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Radial Internal Clearance

Radial internal clearance is the distance that one ring can be moved relative to the other in the radial direction. This is measured in microns or thousandths of a millimetre. Bearings can be classified in several standard clearance ranges. For 608 bearings, standard ranges are C2 (0 - 7 microns), normal (2 - 13 microns), and C3 (8 - 23 microns). When a 608 is placed in the skate wheel, the outer ring is slightly compressed and there is a slight reduction in the clearance. Too little residual clearance (the amount of clearance left after pushing the bearing into the wheel) will result in heavy drag and decreased performance. If there is no clearance in the bearing, the bearing may actually seize. On the other hand, too much clearance will result in a noisy and unsteady bearing. [Top^]

Wheel and Spacer Fits

Ideally, bearings would be tightly pressed into the wheel. Practically, they can't be. That means there will be some slippage, which will lead to wear and reduced performance. The same philosophy can be applied to the inner ring spacers. In both cases, follow your owner's manual and replace components when they become worn. [Top^]

Cage type (also known as a retainer or separator)

What does it do?
The cage keeps the balls from bunching together on one side of the bearing.

What is made of?
Steel and plastic. For 608 bearings, steel cages are assembled in two forms: welded and tab-type. For a welded cage, the two cage halves are welded together between balls to keep the halves from separating.

For tab-type cages, the two cages halves are held together with tabs. The tabs are placed between cage pockets on one cage half and fold over the opposite cage half.

Plastic cages are manufactured from a variety of material including nylon, phenol, Teflon, etc. Most are reinforced with glass fibers which gives the plastic extra strength.

	Plastic cages are manufactured from a variety of material including nylon, phenol, Teflon, etc. Most are reinforced with glass fibers which gives the plastic extra strength.
STEEL CAGE	PLASTIC CAGE
PRO's	PRO's
Impenetrable to liquids.	Natural lubrication.
Lower frictional stress.
CON's	CON's
Tab-type steel cages weaken by vibration, whereas welded steel cages are not affected by vibration.	Can be cracked by vibration.
No natural lubrication properties.	Higher frictional stress due to tight fit of fingers capturing balls.
	[Top^]

Closures

Closures keep grease in the bearing and keep contaminants out. There are 3 main types of closures: Shields, contact seals and non-contact seals.

Shields
	Shields are disk-shaped covers stamped out of sheet steel (see Figure 1). The manufacturing method makes them relatively inexpensive. There is no contact between the shield and the inner ring of the bearing, thus there is no friction and no effect on bearing speed. When grease fills the inner ring groove, it forms what is called a labyrinth seal. This seal acts as a barrier to larger particles but it does not protect against water. Not all manufacturers include this special groove on the inner ring or the special "L" shape at the edge of the shield. (NTN 608Z=608 with 1 shield, NTN 608ZZ=608 with 2 shields).
Contact Seals
	NTN LU seals use two lips to provide a contact seal against the inner ring seal groove. The inner lip keeps grease in while the outer lip keeps contaminants out. Because of the heavy contact, this seal not only keeps out dirt, but also water. Unfortunately, the seal contact increases the torque and increases drag. (NTN 608LU=608 with 1 seal, NTN 608LLU=608 with 2 seals).
Non-contact Seals
	Seals are made by precision moulding rubber onto a steel backing plate. The precision moulding gives us much better control over the final dimension of the LB seal. As a result, the NTN LB seal can run much closer to the inner ring without touching it. The LB seal creates a labyrinth seal like a shielded bearing but the tighter tolerances of the rubber seal allows for a smaller gap between the seal lip and inner ring seal groove. In general, it can keep smaller dust particles away from the bearing interior better than a standard metal shield yet it is not very good against water. (NTN 608LB=608 with 1 seal, NTN 608LLB=608 with 2 seals).
	[Top^]

Lubricant

The two most common lubricants are grease and oil. Grease is basically oil with a thickener or soap. The thickener acts like a sponge to soak up the oil when not in use. A lubricant can keep metal parts from wearing against one another, and are effective in keeping dirt away from the sensitive inner workings of a bearing.

From the above, you can see that a greased bearing requires little servicing yet cannot run as fast as an oiled bearing. An oiled bearing is susceptible to dust and contamination so it needs to be serviced more often (generally after every use). As grease is thicker, it acts as a seal against dirt, but at the same time it can increase torque and slow down the bearing.

In the end, how fast you want to go depends on how often you're willing to service your bearings. While greased bearings require little servicing, oiled bearings perform best when they are cleaned and re-oiled after each use. [Top^]

Contaminant

The most prominent problem with bearings is not failure due to wear but failure due to contamination. Contamination is a broad term which includes dust, dirt and water.

How does dirt and water affect bearing performance? Dirt and water break down the surfaces of the bearing resulting in dents and rust of ball and raceway surfaces. These rough surfaces eventually break down the lubricant and reduce performance.

You can extend bearing life by simply wiping dirt off the face of the bearing - and you don't even have to take it out of the wheel. If dirt and dust build up on the outside of the bearing, they will eventually get in and affect the performance of the bearing. [Top^]