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 Loaded Seal
by Sara McCaslin, PhD Sara McCaslin, PhD No Comments

High-Performance Polymer Bearings: Materials, Advantages, and Use Cases

High-performance polymer bearings are replacing metal in increasingly demanding applications. The low-friction, dry-running capabilities, chemical resistance, and weight savings are just a few of the reasons why polymer bearings are gaining favor among engineers. This blog post focuses on three materials — PEEK, PPS, PAI, and PTFE — and their applicability to plane bearing applications.

Advantages of High-Performance Polymer Bearings

Bearing-grade engineering polymers such as PEEK, PPS, PAI, and PTFE are excellent options for bearing design. Compared to traditional metal bearings, these can run dry with minimal lubrication, generate less frictional heat, and experience less wear. They also offer corrosion resistance and are resistant to many problematic chemicals, along with significant weight savings. They also serve as natural electrical insulators and offer good vibration damping. High-performance polymer bearings are also compatible with clean environments (e.g., medical and semiconductor).

Understanding High-Performance Polymer Bearings

What makes a bearing material high-performance? First is low friction and natural lubricity, which reduces friction and the amount of heat generated by friction. Next is resistance: resistance to wear, resistance to chemicals, and resistance to absorbing moisture. Dimensional stability is also key, as well as the ability to maintain their load capacity under heat. Finally, PV limits serve as a benchmark for how suitable a high-performance polymer is for a particular application.

Material Profiles for High-Performance Polymer Bearings

1. PEEK (Polyetheretherketone)

PEEK is a semicrystalline, high-performance thermoplastic that is best known to engineers for its balance of strength, temperature resistance, and dimensional stability. It has excellent mechanical strength, with bearing grades able to withstand up to 6,00 psi. It also possesses excellent high-temperature performance up to 480°F continuous. In addition, PEEK has excellent chemical resistance that includes fuels, oils, solvents, and corrosive media. It also provides good creep resistance and dimensional stability even under thermal cycling.

Depending on the grade chosen, PEEK’s PV rating is 100,000 PV (Fluorolon 3015, PEEK BG) or 50,000 PV (Fluorolon 3010), with velocities up to 600 SFM and low friction (especially when graphite is used as a filler).

PEEK is often used with high-speed actuators, pumps, compressors, downhole and energy-sector applications, and aerospace mechanical linkages.

2. PPS (Polyphenylene Sulfide)

PPS is a rigid, aromatic polymer known for excellent chemical inertness and good dimensional stability, especially in corrosive environments. PPS is resistant to nearly all solvents, acids, bases, fuels, and process chemicals and has a continuous-use temperature of up to 400°F. It has naturally low friction and low moisture absorption, but is more brittle and less impact resistant than PEEK or PAI. 

PPS good PV ratings, depending on the grade used (i.e, 25,000 PV for Fluorolon 5065 and 11,000–12,000 PV  for Fluorolon 5025, 5010). It also possesses a load capacity of up to 2,000 psi, dependent on the grade chosen. 

PPS works very well in chemical processing equipment, automotive components exposed to aggressive chemicals, pumps and valves with moderate loads, and applications where chemical performance and corrosion resistance outweigh mechanical requirements.

3. PAI (Polyamide-Imide)

Torlon, or PAI, is the highest-strength thermoplastic available for bearing applications. Its imide backbone provides exceptional thermal, mechanical, and creep resistance. It has excellent compressive strength and fatigue resistance, with a continuous use temperature up to 500°F. It also has low friction in its graphite-filled grades, like Torlon 4301. 

PAI has good PV ratings, with 100,000 PV  for Torlon 4435 and  50,000 PV for Torlon 4301. Its velocity limit is 850–900 SFM, one of the highest speeds for non-metal bearings. And its load capacity is up to 1,000 psi. 

PAI bearings are commonly used for aerospace linkages and flap actuators, industrial machinery with extreme loads, robotic joints and linear motion systems, and high-temperature turbine or compressor environments. 

4. PTFE (Polytetrafluoroethylene, Filled and Unfilled)

PTFE is the lowest-friction engineering material available. It offers near-universal chemical resistance, but has lower strength and PV capability than PEEK or PAI. In fact, its ultra-low coefficient of friction makes it often used in dry-running or low-lubrication conditions. PTFE also has exceptional chemical resistance as it is inert to nearly all chemicals. High thermal resistance is another key property of PTFE: depending on the grade, it can be between
500°F–550°F depending on grade. However, it exhibits poor mechanical strength and creep resistance unless fillers are used.

Its PV rating depends heavily on fillers: 10,000 PV for filled PTFE grades like Rulon LR, J, W2, and only 1,000 PV  for unfilled PTFE: Fluorolon 1000. PTFE’s velocity limits are up to 400 SFM. For filled PTFE grades, the load capacity ranges from 750 to 1,000 psi.

PTFE bearings work best as low-load, low-to-moderate speed bearings. Common areas of application include chemical processing, where exposure to highly corrosive chemicals is expected, and semiconductor and cleanroom applications, where contamination is not acceptable.

Conclusion

High-performance polymer bearings made from PEEK, PPS, PAI, and PTFE offer significant advantages over traditional metal counterparts, including low friction, chemical resistance, and weight savings. And polymer bearings extend equipment life, improve reliability, and increase efficiency. 

If you’re evaluating materials for demanding bearing applications, our engineers can help you identify the best polymer solution for your requirements. Contact Advanced EMC today to discuss your design challenges and request a consultation.