Metal working – Method of mechanical manufacture – Gear making
Reexamination Certificate
2002-01-24
2004-08-24
Omgba, Essama (Department: 3726)
Metal working
Method of mechanical manufacture
Gear making
C029S893300, C029S893340, C072S108000
Reexamination Certificate
active
06779270
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a technique for precision form finishing of the entire contour of a machine element, typically the teeth of a spur or helical gear or of a sprocket, made of wrought or forged alloyed carbon steels, including the active contacting surfaces and the trochoidal root/fillet regions, thereby inducing material flow in the critical regions of the teeth. Full form finishing by plastically deforming these regions results in improved surface finish, higher strength and accuracy of the teeth of the machine element. Throughout the ensuing disclosure, the mention, for example, of gears or of helical gears is not to be taken in a limiting manner but only for purposes of description.
2. Description of the Prior Art
Highly loaded transmission gears used for automotive and aerospace applications are normally manufactured using wrought or forged low carbon low-to-medium alloyed steels, by blank machining to produce the gear teeth, followed by carburizing and hardening heat treatments to impart high surface strength and hardness combined with adequate toughness of the core. Alternate to above carburizing grade low carbon alloyed steels are medium-to-high carbon and alloyed through-hardening type steels, which do not require the carburizing cycle. Alternate methods for producing the gear teeth include near net forging. Aerospace gears, and some automotive gears, are then hard finished by grinding after heat treatment to impart the required dimensional accuracy and surface finish. However, cost considerations preclude expensive hard finishing operations for most automotive gears, and instead, pre-finishing techniques such as gear roll finishing and shaving are often used prior to heat treatment. Gear shaving is a free-cutting material removal process that improves the gear tooth accuracy and surface finish by machining a thin layer of stock (0.001″ 0.003″ per tooth flank) from the tooth surfaces. On the other hand, gear roll finishing is a form-finishing process that improves accuracy and surface finish by plastically deforming and moving a thin layer of stock (0.001″-0.002″ per tooth flank) across the gear tooth surfaces. Roll finishing produces much finer surface finish of 4-6 &mgr;in Ra as compared to 25 &mgr;in Ra achieved by shaving. Both gear shaving and conventional gear rolling processes finish only the active contacting tooth surfaces, and do not touch the trochoidal root and fillet regions of the gear teeth. Therefore, for rolling or shaving operations, the gears are produced with rolling or shaving stock only on the tooth flanks, and not on the root/fillet regions. The rolling dies used for conventional roll finishing are designed with tip clearance to avoid contacting the fillet and root regions of the gear teeth.
If the roll finishing operation were extended to finish the root/fillet regions in addition to the active contacting surfaces of the gear teeth made of wrought or forged alloyed carbon steels, then the surface finish and bending fatigue strength of the gear teeth would be substantially improved. Root fillet regions of gear teeth experience the maximum bending stress. Roll finishing of the root/fillet regions will improve the surface finish, thereby reducing the stress concentration, and enhance the fatigue resistance of the material due to plastic deformation and flow of the rolling stock.
Therefore, to produce wrought or forged steels gears with improved accuracy, surface finish and enhanced load carrying capacity, the gear roll finishing process must be applied to both the active contacting surfaces as well as the trochoidal root fillet regions of the helical gear teeth.
A number of patents are definitive of the prior art in this regard. For example, U.S. Pat. No. 3,659,335 to Bregi et al. discloses a combined gear shaving and rolling machine. Provision is made for relative traverse while shaving in a direction parallel to the axes of the gear and tool and for incremental in-feed during shaving and continuous in-feed during gear rolling.
The process of roll finishing of gears is covered by U.S. Pat. No. 3,362,059 to DiPonio et al.
U.S. Pat. No. 5,221,513 to Amateau et al. discloses a system for the thermomechanical processing of gears in which precise control of the thermal conditions, the environment and mechanical actions during the forming process is maintained. The essence of the patent resides in the process control methods and architecture for accomplishing precision motions, thermal control, and environmental control using a unique combination of sensors, mechanisms, and software. The apparatus includes an induction heating system which reaustenitizes the surface of the gear with minimum decarburization, a material transfer system which provides timely operations on the work piece, tooling and fixture adjustments which provide accurate initial conditions for forming, and a process control architecture that provides the precise sequence and timing necessary to achieve metallurgically sound and dimensionally accurate gears. Both through-feed and in-feed motion are simultaneously controlled by load, position, and velocity transducers which provide feedback information to a supervising microprocessor.
U.S. Pat. No. 5,451,275 also to Amateau et al. is an improvement on the '513 patent and provides an apparatus for precision gear finishing by controlled deformation using a fixed axis through-feed and coordinated and controlled moving axes in-feed of two rolling dies positioned on diametrically opposing sides of the workpiece. As with its predecessor technique, this later patented invention also includes apparatus for achieving controlled deformation, apparatus for providing precise adjustment of the axes of the two rolling dies from a remote location while the rolling apparatus is thermally stabilized and maintained at the forming temperature and under an inert atmosphere, and apparatus for performing a timely transfer of the workpiece to achieve the optimum metallurgical condition at each stage of the thermomechanical gear finishing process. The essence of this later invention is the concept of using two rolling dies, and process control methods and architecture for accomplishing precision motions, thermal control, and environmental control with a combination of sensors, mechanisms and a software controlled sequence of operations. The control architecture allows precise mechanical movements of the through-feed motion of the workpiece and the in-feed motions of the two rolling dies in either the load control or position control mode of operation. Appropriate transducers and sensors are used to monitor each of these motions and loads, and are used to generate feedback signals, and thereby, the error signals used to drive the servo-controlled actuators for the in-feed and through-feed motions.
It was with knowledge of the foregoing state of the technology that the present invention has been conceived and is now reduced to practice.
SUMMARY OF THE INVENTION
In accordance with the present invention, a methodology is provided for spur and helical gears made of wrought or forged alloyed carbon steels, which utilizes the roll finishing tooling that performs net-shape full form roll finishing of gear teeth in a manner that simultaneously forms the active contacting surfaces of tooth flanks and the trochoidal root/fillet regions of the gear teeth. The essence of the invention is the technique for producing the roll finishing tooling capable of form finishing the entire contoured surface of the helical gear teeth in a single manufacturing operation.
A primary feature, then, of the present invention is the provision of a technique for precision form finishing of the entire contour of a machine element, typically the teeth of gears or sprockets, including the active contacting surfaces and the trochoidal root/fillet regions, thereby inducing material flow in the critical regions of the teeth.
Another feature of the present invention is the provision of such a technique o
Caldwell James V.
Rao Suren B.
Sonti Nagesh
Omgba Essama
The Penn States Research Foundation
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