by Sara McCaslin, PhD Sara McCaslin, PhD No Comments

Startup and Running Friction in Polymer Bearings

Startup and running friction can vary based on several different factors, and in this article we focus on what they are, why their values differ, and typical coefficients, as well as a detailed look at why these differences occur. It ends with a discussion of what impact startup and running friction have on bearing design.

Startup and Running Friction

Polymer bearings tend to exhibit higher breakaway (startup) friction than steady-state running friction due to factors such as static adhesion, microasperity interlocking, and transfer-film formation dynamics. 

The startup friction coefficient µₛ is measured at the onset of motion and represents static friction. The term “startup” does not refer solely to time zero, however. It represents the peak friction force or torque required to break the bearing free after a period of rest. Startup friction is actually the regime of dry contact when the polymer surface is still unconditioned. Unconditioned  means that the transfer film on the counterface is incomplete or patchy

The running friction coefficient µₖ represents a kinetic or dynamic measure of friction. It takes place when steady sliding motion is established and is represented by. Running friction relates to the frictional resistance that exists when two surfaces are in motion, steadily sliding against each other. 

Running friction applies after the initial breakaway event has occurred and the system has moved past issues such as static adhesion and micro-locking. As a result, the coefficient of running friction is typically lower and more stable than startup friction, especially for materials such as PTFE and UHMW-PE.

Why Startup and Running Friction Can Differ in a Polymer

There are some key factors that differentiate startup friction from running friction in polymers. For example, at rest, there is adhesion and junction growth. Polymer chains can increase the real contact load at under load rest (creep/relaxation), thereby increasing µₛ.In addition, at startup, there will be surface roughness and plowing. The roughness increases issues with mechanical interlocking and plowing. These two effects also raise the starting friction value.

In running friction conditions, materials like PTFE form a transfer film that reduces the effect of asperities and surface roughness, which reduces running friction. There is, however, a risk of stick-slip. This phenomenon is more likely to occur when the stiffness of the system is low, the speed is low, and the µₛ / µₖ ratio is high.

Typical Coefficients of Friction

The values below represent commonly used engineering polymers and are typical dry sliding vs steel values. These values can vary with pressure, speed, temperature, finish, fillers, and test method.

  • PTFE (virgin)
    • Startup friction (µₛ): ~0.05–0.10, often nearly identical to running friction
    • Running friction (µₖ): ~0.05–0.10
    • Minimal difference between startup and running friction
  • PEEK (unfilled)
    • Startup friction (µₛ): ~0.20
    • Running friction (µₖ): ~0.25
    • Exhibits a noticeable increase from startup to running friction
  • UHMW-PE
    • Startup friction (µₛ): ~0.15–0.20
    • Running friction (µₖ): ~0.10–0.20
    • Running friction can be equal to or lower than startup friction
  • Nylon 66 (PA66)
    • Startup friction (µₛ): ~0.20 (against steel)
    • Running friction (µₖ): ~0.15–0.25 (typical)
    • Moderate variability depending on surface finish and condition

What Is Behind the Difference Between Startup and Running Friction

Several factors account for the difference between startup and running friction. Pressure and dwell time, for example, mean that higher loads and long dwell times increase the real contact area and have the potential to raise µₛ. For speed, higher speeds can actually reduce friction after the polymer transfer film stabilizes, but can also raise heat generation. 

Temperatures are known to impact polymer modulus and creep, which can shift both µₛ and µₖ and alter the risk of stick-slip. In addition, the counterface material and hardness will affect the adhesion and transfer film, which is why it is important that the frictional coefficient used in design calculations represents the friction against the counterface material (e.g., PTFE vs steel, PEEK vs aluminum).

Note that PTFE-filled PEEK, MoS₂-filled nylon, and glass/bronze-filled PTFE shift friction and wear differently, often lowering friction but sometimes increasing counterface wear.

Surface finish also has a significant impact. If the surface finish is too rough, plowing will occur, increasing both friction and wear. On the other hand, if the surface finish is too smooth it can increase adhesion issues.

Impact on Bearing Design

Startup and running friction impact material selection, clearance, and surface finish in bearing design. Startup friction is dominated by static friction and adhesion at rest. This fact significantly impacts breakaway torque and can be a limiting factor in low-speed, intermittent, or precision motion systems. In such systems, stick-slip, noise, and control instability are unacceptable. 

Running friction, on the other hand, is governed by dynamic friction. Once motion is established, it controls steady-state heat generation, wear rate, and long-term dimensional stability. It directly influences PV limits and service life. 

Because many polymers exhibit higher startup friction than running friction, engineers need to balance low breakaway forces with acceptable operating temperatures and wear. This is usually accomplished through the use of self-lubricating materials, fillers, or surface texturing to manage both regimes. A successful polymer bearing design accounts for the full friction lifecycle, ensuring reliable motion at startup without sacrificing durability during continuous operation.

Conclusion

Startup and running friction have a significant impact on bearing design, as well as factors such as material fillers, pressure, temperature, and counterface material. If you are looking for a polymer bearing solution, contact the experts at Advanced EMC. Our team of bearing specialists can help you find the best bearing material for your design and can help you select the optimal material from our range of bearing-grade polymers.

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

Wear and Friction in Polymer Bearings

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.

MechanismDescription
AdhesionIntermolecular bonding between polymer and counterface contributes to resistance.
Surface DeformationPolymer deforms under load during sliding, adding to frictional resistance.
Viscoelastic DissipationInternal energy loss from viscoelastic behavior increases the friction coefficient.
Transfer Film FormationA layer of polymer transfers to the counterface, altering friction over time.
Stick-Slip PhenomenonAlternating adhesion and sliding causes vibrations or unstable motion.
Surface Roughness/TextureMicrostructure 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 TypeDescriptionCommon CausesRelevance to Polymers
Abrasive WearHard particles or rough surfaces wear away material.Contaminants, rough counterfaces, high contact stressCommon in dirty or poorly filtered environments.
Adhesive WearSurfaces bond at contact points and then tear apart during movement.High pressure, poor lubricationCan be minimized by using low-friction polymer grades.
Fatigue WearCracks form due to repeated cyclic loading, leading to material removal.Repeated loading/unloading cyclesImportant for dynamic applications (e.g., pumps).
Erosive WearMaterial is gradually removed by impact from particles or fluid flow.High-speed fluids with particulatesLess 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.

MaterialCoefficient of FrictionWear ResistanceLimiting PVTypical Enhancements
PTFEVery Low (0.05–0.10)FairLowGlass, bronze, or carbon fillers
PEEKModerate (0.15–0.30)ExcellentHighCarbon fiber, PTFE, graphite
UHMW-PELow (0.10–0.20)GoodModerateUV stabilizers, glass fiber
Nylon (PA)Moderate (0.15–0.25)Good (in dry conditions)Moderate to High (with lube)Moly disulfide
Filled PTFEVery Low (0.04–0.09)GoodModerateGraphite, MoS₂, or glass fiber blends
Polymer Beads

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.