A Roller Bearing is really an extraordinary piece of mechanical engineering. The smallest types have a diameter of no more than a few millimeters and are used in dentist drilling devices with maximum rotation speeds of more than 30.000 rpm, while the largest ones are used in tunnel drilling equipment and measure more than 7 meters in diameter, while rotating at no more than a few rpm's per day.
Linked is an article about one of Koyo's mega bearings for such an application.
http://www.bearing-news.com/delivers-77m-diameter-slewing-bearing-for-one-of-japans-largest-tunnel-excavation-shield-machines/
