The invention relates to a valve-train of an internal combustion engine with a camshaft that comprises a carrier shaft as well as a cam element that is locked in rotation on this carrier shaft and that can move between two axial positions and that has at least one cam group of directly adjacent cams with different cam lobes and an axial connecting link constructed as a groove with external guide walls for defining two intersecting connecting link pathways, and with an activation pin that can couple into the axial connecting link for moving the cam element in the direction of both connecting link pathways.
Such a valve-train assembly that is used for the variable activation of gas-exchange valves by moveable cam elements and in which a single activation pin is sufficient for each cam element, in order to move the cam element in the direction of both connecting link pathways, is already known from DE 101 48 177 A1, which is considered class-forming. In that publication, two cam elements are disclosed with alternatively constructed axial connecting links, wherein the first axial connecting link has a central guide web for forming inner guide walls for the activation pin and the second axial connecting link consists merely of outer guide walls.
The latter construction has the advantage that the production expense for the axial connecting link is significantly lower due to the elimination of the guide web. One significant risk with respect to the functional safety of the valve-train assembly in the case of this construction is that, however, the displacement process of the cam element is completely finished, i.e., without incorrect switching, only when the inertia of the mass in motion of the cam element is sufficient to move it into its other end position after passing through the intersection region of the connecting link pathways without forced guidance of the activation pin, that is, to a certain extent, in free fall. A prerequisite for the sufficient inertia of the mass in motion of the cam element is obviously a minimum rotational speed of the camshaft that is directly dependent on the friction between the cam element and the carrier shaft. Displacement of a cam element with a rotational speed below this minimum rotational speed could have the result that the cam element remains “at a half-way point” and a cam follower acting on the gas-exchange valve is simultaneously acted upon by several cams of the cam group in an uncontrolled manner and simultaneously under high mechanical loading. In addition, in this case there is no longer the ability to move the cam element through action of the activation pin later into one of the end positions, because in this case there is no longer axial allocation between the activation pin and the outer guide walls.
This functional risk is indeed significantly smaller in the case of the first construction of the axial connecting link with a central guide web whose inner guide walls cause a further accelerating forced guidance if the rotational speed of the cam element is lower than the activation pin. Nevertheless, there is also the risk here that the activation pin does not pathway into the specified connecting link pathway after passing through the intersection region, but instead collides with the end face of the guide web also under high mechanical loading.