Large-diameter bearings have no precise definition. But a bore diameter of 12 in. or greater is used to define the size range that distinguishes large bearings from small ones. Some large bearings, such as steel-mill roll-neck bearings, are manufactured, selected, and mounted in the same way as small bearings. Inner races are interference fits and outer races are supported in rigid housings as in small bearings.
Most often, large-diameter bearings are multiload or combination load bearings. Usually, a single large-diameter bearing provides the entire support for a rotating assembly, in contrast to the conventional small bearing where two or more separate bearing assemblies support each rotating element. Also, a large-diameter bearing usually incorporates an integral means of attachment, either mounting holes or weld bands. Usually integral gearing is provided on the inner or outer ring. Large-diameter bearings of this class are manufactured of steel in the 0.4 to 0.6% carbon range with a typical core hardness of 250 to 300 Bhn and with induction or flame hardening to about Rc 58 in the raceway areas.
In general, large-diameter bearings are less firmly supported by their mounting structures than small bearings. The load distribution around the bearing may be vastly different than that predicted by classic bearing formulas. To properly apply these bearings, the entire bearing-mounting-fastening system must be analyzed.
Capacity: For most applications the effective bearing capacity is some fraction of the theoretical static capacity as defined by AFBMA standards. There is no universal agreement among manufacturers as to what this fraction is. Unless continuous rotation is involved, dynamic capacity is rarely used in applying these products.
All of the AFBMA formulas assume infinitely rigid and perfectly flat mountings. The degree of reduction in theoretical capacity is largely a matter of judgment as to the extent the actual bearing differs from the ideals of infinite rigidity and perfect accuracy. Most design is based on previous experience in similar situations.
For crane-type applications, the manufacturers' stated capacities vary from 50 to 150% of the theoretical static capacity. Service factors, again not universally agreed upon, are applied prior to selecting bearings for higher duty-cycle applications.