Section 4.2: Bearings & Lubrication

Section 4.2: Bearings & Lubrication

Bearings are precision-engineered mechanical components that support rotating shafts, reduce friction between moving parts, and handle structural loads. In United States Postal Service (USPS) mail handling equipment, bearings are found in electric motors, conveyor rollers, drive pulleys, and sorting hubs. Understanding bearing types, loading conditions, lubrication requirements, and failure modes is essential for preventive maintenance and diagnostic troubleshooting.

Classification of Bearings

Bearings are broadly categorized into two families: anti-friction bearings (rolling-element bearings) and plain bearings (sliding-contact, sleeve, or journal bearings).

Rolling-Element (Anti-Friction) Bearings

These bearings use balls or rollers placed between an inner and outer ring (raceways) to convert sliding friction into rolling friction.

• Ball Bearings: They feature spherical rolling elements that make point contact with the raceways. This minimal contact area results in very low friction, making ball bearings ideal for high-speed, low-to-medium load applications. The most common type is the deep-groove ball bearing, which primarily supports radial loads (forces perpendicular to the shaft) but can handle light axial loads. Angular contact ball bearings have offset raceways designed to handle significant axial loads (forces parallel to the shaft, also called thrust loads) in addition to radial loads.
• Roller Bearings: Instead of spheres, these use cylindrical, tapered, or barrel-shaped rollers. Because they make line contact rather than point contact with the raceways, they distribute loads over a larger area, giving them a much higher radial load capacity.
  • Cylindrical Roller Bearings: Designed strictly for high radial loads, offering virtually no axial load resistance. They allow axial expansion of the shaft.
  • Tapered Roller Bearings: Feature conical rollers and matching tapered raceways. They are specifically designed to support heavy combined radial and axial loads. They are commonly mounted in opposing pairs on a shaft to handle thrust loads from both directions.
  • Spherical Roller Bearings: Utilize barrel-shaped rollers in a spherical outer ring. They are uniquely self-aligning, meaning they can tolerate minor shaft deflection or housing misalignment (typically up to 2 degrees) without increased friction or wear.
  • Needle Roller Bearings: Use thin cylindrical rollers where the length is much greater than the diameter. They provide high radial load capacity in an extremely compact radial space, making them perfect for universal joints and tight gearbox housings.

Plain (Sleeve or Journal) Bearings

Plain bearings do not have rolling elements. Instead, the shaft (journal) slides directly over a sleeve, bushing, or liner. They rely on a thin film of lubricant to prevent metal-to-metal contact. Plain bearings are quiet, inexpensive, and have high load-carrying capacities. They are often made of soft metals like bronze, brass, or Babbitt metal (a lead-tin alloy), or synthetic materials like PTFE (polytetrafluoroethylene). When operating, they transition from boundary lubrication (metal contact during start/stop) to hydrodynamic lubrication, where the rotating shaft draws oil underneath itself, creating a high-pressure fluid wedge that floats the shaft off the bearing surface.

Lubrication Principles

Lubricants separate moving parts, dissipate heat, prevent corrosion, and flush away wear debris.

• Viscosity: The most important property of any lubricant is its viscosity—its resistance to flow. Thick, high-viscosity oils are used for heavy loads, slow speeds, and high temperatures, as they form a thicker protective film. Thin, low-viscosity oils are used for light loads, high speeds, and cold temperatures. Viscosity is often measured using the ISO Viscosity Grade (ISO VG) or Saybolt Universal Seconds (SUS).
• Grease vs. Oil: Grease is a semi-solid lubricant composed of a base oil suspended in a thickener (often a lithium, calcium, or polyurea soap) along with performance additives. The thickener acts like a sponge, releasing oil as the bearing rotates. Grease is preferred for sealed-for-life bearings and vertically mounted shafts because it stays in place and seals out contaminants. Oil is a liquid lubricant used where heat dissipation is critical, or where the lubricant must flow through gearboxes, reservoirs, or oil mist systems.
• Over-lubrication: A common maintenance error is over-greasing bearings. Excess grease causes churning, where the rolling elements must fight through too much material. This generates extreme friction and heat, causing the grease to degrade and leading to premature bearing failure.

Bearing Failure Indicators and Diagnostics

Bearings are designed to fail eventually due to fatigue, but premature failure is usually caused by contamination, improper lubrication, misalignment, or electrical damage.

• Spalling (Flaking): The progressive chipping or flaking of the metal surface on the raceways or rolling elements. It begins as microscopic subsurface fatigue cracks and progresses to visible pits.
• Brinelling:
  • True Brinelling: Severe indentations in the raceways caused by a sudden impact or static overload (e.g., hitting a shaft with a hammer during assembly).
  • False Brinelling: Wear marks that resemble brinelling, caused by microscopic vibrations while the shaft is stationary (non-rotating). The vibration squeezes out the lubricant, causing localized fretting wear between the rollers and raceways.
• Galling (Scuffing): Severe adhesive wear caused by localized welding and tearing of metal particles when the lubrication film fails, leading to direct metal-to-metal contact.
• Fluting: A series of parallel grooves or washboard-like ridges etched across the raceways. This is caused by electrical arcing, where stray electrical currents (common in motors driven by Variable Frequency Drives, or VFDs) discharge through the bearing, melting the metal at the contact points.
• Diagnostics:
  • Vibration Analysis: The primary tool for bearing health monitoring. Accelerometers detect high-frequency stress waves generated when a rolling element passes over a defect (like a spall).
  • Thermography: Infrared cameras spot hot bearings indicating friction or lack of lubrication.
  • Oil Analysis (Ferrography): Analyzing wear particles in oil samples to identify wear metals (e.g., copper, iron) before catastrophic failure occurs.

Bearing Characteristics Comparison Table

graph TD
    A[Bearing Loads] --> B(Radial Load)
    A --> C(Axial / Thrust Load)
    A --> D(Combined Load)
    B -->|Perpendicular to Shaft| E[Cylindrical Roller / Deep-Groove Ball]
    C -->|Parallel to Shaft| F[Thrust Ball / Angular Contact]
    D -->|Both Directions| G[Tapered Roller / Spherical Roller]