Journal Bearings

Journal bearings are used to provide lateral stability to rotating shafts and axles.

Consider a rope winch with dry or partially lubricated journal bearings.

The clockwise rotation of the spool causes the shaft to roll up the bearing's inner surface until it slips and undergoes stable rotation.

A free-body diagram of the shaft is drawn. The forces acting on the system include the shaft's weight, the clockwise couple, and the bearing's reaction force.

The non-collinear reaction force is equal and opposite to the weight and acts at an angle relative to the surface normal. This angle is called the angle of kinetic friction.

The line of action of the reaction force is always tangent to the circle of friction.

The moment equilibrium condition about point O yields the shaft moment.

For a small kinetic friction angle, the sine term can be approximated to the tangent of the kinetic friction angle.

As the tangent of the kinetic friction angle equals the coefficient of kinetic friction, the moment required to overcome the bearing's frictional resistance is obtained.