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A hydraulic differential with a variable displacement hydraulic unit of the axial piston type, a fixed displacement hydraulic unit of the axial piston type, a stationary housing for mounting the hydraulic units, an input shaft for coupling to a variable speed power source and an output shaft for cou

A hydraulic differential with a variable displacement hydraulic unit of the axial piston type, a fixed displacement hydraulic unit of the axial piston type, a stationary housing for mounting the hydraulic units, an input shaft for coupling to a variable speed power source and an output shaft for coupling to a constant speed load, comprises: a wobbler and a port plate for the variable displacement hydraulic unit coupled to the stationary housing; an axial block and piston set for the variable displacement hydraulic unit, a port plate for the fixed displacement hydraulic unit and a wobbler for the fixed displacement hydraulic unit coupled to the input shaft; and an axial block and piston set for the fixed displacement hydraulic unit coupled to the output shaft; wherein the variable displacement hydraulic unit port plate and the fixed displacement hydraulic unit port plate couple together through a circumferential interface to minimise hydraulic thrust forces developed by the hydraulic unit.

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The claimed invention is: 1. A hydraulic differential with a variable displacement hydraulic unit of the axial piston type, a fixed displacement hydraulic unit of the axial piston type, a stationary housing for mounting the hydraulic units, an input shaft for coupling to a variable speed power sour

The claimed invention is: 1. A hydraulic differential with a variable displacement hydraulic unit of the axial piston type, a fixed displacement hydraulic unit of the axial piston type, a stationary housing for mounting the hydraulic units, an input shaft for coupling to a variable speed power source and an output shaft for coupling to a constant speed load, wherein the input and output shafts extend out of a single end of the stationary housing, comprising: a wobbler and a port plate for the variable displacement hydraulic unit coupled to the stationary housing; an axial block and piston set for the variable displacement hydraulic unit, a port plate for the fixed displacement hydraulic unit and a wobbler for the fixed displacement hydraulic unit coupled to the input shaft; and an axial block and piston set for the fixed displacement hydraulic unit coupled to the output shaft; wherein the variable displacement hydraulic unit port plate and the fixed displacement hydraulic unit port plate couple together through a circumferential interface to minimise hydraulic thrust forces developed by the hydraulic units. 2. The hydraulic differential of claim 1, wherein the circumferential interface comprises a generally cylindrical orifice in the variable displacement unit port plate that receives a generally cylindrical end of the fixed displacement unit port plate. 3. The hydraulic differential of claim 1, wherein the variable displacement unit port plate receives charging fluid and distributes it to the fixed displacement unit port plate through a charging fluid annulus along the circumferential interface between them. 4. The hydraulic differential of claim 1, wherein working fluid communicates between the variable displacement unit port plate and the fixed displacement unit port plate through a working fluid annulus along the circumferential interface between them. 5. The hydraulic differential of claim 1, wherein the input shaft coaxially mounts within the output shaft. 6. The hydraulic differential of claim 1, wherein the variable displacement hydraulic unit and the fixed displacement hydraulic unit are in radial alignment with the input and output shafts. 7. The hydraulic differential of claim 1, wherein the circumferential interface comprises a bearing for supporting the fixed displacement hydraulic unit port plate with the variable displacement unit port plate. 8. A hydraulic differential with a variable displacement hydraulic unit of the axial piston type, a fixed displacement hydraulic unit of the axial piston type, a stationary housing for mounting the hydraulic units in generally radial alignment with each other, an input shaft for coupling to a variable speed power source and an output shaft for coupling to a constant speed load, comprising: a wobbler and a port plate for the variable displacement hydraulic unit coupled to the stationary housing; an axial block and piston set for the variable displacement hydraulic unit, a port plate for the fixed displacement hydraulic unit and a wobbler for the fixed displacement hydraulic unit coupled to the input shaft; and an axial block and piston set for the fixed displacement hydraulic unit coupled to the output shaft; wherein the variable displacement hydraulic unit port plate and the fixed displacement hydraulic unit port plate couple together through a circumferential interface comprising a generally cylindrical orifice in the variable displacement unit port plate that receives a generally cylindrical end of the fixed displacement unit port plate to minimise hydraulic thrust forces developed by the hydraulic units. 9. The hydraulic differential of claim 8, wherein the variable displacement unit port plate receives charging fluid and distributes it to the fixed displacement unit port plate through a charging fluid annulus along the circumferential interface between them. 10. The hydraulic differential of claim 8, wherein working fluid communicates between the variable displacement unit port plate and the fixed displacement unit port plate through a working fluid annulus along the circumferential interface between them. 11. The hydraulic differential of claim 8, wherein the input shaft coaxially mounts within the output shaft. 12. The hydraulic differential of claim 8, wherein the variable displacement hydraulic unit and the fixed displacement hydraulic unit are in radial alignment with the input and output shafts. 13. The hydraulic differential of claim 8, wherein the input and output shafts extend out of a single end of the stationary housing. 14. The hydraulic differential of claim 8, wherein the circumferential interface comprises a bearing for supporting the fixed displacement hydraulic unit port plate with the variable displacement unit port plate. 15. A hydraulic differential with a variable displacement hydraulic unit of the axial piston type, a fixed displacement hydraulic unit of the axial piston type, a stationary housing for mounting the hydraulic units in generally radial alignment with each other, an input shaft for coupling to a variable speed power source and an output shaft for coupling to a constant speed load, comprising: a wobbler and a port plate for the variable displacement hydraulic unit coupled to the stationary housing; an axial block and piston set for the variable displacement hydraulic unit, a port plate for the fixed displacement hydraulic unit and a wobbler for the fixed displacement hydraulic unit coupled to the input shaft; and an axial block and piston set for the fixed displacement hydraulic unit coupled to the output shaft; wherein the variable displacement hydraulic unit port plate and the fixed displacement hydraulic unit port plate couple together through a circumferential interface comprising a generally cylindrical orifice in the variable displacement unit port plate that receives a generally cylindrical end of the fixed displacement unit port plate, a charging fluid annulus that distributes charging fluid from the variable displacement hydraulic unit port plate to the fixed displacement hydraulic unit port plate, and a working fluid annulus that distributes working fluid between the variable displacement unit port plate and the fixed displacement unit port plate to minimise hydraulic thrust forces developed by the hydraulic units. 16. The hydraulic differential of claim 15, wherein the input shaft coaxially mounts within the output shaft. 17. The hydraulic differential of claim 15, wherein the variable displacement hydraulic unit and the fixed displacement hydraulic unit are in radial alignment with the input and output shafts. 18. The hydraulic differential of claim 15, wherein the input and output shafts extend out of a single end of the stationary housing. 19. The hydraulic differential of claim 15, wherein the circumferential interface comprises a bearing for supporting the fixed displacement hydraulic unit port plate with the variable displacement unit port plate.