I believe in my case heat comes from friction. I use grease on ball screw nuts. When i became aware of the heating was when i was machining 8h per day nonstop. So i greased well separately the ball screws and heat lowered, but still there it was. So now i grease very well these elements very often.

But thats why i think the roller bearing is not very suitable. It will generate much more heat due to much bigger contact area for friction. I love overbuilding but i believe this is not the place to do that. Plus the price of the bearings. How much they cost? The Chinese bearings we use normally are not very expensive. 10-15$