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Limited by the driving conditions of hilly and mountainous terrain, portable digging machine is widely used for fruit tree planting, rubber planting, small tree transplanting and fertilization. The spiral drilling bit of the digging machine can rotate under the drive of the gasoline engine, which can only rotate in one direction. While the lifting and lowering of the spiral drilling bit needs to be done manually, which not only has high labor intensity, but also leads to the inaccurate control of the feed speed and poor digging quality. In addition, the vibration of the gasoline engine is intense and the maneuverability of the digging operation is poor. In order to solve the problem that the gasoline engine can only rotate in one direction, a kind of automatic lifting digging machine was designed based on the three-star gear reversing principle. The automatic lifting digging machine consisted of a gasoline engine, a spiral drill bit, a belt drive mechanism, a three-star gear reversing mechanism, a screw nut lifting mechanism and a frame. By adjusting the position of the reversing handle, the number of meshing gears of the three-star gear reversing mechanism changed, so the single power direction of the gasoline engine could be converted into clockwise and counterclockwise directions. Then the forward and backward rotation of the nut shaft of the screw nut lifting mechanism could be realized. When the digging machine was working, one part of the power of the gasoline engine drove the spiral drill bit to rotate, and the other part of the power drove the screw nut lifting mechanism to move back and forth. Thus the spiral drill bit could feed uniformly. When the digging operation was completed, the spiral drill bit could be reset automatically. In the process of digging, manual operation was required only when changing direction. The average effective digging time of single operation was 57 s. While the manual reversing operation time was only 2 s, accounting for 3.5% of the total time of single pit operation. In the feed stroke and reset stroke of the digging operation, no manual operation would be needed, so it greatly reduced the labor intensity. The average hole depth was 237 mm, and the average hole diameter was 300mm. The digging quality met the requirements of the subsequent fertilization operation. The average maximum throwing radius was 631 mm, and the throwing effect was good which was convenient for soil backfilling. In addition, the middle limit hole of the reversing handle was added. When the spiral drill bit encountered large stones or tree roots during the feed stroke of digging operation, the power transmission path from the gasoline engine to the screw nut lifting mechanism could be temporarily cut off, in order to prevent the screw nut lifting mechanism from getting stuck. So the middle limit hole played a protective role for the whole system. This research also provides design reference for small digging machine to realize automatic lifting, which greatly reduce the labor intensity and improve the digging operation quality. [ABSTRACT FROM AUTHOR]