Gear Grinding Techniques Parallel Axes Cears
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BACK TO BASI'CS ... Gear Grinding Techniques Parallel Axes Cears by John M. lange Miller Associates, Inc. The fundamental purpose of gear Ground Unhardened Gears Gas turbine manufactu.rersemploy grinding is to consistently and econom- Low hardness or "soft" gears. in the abrasive machining to effect considerable ically produce "hard" or "soft" gear tooth range of Rockwell C 25 to 40 are usual- savings in the manufacture of splines and elements within the accuracy required by ly cut by a gear shaper, Shear-speed, or keyed through-hardened spacer rings. the gear functions. These gear elements hobber, and then finish shaved to obtain The solid through-hardened ring blanks include tooth profile, tooth spacing, lead ill relatively high degree of accuracy, i.e .. are stacked on an arbor, then the splines, or parallelism, axial profile, pitch line AGMA 1~11 (American Gear Manu1ac- keyways and peripheral grooves are runout, surface finish, root fillet profile, turers Association quality no.), In some form ground from the solid. These rings and other gear geometry which con- applications, it may be more economical were formerly made by cutting in the tribute to the performance of a gear to grind the "soft" teeth in the solid gear soft-blank, heat treating and then finish train. blank or bar stock, especially in the grinding individually. Form grinding The strength and wear resistance of range of 16 diarnetral pitch and finer. from the solid blanks after hardening ef- gear teeth may be increased by harden- fected savings in excess of 50% of the ing the tooth and root surface. Hardened original cost. gear teeth make it feasible to reduce the High Anoy Gears Ground .FromSolid gear size and increase the transmitted The high temperature high aHoy gears load and speed ..This is highly desirable and splines afford an excellent area in Case Hardened Ground Gears from the standpoint of economy as well which the economics of abrasive rnachin- Case hardened gears are usually as the product efficiency. However, ing of gear teeth may be realized. It has employed in high speed and high load distortions OCCUI during the hardending been demonstrated that a 16/32 DP in- units which require maximum durability process which can be corrected only by ternal spline, which required 12 hours to and reliability as well as high power per- abrasive machining or grinding. shaper cut ina very high nickel-chrome formance. Aircraft, missile systems, alloy, could be form ground from the truck and automotive transmissions, solid i.napproximately 4 hours. In addi- marine speed reducers, gas turbine gear- tion to the time saved, a more accurate ing, farm and off-the-road equipment. spline was produced at considerably less and machining tools are typical of AUTHOR: cost. durable reliable high performance case hardened gear applications. MR. JOHN M. LANGE is the Vice Presi- Gear tooth and root surfaces may be dent of Miller Associates lnc., the United States Medjum Hardness Ground Gears hardened locally by carburizing, Agent for Maag Gear lI\IheelCo. Ltd .. Zurich. Switzer/and. Mr. Lange joined Miller Through-hardened alloy gears in the nitriding, induction hardening, (and a. Associates in 1969 after graduating from range of Rockwell C 4S to 55 are usual- number of shallow surface hardening Carthage College in Kenosha. WI with a BA ly cut, hardened and then finish ground methods which improve resistance to in Business. His gear training began through or honed as required to obtain specified wear, but not the bending strength of enrolling in an apprentice program at Maags accuracy and surface finish. Although fatigue life at the critical root fillet plant in Zundl in 1971. and has continued ever since through his exposure to the Gear gears harder than Rockwell C 40 are not section.) technology of hundreds of customers nation- usually cut, it is possible to shaper cut wide. Mr. !.ange has presented papers at SMEs through hardened gears up to about Gear Processing Manufacturing Seminars and Rockwell C so. This procedure is very Carburized And Hardened AGMAs Gear Manufacturing Semposiums. costly due to rapid cutter wear and Ground Gears He is presently ,active in AGMA as a member of.the Gear Manufacturing Committee and the should be restricted to those applications Due to their high impactand bending Chairma» of the Metric Resource and Ad- where grinding after hardening is not load capacity and their good wear resis- visory Committee. possible .. tant properties, carburizedand hardened 34 Gear Jechnol'ogy gears are the most widely used case hardened gears, where rugged high per- formance is required. 'Gears may be hardened all over, or masked off during carburizing to, prevent hardening of the hub, web, and rim areas, Carburized gear blanks are frequently copper plated all-over prior to cutting the teeth and finish machining those surfaces to be hardened, such as functional bearing or mounting surfaces. The gear is then heated above the critical temperature, approximately 1700DF. in a carbon pack, sah bath or a carbon atmosphere, and held for the time required for the exposed machined surfaces to absorb carbon to Fig. lA-Shallow case depth in root fillets. the desired depth. The carburized case depth may vary from 0.010 to 0.020 inch for 20 diametral pitch gears, up to 0.025 to 0.035 inch depth for U diametral pitch, and 0.075 to 0.100 inch depth for gears of 2 diametral pitch and coarser. A carburized gear may be hardened by quenching directly from the carburizing furnace, or it may be slew ceo led in a protective atmosphere, then reheated in a. protective atmosphere and quenched for grain refinement and case hardening. The case wiU quench out at about Rockwell C 63 to 65 and is then tempered to RockweU C 58 or 60 te 63 before finish grinding or honing. Since about 85% of the distostion takes place during car- hurizing, it issomet.imes advantageous to Fig. ] B - Excessive case removal in .root fillet reducesbending sf rength. temper the carburized gear below Rockwell C 40, then finish shave prior to hardening. This procedure may make it possible to hone for surface finish only, gear can be indicated after hardending shows the resulting removal of most .of or mlnimizethe necessarygrinding stock. and used to locate the centers, the bore, the case hardened fillet during grinding. Most automotive transmission gears or the operating journals which will Note the thinness of the hardened section are small and of simple design, which determine the gear axis during additional at the root fillet of the gear tooth which makes it possible to shave them prior to operations including gear tooth grinding. will fail in bending fatigue due to insuf- carburizing and hardending. They only Since the pitch circle. distored in car- ficient case depth in this cnticalarea. require honing to correct minor heat treat burize and hardening, may be difficult to Sharp root fillets or steps should be distortions. Unljke grinding, honing is define, a proof circle near the root avoided whenever possible, especially on not a meta] removing process and should diameter, and a reference face perpen- case hardened gears .. he used only to improve surface finish dicular to the axis, should be provided Fig. 2(A) illustrates schematically a and correct very miner thermal distor- to facilitate optimizing the concentricity protuberance-cut full-fillet gear tooth and tions. Honing will also remove handling and squareness of the gear teeth before shows that the tendency towards a lean nicks and burrs. finish grinding. Another popular ap- case in the fillet is greatly reduced. View Other high speed high performance proach is to use a pitch line chuck with 2(8) shows that adequate case depth re- gears are cut, carburized. hardened, and either three or six pins to re-establish the mains after grinding in the root and Hllet finish ground, The grinding stock should center line in relation to the pitch circle. of a carburized protuberance-cut full- not exceed 0'.005 to 0.007 inch for case It is absolutely essential that excessive fill et gear. depths up to 0.025 to 0.035 inch and case removal in.the root fillet be avoided. 0.010 inch in the 0.075 to 0.100 inch Fig. I(A) illustrates the shallow case Fig. 3(A) and 3(8) Illustratea pro- depth range .. which occurs in a sharp root fillet asa tuberancecutand a full .fillet after grin- In order to minimize the grinding stock result of the small entrance area in the ding only the tooth profiles. These roots removal. the nominal pitch circle of the fillet during carburizing. View l(B) remain unground . .fully hardened and. Moren/April 11985 35 peening the tooth and fillet surfaces before finish grinding of the tooth pro- file, only, Nitrided Case Hardened and Ground! Gears Nitriding steels contain nitride-forming elements such as aluminum, chromium, molybdenum, vanadium or tungsten which combine with atomic nitrogen (N) when heated from 925°F to llOO°F in an atmosphere of dissociated ammonia. The nitriding cycle requires 25 to 48 hours to produce a 0.015 to 0.022 inch deep case, which is very hard - Rockwell 15N 90 to 93 - and highly resistant to wear. The Fig. 2A - Protuberance cut full fillet improves case depth in fillet. hardness of the relatively thin case drops off rapidly leaving a hard surface which is not well suited for application where impact loadings will be encountered. Since nitriding is done at a temperature below the critical, heat treat distortions are minimum. In many cases, grinding after nitriding would not be necessary ex- cept to insure removal of the "white layer" from the tooth and fillet surfaces.