During operation, balls or rollers in a rolling bearing are separated from the bearing raceway by a microscopically thin film, the bearing lubricant. Proper lubrication reduces friction and prevents metal-to-metal contact between moving components. It also protects the bearing rings and rolling elements from corrosion.

Although the fundamentals of bearing lubrication are not difficult to learn, maintenance technicians often overlook the basics. Errors in lubricants selection and application are commonplace. Luckily, many such errors can be easily corrected at little expense to the bearing user. Here are ten of the most common errors, and how to correct or prevent them:

1. Using a Lubricant with the Wrong Viscosity.

Viscosity is a measurement of the thickness, or ability to flow, of a lubricating oil. It is one of the most important factors to consider when selecting a bearing lubricant. In the United States, a common unit of measurement for viscosity is Saybolt Universal Seconds (SUS). The recommended minimum viscosity varies according to bearing type, ranging from 70 SUS for a ball bearing to 100 SUS at the operating temperature for a spherical roller bearing, and 150 SUS for a spherical thrust bearing.

Many plants continue to use a single all-purpose lubricant for all bearing applications. A generic lubricant may be acceptable for the low-viscosity needs of the typical ball bearing, but the same lubricant may be inadequate for the high-viscosity requirements of a spherical roller bearing. A lubricant's viscosity declines rapidly as the temperature rises, so it's important to know the approximate operating temperature of a bearing application and choose your lubricant accordingly.

2. Misuse of Grease and Oil.

In bearing applications, the actual lubricating fluid is generally a mineral oil or a synthetic oil. This oil can be used alone, or in conjunction with a thickener to form a lubricating grease. The thickener (in most cases, lithium, calcium, or sodium soap) forms a lattice which carries the oil between its fibers.

Bearing users sometimes use and oil where a grease is called for, or grease where oil is indicated. For example, in food processing applications, oil dripping from housings can contaminate the food product; grease is generally the preferred lubricant. Grease may also be called for in situations where the bearing housing is inaccessible and an oil slight gauge can't be routinely checked. On the other hand, oil is generally used when other components in the arrangement are oil-lubricated.

3. Over-Lubrication of Bearings.

Too much oil or grease in a bearing or its housing causes an effect called churning resulting in a sharp temperature rise and often premature lubricant and/or bearing failure. On start-up, grease lubricated bearings expel grease into the housing. To prevent churning there must be sufficient empty space in the housing to accommodate this grease. Therefore, when using grease, fill the bearing completely, but fill only one-third of the bearing housing.

4. Using an Ineffective Lubrication System.

When oil is the preferred lubricant, there are several methods of delivering oil to the bearings, including static oil, circulating oil and the spray-mist system. In Static oil or oil bath systems, the bath should be filled to a level just below the middle of the bottom rolling element.

With a circulating oil system, oil is pumped from a separate reservoir, where it cools down and lubricates simultaneously. The heated oil is then returned to the reservoir, where it cools down again. Filters in the system remove contaminants from the oil as it circulates. A circulating oil system can often greatly increase bearing life expectancy.

Circulating oil, however, should not be used in high-speed applications, such as machines tool spindles, because of unacceptable friction losses as the lubricant moves through the bearing. In these applications, only a small amount of oil is needed, and a spray-mist system is generally preferred.

5. Under-Lubrication of Bearings.

Even with the right lubricant system, under-lubrication can occur. The consequences are excessive heat and eventually metal-to-metal contact between bearing components. The reasons for underlubrication vary. When grease or static oil system is used, a small amount of lubricant can evaporate over time, leading to underlubrication. But underlubrication can also happen with circulating oil systems.

Recently, at a paper mill, several bearings failed because there was insufficient oil in circulation. Maintenance technicians had decreased the rate of oil flow because at a higher rate oil had run out of the housing and damaged the paper. The problem was eventually diagnosed as poor oil drainage from the housing. The solution involved installing a larger drainage pipe in the system so that oil would drain more quickly from the housing instead of leaking.

6. Failure to Prevent Lubricant Contamination.

vLubricant contamination is a leading cause of bearing failure. Dirt particles and other contaminants can damage bearings by leaving dents in rotating elements and raceways. Contamination can occur due to improper handling of the bearing from it packaging until ready for mounting, and clean dirt and debris from the housing. During installation, minimize the amount of time the bearing lubricant is exposed to the air.

7. Using the Wrong Seal.

Improper sealing can also cause contamination. Labyrinth seals are effective in many applications. But mine, steel mills, brick mills and other harsh environments, ambient dust may work its way past these seals and into the bearings, causing contamination and wear. This problem can be prevented by using a higher-grade seal, such as a rubber seal, garter spring seal or a taconite seal.

8. Failure to Relubricate Bearings.

Even without exposure to contaminates, lubricant quality can deteriorate over time. It's often impossible to determine grease quality in a sealed bearing by visual inspection. Relubricating every month with a small amount of fresh grease will replace the lost grease and refill the bearing. SKF recommends changing the grease completely every year, even sooner in some extreme cases. Usually, the grease is injected via a grease gun, automatically expelling the used grease and any contaminants it may carry.

In oil lubrication systems, visual inspection can be useful. Monitoring sight gauges and oil reservoirs can help determine relubrication intervals. The oil in oil-bath systems is generally changed once a year. More frequent oil changes may be necessary in applications with high temperatures or heavy contamination.

9. Mixing Incompatible Lubricants.

During bearing relubrication, maintenance technicians may sometimes inject a different lubricant into a bearing than the one used originally, inadvertently mixing the two lubricants. If the two lubricants are incompatible, there will be a deterioration in lubricating capability.

Never mix greases with different thickeners, such as a lithium-based grease with a sodium-based grease, before checking with the suppliers. Never mix a mineral oil with a synthetic oil. Some lubricants are compatible, but assessing the compatibility of two lubricants can be difficult. As a general rule, always relubricate a bearing with the same lubricant used originally. If a different lubricant must be used, carefully remove all traces of the old lubricant from the bearing and housing before relubricating.

10. Failure to Provide Relubrication Training.

Maintenance technicians commonly receive training or bearing selection and installation, but not lubrication. Plant supervisors should make sure that technicians are thoroughly trained in lubrication fundamentals.

There are many opportunities for training. Some bearing manufacturers offer seminars that cover lubrication as part of course curriculum. Sales representatives of bearing companies and lubricant suppliers can often provide technical training at customer sites.