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A hydraulic machine includes first and second opposing motors. Displacement of the first and second motors is controlled such that while the sum of the displacements of the first and second motors is below a threshold, the displacement of the first motor is substantially equal to the sum, and the di

A hydraulic machine includes first and second opposing motors. Displacement of the first and second motors is controlled such that while the sum of the displacements of the first and second motors is below a threshold, the displacement of the first motor is substantially equal to the sum, and the displacement of the second motor is substantially equal to zero. While the sum of the displacements is above the threshold, the displacement of the first motor may be substantially equal to the displacement of the second motor, either as a displacement percentage or as a displacement volume. The first motor may be equal in capacity, or smaller than the second motor.

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1. A hydraulic machine comprising: a first variable displacement hydraulic motor having a first displacement capacity and a first output to which the first motor is configured to apply a varying degree of torque according to a degree of displacement of the first motor;a second variable displacement

1. A hydraulic machine comprising: a first variable displacement hydraulic motor having a first displacement capacity and a first output to which the first motor is configured to apply a varying degree of torque according to a degree of displacement of the first motor;a second variable displacement hydraulic motor having a second displacement capacity, and a second output to which the second motor is configured to apply a varying degree of torque according to a degree of displacement of the second motor;an output shaft to which the outputs of the first and second motors are rigidly coupled at respective ends thereof such that the first and second outputs rotate at a common rate; anda displacement control unit configured to control displacement of the first and second motors such that: while the sum of the displacements of the first and second motors is below a first value, the machine operates in a first mode, in which the displacement of the first motor is substantially equal to the sum, and the displacement of the second motor is substantially equal to zero,if the sum of the displacements increases from below the first value to beyond the first value, without exceeding a second value, greater than the first value, the machine continues to operate in the first mode,if the sum of the displacements increases to beyond the second value, the machine operates in a second mode, andonce having increased beyond the second value, the machine does not return to operation in the first mode until the sum of the displacements drops below the first value. 2. The machine of claim 1 wherein the control unit is further configured to control displacement of the first and second motors such that while the sum of the displacements of the first and second motors is above the first value, a displacement percentage of the first motor is substantially equal to a displacement percentage of the second motor. 3. The machine of claim 1 wherein the first value is equal to about 50% of a sum of the displacement capacities of the first and second motors. 4. The machine of claim 1 wherein the first value is equal to about 38% of a sum of the displacement capacities of the first and second motors. 5. The machine of claim 1 wherein the first value falls in a range between about 20% and 60% of a sum of the displacement capacities of the first and second motors. 6. The machine of claim 1 wherein the control unit is further configured to control displacement of the first and second motors such that if the sum of the displacements increases to beyond a second value, greater than the first value, a displacement percentage of the first motor is substantially equal to a displacement percentage of the second motor while the sum of the displacements of the first and second motors remains above the first value, and if the sum of the displacements decreases to below the first value, the displacement of the first motor is substantially equal to the sum, and the displacement of the second motor is substantially equal to zero while the sum of the displacements remains below the second value. 7. The machine of claim 1 wherein the control unit is further configured to control displacement of the first and second motors such that if the sum of the displacements increases to beyond the first value, a displacement percentage of the first motor is substantially equal to a displacement percentage of the second motor for at least a selected time period, regardless of subsequent changes of the sum of the displacements. 8. The machine of claim 1 wherein the first displacement capacity is substantially equal to the second displacement capacity. 9. The machine of claim 1 wherein the first displacement capacity is less than the second displacement capacity. 10. The machine of claim 1 wherein the first displacement capacity is approximately equal to half the second displacement capacity. 11. The machine of claim 1, further comprising a drive gear positioned on the output shaft. 12. The machine of claim 1, further comprising bearing means for receiving unequal axial loads on the output shaft from the first motor and second motors. 13. The machine of claim 1 wherein the control unit is further configured to control displacement of the first and second motors such that while the sum of the displacements is greater than the first value and less than a second value, the displacement of the second motor is substantially equal to the sum, and the displacement of the first motor is substantially equal to zero. 14. The machine of claim 9 wherein the first displacement capacity is equal to approximately 60% of the second displacement capacity. 15. The machine of claim 13 wherein the control unit is further configured to control displacement of the first and second motors such that while the sum of the displacements is greater than the second value a displacement percentage of the first motor is substantially equal to a displacement percentage of the second motor. 16. A hybrid vehicle system comprising: a drive machine having first and second opposing and rotationally coupled motors; anda control unit configured to independently control output torque of the first and second motors according to an operating range in which a total output torque of the machine falls, such that the first and second motors transition from a first mode of operation to a second mode of operation as the total output torque of the machine rises past a first threshold, and transition from the second mode of operation to the first mode of operation as the total output torque of the machine drops below a second threshold that is lower than the first threshold. 17. The system of claim 16 wherein the control unit is configured to control the output torque of the first motor to be substantially equal to the total output torque of the machine and the output torque of the second motor to be substantially equal to zero while in the first mode of operation. 18. The system of claim 16, further comprising a vehicle drive train coupled to an output shaft of the drive machine. 19. The system of claim 16 wherein the first motor has an output torque capacity that is less than an output torque capacity of the second motor. 20. The system of claim 16 wherein the first motor has an output torque capacity of approximately 60% an output torque capacity of the second motor. 21. The system of claim 16 wherein the first motor has an output torque capacity approximately equal to an output torque capacity of the second motor. 22. The system of claim 16 wherein the first and second motors are hydraulic motors. 23. The system of claim 16 wherein the first and second motors are electric motors. 24. The system of claim 16 wherein output shafts of the first and second opposing and rotationally coupled motors are rotationally locked so as to rotate at a same rate. 25. The system of claim 17 wherein the control unit is configured to control the output torque of the first motor to be substantially equal to the output torque of the second motor while in the second mode of operation. 26. The system of claim 17 wherein the control unit is configured to control an output torque percentage of the first motor to be substantially equal to an output torque percentage of the second motor while in the second mode of operation. 27. A method comprising: varying an output torque of a first motor of a machine and maintaining a second motor of the machine, rotationally coupled to the first motor, at an output torque substantially equal to zero while a total output torque of the machine is within a first range;varying the output torque of the first motor of the machine and maintaining the second motor of the machine at an output torque substantially equal to zero while the total output torque of the machine is within a second range, greater than the first range, if the total output torque of the machine increases from the first range to the second range;varying the output torque of the second motor while the total output torque of the machine is within a third range, greater than the second range; andvarying the output torque of the second motor while the total output torque of the machine is within the second range if the total output torque of the machine decreases from the third range to the second range. 28. The method of claim 27 wherein varying the output torque of the second motor comprises varying the output torques of the first and second motors substantially equally. 29. The method of claim 27, further comprising bringing the output torques of the first and second motors to substantially equal values when the total output torque of the machine increases to beyond a first selected threshold, and bringing the second motor to an output torque substantially equal to zero while varying the output torque of the first motor when the total output torque decreases to below a second threshold, lower than the first threshold. 30. The system of claim 27 wherein output shafts of the first and second motors are rotationally locked so as to rotate at a same rate. 31. The method of claim 27, wherein varying the output torque of the second motor comprises varying the output torque of the second motor while maintaining the first motor at an output torque substantially equal to zero, the method further comprising varying the output torques of the first and second motors substantially equally while the total output torque of the machine is within a fourth range, greater than the third range. 32. The method of claim 27 wherein the total output torque of the machine is equal to a sum of the output torque of the first motor and the output torque of the second motor. 33. The method of claim 28 wherein the varying the output torques of the first and second motors substantially equally comprises varying output torque percentages of the first and second motors substantially equally. 34. The method of claim 33 wherein an output torque capacity of the first motor is less than an output torque capacity of the second motor. 35. A hydraulic machine comprising: a first variable displacement hydraulic motor having a first displacement capacity and a first output to which the first motor is configured to apply a varying degree of torque according to a degree of displacement of the first motor;a second variable displacement hydraulic motor having a second displacement capacity, and a second output to which the second motor is configured to apply a varying degree of torque according to a degree of displacement of the second motor;an output shaft to which the outputs of the first and second motors are rigidly coupled at respective ends thereof such that the first and second outputs rotate at a common rate; anda displacement control unit configured to control displacement of the first and second motors such that while the sum of the displacements of the first and second motors is below a threshold value, the displacement of the first motor is substantially equal to the sum, and the displacement of the second motor is substantially equal to zero, and while the sum of the displacements of the first and second motors is above the threshold value, a displacement percentage of the first motor is substantially equal to a displacement percentage of the second motor. 36. A hybrid vehicle system comprising: a drive machine having a first motor and a second motor, outputs of the first and second motors being rotationally coupled; anda control unit configured to independently control output torque of the first and second motors according to an operating range in which a total output torque of the machine falls, such that the output torque of the first motor is substantially equal to the total output torque of the machine and the output torque of the second motor is substantially equal to zero while the total output torque lies in a first range of operation, and the output torque percentage of the first motor is substantially equal to an output torque percentage of the second motor while the total output torque lies in a second range of operation. 37. The hybrid vehicle system of claim 36 wherein the control unit is configured to control output torque of the first and second motors such that the output torque of the second motor is substantially equal to the total output torque of the machine and the output torque of the first motor is substantially equal to zero while the total output torque lies in a third range of operation. 38. A method comprising: in a machine having a first motor with an output rotationally coupled to an output of the machine and a second motor with an output rotationally coupled to the output of the machine, varying an output torque of the first motor of the machine and maintaining the second motor of the machine at an output torque substantially equal to zero while a total output torque of the machine is within a first range; andvarying output torque percentages of the first and second motors substantially equally while the total output torque of the machine is within a second range. 39. The method of claim 38, comprising varying the output torque of the second motor of the machine and maintaining the first motor of the machine at an output torque substantially equal to zero while the total output torque of the machine is within a third range. 40. A hybrid vehicle system comprising: a drive machine having first and second rotationally coupled motors; anda control unit configured to independently control output torque of the first and second motors according to an operating range in which a total output torque of the machine falls, such that: the first and second motors transition from a first mode of operation, in which the output torque of the first motor is substantially equal to the total output torque of the machine and the output torque of the second motor to be substantially equal to zero, to a second mode of operation, in which the output torque of the first motor is substantially equal to the output torque of the second motor, as the total output torque of the machine rises past a first threshold, andthe first and second motors transition from the second mode of operation to the first mode of operation as the total output torque of the machine drops below a second threshold. 41. The system of claim 40 wherein the first threshold is equal to the second threshold. 42. The system of claim 40 wherein the first threshold is greater than the second threshold. 43. The system of claim 40 wherein the first and second motors are hydraulic motors.