Plane Polymer Bushings

When bearings fail, friction and wear are often the primary causes. Friction and wear in polymer bearings are critical factors in their design and performance. In fact, friction and wear influence everything from energy efficiency to service life. 

In this article, we examine the fundamentals of friction and wear, their impact on polymer bearing performance, and why understanding PV values is crucial for selecting the appropriate material. Whether you work in aerospace, medical devices, or industrial automation, this guide will help you make informed engineering decisions.

Understanding Friction

One way to define friction is the resistive force that occurs when two surfaces move against each other. In the context of polymer bearings, friction plays a key role in determining energy efficiency, heat generation, and overall wear behavior. 

Friction arises from surface interactions at the microscopic level. Even polished surfaces have asperities (tiny peaks and valleys) that cause mechanical interlocking and adhesion. In polymer bearings, friction behavior is influenced not only by surface roughness and contact pressure but also by factors such as lubrication, temperature, and material composition. The table below summarizes the main friction mechanisms at work in polymer bearings.

Mechanism	Description
Adhesion	Intermolecular bonding between polymer and counterface contributes to resistance.
Surface Deformation	Polymer deforms under load during sliding, adding to frictional resistance.
Viscoelastic Dissipation	Internal energy loss from viscoelastic behavior increases the friction coefficient.
Transfer Film Formation	A layer of polymer transfers to the counterface, altering friction over time.
Stick-Slip Phenomenon	Alternating adhesion and sliding causes vibrations or unstable motion.
Surface Roughness/Texture	Microstructure of surfaces influences contact area and frictional behavior.

High friction in polymer bearings can lead to excessive heat buildup, accelerated wear, and energy loss in mechanical systems. However, polymers often offer lower coefficients of friction than metals, which makes them an excellent option for reducing drag and operating efficiently without the need for continuous lubrication. This is especially advantageous in applications where maintenance access is limited, the environment is cryogenic, or cleanliness and sanitation are critical.

Understanding Wear

Wear is the gradual removal or deformation of material at solid surfaces due to mechanical action. In polymer bearings, wear not only affects part longevity but can also significantly alter dimensions and negatively impact performance over time, potentially leading to even increased friction, misalignment, or failure.

Wear occurs through various mechanisms, including abrasion, adhesion, fatigue, and erosion. In polymer bearings, the dominant type of wear often depends on operating conditions such as load, speed, temperature, and the presence (or absence) of lubricants. The main wear types are summarized in the table below.

Wear Type	Description	Common Causes	Relevance to Polymers
Abrasive Wear	Hard particles or rough surfaces wear away material.	Contaminants, rough counterfaces, high contact stress	Common in dirty or poorly filtered environments.
Adhesive Wear	Surfaces bond at contact points and then tear apart during movement.	High pressure, poor lubrication	Can be minimized by using low-friction polymer grades.
Fatigue Wear	Cracks form due to repeated cyclic loading, leading to material removal.	Repeated loading/unloading cycles	Important for dynamic applications (e.g., pumps).
Erosive Wear	Material is gradually removed by impact from particles or fluid flow.	High-speed fluids with particulates	Less common, but relevant in slurry or fluid transport.

Polymers typically have a lower elastic modulus and can deform under load, which helps reduce localized stress and delay wear, provided the material is selected correctly and the PV value remains within limits.

Friction and Wear in Polymer Bearings

Polymer bearings behave differently than metal under friction and wear. Their unique tribological properties—including low friction coefficients, self-lubricating capabilities, and tolerance for misalignment—make them ideal for applications where conventional metal bearings would struggle to perform.

PV Value: What It Means and Why It Matters

A key metric used in evaluating polymer bearing performance is the PV value, which stands for Pressure × Velocity. It quantifies the combination of load (P, in psi or MPa) and surface speed (V, in ft/min or m/s) that a bearing can withstand before experiencing excessive wear or thermal failure. In general, higher PV values indicate that the bearing can withstand greater stress and speed. Exceeding the limiting PV can lead to thermal softening, deformation, or accelerated wear of the polymer material.

Every polymer material has a limiting PV, which is the maximum combination of pressure and velocity it can handle under steady conditions. Engineers must stay below this threshold when designing systems to prevent performance breakdowns. Factors that affect limiting PV include:

• Material type (e.g., PTFE, PEEK, UHMW)
• Lubrication conditions
• Heat dissipation
• Bearing geometry and clearance
  

Choosing a polymer with a suitable PV rating is essential when operating at high loads, high speeds, or both. Some high-performance polymers even incorporate fillers—such as glass, carbon, or graphite—to increase wear resistance and raise the limiting power-to-weight (PV) threshold. Typical values are in the table below.

Material	Coefficient of Friction	Wear Resistance	Limiting PV	Typical Enhancements
PTFE	Very Low (0.05–0.10)	Fair	Low	Glass, bronze, or carbon fillers
PEEK	Moderate (0.15–0.30)	Excellent	High	Carbon fiber, PTFE, graphite
UHMW-PE	Low (0.10–0.20)	Good	Moderate	UV stabilizers, glass fiber
Nylon (PA)	Moderate (0.15–0.25)	Good (in dry conditions)	Moderate to High (with lube)	Moly disulfide
Filled PTFE	Very Low (0.04–0.09)	Good	Moderate	Graphite, MoS₂, or glass fiber blends

Choosing the Right Polymer for Friction and Wear

Selecting the optimal polymer for a bearing application requires more than just knowing the coefficient of friction. It involves understanding how various operating conditions interact with the material’s tribological profile.

Here are the major factors engineers should consider:

Load and Speed (PV Value): The combined pressure and velocity—expressed as the PV value—should stay below the material’s limiting PV to prevent overheating, deformation, or rapid wear. For high-load, high-speed applications, advanced materials like PEEK or filled PTFE are often necessary.

Temperature Range: Thermal stability varies widely across polymers. PEEK and PTFE perform well in high-temperature environments, while materials like UHMW-PE or nylon may soften or creep under prolonged exposure to heat.

Lubrication Conditions: Some applications operate with continuous lubrication, while others require dry-running performance. PTFE and UHMW-PE offer excellent dry lubricity, making them ideal for maintenance-free or clean-room conditions.

Chemical Exposure: Exposure to aggressive chemicals or cleaners can cause degradation of many plastics. PTFE offers broad chemical resistance, while nylon and acetal may be more limited depending on the environment.

Wear Resistance and Service Life: If durability is a primary concern, choose polymers that exhibit low wear rates under dynamic conditions. Fillers such as carbon fiber, glass, and graphite can significantly improve wear resistance without sacrificing too much in terms of friction.

Cost and Availability: High-performance polymers, such as PEEK and specialty-filled PTFE blends, come at a cost, however. For less demanding applications, materials like nylon or acetal may offer a more cost-effective solution.

Conclusion

Friction and wear are critical in polymer bearing design and specification. These factors directly impact performance, efficiency, and service life. Understanding how such friction and wear operate, along with the impact of PV value, temperature, lubrication, and material composition on behavior, empowers engineers to make informed decisions. Whether the application requires high-speed, dry-running conditions or chemical resistance in a corrosive environment, selecting the right polymer is crucial. At Advanced EMC Technologies, we specialize in high-performance polymer solutions engineered for demanding tribological environments. Contact us today to find the right material for your next bearing application.