Why a robot arm's shoulder works hardest

An industrial arm stacks six motorized joints: the base turns, the shoulder and elbow swing the arm up and out, and three wrist axes point and twist the tool. Every joint must hold up everything beyond it, so the shoulder carries the payload on the longest lever. Its static torque is roughly the payload's weight times its horizontal distance, which is why rated payload drops as the reach grows.

Controls

Labels

Animation speed

Payload mass

Arm reach

Cycle time per part

Step through it

Axis 1: the baseThe whole arm turns on its base. Here it swings a quarter turn between the infeed and outfeed conveyors on every cycle.
Axes 2 and 3: shoulder and elbowThese two big joints place the wrist in space. Software solves the arm's triangle each instant so the gripper drops straight down onto the part.
Axes 4, 5 and 6: the wristForearm roll, wrist pitch and tool roll set the gripper's angle. Here the tool roll turns the part a quarter turn so it lands lined up with the outfeed belt.
Heavier and fartherA 25 kg part held 1.5 m out needs about 370 N·m at the shoulder, more than this arm's 300 N·m rating. The ring at the shoulder turns red.
Teach pendantTechnicians jog each axis with a handheld pendant, record the pick and place points, and set the speed that fixes the cycle time and parts per hour.

Static torque at the shoulder

Torque is force times lever arm. A payload of mass m pulls down with m times g, and when it is held a horizontal distance R from the shoulder axis, the shoulder motor must supply about m times g times R just to hold it still. A 10 kg part at 1.2 m needs about 118 N·m.

This readout is a simplified worst case with the arm stretched level. Real sizing also adds the weight of the links, the gripper and the motors beyond the shoulder, plus extra torque to accelerate the load, which can exceed the static value on fast cycles. The 300 N·m rating is an illustrative figure, not a specific product.

Try it: hold the payload at 15 kg and slide the reach from 0.8 m to 2.0 m. Torque grows in a straight line from about 118 to about 294 N·m, so the same motor that lifts 15 kg at full stretch could lift far more close in.

Six axes, six jobs

Three axes are enough to put a point anywhere in the work zone: base, shoulder and elbow. Three more at the wrist let the tool reach that point at any angle. Axes nearer the base carry more load, so their motors and gearboxes are the largest, and the wrist motors are small.

Parts per hour is simply 3600 seconds divided by the cycle time. Faster cycles demand faster accelerations, which is one more reason heavy payloads usually run slower.

Where Avid uses this

Avid's agricultural robotics work faces the same sizing question as any arm: how much load at how much reach, before the joints run out of torque.

Sources

Kinematics, Statics, and Dynamics of Two-Dimensional ManipulatorsMassachusetts Institute of Technology, CSAIL (B. K. P. Horn)
Survey of Industrial Manipulation Technologies for Autonomous Assembly Applications (NISTIR 7844)National Institute of Standards and Technology

How to cite this model

Avid Solutions International. (2026, November 11). Why a robot arm's shoulder works hardest [Interactive 3D model]. Avid Field Lab. https://avidsolutionsintl.com/explore/six-axis-robot-arm

Last updated November 11, 2026

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