Reading a robot actuator datasheet line by line

By Tolerance Labs engineering team · Updated 18 October 2026

Two actuators with the same headline torque can behave very differently in a robot. The difference is usually in the conditions behind each number. This guide goes through a typical joint datasheet line by line.

Torque and speed

Rated torque, also called continuous or nominal torque, is what the joint can deliver indefinitely without overheating. It means something only with its conditions: ambient temperature, how the joint was mounted or heat-sunk, the speed, and the allowed winding temperature. A rating taken on a large aluminum plate at 25 °C will not hold inside a closed leg in the sun.

Peak torque is what it can deliver for a short time. It should come with a time limit, such as a few seconds, and ideally a starting temperature. It may be set by the motor, by the driver’s current limit or by the gearbox’s momentary rating; ask which. Peak is typically 2 to 4 times rated on joint modules.

Stall torque has two meanings. On joint datasheets it often means continuous torque at zero speed, which may be above or below rated torque: rated torque is measured at speed, where iron and friction losses add heat, but at standstill one phase can carry peak current continuously and heats faster. Use the stall figure for holding. On some motor datasheets it means the theoretical torque with full voltage across a locked rotor, which the motor cannot survive. Ask which one is meant, and whether each torque figure is at the motor or at the output, with or without gearbox losses.

No-load speed is the top speed at a stated voltage with nothing attached. Rated speed is the speed at which rated torque was measured, and it is always lower. Neither tells you what the joint does at peak torque. A torque-speed curve at your bus voltage does, so ask for it if it is missing. Speed falls as a battery sags, so check at your lowest working voltage. How to size a robot joint motor shows the back-EMF check.

Electrical constants

Kt (N·m/A) and KV (rpm/V) compare across datasheets only when the definitions match:

• Kt per amp RMS is √2 ≈ 1.41 times Kt per amp peak for the same motor. Drivers often report q-axis current as peak phase current.
• KV may be referenced to DC bus voltage, to line-to-line peak voltage or to line-to-line RMS voltage, and the numbers differ.
• Kt may be motor-side or output-side, with or without gearbox losses. An output Kt divided by the ratio should land near the motor Kt.
• Resistance and inductance may be phase values or line-to-line values. In a wye winding, line-to-line is twice the phase value.

Mixing definitions doubles errors. Copper loss in a wye motor is 1.5 × I_pk² × R_phase, where I_pk is the peak phase current. With I_pk = 10 A and a datasheet line-to-line resistance of 0.4 Ω, R_phase is 0.2 Ω and the loss is 1.5 × 100 × 0.2 = 30 W. Using the line-to-line value as phase resistance gives 60 W. Inductance matters for the driver: low inductance needs a higher PWM frequency to keep current ripple down, and L ÷ R sets the electrical time constant.

Mechanical figures

• Rotor inertia. Motor side or reflected to the output? Reflected inertia is rotor inertia times the ratio squared. Watch the unit: 1 g·cm² is 10⁻⁷ kg·m², and 1 kg·cm² is 10⁻⁴ kg·m².
• Weight. Does it include the driver board, encoder, brake, cable and connector? Two joints 100 g apart on paper can be equal once the missing parts are added.
• Backlash. Usually in arcmin. Ask for the method and the test torque. Near-zero backlash still allows torsional windup under load.
• Torsional stiffness. In N·m/rad or N·m/arcmin. A single value hides that stiffness is often lower near zero torque. See backlash and torsional stiffness.
• Output bearing ratings. Axial, radial and tilting-moment limits for the load the next link puts on the joint. Often missing, and often the real limit in an arm.

Feedback, communication and power

• Encoder resolution and accuracy. Resolution is the count size; accuracy is how close a reading is to the true angle. A 17-bit encoder may be accurate only to a fraction of a degree. Also note motor or output side, single or multi-turn, and whether multi-turn needs a battery. See encoders for robot joints.
• Communication. Bus type, bit rate, control modes and maximum update rate. Ask whether that rate is per joint or for the whole bus, what the latency is from command to torque, and whether the joint offers impedance control. See CAN, CAN FD, EtherCAT or RS485.
• Voltage range. Nominal, minimum and absolute maximum. Braking returns energy and raises bus voltage, so the absolute maximum must cover it; see regenerative braking and bus overvoltage.
• Supply current. Peak and continuous current drawn from the bus, for sizing the power supply or battery.

Thermal ratings, duty, protection and sealing

Insulation class sets the maximum winding temperature: Class B is 130 °C, Class F 155 °C and Class H 180 °C. Ask where the temperature sensor sits. A sensor on the driver board can lag the winding by tens of degrees during a peak. The thermal time constant tells you how long a peak can last, and RMS torque sizing holds only for cycles much shorter than it. Duty ratings may use the IEC 60034-1 duty types: S1 is continuous, S2 short-time and S3 intermittent periodic. Thermal management of robot joint actuators covers the heat path.

Protections should list over-current, over-voltage, under-voltage, over-temperature and a communication timeout, and say what the joint does when one trips: go limp, damp or apply a brake. The IP rating gives protection against solids (first digit) and water (second digit). Ask whether it covers the connectors and the output shaft seal. See sealing and IP ratings.

Field by field: what to ask

Red flags

• Peak torque with no time limit or starting temperature.
• Rated torque with no ambient temperature, mounting or speed.
• Kt and KV that disagree with Kt ≈ 9.55 ÷ KV by more than a definition factor such as √2 or √3.
• Resolution quoted where accuracy should be, or zero backlash with no test method.
• Maximum power computed as peak torque times no-load speed. The joint cannot reach both at once.
• Weight with no list of what is included.
• Identical figures across different sizes in a series, which suggests copied values.
• An IP rating on a joint with exposed connectors or an unsealed output.

A missing condition is not proof that a number is wrong. It means you should ask, and test the joint before designing around it.

Checklist

• Write the test conditions next to every torque figure.
• Convert Kt, KV, resistance and inductance to one set of definitions.
• Recompute reflected inertia at the output in kg·m².
• Add the parts missing from the weight.
• Check that the voltage range covers your battery and braking energy.
• Ask for the torque-speed curve, thermal time constant and output bearing ratings.
• Confirm encoder accuracy, side and multi-turn method.
• List the protections and what the joint does when each one trips.

Use Robot Parts to compare datasheet fields side by side across robot joint modules and robot motors, and ask us for missing conditions when you request a quote.