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Comparison of pros and cons of "intubation type outer circulation ball screw" and "floating inner circulation ball screw" by foreign engineers

Views:0     Author:Site Editor     Publish Time: 2021-01-13      Origin:Site

One challenge in ball screw design is the ball circulation system. From a historical point of view, the cannula protruding on the nut is the most commonly used. The cannula provides a circulating passage for the balls from the starting point of the internal thread raceway of the nut. But the intubation cycle requires each ball to be turned 90° at the end of the intubation tube. This violent action leads to higher friction loss and increases the risk of blockage of the steel ball. It also causes fluctuations in friction torque. Intubation can also cause lubricant loss. It is difficult to seal the joint between the nut body and the intubation tube, which easily leads to lubricant leakage, increases the cost of repair and maintenance, and causes oil pollution. Intubation type circulation system is not suitable for nut rotating ball screw. The reason is that an asymmetric radial force is generated when the cannula nut rotates. This radial force can cause vibration and instability at high speeds.

In the past few decades, ball screws have been most commonly used in the CNC machine tool industry. The CNC machine tool industry requires pre-tightening force, which is a challenge for the intubation type circulation system. An important measure of the ball screw is the speed rating (DN), which is the diameter of the screw x maximum speed. The maximum DN value of the intubation type ball screw pair is only about 70,000.

In addition, the pin that deflects the steel ball that must be used in the cannula circulation system has the risk of failure.

                    Intubation type outer circulation ball screw                                           Floating inner circulation ball screw

                        Intubation type outer circulation ball screw                                                             Floating inner circulation ball screw

Internal circulation system

In Germany, engineers prefer the internal circulation ball circulation system to the intubation circulation system. When they design the circulation system, the first consideration is the natural trajectory of the steel ball. Therefore, the German circulation system is designed based on tangential deflection. This can minimize the deflection force and help improve efficiency and reliability. Since there is no cannula connection, there is no risk of lubricant leaking from the nut.

This design is the most basic and most widely used ball circulation system, suitable for various screw diameters and nut styles.

In this way, a deflector (returner) is used to cross the ball from the outer diameter of the screw shaft and then guide the ball into the next raceway (or the previous raceway). Each returner is responsible for one circle of balls, that is, one cycle.

The return device can be made of metal or plastic, and is fixed in the hole on the nut to prevent lubricant leakage. The return devices are evenly and symmetrically distributed, which can balance the radial force and are flush with the outer diameter of the nut.

In all circulation systems, the nut diameter of this ball circulation system can be the smallest, which is a compact type. This design is also more suitable for smaller diameter balls and occasions where the "lead to diameter ratio" is less than 0.5. The DN value can reach 120,000.


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