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Journal bearings support and provide lateral stability to rotating shafts and axles while reducing friction, wear, and vibration. They work by forming a thin lubricant film between the bearing surface and the rotating shaft, which minimizes direct contact and reduces frictional forces in machinery such as engines, turbines, and pumps.

As the shaft rotates clockwise, it initially rolls up the inner surface of the bearing until it slips and undergoes stable rotation. During this process, the shaft experiences forces including its weight acting downward, the applied clockwise couple representing torque, and the bearing's reaction force, which acts at an angle relative to the surface normal.

The angle of kinetic friction is the angle at which the bearing's non-collinear reaction force acts relative to the surface normal. The line of action of this reaction force is always tangent to the circle of friction, which represents the locus of all possible contact points between the shaft and bearing surface.

By applying moment equilibrium about the shaft center, the required moment can be determined. For small kinetic friction angles, the sine term approximates to the tangent of the kinetic friction angle. Since the tangent of the kinetic friction angle equals the coefficient of kinetic friction, this relationship yields the moment needed to overcome the bearing's frictional resistance.

A free-body diagram of a shaft in a journal bearing includes three main forces: the shaft's weight acting vertically downward, the clockwise couple representing applied torque, and the bearing's reaction force. The reaction force is equal and opposite to the weight and acts at the angle of kinetic friction relative to the surface normal.

The circle of friction represents the locus of all possible contact points between the shaft and bearing surface. The reaction force's line of action is always tangent to this circle, making it essential for determining how forces distribute during rotation and for calculating the moment required to maintain stable shaft rotation.

Lubrication creates a thin film between the bearing surface and rotating shaft, minimizing direct contact and significantly reducing frictional forces. This film reduces wear and vibration in the system, improving overall machinery efficiency and performance while lowering the moment required to overcome bearing friction.