How to Select a Plain Bearing

 


Introduction to Plain Bearings

Plain bearings (also called bushings or sleeve bearings) are simple, cost-effective components that use sliding friction between a shaft and a lubricated surface. Unlike rolling bearings, they have no balls or rollers, making them ideal for low-speed, oscillating, or heavy-load applications. Selecting the right one involves evaluating application needs to ensure durability, efficiency, and minimal maintenance. Below is a step-by-step guide based on engineering standards like ISO 3547.

Key Factors for Selection

Before choosing, assess these parameters:

  • Load Type and Magnitude: Radial (perpendicular to shaft) or axial (along shaft). Plain bearings handle high loads but require lubrication to prevent seizing.
  • Speed: Low to moderate (up to 5 m/s sliding speed); high speeds can cause overheating.
  • Operating Environment: Temperature (-200°C to 300°C), humidity, corrosion, or contaminants.
  • Lubrication: Self-lubricating (e.g., PTFE-lined) or external (oil/grease).
  • Shaft Material and Finish: Compatibility to avoid wear; hardness >50 HRC recommended.
  • Size and Fit: Inner diameter (ID), outer diameter (OD), length; tolerances for interference fit.
  • Cost and Availability: Budget for materials like bronze (durable) vs. plastic (lightweight).

Step-by-Step Selection Process

  1. Define Application Requirements:

    • Calculate loads: Use formulas like PV factor (Pressure × Velocity) < 1.75 MPa·m/s for safe operation. For example, a 500 N radial load at 1 m/s speed gives PV = 0.5 MPa·m/s.
    • Note speed, temperature, and motion type (continuous rotation vs. oscillation).

  2. Choose Bearing Type:

    • Sleeve Bearings: Cylindrical for radial loads (e.g., piston pins).
    • Flanged Bearings: With a collar for axial retention (e.g., automotive suspensions).
    • Thrust Washers: Flat for pure axial loads (e.g., gearboxes).
    • Self-Lubricating Types: Impregnated with graphite or PTFE for dry operation (e.g., food machinery).

  3. Select Material:

    • Bronze or Brass: High load capacity, good for high temps; e.g., phosphor bronze for marine use.
    • Plastic (e.g., Nylon, PTFE): Corrosion-resistant, low friction; ideal for water or chemicals.
    • Composite or Metal-Polymer: For extreme conditions; e.g., DU bearings (steel-backed PTFE) for automotive.
    • Match to environment: Stainless steel for corrosion, ceramics for high wear.

  4. Determine Dimensions and Tolerances:

    • Use shaft diameter to size ID (e.g., for a 20 mm shaft, ID = 20.02-20.05 mm for press fit).
    • Ensure OD and length fit housing; check clearance (0.01-0.05 mm radial) to allow lubrication flow.

  5. Evaluate Lubrication and Maintenance:

    • For lubricated types, specify oil/grease type (e.g., mineral oil for general use).
    • Opt for self-lubricating if maintenance is an issue; they last 10,000-50,000 hours.

  6. Check Standards and Testing:

    • Refer to ISO 3547 for dimensions or ASTM for materials.
    • Test prototypes if possible; calculate life using L = (PV limit / actual PV) × time factor.

  7. Consider Alternatives and Cost:

    • If high speed or precision is needed, switch to rolling bearings.
    • Compare suppliers (e.g., SKF, Igus) for quality vs. price; expect 50 per unit.

Common Mistakes to Avoid

  • Overloading without proper lubrication leads to galling (seizing).
  • Ignoring misalignment; plain bearings tolerate some but not much.
  • Using in high-speed apps (>2 m/s) without cooling, causing thermal runaway.

Tips and Resources

  • Use online calculators from bearing manufacturers for quick sizing.
  • For custom needs, consult engineers; life expectancy can be 5-20 years with care.
  • Resources: SKF catalog, Igus design guides, or ISO standards.

If you provide details like load, speed, or application, I can help refine a selection!



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