Which type of casualty is indicated by a reduction gear journal bearing showing a dramatic increase in temperature and then decreases to ambient temperature?

The situation described refers to a wiped bearing. This occurs when there is a loss of lubrication or an inadequate oil supply, leading to metal-to-metal contact between the bearing surfaces. The dramatic increase in temperature indicates excessive friction and heat generated by the damaged bearing.

As the bearing continues to operate under these extreme conditions, it can quickly degrade, leading to a point where the damage is so severe that the bearing surface may start to lose material, hence the term "wiped." If the bearing were to cool to ambient temperature following this event, it suggests that the machine may have momentarily stopped or significantly reduced load, allowing the bearing to cool down but not restoring it to a functional state.

This scenario is distinct from other types of bearing failures. For instance, worn bearings usually exhibit gradual wear and a more consistent increase in temperature over time rather than a sudden peak and drop. Overloaded bearings would not typically cool down suddenly unless the load was removed completely or the bearing failed. A seized bearing would result in a complete halt of movement rather than a drop in temperature after high heat and would not adequately explain the cooling down described.

Therefore, the details provided align most closely with the characteristics of a wiped bearing.