of carbon materials. Because of its production process and the designability of raw materials, graphite bearings provide certain space for design and manufacturing, so as to produce graphite bearings with different properties and suitable for a variety of technical conditions and use environment. PV value is an important parameter in bearing design PV=P*V, P-- unit area load pressure, MPa; V is the linear velocity of the axis,m/s
PV value exceeds the limit value of various materials, it will cause abnormal temperature rise and wear, the general design should have a margin, the PV value should be in the limit value of 50%- 70%. In actual work, bearing operation can be divided into dry operation and wet operation due to different environment. In dry operation, the maximum load allowed on graphite bearings is limited by the strength of the bearings and the allowable wear rate. The maximum allowable linear velocity is limited by the heat generated on the friction surface of the bearing. The wear rate is affected by bearing material, shaft and bearing surface machining, load, speed and surrounding media. In the case of dry running, the allowable PV value of the bearing is also directly affected by the bearing material.
Graphite bearing design types are cylindrical, cylindrical generation flange shape, circular, special structure, but from the strength of graphite bearing, the thickness of carbon bearing must be larger than metal bearing, thickness with inner diameter 1/5-1/7 is appropriate, the minimum limit is more than 3mm, the length of the bearing is more than 2 times the inner diameter. The temperature of the bearing will rise when it is running. The running clearance can be determined according to the thermal expansion of the shaft and bearing, which is generally about 0.3% of the shaft diameter.
The manufacturing process of carbon graphite bearings is roughly the same as that of other carbon products. The basic process includes: Selection of raw materials, pretreatment of raw materials, crushing of raw materials, screening classification, ingredients of granular and powdered materials, selection of binder, mixing, kneading, roller pressing, grinding molding, roasting, artificial graphite products must also be graphitized, impregnated, mechanical processing, some graphite bearings in order to increase mechanical strength and impact resistance, Also need metal (such as antimony, copper, lead, etc.) impregnation, finally achieve the required technical properties, and then machining into the required geometric size.
4. Discuss
In summary, the carbon/graphite bearings are briefly studied and analyzed, which confirms the excellent characteristics of carbon graphite bearings. According to the special lubrication requirements under different conditions, sliding graphite bearings with different properties and suitable for various conditions can be researched and manufactured. At present in food, textile, printing and dyeing, chemical industry conveyor bearing, centrifugal pump bearing, submersible pump bearing, gear bearing and automobile, aviation, agricultural machinery and other aspects have been applied to varying degrees.
Furthermore, the modification of carbon materials can be further studied and some new materials can be developed. If the carbon alloy method is used to chemically modify the surface and body of carbon materials, the result will certainly affect the structure and properties of the obtained materials, and it is very possible to create new functional materials for the manufacture of lubricating graphite bearings. Another example is the study of carbonized silicon bearing, which has strong hardness, high temperature resistance and other characteristics, but the cost is very high and has not been promoted. The traditional method of powder metallurgy is used to manufacture bearings. If it can further improve the compressive strength and accuracy, it has great development.
Carbon graphite bearings have strong vitality. At the same time of the development of graphite bearings, new technologies such as intercalated graphite and graphite coated bearings will be studied to expand their research and application fields. Graphite bearings will be developed in a larger, more extensive and updated way in the future.