Method of making a compensation shaft

A method of making a compensation shaft (1) out of a drop-forged shaft blank (11) is provided. The compensation shaft includes a shaft section (5) with a base body (16) and a rib (15) with a height H starting from the base body. The method includes separating the forging ridges (14) formed in the die (9, 10) by trimming the forged shaft blank into a shape close to the final contour of the base body at trimming points, with the distance S2 between these trimming points being substantially smaller than the distance S1 between the forging ridges prior to separation.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of German Patent Application No. 102009036067.0, filed Aug. 4, 2009, which is incorporated herein by reference as if fully set forth.

BACKGROUND

The invention concerns a method of making a compensation shaft out of a drop-forged shaft blank. The compensation shaft comprises a shaft section with a mass center of gravity which extends eccentric to the axis of rotation of the compensation shaft and which, together with the axis of rotation of the compensation shaft, creates an unbalanced mass plane which extends substantially perpendicularly to the die parting plane. The shaft section comprises a base body and a rib with a height H starting from the base body and extending, with regard to the axis of rotation, in a direction of the unbalanced mass plane oriented away from the mass center of gravity.

A compensation shaft of the above-noted type is used in internal combustion engines for providing a partial or complete compensation of the free mass forces and/or mass torques. The radial mounting of the compensation shaft is increasingly accomplished through low-friction needle roller bearings that are in direct rolling contact with the bearing journal of the compensation shaft, so that needle roller-mounted compensation shafts are generally configured as forgings with a high tribological load bearing capacity. This is also the case in DE 10 2007 019 008 A1 in which, with a view to a homogenous fiber orientation in the region of the highly loaded tribological load zone of the bearing journal, it is proposed to orient the parting plane of the die in a direction perpendicular to the unbalanced mass plane of the compensation shaft.

The shaft sections extending between and next to the bearing journals and decisively participating in the unbalanced mass action of the compensation shaft comprise stiffening ribs in direction of the unbalanced mass plane, which ribs are formed through the hollow mold of one of the die halves because the die parting plane extends perpendicularly to the unbalanced mass plane. This flow of material, known in forging techniques as swelling, occurs in opposition to the lift movement of the die halves and, with a view to completely filling the hollow rib mold, it is required that a sufficient volume of material be available also in direction of the die parting plane, which material volume is pressed under very high pressure on the one hand in direction of the hollow rib mold and on the other hand in direction of the ridge gaps of the die with formation of the forging ridges. Consequently, the forgeable height of the rib, i.e. its extent in direction of the unbalanced mass is limited by the width of the forged shaft blank, which width extends in direction of the die parting plane.

SUMMARY

The object of the present invention is to improve the manufacturing method of a compensation shaft forged in a drop forging die, so that the height of the rib relative to the width of the base body from which the rib rises can be maximized.

This objective is achieved through the method according to the invention. Advantageous developments and embodiments of the invention will become apparent from the following description and claims. The invention provides the following method step: separation of the forging ridges formed in the die by trimming the forged shaft blank into a shape close to the final contour of the base body at trimming points on both sides of the unbalanced mass plane, a distance S2between the trimming points being substantially smaller than a distance S1between the forging ridges prior to separation.

In other words, deburring of the forged shaft blank is not restricted as heretofore to the separation of the thin-walled forging ridges but is effected in the thick-walled region of the base body, so that cut edges with comparatively large cut surfaces are formed that extend substantially parallel to the unbalanced mass plane. The degree of freedom created in this way makes it possible to increase, if necessary, the width of the hollow forging molds extending in direction of the die parting plane in correspondence with the required filling height of the hollow rib mold because the material not required in the width is separated thick-walled and preferably without chip removal after forging. The thickness D of the cut edges, i.e. the material thickness which has to be cut through is preferably at least half as large as the distance S2between the trimming points: D:S2>0.5.

According to a preferred development of the invention, the following ratios are true for the distance S1between the forging ridges, the distance S2between the trimming points and the height H of the rib: S2:S1<0.5 and S1:H>2, preferably>3. In other words, after trimming, the base body should be, at the most, half as wide as the forged shaft blank if this has only been deburred, and the hollow forging molds minus the ridge lands should have at least double, preferably triple the width of the hollow rib mold.

With a view to achieving a trimming method using as low a force as possible, the invention further provides for the trimming to be done in the still warm state of the shaft blank, i.e. within a short time after forging.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 4shows a compensation shaft1made according to the method of the invention, in finished state. The compensation shaft1forms a part of a mass balancing mechanism which serves for compensation of the free mass forces of the second order of an internal combustion engine of a four-cylinder in-line type. In this case, the mass balancing mechanism comprises two such compensation shafts1which rotate in opposite directions at double the crankshaft speed. For the sake of a simplified illustration, the attached parts required for driving the compensation shaft1, such as, for example, a chain sprocket or a pinion have been omitted.

The forged compensation shaft1comprises two bearing journals2and3of circular cross-section. The bearing journals2,3are finished by machining and induction-hardened and serve as inner raceways for the needle rollers of respective needle roller bearings through which the compensation shaft1is radially roller bearing-mounted in the internal combustion engine. Shaft sections4to6extending on both sides of the bearing journals2,3are configured, with a view to their function, as unbalanced mass sections so that the compensation shaft1has a mass center of gravity8which is eccentric to its axis of rotation7and produces the unbalanced mass. The mass center of gravity8together with the axis of rotation7creates an unbalanced mass plane (seeFIG. 3). A part of the unbalanced mass action is also produced by the geometry of the two bearing journals2,3because, in peripheral direction, these journals have a variable width matched to the so-called lumped loading of the inner raceway. This configuration of the bearing journals2,3is known, per se, from EP 1 775 484 A2 and is explained here only in so far as, during rotation of the compensation shaft1, the bearing journals2,3are loaded by the simultaneously rotating unbalanced mass, so that a quasi stationary load zone, that is to say a load zone which is immobile relative to the rotating inner raceway, is formed on each inner raceway. This load zone extends within the wider fractional periphery of the bearing journals2,3on the side of the mass center of gravity8and is substantially symmetric to the unbalanced mass plane. The low load outside of the load zone permits a clear tapering of the inner raceways outside of the load zone.

FIG. 1shows the cross-section corresponding to the section I-I ofFIG. 4through a forging die9,10with a schematic representation of the finished, forged shaft blank11situated therein;FIG. 2shows the lower die10. The die is a so-called ridge die in which a part of the shaped material of the shaft blank11is displaced—through narrow cross-section ridge gaps12situated in the region of the die parting plane13which is shown as a dot-dash line—out of the hollow molds of the upper and lower dies9and10outwards with formation of the forging ridges14to be separated subsequently. This promotes the complete filling of the die9,10and, in particular, the hollow rib mold of the upper die9in which a rib15for stiffening the rapidly rotating compensation shaft1is formed. The rib15starts from a the base body16of the shaft section5and has a height H (seeFIG. 3) while extending, with regard to the axis of rotation7of the compensation shaft1, in a direction of the unbalanced mass plane oriented away from the mass center of gravity8. The die parting plane13extends perpendicular to and within the base body16.

As already mentioned above, the so-called swelling of the material into the hollow rib mold requires a material distribution in the die9,10such that an adequate volume of pressurized material extends in direction of the die parting plane13. This is achieved in the present case by the fact that the forged width of the shaft blank11, i.e. the distance S1between the forging ridges14is at least three-times the height H of the rib15(the forging ridges14formed between the ridge lands17are not included in the dimension S1).

As can be seen inFIGS. 3 and 4, this initially forged width S1is no longer existent on the shaft section5of the finished compensation shaft1. Rather, the separation of the forging ridges14(with a separating tool, not shown) is performed at trimming points on both sides of the unbalanced mass plane, a distance S2between the trimming points being clearly smaller than the distance S1between the forging ridges14prior to separation (see dotted line inFIG. 3). Because, in the present case, the ratio S2:S1is 0.5, trimming of the forged shaft blank11is performed in the thick-walled region of the base body16and produces the wide cut surfaces18that are clearly shown inFIG. 4. The thickness D of the cut surfaces (measured in direction of the unbalanced mass plane) in the finished compensation shaft1is approximately 8 mm with S2being 14 mm, so that a ratio D:S2=0.57 is obtained.

The step of the simultaneous non-chipping trimming of the shaft blank11at both trimming points follows the forging step very closely in time, so that, due to the still heated shaft blank11, the cutting forces of the tool can be minimized. A finishing of the cut surfaces18by machining is not provided by the invention.

LIST OF REFERENCE NUMERALS

7Axis of rotation

8Mass center of gravity

S1Inner distance between the forging ridges

S2Distance between the trimming points

H Height of the rib

D Thickness of the cut surface