Quantifying Friction: Lubrication Regimes Drive COF in Plain Bearings

Unlike rolling bearings, the Coefficient of Friction in Bearings for plain and self-lubricating types is highly variable and critically dependent on the lubrication regime existing between the sliding surfaces.

Key Factors Influencing COF in Plain & Self-Lubricating Bearings

The actual Coefficient of Friction in Bearings experienced is a result of complex interactions:

1. Lubrication Regime (The Dominant Factor): As shown above, whether the surfaces are fully separated by fluid (hydrodynamic), partially separated (mixed), in significant contact (boundary), or relying on a solid lubricant transfer film (self-lubricating) dramatically changes the COF. This regime is determined by speed, load, and lubricant properties (or solid lubricant characteristics).

2. Lubricant Properties (Fluid or Solid):

   • Fluid Viscosity: For lubricated bearings, viscosity is key to forming the hydrodynamic film. It’s highly temperature-dependent.

   • Solid Lubricant Type & Quality: For self-lubricating bearings (like our bronze graphite), the type of solid lubricant (Graphite, PTFE, MoS2), its concentration, and how effectively it forms a transfer film on the mating surface are critical.

3. Operating Conditions:

   • Load (Pressure P): High loads tend to squeeze out fluid films, increasing COF in lubricated bearings. In self-lubricating types, high loads increase pressure on the contact points and can accelerate wear if the material’s limit is exceeded.

   • Speed (Velocity V): Essential for generating hydrodynamic pressure in fluid-lubricated bearings. In self-lubricating bearings, speed influences heat generation and the dynamics of the transfer film. The PV value (Pressure x Velocity) is a critical limit for self-lubricating materials.

   • Temperature: Affects fluid viscosity drastically. For self-lubricating bearings, it impacts the structural integrity of the base material (e.g., bronze) and the solid lubricant itself (e.g., graphite oxidation limits, polymer softening points). Frictional heat needs effective dissipation.

4. Materials & Surface Finish (Crucial for Self-Lubricating):

   • Bearing Material: The base material (e.g., Tin bronze W-12 (EN 1982 CC483K / CuSn12-C), Aluminium bronzes (CC333G / CC331G), High tensile brasses EN 1982 CC762S) provides support and influences heat transfer and wear resistance. The embedded solid lubricant provides the low-friction interface.

   • Mating Surface Material & Hardness: The shaft or slide must be sufficiently hard (often >200 HB, sometimes >300 HB or even hardened steel HRC 45-60) to resist abrasion from the bearing material or trapped debris, ensuring the transfer film isn’t damaged.

   • Mating Surface Roughness (Ra): Critically important for self-lubricating bearings. A surface that is too rough (e.g., > 0.8 μm Ra) acts like a file, causing high initial wear and friction. A surface that is too smooth (e.g., < 0.4 μm Ra) may not provide enough microscopic “anchors” for the solid lubricant transfer film to adhere effectively. The range Ra 0.4 – 0.8 μm is often recommended.

5. Bearing Design: Clearance, length-to-diameter ratio (L/D), and features like grooves (if intended for grease distribution in some plain types) influence performance.

6. Installation & Alignment: Misalignment causes edge loading, drastically increasing local pressure, disrupting lubrication/transfer films, and leading to high friction and rapid failure.

Calculating the Coefficient of Friction: Beyond Simple Formulas

How is the coefficient of friction calculated for plain and self-lubricating bearings? It’s complex. While μ = Ff / Fn is the definition, predicting the operational COF involves more than a single bearing coefficient of friction formula.

• Lubrication Theory: For hydrodynamic bearings, calculations involving the Sommerfeld number can predict COF, but these rely on many assumptions. Predicting COF in boundary, mixed, or solid-lubricated regimes is much harder theoretically. There’s no simple universal formula for coefficient of friction in journal bearing that covers all regimes accurately, especially boundary and self-lubricating conditions.

• Empirical Testing: This is paramount, especially for self-lubricating materials. Manufacturers like us conduct extensive tests under various load, speed, temperature, and environmental conditions to characterize the COF of specific materials like our bronze graphite range.

• Design Estimates: Engineers often use manufacturer-provided typical COF values or conservative estimates (e.g., assuming boundary conditions for start-up torque calculations) to handle coefficient of friction problems during the design phase.

Bronze Graphite Self-Lubricating Bearings

As specialists, let’s focus on common friction bearing examples like bronze graphite bearings:

• Mechanism: Graphite particles embedded in a porous bronze matrix are exposed at the sliding surface. During operation, graphite transfers to the mating shaft, forming a low-friction solid lubricant film.

• Typical COF: Generally 0.05 to 0.20, heavily dependent on conditions. A design value of 0.10 or 0.15 might be used for conservative calculations.

• Key Influences:

  • Humidity: Graphite relies on adsorbed moisture (like water vapor) for optimal low friction. In very dry air or vacuum, its COF can increase significantly (>0.3 is possible).

  • Temperature: Bronze provides good thermal conductivity, but has temperature limits. Graphite itself is excellent at high temperatures (in non-oxidizing atmospheres) but oxidizes in air above ~450-500°C.

  • Mating Surface: Hardness (>200 HB essential) and roughness (Ra 0.4-0.8 μm vital) are non-negotiable for good transfer film formation and long life.

  • PV Limit: Exceeding the Pressure x Velocity limit leads to rapid temperature rise and failure.