A grade Official document, standard, or first-party source

'This servo does 20 kg·cm': can the datasheet figure be a design figure?

The claim: A product page lists a peak torque (say 20 kg·cm), and it gets used as 'this servo outputs 20 kg·cm' for sizing and design.
Reality: That number is almost always stall torque: the instantaneous value at nominal voltage with the output shaft fully locked. It is not a sustained capability, and the measuring process itself heads toward damage — stall current overheats the motor and driver within seconds. Usable continuous torque is a third to a quarter of the figure; a few-second transient can reach half. The other ignored dimension is speed: peak torque is measured at near-zero motion, and torque falls along the curve as speed rises.
Where the gap is: The gap is that 'peak' is a capability ceiling and 'continuous' is a design basis — and they differ by three to four times. Printing a large peak on the datasheet is not wrong; it is a fact about the device. The error is using it as the design value. Machines designed this way look strong unloaded, then stall and overheat under real loads — and the builder usually diagnoses 'broken servo' when the actual diagnosis is 'wrong column of the datasheet'.
When it closes: When the figure can be used as a reference: ① the motion is transient (the instant of a gripper closing, one launch step); ② there is thermal headroom (metal case, airflow); ③ there is current limiting (the driver protects against stall burn). With all three, the peak works as a capability ceiling. The continuous design value is still a third of it. Joint-motor datasheets usually list both peak and rated columns — the rated column is the design value, and that runs from the SG90 up to Robstride and beyond.

This site's call

The design move: size by a third of the claimed figure, and use the peak only to sanity-check transients. Add a column to your selection table — 'continuous = claimed ÷ 3.5' — and a surprising number of 'not enough torque' verdicts turn into 'actually enough'. The same arithmetic explains why one servo is fine in project A and burns in project B: not quality, duty cycle.

How the three columns relate

Figure	Meaning	Sustains for	Used for
Stall / peak	Shaft locked, nominal voltage, instantaneous	Seconds, heading into overheat	Sanity-checking transient motions
Usable transient	About half of peak	Seconds	Launches, gripper closures
Continuous / rated	About a third to a quarter of peak	Indefinitely	Selection and design

Sources

Servo and joint-motor datasheet torque definitions (stall / peak / rated)

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Last checked 2026-09-28