GB2212231A - Limited slip differential gearing with floating worm wheels - Google Patents

Abstract

A differential gear mechanism having a gear carrier casing (1) mounted for rotation about an output axis (X) of the differential, a pair of spaced coaxial helical output gears (4A, 4B) mounted for rotation in the carrier about the axis thereof, and gear trains mounted in the gear carrier and interconnecting the two output gears, each gear train comprising two worm members (5A, 5C, 5B, 5D). The worm members each have a helically toothed formation meshing with a respective worm wheel (7) at diametrically opposite portions thereof and each worm member has an extension of its helically toothed formation meshing also with a respective one of the output gears (4A, 4B). The worm wheels (7) are mounted so as to be rotatable about the carrier axis between their respective enmeshed worm members, either by virtue of the clearance of their mountings as in the embodiment shown or alternatively by having a two part mounting pin for the worm wheels having an arcuate groove guided on a stub shaft extension of the output gears. <IMAGE>

Description

DIFFERENTIAL GEAR MECHANISM This invention relates to differential gear mechanisms for vehicles of the kind known for example in European Patent Specification No 0 148 641.

More specifically, the invention is concerned with such mechanisms which comprise a gear carrier mounted for rotation about the output axis of the differential, a pair of spaced co-axial helical output gears mounted for rotation in the carrier about the axis thereof, and gear trains mounted in the gear carrier and interconnecting the two output gears, each gear train comprising two worm members and a worm wheel, the worm members each having a helically toothed formation meshing with the worm wheel at diametrically opposite portions thereof and each worm member having an extension of the helically toothed formation meshing also with a respective one of the output gears.

The worm member and worm wheel meshing conditions have to be accurate for satisfactory operation of the mechanism, a condition which is exacerbated by the high sliding velocity at the contact surfaces, since otherwise inaccurate meshing will overload the surface. This is particularly the case in relation to the Applicants' mechanism disclosed in EP-A-O 148 641 because there the worm wheel is in mesh with the two worm members i.e. in 'double' mesh. Thus, difficulties in manufacture are envisaged because the two worm member axes must be exactly the same distance from the worm wheel axis to give proper meshing. A further complication arises in that there is a parallel set of worm members and worm wheels which must share the torque loading of the mechanism between the two output gears.Any centre distance inaccuracy will result in uneven loading, and possible overloading, of the individual components.

It has been found that this problem can be alleviated by allowing the worm wheels to 'float' between the worm members, so that they are not constrained into a fixed position in the casing. Ideally, each worm wheel should be allowed to 'float' circumferentially, that is, about the main axis of the carrier, between its meshing worm members so as to be able to find the position where tooth loadings are balanced.

To this end, the invention provides a gear carrier casing mounted for rotation about an output axis of the differential, a pair of spaced coaxial helical output gears mounted for rotation in the carrier about the axis thereof, and gear trains mounted in the gear carrier and interconnecting the two output gears'. each gear train comprising two worm members and a worm wheel, the worm members each having a helically toothed formation meshing with the worm wheel at diametrically opposite portions thereof and each worm member having an extension of the helically toothed formation meshing also with a respective one of the output gears, characterised in that the worm wheels are mounted so as to be rotatable about said carrier axis between their respective enmeshed worm members.

According to a feature of the invention, both worm wheels may be mounted for rotation together about the carrier axis.

In one construction where this feature is adopted a single pin carrying both worm wheels may be mounted such that limited movement about the main carrier axis is permitted.

However, in another construction, a split two-part worm wheel support pin may be mounted for rotation about the axis of the carrier each of the pin parts carrying one of the worm wheels so that the worm wheels are disposed at diametrically opposed locations and in sliding contact with the differential casing whereby the differential casing absorbs all radially outward thrust loads.

In order for radially outward thrust loads to be transmitted to the differential casing, it is preferred that each worm wheel is provided with a cylindrically faced thrust washer which seats in a mating cylindrical bearing surface provided by an adjacent part of the differential casing, the cylindrical radius of curvature being centred on the main carrier axis thereby allowing each wheel to move circumferentially (rotation about the carrier axis) at the point of worm member/wheel tooth mesh.

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 longitudinal section on the line I-I of Figure 2 of a differential gear mechanism for a vehicle; FIGURE 2 is a cross section on the line II-II of Figure 1; FIGURE 3A is a cross-sectional sketch of a modified mechanism taken through a plane containing the main axis of the differential casing; FIGURES 3B, 3C and 3D are perspective sketches of bearing components of the modified mechanism; and FIGURE 3E is a cross-sectional view similar to Figure 2 but of the modified mechanism.

Referring first to the differential gear mechanism shown in Figures 1 and 2 of the drawings, the mechanism comprises a gear carrier 1 formed into two portions 1A, 1B secured together. The casing 1A has a flange 2 which may form the output gear wheel of a transfer gear box or have a bevel crown wheel bolted to it and forms the input member for the mechanism. Two co-axial output shafts 3A and 3B are rotatably supported in the gear carrier 1 and terminate at their inner ends in helically toothed output gears 4A and 4B each having fourteen teeth. Four identical seven-start worm members 5A, SB, 5C and 5D are rotatably mounted on respective pins 6 in the carrier 1.

The worm members SA and 5B mesh with output gear 4B while the worm members 5C and 5D mesh with output gear 4A. Each of the worm members and output gears have a helix angle of 46.9 degrees.

The helical portions of the worm members 5 are sufficiently long to enable them to mesh also with worm wheels 7A,C and 7B,D. As can be seen in Figure 1, the worm members 5A and 5C mesh with the worm wheel 7A,C at diametrically opposite portions thereof while the worm members 5B and 5D mesh with worm wheel 7B,D also at diametrically opposite portions thereof.

The worm wheels 7 are both mounted on a common transverse pin 8 the latter being trapped axially within the gear carrier 1. The worm wheels 7 are positioned axially on the pin 8 at their outer lateral faces against the adjacent faces lB, 1C of the carrier casing. At their inner faces the worm wheels are axially positioned by opposed bearing faces on a spacer block 's' which also presents two further bearing faces to the inner end faces of the output gears 4A and 4B (Figure 1).

The whole worm wheel assembly comprising the worm wheels 7, pin 8, and spacer block 'S' have sufficient clearance to be able to rotate as a unit about the main axis 'X' of the carrier 1. Thus the worm wheels can 'float' by circumferential movement automatically to find the position where tooth loadings are substantially balanced, while inward and outward radial thrust loads transmitted from the worm wheels 7 are supported by the casing 1.

Referring now to Figures 3A, 3B, 3C, 3D and 3E a modified mechanism is shown which provides an alternative 'floating worm wheel' assembly in which like parts to those of the previous embodiment are designated like reference numerals with the addition of suffix " ' ". In this modified construction, the spacer block 's' is omitted and substituted by a pair of stub shafts 13 and 14 extending to the output gears 4A' and 4B', respectively and which have mating bearing end faces. The stub shafts terminate in bearing discs 'D' which bear against the adjacent face of the respective output gear. Further, the single transverse pin is replaced by a split two-part pin 15, 15A having a convex end portion formed with an arcuate groove pivotal on the stub shafts 13, 14 so that they can articulate thereon. Each of the pin parts also is formed with a radially outwardly extending bearing shaft 16, 16A on which respective ones of the worm wheels 7' are rotatably mounted and which accommodate radially inward thrust loads transmitted by the associated worm wheel. In order to accommodate radially outward thrust loads, each worm wheel bears against a radially outer cylindrically convex washer 17 which seats in a mating cylindrically concave thrust face 18 provided by an adjacent portion of the carrier casing. Thus, radially outward thrust loads are accommodated by the casing. The cylindrical radius of a a the curvature is centred on the main differential axis 'x' thereby allowing the worm wheel to rotate about axis i.e. to float circumferentially at the worm member/wheel tooth meshing point.

Claims (6)

1. A differential gear mechanism comprising a gear carrier casing mounted for rotation about an output axis of the differential, a pair of spaced coaxial helical output gears mounted for rotation in the carrier about the axis thereof, and gear trains mounted in the gear carrier and interconnecting the two output gears, each gear train comprising two worm members and a worm wheel, the worm members each having a helically toothed formation meshing with the worm wheel at diametrically opposite portions thereof and each worm member having an extension of the helically toothed formation meshing also with a respective one of the output gears, characterised in that the worm wheels are mounted so as to be rotatable about said carrier axis between their respective enmeshed worm members.

2. A differential gear mechanism according to claim 1, further characterised in that both worm wheels may be mounted for rotation together about the carrier axis.

3. A differential gear mechanism according to claim 2, further characterised in that a single pin carrying both worm wheels is mounted such that limited movement about the main carrier axis is permitted.

4. A differential gear mechanism according to claim 2 further characterised in that a split two-part worm wheel support pin may be mounted for rotation about the axis of the carrier each of the pin parts carrying one of the worm wheels so that the worm wheels are disposed at diametrically opposed locations and in sliding contact with the differential casing whereby the differential casing absorbs all radially outward thrust loads.

5. A differential gear mechanism according to claim 4, further characterised in that each worm wheel is provided with a cylindrically faced thrust washer which seats in a mating cylindrical bearing surface provided by an adjacent part of the differential casing, the cylindrical radius of curvature being centred on the main carrier axis thereby allowing each wheel to move circumferentially at the point of worm member/wheel tooth mesh so that radially outward thrust loads are transmitted to the casing.

6. A differential gear mechanism substantially as hereinbefore described with reference to and as shown in Figure 1 and 2 or Figures 3A, 3B, 3C, 3D and 3E of the accompanying drawings.