NZ212702A - Limited slip differential incorporating worm gears - Google Patents

Description

2 12702 Priority Date(s): Complete Specification. Filed: ^ Class: -f y&ys. y.l.4-.^.

Publication Date: ...

P.O. Journal, No: .O Patents Form No. 5 PATENTS ACT 1953 COMPLETE SPECIFICATION "DIFFERENTIALS" WE, JACK KNIGHT (DEVELOPMENTS) LIMITED, of Butts Road, Woking, Surrey, GU21 1JU, England, a British Company, hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- (followed by page la) - INDIFFERENT IALS 212702 r, - - ^ r This invention relates to limited slip differentials for motor vehicles for use in a driving axle or between two driving axles.

Limited slip differentials incorporating reversible worm-wormwheel drives are known, as described in US Patent Specifications 5 Nos. 2 559 916, 2 631 475, 2 859 641 and 4 191 071. Such differentials are of the "cross-axis planetary" type having co-axial output shafts each carrying a sun gear in the form of a worm-wheel meshing with an associated worm or set of worms. The axis of each worm is thus transverse to and displaced radially from the axis of 2Q the sun gears. A spur pinion is integral with each end of each worm and meshes with the corresponding spur pinion of a worm meshing with the other sun gear. These spur pinions further increase the radial bulk of the differential. Also, the correct positioning of the transverse worm assemblies presents manufacturing difficulties. ^5 Moreover, as indicated in US Patent 3 849 862 such differentials could be noisy under certain test conditions, somewhat inefficient in delivering power therethrough and did not always give agood division of power between the left and right drive wheels of a vehicle. As indicated in US 3 849 862 additional steps are required 20 to overcome these problems.

According to the present invention there is provided a limited slip differential for a vehicle, comprising a gear carrier mountable for rotation about a longitudinal axis of the carrier, a pin extending within the carrier, the pin having its axis transverse to 25 the carrier axis and being mounted by its ends in the carrier, first and second worm wheels rotatably mounted on the pin, four helically toothed worm members rotatably mounted in the carrier with the axis of each said worm member extending longitudinally in the carrier, the first and the third of the worm members meshing with a first of 30 the wormwheels on opposite portions of the circumference of the said one worm wheel, the second and the fourth of the worm members meshing with the second of the wormwheels on opposite portions of the circumference of the second wormwheel, a spacer block on the pin, the spacer block having on two opposed faces thereof worm 35 wheel locating surfaces each to engage a first side surface of a respective one of the wormwheels, the carrier having two further S wormwheel locating surfaces to engage second side surfaces of the "\\ worm wheels, and first and second helical 6un gears coaxial with the AUG 1987$ carrier axis, each Bun gear being rotatably mounted with respect to *-3/^ J -2 - 91270? the carrier, the first sun gear meshing with extension portions of the helical teeth on. the second and third worm members, the second sun gear meshing with extension portions of the helical teeth on the first and fourth worm members, the Bpacer block spacing 6aid sun gears from each other.

An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings, in which Figure 1 is a chordal sectional view of a differential according to the invention on the line I-I of Figure 2; Figure 2 is a cross sectional view along the lines II of Figure 3; and Figure 3 is a axial sectional view along the line III-III of Figure 2.

The differential shown in the drawings transmits drive from 25 transfer gearing (not shown) to half-shafts 2 and 4 of a driven axle, for example of a motor car.

The differential has a gear carrier 6 formed by a bell-shaped housing 7 and a cover member 8 having a flange 8A and a hub portion 9 snugly engaged in the mouth of the housing 7. The flanges 8A may 2o be formed with a ring of gear teeth or, as shown here, with a ring of holes 8B for bolts securing a bevel crown wheel or a transfer gear (not shown). The housing 7 and cover member 8 are secured together by suitable bolts or screws (not shown) to form the gear carrier 6.

Four worm members 10A-10D are rotatably mounted on hollow pins 11 supported in the gear carrier 6 at each of their ends. The worm members 10 and their pins 11 are parallel and equally spaced from the axis of the shafts 2 and 4. Each pin 11 is formed with a seating 11A for a grub screw 11B. Two of the worm members, 10A and 30 IOC, mesh with a worm wheel 12 at diametrically opposed positions.

.The worm wheel 12 is located between the worm members 10A and IOC and is rotatably supported on a hollow support pin 14 mounted in the housing 7. The other two worm members, 10B and 10D, mesh with a further worm wheel 16 located between them, and also mounted on the 35 support pin 14 but spaced along the support pin 14 by means of a spacer block 17. The pin 14 has a seating 14A for a securing grub screw 15 in the spacer block 17.

^ £ N ^ worm wheels 12 and 16, the spacer block 17 and the support ' --©in 14 are aligned on an axis YY which intersects the axis XX at °f 12 MAY 1987^ ight angles.

// Half-shaft sun-gears 18 and 20 are respectively splined to (or 212702 formed integrally with) the inner ends of the half-shafts 2 and 4 and are aligned on the axis XX. Each half-shaft sun gear is within the gear carrier 6 and meshes with an extension one of the worm members of each pair of worm members meshing with a respective worm 5 wheel. Thus, the half-shaft sun gear 18 meshes with worm members 10B and IOC which mesh respectively with worm wheels 16 and 12. The half-shaft sun gear 20 meshes with worm members 10A and 10D which mesh respectively with worm wheels 12 and 16.

Each worm member thus has a worm portion 22, having a helical 10 thread and meshing with a worm wheel, and an extension forming a pinion portion 24 meshing with a half-shaft sun gear. The helix angle of the worm portions 22 and worm wheels 12 and 16 is such that the worm drive is reversible (in this case 43.1° to the axis, for the worms) i.e. operates bi-directionally. In this embodiment the 15 half-shaft sun gears 18 and 20 and the pinion portions 24 of the worm members are helical, the pinion portion 24 being formed by continuations of the worm formation of the worm portions 22.

Each worm member also has a reduced-diameter untoothed portion 25 adjoining the worm portion to avoid engagement with the other 20 half-shaft sun gear.

In operation, when the vehicle is travelling straight ahead with the road wheels turning at the same speed the gear carrier 6 is rotated about the axis XX. The rotary motion is transmitted to the half-shafts 2 and 4. The half-shafts, the gear carrier 6 and the 25 gearing within the gear carrier rotate as a unit. There is no movement of the worms 10 or worm wheels 12, 16 about their axes.

When the vehicle corners, one half-shaft must rotate faster than the gear carrier 6 and the other half-shaft slower than the gear carrier. If the half-shaft 2 rotates faster than the gear 30 carrier 6 it, through the half-shaft sun gear 18, causes the worms 10B and IOC to rotate about their axes in the gear carrier 6. The worms 10B and IOC in turn cause the worm wheels 16 and 12 to turn about the support pin 14. Because the worm drive is reversible, the worm wheels 16 and 12 cause the worms 10D and 10A to rotate in 35 the opposite sense to worms 10B and IOC. Although the worm drive is reversible, drive is not transmitted with 100% efficiency from the worm wheels to the worm members. In this embodiment the efficiency 2 1270 2 Is about 70%. As a result, there Is frictlonal resistance to operation of the device and it functions as a limited slip differential. The rotation of the worm members 10D and 10A causes the sun gear 20 and the half-shaft 4 turn backwards relative to the 5 carrier 6.

The steel worm members, worm wheels and spacer block must be robust and wear resistant and in this embodiment the worm wheels (and advantageously the worms and the spacer block especially where engine oil is the only available lubricant) can be case hardened and 10 then receive anti-scuffing treatment such as 'No Scuff' available from British Heat Treatments Limited of Lowestoft.

The helix angle of the worms to the worm axis may be chosen to equal the angle of friction between the tyres and a wet road.

The housing 7 is formed with large openings 26 both to reduce 15 mass and to assist the circulation of lubricating oil. Preferably, the lubricating oil is hypoid gear oil. Lubrication is ensured by oil passages L drilled in the various components.

The limited slip differential described above may be used to transmit drive to two further such differentials in the two driving 20 axles (e.g. front and rear) of a four-wheel drive vehicle.

In a modification (not shown in the drawings), the grub screws 11B and seatings 11A are omitted and the untoothed portions 25 of the worms are replaced by flanges of corresponding diameter and axial length on the pins to provide axial location of the latter 25 while allowing them to rotate.

If desired, further friction in the differential can be generated by axially biassing the worm members and/or one or both sun gears and/or the worm wheels, for example by means of Belleville washers located between the worm members and either the casing 30 or the pin flanges (if used), between worm wheels and the casing, and/or one or both sun gears and the casing. Further friction may also be generated by the use of frusto-conical end surfaces on appropriate ones of the helically toothed members. .

Claims (7)

WHAT WE CLAIM IS: - 5 - 212702

1. A limited elip differential for a vehicle, comprising: a gear carrier mountable for rotation about a longitudinal axi6 of the carrier, a pin extending within the carrier, the pin having its axis transverse to the carrier axis and being mounted by its ends in the carrier, first and second worm wheels rotatably mounted on the pin, four helically toothed worm members rotatably mounted in the carrier with the axis of each said worm member extending longitudinally in the carrier, the first and the third of the worm members meshing with a first of the wormwheels on opposite portions of the circumference of the said one worm wheel, the second and the fourth of the worm members meshing with the second of the wormwheels on opposite portions of the circumference of the second wormwheel, a spacer block on the pin, the spacer block having on two opposed faces thereof worm wheel locating surfaces each to engage a first side surface of a respective one of the wormwheels, the carrier having two further wormwheel locating surfaces to engage second side surfaces of the worm wheels, and first and second helical sun gears coaxial with the carrier axis, each sun gear being rotatably mounted with respect to the carri er, helical teeth on the second and third worm members, the second sun gear meshing with extension portions of the helical teeth on the first and fourth worm members, the spacer block spacing said sun gears from each other.

2. A differential according to claim 1, wherein the distances between the axes of the first and second worm members and between the axes of the third and fourth worm members are each less than the distances between the axes of the first and third worm members and between the axes of the second and fourth worm members. the first sun gear meshing with extension portions of the -6- 212702

3. A differential according to claim 1 or 2, wherein the spacer block is secured to the pin.

4. A limited slip differential according to any of the preceding claims comprising means for axially biassing the worm members relative to the carrier to generate additional friction in the differential.

5. A limited slip differential according to any one of claims 1 to 3 comprising means for axially biassing at least one of the sun gears and/or the worm wheels relative to the carrier to generate additional friction in the differential.

6. A limited slip differential according to claims 4 or 5 wherein the axial biassing means comprises Belleville washers.

7. A limited slip differential arranged and constructed and operated substantially as hereinbefore described with reference to the accompanying drawings. JACK KNIGHT (DEVELOPMENTS) LIMITED, By Their Authorised Attorneys, n JOHN A. KEMMINGION & ASSOCIATES