EEI MASS Practice Test 3

Why Bearings Fail: The Role of Lubrication

¹Pumps, fans, motors, and gearboxes throughout a power plant turn on bearings, and nearly all of those bearings depend on a lubricant. Ball and roller bearings look smooth, but under a microscope their surfaces are rough. Without a lubricant, the high points of the rolling parts and the races would rub, heat up, and tear each other. The main job of oil or grease is to keep a thin film between these surfaces so that metal never quite touches metal. The lubricant also carries away heat and helps keep out dirt and moisture.

²Most bearings that fail early do not simply wear out from age. They fail because something went wrong with the lubricant. Too little lubricant lets the film break down, and the bearing runs hot. The wrong lubricant can cause just as much harm. An oil that is too thin for the load and speed cannot hold the surfaces apart, while one that is too thick may not flow into the bearing quickly enough, especially during a cold start.

³Contamination is the leading problem. A few grains of dirt, sand, or metal can dent the polished races, and each dent becomes a starting point for cracks and flaking. Water is just as harmful. Even a small amount of water in oil weakens the film and causes rust on the bearing surfaces. For this reason, mechanics wipe grease fittings clean before attaching the grease gun, keep oil containers capped, and use a separate clean container for each type of lubricant.

⁴More is not always better. Packing a bearing housing completely full of grease forces the rolling parts to churn through it. The churning creates friction and heat, the grease breaks down, and the bearing can fail from overheating even though it was "well greased." Extra grease can also push past seals into places it does not belong, such as the windings of an electric motor. Lubrication schedules state both how often and how much to add, and both numbers matter.

⁵Bearings usually give a warning before they fail. A rise in running temperature, a new grinding or whining noise, and an increase in vibration are common signs. Oil drained from the housing may look dark, milky, or cloudy, or it may contain shiny bits of metal. Many plants send oil samples to a laboratory on a regular schedule so that wear and contamination can be found while the repair is still small. Catching a problem early turns an emergency shutdown into planned maintenance.