Does internal clearance effect the bearing precision?
Internal clearance is the amount of internal free space between the rolling elements and the raceways of a bearing. The ranges of internal clearance are governed by ISO and the
ABMA and are denoted by the NTN suffixes: C1, C2, C3, C4 and C5, where no suffix indicates normal clearance. C1 is a range less than C2. C2 is a range less than normal. C3 is a range
greater than normal, C4 is a range greater than C3 and C5 is a range greater than C4.
Precision is a range of tolerance on bearing dimensions. ISO and ABMA govern the precision ranges. ISO uses Class 0, Class 6, Class 5, Class 4, and Class 2 and ABMA uses
ABEC 1, ABEC 3, ABEC 5, ABEC 7 and ABEC 9, respectively the systems are generally interchangeable. NTN uses P0 (usually not added to the part number), P6, P5, P4 and P2 to denote Class 0,
Class 6, Class 5, Class 4 and Class 2, respectively.
Ultimately, clearance does not affect tolerance and tolerance does not affect clearance. (TOP)
Is it safe to use a C3 fit in place of a standard fit?
C3 is typically not referred to as a fit; it is the internal clearance in the bearing as defined above. A bearing fit is how tight or loose the shaft and housing hold the
bearing in place.
When either of the rings of a bearing is tight fitted to its mating component (i.e. the shaft or housing), the resultant deformation of the ring causes a reduction in the amount
of clearance in the bearing. A rotating bearing also produces heat due to material stress and through friction from rolling and sliding contact, lubricant shear, and seal contact.
The bearing housing is usually stationary and is therefore better able to conduct heat away from the outer ring of the bearing. As a result, the temperature of the inner ring and rolling
elements is usually 5℃ to 10℃ higher than that of the outer ring. This results in greater thermal expansion of the inner ring and thus reduces the clearance in the bearing.
If the shaft is being heated or if the housing is being cooled, the temperature difference will be even greater.
Selection of the proper initial bearing internal clearance must take all the foregoing factors into consideration. Assuming this has been done, it is never advisable to replace a bearing
with one having a lower initial clearance. By the same argument, it is usually safe to substitute a larger clearance when the desired clearance is not available. (TOP)
What is preload?
Preload is an initial load or "negative clearance" given to a bearing before operation. This results in the rolling element and raceway surfaces being under constant elastic
compressive forces at their contact points. This has the effect of making the bearing extremely rigid so that even when load is applied to the bearing, radial or axial shaft displacement
does not occur. (TOP)