The shaft is a cylindrical object that goes through the middle of the bearing or the middle of the wheel or the middle of the gear, but there are also a few that are square. A shaft is a mechanical part that supports rotating parts and rotates with them to transmit motion, torque or bending moment. It is generally in the shape of a metal round rod, and each segment can have a different diameter. The parts that perform rotary motion in the machine are mounted on the shaft.
Materials for shaft parts:
1. High-quality carbon structural steels such as carbon steel 35, 45, and 50 have many applications due to 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 quenched and tempered. For shafts that are not important or have small stress, carbon structural steel such as Q235 and Q275 can be used.
2. Alloy steel Alloy steel has high mechanical properties, but is more expensive and is mostly used for shafts with special requirements. For example, high-speed shafts using sliding bearings often use low-carbon alloy structural steels such as 20Cr and 20CrMnTi. After carburizing and quenching, the wear resistance of the journal can be improved; the machine rotor shaft must have good wear resistance when working under high temperature, high speed and heavy load conditions. For high temperature mechanical properties, alloy structural steels such as 40CrNi and 38CrMoAlA are often used. For the shaft blank, forgings are given priority, followed by steel; for larger sizes or complex structures, cast steel or ductile iron can be considered.
For example, using ductile iron to manufacture crankshafts and camshafts has the advantages of low cost, good vibration absorption, low sensitivity to stress concentration, and good strength. The mechanical model of the shaft is a beam, and most of it rotates, so its stress is usually symmetrical and cyclic. Possible failure modes include: fatigue fracture, overload fracture, excessive elastic deformation, etc. Some parts with hubs are usually installed on the shaft, so most shafts should be made into stepped shafts, which requires a large amount of cutting.
The formulation of process procedures for shaft parts is directly related to workpiece quality, labor productivity and economic benefits. A part can have several different processing methods, but only one is more reasonable. When formulating the machining process regulations, the following points must be paid attention to:
1. During the process analysis of the part drawing, it is necessary to understand Part structural characteristics, accuracy, material, heat treatment and other technical requirements, and product assembly drawings, component assembly drawings and acceptance standards must be studied.
2. The processing route of carburized parts is generally: blanking → forging → normalizing → rough machining → semi-finishing → carburizing → decarburization processing (for parts that do not need to increase the hardness) → quenching → Threading, drilling or milling → rough grinding → low temperature aging → semi-fine grinding → low temperature aging → fine grinding.
3. Coarse datum selection: If there is a non-machined surface, the non-machined surface should be selected as the rough datum. For casting shafts that require machining on all surfaces, align the surface according to the smallest machining allowance. And choose a flat and smooth surface, leaving the gate open. Choose a solid and reliable surface as the rough datum. At the same time, the rough datum cannot be reused.
4. Fine datum selection: To comply with the principle of datum coincidence, choose design datum or assembly datum as the positioning datum as much as possible. Comply with the principle of benchmark unification. Use the same positioning datum in as many processes as possible. Make the positioning datum coincide with the measurement datum as much as possible. Choose a surface with high precision and stable and reliable installation as the precise benchmark. Whether the process procedures are reasonably formulated directly affects the quality of the workpiece, labor productivity and economic benefits. A part can be manufactured using several different processing methods, but under certain conditions, only one method is more reasonable. Therefore, when formulating process procedures, we must proceed from reality and use advanced processing methods as much as possible to formulate a reasonable process based on specific conditions such as equipment conditions and production types.
Processing technology of shaft parts:
1. Materials of shaft parts
The selection of materials for shaft parts is mainly based on the strength, stiffness, It is determined by wear resistance and manufacturing process, and strives to be economical and reasonable. Commonly used materials for shaft parts include 35, 45, and 50 high-quality carbon steel, with 45 steel being the most widely used. Ordinary carbon steels such as Q235 and Q255 can also be used for shafts with smaller or less important loads. For those with large forces, limited axial size and weight, or some special requirements, alloy steel can be used.
For example, 40Cr alloy steel can be used in working situations with medium precision and high rotation speed. The material has good comprehensive mechanical properties after quenching and tempering treatment; alloy steel such as Cr15 and 65Mn can be used in situations with higher precision and poor working conditions. , these materials have better wear resistance and fatigue resistance after quenching, tempering and surface quenching; if they are shaft parts working under high speed and heavy load conditions, choose low carbon steel such as 20Cr, 20CrMnTi, 20Mn2B or 38CrMoA1A impregnated Carbon steel, after carburizing, quenching or nitriding treatment, these steels not only have high surface hardness, but also have greatly improved core strength, so they have good wear resistance, impact toughness and fatigue resistance. Ductile iron and high-strength cast iron are often used in manufacturing shafts with complex shapes and structures due to their good casting properties and vibration-damping properties. In particular, the rare earth magnesium ductile iron developed in our country has good impact resistance and toughness, and also has the advantages of reducing friction, absorbing vibration, and being less sensitive to stress concentration. It has been used to manufacture important shaft parts for automobiles, tractors, and machine tools. .
2. Blanks of shaft parts
The common blanks of shaft parts include profiles (round bars) and forgings. Large shafts with complex shapes and structures can also be cast. Crankshafts in internal combustion engines are generally made of casting blanks. The profile blanks are divided into hot-rolled or cold-drawn bars, both of which are suitable for smooth shafts or stepped shafts with small diameter differences. After the forging blank is heated and forged, the internal fiber structure of the metal is distributed along the surface, so it has high tensile, bending and torsion resistance, and is generally used for important shafts.
Processing methods of shaft parts:
1. Processing method and processing accuracy of outer surface
Shaft, sleeve and disc parts have Typical parts with cylindrical surfaces. Commonly used machining methods for the outer surface include turning, grinding and various finishing methods. Turning is the most economical and effective processing method for cylindrical surfaces, but in terms of economic accuracy, it is generally suitable as a rough machining and semi-finishing method for cylindrical surfaces; grinding is the main finishing method for cylindrical surfaces, especially suitable for Suitable for finishing of various high hardness and quenched parts; finishing processing is an ultra-precision processing method (such as rolling, polishing, grinding, etc.) performed after finishing, and is suitable for certain parts with high precision and surface quality requirements . Since various processing methods can achieve different economic processing accuracy, surface roughness, productivity and production costs, a reasonable processing method must be selected according to the specific situation to process qualified parts that meet the requirements on the part drawings.
2. Turning of the cylindrical surface
(1) Form of cylindrical turning The main processing method of the cylindrical surface of shaft parts is turning. The main processing forms are: rough-cut free forgings and large casting blanks, with a large machining allowance. In order to reduce the outer circular shape error and position deviation of the blank and make the machining allowance uniform in subsequent processes, the main purpose is to remove the oxide scale on the outer surface. For outer circle processing, the general removal allowance is 1-3mm on one side. Rough turning of small and medium-sized forging and casting blanks is generally carried out directly for rough turning. Rough turning mainly cuts off most of the remaining stock of the blank (generally turning out a stepped profile). When the stiffness of the process system allows, a larger cutting amount should be used to improve production efficiency. Semi-finishing is generally used as the final machining process for medium-precision surfaces, and can also be used as a pre-processing process for grinding and other machining processes. For blanks with higher precision, semi-finish turning can be done directly without rough turning. The final processing process of finishing the cylindrical surface and the pre-processing before finishing. The final processing process of fine turning with high precision and fine roughness. It is suitable for processing the cylindrical surface of non-ferrous metal parts. However, since non-ferrous metals are not suitable for grinding, fine turning can be used instead of grinding. However, precision lathes require machine tools with high precision, good rigidity, smooth transmission, micro-feeding, and no crawling. Diamond or cemented carbide tools are used in turning. The main declination angle of the tool should be larger (45o-90o), and the tool tip arc radius should be less than 0.1-1.0mm.
(2) Application of turning method
1) Ordinary turning. It is suitable for cylindrical processing of various batches of shaft parts and has a wide range of applications.
For small batches of single pieces, bedroom lathes are often used to complete turning processing; for medium- and large-scale production, automatic, semi-automatic lathes and special lathes are used to complete turning processing.
2) CNC turning. Suitable for single piece small batch and medium batch production. Its application is becoming more and more common, and its main advantages are good flexibility, short equipment adjustment and preparation time when replacing processed parts; less auxiliary time during processing, and efficiency can be improved by optimizing cutting parameters and adaptive control; good processing quality, and few special tools and fixtures , the corresponding production preparation cost is low; the technical requirements for machine tool operation are low and are not affected by the operator's skills, vision, mental, physical strength and other factors. For shaft parts, CNC turning is suitable if they have the following characteristics. Parts with complex structure or shape, difficult ordinary processing operations, long working hours, and low processing efficiency. Parts that require higher processing accuracy and consistency. Parts with changing cutting conditions, such as parts that require grooving, drilling, threading, etc. due to their shape characteristics, need to change the cutting amount many times during processing. The batch is not large, but each batch of parts has a variety of varieties and a certain degree of complexity. For shaft parts with keyways, radial holes (including screw holes), and distributed holes (including screw holes) on the end face, such as the belt method Blue shafts, shafts with keyways or square heads can also be processed on turning machining centers. In addition to ordinary CNC turning, various grooves, holes (including screw holes), surfaces and other surfaces on the parts can also be machined. And can be processed. The process is highly concentrated, its processing efficiency is higher than ordinary CNC turning, and its processing accuracy is more stable and reliable.
3) The method of grinding the outer cylindrical surface using abrasive tools to process the workpiece surface at a high linear speed is called grinding. Grinding is a multi-tool and multi-edge high-speed cutting method, which is used for part finishing and hard surface processing. The grinding process range is very wide and can be divided into rough grinding, fine grinding, fine grinding and mirror grinding. The abrasives (or abrasives) used in grinding have the characteristics of small particles, high hardness, and good heat resistance, so they can process harder metal materials and non-metal materials, such as hardened steel, carbide cutting tools, ceramics, etc. ; During the machining process, there are many particles participating in the cutting motion at the same time, and it can remove extremely thin and fine chips, so the machining accuracy is high and the surface roughness value is small. Grinding, as a finishing method, is widely used in production. Due to the development of powerful grinding, the blank can also be directly ground to the required size and precision, thus achieving higher productivity.
Non-standard products refer to products or equipment that are not manufactured in accordance with unified industry standards and specifications promulgated by the state, but are designed and manufactured by oneself according to their own needs. And the appearance or performance are not in the national equipment product catalog.
In order to meet the needs of large-scale industrial production, mechanical equipment that has been produced and has proven to have good performance is finalized and serialized, called standard products. According to user requirements, products produced, modified or customized on the basis of standard products are called non-standard products.
Standardized and serialized products are mainly suitable for general-purpose mechanical equipment. In fact, when designing a new industrial device, some equipment is specialized and cannot be found in the existing serialized list. It is necessary to It is independently designed and manufactured, and its future versatility will not be high. This type of equipment is called a non-standard product.
At present, the chemical and petrochemical industries are the main areas where non-standard products are mentioned. Because there are many products in this industry and the processes vary widely, there are also many non-standard products.
Non-standard parts are mainly other accessories that are freely controlled by enterprises except that the country has not set strict standard specifications and has no relevant parameter regulations. There are many varieties of non-standard parts, and there is currently no standardized classification. The general classification is as follows:
Metal non-standard parts:
The customer provides drawings, and the manufacturer uses equipment to produce corresponding products according to the drawings, usually mostly molds, with tolerance requirements and smoothness. There is no certain paradigm as specified by the customer. Products completely require corresponding quality control from casting to finishing. The process is complex and highly variable, and the general cost is higher than standard parts.
Non-metal non-standard parts:
It is the processing of some non-metal materials.
Such as plastics, wood, stone, etc. In recent years, the development of plastic molds in the injection molding industry has become increasingly sophisticated. The introduction of curved surface design and programming CNC has greatly improved the cost and tolerance level of non-standard processing.