The Bearing is a vital component of any TT, belt or Direct Drive. It can be seen as the heart of the rotating system. Any high quality drive system will be let down by a substandard bearing. The result is a higher rumble which is a kind of unwanted low frequency noise. 

The bearing itself is at the look of it a very simple device. There is a shaft which is the extension of the center spindle on which the platter rest. This shaft then rotates in a "well". For reduced friction grease or oil is used between the two. In many cases a ball is used in the bottom of the well upon which the shaft then rests. However, there are many bearings that do not use a ball but a slightly pointed end to the shaft instead.

The top picture shows the Pioneer PL1000L bearing well that is integrated into the company's trademark, the "Stable Hanging Rotor". The next picture shows the shaft and the steel ball. Note that Pioneer here uses a reversed bearing layout where the well is integrated in the moving rotor part and the shaft is stationary. This construction is unique for Pioneer and it could be argued that it's superior becuase it's better to have the contact point as high as possible instead of at the bottom as this should increase stability and reduce pressure between the shaft and the walls of the well. 

However, as very often is the case, the simplest devices often turn out to be the most difficult to perfect. The reason for this is that both the material and the tolerances need to be of the absolute highest standards or it will be painfully obvious in degraded sound quality and high Rumble.

The material used for the bearing is often high quality stainless steel in combination with brass for example but other materials can be found such as ceramics, magnesium, etc.

The parts of the bearing has to be machined with the utmost precision to achieve as smooth and hard a surface as possible. This is time consuming and requires both fine machinery and skilled craftsmen. Therefore the bearing can be one of the most expensive parts in a high quality TT.

There are numerous ways of reducing friction in the bearing. One very effective way is to use a magnetic force to repell the shaft from the bottom of the bearing well. This way, the pressure of the shaft against the ball or the bottom of the well is reduced. An easy method of testing the fricion of a bearing is to spin the platter by hand rather fast and then see how long it keeps spinning. A very good TT can spin for 1.5 to 2 minutes or even more. This indicates that there is a low friction in the bearing. Another way is to spin it very slowly and see how long it keeps turning. This method can be even more accurate in exposing poor bearings. 

Another very important factor is the stability and rigidity of the bearing/spindle. If it's not rigid the platter can "wobble" or flex. To prevent this the shaft needs to be thick and made of high quality steel that will not easily flex. This is especially important when a heavy and oversize platter is used. You can get an idea of how rigid a spindle/bearing is by pressing lightly down on the outer edge of the platter and feel how much it flexes. There are big differences between turntables and cheap constructions often flex way too much. Be careful not to press to hard as this could damage the bearing or even  bend the spindle.  

Many DD bearings are not easily disasambled as they are integrated with the motor and FG generator slit disk. But there are some brands that are easy to disasmble such as Pioneer for example. On these you can pull out the rotor/shaft assembly and closely inspect the shaft and well and maybe add a small amount of high quality oil if needed. Most DD models do not need to be oiled however.