String trimmer and trimmer head

A string trimmer, comprising:

• a trimmer head comprising:
• a spool for winding a trimming line, the spool comprising an outer surface having a limiting portion extending upwards from the outer surface, the limiting portion defining a channel;
• a head housing defining an accommodating space for at least partially accommodating the spool, the head housing comprising an outer housing surface defining an aperture though which the limiting portion extends, a circumference of the aperture including a plurality of mating portions extending from the outer housing surface, the plurality of mating portions each having an inclined surface and a plane defined by the inclined surface;
• a transmission member slidably coupled to the spool within the channel, the transmission member comprising a driving portion, the transmission member having a first position where the driving portion mates with at least one of the plurality of mating portions of the head housing to prevent relative rotation between the spool and the head housing and a second position where the driving portion does not mate with at least one of the plurality of mating portions of the head housing to allow relative rotation between the spool and the head housing;
• a biasing element for biasing the transmission member towards the first position;
• and
• a driving device comprising a motor for driving the trimmer head to rotate around a rotation axis;
• wherein the driving portion drives the head housing to rotate synchronously with the spool, the head housing is formed with a mating portion to mate with the driving portion, the mating portion comprises a first teeth distributed around the rotation axis, each of the first teeth comprises a tooth surface, and when the trimmer head rotates around the rotation axis and the transmission member rotates with the spool around the rotation axis, the transmission member generates a centrifugal force along a first straight line, the inclined surface contacts the driving portion and imparts an acting force to the driving portion, the acting force has a first component force along a direction perpendicular to the first straight line and a second component force along the first straight line, and the second component force is also opposite to the direction of the centrifugal force so as to prevent the transmission member from moving in the first straight line, and the trimming line has an effective portion located outside the head housing, and the second component force changes with a length of the effective portion of the trimming line,
• when the trimmer head switches from a working state to a line-feeding state, a rotational speed of the trimmer head does not change, and
• when the trimmer head switches from the working state to the line-feeding state, the trimmer head does not need an indication signal for feeding the trimming line.