Unless I'm mistaken, the main advantages of the bushing over the ball bearing are cost (yes, even in high-end movements), simplicity, size/packaging and a perceived advantage in winding efficiency, as a small diameter bushing theoretically lends greater leverage to the rotor, for better efficiency. It is also known to be very smooth (when properly maintained), compared to bearings, which you can sometimes feel/hear.
The disadvantage of the bushing are less support for the rotor (if the diameter is small, which would be the objective in using a bushing), making the rotor more prone to hitting movement parts/bridges/plates in case of shock, less shock resistance (the bushing can ultimately break...), and necessity of regular maintenance.
As far as I know, leaving aside the winding efficiency element for the time being, the ball bearing is, from a mechanical engineering point of view, superior to the bushing in every way. More support for the rotor, greater durability, less requirement for maintenance, better structural integrity, better shock resistance. And, the clincher: I believe that the technology is there nowadays to make a winding system based on a bearing-supported rotor as efficient as a bushing-based system. As to the packaging element, any new movement designed from scratch can accommodate bearings without issue.
I'm the kind of person who values durability, sound basic design, and the reliability above other criteria, so for me a bearing system is always superior. If I were to design an automatic-winding movement, it would necessarily use bearings for the rotor as opposed to a bushing. Each and every time. It's also probably not a coincidence that most of the new movements from reputable makers (JLC in particular, the recent AP 3120, etc.) all use bearings and that the bushings appear to be found mainly in older designs.
The disadvantage of the bushing are less support for the rotor (if the diameter is small, which would be the objective in using a bushing), making the rotor more prone to hitting movement parts/bridges/plates in case of shock, less shock resistance (the bushing can ultimately break...), and necessity of regular maintenance.
As far as I know, leaving aside the winding efficiency element for the time being, the ball bearing is, from a mechanical engineering point of view, superior to the bushing in every way. More support for the rotor, greater durability, less requirement for maintenance, better structural integrity, better shock resistance. And, the clincher: I believe that the technology is there nowadays to make a winding system based on a bearing-supported rotor as efficient as a bushing-based system. As to the packaging element, any new movement designed from scratch can accommodate bearings without issue.
I'm the kind of person who values durability, sound basic design, and the reliability above other criteria, so for me a bearing system is always superior. If I were to design an automatic-winding movement, it would necessarily use bearings for the rotor as opposed to a bushing. Each and every time. It's also probably not a coincidence that most of the new movements from reputable makers (JLC in particular, the recent AP 3120, etc.) all use bearings and that the bushings appear to be found mainly in older designs.
