Spring Energized PTFE Seal composite bushings
by Sara McCaslin, PhD Sara McCaslin, PhD No Comments

How Self-Lubricating Plain Polymer Bearings Keep Equipment Running Without Oil

Conventional bearings seize when oil runs out, but there are engineering polymer bearings that do not seize, nor do they experience stick-slip behavior. Where oil-free operation is required (e.g., food processing, pharma, wet/submerged environments), self-lubricating, high-performance polymers are the solution. This blog post discusses self-lubricating polymer plain bearings, including how they work, what the best naturally self-lubricating polymer options are, and how to select the right material.

The Problem with Conventional Lubrication

When the oil film fails at the contact surface, serious issues begin to develop for plain bearings, including adhesive wear, heat buildup, and seizure. The cost of failure of plain bearings is expensive and includes not only the cost of repairs but also unplanned downtime, contamination, and potential compliance risks. And while lubrication is necessary, there are some industries where adding lubrication to a bearing is simply impractical. These include food and beverage (NSF H1 and FDA), cleanrooms, and underwater applications. 

How Self-Lubricating Polymer Bearings Work

The core mechanism of self-lubricating bearings lies in the natural self-lubricating nature of the polymer (such as PTFE, UHMW-PE, and POM). There are two phases to the self-lubricating process: the run-in phase and the steady-state phase.

The Run-In (Break-In) Phase: When a new polymer bearing is installed, the metal shaft, no matter how highly polished it may be, has microscopic peaks and valleys called asperities. When the shaft begins to rotate against the bearing under a load, these asperities act like microscopic sandpaper. The asperities shear off a very thin layer of the polymer, and during this phase, the wear rate and friction are slightly higher. The image below shows an example of the asperities and their interaction with the lubricant film using PTFE as an example.

Steady-State Phase: The sheared polymer debris do not disappear. Rather, they get compacted into the valleys of the metal shaft’s surface. This process creates the transfer film. Once this film is fully established, the bearing is no longer rubbing against metal. Instead, it is rubbing against a thin layer of its own polymer material. Because polymer-on-polymer friction is exceptionally low, the wear rate drops dramatically, and the bearing can operate indefinitely without a need for external grease or oil, continuously replenishing the film as needed.

Key Polymer Materials (~150 words)

There are several polymers that have inherent self-lubricating properties due to their molecular structure, with no fillers or additives needed. Four of them are commonly used for plain bearings.

PTFE (Polytetrafluoroethylene)

PTFE has a fluorine-carbon backbone with extremely weak intermolecular forces, giving it one of the lowest coefficients of friction of any solid material (μ ≈ 0.04–0.10). The downside of PTFE for bearings tends to be its poor wear resistance, low load capacity, and tendency to creep in its pure form. However, it is available as a bearing-grade polymer that possesses additives to enhance the strength, stiffness, and wear of unfilled PTFE without sacrificing its low friction and natural lubricity. These fillers include carbon fiber, bronze, and graphite.

UHMWPE (Ultra-High Molecular Weight Polyethylene)

UHMWPE is heavily used in extreme bearing, wear pad, and sliding applications in its virgin, unfilled state. While cross-linked or oil-filled versions exist for specialty uses, its natural abrasion resistance is so remarkably high that it rarely needs compounding to function as a heavy-duty wear surface. Its low friction, excellent toughness, and good wear resistance make it an excellent choice for ebarings, and it is widely used in food processing and orthopedic implants.

Bearing-Grade POM (Acetal/Delrin)

Bearing-grade POM is naturally slippery because of its smooth, crystalline surface and low surface energy. While it is not as low-friction as PTFE, it does offer better dimensional stability and is load-capable without any additives. Virgin POM is hard, slick, and makes an excellent light-duty bearing, but at higher speeds or loads, it can generate excess heat or squeal (caused by slip-stick). The most common bearing-grade acetal has about 10-20% PTFE fibers as an additive. These fibers effectively smear across the contact surface during operation. This further lowers the coefficient of friction and increases the limiting PV  value associated with virgin acetal.

Real-World Payoff

Using a self-lubricating plain polymer bearing eliminates the need for re-lubrication intervals, which leads to significant labor and downtime savings. In addition, self-lubricating bearings pose no issues with lubricant contamination, having a direct impact on product quality as well as compliance benefits. In addition, these beatings result in an extended service life in wet, abrasive, or chemically aggressive environments where oil-lubricated bearings fail rapidly. In fact, as an example, consider a conveyor bushing in a food plant. The voice of a lubrication-free bearing means operation exceeds the service life of traditional greased bronze bearings by 3x.

Conclusion

Self-lubricated plain bearings are a proven engineering solution to bearing lubrication issues, not a compromise. The combination of the right material with correct design and proper run-in can provide you with reliable oil-free operation. Advanced EMC encourages you to evaluate your highest-maintenance lubrication points as retrofit candidates for replacement with self-lubricating solutions. For more information on self-lubricating bearings, contact us today!

Polymer Bushings and Bearings
by Sara McCaslin, PhD Sara McCaslin, PhD No Comments

Plain Polymer Bearings: When Simple Is a Design Advantage, Not a Compromise

Polymer plain bearings are often eliminated early in a design because it is easy equate complexity with performance. But that isn’t always true. While rolling-element bearings are the standard for a reason, the humble plain bearing, specifically when high-performance engineering polymers are used, offers a surprisingly robust alternative. Sometimes, eliminating moving parts is actually the biggest upgrade that can be made. 

This blog post explores what polymer plain bearings are, what their benefits are, and what their limitations are. 

Plain Polymer Bearings

A plain bearing (also known as a plane bearing) is the simplest design of a bearing, primarily because it has no rolling elements. These bearings come in several different forms, including cylindrical sleeves (the simplest), flanged (which can support axial loads), and thrust washers. As simple as their design is, however, there are many instances where they can outperform their rolling-element bearing counterparts. 

Plain Polymer Bearings vs Rolling-Element Bearings

What follows are some of the key benefits of simple plain bearing designs as compared to their rolling-element bearing counterparts. 

Superior Load Capacity and Shock Resistance: Plain bearings have a much larger surface-contact area than rolling element bearings possess. Because of this, plain bearings distribute loads much more effectively. Under shock loads, rolling-element bearings concentrate force into point loads that can cause fatigue or brinnelling. Plain bearings, however, can absorb shock and safely spread these impacts.

Predictable, Non-Catastrophic Failure: When a ball bearing fails, it seizes and can shatter and send problematic metal shrapnel into the system, a form of catastrophic failure that does serious damage beyond just the bearing itself. Plain bearings, on the other hand, experience wear at an even, predictable rate. This non-catastrophic failure mode is critical for applications in industries such as food processing and pharmaceutical applications, where contamination is a serious event.

Weight, Space, and Cost Savings: A typical plain bearing weighs less than half of a comparable needle-roller bearing, especially when a polymer bearing is used to replace a metal rolling-element bearing  (sometimes up to 14 times less than a deep-groove ball bearing). Their single-piece design eliminates the need for components such as snap rings and complex machined shoulders. This, in turn, enables engineers to shrink housing sizes and significantly reduce manufacturing costs.

Vibration Damping and Silence: Because they have no internal moving parts to rattle or slop around, plain bearings run quietly, and they naturally absorb a significant portion of mechanical vibration. This feature makes them extremely quiet when compared to rolling-element bearings.

Self-Lubrication: Perhaps the biggest advantage of plain engineering polymer bearings, such as PTFE and PEEK, is their maintenance-free nature. Instead of requiring continuous oil or grease, they rely on material transfer lubrication. As the shaft makes its first few rotations, the polymer smears a microscopic transfer film onto the mating metal shaft. This transfer film fills the microscopic recesses on the mating shaft, creating a dry-running, low-friction surface that prevents the messy buildup of debris common with greased bearings. 

When Polymer Plain Bearings are Not the Solution

Are plain bearings always the solution? No, and here’s a quick summary of where rolling-element bearings remain the recommended choice. For example, plain bearings are generally optimized for single-speed applications, but rolling-element bearings can tolerate a wider speed range, making them more suitable for applications with variable-speed drives. Start-stop cycles can also be a problem for plain bearings because it takes time for their lubricating transfer film to form, but rolling-element bearings are more efficient in such environments. In addition, high-precision applications work best with rolling-element bearings because polymer plain bearings require a radial clearance. Polymer bearings cannot handle the same level of edge loading as their equivalent rolling-element bearings. Finally, polymer plain bearings have much stricter mating shaft surface requirements.

Conclusion

Switching to plain bearings is not a solution for every design challenge. If you need extreme precision or are dealing with high-speed variability, rolling elements remain the best option. That said, the benefits of going simple are hard to ignore. Removing the risk of catastrophic seizure and reducing maintenance, polymer bearings solve the issues traditional rolling element metal bearings create. It comes down to knowing exactly what your application can tolerate.

If you are trying to decide between plain and rolling-element bearings, Advanced EMC can help. Contact us today and talk to one of our experienced bearing engineers.