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.

Torlon PAI Molecule
by Sara McCaslin, PhD Sara McCaslin, PhD No Comments

Torlon Bearings: Properties, Performance, and Engineering Applications

Torlon bearings are high-performance solutions designed for extreme performance even under extreme loads, high temperatures, and aggressive wear conditions. In this blog post, learn why more and more engineers are opting for Torlon bearings based on Torlon’s properties, bearing performance characteristics, and diverse areas of applications.

Properties of Torlon Bearings

Chemical Structure and Material Classification

Torlon® (polyamide-imide, PAI) is a high-performance engineering polymer. It is a hybrid polymer that combines the characteristics of both polyamides and polyimides, offering excellent toughness, thermal resistance, and mechanical strength. Its aromatic monomers, which reduce molecular mobility, give it stiffness, creep resistance, and high-temperature capabilities. 

The imide linkages in Torlon give it a high bond strength, allowing it to resist chain scission at extremely high temperatures. In addition, the imide groups provide excellent resistance to oxidative degradation, extending their service life in oxygen-exposed environments.

This combination of aromatic monomers with imide linkages enables it to have a high continuous-use temperature of up to 500°F without losing its key mechanical properties. Torlon’s combined imide and aromatic bonding networks also enhance its wear resistance, which is critical in poor-lubrication or dry-running operating conditions.

Torlon’s molecular architecture enables bearings to provide reliable performance even under high PV loads and improves their compressive strength to support high-load-bearing applications. In addition, its chemical makeup enables Torlon to outperform many other engineering polymers in applications that demand dimensional stability under heavy mechanical cycling or extreme thermal spikes.

Thermal Properties

Torlon is known for its continuous-use temperatures approaching 260°C (500°F), as well as its excellent retention of mechanical strength at such elevated temperatures. Torlon also exhibits very low thermal expansion. Its primary thermal drawback lies in manufacturing: it is primarily limited to compression molding and machining because of its high melt processing temperature. 

Mechanical Properties

Torlon is known for its excellent strength and stiffness, including a high load capacity. Two of the bearing grades of Torlon are …

  • Torlon 4435: high-temperature, low-friction, high-PV
  • Torlon 4301: high strength, low friction, graphite-filled

Tribological Properties

Naturally low friction coefficients are another key property of Torlon. These values can be improved by using PTFE- or graphite-filled material. Moreover, Torlon has excellent wear resistance even in environments with minimal lubrication. Finally, it exhibits very low levels of deformation and creep even under sustained stress.

Chemical & Environmental Resistance

Torlon has excellent resistance to chemical attack, including automotive fluids, solvents, and hydraulic oils, and good resistance to hydrolysis.

Performance of Torlon Bearings

Load-Carrying Capacity

Its excellent compressive strength means that Torlon bearings can be used in high-load bearing applications, with PV ratings up to 100,000 for Torlon 4435.

Temperature Performance

Torlon bearings maintain excellent structural and wear performance even under continuous high temperatures. It is suitable for operating conditions where metal bearings seize.

PV (Pressure–Velocity) Capability

The high PV capability of these bearings makes them well suited for high-speed, high-load applications where heat generation is significant. For example …

  • Torlon 4435
    • Max P (Pressure): 1,000 psi
    • Max V (Velocity): 850 sfm
    • Max PV: 100,000 (psi·ft/min)
    • Service Temperature: 500°F
  • Torlon 4301
    • Max P (Pressure): 1,000 psi
    • Max V (Velocity): 900 sfm
    • Max PV: 50,000 (psi·ft/min)
    • Service Temperature: 500°F

Wear and Friction Behavior

Because these bearings have such a low coefficient of friction, startup wear and energy losses are very low, and they exhibit excellent dry-running performance. In addition, friction is stable across a wide range of loads and temperatures.

Dimensional Stability & Creep Resistance

Low thermal expansion and minimal creep make Torlon bearings an excellent option for precision applications, tight clearances, and long service life.

Applications for Torlon Bearings

Torlon bearings are used in aerospace systems such as flap actuators and landing gear, where low weight, high wear resistance, and the ability to withstand extreme thermal cycling are essential. In oil and gas equipment, Torlon provides reliable performance in HPHT environments for components like downhole tools, pumps, and valves. Industrial machinery relies on Torlon for bushings, wear rings, and thrust washers that offer low friction and long life, where metal bearings wear out quickly.

In automotive and transportation systems, Torlon supports transmissions, pumps, and electric motors with strong thermal resistance and low wear. Semiconductor and precision equipment benefit from Torlon’s low outgassing, chemical resistance, and dimensional stability in clean, tightly controlled environments. Automation and robotics use Torlon bearings in high-load joints and guides, where strength, low wear, and minimal lubrication improve system reliability.

Conclusion

Torlon bearings offer unique advantages for extreme mechanical, thermal, and environmental operating conditions. Their high load capacity, excellent wear behavior, superior PV performance, and broad industrial applicability have made them the choice for many bearing solutions across a range of industries and applications. If you are interested in bearing solutions for aggressive operating environments, contact Advanced EMC today for custom Torlon bearing design, material selection, and manufacturing guidance.