Classifications

• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F16—ENGINEERING ELEMENTS AND UNITS; GENERAL MEASURES FOR PRODUCING AND MAINTAINING EFFECTIVE FUNCTIONING OF MACHINES OR INSTALLATIONS; THERMAL INSULATION IN GENERAL
  • F16H—GEARING
  • F16H48/00—Differential gearings
  • F16H48/20—Arrangements for suppressing or influencing the differential action, e.g. locking devices
  • F16H48/28—Arrangements for suppressing or influencing the differential action, e.g. locking devices using self-locking gears or self-braking gears
  • F16H48/29—Arrangements for suppressing or influencing the differential action, e.g. locking devices using self-locking gears or self-braking gears with self-braking intermeshing gears having perpendicular arranged axes and having worms or helical teeth
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
  • B23F—MAKING GEARS OR TOOTHED RACKS
  • B23F13/00—Making worms by methods essentially requiring the use of machines of the gear-cutting type
  • B23F13/06—Making worms of globoidal shape
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
  • B23F—MAKING GEARS OR TOOTHED RACKS
  • B23F19/00—Finishing gear teeth by other tools than those used for manufacturing gear teeth
  • B23F19/002—Modifying the theoretical tooth flank form, e.g. crowning
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F16—ENGINEERING ELEMENTS AND UNITS; GENERAL MEASURES FOR PRODUCING AND MAINTAINING EFFECTIVE FUNCTIONING OF MACHINES OR INSTALLATIONS; THERMAL INSULATION IN GENERAL
  • F16H—GEARING
  • F16H1/00—Toothed gearings for conveying rotary motion
  • F16H1/02—Toothed gearings for conveying rotary motion without gears having orbital motion
  • F16H1/04—Toothed gearings for conveying rotary motion without gears having orbital motion involving only two intermeshing members
  • F16H1/12—Toothed gearings for conveying rotary motion without gears having orbital motion involving only two intermeshing members with non-parallel axes
  • F16H1/16—Toothed gearings for conveying rotary motion without gears having orbital motion involving only two intermeshing members with non-parallel axes comprising worm and worm-wheel
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F16—ENGINEERING ELEMENTS AND UNITS; GENERAL MEASURES FOR PRODUCING AND MAINTAINING EFFECTIVE FUNCTIONING OF MACHINES OR INSTALLATIONS; THERMAL INSULATION IN GENERAL
  • F16H—GEARING
  • F16H55/00—Elements with teeth or friction surfaces for conveying motion; Worms, pulleys or sheaves for gearing mechanisms
  • F16H55/02—Toothed members; Worms
  • F16H55/22—Toothed members; Worms for transmissions with crossing shafts, especially worms, worm-gears
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
  • Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
  • Y10T409/00—Gear cutting, milling, or planing
  • Y10T409/10—Gear cutting
  • Y10T409/101431—Gear tooth shape generating
  • Y10T409/10159—Hobbing
  • Y10T409/101749—Process
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
  • Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
  • Y10T74/00—Machine element or mechanism
  • Y10T74/19—Gearing
  • Y10T74/19949—Teeth
  • Y10T74/19953—Worm and helical

Definitions

• This invention relates to gear hobbing in general, and to a novel technique for hobbing an hourglass worm wheel in particular.
• the invention specifically relates to the use of a novel hobbing technique for the purpose of improving the performance of differential worm gear assemblies of the type shown in U.S. Patent No. 2 . 859,641. granted November 11, 1958 to Vernon E. Gleasman.
• the worm-type does not utilize a bevel gear arrangement. Rather, it includes worm (or helical) gears, coupled to each inner axle end as drive gears, together with so called balancing or transfer gears associated with each of the worm gears and in mesh with each other for transferring and dividing torque between the axle ends.
• the transfer gears, or combination gears as they are referred to herein, are mounted in pairs, and each combination gear of a pair rotates on an axis of rotation that is substantially parallel to a tangent of the envelope of an associated axle drive, or side gear.
• Each combination gear consists of a centrally located hourglass worm wheel portion flanked at either end by an integral spur gear portion.
• This invention has particular applicability to the hobbing processes used to rough form or finish form hourglass worm wheels or hourglass worm wheel portions of combination gears such as are used in worm-type differential assemblies.
• Tip relief for example, is an arbitrary modification of tooth profile whereby a small amount of material is removed near the tip of a gear tooth.
• Crowning where material is removed from a gear tooth in the lengthwise direction, is another example of conventional gear modifications.
• the finishing hob is given a slight modification in form.
• the present invention concerns a novel technique for removing material from a specific type of gear, i.e., an hourglass worm wheel, to provide a predetermined degree of mismatch between the hourglass worm wheel and a mating helical gear for the purpose of improving overall gear performance.
• hourglass refers generally to the trace of tooth root surfaces in a worm wheel blank, the outer diameter shape of which may be of the form of an hourglass or a cylinder.
• Hourglass worm wheels are conventionally produced by what is known as the radial infeed, or plunge, cutting method.
• the rotating hob which duplicates the tooth form of the helical gear that ultimately is to mesh with the worm wheel (except that the tooth height may be increased for clearance and the tooth thickness may be varied for backlash) is fed in a radial direction toward the axis of the rotating hourglass blank.
• the hob is withdrawn in a radially outward direction. This same procedure is used for both rough forming and finish forming operations, using the appropriate class of hob.
• helical gears are typically formed by passing the rotating hobbing tool axially along the face of the rotating blank in a direction parallel to the longit ⁇ dinal axis of the blank.
• oversize hob selected in accordance with known formulas. It has been found, however, that when oversize hobs are used, i.e., a hob larger than the helical gear it - represents, backlash (the amount by which the width of a tooth space exceeds the thickness of the engaging tooth on the operating pitch circle) is eliminated or rapidly taken up upon slight axial shifting of the worm wheel. Absent sufficient backlash under load, noise generating edge contacts and/or interferences are experienced, leading to premature surface deterioration.
• the present invention seeks not only to eliminate such problems which accompany prior art hobbing techniques, but also to improve the overall performance characteristics of worm-type differential assemblies.
• worm-type differential assemblies can be improved by combining conventional radial infeed and axial feed hobbing techniques in the formation of hourglass worm wheels used in such assemblies.
• the hobbing cycle includes plunging, or radially infeeding, the rotating hob from one side of the rotating worm wheel blank toward the longitudinal axis of the blank at a point slightly offset from its lengthwise or axial mid-point.
• the hob is fed axially in the direction of the longitudinal axis of the blank to form an axial cut zone of predetermined length.
• the hob is subsequently withdrawn in a radially outward direction.
• the length of the axial cut zone is critical.
• the axial feed of the hob is limited to the extent that, under load, the zone of tooth contact extends into the curved, or hourglass-shaped portions of the gear on either side of the axial feed zone.
• the axial motion of the hob may be bi-directional so as to achieve the desired full extent of the axial motion.
• the theory behind the new hobbing cycle is that the critically sized axial cut zone of the worm wheel teeth produces a generated involute helicoidal tooth form which remains more conjugate in normal section with its mating helical gear over a range of displacements in its mounted position.
• the axial cut zone provides the desired lengthwise mismatch between the worm wheel and helical gear at tooth ends so as to permit greater axial displacement of the worm wheel under load without substantial loss of backlash and without occurrences of undesirable interferences and edge contacts which occur with worm wheel and helical gear sets formed in the conventional manner.
• FIGURE 1 is a perspective view of a known type of differential gear assembly, with parts broken away in section to show the internal gear structure;
• FIGURES 2 and 3 are schematic diagrams illustrating prior art hobbing machine cycles for forming and/or finishing hourglass worm wheels and cylindrical helical gears;
• FIGURE 4 is a schematic diagram of the hourglass worm wheel hobbing process of an exemplary embodiment of this invention
• FIGURE 5 is a schematic diagram illustrating the paths of travel of the hobbing tool in accordance with an exemplary embodiment of this invention.
• FIGURE 6 is a cross-sectional view of an hourglass worm wheel formed according to the present invention, showing the cylindrical mid-portion of the wheel in exaggerated form.
• the known worm-type differential assembly includes a differential case 4 provided with a flange 6 to which a ring gear (not shown) is suitably attached.
• the ring gear receives power input from the drive train of a vehicle in a conventional manner.
• the differential case 4 also includes a pair of spaced apart trunions 8 (only one of which is shown) which are adapted to receive bearings (not shown) by which the differential case is rotatably mounted inside a conventional differential or axle housing (not shown).
• the differential case 4 also includes a pair of axle receiving bores 10 which are at least partially defined by the trunions 8 and through which axle shaft ends 12, 14 extend into engagement with the differential gear assembly inside the case.
• each axle shaft end includes external splines 16 which engage mating internal splines of helical worm, or side, gear 18.
• Each side gear 18 meshes with three balancing or transfer gears 20 which are arranged at 120° intervals about the periphery of the side gear, and which are • arranged generally tangentially to, and engaged with, the pitch surface of the side gear. It is understood with reference to Figure 1, that only two of the three transfer gears associated with each side gear 18 are shown. These transfer gears, which are hereinafter referred to as combination gears, are formed with a middle portion which constitutes an hourglass worm wheel portion 22 and integral end portions which constitute spur gear portions 24. It can be seen that each side gear 18 meshes with worm wheel portions of three associated combination gears 20. At the same time, the spur gear portions 24 of each combination gear associated with one side gear mesh with the spur gear portions of adjacent combination gears associated with the other of the two side gears. It is this ⁇ 11-
• each combination gear 20 is mounted for rotation about a shaft 26, the ends of which extend beyond the gear and serve to mount the gear within the gear case 4. It can be seen that each side gear 18 meshes with worm wheel portions 22 of a set of three associated combination gears 20. At the same time, the spur gear portions 24 of each combination gear associated with one side gear meshes with the spur gear portions of adjacent combination gears associated with the other of the two side gears. Thus, as clearly illustrated in Figure 1, each of the three combination gears 20 associated with one side gear 18 is paired with a combination gear associated with the other side gear.
• the case 4 is formed with three peripherally arranged “windows” or slots 28 (only partially shown in Figure 1) extending radially inwardly from the periphery of the case 4, each window or slot 28 receiving one pair of combination gears.
• FIG. 2 shows, in schematic form, the conventional hobbing machine cycle for rough forming and/or finishing the teeth on an hourglass worm wheel blank.
• the blank 30, which in this case is for a combination gear having a centrally located hourglass worm wheel portion and spur gear portions at either end thereof, is mounted for rotation about a vertical axis 32.
• a hobbing tool 36 which has substantially the same tooth form as the helical worm, or side gear that the combination gear will ultimately mesh with in the the differential assembly, is moved radially inwardly along a straight path RF toward the center of the blank while rotating about a horizontal axis 38. It is understood that rotation of the hobbing tool and blank are synchronized through gearing in a manner well understood in the gear making art.
• the hobbing tool After the hobbing tool has rough formed or finish formed the worm wheel teeth via radial infeed a predetermined distance, the tool is dwelled for a predetermined number of revolutions and is then radially outwardly withdrawn along the same path.
• FIG. 3 there is shown a conventional hobbing machine cycle for rough or finish forming teeth on a cylindrical helical gear.
• a helical gear blank 40 is shown mounted for rotation about vertical axis 46.
• a hobbing tool 42 is shown mounted for rotation about a horizontal axis 44. The hobbing tool is fed axially along a path AF parallel to the axis 46 of the helical gear blank until the hob passes across the entire face of the gear blank.
• rotation of the gear blank and hob is synchronized.
• a supplemental relative motion is provided between the axial feed of the hobbing tool and the rotation of the gear blank to form gear teeth in the blank at the desired helix angle.
• Combination gear blank 50 having a centrally located hourglass worm wheel portion 52 and spur gear portions 54, is shown mounted for rotation about a vertical axis 56.
• a hobbing tool 58 having substantially the same tooth form as the helical side gear 18 is mounted for rotation about a horizontal axis 60 and for radial infeed along a path 62.
• the hob 58 is smaller than the typical oversized hob normally used to obtain a desired amount of mismatch.
• the hob 58 may be the same size as the mating helical gear it represents, or slightly larger for purposes of achieving desired clearance and backlash.
• the path 62 is offset from the axial center or mid-point C of the worm wheel portion (C also represents the center point of a worm wheel tooth) by a slight amount designated "A" in the drawings, and further described below.
• the net effect of the described hob tool movement is to create curved sections on the hourglass worm wheel, defined by radii R.,, R_, on either side of a cylindrical middle portion defined by the axial dimension B.
• the hob tool is then moved radially outwardly away from the gear along a path 66.
• the hob may be returned to its starting position along a path 68. It is thus seen that the hob may follow an essentially rectangular path during a complete hobbing cycle in this exemplary embodiment.
• the preferred length B of the axial cut zone on the worm wheel portion of the gear is between about .030 inches (.762 mm) and .040 inches (1.016 mm) but less than about .050 inches (1.270 mm). Therefore, the dimension A, representing the amount of offset between the hob tool infeed path and the axial center C of the worm wheel portion is equal to the dimension - ⁇ , or about .015-.025 inches (.381-.635 mm), and preferably about .015-.020 inches (.381-.508 mm). This ensures that the axial cut zone is located centrally of the worm wheel portion of the blank.
• FIG. 6 An exaggerated depiction of the axial cut zone formed in accordance with the hobbing machine cycle described hereinabove is shown in Figure 6.
• the hourglass worm wheel portion has formed thereon a tooth surface area 70.
• Centrally disposed therealong between curved portions defined by radii R.. , R_ is an axial cut zone B wherein both the root surface 72 and outside helix surface 74 are flat.
• the significant criteria for establishing the length of the axial feed zone is that, under load, tooth contact between the mating helical gear and worm wheel teeth extends beyond the length of the axial feed zone.
• the contact zone will extend approximately equally on either side of the zone B, as illustrated, for example, by dimension D.
• the axial feed zone represents a generated involute helicoidal tooth form which is more conjugate with a mating side gear 18 in a worm-type differential as disclosed in Figure 1.
• the increased lengthwise mismatch provided by the axial cut zone permits greater axial displacement of the combination gears under load without substantial loss of necessary backlash, and without occurrences of undesirable interferences and tooth edge contacts.
• an improved worm-type differential is provided with enhanced performance characteristics.
• the axial .feed technique of this invention may be utilized to keep mismatch constant where sharpening of an oversize hob otherwise results in an increase or decrease in mismatch. For example, if mismatch is decreased as a result of hob sharpenings, an increased amount of feed will compensate and provide the desired mismatch.
• the novel hobbing machine cycle of this invention has been disclosed in conjunction with its application to gears utilized in a specific worm-type differential gear assembly, it should be understood that this novel hobbing cycle may provide similar beneficial, results in any worm wheel/helical gear set.
• various changes may be made to the basic concepts disclosed herein which would nevertheless remain within the scope of the invention as defined by the claims which follow.

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• Engineering & Computer Science (AREA)
• General Engineering & Computer Science (AREA)
• Mechanical Engineering (AREA)
• Gears, Cams (AREA)
• Retarders (AREA)
• Gear Transmission (AREA)

Priority Applications (6)

Applications Claiming Priority (1)

Publications (1)

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ID=22195669

Family Applications (1)

Country Status (6)

Cited By (1)

Families Citing this family (15)

Citations (7)

Family Cites Families (5)

• 1986
  • 1986-10-15 EP EP86906608A patent/EP0325579B1/en not_active Expired - Lifetime
  • 1986-10-15 US US07/363,910 patent/US4926712A/en not_active Expired - Fee Related
  • 1986-10-15 WO PCT/US1986/002161 patent/WO1988002829A1/en active IP Right Grant
  • 1986-10-15 DE DE86906608T patent/DE3683300D1/de not_active Expired - Lifetime
  • 1986-10-15 JP JP1989600023U patent/JPH02500018U/ja active Pending
• 1987
  • 1987-09-23 CA CA000547632A patent/CA1289772C/en not_active Expired - Lifetime

Patent Citations (7)

Non-Patent Citations (1)

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