Introduction to Internal and External Circulation of Ball Screws

Introduction to Internal and External Circulation of Ball Screws

Ball screws can be categorized into two types of circulation based on how the balls move within the system: external circulation and internal circulation. In external circulation, the balls sometimes disengage from the screw during the cycle. In internal circulation, the balls remain in contact with the screw throughout the cycle. Below is a detailed introduction to both types of circulation in ball screws.

External Circulation

External circulation involves the balls returning to the screw-nut interface via spiral grooves or return tubes on the outer surface of the nut after completing a cycle. There are three main types of external circulation based on the return method: end cap type, return tube type, and spiral groove type.

1. End Cap Type

• A longitudinal hole is machined in the nut to serve as the return channel for the balls. The end caps on both sides of the nut have openings for the balls to enter the return channel, forming a complete cycle.

2. Return Tube Type

• Bent tubes are used as return channels. This design is easy to manufacture, but the protruding tubes increase the radial dimensions of the nut.

3. Spiral Groove Type

• Spiral grooves are milled on the outer circumference of the nut, with holes drilled at the ends of the grooves to connect with the thread raceways, forming the return channels. This design has smaller radial dimensions compared to the return tube type but is more complex to manufacture.

External circulation structures are simple and widely used. However, the joints in the ball raceways are difficult to make smooth, which can affect the smoothness of the ball movement.

Internal Circulation

Internal circulation uses a reversing mechanism to achieve the ball cycle. There are two main types of reversing mechanisms: cylindrical key reversing and flat round insert block reversing.

1. Cylindrical Key Reversing

• The cylindrical part is embedded in the nut, and the end has a reversing slot. The reversing slot is positioned by the outer cylindrical surface and a circular key at the top to ensure alignment with the thread raceway.

2. Flat Round Insert Block Reversing

• The reversing mechanism is a generally cylindrical head with a flat key, embedded in a cut slot in the nut. The end has a reversing slot, and the external contour of the insert block ensures proper positioning. Compared to the cylindrical key reversing, this type has smaller dimensions, reducing the radial and axial dimensions of the nut. However, the dimensional accuracy of the external contour of the insert block and the cut slot in the nut must be very high.

By comparing the two types of reversing mechanisms, the flat round insert block reversing offers smaller dimensions but requires higher precision in manufacturing.