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 ARB 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. ARB 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 tolrance does not affect clearance.
C3 is typically not referred to as a fit; it is the internal clearance in the bearings 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 thought 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° C 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 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.
Preload is an intial 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.
A deep groove ball bearing is a non-separable type ball bearing with the line connecting contact points of the steel ball and the raceways of 0° in the radial direction. They are designed to take mainly radial loads but can handle some axial load in either direction. An angular contact bearing is also a non-separable type. The line connecting contact points of the steel ball and raceways create an angle with the line drawn in the radial direction called the contact angle. In addition to radial loads, an angular contact bearing can accommodate relatively heavy axial loads in one direction. The larger the contact angle, the larger the axial loadingcapacity becomes. Angular contact ball bearings are offered with a 15° , 30° or 40° contact angle.
Back-to-back (DB) or face-to-face (DF) are duplex arrangements of angular contact bearings (that can also refer to tapered roller bearings). Radial loads and axial loads in either direction can be accommodated by these arrangements. DB should be selected when shaft mounting rigidity is the primary factor. Select DF when shaft misalignment is a concern. The DB arrangement requires the inner rings to be clamped whereas the DF arrangement requires the outer rings to be clamped.
ARB tapered roller bearings with the prefix 4T - or ET - and all ARB-Bower TRBs are Timken interchangeable. Please note that it is not recommended to mix TRB components between different manufacturers.
In general, steel varieties which can be handened not just on the surface but also deep hardened by the so-called "through hardening method" are used for the raceways and rolling elements of bearings. Foremost among these is high carbon chormium bearing steel, which is widely used. Case hardened steel has excellent efficiency shock loads because of its combination of a hard surface layer which has been carburized and hardened to an appropriate depth and a relatively pliable inner core. ARB uses case hardened steel for almost all of its tapered roller bearings and is denoted in ARB nomenclature with 4T - or ET - prefixes.
The "B" typs spherical has asymmetric rollers guided by three fixed inner ring ribs that provide better skewing control. The retainer in this bearing is finger type and provides optimum lubrication access. The "C" type spherical employs symmetric rollers and utilizes a floating center guide ring. As no fixed guide rings are used in this design, a window type cage is necessary to guide the rollers.
ARB's standard spherical roller bearing, over 50mm bore, is a B-type design. This type of bearing has asymmetric rollers that have the advantage of reduced roller skewing. This allows the bearing to run cooler at higher speeds than other spherical designs.
V#, where # is a whole number, indicates a varication on the basic part number. This suffix does not indicate any specfic feature(s), therefore a drawing will be required to determine the special features.
For general applications a grease fill of 30% of the free space in the bearing seems to be accepted as standard by most manufacturers. Slow speed applications inn harsh environments (such as agricultural machinery) can sometimes require a larger fill. High speed applications (i.e. hand tools) can require a lesser fill.
ARB distributes ARB Bearing Unit grease in 400g cartridge format. Standard bearing unit inserts are prelubricated with a lithium base grease (Shell Alvania 3). If ARB Bearing Unit grease is not available at the time of relubrication, one can use a grease with the same base (lithium) and an NLGI grade 2 or 3.