The invention relates mainly to an assembly comprising a brush cage and a spiral spring (36), the spiral spring (36) comprising an axis (Y), a wound part (37) wound about the axis (Y) and an arm (39) extending from one end of the wound part (37), the brush cage comprising at least one lateral wall provided with an opening for the passage of the arm (39) through the lateral wall so that one end of the arm (39) can press against at least part of an end face of a brush, wherein the arm (39) comprises a first portion (391) of which the width (L5) measured along the axis (Y) is different from the width (L6) of the wound part (37) measured along the axis (Y) of the spiral spring (36).