Steering transmission for tracked vehicles

A hydromechanical drive and steering mechanism for a tracked vehicle, the mechanism comprising:

• first and second driving outputs;
• first and second power inputs arranged for driving connection to a prime mover;
• a hydraulic drive mechanism comprising first and second bidirectional hydrostatic power units connected to be driven by the first power input and arranged to provide an infinitely variable transmission output speed ratio between the first power input and the first and second driving outputs;
• the first driving output coupled to the first hydrostatic power unit output of the first hydrostatic power unit;
• the second driving output connected to the hydrostatic power unit output of the second hydrostatic power unit;
• a steering differential including:
• a first steering differential input coupled to the first hydrostatic power unit output,
• a second steering differential input coupled to the second hydrostatic power unit output, and
• a steering differential output providing an average output rotation of the first and second steering differential inputs;
• a first selective coupling mechanism for selectively coupling the second power input to the first and second driving outputs;
• and
• a second selective coupling mechanism for selectively coupling the steering differential output to the second power input;
• wherein selective actuation of the first and second selective coupling mechanisms causes the transmission to drive in distinct ratio ranges;
• wherein the second power input includes a first mechanical input comprising a first gear element and interconnected first and second input planetary gear sets connected to the first gear element, the first and second input planetary gear sets being connected to the output of the steer differential by a first spur gear arrangement for delivering power to, and receiving power from, the steer differential;
• the first and second input planetary gear sets also being connected by a second spur gear arrangement to the first driving output and connected by a third spur gear arrangement to the second driving output;
• the first and second input planetary gear sets being shiftable to produce low, first and third ratio ranges at each of the first and second driving outputs;
• and
• wherein the second power input further includes a second mechanical input comprising a second gear element connected to interconnected third and fourth input planetary gear sets;
• the third and fourth input planetary gear sets being connected to the first driving output by the second spur gear arrangement and to the second driving output by the third spur gear arrangement;
• the third and fourth input planetary gear sets being shiftable to produce second-forward, second-reverse, and fourth ranges at each of the first and second driving outputs.