The main difference between a swing yarder and a tower yarder is that the upperworks is mounted on a large slewing bearing.
yaw bearing | Slewing bearing | Needle roller bearing | Geeks Bearing Gifts | Fluid bearing | Bearing rein | ball bearing | Badger Books SF-58 and SF-61, both dating from 1961 and bearing the byline "John E. Muller" -- but "The Mind Makers" is by Lionel Fanthorpe |