Which of the following is usually a primary cause of bearing trouble?

Misalignment of the shaft and bearing is indeed a primary cause of bearing trouble. When the alignment is off, it can cause uneven distribution of load on the bearing surfaces, leading to increased wear and tear. This misalignment can result from several factors, such as improper installation, wear in the components, or changes in the mounting base. As the bearing operates under these misaligned conditions, it experiences excessive friction and heat, which can lead to premature failure.

The operational stability and longevity of bearings heavily rely on proper alignment. If the shaft is not perfectly aligned with the bearings, it can lead to vibration, increased load, and eventual breakdown of the bearing. Identifying and correcting misalignment in a timely manner is crucial for maintaining the integrity of the machinery and preventing significant downtime or costly repairs.

Other options, while they can lead to problems, are generally not as directly impactful on bearing performance as misalignment. For instance, excessive lubrication might lead to issues such as overheating or contamination, and using the wrong type of bearing can lead to suboptimal performance but may not immediately cause a breakdown. Low operating temperature can actually prolong bearing life, contrary to causing trouble. Thus, misalignment stands out as a significant root cause of bearing issues.