Materials

Inner and outer races and balls

  • SNFA bearings are manufactured in SAE 52100 (ISO 100 Cr 6) chrome steel, this being a particularly clean material offering high reliability.
  • It is hardened and tempered to a process which allows safe operation up to 150º C.
  • Where the operation conditions are severe, bearings may have ceramic balls.

Cage

  • SNFA design cages to be of low weight and with low friction suitable for operation at high speed.
  • The most commonly used material is cotton reinforced phenolic resin.
  • Some bearing series are equipped with glass fiber reinforced polyamide cages.
  • Brass or aluminum cages are available for special applications on request.

Contact Angle

contact_angle1

In practice, the following may affect the bearing contact angle at critical operating speeds and temperature conditions:

  • high interference fitting of races
  • temperature differential between the outer and inner races
  • centrifugal force effects on the balls and rotating race

Neglecting these possible problems could lead to rapid bearing failure.
Where there is concern about contact angle changes, SNFA will analyze the situation using the most up to date methods.
The choice of the contact angle requires careful application analysis keeping in mind at all times the final performance requirements.

[twocol_one]Contact Angle[/twocol_one] [twocol_one_last]SNFA Code[/twocol_one_last]

[twocol_one]12°[/twocol_one] [twocol_one_last]0[/twocol_one_last]

[twocol_one]15°[/twocol_one] [twocol_one_last]1[/twocol_one_last]

[twocol_one]18°[/twocol_one] [twocol_one_last]2[/twocol_one_last]

[twocol_one]25°[/twocol_one] [twocol_one_last]3[/twocol_one_last]

[twocol_one]62°[/twocol_one] [twocol_one_last]62[/twocol_one_last]

Limiting Speeds

Inner and outer races and balls

Angular contact ball bearings feature the highest rotational speed capabilities of all precision bearings.

The speeds illustrated in the SNFA catalog indicate the limit at which bearings can operate continuously, at a safe, constant temperature (thermal reference speed).  This temperature varies according to the type of lubricant used.  Limiting temperature for grease lubricated bearings is lower than for oil because of greater lubricant deterioration. Limiting speed for a grease lubrication is consequently about 65% of the value achievable with oil.

To ensure high reliability, the temperature limits referred to in setting SNFA bearing speeds are lower than those commonly used for standard non-precision bearings to ensure high reliability.  Bearing limiting speeds depend on their class, series and internal configuration.

The following are further elements that influence bearing limit speed:

  • preload, which affects operating temperature directly
  • precision
  • composition of the set and type of mounting: Face-to-face arrangement (FF; TF, 3TF, TFT) has a lower speed limit.
  • Contact angle

Achievement of maximum speed is of course affected by correct assembly of the bearings in their application.

The following must therefore be provided:

  • housing and spindle components and bearing seats within suitable tolerance limits
  • proper dynamic balancing of rotating parts
  • efficient lubrication.

 Bearing Operating Temperature

Maximum allowable operating temperature for angular contact precision ball bearings corresponds to the limiting value below which no geometrical and structural changes occur. This value depends on material and relevant heat treatment adopted.

SNFA bearing rings are stabilized to attain a temperature of 150° C; stratified textile – resin or polyamide cages can attain a maximum temperature of 120° C.

During operation, the internal bearing temperature is much brighter than that measured by probes placed close to the rings.

This situation is particularly critical in the case of rigidly preloaded bearings operating at high speed since danger local overheating at the ball to race contact area can occur.

This event, that leads to bearing seizure, can be avoided by ensuring that the temperature measured by the probe is well below the values indicated above. In the case of grease lubricated bearings, the situation is already critical when the temperature of the outer and/or inner ring is over 55° C, as the grease deteriorates rapidly.

Temperature limits are much higher with oil lubrication and depend on the lubrication system used.

Vibration and Noise

The vibration and noise level of a bearing expresses component quality and bearing cleanliness.

SNFA bearings are inspected during final manufacturing stage with special three-frequency band test equipment to get further confirmation of the quality requirements.

This equipment also permits vibration analysis on frequency spectra to analyze the behavior of each bearing component.

Typical bearing frequencies are closely correlated to their design and can be calculated with the following formulas:

BPFO: Frequency of ball passing over the outer ring.
BPFI: Frequency of ball passing over the inner ring.
BSF: Frequency of ball rotation about its axis.
FTF: Frequency of cage rotation.
(where “f” is the frequency, in Hz, of the relative bearing ring rotation.