By Joyce Laird, contributing editor
The most common bearing lubricants are greases and oils. They circulate within the bearing and coat the components with a film of oil. In contrast to that, solid film lubricants consist of a dray surface coating that is typically provided in the form of a chemically bonded surface coating or as a solid structure that dispenses a dry film as it rubs against bearing components.
According to Nadeem Bhatti, Applications Engineer at NSK Corporation, solid film lubricants are used in special cases where it is not recommended to use a normal grease or oil.
“One example of a special case is extreme temperature. The temperature of the application where the bearing is being used may be too high or too low for any standard bearing lubricant. Extreme temperatures can potentially degrade the grease or oil, creating a viscosity that would be too high or too low for the lubricant to function effectively,” Bhatti says.
“For example, bearings used in liquid nitrogen or liquid natural gas pumps would benefit from solid film lubricants. These bearings are exposed to cold temperatures resulting from the liquid being handled by the pump. The viscosity of normal grease would get so high that the grease would literally turn into a rock. Therefore, a special cage is used that contains a solid film lubricant to help lubricate the bearing during operation.”
How to make a perfect application fit
The most common condition to recommend use of solid lube bearings involves temperature, but there are also other applications that benefit from solid film lubrication. Bhatti says that this includes bearings working in very highly corrosive environments. “When a bearing is exposed to large amounts of water, cleaning solution or even acid, we would advise using a specific dry lubricant or solid film lubricant based on what the corrosive material is. It’s important to evaluate underlying environmental conditions for each application to determine the best lubricant.”
“Semiconductor production is an application where very clean, low outgassing bearings is required. Use of a standard grease releases gas and particles that contaminate and compromise the application,” he adds. “For example, NSK DFO bearing s are used in clean room environments (compliant with Class 10) and vacuum environments (up to 10-7 Pa) where outgassing particle generation becomes a problem if standard bearings are used. DFO bearings are coated with solid lube on the rings, ball and cage.”
One of the more exotic applications is in the area of liquid and liquefied gas applications. “These are transferred and kept at somewhere around -100•C,” Bhatti says. “The bearings used in the pumps in these systems get extremely cold. The liquid gas that flows through them and would freeze any normal lubricant into a rock. In this case, we use a cage so a small layer of lubricant is deposited onto the raceway continually to keep the bearings rolling. It is also used in many other oil and gas type applications.”
“On the other end of the spectrum, furnace conveyor bearings see temperatures up to 400C. Normally, the user would use high-temperature grease or no grease at all for these bearings. However, grease degrades quickly and using no grease at all results in premature bearing failure. For high-temperature applications, we recommend the spacer joints that fit between each bearing like a little puck. They are made out of a solid lubricant material and keep everything lubricated and working well. Dry lubricant spacer joints extend bearing life in high-temperature applications.”
YS bearings with spacer joints made of alloy-based, self-lubricating material (sintered alloy) between balls. They are suitable for high-temperature (up to 350•C) and vacuum environments.
SJ bearings have solid lubricant spacer joints mounted between two balls in a “peapod” cage design structure. These bearings are suitable for high temperature(up to 400C) in normal atmosphere or under vacuum conditions.
“The equipment requiring these bearing is very valuable. Bearings that require solid lubrication are more expensive than a standard bearing assembly, but they are necessary to maintain operation. Overall, the initial investment to install specialty bearings is only a small fraction of the overall cost. In many cases, the reduced downtime when moving from standard to specialized bearings reduces maintenance up to four-fold on productions,” Bhatti says.
Working together is always the perfect solution
Whenever there is a question about what bearing lubricant method is best, it goes back to understanding the full scope of the application.
“The customer should contact the bearing company. When working with NSK for example, we want to know the whole application. Then we can guide the end user through the process and suggest the best solutions. Many of the solutions that all beating manufacturers have available today were created as a result of a new customer requirements.