Manufacturing of gears with tooth tip and / or root modifications

Abstract

A method for generating a tooth profile surface on a workpiece includes providing a gear cutting tool and rotating the gear cutting tool about an axis of rotation. The cutting tool and the workpiece are engaged and roll together following generating rolls to generate the tooth profile surface. The generating rolls include a first roll ratio and a second roll ratio, the first roll ratio and the second roll ratio being different from each other, and one of the first roll ratio and the second roll ratio resulting in a modified section being formed on the tooth profile surface.

Description

[Technical Field]

[0001] The present invention relates to the manufacture of gears, in particular straight bevel gears, and to providing tip and / or root modifications on the tooth flanks of the gears. [Background technology]

[0002] In the manufacture of gears, particularly bevel gears, two types of processes are commonly used: generating and non-generating. Both processes can be divided into two categories: face milling (intermittent indexing), in which one tooth slot is formed with each plunge of the tool, and face hobbing (continuous indexing), in which the tool and workpiece rotate in a timed relationship and the tool is fed to a depth whereby all the tooth spaces are formed with a single plunge of the tool.

[0003] In the generating process, a rotating tool is fed into the workpiece to a predetermined depth. Once at this depth, the tool and workpiece roll together in a predetermined relative rolling motion known as generating roll, as if the workpiece were rotating in mesh with a theoretical generating gear, the teeth of which are represented by the stock removal surface of the tool. The tooth profile is formed by the relative motion of the tool and workpiece during generating roll.

[0004] A non-generating process is one in which the tooth profile on the workpiece is produced directly from the profile on the tool. The tool is fed into the workpiece and the profile on the tool is imparted to the workpiece. No generating rolls are used.

[0005] Differential gears are typically straight-tooth bevel gears with a small number of teeth. Differential gears are coarse-pitch ("pitch" is the distance between similarly evenly spaced tooth surfaces along a given line or curve) and typically have a pressure angle of about 25° or greater. The term "coarse pitch" is used when the number of teeth relative to the gear diameter is small. For example, 10 teeth on a gear with a 100 mm diameter (module = 100 / 10 = 10 mm) is considered coarse-pitch, while 10 teeth on a gear with a 30 mm diameter (module = 30 / 10 = 3 mm) is considered fine-pitch. Those skilled in the art generally consider teeth (or gears) with a module less than 5 mm to be "fine-pitch" and teeth (or gears) with a module of 5 mm or greater to be "coarse-pitch."

[0006] 1 shows an example of a straight bevel differential gear 2 having a plurality of teeth 4, each tooth having a top land 6, a root portion 8, and a pair of tooth flank surfaces 10. The area 11 between a pair of consecutive teeth is known as a tooth "slot" or "space," and the root portion 8 coincides with the bottom of the tooth slot.

[0007] Differential gears have been cut in a variety of ways. FIG. 2 illustrates cutting a straight bevel gear with a pair of inclined rotary disc cutters 12, 14 (commonly referred to as the upper and lower cutters, respectively) having cutting blades 16 for cutting tooth slots in a workpiece 18. Cutter 12 is rotatable about axis 22, and cutter 14 is rotatable about axis 20. In a generating process on a conventional mechanical cradle-type machine, the inclined cutters 12, 14 are typically fed into the workpiece to a predetermined depth, and generating rolls on the machine cradle (not shown) are initiated synchronously with the rotation of the workpiece 18 to generate tooth profile surfaces 24, 26. The rotating cutting blades 16 are effectively ganged together to simultaneously cut the same tooth slots (e.g., U.S. Pat. No. 2,567,273).

[0008] 3 and 4 illustrate an example tool and method for manufacturing straight bevel differential gears using a large circular cutter 30 having a cutter diameter of, for example, 18 inches, 21 inches, or 25 inches (460 mm, 535 mm, or 635 mm). See, for example, U.S. Pat. No. 2,267,181, the entire disclosure of which is incorporated herein by reference. The cutting blades 32 are oriented around the periphery of the cutter body 34, as seen, for example, in FIG. 3, and are classified as roughing, semi-finishing, and finishing blades. The cutter operates in a single indexing process, performing only one revolution while cutting a complete tooth slot of the gear 38. The cutter is positioned at the toe end of the tooth slot and moves from toe to heel during the roughing and semi-finishing portions of the cycle in a conventional cutting process. When the cutter reaches the heel end of the tooth slot, all of the roughing and semi-finishing blades have been used. The cutter then returns to the toe end to finish the tooth slot with the finishing blades in an upcut process. Between the last finishing tooth and the first roughing tooth there is a larger space 36 which allows the machine to index the workpiece to the next tooth slot location without having to stop the cutter from rotating and without requiring a retracting motion of the cutter. The tool material is preferably high speed steel and the applied surface speed is typically 20-40 m / min, which makes this cutting process a type of broaching process.

[0009] The fact that one cutter revolution finishes one slot, including indexing time, makes the circular broaching process described above extremely fast. Compared to cutting straight bevel gears with a pair of inclined rotary cutting tools (Figure 2), where the rotating cutting blades effectively interlock to simultaneously cut identical tooth slots using the same high-speed steel tool material, the cutting time of the circular broaching process is a small fraction (e.g., 15-20%) of the interlocking rotary tool process.

[0010] A disadvantage of the circular broaching process is that the workpiece tooth profile is formed by a profile cutting process, which does not allow for the creation of precise octoid tooth profile shapes for conjugate engagement with low kinematic error. Another disadvantage is that the circular broach cutting edge profile is circular instead of involute or involute approximation. Yet another disadvantage of circular broaching is that the process lacks the degrees of freedom available for flank form correction. Profile cutting with a circular cutting edge profile generates a certain amount of length crowning (i.e., in the direction of the tooth length). Selection of the tooth surface profile curvature radius can create profile crowning. The profile (i.e., tooth height, root-to-tip direction) crowning must be large enough to hide any kinematic inaccuracies present due to the profile cutting process. Fine-tuning the tooth surface to optimize rotational performance is nearly impossible without redefining the cutting edge profile and manufacturing a new cutter.

[0011] After heat treatment, the gear tooth surfaces cut by the circular broaching process are not hard finished and are used with distortion from the heat treatment process. This is sufficient for most practical applications. However, with increasing demands for high power density and quiet operation coming from manufacturers of electric vehicle drivetrains, the need for hard finishing operations is becoming a requirement in many applications.

[0012] Another method for manufacturing differential gears that made industrial advances in the 1970s is forging. In forging, a steel billet with a temperature exceeding 2000°F (1093°C) is pressed into a hard steel die. The die has a negative shape of the toothed side of the differential gear. The bore and backside of the forged part are machined after the forging process. Some forging processes apply calibration as a finishing process. Calibration is performed after forging to improve the surface finish and tooth indexing quality. Today, forging achieves high-quality differential gears in a very cost-effective manufacturing process. Advantages of forging are low manufacturing costs, the production of parts with high bending and impact integrity, and the possibility to apply modifications such as the placement of reinforcing webs at the toe and heel bases, as seen in the gear set of FIG. 5 with pinion 40 and side gear 42 (sometimes referred to as the "gear" member of the differential gear set). Some disadvantages are that the stiffening webs restrain the teeth from elastic bending, which can lead to surface damage such as pitting and even cracks at the tooth root under high load conditions. Additionally, the presence of the stiffening webs makes the root line impossible to machine by any of the prior art gear machining processes because it is not straight or curved. Machining must be performed by slow processes using, for example, ball nose end mills and multi-axis machining centers.

[0013] Forged gears have a thin outer layer of scale, which has a higher hardness and a different steel structure. Forging scale also contributes to surface fracture under high loads. Forged gears have some variation in tooth thickness between the beginning and end of the die tool life. This variation results in backlash that changes after assembly and cannot be controlled. Differential gears forged at the beginning of the die tool life are too tight, reducing efficiency. Gears forged at the end of the die tool life have excessive backlash, which leads to rattle and excessive drivetrain backlash.

[0014] Yet another method for manufacturing straight bevel differential gears is disclosed in U.S. Pat. No. 7,364,391, the entire disclosure of which is incorporated herein by reference. It involves a one-sided cutting process in which the first flanks of all of the teeth are rough-cut and finish-cut in a first step (e.g., FIG. 6(a)), and then the cutter position is changed to finish-cut the second flanks of all of the teeth in a second step (e.g., FIG. 6(b)). The two-stage process may be performed on a computer-controlled multi-axis gear manufacturing machine, such as that disclosed in U.S. Pat. No. 6,712,566, the entire disclosure of which is incorporated herein by reference. The two-stage process produces precise involutes (octoids) and allows for flank form modification. After heat treatment, the differential gears can be ground using a CBN grinding process in a manner similar to cutting. The two-stage process offers various advantages over the circular broaching or forging methods described above, particularly for differentials for electric vehicle drivetrains. The disadvantage of the two-stage process is its lower productivity for differential gears compared to circular broaching or forging.

[0015] A suitable cutting tool for carrying out the above-described two-stage process is shown in Figure 7, which shows a peripheral cutting tool 50 removably secured to the spindle 48 of a machine tool (not shown), such as that disclosed in U.S. Patent No. 6,712,566. The cutting tool 50 includes a cutter head 52 having a plurality of stick blades 54. A clamping block 56 is positioned above each stick blade. The cutter of Figure 7 has an upper ring 58 above the clamping block 56 with an integral clamping screw 59.

[0016] High power density and low rotational noise are the primary requirements for differential gears in electric vehicles. State-of-the-art differential gears, either cut by a circular broaching process, forged, or cut by a two-stage process, typically have large profile crowning, which leads to large motion errors (300 μrad to 2000 μrad) and low effective contact ratios (ε less than 1.0). γ ) results. Summary of the Invention

[0017] The present invention relates to a method for generating teeth on a workpiece, the teeth having a tooth profile surface including tip and root portions, a toe end, a heel end, and a face width extending between the toe and heel ends. The method includes providing a gear cutting tool and rotating the gear cutting tool about an axis of rotation. The cutting tool and the workpiece are engaged and roll together following generating rolls to generate the tooth profile surface. The generating rolls include a first roll ratio and a second roll ratio, the first roll ratio and the second roll ratio being different from each other, and one of the first roll ratio and the second roll ratio resulting in a modified section being formed on the tooth profile surface. [Brief explanation of the drawings]

[0018] [Figure 1] An example of a differential gear is shown. [Figure 2] FIG. 10 is a diagram of a circular broach cutter in the process of cutting a tooth slot for a differential gear, showing the interlocking arrangement of a pair of angled cutters cutting a tooth slot in a workpiece. [Figure 3] Circular broach cutter shown. [Figure 4] 10 is an image of a circular broach cutter cutting a differential gear tooth slot. [Figure 5] FIG. 1 shows a cross-sectional view of a forged differential gear set. [Figure 6(a)] A two-stage process for manufacturing straight bevel gears is shown. [Figure 6(b)] A two-stage process for manufacturing straight bevel gears is shown. [Figure 7] 6(a) and 6(b) show a peripheral cutting tool for carrying out the two-stage process of FIG. 6(a) and FIG. 6(b). [Figure 8] 1 shows an example of motion error and tooth contact of a differential gear pair. [Figure 9] The contact analysis of a differential gear pair without profile crowning is presented. [Figure 10] 1 shows a cutting edge profile with a straight edge modification. [Figure 11] 1 shows a cutting edge profile with rounded edge modifications. [Figure 12] 1 shows the contact analysis of a differential gear pair cut with cutting edge modification. [Figure 13] 1 shows a cutting edge profile with linear ridges. [Figure 14] 1 shows a cutting edge profile with curved ridges. [Figure 15] 1 shows the contact analysis of a differential gear pair cut with raised blades. [Figure 16] 1 shows a tooth tip modification with a modified roll ratio tooth tip section. [Figure 17] 1 shows the contact analysis of a differential gear pair cut with a modified roll ratio section. DETAILED DESCRIPTION OF THE INVENTION

[0019] The terms "invention," "the invention," and "the present invention" as used herein are intended to broadly refer to all of the subject matter of this specification and any claims that follow. Statements containing these terms should not be understood to limit the subject matter described herein or to limit the meaning or scope of any claims that follow. Furthermore, this specification does not seek to describe or limit the subject matter covered by any claim in any particular part, paragraph, statement, or drawing of this application. The subject matter should be understood by reference to this entire specification, all drawings, and any claims that follow. The invention is capable of other configurations and of being practiced or carried out in various ways. It is also understood that the phraseology and terminology used herein are for the purpose of description and should not be regarded as limiting.

[0020] The details of the invention will now be discussed, by way of example only, with reference to the accompanying drawings which illustrate the invention, in which like features or components are referred to by like reference numerals, and in which the size and relative sizes of certain aspects or elements may be exaggerated for clarity or illustrative purposes.

[0021] The use of "comprises," "has," and "having," and variations thereof herein is meant to encompass the subsequently listed items and equivalents thereof, as well as additional items. The use of letters or numbers to identify elements of a method or process is for identification purposes only and does not imply that the elements should be performed in a particular order. As used herein, the singular forms "a," "an," and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise, and the term "and / or" includes any and all combinations of one or more of the associated listed items.

[0022] Hereinafter, when describing the drawings, reference may be made to directions such as top, bottom, upward, downward, rearward, bottom, top, front, and rear, and these are referred to relative to the drawings (as they would normally be viewed) for convenience. These directions are not intended to be interpreted literally or to limit the invention in any way. Additionally, terms such as "first," "second," and "third" are used herein for descriptive purposes and are not intended to denote or imply any importance or significance unless expressly stated.

[0023] Figure 8 shows an ease-off graph for an example of a state-of-the-art differential gear pair at the top. Ease-off represents tooth flank form modifications, such as length crowning and profile crowning, which are required to prevent edge contact under load and in the presence of deflections and manufacturing tolerances. The magnitude of ease-off along the labeled profile section represents the profile crowning for this example gearset. This profile crowning is equivalent to the kinematic error shown in the center of Figure 8. The kinematic error is represented by three consecutive parabolas (representing three consecutive pairs of meshing teeth). The rated kinematic error value is the distance from the top line to the intersection of the parabolas. In this example, the rated kinematic error is 900 μrad. A large kinematic error increases operating noise and reduces power density. To reduce the kinematic error, the profile crowning must be reduced, which would produce a nearly conjugate profile. The tooth contact pattern is shown below the kinematic error graph. The tooth contact is centered between the heel and toe ends of the teeth.

[0024] FIG. 9 shows an ease-off graph for a differential gear pair with nearly conjugate profile sections at the top. The profiles were intentionally made not exactly conjugate to provide a small amount of profile crowning. Conjugate tooth profiles have no crowning in the profile direction, which results in zero motion error. The nearly conjugate profiles of FIG. 9 result in a motion error of 10 μrad. The lack of significant ease-off in the tooth profiles of FIG. 9 would result in edge contact at the pinion and gear tooth tips under load and deflection. The bottom portion of FIG. 9 shows the tooth contact pattern.

[0025] To avoid edge contact between the mating teeth, tooth tip modifications can be implemented on the pinion teeth and gear teeth. Cutting blades with a linear edge modification, as shown in FIG. 10, can be applied to achieve the tooth tip modification. FIG. 10 shows a cutting edge profile with the tip of the blade at the top and the end of the blade at the bottom. The edge modification begins at HKOW from the tip and has an angle DKOW. The starting point HKOW and the blade modification angle DKOW are selected to achieve the desired tip modification on the cut tooth. One example of a machine suitable for grinding cutting blades to apply the above-mentioned modifications is a Blade Profile Grinding Machine (BPG) commercially available from The Gleason Works (Rochester, New York).

[0026] Circular tip modifications are also possible, as shown in Figure 11. The tip modification has a starting point (located at HKOW from the tip of the tooth) and a tooth modification radius RKOW. The starting point HKOW and the tooth modification radius RKOW are selected to achieve the desired tip modification on the cut tooth.

[0027] Figure 12 shows the ease-off graph for a differential gear pair with pinion and gear cutting blade tip modifications at the top. Because the blade tips follow the root line of the respective gears, and because the top of the differential gear (face angle) tapers to the root line due to the tapered tooth depth of the spur bevel gear, the pinion blade tip modification creates a modification section that is large at the heel (compared to Figure 9) and reduces in magnitude toward the toe. Between the center and the toe, there is a position where the modification is zero. The modification at the top of the ease-off is created by the gear cutting blade, and the modification at the root is created by the pinion cutting blade. The motion error at the center of Figure 12 is 240 μrad because the profile section "cuts" away part of the modification area. The bottom graph shows the tooth contact pattern.

[0028] Another method of creating modifications on the tip and root ends of the ease-off is to use a blade with a ridge as shown in Figure 13, which shows the profile of a cutting blade with a straight ridge. The ridge has a start point (at HPRW from the tip of the blade) and a ridge angle DALW relative to the cutting edge. The start point HKOW and ridge angle DALW are selected to achieve the desired root modification on the cut tooth.

[0029] It is also possible to apply curved ridges, as shown in Figure 14, which shows the profile of a cutting blade with a circular ridge. The ridge has a starting point (HPRW from the tip of the blade) and a ridge radius DPRW. The starting point HKOW and ridge radius DPRW are selected to achieve the desired root modification on the cut tooth.

[0030] The ridges result in a nearly uniform undulation along the face width on both sides of the easy-off, as shown in Figure 15 (compare with Figure 9). However, root modifications are undesirable because they weaken the root and reduce power density. The kinematic error caused by the raised teeth is 310 μrad. All tooth modifications, such as tooth end modifications and ridges, require complex tooth profile modifications that are custom developed for one specific gearset design. Integration of teeth or cutterheads, as is commonly done for straight bevel gears, would not be possible after job-design-specific tooth modifications (end modifications or ridges) have been made.

[0031] FIG. 16 shows a cross-sectional view of a differential gear profile. The straight line represents the cutting edge profile of the cutting blade, which is a straight line without any modification. The two solid lines represent the cutting blade that produces a tooth profile with the correct (i.e., theoretical) roll ratio for a particular tooth. The "roll ratio" is the ratio between the number of teeth of the theoretical generating gear and the number of teeth of the gear being cut. At the start of the tooth tip modification, the roll ratio is changed, and the dashed line shows the effect of the modified roll ratio with the marked modification section. Straight bevel gear tooth profiles (e.g., involutes) are generally produced with a constant roll ratio. The method of the present invention is based on the change from a constant roll ratio to a modified roll ratio at a specific roll position.

[0032] FIG. 16 shows a schematic diagram of how a cutting blade profile generates an involute tooth profile with a first (e.g., constant) roll ratio (solid cutting edge line) and a tooth tip modification with a second (e.g., modified) roll ratio (dashed cutting edge line). The “modified” second roll ratio is a modification to the first roll ratio. The modified roll ratio begins at the start point of the modification ( FIG. 16 ) and ends at the tooth crest (or a slightly higher roll position), where the machine kinematics terminate the generating roll. This end position is often referred to as the starting roll position, as the machining process moves the cutter blade roll from tooth tip to tooth root. Both root-to-tip rolling and tip-to-root rolling are possible within the available cutting processes. The modified roll ratio may be constant (as opposed to the constant first roll ratio) or calculated as a higher-order function of roll position. For example, each polynomial can be written as a Taylor series expansion. RA=RA0·{1-(c / 2!)}·Δq-(d / 3!)·Δq 2 -(e / 4!) Δq 3 -(f / 6!) Δq 4 ...}(1) Δq=qq T (q S T ) → {q S ≦q≦q T} (q S >q T ) → {q T ≦q≦q S} During the ceremony, RA...effective roll ratio [-] RA0...Basic roll ratio [-] c...linear coefficient [1 / (°)] d...quadratic coefficient [1 / (°) 2 ] e...cubic coefficient [1 / (°) 3 ] f...fourth order coefficient [1 / (°) 4 ] ​Δq...Roll ratio difference [°] q...actual roll position [°] q T ...tooth tip modification start position [°] q S ...Top roll position (end position of tooth tip modification) [°]

[0033] Alternatively, for example, the modified roll ratio may be determined by a general high order polynomial, a spline function, a circular function, or an elliptical function.

[0034] The tip modification start location and coefficient magnitude are calculated or found experimentally to produce the desired amount of modification in the desired area.

[0035] Figure 17 shows an ease-off graph for a differential gear pair (i.e., gear and pinion members) manufactured using a modified roll ratio section. The top modification of the ease-off profile is the result of a tooth tip modification on the gear member, and the bottom modification of the ease-off profile is the result of a tooth tip modification on the pinion member. The modifications are larger at the heel and smaller at the toe, but they are present along the entire face width. The variation in the magnitude of the modifications along the tooth width is caused by the orientation of the generating marks, which have slightly different angles from the face angles of the respective members. In the context of straight bevel gears, this has practical advantages because it produces small motion errors and also provides good protection against edge contact toward the heel. Under high loads, contact extends toward the heel. The modified roll ratio section provides an ideal combination of a differential gear that runs quietly under low loads and prevents edge contact under both low and high loads.

[0036] The results of the tip modifications on the pinion and gear teeth are shown in Figure 17 (compared to Figure 9). The modifications are nearly uniform and appear similar to the ridge modifications in Figure 15. The resulting motion error is 13 μrad. The advantage of a tip modification that is larger toward the heel and smaller toward the toe is that when a load is applied, the contact area moves to the heel, and therefore more modification is desired at the heel. This ensures that the modification at the mid-face results in a lower motion error magnitude. The advantage of a tip modification over a root modification (ridge) is that no root weakening occurs, and the modified roll ratio can be optimized or changed simply by changing the respective machining parameters, without the need for expensive blade modifications. The graph below shows the tooth contact pattern for this example.

[0037] While the method of the present invention has been described and illustrated with respect to gear tip modifications, the present invention is equally applicable to producing gear root modifications. Additionally, the modified roll ratio may be further modified by adjusting machine settings during the generating roll, including cutter tilt, swivel, and / or cutter reference height. For example, a defined quadratic combination of cutter tilt and swivel may be utilized to provide cutter twisting motion to balance the modification effect between the heel and toe of the teeth.

[0038] Additionally, the method of the present invention also contemplates grinding. A grinding wheel is considered to be a cutting tool having an undefined cutting edge.

[0039] While the present invention has been described with reference to preferred embodiments, it should be understood that the invention is not limited to these particular embodiments. The present invention is intended to include modifications that would be apparent to those skilled in the art to which the present subject matter pertains without departing from the spirit and scope of the appended claims.

Claims

1. 1. A method of producing teeth on a workpiece, the teeth having a tooth profile surface including tip and root portions, a toe end, a heel end, and a face width extending between the toe end and the heel end, the method comprising: rotating the gear cutting tool about an axis of rotation; engaging the cutting tool with the workpiece; rotating the cutting tool together with the workpiece according to a generating roll to generate a contour surface of the tooth; the generating rolls include a first roll ratio and a second roll ratio, the first roll ratio and the second roll ratio being different from one another, and one of the first roll ratio and the second roll ratio resulting in a modified section being formed on the tooth profile surface.

2. The method of claim 1 , wherein the modification section includes a modification section at the tip portion of the tooth profile surface.

3. The method of claim 1 , wherein the modification section includes a modification section at the root portion of the tooth contour surface.

4. The method of claim 1 , wherein the first roll ratio is a constant.

5. The method of claim 1 , wherein the second roll ratio is a constant.

6. The method of claim 1 , wherein one of the first roll ratio and the second roll ratio is defined as a higher order function.

7. The method of claim 1 , wherein the tooth contour surface is an involute.

8. The method of claim 1 , wherein the modification section extends along the entire face width of the tooth.

9. The method of claim 1 , wherein the modified section is larger at the heel end of the tooth than at the toe end of the tooth.

10. The method of claim 1 , wherein the gear cutting tool comprises a grinding wheel or a plurality of cutting blades.

11. 2. The method of claim 1, wherein the one of the first roll ratio and the second roll ratio that results in a modified section formed on the tooth profile surface is further modified by adjusting a machine setting during the generating roll.

12. The method of claim 11 , wherein the machine settings include at least one of a cutter tilt, a cutter swivel, and a cutter reference height.

13. The method of claim 1 , wherein the workpiece comprises at least one of a gear member and a pinion member of a gear pair.

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