The invention discloses a planetary gear speed
reducer which comprises output torque shell, a bracket
flange, a spline housing, a primary planetary
gear train, a secondary planetary
gear train and a
tertiary planetary
gear train, wherein the bracket
flange and the shell are assembled together through a bearings; the spline housing is utilized as the high-speed input shaft of the speed
reducer; the primary planetary gear
train, the secondary planetary gear
train and the
tertiary planetary gear
train transfer an input torque to the shell through a gear in the shell; a
tertiary planetary frame of the tertiary planetary gear train is provided with a tertiary planetary frame side plate and five
cantilever beam cylinders which are cast together with a planetary frame body; and the bearings at the joint of the shell and the bracket
flange are two single-row tapered roller bearings. According to the invention, by utilizing one planetary side plate, the intensity of the planetary frame is increased, and the service life is improved; through the two single-row tapered roller bearings which are installed together correspondingly at an adjustable interval, the loading supporting points of the bearings can be enlarged, and the
static loading of the whole planetary gear speed
reducer can be improved; and through changing gear materials, a
heat processing technology and a manufacturing process, the gear module can be reduced while the gear intensity is improved.