Why "Worst-Case" Torque Matters More Than Average Torque
Sizing an actuator for a robotic arm joint based on average expected load is a real design mistake — a joint motor needs to handle its worst-case static holding torque, which for a multi-link arm occurs at a specific arm configuration (usually fully extended horizontally), not at some typical or average position. An undersized actuator that works fine in most poses can stall, overheat, or fail to hold position entirely the moment the arm reaches its actual peak-torque configuration.
This worst case compounds through the arm's link structure: each joint must support not only its own link's weight and any payload, but also the torque contribution from every link and joint further out along the arm — which is exactly why the base joint (Joint 1) of a multi-link arm typically needs dramatically more torque capacity than the joints closer to the end-effector, since it's supporting the combined weight and moment arm of everything beyond it.
Static holding torque (needed just to hold a position against gravity, with the arm not moving) is also a different, generally lower, requirement than dynamic torque (needed to actively accelerate the arm through motion) — a full actuator sizing exercise needs to check both, since a joint that adequately holds a static pose can still be undersized for the higher torque demands of rapid movement.