This is the plastic nut (usually white in color) that is inside the actuator tube (where you can’t really see it).  The worm screw goes through the middle of the nut, and the nut has threads on the outside that the stroke tube screws onto; as the motor turns the worm screw, the nut rides up and down the worm screw, pushing and pulling the stroke tube and making the chair go up and down.

IMPORTANT NOTE: Please check the manufacturer of your motor (Okin or Dewert) before ordering this part – the part shown here will only work in a Dewert motor – it will not fit any other brand motor.  Installation instructions are listed on the Dewert Troubleshooting Guide.