Wheel assembly for a self-propelling trolley, which wheel assembly is arranged to be mounted on said trolley and comprises a wheel (111,112); an electric rotary motor (151,152); an electric connector, arranged to connect the motor (151,152) to a battery for powering the electric motor (151,152), which battery may or may not be comprised in the wheel assembly; and a mechanical brake comprising an engagement part associated with an engaged state, in which the mechanical brake is activated so that a rotational movement of the wheel (111,112) is limited by the mechanical brake, and a disengaged state, in which the mechanical brake is inactivated so that the wheel (111,112) can rotate freely, which electric motor (151,152) is arranged to perform a rocking drive pattern of the trolley by the motor (151,152) propelling the wheel (111,112) such that the trolley moves backwards and forwards; characterised in that the mechanical brake is arranged so that, in said engaged state of the engagement part, the wheel (111,112) can turn freely in relation to the trolley between a first rotary limit position and a second rotary limit position but not beyond said first and second limit positions, the first and second rotary limit positions being arranged in relation to a radius of the wheel (111,112) so that the trolley is allowed to move freely across a ground distance of between 5 and 20 cm in said engaged state of the engagement part, in that the engagement part comprises a pin which is arranged to be inserted into a corresponding cavity to achieve said engaged state, in that the cavity comprises at least two elongated tracks of respective circular arc shape arranged along one and the same circle which circle is concentric with the wheel (111,112) when the wheel (111,112) is mounted on the trolley, in that each of said tracks comprises a respective first side wall, defining said first rotary limit position, and a respective second side wall, defining said second rotary limit position, and in that each of said tracks is arranged to allow said pin to, in said engaged state, travel along said track in question between said respective first and second side walls. a wheel (111,112); an electric rotary motor (151,152); an electric connector, arranged to connect the motor (151,152) to a battery for powering the electric motor (151,152), which battery may or may not be comprised in the wheel assembly; and a mechanical brake comprising an engagement part associated with an engaged state, in which the mechanical brake is activated so that a rotational movement of the wheel (111,112) is limited by the mechanical brake, and a disengaged state, in which the mechanical brake is inactivated so that the wheel (111,112) can rotate freely, which electric motor (151,152) is arranged to perform a rocking drive pattern of the trolley by the motor (151,152) propelling the wheel (111,112) such that the trolley moves backwards and forwards; characterised in that the mechanical brake is arranged so that, in said engaged state of the engagement part, the wheel (111,112) can turn freely in relation to the trolley between a first rotary limit position and a second rotary limit position but not beyond said first and second limit positions, the first and second rotary limit positions being arranged in relation to a radius of the wheel (111,112) so that the trolley is allowed to move freely across a ground distance of between 5 and 20 cm in said engaged state of the engagement part, in that the engagement part comprises a pin which is arranged to be inserted into a corresponding cavity to achieve said engaged state, in that the cavity comprises at least two elongated tracks of respective circular arc shape arranged along one and the same circle which circle is concentric with the wheel (111,112) when the wheel (111,112) is mounted on the trolley, in that each of said tracks comprises a respective first side wall, defining said first rotary limit position, and a respective second side wall, defining said second rotary limit position, and in that each of said tracks is arranged to allow said pin to, in said engaged state, travel along said track in question between said respective first and second side walls.