CAMSHAFT FOR A VARIABLE-STROKE EXCHANGE VALVE TRAIN

A camshaft of a sliding cam valve train in an internal combustion engine is provided. A cam piece (2), which has cam strokes of different lengths and can be displaced on a carrier shaft (1), is mounted on a rolling bearing at a camshaft bearing point (12) of the internal combustion engine. The outer race (16) of the rolling bearing (13) is formed by a one-piece bearing ring (17) which surrounds the cam piece and said race is smaller than a revolution radius of the longer stroke h2.

BACKGROUND

The invention relates to a camshaft of a variable stroke gas exchange valve train of an internal combustion engine. The camshaft comprises a carrier shaft and a cam part that is supported locked in rotation and axially displaceable on the carrier shaft and has at least one first cam group of at least two directly adjacent cams with different strokes and an axial connecting link in which an actuation element for shifting the cam part on the carrier shaft can be coupled, as well as a bearing journal that runs between the first cam group and the axial connecting link and on which a roller bearing supporting the cam part is held so that it can rotate in a camshaft bearing point of the internal combustion engine.

A gas exchange valve train which is also often called a sliding cam valve train with such a camshaft is known from DE 10 2009 030 373 A1. The cam parts supported with a central bearing journal between two intake or exhaust valves of an engine cylinder comprise two identical cam groups each with three cams and an end-side axial connecting link in which two actuation pins for shifting the cam part into the three axial positions can be coupled selectively. The publication alternatively proposes a roller bearing for the cam part in addition to the hydrodynamic sliding bearing.

Despite the three-stage stroke variability, the known valve train has a very compact axial construction. This is achieved in that the cams adjacent to the bearing journals can dip into the camshaft bearing point when they are not instantaneously active. A geometric requirement here is a corresponding dimensioning of the rotational bearing, whose diameter must be greater than the surrounding circle of the cams entering within this circle. In the case of a roller bearing of the cam part, this dimensioning would, however, lead to a camshaft bearing point with an undesirably large radial construction.

SUMMARY

The present invention is based on the objective of improving a camshaft of the type noted above such that, despite the roller bearing of the cam part, it allows the most compact radial construction possible.

This objective is met in that the inner raceway of the roller bearing is formed by the bearing journal, the outer raceway of the roller bearing is formed by a one-part bearing ring, and the cam part and the bearing ring have the following geometric properties:

h1=Stroke of the cam adjacent to the bearing journal in the first cam group

h2=Stroke of the other cam of the first cam group

rHK=Common enveloping circle radius of the cams of the first cam group

rGK=Base circle radius of the cams of the first cam group

rL=Radius of the outer raceway

rAK=Circumferential radius of the axial connecting link

These geometric relationships allow the radius rLof the outer raceway to be less than the circumferential radius rGK+h2of the large stroke h2but nevertheless the one-part bearing ring can be mounted on the bearing journal past the cams. The installation is performed such that the bearing ring is first threaded into a position on the cams eccentric to the rotational axis of the cam part and then centered on the rolling bodies in the area of the bearing journal. The rolling bodies are advantageously held in a roller body cage that is open on the periphery and is mounted on the bearing journal before installation of the bearing ring in the elastically expanded state. Alternatively, a multiple-part cage could also be used.

The invention could also be used in non-variable, roller-supported standard camshafts.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention will be explained starting fromFIG. 1, which shows a three-stage variable stroke gas exchange valve train of an internal combustion engine. The basic functional principle of this known valve train can be summarized in that a conventionally rigid camshaft is replaced by a camshaft with a carrier shaft1with external teeth and—cam parts102arranged locked in rotation on this carrier shaft—corresponding to the number of cylinders of the internal combustion engine—and displaceable between three axial positions. Each cam part has two identical groups of directly adjacent cams103to105that have different strokes with identical reference circle radii. The cam lift is performed by roller cam followers6that transfer the cam travels selectively to gas exchange valves7.

The displacement of the cam part102required for operating point-dependent activation of each cam103,104, or105on the carrier shaft1is realized by a groove-shaped axial connecting link108in which, depending on the instantaneous axial position of the cam part, one of two pin-shaped actuation elements9,10of an electromagnetic actuator (not shown) is coupled, in order to displace the cam part within the common reference circle phase of the cam. To stabilize the cam part in the axial positions, a locking device is used that runs (not visible here) in the interior of the carrier shaft1and locks in the interior of the cam part.

For the radial support of the camshaft in the internal combustion engine, the cam part102is provided between the two cam groups with a bearing journal111that is supported so that it can rotate in a camshaft bearing point12arranged locked in position and cylindrically centered in the internal combustion engine. This is a split camshaft bearing point with a screwed-on bearing cover not shown here. Shown is a hydrodynamic sliding bearing of the bearing journal in the camshaft bearing point, wherein the bearing could also be formed in a known way as a roller bearing.

FIG. 2shows a first embodiment of a cam part202for a two-stage variable stroke gas exchange valve, wherein the bearing journal211of the cam part is enclosed by a roller bearing213in a way according to the invention. Differently than in the known cam part102inFIG. 1, the cam strokes of the first cam group arranged at a distance to the axial connecting link208are oriented so that the stroke h1of the one cam203adjacent to the bearing journal is smaller than the stroke h2of the other cam204. For this first cam group, the following relationship is then applicable:

The cutaway and greatly simplified roller bearing213is a needle bearing with cage-supported needles214, whose inner raceway215is formed by the bearing journal211and whose outer raceway216is formed by a one-part bearing ring217drawn onto the cam part202. The plastic needle cage218is shown as needle ring19below in connection with the needles held therein, i.e., as219inFIG. 2. The width of the bearing ring217is approximately twice as large as the length of the needles214that roll, depending on the axial position of the cam part, either in one axial raceway half or in the other axial raceway half of the bearing ring mounted in the internal combustion engine.

FIG. 3illustrates the geometric radii or diameter relationships on the cam part202(cam parts402to702accordingly) that are projected according to view A inFIG. 2in the plane of the sheet. The two cams203and204have a common enveloping circle with radius rHKthat encloses the same reference circle radius rGKand the two different strokes h1and h2of this cam. In the shown case, however, the smaller stroke h1runs, with respect to its angular position, not completely within the larger stroke h2, and the circumferential radius rGK+h1of the cam203rotating about the camshaft axis20is greater than the common enveloping circle radius rHK. The smallest possible radius rLof the outer raceway216for the installation of the bearing ring217is now generally the larger value of these two radii rGK+h1and rHK, so that the following relationship is also applicable:

On the other hand, in order to keep the radial installation space of the cam part202with the needle bearing213as small as possible, the relationship, according to which the outer raceway radius rLis smaller than the revolution radius of the greater cam204about the camshaft axis20, is also applicable:

Example calculation for the outer raceway radius rLfor the cam geometry according toFIG. 3:

gives

and the following size relationship is applicable for the outer raceway radius:

The drawing of the bearing ring217on the cam part202will be explained with reference toFIGS. 2ato2e.

FIG. 2a: The needle ring219open on the periphery is expanded elastically in the radial direction and is snapped onto the bearing journal211from the lateral direction. The width of the needle cage218and the width of the bearing journal are essentially the same size, so that the needle cage is supported directly in the axial direction from the facing end sides221and222of the inner cam204of the second cam group and cam203, respectively, adjacent to the bearing journal.

FIG. 2b: The bearing ring217is threaded eccentric to the camshaft axis20onto the outer cam204. In this case, the smaller stroke h1extends with respect to its angular position completely within the larger stroke h2, and the radius rLof the outer raceway216is slightly greater than the common enveloping circle radius rHKof the cam203,204with rHK=½ (2 rGK+h2). The drawn radius rAKdesignates the outer circumferential radius of the axial connecting link208and is also greater than the outer raceway radius rL:

FIG. 2c: The bearing ring217is pushed into the eccentric position over the outer cams203,204until the outer cam204is free.

FIG. 2d: The bearing ring217is centered on the camshaft axis20.

FIG. 2e: The bearing ring217is pushed over the needles214.

FIG. 4shows a second embodiment of a cam part402according to the invention. This differs from the previously explained cam part202essentially by the axial support of the needle cage418. Pulling the bearing ring417onto the cam part will be explained with reference toFIGS. 4ato4f.

FIG. 4a: The bearing ring417is threaded eccentric to the camshaft axis20onto the outer cam404and pushed over the outer cams403and404until the outer cam404is free. The needle ring419is pushed toward the inner cam group to create an axial opening for the bearing ring417.

FIG. 4b: The needle ring419and the bearing ring417centered relative to the camshaft axis20are pushed one onto the other.

FIG. 4c: A plastic spacer ring423that is open on the periphery is expanded elastically in the radial direction and snapped onto the bearing journal411from the lateral direction.

FIG. 4d: The spacer ring423is inserted into a ring groove424between the inner raceway415and the inner cam404of the second cam group.

FIG. 4e: The bearing ring417is displaced toward the inner cam404until another ring groove425is freely accessible between the inner raceway415and the outer cam403.

FIG. 4f: As inFIG. 4c, another spacer ring423is inserted into the other ring groove425so that the facing cam end sides421and422support the needle cage418in the axial direction by means of the spacer rings423.

FIG. 5shows a third embodiment of a cam part502according to the invention. In this case, the bearing journal511is formed with large steps in the radial direction with a raised inner raceway515, wherein both end sides526and527of the radial step support the needle cage518surrounding these sides in the axial direction.

FIG. 6shows a fourth embodiment of a cam part602according to the invention. In this fourth embodiment, the axial support of the needle cage618is formed from a combination of the support in the second embodiment and the third embodiment: The axial support is realized on one side on the cam end side621by means of the spacer ring623and on the other side directly on the end side626of the radial step.

FIG. 7shows a fifth embodiment of a cam part702according to the invention. This fifth embodiment is a variant of the fourth embodiment in the way that the spacer ring723is formed in one piece on the needle cage718.

LIST OF REFERENCE NUMBERS