Hydraulic transmission device allowing relatively quiet coupling

Hydraulic transmission apparatus comprising at least one hydraulic motor having radial pistons, said motor comprising:

• a casing;
• inside the casing, a cylinder block that comprises a plurality of cylinders each cylinder containing a slidably mounted piston and being disposed radially relative to an axis of rotation of the motor;
• and
• an undulating cam for the pistons;
• the apparatus further comprising:
• two motor ducts respectively for the fluid feed and for the fluid discharge of the motor, said motor ducts being suitable for being put into communication with said cylinders;
• and
• a casing duct connected to an internal space provided inside the casing and in which a “casing pressure” prevails;
• the motor being suitable for being clutched, in which state the pistons slide in the cylinders while being held in contact with the cam, and the motor is suitable for generating outlet torque under the effect of a difference in pressure between the motor ducts, and for being declutched, in which state the pistons are held in retracted position inside the cylinders;
• the apparatus comprising at least one valve and at least one pump, which are configured, in order to perform the clutching of the motor, to bring a first one of the motor ducts to a “clutching” first pressure that is higher than casing pressure while also making it possible to remove fluid via the casing duct;
• the apparatus comprising, disposed on the casing duct, a limiting valve or a restrictor configured to limit the flow of fluid in said casing duct, in a manner such as to cause the pressure inside the casing to increase during the clutching operation;
• and wherein none of the two motor ducts is connected with said limiting valve or restrictor.