end of the output shaft 4 and engages with the stirring tool the output shaft and the input shaft are coaxially arranged the constant transmission assembly 5 is a one way bearing i which is locked when rotates clockwise the inner ring and the outer ring of one way bearing i respectively engages with the input shaft and the output shaft and rotate coaxially the one way bearing i is locked when rotates clockwise means that the one way bearing i transmits rotation to the output shaft 4 when the input shaft rotates clockwise and the rollers of the one way bearing i slip or free wheel when the inner ring rotates anticlockwise thus cannot transmit rotation to the output shaft the upper end of the output shaft defines a counter bore 41 the outer ring of the one way bearing i is fixed in the counter bore the differential transmission assembly 6 includes a sun gear 61 sleeved on the input shaft and capable of rotating clockwise along with the input shaft a planet gear set 62 engaging with the sun gear and a planetary carrier 63 for receiving the planet gear set the planet gear set 62 constitute of 3 u02dc 5 planet gears the planet gears are all circumferential distributed uniformly in the lower end surface of the planetary carrier via vertical spindles.