Hydraulic drive system

1. A hydraulic transmission comprising a first rotary hydraulic machine to be driven by a prime mover, at least one hydraulic drive unit, a hydraulic conduit connecting said first machine and said drive unit, a control operable to control delivery of fluid between said first rotary machine and said drive unit, an additional variable capacity machine mechanically connected to said first machine to supply fluid to an accumulator and a sensor to monitor rotational speed of said first machine, said control operable to vary the capacity of said additional machine and thereby maintain said rotational speed at a desired level.

2. The transmission according to claim 1 wherein said additional machine is reversible to move between a pumping mode in which energy is absorbed from said first machine and a motoring mode in which energy is delivered to said first machine.

3. The transmission according to claim 2 wherein the capacity of said additional machine in either mode is modulated to maintain said rotational speed at a desired level.

4. The transmission according to claim 3 wherein said capacity is increased in either mode when said speed departs from said desired level.

5. The transmission according to claim 3 wherein the capacity of said additional machine is minimized when said rotational speed is at said desired level.

6. The transmission according to claim 3 wherein said prime mover is an internal combustion engine and said control operates to reduce fuel supply to said engine upon an increase in said rotational speed prior to modulation of the capacity of said additional machine.

7. The transmission according to claim 6 wherein said first machine is a variable capacity machine and has a pressure compensating control to vary the capacity of said machine to maintain a predetermined pressure in said conduit.

8. A method of operating a hydraulic transmission having a first rotary hydraulic machine to be driven by a prime mover, at least one hydraulic drive unit, a hydraulic conduit connecting said first machine and said drive unit, a control operable to control delivery of fluid between said first rotary machine and said drive unit, an additional variable capacity machine mechanically connected to said first machine to supply fluid to an accumulator and a sensor to monitor rotational speed of said first machine, said method comprising the steps of monitoring said speed of rotation and varying the capacity of said additional machine to maintain said speed at said desired level.

9. The method according to claim 8 including the step of minimizing the capacity of said additional machine when said speed is at said desired level.

10. The method according to claim 9 wherein said additional machine is reversible and said method includes the steps of conditioning said additional machine to a pumping mode if said speed exceeds said desired level and conditioning said additional machine to a motoring mode if said speed is below said desired level.

11. The method according to claim 10 wherein said prime mover is an internal combustion engine and said method includes the step of reducing fuel supply to said engine upon said speed exceeding said desired level prior to conditioning said additional machine to a pumping mode.

12. The method according to claim 11 including the step of conditioning said additional machine to a motoring mode to provide energy from said accumulator prior to increasing the supply of fuel to said engine.

13. A hydraulic transmission for a vehicle having a pair of drive members to propel the vehicle, said transmission comprising a first variable capacity reversible hydraulic machine to be driven by a prime mover, a plurality of variable capacity reversible hydraulic drive units, each associated with a respective one of the drive members, a hydraulic conduit connecting said first machine and said drive units, a pressure compensating control operable on said first machine to vary the capacity thereof and maintain a predetermined pressure in said conduit and an external control operable upon said drive units and vary the capacity thereof to vary the torque generated by respective ones of said drive units and a sensor associated with each of said drive members to determine the rotational speed thereof, said external control being operable to vary the capacity of respective ones of said drive units and change between a driving condition in which said drive unit consumes fluid in said conduit and a braking condition in which it delivers fluid to said conduit, and said pressure compensating control being operable to cause said first machine to reverse the mode of operation to maintain said predetermined pressure in said conduit, said control monitoring the relative speeds of rotation of said drive members and adjusting the capacity of said drive units to maintain a predetermined relationship therebetween.

14. The transmission according to claim 13 wherein with said drive units in a drive condition, said control operates to reduce the capacity of one of said drive units when said sensor detects that rotation of said one drive unit exceeds that established by said predetermined relationship.

15. The transmission according to claim 13 wherein with said drive units in a braking condition, said control operates to reduce the capacity of one of said drive units when said sensor detects that rotation of said one drive unit is less than that established by said predetermined relationship.

16. The transmission according to claim 13 wherein said control may vary said predetermined relationship and thereby effect steering of said vehicle.