Thanks guys, for your kind comments
There is no need for explicit tension control, I think.
With this design, the position of ball bearings at sides is precisely determined, but it can be additionally adjusted by means of excentric shafts so she tube is only slightly elastically deformed between the 5 pressure points, and this gives me enough friction so the tube doesn't move on it's own.
Rowland, no, unfortunately I still don't because I don't have my own machine shop yet (it is going to happen once I retire.. at the moment, for jobs like this I am using friend's workshop where I work for him occasionally on Saturdays as an advisor on anything electronic, and he is helping me sometimes with those mechanical parts).