Excessively hot pump bearings are most likely caused by which condition?

• A. the speed being too slow
• B. a worn impeller
• C. over lubrication
• D. a worn packing

Answer: (C)

Over lubrication is the main reason bearings in a pump would run excessively hot. When you add too much lubricant, the oil film becomes thicker and more viscous, which creates more resistance as the shaft spins. That extra drag forces the bearing and shaft to work harder, turning more energy into heat. The excess oil can also trap heat and may foam or not circulate properly, reducing the effectiveness of cooling and lubrication exactly where the heat tends to build up.

Other scenarios don’t directly cause bearing overheating in the same way. Slower speeds typically reduce load and energy input, so overheating is less likely. A worn impeller reduces efficiency and can cause other symptoms like vibration or reduced head, but it isn’t the primary cause of excessive bearing heat. A worn packing mainly increases shaft leakage and packing friction at the seal area; while that can generate heat, it’s not the same direct effect on bearing temperature as over lubrication.

So the best fit is that too much lubrication makes the bearings work harder and traps heat, leading to excessive bearing temperature.