The Science of Self-Lubricating Plain Bearings
Simplicity® linear plain bearings exhibit real benefits, when compared to over other styles of linear bearings; such as the self-lubricating quality of the bearing’s Frelon® liner, as well as the wiping action on the shaft which enables smooth linear motion. These capabilities allow a linear motion system to not only avoid catastrophic failure, but to also lengthen the life of the system.
What is self-lubrication?
Self-lubrication is characterized by the bearings ability to transfer microscopic amounts of material to the mating surface. This transfer process creates a film that provides lubrication and reduces friction over the length of the rail or shaft. Self-lubrication has several advantages over traditional lubricated bearings. Self-lubricating bearings save time and money on preventative maintenance, and require no hazardous waste from the lubricant, disposal, or cleanup to handle. Self-lubrication ensures frictional forces on the bearings and drive system remain consistent, and no added grease or oil is required that can attract contaminants that destroy conventional bearings.
The transfer process
The transfer process is an ongoing dynamic function of the self-lubricating bearing that will continue 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 to the mating surface takes place. The amount of bearing material affected during the transfer is dependent upon multiple factors including the speed, load, and length of stroke, etc. for the application. Typically the initial transfer process will be accomplished in 50-100 strokes of continuous operation.
The secondary and ongoing phase of the transfer is where the self-lubrication is most effective.
What makes a system self-lubricating?
- The lubrication is an integral component of the bearing material.
- The lubrication (typically oil or grease) is NOT added to the original bearing design.
- The lubrication will NOT breakdown and be ineffective over time (lubricant aging).
- The lubrication is consistently applied to the shaft surface.
- Additional components do not add cost to the overall system.
To truly be self-lubricating, a bearing system must do exactly what the name implies. It must provide its own lubrication throughout the life of the system and not have some external source facilitating lubrication for a period of time. It must be designed and manufactured into the bearing material from the beginning.
Common misconceptions of self-lubrication
Clever advertising gimmicks and inaccurate training materials often claim “self-lubricating” or “lubed for life” capabilities for systems or components that do not fit the definition of the lubrication being an integral element of the bearing material. While these systems may be temporarily “self-lubricating”, the lubricant will eventually be used and need to be replaced. Many “lubed for life” bearings are not really lubed for life, they are simply “lubed for a long time”.
Common non self-lubricating systems
Rolling Element Systems: These include rotary (ball and roller) bearings, round-way linear ball bearings, and rolling element profile type monorail designs. All of these systems require some kind of external lubrication to operate. The metal-to-metal contact of the rolling element against the raceway necessitates that there be grease or oil present at all times. If this external lubricant is not present, the ball or roller will begin to make direct contact with the shaft or rail material resulting in galling and brinelling damage. Many manufacturers attempt to overcome this weakness in the design by adding oil-impregnated seals to the ends of the bearing or housing. This approach can result in some benefit to the life a rolling element bearing.
Oil impregnated bronze: Bronze bearings are very porous and have lightweight oil soaked into the material. Under the optimum conditions, this oil is drawn to the bearing surface creating a lubricated layer between the bearing and the shaft.
Graphite plugged bronze: Graphite is a good solid lubricant that is normally added to a bronze type bearing. Solid plugs of graphite are usually inserted into holes in the base bronze material.
Teflon coated materials: PTFE can be used to coat bearing surfaces in several ways. It can be applied as either a part of a powder simply dusting the bearing. It can be a sprayed mixture that adheres to the bearing surface. Or it can be part of a liquid or grease compound applied to the bearing. Any of these methods results in a very thin layer of actual lubricant that is quickly worn away and becomes ineffective.
Oil impregnated plastic: Here again, lightweight oil is added to the base material to aid in bearing lubrication. The result is decreased friction initially, but lubricant aging and dissipation quickly reduce its effectiveness.
The break-in process
The Frelon break-in and transfer process of a self-lubricating linear bearing ensures operation at maximum efficiency. In order to properly integrate the bearing into a linear motion system, the linear shafting needs to be cleaned with a 3-in-1® type of oil to ensure a clean transfer of material. Then the bearing must run multiple cycles over the shafting for the frelon to deposit a microscopic film onto the shaft, filling the valleys in the surface finish and creating a frelon-on-frelon running condition that is truly self-lubricating.
In certain situations, a plain bearing may need additional lubrication. For PBC Linear’s Simplicity bearings, the following lubricants are recommended:
- Waylube oils
- Lightweight, petroleum-based oils
- 3-in-1 type oils
- Lightweight, petroleum-based greases
Some lubricants that should be avoided when using Simplicity bearings include:
- PTFE sprays
- Oils, greases or sprays that contain fluorocarbons or silicone
- Motor oils/oils with additives
Simplicity bearings can be configured with an optional lubrication system machined-in for extra lubrication. A felt wick is machined into the ID of the bearing that is intended only for use with oils—when using grease, remove the felt wick to create a reservoir for the grease lubrication. Housed units are configured with a zerk-fitting in the wall allowing for the introduction of lubricant to the felt-wick on the inside of the bearing.
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