Backdrive capability in Gearboxes: Everything You Need to Know

Backdrive capability in gearboxes, also known as reversibility, describes the ability to operate in both directions—both driven by the motor (input side) and driven in reverse by the load (output side). Simply put: If torque is applied to the output shaft, the gearbox can move backward instead of mechanically locking up. Backdrive-capable gearboxes are therefore not self-locking and allow for bidirectional operation.

Why some gearboxes are backdrive-capable and others are not

Whether a gearbox can be operated in both directions depends primarily on the type of components. Strain wave gears and cycloidal gears are fundamentally reversible—when reversing, only the friction needs to be overcome. An example of a non-reversible gear is the worm gear. Depending on the design, its construction and the friction between the components may prevent it from being reversed from the output side. The gear then acts as a self-locking mechanism.

Which types of gearboxes are backdrive-capable?

The following overview shows typical types of gearboxes and their basic Backdrive capability. The specific design of the gearbox and the overall system is always the decisive factor.

Advantages of backdrive-capable gearboxes

• Greater safety in human-machine interaction: In an emergency, a de-energized system can still be moved manually to free trapped persons or remove obstructing objects.
• Manual mobility in a de-energized state: Depending on the system design, axes or mechanisms can be moved even without an active motor drive, for example during setup via manual teach-in, maintenance, emergency stop situations, or service calls.
• Protection of mechanics and drive: In the event of external forces, backdrive capability helps prevent immediate overloads or damage to components. The resulting motion can be braked in a controlled manner via motor control with a defined decay curve.
• More sensitive control of dynamic systems: In combination with sensors and servo drives, backdrive-capable gearboxes enable a more sensitive response to external forces or load changes, for example in collaborative robots, exoskeletons, or test benches.
• Defined behavior under overload: Instead of locking up abruptly, the system can yield in a controlled manner. This is particularly advantageous when the back-drive torque is adjustable and can thus be adapted to the application.

Strain wave gears with adjustable backdriving torque

Nabtesco offers strain wave gears with an adjustable backdriving torque. This allows the behavior of the axis to be specifically adapted to the application: from easily backdrive-capable for manual guidance to higher resistance against uncontrolled reverse movements. A wide range of adjustment values is possible, but complete self-locking cannot be guaranteed. The specific values depend on the respective size and the customer-specific design. In cycloidal gears, the backdriving torque is not adjustable, but they are generally backdrivable. The specific backdriving torque is determined by the size and gear ratio.

When is which backdriving torque appropriate?

Whether a high or low backdriving torque is appropriate depends on the specific application. The decisive factor is whether a system should yield or whether it must hold its position as reliably as possible.

A high backdriving torque is often required for safety reasons to limit uncontrolled movements. In a robotic arm, for example, it helps ensure that a load does not drop freely when the power is off. Medical devices also benefit from this when unintended movements need to be reduced in the event of an input side failure. In dynamic applications, a higher backdriving torque can also be advantageous, as it enhances the system’s rigidity.

A low backdriving torque, on the other hand, is useful when axes are to be moved easily from the outside. This applies in particular to collaborative robots. Here, it facilitates precise manual guidance, delicate movements, and intuitive operating concepts. During teaching, where motion paths are defined by manually guiding the axes, good backdrive capability is particularly important: The robot can be moved in a realistic manner without unnecessary mechanical resistance.

Comparison of High and Low Backdriving Torque

The following overview shows the key differences between high and low backdriving torque.

Self-locking gearboxes

In applications where a load must be held securely without additional motor power after reaching the target position, self-locking gearboxes are advantageous. Typical examples include cranes, winches, or gate drives. In these cases, backdrive capability is undesirable because the load should not be moved backward by external forces or its own weight.

The application decides

Backdrive capability is not a universal advantage, but rather a matter of application. A low backdriving torque supports delicate movements, manual guidance, and direct human-machine interaction. A high backdriving torque, on the other hand, offers greater resistance to uncontrolled reverse movements and can be useful for holding loads or safety-critical axes. The key is to select the back-drive torque appropriate for the specific function. With adjustable backdriving torques, Nabtesco’s strain wave gears offer additional design flexibility here.

Would you like to know which backdriving torque is right for your application? Our experts will assist you in selecting and designing the appropriate gearbox. Contact us now!