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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.

Engineers often rely on bearing-grade polymers for critical aerospace applications. Bearing-grade materials, including PEEK, Torlon, and polyimide composites, are able to offer the strength and stability needed to replace metals in aerospace mechanisms.

This blog post is going to explore the defining properties, performance advantages, and design considerations of bearing-grade polymers in aerospace mechanisms.

The Engineering Challenge: Bearings in Aerospace Systems

Bearings in aerospace mechanisms face extreme operatging conditions. They must operate under high loads and speeds, withstand temperature extremes from cryogenic levels to over 500°F, and perform in vacuum or radiation environments without failure. Traditional metal bearings—though strong—can corrode, seize, or wear rapidly under these conditions. They often require lubrication, which is problematic in vacuum or high-temperature applications. Given these operating conditions, bearing-grade polymers are an attractive alternative to traditional metal bearings.

What is a Bearing-Grade Polymer?

Bearing-grade polymers are engineered plastics formulated specifically for high-performance bearing and bushing applications. Many of their enhanced properties are the result of additives such as graphite, PTFE, carbon fiber, glass, or molybdenum disulfide (MoS₂). Key parameters that define their performance include maximum pressure (P), velocity (V), and the combined PV limit, which measures load-speed endurance.

Advanced EMC’s Bearing Material Guide identifies several polymer families optimized for aerospace use, including:

Fluorolon 3015 (PEEK BG): it has a high PV capability, good chemical resistance, and good thermal stability.
Torlon 4435: known for its excellent high-temperature performance and low friction under high loads.
Fluorolon 4031–4033 (Polyimide-based): exhibits outstanding thermal resistance and dry-running capabilities.
Fluorocomp 6000/6010 (Polyimide Composites): has superior load-bearing and temperature tolerance with low wear.

Advantages of Bearing-Grade Polymers in Aerospace

Weight Reduction and Fuel Efficiency

Bearing-grade polymers are up to 80% lighter than metal counterparts. This type of weight savings directly contributes to SWaP objectives and offers much better payload and fuel efficiency.

Self-Lubrication and Maintenance-Free Operation

It is possible to obtain aerospace-grade polymers that are either naturally self-lubricating or feature built-in lubricants. This material property effectively eliminates the need for external lubrication, which is ideal for vacuum environments and reduces maintenance requirements.

Dimensional Stability and Low Thermal Expansion

Bearing-grade polymers have excellent dimensional stability and low thermal expansion. Such material properties allow them to maintain consistent clearances across a wide tempreature range. This stability prevents problems with binding or deformation that is common with metal bearings during rapid temperature shifts.

Chemical and Radiation Resistance

Polymer-grade materials include those that exhibit excellent performance even in the presence of chemicals such as hydraulic fluids, de-icing agents, fuels, and radiation without exhibiting degradation. Their chemical and radiation resistance helps ensure a long service life even in highly aggressive environments.

Vibration Damping and Noise Reduction

In addition to mechanical durability, polymers also provide vibration damping and noise attenuation. Not only can this enhance comfort but it can also reduce wear in sensitive control systems.

Common Aerospace Applications

There are a host of aerospace applications that depend on bearing-grade polymer solutions. These include ….

Actuation Systems – Bearings in flight control, flap, and slat actuators benefit from low friction and dry-running capability.
Landing Gear Components – Lightweight polymer bushings withstand impact loads and resist corrosion in outdoor conditions.
Satellite and Spacecraft Mechanisms – Polyimide and PEEK bearings can perform reliably in vacuum and cryogenic environments.
Environmental Control Systems (ECS) – Bearings resist thermal cycling in high-speed air-handling systems.
Turbomachinery and Pumps – High-PV polymers operate effectively without lubrication in auxiliary pumps and gear mechanisms.

Comparative Overview: Bearing-Grade Polymer Families

The table below offers an overview of the most commonly used bearing-grade polymers.

Material	Temperature Limit (°F)	Max PV (psi·ft/min)	Attributes	Typical Aerospace Use
Polyimide (Fluorolon 403x)	570	250,000–300,000	Low friction, high temperature, chemical resistance	Space mechanisms, dry-running bearings
PEEK (Fluorolon 3015)	480	100,000	High PV, chemical resistance, thermally stable	Aircraft actuators, gearboxes
Torlon 4435	500	100,000	High temp, high strength, low wear	Landing gear bushings, structural bearings
PPS (Fluorolon 5065)	400	25,000	Low friction, moderate load	Cabin systems, auxiliary components
Composite (Fluorocomp 6000)	550	80,000	Polyimide-carbon composite, high load, high temp	Dry-running or high-stress joints

Design and Integration Considerations

When designing aerospace compoents from bearing-grade polymer materials, engineers must account for issues such as creep, outgassing, and thermal expansion. Attention must also go into ensuring proper clearance, wall thickness, and housing interference, all of which are crucial to maintaining alignment and preload in the presence of fluctuating temperatures. Surface finish and counterface material also have an impact on wear performance.

Note that manufacturers such as Advanced EMC are capable of ensureing consistency through precision machining, molding, and post-processing that are in compliance with AS9100 and NASA outgassing standards.

Conclusion

Bearing-grade polymers reduce weight, extend component life, and perform where metals cannot, whether their enviroment is cryogenic vacuum conditions or at the heart of high-speed actuation systems. And as aerospace systems continue to rapidly move toward greater efficiency and autonomy, polymer bearing technology will remain a cornerstone of reliability and innovation.

Advanced EMC provides engineered polymer bearing solutions optimized for aerospace performance. Contact our knowledgeable team today to learn how high-performance materials like PEEK, Torlon, and polyimide composites can enhance your next aerospace design.

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