Journal bearings are mechanical components that support and provide lateral stability to rotating shafts and axles. They are crucial in reducing friction, wear, and vibration in machinery such as engines, turbines, and pumps. The principle behind journal bearings is forming a thin lubricant film between the bearing surface and the rotating shaft, which minimizes direct contact and reduces frictional forces.

To better understand the concept of journal bearings, consider a rope winch with dry or partially lubricated journal bearings supporting its rotating shaft. As the spool rotates clockwise, the shaft rolls up the inner surface of the bearing until it slips and undergoes stable rotation.

A free-body diagram of the shaft can be drawn to analyze the forces acting on the system. These forces include the weight of the shaft, acting vertically downward, the clockwise couple, representing the torque applied to the shaft, and the reaction force exerted by the bearing on the shaft. The non-collinear reaction force is equal and opposite to the weight of the shaft and acts at an angle relative to the surface normal. This angle is referred to as the angle of kinetic friction. The line of action of the reaction force is always tangent to the circle of friction, which represents the locus of all possible points of contact between the shaft and the bearing. By applying the moment equilibrium condition about point O (the center of the shaft), we can determine the shaft moment needed to maintain stable rotation.

The sine term in the equation can be approximated to the tangent of the kinetic friction angle for a small kinetic friction angle.

As the tangent of the kinetic friction angle equals the coefficient of kinetic friction, we can calculate the moment required to overcome the bearing's frictional resistance. This moment is an essential factor in determining the efficiency and performance of the system.