1. Field of the Invention
This invention generally relates to a multi-stage or multi-speed planet wheel of a sun and planet gear mechanism having at least two planet wheels, in particular for a bicycle wheel hub.
2. Background Information
Such bicycle wheel hubs with multi-stage planet sun and planet gear mechanisms are disclosed, for example, in United Kingdom Patent No. 21 66 502 B, Federal Republic of Germany Patent No. 814 982, Federal Republic of Germany Laid Open Patent Application No. 41 42 867A1. The individual speeds of the planetary gear are generally selected by momentarily fixing one of the sun wheels, which sun wheels are always engaged with the planet wheels on the stationary hub axle, while the other sun wheels remain free to rotate. An additional shifting capability is frequently created by optionally coupling the planet wheel carrier, generally corresponding to high gears or speeds, or the ring gear, generally corresponding to low gears or speeds, are coupled to the driving toothed wheel of the planetary gear, so that, accordingly, a two-stage planetary gear makes available two high speeds and two low speeds. Frequently, there is also a direct speed, so that a total of five speeds are available. In the case of a three-stage planetary gear, there are accordingly seven speeds.
In conventional planetary transmissions, at least three planet wheels are generally used, to achieve a uniform distribution of force. Low manufacturing costs are important, in particular when the planetary gear is used in a mass-produced item such as a bicycle wheel hub. At the same time, care must be taken to achieve sufficient mechanical strength and the least possible wear. These requirements can be met by using sintered moldings. Sintered moldings are manufactured using non-cutting methods, by compressing metal powder at high temperatures, and they tend not to require any finishing operations. If necessary, the material can also be hardened. But there are limitations with regard to the shapes which can be molded, above all because the powder primary materials are pressed inside the mold by the dies which are opposite one another, and are moved toward one another during the molding operation, essentially only in the axial direction. Any shoulders on the mold which project radially inward can have an adverse effect on the compression process. Other shaping methods, e.g. powder metallurgy, injection molding or lost-wax or precision casting, are generally too expensive for mass-produced items such as bicycle wheel hubs.
This is also true for potential manufacturing methods which utilize machining operations. On account of the different number of teeth of the planet wheel stages, in connection with the mold gate required for each stage, the economical cutting processes, e.g. cutting with a solid cylindrical milling cutter, are practically out of the question. The manufacture of the multi-stage planet wheel by a numerically-controlled slotting machine is of course possible in itself, but again it is too expensive.
Making the planet wheel in several parts tends not to be coherent with the objective of economical manufacture, because it should generally be absolutely guaranteed that all the planet wheels of the planetary transmission have as close to exactly the same orientation of the gear teeth of the two parts relative to one another as possible. Otherwise there can be deformations inside the planetary transmission, since the planet wheels are essentially constantly engaged in the common ring gear and in the common sun wheels. It should generally therefore be expected that when the planet wheels are designed as two-piece parts, some measures have to be taken to guarantee the substantially exact mutual angular orientation of the two parts when they are joined. But that, in turn, is generally not coherent with the objective of the most economical manufacturing process possible.