Trimmer head and spool for a trimmer head

A trimmer head for a rotary trimmer apparatus, comprising:

• a housing having a first end wall, a second end wall and a side wall extending between the first end wall and the second end wall and around a rotation axis of the housing;
• trimmer line exit openings in the side wall;
• a spool, received in the housing and axially movable within said housing;
• a spring operatively connected to the spool and axially biasing the spool in a first operating position;
• and
• a kit comprising a first ring member and a second ring member configured to be mounted alternatively on the trimmer apparatus, said first ring member, having a first set of inclined teeth and configured to be coupled to one of said housing and said spool and co-acting with projections on the other of said housing and said spool, wherein said first set of inclined teeth and said projections are configured such that, when the first ring member is coupled to said one of the housing and the spool, the first set of inclined teeth and the projections prevent rotation of the spool with respect to the housing in a first rotation direction, and cause an axial displacement of the spool against the spring, away from the first operating position towards a second operating position, when the spool rotates in a second rotation direction with respect to the housing, said second ring member, having a second set of inclined teeth and configured to be coupled to said one of said housing and said spool and co-acting with the projections on the other of said housing and said spool, wherein said second set of inclined teeth and said projections are configured such that, when the second ring member is coupled to said one of the housing and the spool, the second set of inclined teeth and the projections prevent rotation of the spool with respect to the housing in the second rotation direction, and cause an axial displacement of the spool against the spring, away from the first operating position towards a second operating position, when the spool rotates in the first rotation direction with respect to the housing, wherein each said first inclined teeth and second inclined teeth has a first edge substantially orthogonal to a plane parallel to the ring member, and a second edge, forming an angle smaller than 90° and larger than 0° with said plane parallel to the ring member, wherein the first ring member and the second ring member are configured to be alternatively constrained to one of the first end wall and the second end wall of the housing, and wherein said end wall to which the first ring member and the second ring member are configured to be alternatively constrained is provided with a plurality of projections, operatively associated to the inclined teeth of the first ring member and second ring member;
• and wherein when the first ring member or the second ring member is constrained to the housing, the first edges are approximately flush with said projections.