Ball Screws loose not self-locking reasons
In the field of mechanical transmission, Ball Screws are widely used for its high efficiency and accuracy, but it has inherent shortcomings in self-locking performance. Unlike some of the transmission components that rely on their own structure to achieve static locking, the self-locking of ball screws often need to rely on external auxiliary devices, which can not rely on their own mechanical structure to achieve stable self-locking reasons, mainly from the following two key factors.
First, from the mechanical point of view: the relationship between the friction angle and the helix angle
From the mechanical principle, the ball screw in operation, its friction angle is significantly smaller than the helix angle. In the process of rolling the ball along the screw spiral groove, the resistance generated by rolling friction is not enough to counteract the driving force brought about by the spiral structure. When the ball screw is at a standstill or the external force stops acting, the friction cannot form an effective resistance moment to offset the downward trend generated by the helix angle, making it difficult to keep the ball in a fixed position, and thus failing to realize self-locking. For example, in the vertical installation of ball screw application scenarios, if there is a lack of additional braking measures, the load will be due to the role of gravity down along the screw, which is a visual embodiment of this principle.
Efficiency: Self-locking defects due to high transmission efficiency
The high transmission efficiency of Ball Screws (typically up to 90% - 96%) is an advantage, but it is also a key factor in the lack of self-locking. High transmission efficiency means that there is very little energy loss during movement and low frictional resistance between the balls and the screw. When the externally applied torque disappears, the residual friction inside the system is not able to provide enough braking torque to quickly stop the movement of the balls and lock them in position. This phenomenon is particularly evident in dynamically operating equipment. Once the power source is interrupted, the lack of internal frictional resistance effectively binds the ballscrew to maintain a stationary state, resulting in uncontrolled displacement of the load.
It is because of the above characteristics, in the actual engineering applications, especially in the vertical transmission, elevation mechanism and other self-locking security requirements of the higher scenarios, if the use of ballscrews as drive components, must be additionally configured with braking devices (such as servomotor brakes, electromagnetic brakes, etc.). These devices are quickly activated when the equipment loses power or stops running, and by means of mechanical locking or electromagnetic braking, the reverse movement of the ballscrew is forcibly prevented, thus avoiding safety accidents caused by falling loads. In contrast, although the transmission efficiency of ordinary sliding screw is low (only 26% - 46%), the higher frictional resistance makes it have a certain self-locking ability, which also explains the applicability differences of different transmission methods under certain specific working conditions.
In summary, the structural mechanical properties of the ball screw itself and the advantages of efficient transmission, decided that it does not have a natural self-locking function. In the mechanical design and selection, engineers need to fully consider this feature, combined with the actual application scenarios to choose the appropriate transmission program, if necessary, through external auxiliary devices to make up for the lack of self-locking performance, to ensure the safety and reliability of equipment operation.
