1、 STD-ALMA 94FTM3-ENGL 1994 E b87575 0004547 b113 94FTM3 Application of Ausforming to Gear Finishing - Process, Design and Manufacturability Issues by: N. Sonti, A.J. Lemanski and S.B. Rao Pennsylvania State University American Gear Manufacturers TECHNICAL PAPER COPYRIGHT American Gear Manufacturers
2、Association, Inc.Licensed by Information Handling ServicesSTD-AGMA 94FTM3-ENGL 1774 m Ob87575 0004550 3b5 m Application of Ausforming to Gear Finishing - Process, Design and Manufacturability Issues N. Sonti, A. J. Lemanski and S.B. Rao, Pennsylvania State University The statements and opinions cont
3、ained herein are those of the author and should not be construed as an official action or opinion of the American Gear Manufacturers Association. ABSTRACT: Ausforming is a thermomechanicai process which integrales surface induction heating with precision hot gear roll finishing, and is capable of fi
4、nishing spur and helical gears, theEby eliminating tooth-grinding and some associated operations. Gears are in the hobbed, carburized and hardened condition prior to ausform finishing. Ausforming involves controlled execution of the following four processing steps: (1) contour induction hearing of t
5、he hardened gear tooth surface layers, followed by (2) rapid quenching of the gear in marquenching oil maintained at 300-450OF to achieve a metastable austenitic condition, (3) roll finishing of gear teeth in the metastable austenitic condition in the processing tank containing the hot marquenching
6、oil, to final tooth dimensions using power-dnvenprecision gearroll finishing dies, and (4) final quenching to martensite. Ausforming is applicable to carburizing grade low alloyed steels, usedin a broad spectrum of gearapplications, as well as through-hardening steels. Potential benefits of ausformi
7、ng as demonstrated on the existing singleae ausforming machine at Penn State include improved gear tooth accuracy up to AGMA 12, enhanced surface finish of 3 to 6 pin Ra, and improved metallurgical features such as ausform strengthening induced in the surface layers, finer grain microstructure, and
8、higher compressive residual stresses on gear tooth surfaces. Metallurgical characterization of ausformed gear tooth surfaces and performance testing of ausfomed surfaces is in process. This paper discusses the potential applications of ausform finishing of spur and helical gears, including process d
9、esign and related manufacturability issues. Ausforming can be applied for finishing of gears with bores, integral shaft gears, cluster gears, and double helical herringbone gears. Examples are presenteddescribing the flexible tooling arrangements possible to process a variety of gear geometries. Imp
10、lementation of ausforming for a given application requires integrated tooling design of the hob or shaper cutter to produces appropriate stock allowance for optimal roii finishing. Machine design, controls and material handling features of the production-capable double die gear ausform finishing mac
11、hine currently being built are briefly described showing the flexibility in tool arrangements and processing of a variety of gear geometries. Copyright O 1994 American Gear Manufacturers Association 1500 King Street, Suite 201 Alexandria, Virginia, 223 14 Oc tober, 1994 ISBN 1-55589437-5 COPYRIGHT A
12、merican Gear Manufacturers Association, Inc.Licensed by Information Handling ServicesSTD-AGMA SLiFTM3-ENGL LLi = Ob87575 0004553 2TL m Application of Ausforming to -ar Finishing - Process Design and tdanufacturability Issues N. Sonti, Research Associate A. J. Lemanski, Associate Director S. B. Rao,
13、Director National Center for Advanced Gear Manufacturing Technologies Applied Research Laboratory The Pennsylvania State University, University Park, PA 16804. 1 Introduction Over the past several years, the US gear industry has encountered considerable challenges from the industrialized countries i
14、n Europe and far east The competitive position of the US gear industry has been substantially weakened and the share of imports to the US gear market is continuing to rise. l Investment in gear machine tools and infrastructure in the US gear industry is steadily declining, and use of out-dated equip
15、ment and processes only worsens the problem of delivery times, quality and manufacturing costs.2 There is a strong need for the development of new and improved gear manufacturing techniques and equipment to improve the competitive status of the US gear industry The National Center for Advanced Gear
16、Manufacturing Technologies (NCAGMT) at Penn State is developing innovative techniques in gear manufacturing and metrology to strengthen the technological base of the gear industry. One of the research and development programs in the area of advanced gear manufacturing is Gear Finishing by Aucforming
17、. The ausfornung process is capable of finishing case-hardened spur and helical gears to achieve improved strength accuracy, surface finish, and through-put production rates. In this paper, the process, and manufacturability issues involved in applying ausfornung to the finishing of high performance
18、 transmission gears are discussed. 2. Ausforming Process Details 2.1 Process Description Ausform finishing integrates three of the widely used gear manufacturing processes, namely, (1) induction heat treating, (2) martempering (also called marquenching), and (3) gear roll finishing, into a single in
19、-line automated manufacturing operation which is capable of finishing spur and helical gears to AGMA quality rating of 12 or better.4 Prior to ausform finishing, gears are hobbed, carburized and hardened to specifications required for the service requirements. The ausform finishing process involves
20、the following processing steps: b Contour induction heating of the carburized surface layers of the gear teeth (including profile, fillet and root areas) to austenitize only the case region; Rapid quenching of the gear into hot marquenching oil maintained at 300 to 45OoF to achieve a metastable aust
21、enitic condition in the carburized surface layers of the gear teeth; a Precision gear roll finishing of the gear teeth in hot marquenching oil to final dimensions using master gear rolling dies and a feed-back controlled servo-hydraulic system; 0 Final quenching of roll finished gear to 120- 150F fo
22、r transformation of austenite to martensite. The thermal excursion of gear tooth surface temperature during the above four processing steps involved in gear ausform finishing is shown schematically in a Time-Temperature-Transformation (TTT) diagram (Figure 1) for a typical carburizing grade low allo
23、yed steel. It is important to note that gears must be in the carburized and hardened condition prior to ausforming in order to achieve the core properties, especially the combination of microstructure, hardness and toughness required. Ausform finishing is preferably performed prior to the tempering
24、cycle. This facilitates optimization of the induction heating cycle and to ensure rapid solutionization of alloy carbides and complete austenitization of the gear tooth case region prior to the roll finishing step. Line a-b (Figure 1) represents the induction heating step of ausforming when the gear
25、 tooth surfaces are austenitized by heating to above the appropriate temperature. The induction heating cycle takes only a few seconds to complete. It is optimized to ensure that the heat 1 COPYRIGHT American Gear Manufacturers Association, Inc.Licensed by Information Handling Services STD-AGHA 94FT
26、fl3-ENGL 1994 D b87575 0004552 138 D affected zone below the carburized case is minimal to retain the sub-case properties already established in prior processing. The induction heating cycle is carefully controlled using a high resolution optical pyrometer for monitoring the gear tooth surface tempe
27、rature and controlling the power and time of induction heating. Following the induction austenitization of the gear tooth surface layers, the work gear is then rapidly quenched into hot marquenching oil maintained at a suitable temperature (typically 50 to 150F over the MS, the martensite start temp
28、erature), as represented by line b-c in figure 1, making sure that no diffusional transformations occur. Gear tooth surfaces are in the metastable austenitic condition at this stage, represented by the bay region of the TTT diagram. The gear temperature is allowed to stabilize (line c-d in figure 11
29、, prior to the subsequent roll finishing step, and the gear teeth therefore undergo martempering prior to the roll finishing operation. Martempering is widely used in the gear industry, especially for tool steels, and is a heat treatment procedure designed to minimize heat treat distortion and crack
30、ing.5 Ausforming process therefore takes advantage of the beneficial effects of martempering. Gears are then roll finished, while still in the metastable austenitic condition in the processing tank containing the hot marquenching oil, using precision ground master gear rolling dies (line d-e in Figu
31、re 1). Conventional gear rolling, widely used in the automotive gear industry as a less expensive alternative to the gear shaving process, is performed prior to heat treatment. Accuracy improvements and enhanced surface finish obtained by conventional automotive gear rolling are therefore substantia
32、lly lost during heat treatment. Conventional gear rolling also utilizes open-loop hard-stop type machine controls, which do not recognize the variations in gear tooth dimensions. By contrast, ausforming is performed after gear heat treatment. Furthermore, ausforming utilizes a feedback controlled se
33、rvo-hydraulic system for precise control of the deformation process. The rolling stock on gear teeth is moved both axially and up and down the gear tooth profiles in a precise manner by controlling the amount of infeeding of the rolling dies after tight-mesh with the work gear. In other words, the r
34、olling dies and the work gear are first engaged in a tight mesh condition, and then the center distance between the dies and the work gear is decreased by a precise amount using feedback controlled servo-hydraulic actuators. The roll finished gear is then removed from the processing tank and quenche
35、d to transform the metastable austenite to fine grain martensite (line e-f in Figure li. 2.2 tdmtallurgical maturcrr of Ausforpping Ausforming, a thennomechanical strengthening process involving plastic deformation of austenite, was originally developed in the 1950s, and has been demonstrated to imp
36、art significant improvement in mechanical properties of ausfonned martensite. The strengthening effect of ausforming is attributed to favorable residual dislocation structure and carbide distribution generated in the deformed metastable austenite, and inherited by the final martensite. A fine disper
37、sion of carbides is formed during the working of austenite, stabilizing not only the grain size, but also the subgrain size as well. Ausforming results in a very high dislocation density in the final martensite. The dislocation networks produced are not the normal ones in which the dislocations are
38、concentrated at the cell walls, but rather are dispersed uniformly. Larger scale microstructural effects also play an important role in the strengthening process. The low temperature of working (up to a few hundred degrees above the Improved accuracy of gear teeth from AGMA quality rating of 8-9 pri
39、or to ausforming to about 12 after ausform finishing; Enhanced gear tooth surface finish of 3 to 6 pin Ra; Elimination of associated labor intensive off line processes such as nital-etch inspection, refrigeration, and shot peening; Decrease in gear finishing process tfme to less than a minute, as co
40、mpared to grinding which can take several minutes; More consistent gear quality derived from induction heating and gear roll finishing processes which are inherently repeatable and consistent; Prolonged life of roll finishing dies. Typical . roll finishing dies can finish over half a million gears;
41、Reduction in gear processing cycle time and COPYRIGHT American Gear Manufacturers Association, Inc.Licensed by Information Handling ServicesSTD.AGMA SYFTM3-ENGL 1994 Ob87575 0004553 07q m . costs for hardened and ground gears; Improved surface strengthening due to ausfodng without degrading toughnes
42、s or ductility; a Retained austenite level of 6 to 0% achieved after ausforming without refrigeration; Improvements in compressive residual stresses of over 40% in the ausformed condition as compared to conventionally quenched and tempered gears; Retention of beneficial surface layers with high comp
43、ressive residual stresses. 3. Iquipiunt C Proceaa hsign Issues 3.1 Doublc Dir war Ausforming Machine A production-capable double die gear ausform finishing is currently being built for computer controlled execution of the various processing steps for ausform finishing of spur and helical gears. Figu
44、re 4 shows a schematic front elevation of the double die gear roll finishing machine subsystem being developed. The workpiece gear is held by a hydraulically operated expanding chuck mounted on the throughfeed spindle. The vertical throughfeed motion of the work gear is provided by the throughfeed h
45、ydraulic actuator, flange mounted on the machine frame. The throughfeed axis is laterally fixed, and the two rolling dies assemblies on diametrically opposing sides of the work gear are fed in to achieve the relative displacement between the work gear and the rolling die axes, thereby resulting in p
46、lastic deformation of the gear tooth surface layers. The rolling dies are power driven from the top via a dual output gear box. To ensure precise orientation of the rolling dies with respect to the work gear, adjus,tment capability is incorporated for the precise rotational, axial, in-plane and out-
47、of-plane angular adlustment of the two rolling dies. The machine design allows these adjustments to be made and monitored remotely even when the ausforming processing tank is at operating temperature. The ausforming processing tank is designed for operation up to 500“F, and contains the two die asse
48、mblies, infeed tension bars and the die axes adjustment mechanisms, thereby maintaining a thermally stable environment with minimal gradients. The degree of deformation must be controlled to close tolerances by controlling the infeed motion of the two rolling dies and throughfeed motion of the work
49、gear. Linear encoders are used to precisely monitor and control the infeed (die-to-die) and throughfeed positions and loads including the drive torque. Figure 5 shows a schematic side view of the gear roll finishing machine and the material handling equipment required for mechanized operation of the ausforming processing steps including gear loading, dual frequency contour gear induction heating, rapid marquenching and roll finishing in the ausforming tank, and final quench to martensite. A microprocessor based ausforming process and machine controller is used t
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