ORCA-Servo: A Transparent Two-Check Factor Method for Rotary Servo Motor Sizing

Servo motor sizing guidance reaches the practitioner in fragments, and the same application handed to two engineers yieldstwo motors with no record of why either was chosen. This paper presents ORCA-Servo, a transparent factor method for the first selectionof a rotary servo motor, made before a motion profile exists. From moving mass, stroke, move time, dwell, mechanism geometry, andambient temperature, the method computes peak torque and a closed-form root mean square torque, then applies two checks and twogates: a thermal check scaled by ambient and margin factors, a peak check scaled for shock loading, and gates on reflected inertia ratioand motor speed. Every factor is recorded with its value and its source. The paper also documents the Alternative, developed withGeorge Bennett, which selects the gear ratio to match load and rotor inertia and applies a load margin to gravity-loaded axes only. Thetorque model reproduces the Alternative method’s units spreadsheet to four significant figures, and the same case shows how omitting thegravitational constant from weight-based inertia inflates torque 32-fold. Three worked examples show that for geared point-to-point axesthe inertia gate, rather than heat, usually sets the frame. A companion Monte Carlo study evaluates all three procedures against as-builtconditions.