Differential gear mechanism

Details Differential gear mechanism Differential gear mechanism

1988-04-11

1990-07-03

Braun, Leslie A.

Planetary gear transmission systems or components

Differential planetary gearing

Planet pinion is worm gear

F16H 138

Patent

active

049380990

DESCRIPTION:

BRIEF SUMMARY
TECHNICAL FIELD

This invention relates to differential gear mechanisms for vehicles of the kind known for example in European patent specification No. 0 148 641.
Specifically the invention is concerned with such mechanisms of which comprise a gear casing 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 positions thereof and each worm member having an extension of the helically toothed formation meshing also with a respective one of the output gears.
Preferably, the worms and worm wheels are of the same or similar material in respect of composition and hardness.
In practice, these will either be two or three gear trains since a single train would result in unbalanced loading of the output gears while designs with more than three gear trains are unlikely to be useful in view of the resulting complexity of the multiple gear assembly.


DISCLOSURE OF THE INVENTION

A differential gear mechanism according to the invention is characterised by the following equations: ##EQU2## where R.sub.W is the ratio of the number of teeth (starts) of each worm to the number of teeth of the worm wheel, number of teeth of the worm wheel, wheel and each output gear measured axially of the worm wheel and output gear respectively,


__________________________________________________________________________ NO. OF DEGREESlpha. K1 GEAR NOMINAL RANGE NOMINAL RANGE TRAINS VALUE POSSIBLE PREFERRED VALUE POSSIBLE PREFERRED __________________________________________________________________________ 2 62 .+-.6 .+-.4 0.65 .+-.0.15 .+-.0.05 3 38 .+-.3 .+-.2 0.5 .+-.0.15 .+-.0.05 __________________________________________________________________________
The above defined equations help to ensure that despite the mixture of helical and worm and wheel operations, the various components have comparably long lives, thus avoiding premature wearing out of one component.
Preferably, the aspect ratio AR of the gearing, i.e. the ratio of the diameter of the imaginary cylinder enveloping the worms to the axial length between the outer ends of the teeth of the output gear is given by ##EQU3##


BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings, in which:
FIG. 1 is a longitudinal section on the line II of FIG. 2 of a differential gear mechanism for a vehicle; and
FIG. 2 is a cross section on the line II--II of FIG. 1.


BEST MODE FOR CARRYING OUT THE INVENTION

The differential gear mechanism shown in the drawings, comprises a gear carrier 1 formed into two portions 1A, 1B secured together. The casing part 1A has a flange 2 which may form the output gear wheel of a transfer gearbox 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, 5B, 5C and 5D are rotatably mounted on respective pins 6 in the carrier 1. The worm members 5A and 5B mesh with output gear 4B while the worm members 5C and 5D mesh with the 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 FIG. 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

REFERENCES:
patent: 4625585 (1986-12-01), Dissett
patent: 4677876 (1987-07-01), Dissett
patent: 4751853 (1988-06-01), Dissett
European Patent Application Publication No. EP O 148 641, published 7/17/85.
International Search Report for International Appln. No. PCT/GB 87/00560.

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