Hybrid bearings
Hybrid angular contact ball bearings (designation suffix HC) have rings made of bearing steel and rolling elements made of bearing grade silicon nitride (ceramic). As ceramic balls are lighter and have a higher modulus of elasticity and lower coefficient of thermal expansion than steel balls, hybrid bearings can provide the following advantages:
  • higher degree of rigidity
  • higher speed capability
  • reduced centrifugal and inertial forces within the bearing
  • minimized stress at the outer ring rolling contacts at high speeds
  • reduced frictional heat
  • less energy consumption
  • extended bearing and grease service life
  • less prone to skid smearing damage and cage damage when subject to frequent rapid starts and stops
  • less sensitive to temperature differences within the bearing
  • more accurate preload/clearance control

For additional information about silicon nitride, refer to Materials for bearing rings and rolling elements.

Sealing solutions

The most common bearings can be supplied with an integral seal fitted on both sides (designation prefix S). The seal forms an extremely narrow gap with the inner ring shoulder (fig. 1), and therefore speed capability is not compromised.

The seals are made standard of an oil- and wear-resistant NBR and are reinforced with sheet steel. On request, bearings can be supplied with seals made of FKM. For additional information, refer to Seal materials.

Sealed bearings are filled as standard with a high-quality, low-viscosity grease that has a lithium soap thickener and a synthetic ester base oil. The quantity of grease fills ~ 15% of the free space in the bearing. The temperature range for the grease is –55 to +110 °C (–65 to +230 °F). On request, the bearings can be supplied with other greases. For additional information, contact the SKF application engineering service.

When compared to bearing arrangements with open bearings and external seals, sealed bearings can provide a number of advantages, including:

  • potential for extended bearing service life
  • extended maintenance intervals
  • reduced inventory
  • reduced risk of lubricant contamination during mounting and operation

Sealed bearings are lubricated for life. They should not be washed or heated to temperatures above 80 °C (175 °F). If a sealed bearing is to be heated for mounting, an induction heater must be used and the bearing should be fitted immediately to minimize the time that the bearing is exposed to high temperatures. For information about the storage interval of sealed bearings, refer to Shelf life of sealed bearings.

Direct oil-air lubrication
Some very high speed applications require open bearings in the 719 .. D and 70 .. D series, 719 .. E and 70 .. E series, and 719 .. B and 70 .. B series to be lubricated with minimal amounts of oil, directly through their outer rings.

On request, bearings can be supplied with two lubrication holes in their outer rings. Bearings with an annular groove or an annular groove and two annular O-ring grooves, complete with O-rings to seal to the bearing housing bore, are also available (fig. 2). The positions of these features are listed in the product table.

Bearings made of NitroMax steel
The rings of conventional super-precision hybrid angular contact ball bearings are made of carbon chromium steel. Hybrid bearings can, however, be supplied with rings made of NitroMax steel (designation prefix V), a new generation high-nitrogen stainless steel. Bearing rings made of this material have superior corrosion resistance, high wear resistance and enhanced fatigue strength, a high modulus of elasticity and a high degree of hardness and impact toughness.

The combined properties of NitroMax steel rings and balls made of bearing grade silicon nitride greatly improve bearing performance, enabling the bearings to run up to three times longer than conventional hybrid bearings, depending on the lubrication conditions.

These bearings are particularly suitable for very demanding applications such as high-speed machining centres and milling machines, where speed, rigidity, and bearing service life are key operational parameters.

For additional information about carbon chromium steel, ceramics, and NitroMax steel, refer to Materials for bearing rings and rolling elements.

SKF Silent Series bearings
Where quieter running in machine tool applications is desired, SKF Silent Series bearings can offer a number of advantages, including:
  • reduced noise and vibration
  • easier mounting and reduced installation time
  • improved load sharing for matched sets

The bearings are manufactured to a new tolerance class P4B, and have a narrower tolerance range on the bore and outside diameter compared to bearings with a P4A tolerance class (→ Precision). Optimized ball centred polyamide cages in D design bearings and special guidance PEEK cages in E design bearings reduce the "cage rattling" phenomenon allowing more silent running. For grease lubricated applications, SKF Silent Series bearings can achieve the same high speeds as bearings with outer ring centred cages.

This variant is available for certain sizes in the 719 .. D, 70 .. D and 70 .. E series.

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