A shaft with an aspect ratio of less than 5 is called a short shaft, and a shaft with an aspect ratio of more than 20 is called a slender shaft, with most shafts in between.
The shaft is supported by the bearing, and the shaft section matched with the bearing is called the journal. Journal is the assembly benchmark of shaft, and their accuracy and surface quality are generally required. Their technical requirements are generally formulated according to the main functions and working conditions of the shaft, and usually include the following items:
1, surface roughness
Generally, the surface roughness of shaft diameter matched with transmission parts is Ra2.5~0.63μm, and that of supporting shaft diameter matched with bearings is Ra0.63~0. 16μm+06 μ m. ..
2, the accuracy of the mutual position
The position accuracy requirement of shaft parts is mainly determined by the position and function of shaft in machinery. Usually, it is necessary to ensure the coaxiality between the journal of the assembly transmission parts and the supporting journal, otherwise the transmission accuracy of the transmission parts (gears, etc.) will be affected. ) and produce noise. For ordinary precision shafts, the radial runout of the matching shaft section to the supporting journal is generally 0.0 1~0.03mm, and for high-precision shafts (such as spindles), it is generally 0.001~ 0.005 mm. ..
3, geometric shape accuracy
Geometric shape accuracy of shaft parts mainly refers to roundness and cylindricity of journal, outer conical surface and Morse cone hole. , its tolerance should generally be limited to the size tolerance range. For the inner and outer circular surfaces with high precision requirements, the allowable deviation should be marked on the drawings.
4, dimensional accuracy
In order to determine the position of the shaft, bearing journals usually require high dimensional accuracy (IT5~IT7). Generally, the journal dimension accuracy of assembly transmission parts is low (IT6~IT9).
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.
Eccentric CNC lathe (eccentric/slitting lathe) is a kind of CNC machine tool, which is mainly used for precision machining of shafts and non-standard shafts. Compared with CNC lathe, it has made a qualitative leap in machining efficiency and accuracy. The machining cycle time is greatly reduced due to the use of tools arranged in two axes. By shortening the tool change time between the cutter row and the opposite tool table, the overlapping function of multiple tool tables, the overlapping function of the effective axis movement of thread chips and the direct indexing function of the spindle during secondary machining, the blank is realized. The chip cutter is always machined at the clamping position between the spindle and the workpiece, which ensures the machining accuracy unchanged. At present, the maximum processing diameter of centrifuges on the market is 42mm, which has great advantages in the precision shaft processing market. This series of machine tools can be equipped with automatic feeding device to realize automatic production of a single machine tool and reduce labor cost and product defect rate. It is very suitable for mass production of precision shaft parts.