Titanium MATERIALS You will find the figures mentioned in this article in the German issue of MTZ 3/2002 beginning on page 172.
Die Nockenwelle im Wandel der Zeit
The Camshaft in the Course of Time
By Manfred D. Röhrle
MTZ worldwide 3/2002 Volume 63
This paper describes the design and material-related changes of a component that has been successfully applied in valvecontrolled internal combustion engines: the camshaft. Apart from the engine block, only a few engine components have undergone such a fundamental change in design options, providing the engine designers with a suitable specific solution for each respective area of application. The most recent development from Mahle Ventiltrieb GmbH is a cast steel camshaft produced using the lost foam process.
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DEVELOPMENT
Valve Train
1 Introduction
The function of the camshaft with its cams, the contours of which include curved and straight lines, is to open and close the intake and exhaust valves of an internal combustion engine at specific intervals. This function has remained unaffected for years, ever since valve-controlled internal combustion engines have been operated. Even today, the camshaft rotates at half the crankshaft speed. The position of the camshaft in modern engines featuring inverted valves has changed from a drive via push rods, spur gears and chain drive to a drive by rocker arms and roller lifters, i.e. it was moved from underneath the cylinder head to overhead. In addition, the number of camshafts in an internal combustion engine has been increased and adjustable camshaft drives with varying methods of operation have been developed and introduced. Furthermore, the material and manufacturing methods of the camshaft with its cams have changed. This development process has yet to reach its end, because internal combustion engines without camshafts, the valves of which will be controlled via magnetic systems and which are under development at some companies, will reach series production readiness only in a couple of years. Camshafts are rotating masses that are continuously accelerated and decelerated in the drive unit of an internal combustion engine. By reducing the weight of the camshaft, the energy input into an internal combustion engine is favourably affected. 2 Chilled Cast Iron Camshafts
Camshafts made of special cast iron, Figure 1, nodular or malleable cast iron and produced using the chilled casting process often represent the standard valve train equipment in today’s gasoline engines. A sand cast is formed using plastic camshaft models, after which the camshaft blanks are cast, using chill irons to achieve a locally selective faster solidification. By this, cam surfaces with a Rockwell C hardness exceeding 45 are produced, thus achieving excellent wear characteristics during operation. In order to reduce the camshaft weight, hollow-cast camshafts have been produced for many years by companies such as Mahle-Wizemann-Pleuco, Gaildorf (today Mahle Ventiltrieb GmbH). In this production process, a glass tube is inserted in the mould, Figure 2, and removed after casting and mould opening. Hollow-cast camshafts are characterized by a considerable weight reduction compared with conventionally cast camshafts. In some engines, the longi-
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tudinal hole supplies the camshaft bearings with oil. However, one of the disadvantages of the two designs is the subsequent, complex finish machining performed on modern high-performance machining centres, Figure 3, and cam contour grinding machines. Chilled cast iron can endure Hertz pressures of approximately 1,000 MPa. Camshafts of cast iron, the cam surface of which is later subjected to remelting to achieve a ledeburitic structure, endure Hertz pressures of approximately 1,200 MPa (WIG) and 1,400 to 1,800 MPa (electron beam), depending on the cast iron base material. 3 Forged Steel Camshafts
Even when using the suitable cam material and manufacturing process of the camshaft, the Hertz pressure on the cam surface plays a crucial role. While camshafts of special cast iron and nodular cast iron that are produced in the chilled casting process are limited to Hertz pressures of 1,000 MPa, 2,000 to 2,500 MPa are possible when using cast steel cams. In a forged condition, steel cams face their loading capacity limit at 1,800 to 2,000 MPa. Since higher specific pressures occur in diesel engines, forged camshafts are primarily used in these engines. But their machining is complex, especially when producing the weight-reducing cavities by deep hole drilling. Thus, alternative solutions are being sought. 4 Composite Camshafts
The possibility to produce camshafts of one material only, because of the manufacturing processes ‘casting’ or ‘forging’, has increasingly revealed the limits of the respective applications. The composite camshaft, Figure 4, allows the combination of various materials and at the same time a reduction in camshaft weight. As early as 1982, Mahle/Süko began developing the composite camshaft for passenger-car gasoline engines. This composite camshaft is applied to valve drives with bucket tappets. Composite camshafts may be produced in two ways, subject to the mounting technique selected. This process uses either a steel tube that is formed from, for example, a precision steel band of high material quality and cold drawn after pressure welding, achieving a strength of at least 800 N per mm2 at a surface hardness of HV = 230 – 280, or a cold-drawn, welded, mild precision steel tube (ST 52-3) with a wall thickness of 4 mm, which is subsequently hardened on the bearing locations, Figure 5.
To date, Mahle/Süko, recently reorganized to Mahle Ventiltrieb GmbH, has considered the classic shrink fit between tube and cams to be the best mounting technique, Figure 6. Other manufacturers (Krupp-Presta) select a mild tube that is serrated at the cam positions to press on the outer parts (cams, rings) in a form-locking and force-locking press fit, Figure 7 (top). Nippon Piston Ring (NPR) manufactures composite camshafts by brazing / sintering, while mechanical internal expansion is applied by Torrington, USA, General Motors, Emitec, and GSA to produce composite camshafts (Figure 7, centre and bottom). Bleistahl, Chemnitz offers the composite, high internal pressure-formed camshaft featuring already hardened cam rings that are mounted onto the shaft of mild steel using the high internal pressure-forming process to achieve a form-locking and forcelocking connection. In this process, the inner contour of the cam ring shows considerable deviations from circularity to fulfil the closing shape. The cams are axially retained through a pressing of the tube material close to the cams towards the outside in the range of a tenth of a mm. In shrink fitting, the composite camshaft is produced by the thermal mounting process, i.e. heating of the individual components and cooling of the tube, Figure 6. The process allows mounting at short intervals, with a high precision of the axial and angular dimensions. The cams and individual parts (bearing rings, eccentric pieces) are successively assembled on NC-controlled special-purpose machines to form a complete camshaft. The mounting precision is +/- 20 - 30 angular minutes for axial dimensions of +/- 0.2 - 0.3 mm. Composite camshafts allow a weight reduction of up to 40 % compared to cast or forged conventional camshafts. The cam material (100 Cr 6 steel or sintered materials) may be selected corresponding to the contact material. The cams are manufactured by sintering or forging in four steps. For grinding, only a slight stock size on the cams is required. Even narrow cam distances can be produced without any problems. The cam position may be randomly selected. If precision sintered cams are used, grinding can even be omitted. In the case of PM materials, the limits of Hertz pressure are 1,200 MPa and, subject to the composition of the sintered alloy, even up to 2,000 MPa. Composite camshafts are currently applied in the volume production of Ford gasoline and diesel engines, as well as for the Smart and other vehicles. They are also successfully applied in Formula 1 racing engines.
MTZ worldwide 3/2002 Volume 63
IMPRINT 5 Cast Steel Camshafts
In newly developed gasoline and diesel engines with roller tappets, Hertz pressures on the cams which exceed the previous limits of chilled cast iron and sintered cams have nowadays been reached. With regard to lower pressures, each company has its own philosophy on whether to use cast conventional shafts of chilled cast iron, forged conventional steel shafts or composite camshafts. This induced Mahle Ventiltrieb GmbH in Gaildorf to develop an inexpensive alternative to forged or conventionally machined steel camshafts with reduced weight and higher design freedom, yet at the same time with good tribological properties, that can be produced in large volumes. For manufacturing, the lost foam process was selected. This process has previously been mainly applied to aluminium engine blocks and cylinder heads, as well as intake pipes, ventilated brake disks and various small parts made of different materials. This method allows the casting of an outer contour with a small machining allowance and a camshaft with a continuous cylindrical hole or cylindrical blind hole to reduce weight. The only prerequisite is that the design is suitable for the lost foam technique. The lost foam casting process, which was patented by H.F. Schroyer in 1958, was initially applied in art casting. Today, it is primarily used for aluminium parts. The model of the casting and the gate system is produced using polyurethane foam beads (T beads) with diameters ranging from 0.3 to 0.5 mm and 5 - 8 % pentane as a propellant by pre-foaming and finish foaming under water vapour, Figure 8. In this process, the shrinkage allowance for the casting must be considered. In order to be able to produce cutbacks, the models are glued together or assembled from several segments. It is crucial that the layer of glue is uniformly thin. A central pouring-in channel is connected to the star-shaped riser system at the lower end of the grape-shaped model consisting of several branches. The whole Styrofoam structure is provided with a thin ceramic coating by immersion into a liquid black wash, Figure 9. During casting, the black wash assumes the support effect in the gas-filled area between the model and the casting metal. The permeability to gas of the black wash determines the gasification process of the model. After drying, the model is covered with sand in a vibrating casting cylinder. The casting process can then be performed. Figure 10 shows a cast steel camshaft produced using the lost foam process. For the casting process, Mahle Ventiltrieb GmbH, Gaildorf, cooperates with a cast steel specialist. The first steel camshaft cast in 100 Cr 6 using the lost foam process, with a wall thickness of 5 to 6 mm and a hardness of HB 230, is currently being tested by a German engine manufacturer.
MTZ
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6 Summary
This article presents the changes to the camshaft, a component that had previously remained unchanged for a long time. This development was triggered by the increase in specific pressure, and the subsequent demand for special materials for this purpose, combined with the need to reduce component weight. An overview of the process is presented, starting with chilled cast iron camshafts made of special cast iron or nodular cast iron and going on to forged steel camshafts, focussing on composite camshafts and their different manufacturing processes. The article concludes with a presentation of the most recent development, cast steel camshafts produced using the lost foam process. ■
MTZ worldwide 3/2002 Volume 63
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