The reasons why the ball screw can not self-lock can mainly be attributed to the following points：

one、Relation between friction Angle and spiral Angle

• The friction Angle is less than the spiral Angle：The friction Angle of the ball screw is smaller than its spiral Angle，This means that when the ball is rolling on the screw，The friction generated cannot overcome the screw helix Angle，Therefore, it cannot form a self-locking effect。

two、The effect of transmission efficiency

• High transmission efficiency：The transmission efficiency of the ball screw is very high，Usually in90%96%among，Much higher than the ordinary lead screw26%46%。This high efficiency allows the ball screw to complete a complete rotation in a very short time，It also means that when the external torque disappears，Internal friction does not allow the ball to stay on the screw for long，As a result, it cannot lock itself。

three、Contact characteristics of ball and thread

• Dynamic contact and unstable friction：Ball screw in transmission，The contact point between the ball and the thread is constantly changing，Unable to form stable friction to close the loop。This dynamic contact mode makes it difficult for the ball screw to generate sufficient self-locking force during transmission。

four、Other factors

• Load and accuracy：The load of the ball screw is too large or the precision of the screw thread is not enough。Excessive load will increase the friction between the ball and the screw，But this increase is not enough to form a self-lock；The insufficient precision of the screw thread will affect the mating accuracy between the ball and the screw，Thus, the self-locking performance is affected。

solution

Because the ball screw can not lock itself，Especially when placed vertically，Measures must be taken to prevent reversals due to weight。Common solutions include the use of servo motors to hold the brake、Loading brake、Use overrunning clutch or can not reverse drive motor, etc。These measures can effectively prevent the ball screw from reversing in the event of power failure or shutdown，Ensure safe operation of equipment。

To sum up，The reason why the ball screw can not self-lock mainly includes that the friction Angle is less than the spiral Angle、High transmission efficiency、The contact characteristics of the ball and thread, as well as load and precision。In practical application，The right solution needs to be selected according to the specific situation to ensure the stability and safety of the ball screw。