A ball bearing has three primary components - (I) Ring: Outer & Inner, (II) Cage, and (III) Balls. Besides, these three, there is optional ancillary sealing for further protection. Off the primary elements, probably the most critical is the Cage. The cage is the ball bearing component that separates and positions the balls at equal intervals around the bearing's raceway.

Preference of cage material of the bearing depends on application. Dental Air-Turbines are high speed applications - speed up to 420,000 RPM. In comparison, a ceiling fan runs at a mere 400 RPM, the aero-engine of commercial aircraft at 3000 RPM, and the F1 cars at 16000 RPM.  At such staggering speed, the cage material is designed to offer very high performance parameters. Bearings used in air-rotor handpiece use high performance polymer or its composites as cage material. They are either Poly-Amide-Imide (PAI) or Phenolic-Resin bonded cotton fiber composites.

Bearings used in air-rotor handpiece use high performance polymer or its composites as cage material. They are either Poly-Amide-Imide (PAI) or Phenolic-Resin bonded cotton fiber composites. They may be easily distinguished by their color - as shown in figure, PAI is black and Phenolic is Yellowish-Red. 

PAI is virtually unaffected by aliphatic and aromatic hydrocarbons, chlorinated and fluorinated hydrocarbons, and most acids at moderate temperatures making it the most preferred bearing cage material for dental air-rotor applications. Torlon®, manufactured by Solvay SA, Belgium, is the most common cage material. Still all cage materials are susceptible to stronger inorganic bases, i.e. Sodium Hypochlorite etc.

ASTM F2953-12, sets the standard for phenolic cage materials. Phenolic cage is produced from phenolic-resin laminated cotton fabrics - basically a composite of microfibers and phenolic resin. Due to the microfiber matrix, Phenolic cages are little porous and thus act as a reservoir to lubricant. Air-Rotor that uses bearing with Phenolic cage is little resistant to irregular maintenance. Phenolic cage is preferred when there is lack of consistent lubrication.

In almost all cases, the bearings failure is attributed to the failure of cage. Though fairly chemical resistant, the cage materials, however, may be attacked by bases. Yet, the most detrimental to cage of any high speed dental air-rotor bearing is crystalline foreign solid particles, most of which are either of:

• Crystalline carbon forming inside the cartridge due to the burning of residual lubricant oil
• Enamel/dentine dust reverse aspirated onto the cartridge
• Rust (Iron Oxides) from the Air-Rotor components

Recently, further significant improvements have been made. Two of them are particularly impressive. One by MinebeaMitsumi group company NHBB, and the other by their competetitor GRW GmbH.

• GRW GmbH have improved the PAI cage material by chemically bonding a dry lubricant in XTRAlon®  that helped in increased tolerance of high temperatures and minimising friction and wear.
• NHBB Inc. has come-up with a novel Silver coated composite metal polymer cage for Ultra-high speed capability.