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There are two commonly used circulation methods: outer circulation and inner circulation. The ball is sometimes out of contact with the screw during the circulation process is called the outer loop; the ball is always in contact with the screw is called the inner loop. Each closed loop of the ball is called a row, and the number of leads contained in the closed loop of each ball is called the number of turns. There are 2 rows, 3 rows, 4 rows and 5 rows of each nut of the inner circulation ball screw pair, and each row has only one circle; each row of the outer circulation has 1.5 circles, 2.5 circles and 3.5 circles and so on.

1) External circulation: The external circulation is that the balls re-enter the circulation through the spiral groove or cannula on the outer surface of the nut and return to the screw and nut after the end of the circulation process. The return methods of the external circulation ball screw nut pair according to the ball circulation mainly include end cap type, intubation type and spiral groove type. Commonly used external circulation methods are end cap type; intubation type; spiral groove type. For the end cover type, a longitudinal hole is machined on the nut as a return passage for the balls. The cover plates at both ends of the nut are provided with return openings for the balls, from which the balls enter the return pipe to form a circulation. Intubation type, which uses elbows as return pipes, this structure has good manufacturability, but because the pipes protrude from the outside of the nut, the radial size is large. Spiral groove type, which mills a spiral groove on the outer circle of the nut, and drills through holes at both ends of the groove and is tangent to the threaded raceway to form a return channel. This structure is smaller than the intubation structure in radial size, but Manufacturing is more complicated. The outer circulation ball screw has a simple structure and manufacturing process and is widely used. The disadvantage is that it is difficult to smooth the seam of the raceway, which affects the stability of the ball raceway.

2) Inner circulation: Inner circulation adopts reverser to realize ball circulation, and there are two types of reverser. Cylindrical convex key reverser, its cylindrical part is embedded in the nut, and a reverse groove 2 is opened at the end. The reverse groove is positioned by the cylindrical outer surface and the round key 1 on the upper end to ensure the alignment of the thread raceway direction. The oblate insert reverser, the reverser is a general round head flat key insert, the insert is embedded in the groove of the nut, and the end is provided with a reverse groove 3, which is positioned by the outer contour of the insert. Comparing the two reversers, the latter is smaller in size, thereby reducing the radial dimension of the nut and shortening the axial dimension. However, the dimensional accuracy of the outer contour of the reverser and the groove on the nut is relatively high.