The camshaft is an important part in the valve train of an automobile engine. The structural design and processing quality of the camshaft directly affect the performance of the engine. In recent years, due to the needs of environmental protection, engines with low fuel consumption and low pollution are being developed. In order to solve the problem of non-polluting automobile exhaust emissions and achieve high engine speed and high output power, many engines adopt multi-valve and valve phase, variable valve lift structures, which increases the load of valve springs. At the same time, in order to reduce fuel consumption and friction loss, a roller structure is used between the cam and the rocker arm, and the contact surface between the cam and the roller forms a high pressure area, which affects the stability of the camshaft movement, dynamic balance, wear resistance and torsional strength. put forward higher requirements. In addition, in order to achieve the purpose of light weight and low cost of automobiles, without affecting the performance requirements of each part, the parts should be simplified as much as possible, the weight should be reduced, and the use of materials should be more reasonable.
Generally speaking, in an in-line engine, one cam corresponds to one valve. A V-engine or boxer engine shares a cam for every two valves. The rotary engine and the valveless engine do not need a cam due to their special structure.
In order to achieve light weight, easy processing and low cost, as well as high engine speed and high output power, the design of engine components, especially camshafts, must be reconsidered, requiring them to be compact in structure, highly concentrated in functions, light in weight, and able to withstand higher contact pressure. In the current camshaft application, the cylinder head lubrication system has been concentrated on the hollow camshaft, the fuel pump drive components that realize direct injection in the cylinder are implemented on the camshaft, and the VVT (variable timing valve) is implemented on the camshaft. used at the end. In the valve train, the performance requirements for each part of the camshaft are different. For the cam and fuel pump drive wheel, it is required to be resistant to wear, adhesion and pitting; for the journal, it requires good sliding performance; for the shaft, it requires good rigidity, bending and torsion performance.
With the development of lightweight automobiles, camshafts are developing in the direction of lightweight, highly concentrated functions and low cost, and the advantages of assembling camshafts are gradually recognized and accepted by people.
The shaft and cam of a fabricated camshaft are manufactured separately and then assembled together. The cam is generally made of carbon steel or powder metallurgy material, the journal is made of powder metallurgy parts or the steel pipe concentrated on the mandrel, and the mandrel is made of cold-drawn thin-walled seamless steel pipe. Carbon steel cams can be subjected to high frequency quenching or carburizing treatment, and have high resistance to adhesion and pitting corrosion.
In terms of design, the assembled camshaft can be designed with a narrow cam width and a small interval, and the arrangement of the cams is very compact. Compared with the traditional camshaft, it has the advantages of light weight, low processing cost and reasonable material utilization, and the weight is reduced by up to 45% compared with the solid shaft camshaft.
The key technology of the assembled camshaft is the connection method, and the manufacturing process and equipment are determined by the different connection methods. The assembled camshaft began in the 1980s, and the first developed was the welded connection camshaft. In the mid-1980s, the sintered connection camshaft was put into use, and the camshaft produced by the expansion method appeared in the same period. In the late 1990s, the development of knurled camshafts began. Fabricated camshaft manufacturing techniques are constantly being updated as new connection methods emerge.
Welding-connected camshafts are prone to thermal deformation during welding, which reduces the dimensional accuracy of the camshafts, and severe thermal changes are also prone to cracks in the welded parts, making it difficult to guarantee the quality. , The cam must be connected to the steel pipe by diffusion in the liquid phase. This process must be carried out in a sintering furnace above 1000 ° C. At high temperature, the shaft is prone to bending, resulting in dimensional accuracy errors, and during sintering, there are restrictions on the performance of the material. , requires a large sintering furnace, and the thermal efficiency is not high; the expansion method first makes the cam and the steel pipe cooperate, and then adds hydraulic or mechanical expansion from the inside of the pipe. In order to withstand the internal pressure of the expansion pipe, the cam wall must have a certain thickness. The pipe is easy to carry out, and thin-walled steel pipes must be used, and due to the special requirements of high-pressure operations, the equipment is also large-scale; although the hot and cold sleeve interference connection method is widely used in the production of mechanical parts, it may not be suitable in the production of camshafts. , because when the cam is connected with the shaft heat sleeve, the cam is heated, resulting in softening, and it is difficult to ensure the wear resistance during friction. Many cams will conduct heat to the shaft end when working, making the connection between the initial and the end of the work. If the quantity changes, the connector degree cannot be kept consistent; the knurling method has certain advantages in reliability, accuracy, equipment, energy consumption, etc.
Further development of the combined camshaft will focus on developing materials and optimizing processing techniques. The market is currently vigorously developing and applying composite materials, such as ceramic materials to manufacture camshafts. MAHLE is working on the development of extended functions for the combined camshaft. For example, the integration of fuel pump actuation and sensor components is also gaining traction. Synchronization of the inlet or outlet cam segments on SOHC camshafts is achieved with the MAHLE CamInCam™ nested camshafts.
Assembled camshafts are currently developing at a rapid rate and are mainly used in high-performance engines. With the improvement of the production technology of assembled camshafts, assembled camshafts with better performance, lower cost and diversified technologies will emerge.
