Composite Bushings: The Best of Both Worlds?

Composites combine different materials – often leveraging the strengths of metals and polymers – to achieve performance characteristics unattainable by a single material type. They represent engineered solutions for demanding applications where standard materials fall short.

• Metal-Backed Polymer Bushings:

  • Structure: These are multi-layered bearings. A strong metal backing (typically steel for strength and cost-effectiveness, or bronze for better corrosion resistance and thermal conductivity) provides rigidity and dimensional stability. Bonded to this is an intermediate porous layer, often sintered bronze. This layer serves multiple purposes: it mechanically locks the polymer layer to the backing, helps transfer heat away from the bearing surface, and can act as a reservoir for lubricant impregnation during manufacturing. Finally, a polymer-based overlay (the actual bearing surface) is applied and impregnated into the bronze layer.

  • Common Overlays:

    • PTFE-Based : Incorporates PTFE and other fillers (like lead in original DU, now often lead-free fillers) for extremely low friction, self-lubrication, and good chemical resistance. Ideal for dry running or minimal lubrication applications across a wide temperature range. Requires a transfer film to form on the shaft for optimal performance.

    • Acetal (POM)-Based (Pom series): Features an acetal copolymer overlay, often with lubricant pockets indented into the surface. Designed for marginally lubricated or grease-lubricated applications. Offers good wear resistance and load capacity but generally lower temperature limits and slightly higher friction than PTFE-based types.

  • Key Benefits: High load capacity (static and dynamic) due to the metal backing, excellent heat dissipation, very low friction (especially PTFE-based), self-lubricating, compact dimensions, good chemical resistance (depending on overlay).

  • Limitations: Can be susceptible to edge loading if misalignment is severe, corrosion resistance depends on the backing material and plating (if any). Performance relies heavily on the integrity of the thin polymer layer.

• Fiber-Reinforced Polymer Bushings:

  • Structure: These composites consist of high-strength fibers embedded within a polymer matrix. The fibers act as the primary load-bearing elements, while the polymer matrix binds them together, transfers load between fibers, and provides the bearing surface’s characteristics (chemical resistance, base friction).

  • Common Fibers:

    • Glass Fibers: Offer a good balance of increased strength, stiffness, and relatively low cost. Most common reinforcement.

    • Carbon Fibers: Provide significantly higher stiffness, strength, and lower weight compared to glass fibers, plus excellent fatigue resistance and low thermal expansion. More expensive.

    • Aramid Fibers (e.g., Kevlar®): Known for exceptional toughness, impact resistance, and vibration damping, but lower compressive strength than glass or carbon.

    • Polyester Fibers: Lower cost option for moderate strength improvements.

  • Common Polymer Matrices: Nylon (PA), PEEK, Epoxy Resins, Phenolic Resins, Polyester, Polyurethane (PU). The matrix largely determines temperature limits, chemical resistance, and inherent friction/wear properties. Lubricating fillers (PTFE, graphite, MoS2) are often added.

  • Manufacturing: Can be made through injection molding (with chopped fibers), filament winding (continuous fibers wound onto a mandrel, often with epoxy resin, creating very high strength in specific directions), or lamination processes.

  • Key Benefits: High strength-to-weight ratio, excellent corrosion resistance (no metal components), good wear resistance (can be tailored), potential for complex shapes (molded), good vibration damping, and often self-lubricating. Can operate in abrasive or wet environments where metals struggle.

  • Limitations: Generally lower thermal conductivity than metal-backed types (heat dissipation can be a concern at high speed/load), potentially higher cost, can be abrasive to softer shaft materials (depending on fibers/fillers), properties can be anisotropic (direction-dependent) especially in filament-wound types, lower tolerance for sharp impacts compared to ductile metals (though toughness can be high).

Table 4: Contrasting Metal-Backed vs. Fiber-Reinforced Composite Bushings

Composite bushings bridge the gap between the limitations of simple metals and polymers, offering engineered solutions for applications demanding high load capacity and low friction, or high strength and excellent corrosion resistance, often with self-lubricating properties.

Factors Influencing Material Selection: The PV Factor and Beyond

A crucial concept in bearing selection, especially for polymer and composite bearings, is the PV factor.

• P = Pressure (Load per Unit Area) (psi or N/mm²)

• V = Surface Velocity (Sliding Speed) (fpm or m/s)

The PV value (P x V) represents the combined mechanical and thermal stress on the bearing. Every bearing material has a maximum limiting PV value, above which wear rate increases dramatically, and thermal failure (melting, degradation) becomes likely.

• High P, Low V: Requires high compressive strength, good wear resistance (e.g., Aluminum Bronze, Fiber-Reinforced Composites).

• Low P, High V: Requires low friction coefficient, good thermal conductivity/heat resistance (e.g., Babbitt, PTFE-based Composites, Carbon Graphite).

• High P, High V: The most demanding scenario. Requires materials with high limiting PV values, excellent wear resistance, and often external lubrication/cooling (e.g., Metal-Backed Composites, High-Temp FRPs, sometimes specially treated Bronzes).

Beyond PV, Consider:

1. Operating Temperature: Min/Max temps affect material strength, clearance (thermal expansion), and lubricant viscosity/integrity.

2. Environment: Presence of moisture, chemicals, dirt, radiation. Dictates need for corrosion resistance, sealing, embeddability.

3. Lubrication:

   • Fully Lubricated (Hydrodynamic): Material properties are less critical as shaft & bearing are separated by oil film. Fatigue strength and corrosion resistance matter.

   • Marginally Lubricated (Boundary/Mixed Film): Bearing material properties are crucial! Low friction, wear resistance, compatibility with lubricant are key. Self-lubricating materials excel here.

   • Dry Running: Requires inherently low friction, self-lubricating materials (Polymers, Composites, Graphite).

4. Shaft Material & Hardness: The bearing and shaft form a system. Harder shafts generally allow for higher bearing loads and reduce wear on both components. Some bearing materials (e.g., abrasive FRPs) require very hard shafts. Softer materials (Babbitt, Polymers) are more forgiving of softer shafts.

5. Misalignment: Requires materials with good conformability (Babbitt, Leaded Bronze, some Polymers) or designs incorporating self-aligning features.

6. Cost & Availability: Balance performance needs with budget and lead times for standard or custom bearings.

Conclusion: Making the Informed Choice

Selecting the optimal bush bearing material is not a one-size-fits-all process. It requires a thorough understanding of the application’s demands – load, speed, temperature, environment, lubrication, and shaft characteristics – and matching those demands to the properties of candidate materials.

• Start with Metals (Bronzes) for general-purpose, moderate-to-high loads, and good temperature resistance, especially if lubrication is reliable.

• Consider Polymers (Acetal, Nylon, UHMWPE) for lower loads, corrosion resistance, self-lubrication needs, and cost sensitivity, paying close attention to temperature and moisture limitations.

• Explore Composites (Metal-Backed, Fiber-Reinforced) when pushing performance boundaries – requiring high load capacity with self-lubrication, extreme corrosion resistance, high strength-to-weight, or operation in very demanding environments.

• Don’t forget Babbitt for high-speed, fluid-film applications where shaft protection is paramount.

• Leverage Sintered Metals for cost-effective self-lubrication where loads are moderate.

Using comparison tables, understanding the PV factor, and carefully considering all operating conditions are essential steps. Consulting manufacturer data sheets for specific material grades is crucial, as properties can vary significantly even within the same family (e.g., different bronze alloys or filled PTFE compounds). Investing time in material selection upfront prevents costly failures and ensures the longevity and efficiency of your machinery.