Axles and Bearings
The axle connects the sides of the yo-yo together, and it's where the string attaches to the yo-yo. As such, the axle and the bearing that goes over it are vital parts of the yo-yo that have a huge affect on the style of play.
Wooden Fixed Axle
The axle in the Duncan ProYo and ProFly is a fixed wooden axle. Highly sensitive to the string twist, the wooden spool (axle) needs to be replaced after a while, as the string will eventually smooth out the wooden groove and make the yo-yo unplayable. This is a cheap option, but is inferior in terms of usability and performance, and isn't very common these days. The Hyper Yo-Yo Woodbullet is a good example of a modern wooden fixed axle yo-yo, and is a good option for beginners.
This 44CLASH collector's edition is a full wooden yo-yo.
Click Here for maintenance tips for wooden axle yo-yos.
Metal Fixed Axle
Duncan's Imperial and Wheels, and Hyper Yo-Yo's MonsterShell are made with fixed metal axles. Like the wooden fixed axles, metal fixed axles are sensitive to string twist. Although they are cheap, they still have relatively low sleep times, making them suitable for beginners. Another key point is the lack of required maintenance, aside from replacing strings. By double-wrapping the string around the axle, the yo-yo will become extremely responsive.
Yomega's Fireball and Hyper Yo-Yo's SpinPhoenix are loaded with plastic bearings, a plastic spool that goes over the metal axle. This reduces friction, causing a remarkable difference in performance from the fixed axle yo-yos above. The maintenance required for this yo-yo is limited to oiling the spool, but models such as the Hyper Yo-Yo Hyper Comet and YoYoFactory Firedog cannot be opened, and therefore require no maintenance.
Metal ball bearings are the latest development in bearing tech, and is the most common type of bearing used today.
Ball bearings come in many shapes too!
The surface and interior of the bearing may vary depending on the manufacturer, for different reasons. These are all designed for string play:
Ball bearings are made with small metal balls between two metal tracks. The 10-ball bearing has more balls than the average 8, giving it a unique smoothness and feeling. The surface of the bearing itself is flat. onedrop yo-yos come standard with 10-ball bearings.
The surface of the bearing is rounded to a concave shape, which helps keep the string in the center of the bearing, and away from the body and response system. KonKave bearings are made by Dif-e-Yo, and are one of the most popular metal bearings available. There are other "curved" bearings similar to the KonKave, manufactured by other makers.
Center Trac Bearing
YoYoFactory's contribution to bearing technology looks a bit like KonKave, but instead of fully curved, a flat track is set in the center of the bearing surface. The effect is the same as KonKave, keeping the string away from the body and response area.
Italian maker B!st makes the V-cave bearings, with straight slopes to a center point. This creates yet another feeling than the KonKave, but achieves the same general effect.
In order to keep the string in the center of the bearing, yo-yo maker Crucial created the Groove bearing, with a literal groove cut in the center to hold the string in place.
Taking two thin bearings and sandwiching them together, the Beef Cake was one of the first mods for ultra-wide gap settings. Although it's not a type of bearing, this setting is still preferred by some players. The string sits in the small gap created by the edge of the bearings, creating a similar effect to the groove bearing.
The Werrd XXXXL is one of the few yo-yos that comes outfitted with a Beef Cake bearing system.