Servo watts calculation

According to R18:

PWM COTS servos with a maximum power rating of 4W each at 6VDC
Per the Servo Industry, Servo Max Power Rating = (Stall Torque) X (No Load Speed)

I’m wanting to do the calculation for the HS-422 servo. Can someone verify my work?

Assumes 6v power at PWM port on roboRIO.

Torque T = 4.1 kg-cm = .4 N-m
Angular speed w = .16sec @ 60 degrees

Watts W = N-M / s = .4 / .16 = 2.5

You can compare to this spreadsheet. Looks like the only difference is due to rounding.

It’s worth noting that what the servo industry calls “power”, knowledgeable people call “wrong”. Divide that false rating by 4 to get the theoretical maximum power of the servo, which you can use for mechanism design.

(It’s false because there exists no condition1 where a DC brush motor is simultaneously spinning at its free speed and drawing its stall current.)

1 Assuming that the motor is connected only to an electrical power source at steady state.