Title Method of Forming a Gear

Field of the Invention

The invention relates to a method of spin forming a gear having teeth on a radially inward oriented surface.

Background of the Invention Annulus gears are an integral part of most automotive transmissions and are also used for many other industrial applications. These gears are required to have very accurate dimensions as well as a hardened structure to be able to transfer power efficiently without generating noise and without failing under the application loads.

Conventional methods of manufacturing annulus gears include broaching, shaper cutting, or grinding the gears. Each of these are very expensive processes with long cycle times, namely because each tooth is made individually .

Representative of the art is U.S. patent no. 3,777,345 to Brown which discloses a toothed tool for finish forming, by rolling, a cylindrical or helical work gear, said tool having, in the flanks of its teeth, serrations extending perpendicular to the axis of the tool which define flutes in the tooth flanks and intervening lands, the flutes on each tooth flank extending alternately from the tip towards the root and from the root towards the tip and extending along part only of the length of the tooth flank and being so disposed that corresponding flutes on successive tooth flanks are disposed helically on the circumference of the tool.

What is needed is a method of spin forming a gear having teeth on a radially inward oriented surface. The present invention meets this need.

Summary of the Invention

The primary aspect of the invention is to provide a method of spin forming a gear having teeth on a radially inward oriented surface .

Other aspects of the invention will be pointed out or made obvious by the following description of the invention and the accompanying drawings.

The invention comprises a method of forming a gear comprising gripping a blank to be spun formed in a spin forming machine, spin forming a gear having helical teeth disposed on an inner surface, and ejecting the gear by simultaneous extension and rotation of an ejector member and thereby rotation of the gear being ejected.

Brief Description of the Drawings The accompanying drawings, which are incorporated in and form a part of the specification, illustrate preferred embodiments of the present invention, and together with a description, serve to explain the principles of the invention. Fig. 1 is a cross sectional view of a blank.

Fig. 2 is a cross-sectional view of a spun formed part .

Fig. 3 is a cross-sectional view of a trimmed spun part . Fig. 4 is a cross-sectional view of the forming mandrel assembly.

Fig. 5 is a detail of Fig. 4.

Fig. 6 is a detail of the mandrel assembly.

Fig. 7 is a cross-sectional view of Fig. 2 with an overspun portion.

Detailed Description of the Preferred Embodiment Fig. 1 is a cross sectional view of a blank. The first step in manufacturing an annulus gear is spinning a blank, also known in the art as flow forming, on a mandrel. The blank 10 may either comprise a tube or a round flat blank stamped from sheet (plate) metal. The blank material can be chosen from a variety of micro alloys or alloy steels, although for lower load applications carbon steels or low carbon steels can also be used. Some of the blank materials that can be used include, but are not limited to, designated European steel grades including HLB 22, HLB 27, and 27MnV6. Various US steel grades are applicable as well.

Fig. 2 is a cross-sectional view of a spun formed part. Part 20 comprises teeth 21 disposed on a radially inward inner surface. In this embodiment teeth 21 are helical for use in transmissions and the like or any application requiring a helical gear. Teeth 21 have an angle α to a rotational axis. Angle α may be any required by a user. In this embodiment angle α is approximately 20°. Teeth 21 may also be straight cut as required by a user.

Skirt 22 is an artifact of the spin forming process. Skirt 22 may be removed or further shaped in a subsequent trimming step. Trimming may be accomplished by cutting or other suitable process known in the art . Fig. 3 is a cross-sectional view of a trimmed spun part. After spinning, the extra material around the gear skirt 22 is trimmed to the desired shape 220 and the gear is ready for heat treatment. For example, lugs 221 may

be stamped or cut in the part depending upon the desired service .

Portion 223 is also removed by trimming to axially "daylight" teeth 21. Removal of portion 223 may or may not be required depending upon the intended service for the finished gear.

Different practices recommended for the heat treatment process to achieve the required hardness with minimal dimensional distortions include heat treating the core and carburizing the case, heat treating the core and carbo-nitriding the case and through hardening. The heat treatment process utilizes a through hardening process where the material is hardened all the way through its core to a hardness of 30 to 40 Rockwell C (but other hardnesses can be chosen) . A case hardening is also used where a layer of about 0.75 mm (0.030") thick on the surface is hardened to a hardness of about 60 Rockwell C

(but other hardnesses can also be chosen) . This process, which is a known art, gives the part the proper strength and ductility throughout its cross section in addition to a hardened case or skin.

After heat-treating, the part is finished using known machining methods to the tolerances required for transmission applications and to remove the distortion caused by heat treating. The average amount of distortion caused by heat treatment is about 0.2 mm

(0.008") for parts that are about 150 to 200 mm (6 to 8 inches) in diameter. The tolerances for annulus gears are usually very tight and are usually in the hundredths of millimeters range. The finishing process comprises diamond broaching where a broach tool comprising tool steel and covered with diamond particles embedded in chromium or nickel is used. The diamond broach moves by advancing axially for a distance of about 2 to 6 mm, for

example, and then axially retracting a lesser distance of about 1 to 3 mm, for example. The oscillating or pulsing action allows the ground and shaved material to be washed away by the coolant/lubricant in the machine allowing a better, faster, and more accurate broaching operation. The amount of material that is removed is about 0.25 mm which is mostly the result of heat treatment distortion.

Another method that can be used for finishing is hard broaching where a broach needle made of tool steel is forced into the grooves with both axially linear and rotational movement to thereby broach the gear surfaces 21.

Fig. 4 is a cross-sectional view of the forming mandrel assembly. A tail stock 101 and headstock 102 cooperatively engage and grip blank 10. For reference purposes, Fig. 4 depicts the blank in a mandrel after it has been fully spun formed 20 (see Fig. 2) .

The tail stock 101 and headstock 102 are pressed together by a hydraulic cylinder 1010 to grip and rotate the blank. The end of headstock 102 comprises a toothed die 105. Teeth 21 are formed on the outer surface of die 105 as the blank material is spun formed into the die.

The headstock 102 and tailstock 101 rotate the blank. Rotation of the headstock 102 is caused by rotation of spindle 103 which also contains the non- rotating ejector shaft 108 within. Spindle 103 is rotated by a motor 200 which is connected to the spindle by a belt drive 201 or any other suitable drive known in the art. As the blank rotates a roller 104 (or two or more rollers) are radially pressed to the blank to form part 20, see Fig. 5. The ejector shaft 108 and ejector cylinder 1080 do not rotate, instead only the spindle 103, head stock 102 and tail stock 101 rotate.

Once the part 20 is formed it is ejected from the die. Ejector shaft 108 engages part 20 by extension of ejector cylinder 1080. As the ejector cylinder 1080 is axially extended, pin 106 in shaft 108 engages a slot 107 in portion 1070. This causes the ejector shaft 108 and part 20 to rotate, for example, through approximately 20° as shaft 108 extends. Simultaneously, tail stock 101 is retracted to allow ejection. Rotation of part 20 as it is ejected causes teeth 21 to disengage and to be pressed clear of die 105. The rotation angle (or amount of rotation) of shaft 108 substantially matches the angle of the helical gears formed in part 20 to facilitate ejection.

Spinning can be performed on an individual part or a plurality of parts simultaneously. In the case where they are formed simultaneously a single toothed blank is formed in a part as described herein. The intermediate part is then removed from the mandrel and cut to make individual annulus gears. Namely, the formed intermediate part is an elongated version of part 20 as described herein. The elongation is determined as a function of the number of gears to be cut from the formed intermediate part. For example, if three formed gears are desired, the intermediate part has a length totaling the combined length of the three gears to be cut therefrom.

Furthermore, the gear can either be flow formed individually or in combination with other rotating parts of the transmission in a stacked assembly manner to reduce the number of separate parts required. Fig. 7 is a cross-sectional view of Fig. 2 with an overspun portion. A formed gear 20 either before or after heat-treating and finishing can be placed on other flow or spin forming tooling and another component 500 can then be spun over the part 20. Namely, flow forming

of an overlaid component 500 is accomplished on the outer surface 222 of the otherwise spun formed part 20. In this case, to assure a permanent lock between the part 20 and the outer component 500, one of many engagement portions 501, i.e., grooves, splines, blind ended screw threads, and so on can be spun formed, broached or machined on the outside surface 222. In the alternative, component 500 may be cast in place on surface 222 of part 20. In another alternative component 500 may be made separately and then pressed onto surface 222 of part 20. In either of the foregoing alternatives, the engagement portions 501 may or may not be used depending on the service conditions for the finished component.

Fig. 5 is a detail of Fig. 4. Roller* 104 comprises an outer surface 1040. Surface 1040 comprises a shape suitable to form the outer surface 222 of the gear 20. One, two, three, or more shaping rollers 104 may be used simultaneously, although the preferred arrangement is three rollers located in 120 degree arrangement. The three roller configuration avoids creation of any out-of- roundness as the part is being spun formed. Rollers 104 may be moved vertically and horizontally in the "x" or "y" direction during the spin forming process.

Fig. 6 is a detail of the mandrel assembly. Pin 106 is guided by slot 107 as part 20 is ejected. As the part is ejected is rotates in direction "R". Once the part 20 is ejected a new blank 10 can then be placed in position and the process repeated. Ejector shaft 108 is guided within and slides within head stock 012. Although a form of the invention has been described herein, it will be obvious to those skilled in the art that variations may be made in the construction and relation of parts without departing from the spirit and scope of the invention described herein.