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A hydraulic actuator for a hydraulic servomotor 4 is disclosed. The hydraulic servomotor 4 has a first chamber 5 and a second chamber 6 associated therewith. The hydraulic actuator is connected to a fluid source 2 and a fluid drain 3, and it comprises a valve assembly arranged between the fluid sour

A hydraulic actuator for a hydraulic servomotor 4 is disclosed. The hydraulic servomotor 4 has a first chamber 5 and a second chamber 6 associated therewith. The hydraulic actuator is connected to a fluid source 2 and a fluid drain 3, and it comprises a valve assembly arranged between the fluid source 2 and the fluid drain 3 to control fluid pressures in the chambers 5, 6. At least one auxiliary valve 11 is fluidly connected between the valve assembly and the fluid drain 3, said auxiliary valve(s) 11 being of a kind which is normally closed in a de-energized state. In the case of a power cut off the auxiliary valve(s) 11 will close, thereby preventing fluid flow from the valve assembly towards the fluid drain 3. This causes a servomotor piston member 12 to be hydraulically locked in its instantaneous position, thereby locking the hydraulic servomotor 4. The valve assembly may be designed without taking the locking function into consideration.

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What is claimed is: 1. A hydraulic circuit being connected to a fluid source and a fluid drain comprising: a variable displacement unit, fluidly arranged in a hydraulic main line, having a first chamber and a second chamber, a valve assembly arranged between the fluid source and the fluid drain, th

What is claimed is: 1. A hydraulic circuit being connected to a fluid source and a fluid drain comprising: a variable displacement unit, fluidly arranged in a hydraulic main line, having a first chamber and a second chamber, a valve assembly arranged between the fluid source and the fluid drain, the valve assembly further being arranged to control fluid pressures in the first chamber and the second chamber such that the valve assembly controls flow in and out of the first and second chambers to vary a displacement volume of the variable displacement unit, and at least one auxiliary valve fluidly connected between the valve assembly and the fluid drain, said at least one auxiliary valve being of a kind which is normally closed in a de-energized state, wherein the valve assembly comprises at least four valves arranged in a bridge circuit with the variable displacement unit arranged in a diagonal of the bridge circuit. 2. The hydraulic circuit according to claim 1, wherein the at least one auxiliary valve is an electrically operable valve. 3. The hydraulic circuit according to claim 1, wherein the valve assembly comprises: at least one first valve fluidly connected between the fluid source and the first chamber, at least one second valve fluidly connected between the fluid source and the second chamber, at least one third valve fluidly connected between the first chamber and the at least one auxiliary valve, and at least one fourth valve fluidly connected between the second chamber and the at least one auxiliary valve. 4. The hydraulic circuit according to claim 1, wherein the valves of the valve assembly are controlled by means of a closed loop control. 5. The hydraulic circuit according to claim 4, further comprising at least one sensor, said sensor(s) being adapted to provide an input signal to the closed loop control. 6. Use of a hydraulic circuit according to claim 1 in a hydro-mechanical transmission (HMT). 7. A hydraulic circuit being connected to a fluid source and a fluid drain comprising: a variable displacement unit, fluidly arranged in a hydraulic main line, having a first chamber and a second chamber, a valve assembly arranged between the fluid source and the fluid drain, the valve assembly further being arranged to control fluid pressures in the first chamber and the second chamber such that the valve assembly controls flow in and out of the first and second chambers to vary a displacement volume of the variable displacement unit, and at least one auxiliary valve fluidly connected between the valve assembly and the fluid drain, said at least one auxiliary valve being of a kind which is normally closed in a de-energized state, wherein the at least one auxiliary valve is kept open during normal operation. 8. A hydraulic circuit being connected to a fluid source and a fluid drain comprising: a variable displacement unit, fluidly arranged in a hydraulic main line, having a first chamber and a second chamber, a valve assembly arranged between the fluid source and the fluid drain, the valve assembly further being arranged to control fluid pressures in the first chamber and the second chamber such that the valve assembly controls flow in and out of the first and second chambers to vary a displacement volume of the variable displacement unit, and at least one auxiliary valve fluidly connected between the valve assembly and the fluid drain, said at least one auxiliary valve being of a kind which is normally closed in a de-energized state, wherein at least one valve of the valve assembly is driven by a pulse train signal.