Camshaft is a component of piston engine. Its function is to control the opening and closing of the valve. Although the speed of the camshaft in a four-stroke engine is half that of the crankshaft (in a two-stroke engine, the speed of the camshaft is the same as that of the crankshaft), it is still very high and needs to bear a lot of torque, so the design requires high strength and support of the camshaft, which is generally made of special cast iron and occasionally used forgings. Because the law of valve movement is related to the power and working characteristics of the engine, the design of camshaft plays a very important role in the design process of the engine.
Important machining technology of camshaft
3. 1 center hole machining
Generally, the standard center is used for machining center holes (special center holes are treated differently), and the process arrangement is divided into two times.
Before and after heat treatment. The key of this process is to control the tolerance of 60 positioning cone (15 ′ ~ 20 ′) and check it all with special tools (simulating the positioning thimble of subsequent machine tools), so as to control the axial gear opening accuracy of camshaft (0.2 ~ 0.5 mm) and prevent unmapped burrs from appearing during cam milling.
3.2 Heat Treatment +0 Quenching
Induction hardening should be based on different workpiece materials. On the premise of meeting the hardness requirements, find out the boundary conditions of quenching, control the maximum and minimum voltage and current range of output, control the concentration, flow rate and temperature of quenching liquid at the same time, and analyze the cooling rate of quenching liquid regularly as the basis for replacing quenching liquid.
3.2.2 Various camshaft materials and heat treatment processes
Various camshaft materials and heat treatment processes are shown in the following table 1. 3.3 Cam grinding
With the reduction of grinding allowance and the increase of grinding speed (from the traditional 35 ~ 60m/s to 125 ~ 200m/s), it will be an inevitable trend to adopt high-speed grinding. The grinding speed of resin bonded corundum, silicon carbide and cubic boron nitride grinding wheel can reach 125m/s, and the grinding speed of ceramic bonded grinding wheel can reach 200m/s. When grinding with CBN grinding wheel, attention should be paid to selecting appropriate grinding wheel width, concentration, hardness and cutting fluid. The correct selection of cutting fluid types and cooling process parameters will have a great influence on the degree of mechanical wear, chemical erosion and thermal damage of grinding wheels during grinding. Especially when there are concave-convex wheels (also known as negative radius of curvature (NROC) cams) on the camshaft, equipment suppliers generally recommend the use of double-grinding-head CNC grinders. As for the grinding process design of cam, the general users rely on equipment suppliers to solve it. According to the 0 ~ 360 discrete points of the cam provided by the user, the supplier mainly solves the following problems by selecting the appropriate numerical control system: 1) Turn the discrete points into a continuous closed curve. 2) Turn the generated curve into a grinding curve. Because the grinding point and the generating point are not at the same point when grinding the cam, it is necessary to change the mold mathematically, and this transformation is also related to the measurement method of the cam. 3) Establish the C-axis speed regulation curve. In cam grinding, in order to ensure the constant cam acceleration, the speed regulation curve of C-axis speed must be adjusted according to the angle of C-axis. Among them, before the NC program of cam grinding is generated, the key is to compile the calculation program of cam generation curve (discrete points form closed curve through data smoothing) and speed curve, that is, to convert the given cam generation table into grinding curve (C coordinate value and X coordinate value) for grinding.
4. 1 Most traditional camshafts are produced by casting or forging, and some of them are made by cutting carbon steel. Cast camshafts mainly include chilled cast iron and quenched cast iron. In order to reduce the weight, some camshafts are cast with cores to make the shafts hollow. In Japan, chilled cast iron camshafts are dominant, while in the United States, quenched cast iron camshafts are dominant. In order to improve the engine performance, various types of camshafts have been developed in recent years, such as remelting chilled cast iron and quenched ductile iron, but their application scope is limited to a few fields due to cost and other reasons. The forged camshaft mainly adopts carbon steel hot forging, and the cam part adopts high frequency quenching, which is mainly used for large and medium-sized engines. Because of its good pitting corrosion resistance, it is often used in conjunction with the tappet of overhead valve (OHV) mechanism and the rocker arm of overhead cam (OHC) structure of diesel engine. Because the forged camshaft cannot be lightweight, its development potential is very small. It is difficult for the traditional camshaft manufacturing characteristics to meet the different performance requirements of the engine valve train for each part of the camshaft at the same time, and the cam layout is not very compact, the material utilization is unreasonable, and the subsequent processing is complicated, so it is difficult to make a new breakthrough in lightweight and cost reduction.
4.2 Metal-plastic composite camshaft has been applied in the United States. Put the powdered metal molded and ground cam plate and hollow steel shaft into the mold, and inject plastic around the hollow shaft. There is no direct metal contact between the cam plate and the shaft, but it is fixed into a whole with plastic. The cost and weight of this camshaft can be reduced by 40%, the engine noise can be reduced, and the processing preparation time can be shortened from several hours to several minutes.
4.3 At present, the assembled camshaft develops rapidly and is mainly used in high-performance engines. At present, many automobile manufacturers with developed automobile industry in the world are using more and more new technologies for manufacturing assembled camshafts, but the types of assembled camshafts are different due to different levels and methods of technology mastery.
Advantages of assembled camshaft
The shaft and cam of assembled camshaft are manufactured separately and then assembled together. Cams are generally made of carbon steel or powder metallurgy materials, while shafts are made of cold-drawn thin-walled seamless steel pipes. Carbon steel cam can be quenched or carburized at high frequency, and has high adhesion and pitting resistance. Sintered alloy is made of Fe -C-P- Ni-Cr-Mo alloy, which is an ideal material for manufacturing cams. In design, the cam width can be designed to be very narrow, the interval can be very small, and the arrangement of cams is very compact. Compared with the traditional camshaft, it has the advantages of light weight, low processing cost and reasonable material utilization.
4.3.2 Connection form and manufacturing method of assembled camshaft
The connection modes of assembled camshaft are mainly divided into three types: welding, sintering and mechanical, and several methods are also used in combination.
Development of camshaft materials and production technology
Camshafts are made of many materials at home and abroad, including 45 steel, nodular cast iron and alloy cast iron. At present, the commonly used casting method is shell casting, including sand-coated iron mold and shell-filled iron shot, and others, such as EPC. Shell casting was invented by Johanes2Corning in Germany in the 1940s, also known as Cronning method. The castings produced by shell casting have the advantages of low surface roughness and high dimensional accuracy, which can meet the needs of castings with many cold irons and complex shapes, and are suitable for the production of shaft parts such as camshaft and crankshaft. It is an advanced technology in the world and has a broad application prospect.
Tsinghua University Li Shuangshou and others studied the laser surface treatment of nodular cast iron camshaft [10], and found that the surface layer of nodular cast iron camshaft was composed of fused layer, hardened layer and matrix from outside to inside, and its hardness was greater than 58HRC, and the hardness at the lap joint did not decrease. AstashkevichBM and others have studied the application of laser surface treatment in all cast iron, medium carbon steel and tool steel, and pointed out that the hardenability and hardness of materials are limited by some variable parameters of laser treatment, such as energy, beam diameter, beam shape, scanning speed and focusing conditions.
In foreign countries, Chernyshevan and others found that gray cast iron and high-strength cast iron containing spherical graphite were not suitable for surface strengthening by remelting process for three reasons: 1) the output was greatly reduced; 2) The carbon in the molten pool is not completely decomposed, and the wear resistance is reduced; 3) Excessive consumption of tungsten electrode. They pointed out that it is better to replace gray cast iron and high strength cast iron with vermicular cast iron, and also pointed out that the change of parameters will affect the distribution of flaky cementite in the chilled layer. Professor AMitchell of the University of British Columbia has successfully applied electroslag melting and vacuum arc remelting, which are only used for refining, to the solidification control stage of industrial pure iron production.
6. Conclusion
Camshaft is an important part in the valve train of automobile engine. The structural design and machining quality of camshaft directly affect the performance of engine. With the rapid development of science and technology, the manufacturing materials and technology of camshaft are also developing rapidly. As a new camshaft production technology, assembled camshaft is attracting more and more attention. Assembled camshaft processing technology conforms to the principle of lean production, is an advanced production technology with high precision, high efficiency, low cost and high flexibility, is the development and upgrade of camshaft manufacturing technology, and is the key to realize innovation leap. At present, we vigorously advocate environmental protection, develop engines with low energy consumption and no pollution, and realize low cost and light weight. Assembled camshaft has been widely used in automobile field because of its advantages that traditional camshaft does not have, and its development prospect is very broad.