Under what condition should a thrust bearing be used?

A thrust bearing is specifically designed to support axial loads, which are forces that act along the axis of the shaft. This type of bearing helps to manage the forces that can cause the shaft to move in a linear direction, rather than allowing it to rotate freely about an axis.

In applications where a load is applied along the axis of the shaft, thrust bearings are essential because they provide stability and reduce wear on the components by accurately bearing this type of load. For example, in machines such as gear drives or screw conveyors, where axial forces are prevalent, the use of thrust bearings ensures that the setup functions smoothly, without risking damage to the shaft or other components.

On the other hand, other scenarios like horizontal loads, no loads, or stationary conditions pertain to different types of bearings that are designed to handle radial loads or static situations. These conditions do not align with the primary purpose of a thrust bearing, which is why the correct answer highlights the application of thrust bearings strictly when axial loads are involved.