I finished the modification to the crane axis over the weekend, and it is looking good. The motor is now mounted on a swivelling carriage that also has the bottom bearing for the ballscrew. On the arm is another swivelling carriage with the ballnut attached – that’s the bit that travels up and down.
Result: no play at the camera end! This is going to be really solid.
Important thing about positioning the screw seems to be to get the angle close to 90 degrees when the jib arm is level, and to avoid any acute angles at the extreme ends of travel. It required me to attach an extension arm to the bottom of the rotating pillar so I could achieve this. I have seen rigs where the motor carriage is attached close to the pillar, but I think mine is superior.
Yet to test it out as I am waiting for a solid 8/10mm coupler to arrive from China!