two lateral brushes (102, 103) and an upper brush (101), each respectively having a respective supporting rod, which is inserted inside a directive channel (105, 110, and 106, 109) and is connected to a gear (111, 112, and 113) transmission means of a rotary motion, generated by one or more engines, towards said gears (111, 112, and 113) a motion conversion mechanism from a rotary motion around a longitudinal axis to a simultaneous rotary motion and vertical oscillating motion perpendicular to the same longitudinal axis and parallel to the larger axis of teeth, wherein: the lateral supporting rods include a respective rear part connected to a respective gear (139) in a off-center position in respect to the rotary axis (140) the gear (139) transmits a conical motion which is converted into a vertical oscillating motion to the lateral brushes (102, 103).
