Planetary gear transmission systems or components – Planetary gearing or element
Reexamination Certificate
2001-03-19
2003-12-02
Schwartz, Christopher P. (Department: 3683)
Planetary gear transmission systems or components
Planetary gearing or element
C475S230000, C475S248000, C475S901000
Reexamination Certificate
active
06656079
ABSTRACT:
BACKGROUND AND SUMMARY OF INVENTION
This application claims the priority of German patent document No. 100 134 29.7-12, filed Mar. 17, 2000, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a differential gear with a differential housing and with a differential crown wheel, as is used particularly in the automobile sector, and to a method for producing the differential gear.
When a bend is negotiated, the outer wheels of a motor vehicle have to cover a greater distance than the inner wheels. Also, irregular road surfaces cause differences in distance between the individual wheels. When the driving wheels, in these situations, are to roll on the roadway without slipping, they should not be connected to one another by rigid shafts, but must be connected via a differential gear. A differential gear makes it possible to have different rotational speeds of the driving wheels.
The power generated in the engine is transmitted by the drive shaft, at the end of which a driving bevel pinion is located. This driving bevel pinion engages into a differential crown wheel which is connected fixedly in terms of rotation to a housing of the differential gear, the so-called differential housing, and drives the housing. Arranged in the differential housing are differential bevel pinions which are connected to the axle shafts leading to the wheels and which compensate the different speeds of the wheel shafts. Differential housings generally consist of cast iron. In known differential gears, differential crown wheels are manufactured from case-hardened steel. Case-hardened steel refers to steels having a high edge layer hardness, which occurs due to the annealing of the workpiece in carbon-yielding agents and to subsequent quenching. As a result of the annealing operation, the edge layer of the workpieces is enriched with carbon and is hardened from the annealing temperature via the quenching operation.
As a rule, the axis of the drive shaft and of the differential bevel pinion is arranged so as to be offset to the centre of the differential gear (so-called pinion offset), primarily due to the transmission ratio between the driving bevel pinion and the differential crown wheel. In many applications, it is advantageous to avoid a pinion offset, because this leads to the driveshaft running diagonally. This entails various disadvantages. For example, the inclination of the drive shaft is manifested in a higher load on the cardan joints, which, in turn, means higher wear and power loss. Moreover, there are restrictions in the arrangement of the floor assembly, which has effects on construction space and variant diversity.
In known differential gears, the connection between the differential housing and the differential crown wheel is made by screws. The disadvantage of such a screw connection is that a solid flange is necessary on the differential housing. Construction space is required for this flange and also for the screw head and for mounting purposes. Furthermore, a minimum thread depth is necessary for a secure connection. The differential crown wheel therefore has to have a width which can receive the shank of the screw. Moreover, the dimensions of the differential crown wheel must be selected such that it meets stability requirements, particularly because the stress plane of the forces to be transmitted runs through the screw threads. The result of these boundary conditions of construction is that the differential crown wheel must have a minimum size. This minimum size, and also the solid flange and the screw heads, not only have an adverse effect on the weight of the differential gear, but also result in the driving bevel pinion having to be offset in the differential gear thus entailing the disadvantages listed above.
On account of the high carbon contents both in the cast iron of the differential gear and on the surface of the case hardened steel of the differential crown wheel, it is not possible for the structural parts to be welded together. At the concentrations which occur in the case of this material combination, the carbon in the melt forms brittle structural constituents during rapid cooling after welding, which have an adverse influence on the quality of the weld seam, since they may lead to the formation of cracks.
It is known from WO 99/58 287 A1 to weld a case hardened differential crown wheel to a differential housing consisting of cast iron (malleable iron, cast steel or nodular cast iron). For this purpose, prior to welding, the surfaces to be welded together on the otherwise ready-machined structural parts are at least partially stripped for welding preparation, so that, where welding is to be carried out, a narrow groove is obtained. This machining step causes the surface to be removed in the region of the joint on the case hardened differential crown wheel. This region of the structural part has the highest carbon content. Since the carbon fraction in a case hardened steel decreases very sharply at an increasing distance from the surface, the introduction of the groove brings about an extreme reduction in the carbon content at the joint, with the result that the above-described problems in the welding of materials with high carbon contents are reduced considerably. In the differential gear according to WO 99/58 287 A1, welding is carried out, with a welding wire being fed continuously. The weld seam is produced axially with respect to the axis of the differential crown wheel.
This known differential gear entails the disadvantage that both the differential housing and the differential crown wheel must be designed in such a way that they can absorb a shrinkage of the axial weld seam. This is achieved by recesses introduced into the structural parts. These recesses require construction space. Because of the recesses, the structural parts must have correspondingly larger dimensions in order to afford the stability necessary for operation. The result is that, in the known differential gear, the gain in construction space, if there is any at all, is very slight, in comparison with screw-connected differential gears. Nor is this conducive to minimizing the pinion offset.
Shrinkage of the weld seam cannot be avoided, in practice, and, in the known differential gear, entails a tilting of the toothing of the differential crown wheel in spite of the recesses introduced. This leads to inherent stresses in the structural part and also frustrates the aim of achieving a reproducible quality of the differential gear. During operation, the connection is subjected to stress not only by the torsion and the axial and radial loads introduced into the differential crown wheel, but also by inherent stresses. So that the weld seam withstands these stresses, greater welding depths are necessary, which leads to an increased outlay in terms of production and to higher costs.
Another disadvantage of the known differential gear is to be seen in that welding preparation, that is to say the at least partial stripping of the surfaces to be welded together, means an additional work step which has an adverse effect both on the production time and the production costs. Furthermore, the stripping of a case hardened layer entails high machine-cutting costs.
The surfaces of the known gear which are to be welded together consist of two regions: a groove region and a centering region which is arranged below the groove region and at which the differential crown wheel and the differential housing abut on one another. After welding, this centering region has the effect of a notch on the weld-seam root due to shrinkage processes.
Moreover, the continuous feed of welding wire during the welding operation is a disadvantage since this necessitates complicated positioning and control of the speed of the fed welding wire. If there is a fault in these parameters, the additional material is not uniformly distributed over the entire height and length of the weld seam.
Particularly at the seam root, optimum intermixing of the additional material with the melt is not ensu
Eulenstein Tycho
Netuschil Willi
Paasch Rudolf
Daimler-Chrysler AG
Kramer Devon
Schwartz Christopher P.
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