The invention relates to the technical field of speed reducers, in particular to an underground speed
reducer which comprises a shell, a sealing cavity is formed in the shell, a first annular sliding sleeve and a second annular sliding sleeve are arranged at the two ends of the sealing cavity respectively, a first
piston is slidably and hermetically connected into the first annular sliding sleeve, and a first spring is axially arranged in the first annular sliding sleeve; the input shaft penetrates through the middle of the first
piston, a second
piston is connected into the second annular sliding sleeve in a sliding and sealing mode, a second spring is arranged in the second annular sliding sleeve in the axial direction, and the output shaft penetrates through the middle of the second piston. When the
pressure difference is generated between the sealing cavity and the outside of the shell, the first piston and the second piston can compress or enlarge the space of the sealing cavity, then the pressure in the sealing cavity is increased or reduced till the pressure inside and outside the speed
reducer is balanced, the working performance and reliability of the speed
reducer in the underground severe environment are improved, and the service life of the speed reducer is prolonged.
Pressure difference inside and outside the speed reducer can be balanced, and sealing performance is good.