Metal treatment – Stock – Ferrous
Reexamination Certificate
1999-12-31
2001-11-13
Yee, Deborah (Department: 1742)
Metal treatment
Stock
Ferrous
C148S586000, C148S573000
Reexamination Certificate
active
06315841
ABSTRACT:
TECHNICAL FIELD
This invention relates to processes for preparing gears and for the gears so obtained, and more particularly to an induction hardened forged gear and a process for preparing the same.
BACKGROUND OF THE INVENTION
Processes for preparing gears which include forging heat treatment and machining, a low carbon steel, typically containing about 0.2% by weight carbon, are known. Disadvantages of using a low carbon steel of this type are twofold. First, a carburizing heat treatment is required because it is not possible to obtain sufficient hardness otherwise when starting with such a low carbon steel. Secondly, a low carbon steel containing about 0.2% by weight carbon cannot be heat treated by induction heat treating.
It has been reported in the prior art that ring gears for heavy duty automotive vehicles such as trucks can be fabricated by a process essentially as described above, starting with a low to medium carbon steel which can contain anywhere from about 0.05% by weight carbon to 0.4% by weight carbon. It has been further reported that the steel can be precision forged so that no further shaping of the tooth profiles, after the final heat treatment, is required. However, it is necessary to use a carburizing heat treatment in this process.
A process for making an automotive vehicle ring gear which includes forging, machining the result in forging, and induction heating is also known. In this process, a blank is Anear-net@ forged such that no more than about 0.070 inch of metal has to be removed from the sidewalls of each gear tooth. The resulting forging is then machined, and finally, when a high carbon steel is used, the ring gear surface can be contour induction hardened. This process is also disclosed as being applicable to medium carbon steels containing from 0.05% carbon to 0.6% carbon.
Presently known gear fabrication process, whether used for the manufacture of low motion transmission error (MTM) gears such as ring gears for automotive vehicles, or Ahigh MTM@ gears, such as differential gears (e.g. a pinion mate gear or side gear) for automotive vehicles, or for making gears for lighter vehicles such as lawn mowers and golf carts and the like, have generally required either carburizing or machining or both. Such operations add considerably to the expense of the manufacturing. Furthermore, such processes requiring such steps are unnecessary for the manufacture of high MTM torque gears.
SUMMARY OF THE INVENTION
The present invention provides a low cost process for manufacturing of high MTM gears, such as automotive vehicle differential gears, (e.g. a pinion mate gear or side gear), and gears for smaller vehicles such as golf carts and lawnmowers. The gear may be either a straight bevel gear or a spiral bevel gear. Automotive vehicle differential gears are normally of the straight bevel gear type. Other gears manufactured herein may be either of the straight bevel gear or the spiral bevel gear type.
The present process comprises the steps of forming a bevel gear having teeth by forging a steel blank having a carbon content of at least about 0.4% by weight until a bevel gear precursor having essentially the tooth configuration and dimensions of the desired product gear is obtained, and then hardening the gear precursor so obtained by induction hardening to obtain a gear having relatively high surface hardness, e.g., Rockwell C50 or higher, and a softer core, having a Rockwell hardness of about Rockwell C45 or lower.
This invention provides a low-cost manufacturing process, especially suited for differential gears such as pinion mate gears and side gears, and other gears which do not have to be manufactured with low MTM (below 40 micro radians), which eliminates the costly steps machining and carburizing heat treatment.
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Fisher James S.
Smith Roland C.
Dana Corporation
Hahn Loeser + Parks LLP
Oldham Scott M.
Yee Deborah
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