The black ball nut rides up and down the large screw. The ball nut has threads on the end that thread into the painted collar which pivots on the frame. The painted collar has a set screw on the bottom side that secures the ball nut in the collar.
The white plastic cap on the end of the screw is secured with a roll pin that is tapped into place. The white cap is not necessary for this application but the roll pin needs to be there to stop the ball nut at the end of the stroke.
