Self-Locking
Property of a gearbox or screw drive whereby a load acting on the output side cannot drive the gearbox in reverse — an essential safety feature in hoisting and actuator applications.
Definition
Self-locking describes the property of a gearbox (in particular worm or screw gearboxes) whereby a load applied to the output side cannot initiate a reverse movement. The gearbox "locks" due to friction between the active surfaces. This property is desirable in lifting platforms, actuators, and linear axes that must hold their position without a brake.
Self-locking occurs when the lead angle of the gear flanks is smaller than the friction angle. A worm gearbox with a small lead angle (below approximately 6°) is typically self-locking, while ball screws are fundamentally non-self-locking due to their very low friction and therefore require a mechanical brake on vertical axes.
Condition for Self-Locking
γ < arctan(μ)
γ = Lead angle of the gear/spindle [°]
μ = Coefficient of friction between the active surfaces
Typical: μ = 0.10–0.15 → Self-locking at γ < 6–8°
Practical Relevance
Worm gearboxes with a small module and low number of starts are self-locking and are specifically used in lifting platforms, seat adjustments, and valve actuators. Important: Self-locking only applies in the static case — under vibrations or impacts, the locking force can be overcome. In dynamic operation, an additional brake is always recommended.