How many joints does a robot arm need?

How the parts work together

1. Three joints place the gripper

   Base yaw, shoulder pitch and elbow pitch move the gripper to a point. The links never change length, only the angles. With no wrist the gripper points along the forearm, so the joints that choose the point also choose the tilt: drag one slider and Gripper position and Gripper tilt change together.

2. No wrist roll: the cubes arrive turned

   The 4-axis arm adds a wrist pitch, which keeps the gripper pointing straight down as it reaches. Nothing turns the jaws about the gripper's own axis, so each cube turns only as far as the base swings, 87.5 or 80 degrees. Here cube 2 comes down a quarter turn away from its slot's arrow, and cube 1's slot is already red.

3. A wrist roll adds the quarter turn

   The 5-axis arm adds a wrist roll, which spins the gripper about its own axis without moving the tool point. On the way to the box it turns each cube the extra quarter turn, so cube 2 comes down along its slot's arrow and cube 1's slot is green.

4. Five axes cannot lean sideways

   Cube 3 rests on a ramp that slopes sideways, across the arm's upright plane. Wrist pitch tilts the gripper only within that plane and wrist roll only spins it, so the 5-axis arm stops above the cube, the ramp turns red, and the arm moves on without it.

5. A forearm roll leans the gripper

   The 6-axis arm adds a roll about the forearm's own length. Turning it swings the wrist-pitch axis out of line with the shoulder and elbow axes, so pitching the wrist leans the gripper sideways, square to the ramp. Position takes three joints and orientation three more: six in all.

6. Inside a joint

   Each joint is a motor driving its link through a reduction gear. In this cutaway of the elbow, the motor's rotor turns an oval wave generator that flexes a thin 200-tooth cup into a rigid ring of 202 teeth. Each motor turn moves the cup back two teeth, so the motor turns 100 times, the other way, for one turn of the joint. Modern Robotics describes this 100:1 harmonic drive gearing for each joint of the UR5 arm. Move the elbow and watch the motor-turns readout.

The main parts

Base plinth

Bolts the arm to the bench. It holds the yaw bearing and never moves.

Turret

Turns the whole arm about a vertical axis. It carries the shoulder module.

Shoulder module

A motor and reducer at the joint tilt the upper arm forward and back. Its axis is horizontal.

Upper arm

A fixed-length link, 0.40 m between joint centres. It never stretches; only the angles change.

Elbow module

Bends the forearm relative to the upper arm. It bends one way only, 0 to 115 degrees here, so the demo solver always takes the elbow-forward pose. Choose the elbow cutaway to see the motor and gear inside.

Forearm

A fixed-length link, 0.35 m between joint centres.

Forearm roll module

Only on the 6-axis arm. It twists the wrist and gripper about the forearm's own axis, which lets the gripper tilt sideways out of the arm's plane.

Wrist pitch module

Not on the 3-axis arm. It tilts the gripper, so its approach direction can differ from the forearm's.

Wrist roll module

On the 5- and 6-axis arms. It spins the gripper about its own axis. It turns the jaws but does not move the tool point.

Parallel gripper

Two jaws slide together or apart. They are driven separately from the positioning joints.

Questions worth exploring