Improper Fits: The Primary Cause of Failure

Field data indicates that a large percentage of premature bearing failures stem from incorrect fit selection. Improper fits can lead to creeping, fretting corrosion, and even ring cracking.

• Rotating Ring (Usually Inner Ring): Must use an Interference Fit (Tight Fit). If the inner ring slips on the shaft, the resulting friction heat will rapidly damage the shaft journal.

• Stationary Ring (Usually Outer Ring): Typically uses a Transition or Clearance Fit (Loose Fit) to facilitate heat dissipation and accommodate thermal expansion of the shaft.

• Application Scenario	Shaft Tolerance	Housing Bore Tolerance	Suitable Conditions
  General Standard	k5, k6	H7, G7	General motors, gearboxes
  Heavy Load / Impact	m5, m6, n6	G7, H6	Crushers, steel mills, high-load equipment
  Precision / Quiet	js6, h5	H6, JS7	Machine tool spindles, precision instruments

  Calculation of Operating Clearance

• This is often the most overlooked aspect on-site. Many engineers simply apply the "initial clearance" from the catalog, ignoring the significant reduction in clearance caused by fits and temperature differences.
• The logic for calculating operating clearance is as follows:
• Δworking=Δinitial−δfit−δtempΔworking​=Δinitial​−δfit​−δtemp​

• Clearance Reduction due to Fit ( δfitδfit​ ): An interference fit expands the inner ring, typically consuming 70% to 90% of the initial clearance.

• Clearance Reduction due to Temperature ( δtempδtemp​ ): The inner ring is usually hotter than the outer ring ( ΔT≈5−10∘CΔT≈5−10∘C ), and thermal expansion further reduces internal clearance.

• Practical Selection Advice:

• High Speed or High Temp: Clearance larger than CN (Normal) must be selected (e.g., C3, C4).

• Case Studies:

  • General Motors: C3 Clearance is recommended (due to tight fits and higher operating temperatures).

  • Vibrating Screens: C4 or C5 Clearance is mandatory (due to heat generated by vibration and heavy loads).

  • General Gearboxes: CN Clearance is usually sufficient.

Grease Selection Strategy

• Grease selection also depends on the application:

• General Applications: NLGI No. 2 Lithium Soap Grease is recommended (good versatility and pumpability).

• High-Temp Motors: Complex Lithium Soap or Polyurea Grease should be selected (better high-temperature stability).

• Extreme Low Temp: Synthetic oil-based low-temperature greases are required.