EFFECTIVE SPREAD

BEARING REACTIONS 

For a shaft on two supports, bearing radial loads are determined by: • Defining the bearing effective spread. • Resolving forces applied to the shaft into horizontal and vertical components, relative to a convenient reference plane. • Summing moments about the effective center of each of the bearing supports, and solving for the radial and axial reactions at each support.

EFFECTIVE SPREAD

TAPERED ROLLER OR ANGULAR CONTACT BALL BEARINGS

When a load is applied to a tapered roller or angular contact ball bearing, the internal forces at each rolling element-to-outer raceway contact act normal to the raceway. These forces have radial and axial components. With the exception of the special case of pure axial loads, the inner ring and the shaft will experience moments imposed by the asymmetrical axial components of the forces on the rolling elements. The effective center for tapered roller bearings is defined as the point at which the lines of force normal to the outer ring raceway intersect the bearing axis. As an approximation, it also applies to angular contact ball bearings. The effective spread is then defined as the distance between the bearing effective centers for a two-bearing system. It can be demonstrated mathematically that, if the shaft is modeled as being supported at its effective bearing center rather than at its geometric bearing center, the bearing moment may be ignored when calculating radial loads on the bearing. Only externally applied loads need to be considered, and moments are taken about the effective centers of the bearings to determine loads or reactions.

Fig. 65 shows single-row bearings in a direct and indirect mounting configuration. The choice of whether to use direct or indirect mounting depends upon the application. SPHERICAL ROLLER BEARINGS The effective center for each row of spherical rollers intersects the shaft axis at the bearing geometric center as shown in fig. 66. As the distance between effective centers for each row of a bearing is zero (i.e. zero moment arm), a pure couple cannot be generated internal to the bearing. Therefore, when a shaft and housing are misaligned, the inner and outer rings of the bearing rotate up to a few degrees relative to each other, without creating internal forces. This self-aligning capability in turn prevents an external moment load from being supported by the bearing. Therefore, spherical roller bearings can only accommodate external shaft and housing loads through radial and axial reaction forces. Direct Mounting – Tapered Roller Bearings Face-to-Face/DF – Angular Contact Ball Bearings Fig. 65. Choice of mounting configuration for single-row bearings, showing position of effective load-carrying centers. Fig. 66. Spherical roller bearing.

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