What is a shaft
What is a shaft?
Shaft is a cylindrical object that passes through the middle of bearing, wheel or gear, but a small part is 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 section can have different diameters. The rotating parts of the machine are installed on the shaft.
Classification of shafts
Shafts can be classified from the aspects of different loads on the shaft, the shape of the shaft and the application of the shaft.
1) According to the different loads on the shaft, the shaft can be divided into spindle, transmission shaft and rotating shaft.
The shaft that only bears bending moment and does not transmit torque is called mandrel. The spindle can be divided into two types: the fixed spindle in which the shaft does not rotate during operation and the rotating spindle in which the shaft rotates during operation. Spindle is mainly used to support various mechanical parts.
The shaft that only transmits torque and does not bear bending moment is called transmission shaft. The transmission shaft mainly transmits power by bearing torque.
The shaft that transmits torque and bears bending moment is called rotating shaft. All kinds of transmission parts mainly transmit power through the rotating shaft.
2) According to different structural shapes, shafts can be divided into optical shafts, stepped shafts, solid shafts, hollow shafts, etc. Due to the complex manufacturing process of hollow shaft, hollow shaft is usually designed mainly in the occasions with large diameter and weight reduction requirements.
3) According to different geometric axis shapes, shafts can be divided into straight shafts and crankshafts.
In addition, there is a kind of shaft with low structural stiffness – flexible shaft. The flexible shaft is mainly used for transmission when the axes of two transmission parts are not on the same straight line.
Material of shaft
There are many kinds of shaft materials. The design is mainly based on the strength, stiffness, wear resistance and other requirements of the shaft, as well as the heat treatment method used to achieve these requirements, and the manufacturing process is considered. Because the shaft is usually subjected to alternating stress during operation, the most common failure form of the shaft is fracture due to the action of alternating stress. Therefore, the material of the shaft should have certain toughness and good fatigue resistance, which is the basic requirement for the material of the shaft.
The common material of shaft is high-quality carbon structural steel with moderate carbon content. Ordinary carbon structural steel can also be used for shafts with small or less important load. Medium carbon alloy steel can be used for shafts with large stress, limited size and weight of shafts, and some special requirements. Alloy steel has high sensitivity to stress concentration, so the structural shape of the shaft using alloy steel should minimize the source of stress concentration and require low surface roughness.
Due to the poor toughness of cast iron, cast iron should be used as the material of shaft as little as possible. However, for the shaft with complex structure and less important, nodular cast iron or high-strength cast iron can also be selected as the material of the shaft.
Although the elastic modulus of the material with high strength limit is also slightly larger, it is not appropriate to select the material with high strength limit only to improve the stiffness of the shaft because there is little difference in the elastic modulus of all kinds of steel.
The shaft is generally made of rolled round steel or forgings by cutting. The shaft with smaller diameter can be made of rolled round steel. For large diameter or important shafts, forgings are often used.
- 1. High quality carbon structural steels: such as carbon steel 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 quenched and tempered. Q235, Q275 and other carbon structural steels can be used for shafts that are unimportant or less stressed.
- 2. Alloy steel: alloy steel has high mechanical properties, but the price is expensive. It is mostly used for shafts with special requirements. For example, for high-speed shafts with sliding bearings, low-carbon alloy structural steels such as 20Cr and 20CrMnTi are commonly used, which can improve the wear resistance of journal after carburizing and quenching; When the rotor shaft works under high temperature, high speed and heavy load, it must have good high temperature mechanical properties. Alloy structural steels such as 40CrNi and 38CrMoAlA are often used.
Forgings are preferred for shaft blanks, followed by steel; Cast steel or nodular cast iron can be considered for those with large size or complex structure. For example, manufacturing crankshaft and camshaft with nodular cast iron has the advantages of low cost, good vibration absorption, low sensitivity to stress concentration, good strength and so on. The mechanical model of the shaft is a beam, most of which have to rotate, so its stress is usually symmetrical cycle. The possible failure forms are fatigue fracture, overload fracture, excessive elastic deformation and so on. Some parts with hubs are usually installed on the shaft, so most shafts should be made into stepped shafts with large cutting capacity.
Processing technology of shaft
Materials of shaft parts
The selection of materials for shaft parts is mainly determined according to the strength, stiffness, wear resistance and manufacturing process of the shaft, and strive to be economical and reasonable. The commonly used materials of shaft parts are 35#, 45# and 50# high-quality carbon steel, of which 45# steel is the most widely used. Ordinary carbon steels such as Q235 and Q255 can also be used for shafts with small load or less important. Alloy steel can be used for those with large stress, limited ahaft 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 better comprehensive mechanical properties after quenching and tempering treatment; Cr15, 65Mn and other alloy steels can be used in the situation of high precision and poor working conditions. These materials have good wear resistance and fatigue strength after quenching, tempering and surface quenching; For shaft parts working under high-speed and heavy load conditions, 20Cr, 20CrMnTi, 20mn2b and other low-carbon steels or 38CrMoA1A carburized steels are selected. After carburizing, quenching or nitriding treatment, these steels not only have high surface hardness, but also greatly improve their core strength, so they have good wear resistance, impact toughness and fatigue strength. Nodular cast iron and high strength cast iron are often used in manufacturing shafts with complex shape and structure because of their good casting performance and vibration damping performance. In particular, the rare earth magnesium nodular cast iron developed in China has good impact toughness, friction reduction, vibration absorption and low sensitivity to stress concentration. It has been used to manufacture important shaft parts in automobiles, tractors and machine tools.
Blank of shaft parts
The common blanks of shaft parts are profiles (round bars) and forgings. Castings can also be used for large shafts with complex shape and structure. The crankshaft in internal combustion engine generally adopts casting blank. The profile blank is divided into hot-rolled or cold drawn bars, which are suitable for smooth shafts or stepped shafts with small diameter difference. 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 torsional strength. It is generally used for important shafts.
Machining method of shaft
1. Machining method and machining accuracy of cylindrical surface
Shaft, sleeve and disc parts are typical parts with cylindrical surface. The common machining methods of cylindrical surface include turning, grinding and various finishing methods. Turning is the most economical and effective machining method for cylindrical surface, but in terms of its economic accuracy, it is generally suitable for rough machining and semi finishing of cylindrical surface; Grinding is the main finishing method of cylindrical surface, which is especially suitable for finishing of various 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 achieved by various machining methods are different, reasonable machining methods must be selected according to the specific situation, so as to process qualified parts that meet the requirements on the part drawing.
2. Turning of cylindrical surface
(1) The main machining method of the cylindrical surface of shaft parts is turning. The main processing forms are: the blank of free forging and large casting of rough car has a large machining allowance. In order to reduce the shape error and position deviation of the blank outer circle and make the machining allowance of subsequent processes uniform, the outer circle processing mainly removes the oxide scale on the excluded surface, and the cutting allowance is generally 1-3mm on one side. Rough turning of medium and small forging and casting blanks is generally carried out directly. Rough turning mainly cuts off most of the allowance of the blank (generally turning out the step contour). When the rigidity of the process system allows, larger cutting parameters should be selected to improve the production efficiency. Semi finish turning is generally used as the final machining process of medium precision surface, and also as the pre machining of grinding and other machining processes. For the blank with high precision, it can be directly semi refined without rough turning. Finish turning the final machining process of cylindrical surface machining and pre machining before finishing. Fine turning is the final processing process of high-precision and fine roughness surface. It is applicable to the machining of cylindrical surface of non-ferrous metal parts, but because non-ferrous metals are not suitable for grinding, fine turning can be used instead of grinding. However, the precision lathe requires high precision, good rigidity, stable transmission, micro feed and no crawling. Diamond or cemented carbide tools are used in turning. The main deflection angle of the tool is larger (45° – 90°), and the arc radius of the tool tip is less than 0.1-1.0mm.
(2) Application of turning method
- 1) Ordinary turning is applicable to the cylindrical machining of various batches of shaft parts, which is widely used. Single piece small batch often use bedroom lathe to complete turning; For medium batch and mass production, automatic, semi-automatic lathes and special lathes are used to complete turning.
- 2) NC turning is suitable for single piece small batch and medium batch production. Its main advantages are good flexibility and short equipment adjustment and preparation time when replacing machined parts; The auxiliary time is less, and the efficiency can be improved by optimizing cutting parameters and adaptive control; Good processing quality, few special tools and fixtures, and low production preparation cost; The technical requirements for machine tool operation are low and are not affected by the skills, vision, spirit, physical strength and other factors of operators. For shaft parts, NC turning is suitable for the following characteristics. Parts with complex structure or shape, difficult ordinary processing operation, long working hours and low processing efficiency. Parts requiring high machining accuracy consistency. For parts with changeable cutting conditions, such as parts that need to be grooved, drilled and threaded due to shape characteristics, the cutting parameters should be changed many times during machining. The lot size is small, but each batch of parts has various varieties and a certain degree of complexity. For shaft parts with keyway, radial hole (including screw hole) and distributed hole (including screw hole) on the end face, such as shaft with flange, shaft with keyway or square head, they can also be processed on the turning center. In addition to ordinary NC turning, various grooves Holes (including screw holes), surfaces and other machined surfaces can also be machined at the same time. The process is highly concentrated, its machining efficiency is higher than that of ordinary NC turning, and the machining accuracy is more stable and reliable.
- 3) Grinding of cylindrical surface the method of machining the workpiece surface with grinding tools at high linear speed is called grinding. Grinding is a high-speed cutting method with multiple tools and edges. It is used in the finishing of parts and the machining of hard surfaces. Grinding process covers a wide range, which can be divided into rough grinding, fine grinding, fine grinding and mirror grinding. The abrasive tools (or abrasives) used for grinding have the characteristics of small particles, high hardness and good heat resistance, so they can process hard metal and non-metal materials, such as hardened steel, cemented carbide tools, ceramics, etc; In the machining process, there are many particles participating in the cutting movement at the same time, which can remove very thin and fine chips, so the machining precision 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 ground directly to the required size and accuracy, so as to obtain high productivity.
Shaft wear is the most common equipment problem in the process of shaft use. There are many reasons for shaft wear, but the main reason is determined by the metal characteristics used to manufacture the shaft. Although the metal has high hardness, it has poor concession (unable to recover after deformation), poor impact resistance and poor fatigue resistance. Therefore, it is easy to cause adhesive wear, abrasive wear, fatigue wear and fretting wear. Most shaft wear is not easy to detect, Only when there is high temperature, large jumping range and abnormal noise of the machine, people will notice it. However, when people notice it, most of the shafts have been worn, resulting in the shutdown of the machine.
The repair of worn shaft head of large equipment is a problem worthy of attention. When the shaft is made of 45 steel (quenching and tempering treatment), if only surfacing treatment is adopted, the welding internal stress will be generated. Under the condition of heavy load or high-speed operation, cracks and even fractures may appear at the shaft shoulder. If stress relief annealing is adopted, it is difficult to operate, with long processing cycle and high maintenance cost. When the material of the shaft is HT200, cast iron welding is not ideal. Repair welding, shaft sleeve and pitting are generally used for shaft wear in China. If the shutdown time is short and there are spare parts, the shaft will generally be replaced. Some enterprises with high maintenance technology will adopt brush plating, laser welding, micro arc welding or even cold welding. These maintenance technologies need to purchase expensive equipment and hire skilled workers with high salary, Some domestic small and medium-sized enterprises generally help repair high-value axes through outsourcing with high technology, but have to pay high maintenance and transportation costs.
The above repair technologies are not common in European, American, Japanese and Korean enterprises because the effect of traditional technologies is poor, while advanced repair technologies such as laser welding and micro arc welding require high equipment and personnel, and the cost is large. European, American, Japanese and Korean generally adopt carbon nano polymer material technology and nano technology. On site operation not only effectively improves the maintenance efficiency, It greatly reduces the maintenance cost and maintenance intensity. Because the metal material is a “constant relationship”, although the strength is high, the impact resistance and yield are poor, so the long-term operation will lead to the continuous increase of fit clearance and shaft wear. After realizing this key reason, the polymer composite developed by European and American new technology research institutions has the strength and hardness required by the metal, It also has the concession (variable relationship) that metal does not have. Through processes such as “tooling repair”, “component correspondence” and “machining”, the size matching of repair parts and mating parts can be ensured to the greatest extent; At the same time, taking advantage of the comprehensive advantages of compression resistance, bending resistance and ductility of the composite material itself, it can effectively absorb the impact of external forces, greatly resolve and offset the radial impact force of the bearing on the shaft, avoid the possibility of clearance, and avoid the wear of relative movement of the equipment due to the increase of clearance. Therefore, for the static fit between the shaft and the bearing, Composite materials do not rely on “hardness” to solve the wear of equipment, but on changing the relationship between force to meet the operation requirements of equipment.
Correction of shaft bending deformation
If the deformation of the shaft is too large, it can be corrected by cold pressing or local flame heating. The supporting part shall be correct during correction, and special attention shall be paid to avoid stress concentration at the corner of step shaft due to correction.
Repair of journal wear
Usually, the geometric error of the shaft is eliminated by grinding, and then metal spraying or brush plating. In serious cases, it can be repaired by surfacing or insert sleeve. When inserting sleeve, the sleeve and the shaft are interference fit.
Repair of spline, keyway and thread
The worn tooth side can be repaired by gas welding or surfacing, and then the spline can be milled out based on the worn spline. After the keyway is damaged, the keyway can be appropriately enlarged or the old keyway can be welded and blocked, and a new key can be equipped. When the thread on the shaft is damaged, surfacing shall be carried out and the thread shall be re turned.
Structural design of shaft
The structural design of the shaft is an important step to determine the reasonable shape and all 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, manufacturing and assembly process, installation and transportation, and the deformation of the shaft. The designer can design according to the specific requirements of the shaft. If necessary, several schemes can be compared to select the design scheme. The following are the general shaft structure design principles:
- 1. Save materials and reduce weight, and try to adopt the section shape with equal strength overall dimension or large section coefficient;
- 2. It is easy to accurately locate, stabilize, assemble, disassemble and adjust the parts on the shaft;
- 3. Adopt various structural measures to reduce stress concentration and improve strength;
- 4. Easy to manufacture and ensure accuracy.
Shaft positioning and fixing of parts on shaft
The common ahaft positioning and fixing methods of parts on the shaft include shaft shoulder, shaft ring, locking ring, sleeve, round nut and stop washer, elastic ring, shaft end ring and conical surface.
Overview of design method of shaft
Many factors and requirements must be considered in the design of shaft, mainly including material selection, structural design, strength and stiffness analysis. For high-speed shaft, vibration stability should also be considered.
The design of the shaft is based on meeting the structural and functional requirements. Firstly, according to the function of the shaft in the specific system, the structure meeting the functional requirements is designed, and then the corresponding bearing capacity is checked according to the load and working requirements.
In fact, before the specific structure of the shaft is determined, it is difficult to accurately determine the action point of the force on the shaft, and the size and distribution of the bending moment can not be obtained. Therefore, the calculation of shaft is usually checked and calculated after the preliminary structural design. The calculation criteria mainly include the strength criterion, stiffness criterion and vibration stability criterion of shaft. The shaft design is usually carried out in the order of “structural design – bearing capacity checking calculation – structural improvement design – bearing capacity checking calculation…”.
Generally, the specific procedures for shaft design are:
- a. According to the overall layout of the mechanical transmission scheme, draw up the layout and assembly scheme of the parts on the shaft;
- b. Select the material of the shaft;
- c. Estimate the minimum diameter of the shaft;
- d. Carry out the structural design of the shaft;
- e. Carry out bearing capacity checking calculation, usually including strength checking calculation, stiffness checking calculation and vibration stability checking calculation;
- f. According to the checking calculation results of bearing capacity, either determine the design or improve the design;
- g. Draw the part working drawing of the shaft.
In addition to the above design contents, there are also key or spline connection strength check, rolling bearing life check, sliding bearing bearing bearing capacity check and other works, which have a certain relationship with the shaft design and need to be considered together in the shaft design process.
In terms of design method, shaft design can be divided into conventional design and computer-aided design. The main difference between these two kinds of design methods is that the calculation method for the bearing capacity of the shaft in the conventional design mainly adopts a relatively simplified mechanical model, the calculation results are usually inaccurate, and the calculation results usually need to be corrected with empirical data. However, the conventional design method has been familiar to the majority of engineering design, and has accumulated a large number of valuable empirical data, which is still dominant in the current engineering design.
When using computer-aided design, the bearing capacity is mainly calculated by finite element method, which can obtain more accurate calculation results. It has obvious advantages to use the means of computer-aided analysis for the shaft with complex structure.
Generally, the load transmitted by the shaft and the ultimate stress of the shaft have a certain randomness. In the conventional design, these factors are regarded as deterministic variables. When determining the bearing capacity of the shaft, a certain safety factor is included to ensure the safety redundancy of the structure. If the randomness of load and ultimate stress is considered in the design, the probability reliability of safe operation of shaft can be determined, which leads to the reliability design of shaft. The reliability design method is an important content of modern design method.
Source: China Shaft Manufacturer – Yaang Pipe Industry (www.steeljrv.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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