Patent Application
20170138231
The present invention relates to a valve drive having a sliding cam element which is held on a carrier shaft so as to be axially displaceable in an axis of rotation of the carrier shaft and which has at least one cam profile group with cam profile sections of mutually different form, and having a pick-off element by way of which a control movement can be picked off from the cam profile sections and transmitted to a valve.
DE 10 2007 010 155 A1 presents a valve drive having a sliding cam element arranged on a carrier shaft, and the sliding cam element can be displaced axially in the axis of rotation of the carrier shaft. On the sliding cam element there is mounted a cam profile group which has different cam profile sections. Depending on the axial position of the sliding cam element, a pick-off element, illustrated as a roller-type rocker lever, can interact with differently contoured cam profile sections of the cam profile group, such that the valve is actuated in a manner dependent on the selected cam profile section. For example, the cam profile sections can be differently contoured such that the valve can perform opening strokes of different magnitude or different length. The roller-type rocker lever has a roller, and on the side averted from the valve, the roller-type rocker lever is supported by way of a valve play compensation element.
The contact between the roller of the roller-type rocker lever and the cam profile section necessitates a minimum width owing to the actuation forces of the valve, and linear contact is formed between the roller and the cam profile section, which linear contact must thus have a minimum length. If the roller is selected to be too narrow, the Hertzian stresses on the surface of the cam profile section become too high, and premature fatigue or surface wear of the valve drive can occur. It is thus disadvantageously necessary for the sliding cam element to be formed with a minimum width which is determined at least by the required width of the cam profile sections, such that a minimum displacement length of the sliding cam element also becomes necessary in order to change over the cam profile section in contact with the roller. However, a sliding element of wide form lengthens the switching times for switching between different cam profile sections in operative connection with the pick-off element, wherein it is basically sought to provide a valve drive in which the fastest possible changeover of the cam profile sections in contact with the pick-off element is possible. In particular in the case of high rotational speeds of an internal combustion engine, the sliding cam element must be moved axially between two positions in very short times, such that a design of a sliding cam element of narrow construction would be expedient.
DE 10 2007 020 128 A1 presents a pick-off element having two rollers which are in contact with a single cam element. Here, the two rollers, which are arranged spaced apart from one another, effect guidance of a receiving rocker which lies in a cam follower and which permits tilting compensation of the cam follower relative to the cam element.
It is an object of the invention to further develop a valve drive having a sliding cam element on a carrier shaft, which sliding cam element can be displaced in the direction of the axis of rotation of the carrier shaft between different axial positions with short switching times. In particular, it is sought to shorten the displacement length in the direction of the axis of rotation without premature wear of the valve drive occurring.
Said object is achieved, proceeding from a valve drive as per the preamble of claim 1, in conjunction with the characterizing features.
Advantageous refinements are specified in the dependent claims.
The invention encompasses the technical teaching that, on the sliding cam element, there are formed at least two cam profile groups which are mutually identical and which, for the joint control of one valve, interact with a single pick-off element.
Here, the invention is based on the concept of dividing a required contact width between the cam profile sections of a cam profile group with a pick-off element into two individual widths. In this way, it is for example the case that the width of the individual cam profile sections of the cam profile group is halved, wherein mutually identical cam profile sections are in contact with a single pick-off element simultaneously. In this way, a twin sliding cam element is formed, and the adjustment speed of the sliding cam element in the direction of the axis of rotation is greatly increased owing to the shortened travel. In this way, it is also possible for the switching of the sliding cam element to take place even at higher rotational speeds of the camshaft, because the displacement length of the sliding cam element on the carrier shaft is shortened.
Here, the cam profile sections which are formed in pairwise fashion do not imperatively need to have a width half that of a conventional cam profile section with unitary contact between the cam profile section and a pick-off element for the control of a valve. An approximate halving is however possible because the width of the cam profile group can be approximately halved, while there are always two individual contact lines present between the pick-off element and the sliding cam element by way of mutually separately arranged cam profile sections, such that the forces that can be transmitted by way of in each case one contact point are approximately halved per cam profile section. By way of the linear contact between the cam profile section and the pick-off element, a line load is generated here which is approximately equal to a conventional pick-off configuration by way of one pick-off element with a continuous single contact line. A further advantage arises as a result of improved lateral tilting stability owing to the twofold support of the pick-off element on the sliding cam element.
In an advantageous embodiment, the pick-off element may have at least two rollers, wherein a first roller interacts with the cam profile sections of a first cam profile group, and wherein a second roller interacts with the cam profile sections of a second cam profile group. Here, the cam profile groups may be formed adjacent to one another, and so as to adjoin one another, on the sliding cam element, such that the sliding cam element is not made unnecessarily wider.
The rollers on the pick-off element may be held in a common roller axis and may have an identical diameter to one another. For example, the pick-off element may be in the form of a rocker lever, in particular a roller-type rocker lever, and the two rollers may be mounted parallel to one another on a common axis.
The rollers may interact with cam profile sections which have mutually identical cam contours, and furthermore, on the pick-off element, between the rollers, there may be formed an intermediate space which is of such a size that it corresponds at least to the width of a cam profile section or of two cam profile sections of a cam profile group. If, for example, the cam profile group has three different cam profile sections, it is necessary for the intermediate space between the rollers to be of at least such a width that two further cam profile sections which are temporarily not in contact with the pick-off element can lie between the rollers.
The cam profile sections may have a common cam base circle section which has, in particular, a diameter which is constant over the width of the sliding cam element. In this way, the change in position of the sliding cam element in the direction of the axis of rotation can be performed when the pick-off element runs on the cam base circle section, which is preferably of continuous form and thus free from shoulders and step changes in diameter.
Finally, the sliding cam element may be formed in one piece with the two cam profile groups, wherein the two cam profile groups also form a first contact section with a first pick-off element, and a further pair of cam profile groups may be mounted on the same sliding cam element, which controls a further valve by way of a further pick-off element. In this way, the required axial displacement travel in the direction of the axis of rotation of the sliding cam element is not enlarged, because both pick-off elements interact synchronously with their respective pairwise cam profile group.
Further measures which improve the invention will be presented in more detail below in conjunction with the description of a preferred exemplary embodiment of the invention on the basis of the figures, in which:
Two cam profile groups 13 of mutually identical form are formed on the sliding cam element 10. The cam profile groups 13 have, by way of example, three cam profile sections 14, wherein one cam profile section 14 is in the form of a zero-lift cam, and two further cam profile sections 14 are formed with different lift heights. Here, the cam profile sections 14 adjoin one another in the axial direction.
A pick-off element 15 interacts with the cam profile groups 13, which pick-off element transmits the lift information of the cam profile groups 13 to a valve 16. The valve 16 is furthermore shown as having a valve spring 21, and on that side of the pick-off element 15 which is situated opposite the valve 16, there is situated a support element 20 which can serve as a valve play compensation element and on which the pick-off element 15 is supported.
In the exemplary embodiment shown, the pick-off element 15 is in the form of a roller-type rocker lever with rollers 17 which are held on the main body of the pick-off element 15 in a common roller axis 18. The rollers 17 are held on the pick-off element 15 so as to be spaced apart from one another, and here, the spacing corresponds to the spacing of two cam profile sections 14 of identical form from the respective cam profile groups 13 of the sliding cam element 10. Thus, a twin contact configuration between the pick-off element 15 and the sliding cam element 10 is realized.
The cam profile sections 14 of the cam profile groups 13 are formed with a width B which is smaller than a required width of cam profile sections 14 if these form, with one of the two rollers 17, the pick-off contact for the control of a valve 16. For example, the width B of the cam profile sections 14 may be approximately half the magnitude of a width of cam profile sections which form an individual contact point with a pick-off element 15. The rollers 17, too, may have a width approximately half the magnitude of a width that would be required if only one roller 17 of a pick-off element 15 were in contact with a cam profile section 14 and picked off a lift movement therefrom.
Although the sliding cam element 10 has an overall width substantially equal to an overall width of a sliding cam element 10 with cam profile groups 13 of conventional form, the required axial displacement of the sliding cam element 10 in the direction of the axis of rotation is however reduced, because the cam profile sections 14 are narrower, and thus the travel in the direction of the axis of rotation by which the sliding cam element 10 must be displaced in order to perform a changeover of the pick-off of the rollers 17 between different cam profile sections 14 is reduced.
The actuation forces of the pick-off element 15 that are generated by the cam profiles of the cam profile sections 14 are divided equally between cam profile sections 14 which are of mutually identical form and which are in contact with the two rollers 17. Thus, a line load is generated at the contact line between the rollers 17 and the cam profile sections 14, which line load is of the same magnitude as if only one roller 17 were of relatively wide form and interacted with a single cam profile section 14 of an individual cam contour.
Owing to the relatively short axial travel for the adjustment of the sliding cam element 10, a changeover of cam profile sections 14 in contact with the roller 17 can be performed more quickly, wherein the changeover must always be performed when the roller 17 is in contact with the cam base circle section 19, which is provided on that side of the sliding cam element 10 which is situated opposite the cam profile groups 13. Thus, the sliding cam element 10 can be adjusted even at relatively high rotational speeds of the valve drive 1, for example above 4000 rpm.
The invention is not restricted in terms of its embodiment to the preferred exemplary embodiment specified above. Rather, numerous variants are conceivable which make use of the presented solution even in fundamentally different types of embodiment. All of the features and/or advantages which emerge from the claims, from the description or from the drawings, including design details or spatial arrangements, may be essential to the invention both individually and in a wide variety of combinations.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2014 109 243.0 | Jul 2014 | DE | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2015/064930 | 7/1/2015 | WO | 00 |
