Self-Locking and Back-Driving

Self-locking is a condition where the screw cannot be turned by the axial force. That is, it does not need a brake to hold the load.

Self-locking occurs when friction $f\le \mu N$ is large enough to hold $P$ without $F$ in the lowering-down situation.

For a square thread, we have

$$\mu \ge \tan \lambda$$

for self-locking.

Condition for ACME Self-Locking

This means that the worm gear is self-locking if $\mu \ge \tan \lambda \cos {\varphi }_n$.

Back-Driving

Back-driving is a condition under which pushing axially causes the screw to turn.

Worm-Gear Self-Locking

Examples

Example 3: Self-locking or back-driving

Example 4: Worm Gear for Lowering Down