Companies that make bearings and linear guides too often misuse performance buzzwords such as “self‐lubrication,” “maintenance free,” and “lubed for life.” This has led to a broad misunderstanding of what these terms actually mean. This confusion can lead to misapplication of products resulting in failures, downtime, and bottom line losses in productivity and profits.
While innovations such as oil-impregnated seals and wipers—along with long-term lubrication reservoirs and felt wicks—may extend a bearing’s life and performance, they cannot be classified as “self‐lubricating.” They require maintenance attention to oil levels that dissipate, age, and become ineffective over time.
True “lube for life” requires that the lubrication be part of the original bearing material. To be truly self‐lubricating, the lubrication cannot be an add‐on or break down, and it must remain a part of the bearing’s makeup for its entire life without the need for maintenance.
What Does Self-Lubricating Mean?
Self‐lubrication is characterized by the bearing’s ability to transfer microscopic amounts of material, usually a PTFE (Teflon)-based compound, to the mating surface, often a shaft or rail. This transfer process creates a lubricating film that reduces friction over the length of that mating surface.
The transfer process is an ongoing dynamic function of the self‐lubricating bearing that continues throughout its operational life. The first and most critical step in the process is the break‐in period. This is when the initial transfer of material onto the mating surface takes place. The amount of bearing material deposited on the mating surface depends on several factors, including the speed, load, and length of stroke for the application. Typically the initial transfer takes only 50 to 100 continuous operational strokes or revolutions.
The secondary and ongoing phase of the transfer is where the self-lubrication is most effective. The transfer process continually deposits and maintains a microscopic film on the shaft, especially in the valleys of the mating surface, creating a true self-lubricated condition.
Some clever advertising gimmicks and inaccurate training materials claim “self‐lubricating” or “lubed for life” capabilities for components that do not fit the definition. The lubrication is not an integral element of the bearing material. Here’s a look at some often-mislabeled types of components:
•Rolling element devices: These include rotary (ball and roller) bearings, round‐way linear ball bearings, and rolling-element profile-type monorail designs. All of these require some kind of external lubrication to operate. The metal‐to-metal contact of rolling elements against raceways necessitates that there always be grease or oil present.
If this external lubricant is not present, the ball or roller will begin to make direct contact with the shaft or rail, resulting in galling and brinelling damage. Many manufacturers try to overcome this weakness in the design by adding oil‐impregnated seals to the ends of the bearing or housing. This approach does provide some benefit to the life of the bearing, but does not mean lubed for life.
• Oil impregnated bronze bearings: Bronze is porous and these bearings have been soaked in lightweight oil, some of which gets into the bronze. Under the best conditions, the oil gets drawn to the bearing surface when in use where it creates a lubricating layer between the bearing and shaft. Eventually the oil is all used up and needs to be replenished. Hence, these bearings aren’t lubed for life either.
• Graphite plugged bronze bearings: Graphite is a good solid lubricant that is normally added to bronze bearings. Solid plugs of graphite are usually inserted into holes in the base bronze where they provide lubrication as long as the graphite remains. But it gets worn down before the bearing is at the end of its operational life.
• PTFE (Teflon) coated bearings: PTFE can be used to coat bearing surfaces in several ways. It can be dusted onto the bearing as a powder; put into a mixture and sprayed onto bearings where it adheres; or it can be part of a liquid or grease compound applied to bearings. All of these methods results in a thin layer of actual lubricant that is quickly worn away and becomes ineffective.
• Oil impregnated plastics: Here again, lightweight oil is added to the base material to aid in bearing lubrication. The initial result is decreased friction, but lubricant aging and dissipation quickly reduce its effectiveness.
To truly be self-lubricating, bearings must do exactly what the name implies. They must provide their own lubrication throughout their operational life and not have an external lubrication source (automated or manual) for a period of time, or a reservoir that must be replenished. Lubrication that does not break down over time must be designed and manufactured into the bearing material from the beginning.
One example of a lubed for life bearing component is the Simplicity self-lubricating bearing liner from PBC Linear. It is a PTFE-based liner (Frelon) bonded to an aluminum body. This eliminates metal-to-metal contact between bearing and shaft, which, in turn, prevents galling and brinelling. No lubricants need to be added or replenished, so it makes the bearing maintenance/servicing free. As an added plus, it dampens vibrations, letting the bearing operate smoothly and quietly.
In other words, “self‐lubricating” must ensure clean and maintenance‐free operation in the most challenging environments. Designers need to learn to recognize the difference between the various types of lubrication options. Failure to do so will result in costly misapplications and re‐designs.
For more information on lubed for life bearings or any questions, contact PBC.