Worm wheel and method of hobbing same

Claim 2

An hourglass worm wheel having a longitudinal axis, a generally hourglass-shaped root surface and at least one gear tooth formed thereon, said root surface increasing in diameter from opposite ends of a mid zone which is substantially cylindrical and which cylindrical mid zone extends over a predetermined axial distance.

Claim 3

A hourglass worm wheel as defined in claim 1 wherein each gear tooth formed thereon has an outside helix surface, a central region of which, in cross section, substantially corresponds to the mid zone formed in the root surface.

Claim 4

An hourglass worm wheel as defined in claim 1, wherein the mid zone is at least about 0.030 inches (0.762 mm) but less than about 0.050 inches (1.270 mm) in length as measured along the longitudinal axis of the worm wheel.

Claim 5

An hourglass worm wheel as defined in claim 3, wherein the mid zone is about 0.030 inches (0.762 mm) to 0.040 inches (1.016 mm) in length as measured along the longitudinal axis of the worm wheel.

Claim 6

A gear set comprising: (a) an hourglass worm wheel having an axial center lying on a longitudinal axis, and (b) a mating helical gear, wherein, the hourglass worm wheel is formed with a root surface which increases in diameter from opposite ends of a cylindrical portion formed by an axial cut zone of predetermined dimensions spanning the axial center thereof.

Claim 7

A gear set comprising: (a) an hourglass worm wheel having an axial center lying on a longitudinal axis, and (b) a mating helical gear, wherein, the hourglass worm wheel is formed with a root surface which increases in diameter from opposite ends of a cylindrical portion formed by an axial cut zone of predetermined dimensions spanning the axial center thereof.

Claim 8

A gear set as defined in claim 5, wherein said axial cut zone is at least about 0.030 inches (0.762 mm) but less than about 0.050 inches (1.270 mm) in axial length.

Claim 9

A gear set as defined in claim 5, wherein said axial cut zone is about 0.030 inches (0.762 mm) to 0.040 inches (1.016 mm) in axial length.

Claim 10

A gear set as defined in claim 5, wherein teeth of the hourglass worm wheel and mating helical gear mesh in a contact zone which, under load, extends beyond either side of the axial cut zone.

Claim 11

A gear set as defined in claim 8, wherein said axial cut zone extends approximately equally on either side of said axial center of said worm wheel.

Claim 12

A gear set as defined in claim 8, wherein said axial cut zone extends a greater distance on one side of said axial center of said worm wheel than on the other side thereof.

Claim 13

In a combination gear comprising an elongated axially extending body formed with a centrally located elongated hourglass worm wheel portion, and spur gear portions integrally formed at either end thereof, and wherein said elongated worm wheel portion is adapted for meshing engagement with a mating helical gear, the improvement comprising: a cylindrical root surface and outside helix in an axial cut zone located along the length of the hourglass worm wheel portion, said root surface increasing in diameter from opposite ends of said axial cut zone toward the spur gear portion.

Claim 14

In a combination gear comprising an elongated axially extending body formed with a centrally located elongated hourglass worm wheel portion, and spur gear portions integrally formed at either end thereof, and wherein said elongated worm wheel portion is adapted for meshing engagement with a mating helical gear, the improvement comprising: a cylindrical root surface and outside helix in an axial cut zone located along the length of the hourglass worm wheel portion, said root surface increasing in diameter from opposite ends of said axial cut zone toward the spur gear portion.

Claim 15

A combination gear as defined in claim 11, wherein the cylindrical root surface in the axial cut zone extends at least about 0.030 inches (0.762 mm) but less than about 0.050 inches (1.270 mm) in axial length.

Claim 16

A combination gear as defined in claim 11, wherein the cylindrical root surface in the axial cut zone extends between about 0.030 inches (0.762 mm) and 0.040 inches (1.016 mm) in axial length.

Claim 17

A combination gear as defined in claim 11, wherein said axial cut zone has a length such that when said hourglass worm wheel portion is in meshing engagement with said mating helical gear under load, a contact zone is established therebetween which extends beyond the axial cut zone.

Claim 18

In a vehicle differential assembly of the type which includes a differential case having means for receiving a pair of axle ends together with a pair of side gears coupled to said pair of axle ends for rotation therewith, at least two pairs of combination gears associated with said side gears and rotatably carried by said differential case and arranged such that for each pair, one combination gear of a pair associated with one of said side gears meshes with a second combination gear of the pair associated with the other of said side gears, said first pair of combination gears meshing together via spur gears, each of said combination gears including an hourglass worm wheel portion having an axial center, and provided at either end thereof with spur gear portions, the worm wheel portion being formed with an outside helix and root surface which are of substantially hourglass shape, the improvement comprising: a cylindrical root surface and outside helix in an axial cut zone of predetermined dimensions located along the length of the worm wheel portion, and spanning said axial center.

Claim 19

In a vehicle differential assembly of the type which includes a differential case having means for receiving a pair of axle ends together with a pair of side gears coupled to said pair of axle ends for rotation therewith, at least two pairs of combination gears associated with said side gears and rotatably carried by said differential case and arranged such that for each pair, one combination gear of a pair associated with one of said side gears meshes with a second combination gear of the pair associated with the other of said side gears, said first pair of combination gears meshing together via spur gears, each of said combination gears including an hourglass worm wheel portion having an axial center, and provided at either end thereof with spur gear portions, the worm wheel portion being formed with an outside helix and root surface which are of substantially hourglass shape, the improvement comprising: a cylindrical root surface and outside helix in an axial cut zone of predetermined dimensions located along the length of the worm wheel portion, and spanning said axial center.

Claim 20

The differential assembly of claim 15, wherein the axial cut zone is at least about 0.030 inches (0.762 mm) but less than about 0.050 inches (1.270 mm) in axial length.

Claim 21

The differential assembly of claim 15, wherein the hourglass worm wheel portion of each combination gear meshes with an associated side gear in a contact zone which, under load, extends beyond both sides of said axial cut zone.

Claim 22

The differential assembly of claim 17, wherein said axial cut zone extends approximately equally on either side of said axial center.

Claim 23

The differential assembly of claim 17, wherein said axial cut zone extends a greater distance on one side of said axial center than on the other side of said axial center.

Claim 26

A method as defined in claim 20, wherein in step (b), the finish hobbing tool is axially fed a distance of between at least about 0.030 inches (0.762 mm) but less than about 0.050 inches (1.270 mm).

Claim 27

A method as defined in claim 20, wherein in step (b), the finish hobbing tool is axially fed a distance of between about 0.030 inches (0.762 mm) and 0.040 inches (1.016 mm).

Claim 28

A method as defined in claim 20, wherein said predetermined distance is such that the hourglass worm wheel and mating cylindrical worm gear mesh in a contact zone which extends beyond both sides of axial cut zone.

Claim 29

A method as defined in claim 23, wherein in step (b) said hobbing tool is axially fed an equal distance on either side of said axial center of said worm wheel.

Claim 30

A method as defined in claim 23, wherein in step (b), said hobbing tool is axially fed a greater distance on one side of said axial center of said worm wheel than on the other side of said axial center.

Claim 33

A method of hobbing an hourglass worm wheel as defined in claim 26, wherein, in step (d), the hobbing tool is offset from the axial center of the blank a distance of at least about 0.015 inches (0.381 mm) but not more than about 0.025 inches (0.635 mm).

Claim 34

A method of hobbing an hourglass worm wheel as defined in claim 26, wherein, in step (d), the hobbing tool is offset from the axial center of the blank a distance of between about 0.015 (0.381 mm) and 0.020 (0.508 mm) inches.

Claim 35

A method of hobbing an hourglass worm wheel as defined in claim 27, wherein, in step (e), the hobbing tool is axially fed a distance of at least about 0.030 inches (0.762 mm) but less than about 0.050 inches (1.270 mm).

Claim 36

A method of hobbing an hourglass worm wheel as defined in claim 28, wherein, in step (e), the hobbing tool is axially fed a distance of between about 0.030 inches (0.762 mm) and 0.040 inches (1.016 mm).

Claim 37

A method of hobbing an hourglass worm wheel as defined in claim 26, wherein, in step (e), the hobbing tool is fed equal distances on either side of said axial center of said worm wheel.

Claim 38

A method of hobbing an hourglass worm wheel as defined in claim 26, wherein, in step (e), the hobbing tool is fed a greater distance on one side of said axial center of said worm wheel than on the other side thereof.

Claim 39

A method of hobbing an hourglass worm wheel as defined in claim 26, wherein the hobbing tool is of a size larger than a helical gear it is designed to represent by an amount chosen in conjunction with an amount of axial feed practiced in step (e) such that a predetermined amount of lengthwise mismatch between teeth of said helical gear and teeth of said worm wheel is obtained.