Method for testing a hydrostatic transmission

1. A method for testing a hydrostatic transmission of a vehicle, the hydrostatic transmission comprising a hydrostatic unit installed on the vehicle, the method comprising the following steps:

calculating an actual value (Q a ) of a parameter (Q) which is indicative of the volumetric efficiency of the hydrostatic unit, in a working condition, wherein the parameter (Q) is the ratio between a value (η v ) of the volumetric efficiency of the hydrostatic unit determined when a clutch of the vehicle is at least partially engaged and a further value (η v0 ) of the volumetric efficiency of the hydrostatic unit determined when the clutch is disengaged,

wherein the further value (η v0 ) is determined from the rotational speeds (n E0 ; n HST0 ) respectively of a first component associated with an input of the hydrostatic unit and of a second component associated with an output of the hydrostatic unit, measured during a first measuring step in which the vehicle is stationary and its clutch is disengaged, and

wherein the value (η v ) is determined from the rotational speeds (n E ; n HST ) of the first component and of the second component, measured during a second measuring step in which the clutch is at least partially engaged; and

determining an expected value (Q ED ; Q EE ) of the parameter (Q) in the working condition, the actual value (Q a ) being comparable with the expected value (Q ED ; Q EE ) in order to evaluate how the hydrostatic unit is working.

2. A method according to claim 1 , wherein the actual value (Q a ) of the parameter (Q) is calculated by multiplying the ratio (n E0 /n HST0 ) between the rotational speed of the first component and the rotational speed of the second component, detected in the first measuring step, by the ratio (n HST /n E ) between the rotational speed of the second component and the rotational speed of the first component, detected in the second measuring step.

3. A method according to claim 2 , wherein the hydrostatic unit comprises a variable displacement hydraulic pump for driving a hydraulic motor, the hydraulic pump having a swash plate that can be positioned at selectable swivel angles, an engine being provided for driving the hydraulic pump, the engine being connected to the hydrostatic transmission by a damper.

4. A method according to claim 3 , wherein the first component is an output shaft of the hydraulic motor and the second component is a shaft of the engine.

5. A method according to claim 3 , wherein the step of determining the expected value (Q ED ; Q EE ) of the parameter (Q) comprises a step of measuring a torque (M D , M E ) generated by the engine.

6. A method according to claim 5 , wherein a planet gearing is interposed between the hydrostatic unit and the engine, so that the torque (M D , M E ) is proportional to a pressure difference (p) between a high-pressure line of the hydrostatic unit and a low-pressure line of the hydrostatic unit.

7. A method according to claim 5 , wherein the torque (M D , M E ) is measured in the second measuring step.

8. A method according to claim 5 , wherein the step of determining the expected value (Q ED ; Q EE ) of the parameter comprises finding out a stored value (Q ED ; Q EE ) of the parameter which was determined for one or more reference vehicles at the torque (M D , M E ).

9. A method according to claim 5 , wherein the torque (M D ) is measured at the damper.

10. A method according to claim 5 , wherein the torque (M E ) is measured internally of the engine.

11. A method according to claim 3 , wherein the swash plate of the hydraulic pump is kept at its maximum swivel angle while values (n HST0 , n E0 , n HST , n E ) required to calculate the actual value (Q) of the parameter are measured.

12. A method according to claim 1 , further comprising providing a control unit for the vehicle, the control unit being programmed for carrying out the method.

13. A method according to claim 1 , further comprising providing a computer program product for use in the vehicle, which program product is operative to cause a processor to perform the method.