NCCER Millwright Practice Exam

A pillow block bearing failure caused by an undersized shaft is associated with excessive leakage. This type of failure occurs because the fit between the shaft and the bearing is not optimal; an undersized shaft can result in inadequate sealing within the bearing housing, allowing lubricant to escape and external contaminants to enter. This leakage compromises the bearing's lubrication system, which is essential for reducing friction and wear.

In contrast, excessive wear typically involves surface degradation due to insufficient lubrication or misalignment, not solely from an undersized shaft. Excessive vibration could arise from numerous issues, including misalignment or imbalance, but it doesn't directly point to the problem of an undersized shaft. Lastly, excessive heat often results from friction and inadequate lubrication, which can also be linked to wear but is not the primary failure mode due to shaft sizing.