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Hybrid Bearings

Hybrid Bearings

Hybrid bearings have rings of bearing steel and rolling elements of bearing grade silicon nitride (Si3N4). In addition to being excellent electric insulators, hybrid bearings have a higher speed capability and will provide longer service life than all-steel bearings in most applications.

The very good electrical insulating property is one of the essential features of the silicon nitride. This protects the rings from electric current damage and so-called washboarding, and thus increases bearing service life.

The density of silicon nitride is only 40% of the density of bearing steel. Thus the rolling elements weigh less and have lower inertia. This means less cage stresses during rapid starts and stops and also significantly lower friction at high speeds as explained in the section "Friction". Lower friction means cooler running and longer lubricant service life. Hybrid bearings are thus suitable for high rotational speeds.

Under insufficient lubrication conditions there is no smearing between silicon nitride and steel. This enables hybrid bearings to last much longer in applications operating under severe dynamic conditions or lubrication conditions with low operating viscosity (? < 1). For hybrid bearings it is common to apply ? = 1 for running conditions with ? < 1 to estimate life under such conditions. Hybrid bearings may perform well, when lubricated with ultra thin film forming media, such as refrigerants, enabling oil-free designs but care needs to be taken in design and material selection. In such cases it is recommended, to consult the SKF applikation engineering service before deciding upon design and ordering.

Silicon nitride has a higher hardness and higher modulus of elasticity than steel, resulting in increased bearing stiffness and longer bearing service life in contaminated environments.

Silicon nitride rolling elements have a lower thermal expansion than steel rolling elements of similar size. This means less sensitivity to temperature gradients within the bearing and more accurate preload control. When designing bearing arrangements for very low temperature and as to estimate reductions in bearing clearance of hybrid bearings, please contact the SKF application engineering service.

Hybrid deep groove ball bearings

The SKF standard range of hybrid bearings essentially comprises hybrid single row deep groove ball bearings (fig 1). The reason for it is clear: deep groove ball bearings are the most widely used bearing type, especially in electric motors, and are very useful for simple designs utilizing greased-for-life bearings. Deep raceway grooves and the close conformity between the raceways and the balls enable the accommodation of radial loads as well as of axial loads in both directions.

SKF hybrid deep groove ball bearings are available from 5 up to 110 mm bore diameter. They meet most application needs. Larger bearings can also be manufactured by SKF on request.

Bearings up to 45 mm bore diameter, for example, are most suitable for electrical motors in the power range of 0,15 up to 15 kW as well as generators, power tools and high-speed drives.

There is a wide application field for hybrid deep groove ball bearings, consequently SKF produces

  • sealed and greased-for-life bearings
  • open design bearings.

Hybrid deep groove ball bearings - Sealed and greased-for-life bearings

Sealed and greased-for-life SKF hybrid deep groove ball bearings (fig 2) are protected on both sides, either by

  • a low-friction seal of the RSL design (a) fitted to bearings with an outside diameter up to 25 mm, designation suffix 2RSL
  • a low-friction seal of the RSL design (b) to bearings with an outside diameter over 25 mm and up and inclusive 52 mm, designation suffix 2RSL
  • a low-friction seal of the RZ design (c) fitted to bearings with an outside diameter above 52 mm, designation suffix 2RZ
  • a contact seal of the RS1 design (d), designation suffix 2RS1.

Details about the suitability of the different seals for various operating conditions can be found in the section "Deep groove ball bearings".

The seals are made of acrylonitrile-butadiene rubber (NBR) with sheet steel reinforcement. The permissible operating temperature range for these seals is 40 to +100 C and up to +120 C for brief periods.

Sealed bearings are filled as standard with a premium quality grease, synthetic ester oil based using polyurea thickener, bearing designation suffix WT. It has excellent lubrication properties in the temperature range from about +70 to +120 C, offers extremely long life unattainable with other sealed and greased-for-life bearing designs and fits the needs of electrical machinery. The most important properties of the WT grease are listed in table 1.

Regarding the suitability for high temperatures, the permissible temperature ranges of the cage and seals have to be taken into consideration. For SKF hybrid bearings with seals of fluoro rubber, which withstand temperatures up to 180 C please contact the SKF application engineering service.

Hybrid deep groove ball bearings - Open design bearings

In addition to sealed and greased-for-life bearings larger SKF hybrid deep groove ball bearings are also available in open basic design without seals. If smaller open design bearings are required and the quantity is small, SKF recommends ordering sealed hybrid bearings and removing the seals, which can be done quite simply.

Misalignment Hybrid deep groove ball bearings have only limited ability to accommodate misalignment. The permissible angular misalignment between inner and outer rings, which will not produce inadmissibly high additional stresses in the bearing, depends on

  • the radial internal clearance of the bearing in operation
  • the bearing size
  • the forces and moments acting on the bearing.

Depending on the various influences of the factors, the permissible angular misalignment lies between 2 and 10 minutes of arc. Any misalignment will result in increased bearing noise and reduced bearing service life.

Cages Depending on the bearing size, SKF hybrid deep groove ball bearings are fitted with

  • an injection moulded snap-type cage of glass fibre reinforced polyamide 66, ball centred, designation suffix TN9 (fig 3a)
  • a riveted cage of pressed steel, ball centred, no designation suffix (fig 3b).

Hybrid bearings with cage of glass fibre reinforced polyamide 66 can be operated at temperatures up to +120 C.

Minimum load To achieve satisfactory operation, hybrid deep groove ball bearings, like the standard bearings, must always be subjected to a given minimum load. Please refer to the section "Minimum load" of standard deep groove ball bearings.

However, hybrid bearings are generally more resistant to skidding and smearing damages of raceways caused by too light loads. This makes hybrid bearings a good alternative for bearing arrangements subjected to variable load cycles that include light loads.

Axial preloading In order to provide low noise and good highspeed operation it is normal to apply an axial preload to a bearing arrangement comprising two hybrid deep groove ball bearings. A particular simple method to apply the axial preload is by using spring washers, as described in the section "Preloading by springs". The recommended axial preloads can be calculated as provided in this section. For additional information please refer to the section "Bearing preload".

Axial load carrying capacity If deep groove ball bearings are subjected to purely axial load, this axial load should generally not exceed the value of 0,5 C0. Small bearings (bore diameter up to approx. 12 mm) and bearings in the light Diameter Series 0 should not be subjected to an axial load greater than 0,25 C0. Excessive axial loads can lead to a considerable reduction in bearing service life.

Equivalent dynamic bearing load

P = Fr when Fa/Fr = e

P = XFr + YFa when Fa/Fr > e

The factors e and Y depend on the relationship f0Fa/C0, where f0 is a calculation factor (see product data), Fa the axial component of the load and C0 the basic static load rating.

In addition, the factors are influenced by the magnitude of the radial internal clearance. For bearings with C3 internal clearance mounted with the usual fits as listed in table T1, T5 and T6, the values for e and Y are listed in table 5.

Equivalent static bearing load

P0 = 0,6 Fr + 0,5 Fa

If P0 < Fr, P0 = Fr should be used.

Speed capability Hybrid deep groove ball bearings fitted with a polymeric cage can be operated at speeds in excess of the ratings given for all-steel bearings. The limiting speeds listed in the product tables are valid for bearings with the standard cage, seal and grease according to the bearing designation. Hybrid bearings fitted with a cage of polyetheretherketone (PEEK) can be operated at higher speeds and temperatures. For more information please contact the SKF application engineering service.

The values for "Reference speed" shown with the sealed bearings are valid for open basic design bearings and demonstrate the speed capability of these bearings. For sealed bearings the values listed for "Limiting speed" should not be exceeded.

Hybrid bearings perform excellently under vibrating or oscillating conditions. It is therefore not usually necessary to apply special greases or preloads for these conditions.

Silicon nitride properties The properties of the bearing grade silicon nitride (Si3N4) are presented in the section "Materials for rolling bearings".

Electrical properties Hybrid bearings provide effective protection against electric arc damage to the grease and raceways caused by both AC and DC currents. The impedance for a hybrid bearing is high, even for very high frequencies, providing extremely good protection against high frequency current and peaks through the ball/raceway contacts. For small hybrid bearings equipped with a sheet steel reinforced contact seal acrylonitrilebutadiene rubber (NBR), the voltage level when the first arcing occurs through the seal/bearing contact is beyond 2,5 kV DC. For additional information please contact the SKF application engineering service.

Supplementary designations The designation suffixes used to identify certain features of SKF hybrid deep groove ball bearings are explained in the following.

DesignationBearing Features
C3Radial internal clearance greater than Normal
F1Grease filling grade: 10-15% of the free space in the bearing
HC5Rolling elements of silicon nitride
2RS1 Sheet steel reinforced contact seal of acrylonitrile-butadiene rubber (NBR) on both sides of the bearing
2RSH2Sheet steel reinforced contact seal of fluoro rubber (FKM) on both sides of the bearing
2RSLSheet steel reinforced low-friction seal of acrylonitrile-butadiene rubber (NBR) on both sides of the bearing
2RZSheet steel reinforced low-friction seal of acrylonitrile-butadiene rubber (NBR) on both sides of the bearing
TNHInjection moulded snap-type cage of glass fibre reinforced polyetheretherketone (PEEK), ball centred
TN9Injection moulded snap-type cage of glass fibre reinforced polyamide 66, ball centred
WTGrease with polyurea thickener of consistency 23 to the NLGI Scale for a temperature range 40 to +160 C (normal filling grade)

Selection of bearing size When selecting the necessary bearing size of hybrid deep groove ball bearings please follow the procedure for all-steel bearings in the section "Selection of bearing size". Due to the higher modulus of elasticity of ceramic balls the static safety factor s0 should be increased by

s0 hybrid = 1,1 s0 all-steel

The recommended values of s0 for all-steel bearings can be found in table 10.

Lubrication Most of the SKF hybrid deep groove ball bearings are sealed and greased-for-life.

In the case of open bearings and grease lubrication SKF recommends the SKF grease LGHP 2 for electrical motors. For very high-speed applications at temperatures below +70 C the use of SKF grease LGLT 2 is recommended. More about the SKF greases can be found in the section "Lubrication".

Applications requiring long service life at extremely high speeds have to be oil-lubricated. The two recommended lubrication methods in this case are

  • oil jet lubrication
  • oil+air lubrication.

Oil+air lubrication enables reliable lubrication to be achieved with extremely small quantities of oil, which lowers the operating temperatures, enables higher speeds and reduces oil emission to the environment.

For more information about oil+air lubrication systems, refer to the publication "Oil+Air Systems".