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A hydraulic circuit for a tree-cutting machine having a feed mechanism powered by hydraulic fluid at the output of a hydraulic motor. The hydraulic motor may comprise a variable-displacement motor. The hydraulic circuit may comprise elements which provide an anti-stall mode, wherein the feed mecha

A hydraulic circuit for a tree-cutting machine having a feed mechanism powered by hydraulic fluid at the output of a hydraulic motor. The hydraulic motor may comprise a variable-displacement motor. The hydraulic circuit may comprise elements which provide an anti-stall mode, wherein the feed mechanism is operated to withdraw the cutting element upon pressure at an input port of the hydraulic motor exceeding a threshold.

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What is claimed is: 1. A hydraulic system for controlling and actuating a cutting apparatus, the hydraulic system comprising: a hydraulic motor having an input port and an output port; a cutting element coupled to be driven by the hydraulic motor; one or more feed actuators coupled to advance or r

What is claimed is: 1. A hydraulic system for controlling and actuating a cutting apparatus, the hydraulic system comprising: a hydraulic motor having an input port and an output port; a cutting element coupled to be driven by the hydraulic motor; one or more feed actuators coupled to advance or retract the cutting element; and, a control system connected in series with the motor and connected to deliver the hydraulic fluid to the one or more feed actuators at a selected pressure and rate of flow, the selected pressure and rate of flow controlling an advancing movement of the cutting element. 2. A hydraulic system according to claim 1, wherein the motor is a variable displacement motor and the hydraulic system comprises an actuator connected to increase a displacement of the motor when a fluid pressure at a motor input port exceeds a threshold pressure. 3. A hydraulic system according to claim 1 comprising one or more hydraulic elements configured to retract the cutting element when a torque experienced by the cutting element increases above a threshold. 4. A hydraulic system according to claim 3, wherein the one or more hydraulic elements configured to retract the cutting element are configured to stop retracting the cutting element when the torque experienced by the cutting element falls below the threshold. 5. A hydraulic system according to claim 3, wherein the one or more hydraulic elements configured to retract the cutting element comprise a pressure-operated valve connected to redirect a flow of hydraulic fluid in response to a pressure of the hydraulic fluid at a motor input port. 6. A hydraulic system according to claim 1, wherein the control system comprises a pressure relief valve and a flow control valve. 7. A hydraulic system according to claim 6, wherein the pressure relief valve is connected to receive the hydraulic fluid from a motor output port and to maintain the hydraulic fluid at the selected pressure. 8. A hydraulic system according to claim 7, wherein the flow control valve is connected to receive the hydraulic fluid from the motor output port at the selected pressure and to transmit the hydraulic fluid at the selected rate of flow. 9. A hydraulic system according to claim 1, wherein the control system comprises a pressure reducing valve and a flow control valve. 10. A hydraulic system according to claim 9, wherein the pressure reducing valve is connected to receive the hydraulic fluid from a motor input port and to maintain the hydraulic fluid at the selected pressure and the flow control valve is connected to transmit the hydraulic fluid at the selected rate of flow. 11. A hydraulic system according to claim 1, wherein the control system comprises a first flow control valve in series with a check valve, which are connected to maintain the hydraulic fluid at the selected pressure, and a flow control valve, which is connected to transmit the hydraulic fluid at the selected rate of flow. 12. A hydraulic system according to claim 1, wherein the control system comprises one or more valves capable of creating a pressure when they transmit hydraulic fluid, which are connected to maintain the hydraulic fluid at the selected pressure, and a flow control valve, which is connected to transmit the hydraulic fluid at the selected rate of flow. 13. A hydraulic system according to claim 1 comprising one or more hydraulic elements configured to prevent advancement of the cutting element in the absence of pressurized hydraulic fluid at a motor input port. 14. A hydraulic system according to claim 1 comprising a check valve connected between a motor input port and a motor output port. 15. A hydraulic system according to claim 1, wherein the cutting element comprises a rotatable saw blade. 16. A hydraulic system according to claim 1, wherein in the one or more teed actuators comprise one or more hydraulic cylinders. 17. A hydraulic system according to claim 1 comprising a direction control valve which comprises a valve element switchable between: a first configuration, wherein pressurized hydraulic fluid is directed in a first direction to actuate the motor and to advance the cutting element; and a second configuration, wherein pressurized hydraulic fluid is directed in a second direction to retract the cutting element. 18. A cutting apparatus comprising: a rotatable cutting element; a feed mechanism for advancing and retracting the cutting element in a substantially linear manner; and a hydraulic system comprising: a hydraulic motor coupled to rotate the cutting, element, the hydraulic motor having a motor input port and a motor output port; a control system for controlling the feed mechanism, the control system comprising one or more hydraulic elements connected to receive hydraulic fluid from one of the motor input port and the motor output port and connected to deliver the hydraulic fluid to one or more feed actuators at a selected pressure and rate of flow, the one or more feed actuators capable of advancing and retracting the cutting element. 19. A cutting apparatus according to claim 18 comprising a variable displacement motor and the system comprises an actuator connected to increase a displacement of the motor when a fluid pressure at the motor input port exceeds a threshold pressure. 20. A cutting apparatus according to claim 18 comprising one or more hydraulic elements configured to retract the cutting element when a torque experienced by the cutting element increases above a threshold. 21. A cutting apparatus according to claim 20, wherein the one or more hydraulic elements configured to retract the cutting element are configured to stop retracting the cutting element when the torque experienced by the cutting element falls below the threshold. 22. A cutting apparatus according to claim 20, wherein the one or more hydraulic elements configured to retract the cutting element comprise a pressure-operated valve connected to redirect a flow of hydraulic fluid in response to a pressure of the hydraulic fluid at the motor input port. 23. A cutting apparatus according to claim 18, wherein the control system comprises a pressure relief valve and a flow control valve. 24. A cutting apparatus according to claim 23, wherein the pressure relief valve is coupled to receive the hydraulic fluid from the motor output port and to maintain the hydraulic fluid at the selected pressure. 25. A cutting apparatus according to claim 24, wherein the flow control valve is coupled to receive the hydraulic fluid from the motor output port at the selected pressure and to transmit the hydraulic fluid at the selected rate of flow. 26. A cutting apparatus according to claim 18, wherein the control system comprises a pressure reducing valve and a flow control valve. 27. A cutting apparatus according to claim 26, wherein the pressure reducing valve is connected to receive the hydraulic fluid from the motor input port and to maintain the hydraulic fluid at the selected pressure and the flow control valve is connected to transmit the hydraulic fluid at the selected rate of flow. 28. A cutting apparatus according to claim 18, wherein the control system comprises a first flow control valve in series, with a check valve, which are connected to maintain the hydraulic fluid at the selected pressure, and a flow control valve, which is connected to transmit the hydraulic fluid at the selected rate of flow. 29. A cutting apparatus according to claim 18, wherein the control system comprises one or more valves capable of creating a pressure when they transmit hydraulic fluid, which are connected to maintain the hydraulic fluid at the selected pressure, and a flow control valve, which is connected to transmit the hydraulic fluid at the selected rate of flow. 30. A cutting apparatus according to claim 18, wherein the selected pressure and rate of flow of the hydraulic fluid delivered to the one or more feed actuators determine a speed and force of the one or more feed actuators. 31. A hydraulic system for controlling and actuating a tree and log cutting apparatus, the hydraulic system comprising: a variable displacement hydraulic motor which receives pressurized hydraulic fluid at a motor input port; and a cutting element coupled to be driven by the variable displacement hydraulic motor, the variable displacement motor capable of increasing its torque output in response to increases in a cutting pressure experienced by the cutting element and capable of increasing its speed output in response to decreases in the cutting pressure experienced by the cutting element. 32. A hydraulic system according to claim 31 comprising one or more feed actuators coupled to advance and retract the cutting element, and one or more hydraulic elements connected to deliver the hydraulic fluid to the one or more feed actuators at a selected pressure and rate of flow to control an advancing movement of the cutting element. 33. A hydraulic system according to claim 32 comprising one or more hydraulic actuators coupled to retract the cutting element, the hydraulic system configured to deliver hydraulic fluid to operate the hydraulic actuators to retract the cutting element when a torque experienced by the cutting element increases above a threshold. 34. A hydraulic system according to claim 33, wherein the hydraulic system is configured to stop retracting the cutting element when the torque experienced by the cutting element falls below the threshold. 35. A hydraulic system according to claim 34 comprising a pressure-operated valve connected to redirect a flow of hydraulic fluid in response to a pressure of the hydraulic fluid at the motor input port. 36. A hydraulic system according to claim 32, comprising a pressure relief valve having an input connected between the motor and the one or more feed actuators and a flow control valve connected in series between the motor and the one or more feed actuators. 37. A hydraulic system according to claim 32, comprising a pressure reducing valve connected to maintain the hydraulic fluid at the selected pressure, and a flow control valve connected to transmit the hydraulic fluid at the selected rate of flow. 38. A tree cutting machine comprising: a hydraulic system according to claim 31; a source of pressurized fluid connected to the hydraulic system; and, a hydraulic feed mechanism connected in series with the hydraulic motor, the hydraulic feed mechanism coupled to advance the cutting element into a tree being cut. 39. The tree cutting machine of claim 38 comprising a pressure-operated valve having a control input connected to an input port of the motor, the pressure-operated valve connected to redirect a flow of pressurized fluid away from the hydraulic feed mechanism in response to a pressure of the pressurized fluid at the motor input port. 40. A tree cutting machine comprising: a source of pressurized fluid; a hydraulic motor connected to the source of pressurized fluid; a cutting element coupled to be driven by the motor; and a hydraulic feed mechanism connected in series with the hydraulic motors, the hydraulic feed mechanism coupled to advance the cutting element into a tree being cut wherein the motor is a variable displacement motor and the tree cutting machine comprises an actuator connected to increase a displacement of the motor when a fluid pressure at a motor input port exceeds a threshold pressure. 41. A tree cutting apparatus comprising: a rotatable cutting element; a feed mechanism for advancing the cutting element toward a tree to be cut; and a hydraulic system comprising: a variable-displacement hydraulic motor coupled to rotate the cutting element, the hydraulic motor having an input port and an output port; a control system for controlling the feed mechanism, the control system comprising one or more hydraulic elements connected to receive hydraulic fluid from one of the input port and the output port and connected to deliver the hydraulic fluid to the feed mechanism to cause the cutting element to advance at a variable rate dependant on a torque experienced by the hydraulic motor. 42. A tree cutting apparatus according to claim 41 wherein the cutting element comprises a saw blade. 43. A tree cutting machine comprising: a source of pressurized fluid; a hydraulic motor connected to the source of pressurized fluid; a cutting element coupled to be driven by the motor; a hydraulic feed mechanism connected in series with the hydraulic motor, the hydraulic feed mechanism coupled to advance the cutting element into a tree being cut; and, a pressure-operated valve having a control input connected to an input port of the motor, the pressure-operated valve connected to redirect a flow of pressurized fluid away from the hydraulic feed mechanism in response to a pressure of the pressurized fluid at the motor input port. 44. A tree cutting machine according to claim 43 wherein the hydraulic motor is a variable-displacement motor.