The marriage of bearings, lubrication

Year after year, advancements in equipment and new technologies enable machines to meet the ever-increasing demands of productivity. Although it is not often thought of as part of the equipment, lubrication is an integral component. Lubrication can be a weak link if not given proper thought and attention. Increased knowledge of the invaluable role of lubricants has a significant impact on the life, quality and productivity of a machine.

Significance of Lubrication
Proper lubrication of moving parts, especially gears and bearings, is critical to performance as demands on machinery increase. Lubrication serves to carry away heat, reduce wear and friction, minimize debris problems, and protect bearing surfaces from corrosion. Increased production often results in higher operating temperatures, faster speeds and heavier loads. Just as bearings have improved in design, proper lubrication selection and practices must go hand-in-hand to achieve maximum performance.

Too often, companies struggle with ongoing maintenance because of improper lubrication practices. For instance, insufficient lubrication leads to additional friction and increased temperatures, which can cause premature bearing damage. Excess lubrication can generate additional heat, causing the lubricant to break down and lead to potential unexpected and premature bearing damage.

Recognizing early warning signs of lubrication problems, such as increased operating temperatures, lubrication leaks, and unusual noise and vibration can help prevent bearing damage. With inadequate lubrication, temperature gauges often indicate the bearing is running hot – it may even become noisy from resultant metal-to-metal contact. When bearings are overfilled, the lubricant breaks down and darkens in color because of extreme temperatures.

When lubrication failure occurs, it can result in several types of roller bearing damage, including ribs and roller end scoring from metal-to-metal contact. In extreme cases, inadequate lubrication causes the rollers to skew, slide sideways and provoke total bearing lock-up. Recognizing the types of damage caused by inadequate lubrication helps you pinpoint and correct the problem.

Proper lubrication maximizes the
life and performance of bearings,
and leads to enhanced equipment
productivity.

Options in Lubrication
Just as important as acknowledging proper lubrication levels is selecting the correct type of lubricant for a given application. The most common mediums are oil and grease. Each type serves to meet specific application needs, so making the proper bearing selection requires a clear understanding of the demands of the application.

Common oils used in bearing applications are synthetic or mineral. Mineral oil is derived from refining crude petroleum and can contain unstable compounds such as nitrogen, oxygen or sulfur, which can affect service life. Synthetic oils are free of impurities and are designed for use in special circumstances where normal petroleum products can’t cope with temperatures. In most cases, additives are used in the oils to resist oxidation, reduce foaming and improve lubricity.

Although lubricating oils are uniform in many characteristics, knowing and understanding the viscosity of oil is critical in determining its operating limits. Viscosity is the measure of the flowability of a liquid at a definite temperature. The faster the flow, the lower the viscosity, and vice versa. The oil viscosity must be high enough to provide a continuous film but not too high to generate excessive heat.

The flexibility of oil operates better than grease in higher temperature applications because it circulates more freely, resulting in the dissipation of heat. It allows contaminants to be flushed or filtered from the bearing and can allow the bearing to attain higher speeds. Oil levels also are more easily controlled and easier to drain and refill, yet they can be tricky to seal in some situations.

On the other hand, grease – a precise combination of oil, thickeners and additives – is more easily confined to the bearing. Grease acts like a sponge to retain and release oil, but it also performs like a seal against moisture, dirt and other contaminants. With no oil levels to maintain, it requires less frequent lubricating and its thickness makes leakage less likely to occur.

Similar to oil, grease also has its limits for upper and lower operating temperatures. These limits change per the type and amount of thickening agents, and the base oil type and viscosity used in making the grease. Operating speeds also need to be considered. Greases typically are limited to a speed of about 2,500 feet/minute (rib speed) because of heat generation.

Most importantly, what works for one application may not work for another. Each lubricant offers advantages specific to use and operational demands. The selection of lubrication requires careful expert review of these demands to ensure peak performance.

Inadequate lubrication can cause rollers to
skew, slide sideways and, in extreme cases,
lock up the entire bearing (shown here).

Lubrication Delivery
Almost half of bearing failures are due to improper lubrication. In addition to improper fill levels, other factors can contribute to reduced bearing life and performance, including the wrong lubricant or improper maintenance cycles. Experience shows that the application of small amounts of lubricant more frequently results in greater bearing life compared to a large dose over a longer time period. Proper lubrication is an essential component of effective preventive maintenance.

Delivery of Oil
Knowing how to properly deliver and apply the lubricant is important to the performance of that equipment. For oil, there are a variety of methods available. Choosing an oil system is highly dependent on the desired speed of the application. The more heat than can be removed from the system, the higher the attainable speed.

Here are some oil system options:

• Oil bath is a static oil level for low- and moderate-speed applications.
• Drip feed uses controllable sight feed oilers in moderate speed applications.
• Wick feed uses a wick in conjunction with a reservoir of oil.
• Oil splash allows the splash from the gear teeth to sufficiently lubricate the bearings. It is used for specific applications such as gearboxes.
• Circulation pumps assure a proper and constant supply of lubricant to the bearings and are often used for low- to medium-speed, heavy-duty applications.
• Oil mist is used for continuous, high-speed applications and offers a continuous spray of pressurized air and oil to the bearings.
• Oil jet, similar to circulation, may be used for high speeds and heavy loads but requires large drains to prevent the accumulation of oil after passage through the bearings.

Delivery of Grease
Although greases meet different operating demands than oils, they are equally important lubricants and require equally unique but different methods of delivery. For grease, manual delivery is a viable option, but bearings easily can be overfilled or underfilled. When applying the grease, force it between the balls or rollers until the available space inside the bearing is filled completely. Wipe away any excess grease. The space on each side of the bearing in the housing should not be more than half-filled.

If a grease gun is used, regulate pressure so as not to damage any seals or other parts of the machinery. Grease delivery generally follows the one-third rule. This means that the bearing will expel or purge all grease in excess of about one-third of the volume available. This is especially important for bearings with two shields or seals since excess grease can’t get out during operation. Bearings without the shields or seals will expel any excess grease into the free housing space outside of the bearing during operation. This is the reason why you should never completely fill the housing space.

Other delivery options include single-point lubricators and centralized systems. Single-point lubricators provide the option of using several different greases simultaneously on a piece of machinery to compensate for different lubricant requirements precisely where and when needed. Centralized systems offer precise lubrication delivery and intervals, but proper care must be taken to ensure that the ability to pump the grease is not compromised.

Roller end scoring is another common
bearing damage mode that can occur from
poor lubrication practices.

Conclusion
Selecting, delivering, reapplying and maintaining superior lubrication requires following manufacturer’s guidelines and recommendations for each application. By continuing to learn about lubrication and practicing a regular maintenance schedule, bearing failure from lubrication problems can be avoided.

When lubrication is correctly applied and maintained, it increases bearing life and overall machine productivity, and can reduce costly maintenance repairs.

David A. Pierman is a product development specialist for The Timken Company. For more information, contact Timken at 330-438-3000 or visit www.timken.com.