Material of shaft parts:
High-quality carbon structural steels such as 1 and carbon steels 35, 45 and 50 are widely used because of their high comprehensive mechanical properties, among which 45 steel is the most widely used. In order to improve its mechanical properties, it should be normalized or tempered. For unimportant or less stressed shafts, carbon structural steel, such as Q235 and Q275, can be used.
2. Alloy steel Alloy steel has high mechanical properties, but it is more expensive, and is mostly used for shafts with special requirements. For example, the high-speed shaft with sliding bearing is generally made of low-carbon alloy structural steel such as 20Cr and 20CrMnTi, which can improve the wear resistance of the journal after carburizing and quenching. The rotor shaft must have good high-temperature mechanical properties when it works at high temperature, high speed and heavy load. Commonly used alloy structural steels such as 40CrNi and 38CrMoAlA are used. The forging of shaft blank takes priority, followed by steel; For large-size or complex structures, cast steel or ductile iron can be considered.
For example, the crankshaft and camshaft of nodular cast iron have low cost, good vibration absorption, insensitivity to stress concentration and good strength. The mechanical model of the shaft is a beam, most of which is rotating, so its stress is usually symmetrical and cyclic. The possible failure forms are fatigue fracture, overload fracture and excessive elastic deformation. Some parts with hubs are usually installed on shafts, so it takes a lot of cutting to make most shafts into stepped shafts.
Structural design of shaft:
The structural design of the shaft is an important step to determine the reasonable shape and various structural dimensions of the shaft. It is related to the type, size and position of the parts installed on the shaft, the fixing mode of the parts, the nature, direction, size and distribution of the load, the type and size of the bearing, the blank of the shaft, the manufacturing and assembly technology, the installation and transportation, the deformation of the shaft and so on. Designers can design according to the specific requirements of the shaft, and if necessary, they can compare several schemes in order to choose the design scheme. The following are the general design principles of shaft structure: 1, save materials and reduce weight, and try to adopt the cross-section shape with equal strength contour size or large cross-section coefficient; 2. It is convenient for accurate positioning, stability, assembly, disassembly and adjustment of parts on the shaft; 3. Take various structural measures to reduce stress concentration and improve strength; 4. It is convenient to manufacture and ensures the accuracy.
Classification of axes:
Common shafts can be divided into crankshaft, straight shaft, soft shaft, solid shaft, hollow shaft, rigid shaft and soft shaft (soft shaft) according to their structural shapes. The straight shaft can be divided into: ① the rotating shaft, which bears both bending moment and torque when working, is the most common shaft in machinery, such as the shaft in various reducers. (2) The main shaft is used to support the rotating parts, only bearing bending moment and not transmitting torque. Some spindles rotate, such as the shafts of railway vehicles, while others do not, such as the shafts supporting pulleys. (3) The transmission shaft is mainly used to transmit torque without bearing bending moment, such as the long optical axis in the mobile mechanism of crane and the driving shaft of automobile. The material of the shaft is mainly carbon steel or alloy steel, and nodular cast iron or alloy cast iron can also be used. The working capacity of the shaft generally depends on the strength and stiffness, and also depends on the vibration stability at high speed.
Technical requirements of the shaft:
1, machining accuracy
1) dimensional accuracy. The dimensional accuracy of shaft parts mainly refers to the dimensional accuracy of shaft diameter and shaft length. According to the application requirements, the diameter accuracy of the main journal is usually IT6-IT9, and the precision journal can also reach IT5. The shaft length is usually specified as the nominal size, and each step of the stepped shaft can be given a tolerance according to the use requirements.
2) Geometric accuracy. Generally, shaft parts are supported on bearings with two journals, which are called supporting journals and are also the assembly benchmark of shafts. In addition to dimensional accuracy, geometric accuracy (roundness and cylindricity) of the bearing journal is generally required. For the journal with average accuracy, the geometric error should be limited to the diameter tolerance. When the requirements are high, the allowable tolerance value should be specified separately on the part drawing.
3) Accuracy of mutual position. The coaxiality between the matching journal (the journal for assembling transmission parts) and the supporting journal in shaft parts is the general requirement for their mutual position accuracy. Generally speaking, for shafts with ordinary precision, the matching precision of supporting journal radial circular runout is generally 0.0 1-0.03mm, and for shafts with high precision, it is 0.001-0.005mm.. In addition, the mutual position accuracy also includes the coaxiality of the inner and outer cylindrical surfaces, the verticality requirements of the axial positioning end face and the axis, etc.
2. Surface Roughness According to the accuracy and running speed of machinery, the surface roughness requirements of shaft parts are different. In general, the surface roughness Ra value of the bearing journal is 0.63-0.16 μ m; The surface roughness Ra value of the matching journal is 2.5-0.63μ.
Processing technology of shaft parts;
1, material of shaft parts
The material selection of shaft parts is mainly based on the strength, stiffness, wear resistance and manufacturing technology of the shaft, and strives to be economical and reasonable. The commonly used materials for shaft parts are 35, 45 and 50 high-quality carbon steels, of which 45 steel is the most widely used. Ordinary carbon steels such as Q235 and Q255 can also be used for shafts with less or less important loads. Alloy steel can be used for heavy load, limited axial size and weight or some special requirements. For example, 40Cr alloy steel can be used in the workplace with medium precision and high speed, and the material has good comprehensive mechanical properties after quenching and tempering. Cr 15, 65Mn and other alloy steels can be used in the case of high precision and poor working conditions. After quenching and tempering, these materials have good wear resistance and fatigue strength. Low carbon steel such as 20Cr, 20CrMnTi, 20Mn2B or 38CrMoA 1A carburized steel is selected for shaft parts working at high speed and heavy load. After carburizing, quenching or nitriding, these steels not only have high surface hardness, but also greatly improve their central strength, so they have good wear resistance, impact toughness and fatigue strength. Ductile iron and high strength cast iron have good castability and vibration reduction performance, and are often used to manufacture shafts with complex shapes and structures. In particular, the rare earth magnesium nodular cast iron developed in China has the advantages of good impact toughness, friction reduction, vibration reduction and insensitivity to stress concentration, and has been applied to the manufacture of important shaft parts such as automobiles, tractors and machine tools.
2. Shaft parts blank
The common blanks of shaft parts are profiles (round bars) and forgings. Castings can also be used for large shafts with complex shapes and structures. Crankshaft of internal combustion engine generally adopts casting blank. Profiles are divided into hot rolled or cold drawn bars, which are suitable for smooth shafts or stepped shafts with little difference in diameter. After the forging blank is heated and forged, the fiber structure in the metal is distributed along the surface, so it has high tensile, bending and torsional strength and is generally used for important shafts.
Processing method of shaft parts;
1, machining method and machining accuracy of cylindrical surface
Shaft, sleeve and disc parts are typical cylindrical parts. The common machining methods of cylindrical surface are turning, grinding and various finishing methods. Turning is the most economical and effective machining method of cylindrical surface, but in terms of its economic accuracy, it is generally suitable for rough machining and semi-finish machining of cylindrical surface. Grinding is the main finishing method of cylindrical surface, especially suitable for finishing all kinds of high hardness and quenched parts; Finishing is an ultra-precision machining method (such as rolling, polishing, grinding, etc. ) after finishing, it is suitable for some parts with high precision and surface quality requirements. Because the economic machining accuracy, surface roughness, productivity and production cost that can be achieved by various machining methods are different, it is necessary to choose a reasonable machining method according to the specific situation in order to process qualified parts that meet the requirements on part drawings.
2, cylindrical turning processing
(1) The form of cylindrical turning The main machining method of the cylindrical surface of shaft parts is turning. The main processing forms are: turning the blanks of free forgings and large castings, and the machining allowance is very large. In order to reduce the shape error and position deviation of the blank excircle and make the machining allowance of the subsequent process uniform, the machining of the excircle is mainly based on removing the external surface oxide scale, and the general cutting allowance on one side is1-3 mm. Rough turning small and medium-sized forged billets are generally roughed directly. The rougher mainly cuts off most of the allowance of the blank (generally turning out the step outline). If the rigidity of the process system allows, a larger cutting amount should be selected to improve production efficiency. Semi-finish turning is generally used as the final machining process of medium-precision surface, and can also be used as the pretreatment of grinding and other machining processes. For high-precision blanks, semi-finish turning can be directly carried out without rough turning. The final processing procedure of finishing cylindrical surface and pretreatment before finishing. The final processing technology of finishing high precision and high roughness surface. It is suitable for machining cylindrical surfaces of non-ferrous metal parts, but because non-ferrous metals are not suitable for grinding, fine turning can be used instead of grinding. However, finishing requires high precision, good rigidity, smooth transmission, micro-feed and no crawling. When turning, diamond or cemented carbide tools are used, the main deflection angle of the tool is relatively large (45o-90o), and the radius of the tip arc is less than 0.1-kloc-0/.0mm. ..
(2) Application of turning method
1) ordinary turning is suitable for cylindrical machining of various batch shaft parts and is widely used. Bedroom lathe is often used to complete single piece and small batch turning; In medium and large batch production, automatic, semi-automatic lathes and special lathes are used to complete turning.
2) CNC turning is suitable for single piece, small batch and medium batch production. The application is more and more common, and its main advantages are good flexibility and short preparation time for equipment adjustment when replacing machined parts; There is less auxiliary time in machining, and the efficiency can be improved by optimizing cutting parameters and adaptive control. Good processing quality, few special fixtures and low corresponding production preparation cost; The technical requirements for machine tool operation are low and are not affected by the operator's skills, vision, spirit and physical strength. For shaft parts, CNC turning is suitable for the following characteristics. The parts with complex structure or shape are difficult to process in general, with long working hours and low processing efficiency. Parts that require high consistency in machining accuracy. For parts with changeable cutting conditions, such as grooving, turning holes, turning threads, etc. In the process of machining, the cutting parameters have to be changed many times. The batch is not large, but for shaft parts with keyways, radial holes (including screw holes) and distributed holes (including screw holes) on the end face, such as shafts with flanges, shafts with keyways or square heads, it is also possible to process many kinds of parts with a certain degree of complexity in each batch in the turning center. In addition to ordinary CNC turning, we can also process all kinds of grooves, holes (including screw holes) and surfaces on parts together. The process is highly centralized, the machining efficiency is higher than that of ordinary CNC turning, and the machining accuracy is more stable and reliable.
3) Grinding the surface of a workpiece with a grinding tool at high linear speed is called grinding. Grinding is a multi-tool and multi-edge high-speed cutting method, which is used for finishing parts and machining hard surfaces. Grinding has a wide range, which can be divided into rough grinding, fine grinding, fine grinding and mirror grinding. Abrasive tools (or abrasives) used in grinding have the characteristics of small particles, high hardness and good heat resistance, so they can process hard metal materials and non-metal materials, such as hardened steel, cemented carbide tools and ceramics. In the process of machining, many particles participate in cutting movement at the same time, which can remove extremely thin and fine chips, so the machining accuracy is high and the surface roughness value is small. As a finishing method, grinding has been widely used in production. Due to the development of strong grinding, the blank can also be directly ground to the required size and accuracy, thus achieving higher productivity.