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!

Spring Energized Teflon Seals
by Sara McCaslin, PhD Sara McCaslin, PhD No Comments

Inside PEEK Bearing Materials: High Load Capacity, Low Wear

Engineers are replacing metal bearings too often or are dealing with corrosion, weight, or lubrication failures. PEEK is a high-performance polymer alternative that is changing bearing design. In this article, we look at what PEEK is, why it performs well in many bearing applications, and where it genuinely makes sense to use it.

What Is PEEK and Why Does It Matter for Bearings?

PEEK (Polyether ether ketone) is a semi-crystalline thermoplastic that has proven an excellent solution to many bearing problems. It offers a high strength-to-weight ratio, excellent chemical resistance, a low coefficient of friction, and thermal stability up to 250°C continuous. This combination of properties leads to effective bearing performance, offering PEEK an advantage over both metal and many standard polymer solutions. 

High Load Capacity

The standard performance-bearing metric is the PV rating (pressure × velocity). If we look at the PV ratings of commonly used polymer bearing materials, we see that PTFE is severely limited despite its low friction, and nylon and acetal do not do much better. Unfilled PEEK has the best PV ratio, and when filled with carbon fiber, it has a significantly higher PV rating than other engineering polymers. This alone is an excellent reason to consider using PEEK in bearing applications. And note that filled PEEK grades are typically the main choice for demanding applications, while unfilled PEEK is more commonly used where chemical purity or machineability is the priority.

Low Wear Performance

One of the most interesting features of PEEK as a bearing material is its wear resistance, which is critical for many applications. PEEK’s molecular structure resists abrasion under dynamic loading, and it offers excellent performance in dry-running applications because it is self-lubricating, offering an excellent option when lubricants are discouraged or prohibited. Bearing-grade PEEK filled with carbon or graphite further enhances wear performance by significantly reducing wear rates. The low wear performance of PEEK bearings also leads to reduced downtime and longer maintenance intervals. 

Where PEEK Bearings Make Sense

PEEK has excellent properties, but they are not an ideal fit for all bearing applications. PEEK bearings are commonly used for applications such as the following:

  • Medical devices and surgical equipment, where biocompatibility and sterilization resistance prove critical
  • Food and beverage processing, where the self-lubricating property reduces contamination risks
  • Aerospace and defense, where the high strength-to-weight ratio leads to weight savings, reduced energy requirements, and SWaP-friendly designs
  • Chemical processing, where resistance to acids, solvents, and aggressive media exposure is necessary
  • Semiconductor and cleanroom environments, where PEEK exhibits no outgassing 

There are areas where PEEK is not recommended, however. It should not be used in ultra-high load static applications, where steel still dominates. In addition, it’s not well-suited to very high-temperature environments, which are likely beyond PEEK’s thermal ceiling. Finally, PEEK is not a good option for budget-sensitive, low-performance applications where cheaper plastics work well.

Conclusion

PEEK bearings are not a universal replacement for other materials, but rather a viable option for demanding operating environments where their unique properties justify the cost. If you are looking for a sealing solution that sounds like a great application for PEEK, contact the sealing experts here at Advanced EMC and put their knowledge to work for you.