For good trackballs, there are two main kinds of ball retaining/bearing designs: rotating rollers or gliding beads. They're very different in terms of feel: rolled ball easily rotates in two directions, while gliding ball moves in any direction, including twist motion. Both designs have advantages and drawbacks, and there are people who prefer one or the another. It's difficult to say which design is better: both kinds could be good or bad.
1.- Ball bearings. 3 to 6 bearings are used, with ball rolling on the shafts inserted into bearings, on the outer ring of bearings directly, or combination of them. With high-grade instrumental ball bearings, movement feel is quite good. Major drawback is a noise: rotation causes micro vibrations, often amplified by device case. Reason for that is, in most cases, the same design flaw: bearings are mounted (either directly or indirectly) in the middle of thin bottom wall, acting as a piano deck or speaker's membrane, converting microscopic oscillations to loud sounds. Resolution would be very simple: separate ball support assembly from the case via damping inserts, like in any vibration-suppressing device, but manufacturers prefer to ignore that issue. Positive things are lack of "starting force" (i.e., static and dynamic friction is essentially the same), and inertia of freely spinning ball. Formerly common design, but almost extinct nowadays.
2.- Gliding points. Typically, they're small ceramic or ruby spheres, fixed inside ball's cup. Classic scheme uses 3 points; sometimes 4th. is added for stability. Cheaper than ball bearings, and almost completely silent. With polished ball, feeling is nearly the same - but often develop issue known as a 'ball sticking', when small movement from static state feels a bit rough. Require periodic maintenance for proper operation: cleaning and ball lubrication. In some early (and later, in very cheap) models, steel balls were used: initially good, but quickly damaged by constant grinding with mixture of dust particles and natural fat secrete of skin, unavoidable on ball surface. The ability to move in any direction has additional advantage: it's possible to use twisting movement as a separate axis control. Kensington SlimBlade uses it for scrolling function.
Also, there's 3rd. kind: simple rollers in a plastic fixture, but such marginal design is found mostly in very primitive and cheap models.
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