The invention concerns a lever-type cam follower made without chip removal out of a sheet steel for use in a valve train of an internal combustion engine for actuating at least one gas exchange valve.
An internal combustion engine of an automotive vehicle comprises a valve train mechanism for actuating inlet and outlet valves synchronously to the engine rotation. The valve train mechanism generally comprises a camshaft and a cam tappet that converts the rotary motion of the camshaft into a reciprocating motion for axially operating the inlet and outlet valves. The cam tappet comprises a rocker arm that is driven by cams carried by the camshaft. With the progress of automotive vehicle technology in the field of high-performance engines, there is an increasing demand for compact and light-weight engines with a long operating life and a maintenance-free construction.
It is common knowledge in this connection that such lever-type cam followers made without chip removal from sheet steel are generally made of a case-hardening material such as, for example, 16 Mn Cr 5. Case hardening consists of carburizing or carbonitriding followed by hardening either immediately thereafter or after intermediate cooling and re-heating to an adequate hardening temperature. Depending on the required service properties or the requirements for subsequent working, hardening is followed by tempering or by sub-zero cooling and tempering. Case hardening serves to impart a substantially higher hardness to the surface layer of work-pieces made of steel and better mechanical properties to the work-pieces. For this purpose, the surface layer is enriched prior to hardening with carbon (carburizing) or with carbon and nitrogen (carbonitriding). In contrast to carburizing, the additional enrichment with nitrogen, because it modifies the transformation behavior in the surface layer, leads to a higher hardenability and, after hardening, to a higher tempering stability.
A drawback of lever-type cam followers made in this way is that the heat treatment of the case hardening material is very time-consuming and expensive.
It is an object of the invention to provide a decisive reduction of the manufacturing costs of a lever-type cam follower made of sheet metal.
This and other objects and advantages of the invention will become obvious from the following detailed description.
The invention achieves the above objects by the fact of using a cold-working, core-hardening heat-treatable steel.
Due to the transformation hardening over the entire cross-section of the lever-type cain follower, an almost homogeneous hardened zone is obtained that besides having a high strength also possessed good toughness properties. By transformation hardening is not only the strength enhanced but the microstructure is rebuilt and refined. Although, during tempering, the previously achieved increase of strength is partially reversed, the toughness is increased to above the original value. Lever-type cam followers made according to the provisions of the invention can support higher loads without a modification of the cross-section, or can support the same loads with a smaller cross-section. A further advantage of lever-type cain followers of the invention is that, by reason of the different heat treatment, they possess a further saving potential. On the one hand, it is possible to reduce the duration of the heating run and, on the other, the heating temperature. The higher dimensional and shape stability of the lever-type cam followers of the invention is a further advantage.
According to further particularly advantageous features of the invention, the core hardness has a value of ≧600 HV and the surface hardness has a value of ≧680 HV, the core hardness being situated in a range between 600 and 650 HV and the surface hardness in a range between 680 and 700 HV.
Finally, according to another proposition of the invention, the lever-type cam follower is made of a heat-treatable steel of the brand C45M having
0.39-0.46% C, up to 0.15% Si, 0.55-0.7% Mn, up to 0.020% P, up to 0.07% S, 0.25.-0.40% Cr, 0.020-0.060% Al, 0.0040-0.0100% N2. 0.10-0.20% Ni, 0.05-0.10% Mo, up to 0.005% Sn, up to 0.002% Sb, up to 0.15% Cu, total Cu, Ni, Mn, Cr 1.00 to 1.45%.
This cold-forming, core-hardening steel is an isotropic fine grained steel with a high level of purity. Its deep-drawing and shaping capability is comparable with hitherto used cold-rolled strip materials, its hardening ability, however, is distinctly superior to that of conventional steels. Due to its higher core hardness, it can support higher static and dynamic loads than parts made of conventional steels. This reduces plastic deformations at points subjected to high static loading.
The invention will now be described more closely with reference to one example of embodiment illustrated in the appended drawings.
Such a cam follower 1 is made without chip removal from a 3.5 mm thick strip of the heat-treatable steel C45M having the following chemical composition:
0.39-0.46% C, up to 0.15% Si. 0.55-0.70% Mn, up to 0.020% P, up to 0.07% S, 0.25.-0.40% Cr, 0.020-0.060% Al, 0.0040-0.0100% N2. 0.10-0.20% Ni, 0.05-0.10% Mo, up to 0.005% Sn, up to 0.002% Sb, up to 0.15% Cu, total Cu, Ni, Mn, Cr 1.00 to 1.45%.
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If the same cam follower 1 is made of the heat-treatable steel C45M, it is at first hardened with mild carburization by holding for 30 min at 840° C. This is likewise followed by quenching to room temperature and tempering, in this case too, for a duration of 120 min. It can be clearly seen that in the first case, the actual hardening step in case hardening lasts 120 min and in the second case, only 30 min, so that a time saving of 75% is achieved in the hardening step itself. A further advantage is that it is also possible to harden at a temperature that is lower by 40° C. which means that a substantial amount of energy can be saved. A further advantage, finally, is that a cam follower 1 of the invention made of the steel brand C45M compared to a case-hardened cam follower made of the steel brand C16M exhibits much lower distortion and instability of shape and therefore does not require re-working by machining.
Number | Date | Country | Kind |
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10 2004 028 667.1 | Jun 2004 | DE | national |