Back-Drivability
It is the property of the load being able to turn the motor from outside. With spur gears or low-ratio boxes the load can turn the motor easily; in worm-gear and high-ratio boxes friction prevents this and the mechanism becomes self-locking. In lifting applications it directly determines safety.
Locking arises from the worm's thread angle being smaller than the friction angle. In practice worm boxes of approximately 1:50 and above are mostly self-locking, while planetary and spur gear boxes remain back-drivable.
From the driver's point of view two different scenarios arise:
- Non-back-drivable mechanism: When power is removed the load stays where it is. During deceleration the energy the motor produces is limited and the risk of the supply voltage climbing is low.
- Back-drivable mechanism: When power is removed the load starts to fall. The motor turns into a generator, current flows back into the supply and the line voltage rises. A spring-applied mechanical brake is required to hold the load in the air.
Electronic braking, that is short-circuiting the motor terminals, only limits the speed; it does not hold the load in the air indefinitely and does not work when power is removed. In a mechanism that lifts, settle this distinction at the design stage.
The context in which this term is used: DC Motor Driver Selection Guide