Camshaft

A cam shaft, a machine element in the form of a rod ( = " shaft " ), is mounted on the at least one rounded projection ( = " cam "). The shaft rotates around its own axis, by the person or attached to her cam this rotational movement is converted repeatedly in a short longitudinal movement. In standard design, the cam contours run parallel to the camshaft, but it is also " beveled cam" has been realized.

The camshaft is used in cam switches and controls, but mainly in reciprocating engines (internal combustion engine ); here it is part of the valve train, it is used to open the intake and exhaust valves according to constructional timing.

In older injection pump camshaft also serve to fuel metered to the individual suction pipes or cylinders.

History

The technical device for converting rotary into linear movements have been known since ancient times. Since the 11th century, the camshaft in the form of cams or thumb was used to extended corrugated trees or axles on commercial mill wheels. The pressure of the cam to feed pounding left, hammers, walking and Pochstempel lift, while they fell upon further rotation of the shaft and did their work. Following the same principle, bellows and saws were operated. The camshaft has played an important role in the mechanization of numerous industry at the beginning of the High Middle Ages.

Camshafts to control the valves of reciprocating engines

To camshafts of reciprocating piston engines there is usually on each valve an eccentric cam that forces the valve into the open position. When the cam is further rotated, the valve is closed by the valve spring. The cam does not press the exact center of the valve, but slightly to the side. The valve is always turned slightly, and do not form irregular pressure points by the cam. A cam can also activate the more valves; even the actuation of intake and exhaust valves by a single cam for each cylinder has been realized, however, in such constructions, the control times for the inlet and outlet can not be selected independently. A separate opening and closing cam for each valve can be used the forced actuation valve. It camshafts have been designed which do not rotate, but only by less than a full revolution to and fro.

In four-stroke motors in standard design - with the exception of Star Motors - the camshaft rotates at half the speed of the crankshaft, from which it is rarely driven mostly by means of chain drive and timing chain or by timing belt, through a so-called vertical shaft or a set of gears. Since the camshaft rotates at half the speed in relation to the crankshaft, the transmission ratio control of the wheels 2-1.

With star engines, the camshaft is disposed coaxially to the crankshaft; because of a different configuration - short, but the large diameter - it is referred to herein as a " cam ring ", " cam " or " cam ring ". Because all press outlet or inlet cam of a cam drum all outlet or inlet valves of a cylinder star, here arise different speed and gear ratios, see cam drum.

In standard construction camshafts are parallel to the crankshaft, but also arrangements of camshafts, the "stand" perpendicular to the crankshaft have been realized, eg Chater Lea motorcycles and constructions of Richard kitchens, for example, for the Zündapp motorcycle prototype SS 600. This arrangement is often a cause for confusion with bevel.

In the pump-nozzle method for diesel engines ( Volkswagen Group and truck engines) another cam per cylinder actuates the piston of the injection pump. The advantage of the very high injection pressure possible in comparison with the injection by a common rail. Targeted pre-injections are also technically possible with pump-nozzle systems, however, to realize complex. The disadvantage, however, the very high cost associated with high costs and the problematic package, so that the VW group has switched as the original proponent of the system on common- rail injection. Advantages of the common rail system ( originally introduced by Fiat and Alfa Romeo, now adopted by almost all major automotive producers ), however, are the optimization of the combustion process and the engine running characteristics as well as the much lower particulate emissions.

Between the cam and valve is either a follower or a rocker or drag lever to keep the lateral forces of the bumper ( at the bottom camshafts ) or the valve stem. By camshaft adjuster the dates for the opening and closing of the valves can be adjusted to the power requirements during operation. This power and torque can be increased and decreased in the partial load range of consumption. Camshaft adjuster twist the entire camshaft relative to the crankshaft.

All oscillating ( reciprocating ) components must be moved, cost performance and increase the mass forces. When the camshaft directly over the valves incorporating ( single overhead camshaft ) and for each valve series own camshaft used ( double overhead camshaft / DOHC), the number of oscillating components are reduced, the inertia forces are lower and a higher speed is possible.

In calculation, design and manufacture of cam controls compromises must be made: On the one hand one would like to open the valve for a good gas flow rate far; However, this increases the resultant forces on the valve in the circulating drive, also must be avoided, a collision of the valve to the piston. On the other hand you want to keep open ( through a large angle per cycle ) for high gas flow rate, a valve long; but it must on the other hand still be present opportunity for densification.

All the mutual influences set the variation of cam curves (design, tuning or " refacing " ) to increase power narrow limits. An engine with modified towards high power ( " tuned " ) cam parameters will require a higher idle speed, maximum torque is shifted to higher speeds, it will run jerkily and consume more. The sooner more often practiced refacing of the cam profiles is no longer used today, for all standard motors procured one nowadays rather a special camshaft for enhanced performance.

Production

The majority of camshafts for automotive engines is still cast in one piece of cast iron, forged in exceptional cases, also made ​​of steel, then machined and partially cured, why usually the induction hardening process will be used.

For some time, so-called " built " camshafts are made ​​of different materials composite, are used. Advantages of assembled camshafts are lower cost, lower weight, higher -strength materials for the cams (hardened ball bearing steel, sintered steels), greater flexibility in production, but also new cam geometries such as negative radii of the cams are easier to implement.

With a camshaft grinding machine can be produced from a blank or a series camshaft copies ( using so-called Rohmeisternocken ) a template. For the series production of the cam geometries are stored as parameters in the CNC programs. Typically, there is a link between the CAD data, the processing parameters (CAM ) and the associated measurement programs (CAQ ). Such series preparations are carried out on so-called Pendelhubschleifmaschinen ( engl. "Orbital Grinder ").

The surfaces of the cams are hardened between about 0.2 mm and 1.5 mm deep. However, due to hardened cams are often used in assembled camshafts. Through hardening of cams is cheaper and improves the material properties, in particular because of the hardness of the transition region is avoided.

View

In the development and partly already implemented (for example Valvetronic ) are fully variable valve control. While you have the mechanical drive via cam, but each valve electro- hydraulic ( MultiAir technology) or by a mechanical variable ratio is controlled. All systems have their own, that a variable stroke and variable valve timing can be represented. The electro-hydraulic systems can also operate several times the valve during the cam lift. Your degrees of freedom so remember to the injectors, the function ( one thinks of pump nozzle) was adopted in principle. The first electro-hydraulic concept of Fiat found himself from mid-2009 standard, first of the Alfa Romeo MiTo. The mechanical Valvetronic system is on the market since 2000. The fully variable, working without camshaft electromagnetic systems are currently too expensive and immature and are difficult to integrate into the current engine designs (among high electrical demand ).