Electric heating – Inductive heating – Specific heating application
Reexamination Certificate
1999-09-29
2001-07-10
Jeffery, John A. (Department: 3742)
Electric heating
Inductive heating
Specific heating application
C219S661000, C148S573000, C148S526000, C029S888100
Reexamination Certificate
active
06259076
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a method for the hardening of camshafts wherein the cams of the camshaft are hardened simultaneously in groups while the camshaft is rotating, and wherein during the inductively performed heating of a group the zone of the camshaft that is not to be hardened and bordering on the group concerned is shielded from heating by means of a cooling device. In addition, the invention relates to a linear inductor intended to perform the method.
A basic problem in the hardening of closely proximate hardening zones of a camshaft is that the heating of an individual cam can cause the supplementary tempering of the previously hardened and directly proximate cam. When hardening is being performed individually, therefore, it is necessary to shield the cams bordering on the cam being treated. The requisite expenditure on apparatus for this purpose is substantial. Moreover, the hardening of cams individually implies a considerable duration in the processing time required for the hardening of a shaft.
For the purpose of shortening the processing time and eliminating the problem of the additional tempering of previously hardened and directly proximate cams, it has been proposed in the special issue of “elektrowärme international”, number 3/75, “Induktionshärtetechnik für Nockenwellen von Verbrennungsmotoren [induction-hardening technique for camshafts of combustion engines]” that the cams of a camshaft be combined into groups and that the cams of one of these groups at a time be hardened simultaneously. The size of the group and the length of the group are determined by the distance to those zones of the camshaft that should not undergo any hardening. This process permitted a substantial shortening in the processing time required for the hardening of a camshaft. Furthermore, due to the simultaneously effected heating of the closely proximate cams, the problem of the supplementary tempering of previously hardened cams no longer exists.
The requisite expenditure on apparatus for the simultaneous hardening of a group is still substantial even with the process explained above, since an individual inductor, riding as a rule on the cam involved, is used for each cam. For the purpose of reducing this technical expenditure, it has been proposed that the camshaft be hardened by means of a linear inductor extending along the camshaft and positioned in the area of the envelope curve of the camshaft (DE-GM 75 27 498). It has turned out, however, that the use of such a fixed inductor, because of the varying coupling distances in a camshaft rotation, causes overheating of the tips of the cams or insufficient heating of the other sections of the cam being processed.
On the basis of the state of the art explained above, the task of the invention is to provide an economically practicable process for the hardening of camshafts and a particularly suitable inductor for performing the process.
SUMMARY OF THE INVENTION
This task is resolved by the invention in that the groups of cams that are to be hardened are simultaneously hardened by means of one linear inductor each, that the heating of the cams is performed first in a time interval at low power and then in a time interval at high power, and that the intervals are respectively separated from each other by a pause interval during which no power is applied.
In accordance with the invention, it is proposed that not only the cams of an individual group of cams of a camshaft be hardened simultaneously but that the hardening of all cam groups of a shaft be undertaken simultaneously. In this way the processing time and thus by implication the cost involved in the hardening is already reduced. Furthermore, in accordance with the invention, the heating of the cams takes place over at least one time segment at relatively low electrical power. The heat applied in this way lies below the critical limit occasioned by the material characteristics of the cams being processed. During the pause interval, the heat thus applied is distributed so that the heat stored in the cams before the last stage of the heating process is uniformly distributed. Only after this, in order to conclude the heating, is such a high quantity of energy applied that the required surface hardening is achieved by means of the quenching that then occurs. The risk of damage to the cam due to overheating is minimal because of the previously effected uniform heating of the cams. This also applies to the tips of the cams, in which the highest temperature occurs due to the small distance at which they are moved along the heating conductors. This makes it possible to carry out the heating using economical linear inductors.
The number of time intervals in which initially only a low power is applied can be suited to the structural features of the camshaft being processed at a given time. In practical trials of the process of the invention, good working results were obtained when initially two time intervals at low power, separated by a pause interval, were gone through. The repeated interruption of the heating process achieved a gentle and particularly uniform heating of the cams, due to the fact that, between heatings, there was equalization of any non-uniform heat distribution that might have occurred during the applications of power.
A preferred form of the method of the invention is characterized in that the duration of the time intervals and pause intervals decreases from the first to the last interval. This measure, too, favorably affects the uniformity of heat distribution in the cams being processed.
Preferably, the power applied in the last time interval should be twice as great as that applied during the time interval at low power. Similarly, it is-expedient to apply the low power at a different frequency from that of the high power. In this way, the introduction of heat into the work piece can be purposefully controlled. It has turned out in practical experiments that it is advantageous to work at a low frequency, i.e. with a large depth of penetration, during the introduction of the low power and at a high frequency during the concluding interval with the introduction of the high power, so that heating takes place only in those layers of the cam bordering on the surface.
Regarding the practicability of the process of the invention, it is advantageous if the camshafts are arranged essentially horizontally during the performance of the process. In this way, for instance, the cooling of those sections of the camshaft that are not to be hardened is simplified during the heating of the groups of cams that are to be hardened.
Depending on the domain of application and the nature of the camshaft to be processed, it may be expedient to carry out the quenching that follows the heating of the camshaft during a time interval whose duration is shortened in such a way that a residual heat is retained following the quenching. In this way, the residual heat that still exists in the camshaft following the quenching that is thus performed can be used for the tempering of the camshaft, so that a frequently undesired full hardening of the shafts can be safely prevented.
A linear inductor particularly suitable for performing the method of the invention is one having at least two heating conductors extending axially parallel with the camshaft, positioned in the area of the envelope curve of the group of cams, and separated from each other by an angular distance of not more than 180° relative to the axis of rotation of the camshaft, whereby the length of the heating-conductor arms corresponds to the distance between the extreme ends of the respective group of cams, and that each of the heating conductors is formed as a heating-conductor loop with two heating-conductor arms extending axially parallel with the camshaft, the first arm being displaced by an angle of less than 90° relative to the second arm with respect to the axis of rotation of the camshaft.
With its conductor arms, the linear inductor of the invention encompasses at most half of the cylindrical ar
Gezarzick Waldemar
Leisner Hans-Jürgen
Schulte Peter
Elotherm GmbH
Jeffery John A.
Proskauer Rose LLP
LandOfFree
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