Hydraulic motor device for assisting the mechanical transmission of a vehicle

A hydraulic motor device, in particular for assisting the mechanical transmission of a vehicle, which hydraulic motor device includes a hydraulic motor that comprises a cylinder block having cylinders slidably receiving radial pistons, and a reaction cam for the pistons, the cylinder block and the cam being mounted to move in rotation relative to each other about an axis of rotation, the motor further comprising a fluid distributor for the cylinders, a portion of which distributor is constrained in rotation with the cam, and a distribution coupling mounted to be stationary relative to a reference frame of the vehicle and for connecting the distributor to a fluid feed and to a fluid discharge, the motor having a through passage disposed along the axis of rotation for the purpose of receiving a transmission shaft segment, wherein the motor can take up a declutched configuration in which the pistons are radially uncoupled from the cam, the cylinder block is stationary relative to the reference frame of the vehicle, while the cam is rotary, and the cam is constrained in rotation with a transmission shaft segment that is disposed in said through passage.