Metal working – Method of mechanical manufacture – Gear making
Reexamination Certificate
1999-03-24
2001-01-09
Echols, P. W. (Department: 3726)
Metal working
Method of mechanical manufacture
Gear making
C029S090600, C029S404000
Reexamination Certificate
active
06170156
ABSTRACT:
TECHNICAL FIELD
This invention pertains to the manufacture of gears. More specifically, this invention pertains to a new process for smoothing and shaping of gear tooth surfaces. It includes gear run-in or polishing in place for smoothing or re-shaping of tooth surfaces of newly formed gears.
BACKGROUND OF THE INVENTION
Gears have long been used in power transmitting machines and mechanisms to increase or decrease an applied torque or the direction in which a torque is applied. Gears are often formed as wheels, worm wheels or linear racks. Elegant gear manufacturing processes have been developed to form the teeth on the wheel or rack structure.
In the case of gear wheels, the basic gear form with unfinished teeth can be, e.g., cast or forged from a blank of a suitable metal alloy. A hardenable steel, such as AISI 5620, is often a material of choice. Teeth are cut into the circumference of the wheel using a hob or other suitable tool. The surfaces of the hobbed teeth are often then further machine finished or polished so that they are precisely shaped and smooth for good engagement with a counter-gear. Grinding, honing and/or chemical polishing are examples of such gear tooth finishing processes.
In the automotive industry, millions of gears are manufactured each year. In one particularly large manufacturing volume application, e.g., planetary gear sets are commonly used in automatic transaxles. Such planetary gear sets contain at least three main components: a sun gear, a carrier assembly with a plurality of planet pinion gears and an internal gear. The sun gear is located at the center of the planetary gear set and has planet pinion gears revolving around it. These planet pinion gears have gear teeth that are in constant mesh with the sun gear. An internal ring gear encompasses the entire gear set. Torque from the engine (input torque) is transferred to the gear set and forces at least one of these components to rotate. Since all three main components are in constant mesh with each other, the remaining components are often forced to rotate as a reaction to the input torque. After input torque passes through a gear set, it changes to a lower or higher torque value known as output torque. In a front wheel drive automobile transaxle, for example, two such suitably sized gear sets are combined and controlled to provide forward drive ratios and a reverse drive. The output torque from the second gear set then becomes the force that is transmitted to the vehicle's drive axles.
The automobile automatic transaxle is but one example of gear set containing mechanisms that must be carefully designed for minimum cost of manufacture and to sustain high loads over a long product life. The need for continuous improvement in automobile design has required engineers to obtain unreduced or greater output from smaller and lighter robust gear mechanisms.
It is observed that the operating life of a power transmitting mechanism such as an automotive automatic transaxle depends significantly on the fatigue life of the gears. There seem to be two main approaches to increasing the fatigue life of a gear set: improving tooth shape and contact area and increasing the hardness of tooth wear surfaces. The improvement of tooth shape and contact area has been accomplished by expensive machining operations and by unselective natural wear-in or run-in of a newly made and assembled set during the first hours of operation of the mechanism. The increase in the tooth hardness has been accomplished by metallurgical surface hardening, e.g., induction surface hardening of a hardenable steel, or carburization and heat treating of an iron or steel alloy, or by application of a thin coating of hard material such as diamond-like carbon, titanium nitride, boron carbide or the like. While such hardened surfaces increase the fatigue life of a gear set, care must be taken to polish the hard surface or it may cause excessive wear of the mating gear surface by abrasion.
The gear making art requires improvements in the manufacture of suitably shaped gear teeth, and the use of hardened gears, and in the assembly of such gears in a robust power transmission mechanism.
SUMMARY OF THE INVENTION
In a first embodiment, this invention provides an improved method of using a dummy or expendable counter-gear to smooth a hard surface-coated gear before assembly of such gear with an intended counter-gear in a power transmission mechanism. The goal of this smoothing is to remove sharp edges and asperity tips of the hard coating and to reduce the abrasiveness of the coated surface.
In another embodiment of the invention, an expendable hard surface coated counter-gear is used as a low cost and practical tool to run-in and re-shape softer complementary gears before assembly of such gears in a mechanism.
For purposes of illustration, but not limitation, the invention will be described for the case when the changes in the surface of a hard surface-coated sun gear include smoothing, polishing and reduction in its abrasiveness, while the changes in the surface of pinion gears, intended for assembly in a planetary gear set, include polishing and re-shaping.
In one example, an unpolished boron carbide coated sun gear is operated under substantially a design level load and operating temperature against a dummy pinion gear that may be essentially identical to the pinion gears that are to be assembled with the sun gear in a planetary gear set. It is found that a very few rotations of such a sun gear against the expendable pinion smoothes the rough surface asperities of the thin (2-3 micrometers) B
4
C coating. The run-in sun gear is then assembled with design specified pinion gears in the design assembly. The dummy pinion is used to smooth more hard-coated sun gears. From the initial operation of the newly assembled mechanism, the run-in sun gear provides the fatigue life benefits of its hardened teeth surfaces without undesirable abrasion of the pinion teeth.
In the converse example of this invention, a suitable sun gear with hard tooth surface is used to reshape pinion gears. After a group of pinions have been formed by a suitable and practical manufacturing process, one or more at a time are rotated at substantially design load and operating temperature against the dummy sun gear with hard tooth surface. The dummy sun gear is suitably identical to the gear designed for assembly with the pinion(s) and a brief rolling operation gives a “final” shape to the pinions prior to their assembly with the sun gear actually made for the machine.
Other objects and advantages of the invention will become more apparent from a detailed description of the invention which follows. Reference will be had to the drawing figures that are described in the following section.
REFERENCES:
patent: 1989652 (1935-01-01), Drummond
patent: 2147864 (1939-02-01), Thrun
patent: 2423593 (1947-07-01), Gasser
patent: 3606782 (1971-09-01), McNarb et al.
patent: 3894418 (1975-07-01), Hörl
patent: 4-147735 (1992-05-01), None
Harris Stephen Joel
Lev Leonid Charles
Weiner Anita Miriam
Echols P. W.
General Motors Corporation
Grove George A.
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