The coefficient of friction in PTFE slide bearings decreases as bearing pressure increases, provided the pressure stays within acceptable creep limits. This relationship stems from PTFE's unique molecular structure, which allows its polymer chains to align under pressure, creating a smoother sliding surface. Unfilled PTFE exhibits this behavior most prominently, especially when paired with highly polished stainless steel. While pressure is a primary factor, sliding velocity and temperature also influence friction, with high pressure and low velocities generally yielding the lowest friction coefficients. This pressure-dependent friction reduction makes PTFE bearings valuable in applications requiring minimal friction, such as bridge bearings or heavy machinery.
Key Points Explained:
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Inverse Relationship Between Pressure and Friction
- PTFE's coefficient of friction decreases as bearing pressure increases, a phenomenon observed in tests with stainless steel mating surfaces.
- This occurs because higher pressure causes PTFE's polymer chains to orient parallel to the sliding direction, reducing surface asperities.
- The effect plateaus when pressure approaches the material's creep limit, beyond which permanent deformation may occur.
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Optimal Material Conditions for Minimal Friction
- Unfilled PTFE (without additives like glass or bronze) shows the most significant friction reduction under pressure, as fillers can disrupt molecular alignment.
- A highly polished stainless steel counterface further enhances this effect by minimizing surface roughness.
- For custom PTFE parts like bearings, these factors are critical in design specifications.
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Secondary Influencing Factors
- Sliding Velocity: Low velocities complement high pressure to achieve the lowest friction coefficients.
- Temperature: Elevated temperatures can soften PTFE, potentially altering its friction properties, though the effect is less pronounced than pressure.
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Practical Implications
- The pressure-friction relationship makes PTFE ideal for static or slow-moving heavy-load applications (e.g., bridge bearings), where high bearing pressures are common.
- Reduced friction minimizes heat generation, preventing material degradation and maintaining performance in dynamic sealing or washer applications.
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Comparison with Other Materials
- Unlike PTFE, materials like graphite maintain a constant friction coefficient (~0.15) regardless of pressure, highlighting PTFE's unique tribological behavior.
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Design Considerations
- Engineers must balance pressure increases with PTFE's creep resistance to avoid permanent deformation.
- Surface finish and material purity (unfilled PTFE) are often prioritized in high-performance bearing designs.
This understanding enables precise selection of PTFE bearings for scenarios where friction control directly impacts longevity and efficiency, such as in aerospace or medical equipment.
Summary Table:
| Factor | Effect on PTFE Friction | Design Consideration |
|---|---|---|
| Bearing Pressure | Higher pressure reduces friction by aligning PTFE chains; plateaus near creep limits. | Balance pressure with material limits to avoid deformation. |
| Material (Unfilled PTFE) | Pure PTFE shows the most friction reduction under pressure. | Prioritize unfilled PTFE for high-pressure, low-friction applications. |
| Counterface Finish | Polished stainless steel minimizes surface roughness, enhancing friction reduction. | Specify mirror-finish counterfaces for critical sliding interfaces. |
| Sliding Velocity | Low velocities + high pressure yield the lowest friction coefficients. | Ideal for static/slow-moving loads (e.g., bridge bearings). |
| Temperature | Elevated temps may soften PTFE but have less impact than pressure. | Monitor thermal conditions in dynamic applications. |
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