Actuators and Mechanics
Brakes and Backdrivability: What Happens on Power Loss
When power fails, a robot either holds position, sags, or falls. Which one depends on gear ratio and brake design, and it is a safety decision made at purchase.
On power loss, a high-ratio geared arm typically holds by friction alone and is then locked by fail-safe brakes; a backdrivable low-ratio arm collapses under gravity unless its brakes engage within milliseconds. Brakes on robots are almost universally spring-applied and electrically released, so removing power applies them. Understanding which behaviour your arm has is a safety question, not a maintenance detail.
The three behaviours
| Arm type | Typical ratio | Without brakes | With brakes |
|---|---|---|---|
| Industrial arm, harmonic or cycloidal | 80:1 to 200:1 | Holds, or creeps slowly | Locked |
| Collaborative arm | 50:1 to 160:1 | Holds or sags slowly | Locked |
| Quasi-direct drive, legged | 6:1 to 10:1 | Collapses immediately | Locked if the brake acts fast enough |
| Direct drive | 1:1 | Collapses immediately | Requires a substantial brake |
| SCARA vertical axis | ball screw | Descends under load | Locked |
How robot brakes work
The dominant design is spring-applied, electrically released. Springs press a friction disc against a stator; energising a coil pulls the disc clear. Removing power, deliberately or through a fault, applies the brake. This is the correct failure direction and it has three consequences worth knowing.
- Brakes are holding devices, not stopping devices. They are rated to hold a stationary load. Repeatedly stopping a moving arm with them wears the friction surface quickly, which is why controllers decelerate electrically first and apply brakes at standstill.
- Engagement takes time. Typically 20 ms to 150 ms, during which a backdrivable joint is already moving. On a legged robot that window matters.
- They must be tested. A brake that has not held a load in two years may not hold one now. Many controllers include a brake test that applies torque against the engaged brake and checks for movement.
Backdrivability, the deliberate trade
| Property | Backdrivable | Non-backdrivable |
|---|---|---|
| Behaviour on power loss | Collapses | Holds |
| Contact force detection | Excellent, senses through the gearbox | Poor, friction masks the signal |
| Impact absorption | Good, joint gives way | Poor, shock goes into the gearbox |
| Hand guiding | Natural | Requires a sensor and active control |
| Energy efficiency holding a load | Poor, current required | Good, friction holds it |
| Typical use | Legged robots, dynamic manipulation | Industrial arms, machine tending |
The two columns are the same design decision viewed from opposite ends. Backdrivability is what makes a joint safe on contact and unsafe on power loss, and every arm architecture picks a point on that line and then adds brakes to cover the consequence.
What the risk assessment needs
- The arm's actual behaviour on power loss, with the payload fitted, verified rather than assumed.
- Brake engagement time and the distance the tool travels within it.
- Whether a dropped payload is a hazard independent of the arm, since a gripper that opens on power loss creates a falling object even if the arm holds.
- Brake test provision and its interval, because an untested brake is an unverified safety function.
- The release procedure for maintenance, including how a trapped person would be freed, which is a common gap in cell documentation.
Point five appears in audits regularly. If a fault leaves an arm holding a person against a fixture, someone must be able to release the brakes safely and lower the arm in a controlled manner. That procedure needs to exist, be documented and be practised.
Testing brakes without breaking anything
A brake test applies motor torque against the engaged brake and measures whether the joint moves. It is quick, it should be scheduled, and it has a few rules.
| Item | Typical value | Note |
|---|---|---|
| Test torque applied | 80 to 120 % of holding rating | From the controller's own routine where available |
| Movement threshold | 0.05 to 0.5 deg | Measured at the joint, not the tool |
| Duration per joint | 2 to 10 s | Six joints in about a minute |
| Recommended interval | monthly to quarterly | More often on high-payload arms |
| Test with payload | yes | An empty arm proves less than a loaded one |
| After a collision | always | Impact loads can glaze a friction surface |
Recording the measured movement rather than a pass or fail turns the test into a trend. A joint that moved 0.05 degrees last quarter and 0.3 degrees this quarter is degrading, and that is visible long before it fails a threshold.
Frequently asked questions
What happens to a robot when the power fails?
High-ratio geared arms usually hold position through gearbox friction and are then locked by fail-safe brakes. Low-ratio backdrivable arms collapse unless their brakes engage in time, typically within 20 ms to 150 ms.
Are robot brakes fail-safe?
Yes, almost universally. They are spring-applied and electrically released, so removing power applies them. That is the correct failure direction and it is why an unpowered arm is normally locked rather than free.
Can brakes stop a moving robot?
They can, but they are rated as holding devices. Repeated dynamic stops wear the friction surface quickly, so controllers decelerate electrically first and apply the brakes at standstill.
Is gearbox friction enough to hold a load?
Usually in practice, and it is not a safety function. Friction varies with grease temperature, wear, load and vibration, so a risk assessment relies on rated brakes rather than on the transmission.
Why do legged robots need fast brakes?
Because their joints are deliberately backdrivable for contact sensing and impact absorption, so a power loss means immediate collapse. Brake engagement time directly determines how far the machine falls before it locks.
Sources
- ISO 10218-1:2025, Robotics, Safety requirements, Part 1International Organization for Standardization, stop functions, brakes and power loss behaviour
- ISO 12100, safety of machinery, general principles for designRisk assessment obligations covering stored energy and release procedures
- ISO/TS 15066:2016Quasi-static clamping limits relevant to a person trapped by a held arm