Self-locking occurs when the Coefficient of Friction (μ) > Tangent of the Lead Angle (tanλ).
- Self-Locking: If the lead angle is less than about 6 degrees (often found in ratios of 20:1 or higher), the system is usually self-locking. The load cannot back-drive the worm. This is critical for safety in lifts and hoists. Note: Backlash and lubrication affect this.
- Non-Self-Locking: If the lead angle is larger (lower ratios like 5:1 or 10:1), the system is not self-locking. The load can turn the worm, which is often used in positioning systems where you need reversible motion.