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A ball bearing transfers loads through rolling spherical elements between two races. The no-slip conditions at both contacts determine the translation and spin of each ball; these are generally not the same angular velocity.

A mechanical bearing supports relative motion while transmitting constraint forces between machine components. A shaft support can permit rotation while constraining translation, with the permitted motions depending on its construction and arrangement. A ball bearing uses rolling contacts; a journal bearing supports a shaft through a sliding or fluid-film interface. The support reaction is distinct from resistance to the permitted motion: an ideal support can transmit a large transverse force with zero resisting shaft torque.^[ref]

For a simple ideal shaft supported at axial positions0 and $L$, an upward load $P$ applied at $a$ requires reactionsWith $P=200\mathrm{N}$, $L=100\mathrm{mm}$ and $a=40\mathrm{mm}$, the downward support loads are120N and80N. These reactions exist even if the shaft rotates without an applied resisting torque. In a real mechanical bearing, an illustrative frictional resisting moment $M_f=0.020\mathrm{N}\mathrm{m}$ at angular speed $100rad/s$ dissipates $M_f|\omega |=2\mathrm{W}$; force equilibrium alone does not determine that loss.

A locating bearing arrangement fixes the shaft's axial position relative to the housing, while a non-locating bearing support permits the required axial displacement, for example from thermal expansion. The manufacturer distinguishes this arrangement from adjusted and floating arrangements.^[ref] Radial support does not by itself establish which support carries axial thrust.

Selection must therefore consider permitted motion, combined loads, alignment and operating conditions. An idler support reaction determines a shaft-level load; it does not directly give each rolling contact's Hertzian contact pressure or a service-life prediction. Those require the bearing's internal geometry and a suitable load/life model.