nonbackdrivable

Nonbackdrivable refers to a mechanical system or component that resists reverse motion under load when the driving energy is removed. In this sense, backdrivability is the ease with which an externally applied force can move the system in the opposite direction; a nonbackdrivable design prevents such motion, typically through geometry, material friction, or locking elements. Nonbackdrivable actuators can hold a load without continuous power, which can improve safety and energy efficiency for static positioning.

Common mechanisms include worm gear transmissions, where the contact between worm and gear provides high resistance to reverse rotation; friction-based self-locking lead screws, where the lead angle is small enough that thread friction prevents backward motion; and ratchets or one-way clutches that permit forward motion while blocking backward movement. In some hydraulic or pneumatic systems, nonbackdrivability is achieved with valves and locking elements that prevent reverse movement when the drive source is inactive, though such arrangements may be sensitive to leakage and transient loads.

Advantages include maintaining position without power, reducing risk of unintended movement, and simple control for static tasks. Disadvantages include the need for higher input torque to initiate movement in the forward direction when friction is high, reduced efficiency due to friction losses, and potential degradation of lock strength from wear, temperature changes, or contamination. Nonbackdrivable designs are contrasted with backdrivable systems, where external forces can drive motion even when actuators are unpowered. Applications appear in vertical actuators, precision stages, brake systems, and safety mechanisms requiring reliable immobilization.