BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic illustration of mating gear elements of a representative planetary gear set;
FIG. 2 is a fragmentary schematic illustration of a mating gear teeth each having an involute profile and root portions according to the invention;
FIG. 3 is a schematic perspective illustration of an improved ring tool and mating gear element according to the invention; and
FIG. 4 is a fragmentary schematic illustration of a burnishing tooth portion of the improved ring tool.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to the drawings wherein like reference numbers correspond to like or similar components throughout the several figures, there is shown in FIG. 1 a gear set 10, shown here as a representative planetary gear set, consisting of a first external gear element 12, a first internal gear element 13, and a plurality of second external gear elements 14. Although depicted in FIG. 1 as a planetary gear set, gear set 10 may also take the form of, for example, a linear gear train or a more complex planetary gear set having a greater number of meshing gears. Meshing first and second external gear elements 12, 14 are shown in FIG. 1 as a representative sun gear and pinion gear, respectively, which will be used hereinafter for the purpose of explaining the invention. However, the invention would be equally suitable for any pair of mating gear elements within a gear set or gear train.
As shown in FIGS. 1 and 2, first and second external gear elements 12, 14 have a plurality of gear teeth 16a, 16b, respectively, extending radially outward from the gear elements 12, 14. Gear teeth 16a, 16b are equally spaced around the outer perimeter of gear elements 12, 14. Gear element 12 has a plurality of gear teeth 16a that are mutually meshable or engageable with the plurality of identical meshable gear teeth 16b of mating gear element 14. Once so mutually engaged, gear teeth 16a, 16b then cooperate to transmit rotational motion, as represented by the various arrows 18 in FIG. 1. The profiles of gear teeth 16a, 16b are preferably the involute profile common to vehicle transmissions and other high-speed, high-stress systems. Such an involute profile is shown in greater detail in FIG. 2.
The plurality of gear teeth 16a of gear element 12 each have a pair of opposing active surfaces or flanks 20a, the flanks 20a encompassing the entire expanse of contact surfaces between the mating gears. These contact surfaces are represented in FIG. 2 approximately as the line or path AB. Likewise, identical gear teeth 16b each have a pair of flanks 20b opposable with the flanks 20a of gear teeth 16a, with the contact surfaces of flanks 20b described approximately by the line or path CD. Gear element 12 has a generally or substantially semi-circular root portion 22a having a plastically-deformable root surface 24a, shown as a dotted line, disposed between each of gear teeth 16a. Likewise, gear element 14 has a preferably identical generally or substantially semi-circular root portion 22b having a plastically-deformable root surface 24b, shown as a dotted line, disposed between each of the gear teeth 16b.
As mating gear teeth 16a, 16b come into direct dynamic contact and engage, revolve, and subsequently disengage through one complete rotational cycle, mutual opposing force is brought to bear on the active surfaces or flanks 20a, 20b of mating gear elements 12, 14, respectively. Bending fatigue may therefore appear below or radially inward of the flanks 20a, 20b of the respective root portions 22a, 22b due to the mutual opposing force or tensile stress exerted on the flanks 20a, 20b, as represented by arrows 21a, 21b of FIG. 2. Material failure or crack propagation could result approximately along the direction of arrows 21a, 21b unless roots 22a, 22b of gear elements 12, 14 are strong enough to resist the tensile stresses. In accordance with the invention, therefore, the life of gear set 10 is increased by hardening the root portions 22a, 22b to create a plastically-deformed root surface 24a, 24b to thereby increase the bending strength of gear teeth 16a, 16b, respectively, without also thereby contacting the flanks 20a, 20b, and/or altering the geometry of the flanks 20a, 20b.
Looking again to FIG. 2, gear elements 12, 14, have substantially semi-circular root portions 22a, 22b, respectively, each having a plastically-deformable root surface 24a, 24b, as previously described herein. The root portions 22a, 22b are typically milled or hobbed out of a gear blank during the initial gear manufacturing process, with the kinematics of the gear hobbing process alone typically determining the final geometry of the root portions 22a, 22b. By contrast, in accordance with the invention, geometry of the root portions 22a, 22b are determined by broaching and burnishing which isolates hardening to the root portions 22a, 22b of gear elements 12, 14.
To isolate hardening to the root portions 22a, 22b, an improved forming tool or ring tool 30 is provided as shown in FIGS. 3 and 4. Ring tool 30 preferably has a circular perimeter 28 and a plurality of radially-inwardly projecting burnishing teeth 32. Referring to FIG. 4, each burnishing tooth 32 has an identical, generally semi-circular and substantially convex hardened forming surface 38 configured to plastically-deform the opposing root portions 22a, 22b (FIG. 2) and to broach or cut any excess material from the surfaces of mating root portions 22a, 22b. Plastic deformation is provided generally by forming or coating the plurality of burnishing teeth 32 from or with material having a suitably elevated surface hardness relative to the surface hardness of the root portions 22a, 22b of the gear element being formed. Broaching is provided generally by forming a cutting surface or broaching edge 36 (see FIG. 4) along the entirety of the forming surface 38 of burnishing teeth 32.
More specifically, the broaching capability of the invention is provided by a projecting or protruding broaching edge 36 as shown in FIG. 4, the broaching edge 36 spanning approximately the entire span of generally or substantially semicircular burnishing edge 38 and being configured to remove excess stock material from the gear element being formed. Preferably, the broaching edge 36 of each of burnishing teeth 32 is provided by way of a slightly concave face 44 ringed or surrounded by a suitably sharp cutting surface, as shown in FIG. 4. Removal of excess material is accomplished as the broaching edge 36 cuts, shaves, or broaches material in excess of the center diameter 42 of the ring tool 30 as the gear element being formed passes through the ring tool 30, as with representative gear element 12 in FIG. 3. Broaching of excess gear element material, specifically excess material around the root portions 22a is necessary in order to prevent damage to the ring tool 30 and/or gear element 12, due to the presence of excessive material in and around the root portions 22a. Excessive material may be present in the root portions of the gear elements being formed such as 22a, 22b of gear elements 12, 14 due to the larger tolerances used in the formation of the root portions 22a, 22b during manufacturing of the gear blank (not shown), or the material may be left behind after the initial hobbing process used to form the root portions 22a, 22b. Also, heat applied during initial hardening heat treatment may expand and distort the gear blank after formation, increasing the geometry of the gear element until the geometry is no longer within tolerance. Because damage may occur to either or both the gear element and ring tool 30 as the larger than desirable gear element being formed is forced through the ring tool 30, broaching surfaces 36 operable to cut or broach the excess material and thereby prevent such damage are provided in accordance with the invention.
Ring tool 30 also provides a burnishing capability and therefore is appropriately sized and shaped to impart to the gear element to be formed the desired depth of plastic deformation. In FIG. 3, gear element 12 is used as an exemplary or representative gear, although one skilled in the art will recognize that gear element 14 or another external gear element may likewise be used as explained hereinafter. An external gear such as gear element 12 is passable under an external force appropriate for providing a predetermined level of plastic deformation to the root surfaces 24a of contacted root portions 22a. The plastically-deformed layers 24a of root portions 22a of gear teeth 16a, as well as the plastically-deformed layers 24b of root portions 22b of gear teeth 16b, are shown as dotted lines in FIG. 2. As ring tool 30 performs as a forming tool, those skilled in the art of tool making would recognize various methods for holding ring tool 30 stationary with respect to the gear element being formed. For instance, ring tool 30 may be secured within a manual fixture or within a fixture-portion of a piece of capital equipment such as a press, while the gear element, such as gear element 12 of FIG. 3, is moved through the ring tool 30 under a force suitable to impart the desired level of plastic deformation to the root surface 24a.
Variables that will affect the final depth of the plastically-deformable root surfaces 24a, 24b and the geometry of root portions 22a, 22b include both the relative hardness and the geometrical variance and/or tolerance between the mating ring tool 30 and the gear element being formed. Ideally, the gear element being formed is heat-treated prior to burnishing to prevent the loss of the residual stress benefits of burnishing due to martensitic transformation. Likewise, the amount of force used to move the gear element through the ring tool 30 is a function of the desired predetermined level of broaching and/or burnishing, and may be manipulated to produce the desired surface hardness on the root surface such as 24a of gear element 12. As stated earlier herewithin, to ensure proper hardening of the root surfaces such as 24a, the ring tool 30, and in particular the plurality of burnishing teeth 32, are formed of a material having sufficiently greater surface hardness than that of the gear element being hardened, so as to adequately plastically deform the root surface 24a, 24b as a result of mutual, forceful contact between the ring tool 30 and the gear element.
Preferably, at least the forming surface 38 portion of burnishing tooth 32 is constructed using carbide or a suitable high-speed tool steel having Rockwell C hardness (Rc) of at least approximately 5-10 Rc greater than that of the gear element being formed. As representative external gear elements 12, 14 are preferably constructed of 5120 steel or other suitable material having a hardness of approximately 55-65 Rc, the preferred hardness of the mating burnishing teeth 32 is approximately 60-75 Rc, although harder burnishing teeth may also be provided using specialized 75 Rc or harder grades of carbide. Non-forming surface 40 of burnishing tooth 32, describing the expanse of gear tooth surface not including forming surface 38 and represented approximately by the line traced along the curve of the burnishing tooth 32 between points E and F in FIG. 4, does not make contact with any surface of the gear element being formed during the broaching or burnishing process. Therefore, hardening of non-forming surfaces 40 is not essential, although equal hardening of all material surfaces of ring tool 30 is desirable in order to ensure the overall structural integrity of the ring tool 30.
The size of ring tool 30 and the number, shape, and size of burnishing teeth 32 are dependent on the design of the gear element to be formed. That is, as ring tool 30 is a tool or mold through which a formed and heat-treated gear element such as 12 is passed, the geometry of ring tool 30 is configured to match the geometry of the gear element 12 to be hardened. For example, as shown in FIG. 3, if gear element 12 were to have twenty separate gear teeth 16a, a ring tool 30 having twenty separate opposing burnishing teeth 32 would be required, with ring tool 30 being appropriately sized to allow a pre-selected mating gear element 12 to pass through ring tool 30 under the required force necessary to affect the desired changes to the root portion 22a.
As previously discussed herewithin, mating rotating gear elements such as gear elements 12, 14 create mutual tensile stress which may then manifest itself as bending fatigue radiating outward from root portions 22a, 22b, respectively. Each of gear elements 12, 14 possesses a predetermined material strength. If the tensile stress imparted to the gear tooth 16a, 16b exceeds such material strength, one or both of the gear elements 12, 14 will fail in the form of crack initiation, which may then propragate with the continued application of tensile stress. By adding compressive residual stress to the root portions 22a, 22b of representative gear elements 12, 14, the strength of material of the gear elements is thereby increased, with the object of increasing the strength of material to a level above that of the tensile stress imparted on the mating gear elements 12, 14. The use of ring tool 30 as herein described produces the required compressive residual stress in the form of plastic deformation of the root surfaces such as 24a of a gear element such as sun gear 12 after the gear element 12 has passed through the improved ring tool 30.
Those skilled in the art will recognize that excessive plastic deformation may damage the gear element, while insufficient plastic deformation will not produce compressive residual stress levels sufficient to elevate the strength of material of the gear element enough to prevent damage from bending fatigue. To strike the desired balance, the preferred depth of plastic deformation of plastically-deformable root surfaces 24a, 24b in accordance with the invention is approximately 2 to 5 micrometers (μ). To impart such a level of deformation, the profile of gear elements 12, 14 after qualifying broaching are approximately 2 to 5μ larger than the profile of the ring tool 30. In this manner, passing the softer gear element through the harder ring tool 30 will result in the desired 2 to 5μ deformation of the root portions 22a, 22b.
While the invention has been described previously herewithin in relation to an external gear element, those skilled in the art will recognize that the invention is equally applicable to internal gear elements such as ring gear 13 of FIG. 1 by reversing the orientation of burnishing teeth 32 of ring tool 30, that is, by so configuring outwardly-projecting burnishing teeth 32 to burnish only the root portions of the teeth of the ring gear 13.
While the best modes for carrying out the invention have been described in detail, those familiar with the art to which this invention relates will recognize various alternative designs and embodiments for practicing the invention within the scope of the appended claims.
Patent Application
20080032851
