Lubrication and bearing reliability

Proper lubrication is essential to reliable bearing operation. If lubrication is absent, contaminated or insufficient, metal-on-metal contact occurs between anti-friction bearing roll­ers and raceways. When this happens, the components will fatigue prematurely and will likely fail quickly. A lubricant by itself, either grease or oil, is not an enduring substance; it will have a service life requiring replacement in order to allow bearings to reach their allowable life span.

Additionally, grease and lubrication oils are not inexpensive supplies, particularly in view of increasing petroleum prices. The most efficient and cost-effective machinery maintenance program is one that understands and applies the ideal amount of lubrication at the proper operating intervals.

Choosing proper intervals

Choosing the correct re-lubrication interval is essential to achieving maximum life of a bearing. Following the proper re-lubrication interval will ensure that fresh grease is supplied to the bearing, and that old grease and contaminates are being displaced out of the bearing internals. An incorrect re-lubrication interval can decrease the life of the bearing through grease service-life limitations, contamination, and/or heat due to excessive grease in the sealed bearing cavity.

On the other hand, grease acts as a barrier or seal and helps in decreasing the incursion of contaminates into the sealed bearing cavity. When sufficient lubricant is not present, the sealing ability of the bearing is decreased and contaminates can more easily enter the cavity. Once contaminates reach the rolling elements or raceways, total bearing failure is imminent.

In contrast, a too frequent lubrication interval can damage some types of bearings. In high-speed applications, bearings could actually overheat due to the frequency and amount of grease being pumped into the bearing. When excess grease is pumped into the bearing, it causes the bearing to churn the grease. Churning, or continuous mixing of the grease, generates heat within the bearing housing. If this particular re-lubrication interval is continued, the bearing will keep filling with grease and eventually overheat and fail.

However, correct seal design is the prime factor in preventing contaminants from entering a bearing, and re-lubrication at proper prescheduled intervals offers the advantage of purging out any extraneous material from the seals before they have had an opportunity to gain access to the bearings or the housing cavity. The frequency of re-lubrication to avoid corrosion and to aid in purging out any solid or liquid contaminants is difficult to establish since re-lubrication requirements vary with different types of applications.

Anticipating a not-quite-clean to moderately dirty environment as can be assumed to be present in refineries, petrochemical plants and pipeline operations, one authority suggests greasing intervals ranging from 1 to 8 weeks. One should note that the period during which a grease-lubricated bearing will function satisfactorily without re-lubrication depends on the bearing type, size, speed, operating temperature and the grease used. 

Choosing the right amount

If the right amount of lubricant is not added, the benefits of a correct lubrication interval are not fully realized. The bearing can still suffer if either too much or too little lubricant is available inside the bearing cavity. There are three methods to determine the correct re-lubrication quantity. The first method is to contact the bearing manufacturer and request a recommendation based upon the specific application. The second option is to calculate the correct re-lubrication quantity using the bearing manufacturer’s recommended guidelines. Some manufacturers will also have on-line software available for this purpose. The third method would be field trial and error.

Determining the correct re-lubrication quantity and re-lubrication interval are both essential to achieve the maximum bearing life. For example, it has been estimated that 90% of mounted bearing failures are lubricant related. The proper re-lubrication quantity and interval ensure that the bearing rollers and raceways are receiving fresh grease, while the old degraded grease and contaminants are purged out. Taking the time to follow these practices will increase bearing life, and decrease maintenance time as a consequence of bearing failures.