In a host of demanding bearing applications from paper machines to mining equipment, oil is often the recommended lubricant. As industry places increasing demands on operating machinery, the use of oil lubrication is likely to grow.
Grease remains the lubricant of choice in the vast majority of rolling bearing applications because it is easy to use, resistant to leakage and relatively inexpensive. However, difficult operating conditions such as high temperatures, high speeds or heavy contamination can pose serious challenges for bearing greases. For example, extremely high temperatures in paper machines can literally cook bearing grease, causing a loss of lubricant viscosity resulting in metal-to-metal contact and eventual bearing seizure. In fan applications, increasing speed requirements can produce higher bearing operating temperatures, with similar results. In these and other demanding applications, it is advisable for lubrication and maintenance professionals to consider converting from grease to oil lubrication.
There are a few key factors that support the use of oil in bearing applications, including:
1. Oil has the capacity to dissipate heat within rolling bearings.
2. In harsh applications, it flushes away contaminants such as dirt, dust, moisture and wear metals.
3. Oil can also remedy certain load-related problems, such as ball or roller skidding.
There are also some drawbacks to oil lubrication, including:
1. Oil systems tend to be more expensive than grease, both in terms of initial investment and maintenance requirements.
2. There are potential issues involving oil leakage and disposal.
3. The labor required to maintain appropriate levels is greater with oil than with grease.
In many applications, these drawbacks are insignificant when compared with the cost of repeated bearing failures and downtime associated with continued use of grease.
There are two fundamental oil lubrication system design approaches: static oil and circulating oil. Static-oil systems, also known as oil baths, represent an inexpensive first option for bearing users converting from grease to oil lubrication, especially in low- or moderate-speed applications. In static-oil systems, the bearing housing functions as a self-contained oil reservoir. When the bearing is at rest, oil fills the housing to a level just below the center of the lowest ball or roller. During operation, oil is drawn up by the rotating bearing, lubricating bearing components. Afterward, the lubricant drains from the bearing and flows back into the oil bath.
Figure 1. Oil levelers automatically replenish lubricant levels in oil baths, compensating for losses due to leakage or evaporation.
Converting from grease lubrication to an oil bath requires only a small initial investment and a few modifications to existing bearing arrangements. One recommended modification is equipping the bearing housing with a sight glass, which allows lubrication or maintenance technicians to visually monitor oil levels and take immediate action if oil levels drop. In some applications, oil levelers, also called constant level oilers, (Figure 1) can be used to automatically adjust lubricant levels during operation, compensating for oil losses due to leakage or evaporation. One type of oil leveler consists of two interconnected oil reservoirs. The oil level in the lower reservoir is identical to that in the oil bath itself. When oil in the lower reservoir dips below the desired level, replacement oil automatically flows from the upper reservoir to the lower one, replenishing the oil bath. An example of an oil leveler would be a bottle oiler used on ANSI pumps.
In extremely demanding applications, circulating oil is often required. Circulating oil systems find use in the dryer and press roll sections of paper machines and in high-speed or hot-gas industrial fans (Figure 2). They are also employed in shaker screens, shredders and other high-contamination applications in mining, cement manufacturing and other heavy industries.
This is the simplest form of oil lubrication, and can be used at low bearing speeds. When the bearing is at rest, the bath should come to a level just below the center of the bottom rolling element. On rotation, the oil is drawn up by the bearing parts, runs through the bearing and returns to the bath.
The higher the operating temperature, the more rapidly lubricating oil will oxidize. The oil circulation method lengthens oil-change intervals. Before the oil is supplied to the bearing it has to be cleaned, and if necessary, a cooler can be included in the oil circuit.
At high bearing speeds, oil can be forced out of the bearing instead of flowing through it and removing heat. The most efficient way of lubricating is to direct a jet of oil into the bearing. The speed of the jet must be at least 15 m/s (50ft/s) so that the oil will penetrate the air vortex created by bearing rotation.
Figure 2. Circulating oil systems consist of a separate oil storage reservoir, oil pumps, filters and pipes designed to deliver a steady flow of oil to bearing arrangements. Ideally, oil enters the bearing arrangement through an inlet at the top of the housing and exits via two drain holes at the bottom.
Installing a circulating oil system can entail an investment of $10,000 or more, depending on the system’s complexity. The typical system consists of a separate oil storage reservoir, oil pumps, filters and pipes designed to deliver a steady flow of oil to bearing assemblies. After oil circulates through the bearings, it drains from the bearing housings and is piped back to the storage reservoir where the oil cools and is filtered to remove contaminants.
When converting to circulating oil, modifications to existing bearing arrangements are usually required. For example, new drain holes are often drilled in bearing housings, or existing holes enlarged. Ideally, oil should enter a bearing arrangement through an inlet hole positioned at the top of the housing. Housings can be supplied with either wet or dry sumps. For a wet sump, the oil is maintained at a static level, at the middle of the bottom roller, before exiting the housings. With a dry sump, two oil outlets are placed on opposite sides of the housing, at the lowest points. This allows oil to drain immediately after it has passed through the bearing, preventing oil churning and increased bearing operating temperatures. To better determine individual application needs, bearing engineers should be consulted.
Some applications are equipped with only one outlet hole, due to the arrangement’s configuration. Here, the housings should be cross-drilled to allow oil to flow from one housing side to the other. Even after cross-drilling, pressure can build up in one side or section of bearing housings, restricting oil flow. Installing breather vents can equalize pressure inside the housings and help restore proper oil flow.
Figure 3. Seals that make positive contact with the shaft provide added protection against leakage in oil-lubricated systems.
When installing a circulating oil system, technicians should take steps to prevent oil leakage, particularly along the shaft, by upgrading existing seal arrangements. For such applications, SKF often recommends seals with a lightweight aluminum ring equipped with an integrated nitrile rubber O-ring in the seal bore (Figure 3). The seals make positive contact with the shaft during operation and prevent oil leakage. New seals may also be needed in static-oil systems.
Preventing Roller Skidding at Louisiana Mill
Recently, at a Louisiana mill, grease-lubricated spherical roller bearings in a paper machine’s press roll section were experiencing a high rate of failure due to roller skidding. The skidding occurred when the bearings’ rolling elements ceased rotating after leaving the load zone during operation and began to skid instead of rotating when reentering the load zone. Grease was unable to prevent metal-to-metal contact between the bearing rolling elements and roller paths, which eventually caused skid smearing on the roller paths. The end result was increased vibration readings and ultimately shortened service life. Under these conditions, the machine’s press roll bearings, which have a life expectancy of about eight years, were lasting less than a year.
Figure 4. Skid Smearing Damage
The paper mill attempted various solutions, including changing bearing clearances and employing a high-viscosity grease, but the problems persisted. A year ago, the mill converted the press roll section to circulating oil. The circulating oil system supplies a continuous, regulated flow of oil to the machine’s four press roll bearings, preventing metal-to-metal contact between the rolling elements and roller paths. Since the system was installed, there have been no bearing failures in the press roll section, and the vibration readings, an early sign of skidding damage, have been reduced (Figure 4).
Training and Monitoring
The switch from grease to oil may require additional training for lubrication and maintenance personnel. Workers need to know proper methods for disposing of or recycling lubricating oil. Technicians responsible for static-oil systems must learn how to monitor oil levels using a sight glass. Training requirements for circulating oil are more stringent. Here, lubrication and maintenance techs must become familiar with the entire system and with correct maintenance procedures for each of the system’s components, including pumps, pipes and oil filters.
With both static-oil and circulating oil systems, it’s important for lubrication and maintenance personnel to monitor oil quality. This, in combination with other lubrication maintenance practices, will help enhance the quality and effectiveness of the lubricating oil.
Photos courtesy of SKF USA Inc.