Actuators and Mechanics
Direct Drive vs Geared Joints: The Real Trade
Removing the gearbox gives backdrivability, bandwidth and zero backlash, and costs an order of magnitude in torque density. Where each answer is right.
A gearbox multiplies torque by its ratio and divides everything good by roughly the same amount: backdrivability, bandwidth and transparency. Direct drive joints reach only 1 to 4 Nm per kg against 15 to 60 for geared ones, so they appear where force fidelity matters more than payload. Between the two sits quasi-direct drive at 6:1 to 10:1, which is what most modern legged robots use.
What the ratio does to everything
A gear ratio r multiplies output torque by r and divides output speed by r. Two less obvious consequences dominate the design decision.
- Reflected inertia scales with r squared. A motor rotor behind a 100:1 gearbox appears 10,000 times heavier at the output. That is what makes a geared joint feel rigid and unbackdrivable, and it is why an external force cannot easily move it.
- Friction and its variability are multiplied too. Gearbox friction masks the force signal, which is why estimating contact force from motor current works poorly on high-ratio joints and well on low-ratio ones.
| Property | Direct drive | Quasi-direct, 6:1 to 10:1 | Geared, 50:1 to 200:1 |
|---|---|---|---|
| Torque density | 1 to 4 Nm/kg | 4 to 12 Nm/kg | 15 to 60 Nm/kg |
| Backdrivability | excellent | good | poor |
| Backlash | none | small | under 1 to 3 arcmin |
| Force from current estimate | accurate | usable | poor |
| Impact tolerance | excellent | good | poor |
| Holding a load unpowered | no | no | usually yes |
| Efficiency holding a static load | poor | poor | good |
| Control bandwidth | very high | high | moderate |
| Maintenance items | bearings only | few | grease, wear, backlash |
Where each belongs
| Task | Choice | Reason |
|---|---|---|
| Industrial pick and place | Geared, high ratio | Payload and rigidity dominate |
| Machine tending | Geared, high ratio | Must hold a load unpowered |
| Legged locomotion | Quasi-direct drive | Impact tolerance and contact sensing |
| Dynamic catching and throwing | Quasi-direct or direct | Bandwidth and low reflected inertia |
| Haptic devices | Direct drive | Transparency is the whole point |
| Precision rotary positioning | Direct drive torque motor | No backlash, high resolution |
| Collaborative manipulation | Geared with torque sensing | Payload plus sensed compliance |
The last row is worth noting. Collaborative arms did not solve compliance by removing the gearbox; they kept the ratio for payload and added joint torque sensors to recover the force signal the gearbox hides. That is a different answer from the legged robotics one, and both are correct for their problem.
The fourth option: gearbox plus sensing
Rather than choosing a point on the ratio line, several designs keep a high ratio and add instrumentation to recover what the gearbox hides.
| Method | Force resolution | Added mass | Added cost | Limitation |
|---|---|---|---|---|
| Motor current estimate | several N | 0 | 0 | Friction dominates the signal |
| Joint torque sensor | 0.1 to 1 N at the tool | 0.1 to 0.4 kg per joint | moderate | Degrades with distance from the joint |
| Dual encoder, deflection based | 0.5 to 3 N | 0.05 to 0.2 kg | low to moderate | Needs known compliance |
| Flange force torque sensor | 0.02 to 0.2 N | 0.3 to 1.5 kg | high | Adds length, reduces payload |
The joint torque sensor row is what most collaborative arms use, and it explains why they can be both geared and compliant. The flange sensor remains the most accurate and is a separate purchase rather than a joint design choice.
The heat consequence
A direct drive joint holding a static load draws whatever current that torque requires, continuously, and turns most of it into heat. A high-ratio geared joint holds the same load with a fraction of the motor torque, and often with no current at all once the brake engages. For any application where an arm holds a position for long periods, that difference decides the thermal design and often the outcome.
Frequently asked questions
What is direct drive?
A joint where the motor drives the load without a gearbox, so output torque equals motor torque. It gives zero backlash, excellent backdrivability and high bandwidth at roughly a tenth of the torque density of a geared joint.
Why are geared joints hard to backdrive?
Because reflected inertia scales with the square of the ratio, so a rotor behind a 100:1 gearbox appears 10,000 times heavier at the output. Gearbox friction adds to that, which is why an external force cannot easily move the joint.
What is quasi-direct drive?
A joint with a low gear ratio, typically 6:1 to 10:1, recovering much of the torque density while keeping enough backdrivability to absorb impacts and estimate contact force from motor current. It is standard in modern legged robots.
Can a direct drive joint hold a load with the power off?
No. Without gearbox friction there is nothing to resist gravity, so a brake is mandatory and its engagement time determines how far the joint moves before it locks.
Why do collaborative arms keep gearboxes?
Because payload matters and gearboxes deliver it. They recover the force signal the gearbox hides by adding joint torque sensors, which is a different solution from the low-ratio approach used in legged robots.
Sources
- arXiv robotics preprints, actuator design and quasi-direct drivePrimary literature on transmission ratio trade-offs in legged robots
- ISO 8373, robotics vocabularyInternational Organization for Standardization, joint and transmission terminology
- ISO 10218-1:2025, Robotics, Safety requirements, Part 1International Organization for Standardization, brake and power loss requirements