Some applications make bushing lubrication extremely difficult, like food and dairy applications where standard methods of lubrication, such as grease, would often serve as a contaminant risk.
Another problem area is the need for bushings where temperatures drop low enough to cause normal lubricants to freeze or temperatures rise high enough for the lubricants to break down. For that reason, many engineers look to self-lubricating or non-lubricated bearings.
What Is the Difference between Self-Lubricating and Non-Lubricated?
Many engineers will disagree on the difference between self-lubricating bushings and non-lubricated bushings.
Polymer bearings that require no lubrication fall into one of two categories:
1. the nature of the polymer itself achieves lubrication through the transfer of a thin film;
OR
2. the polymer includes a solid lubricant additive such as graphite.
As an example, consider bushings made from a material such as PTFE: the bushing may not require any lubrication at all because PTFEs coefficient of friction is already incredibly low. Other polymers that may not require a lubricant include Nylon polymers and UHMWPE.
These polymers are classified as dry-running and self-lubricating because they can operate without the need for a lubricant. However, others feel that for the same reason, these polymer bushings should be classified as non-lubricated. For the purpose of this blog post, we will differentiate between polymer bushings without a solid lubricant additive (non-lubricated) and polymers bushings that include a solid lubricant additive (self-lubricating).
How Non-Lubricated Bushings Work
Non-lubricated polymer bushings transfer a thin layer of the polymer to the surface of the shaft as soon as relative motion is initiated. This thin film serves the purpose of a lubricant by reducing the friction at the interface and forming a protective layer between the surface and the bushing. It also fills up imperfections on the shaft surface, again reducing the friction between the shaft and the bushing.
The thin-film is especially good at minimizing stick-slip, which can occur every time motion is initiated. In a traditional lubricated bearing, the lubricant needs the momentum of the shaft to distribute itself evenly and may have become somewhat stiff because of the lack of motion. This in turn increases wear but dry-running polymer bushings do not encounter stick-slip.
Dry-running polymer bushings do not encounter stick-slip
Another one of the positive aspects of non-lubricated bushings lies in the initial start-up of the equipment. The bushing is dry as operation begins, but as soon as contact between the bushing and the shaft is initiated, the transfer of the lubricating film begins. These means there are no major issues with the initial start-up because lubrication begins immediately.
How Self-Lubricating Bushings Work
Self-lubricating polymer bushings usually combine a thermoplastic polymer and reinforcing fibers with a solid lubricant additive such as graphite. The bushings release this solid lubricant as soon as relative motion and contact occurs between the bushing and the shaft.
Like non-lubricated bushings, stick-slip is minimized. Also like their counterparts, self-lubricating bushings begin lubricating as soon as contact takes place, meaning there are no major issues with first-time startup of the equipment.
Advantages of Non-Lubricated and Self-Lubricating
There are quite a few advantages to self-lubricating and non-lubricated bushings beyond stick-slip and initial start-up.
- First, the absence of traditional lubricant reduces the probability that abrasive debris will be pulled into the bushing-shaft interface;
- Because no lubricant is required, maintenance is drastically reduced or even eliminated;
- This type of bushing also simplifies the design process by eliminating the need for groove and holes to facilitate the use of lubricant.
Conclusion
Of course, the judicious use of lubricants will extend the life of a bushing. However, lubricants may not be needed when a self-lubricating or non-lubricated polymer bushing can handle the load.
Missed Parts 1 or 2 in the Series? Read them here:
Factors That Influence Polymer Bushing Performance: Mating Hardware – Part 1 in a 3 Part Series
Factors That Influence Polymer Bushing Performance: Speed and Pressure – Part 2 in a 3 Part Series
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