For a bearing to function correctly on a shaft, its inner ring must be securely mounted to the shaft, typically through an interference fit. This tight fit ensures the bearing rotates synchronously with the shaft, preventing slippage. The outer ring is usually fixed within a housing or support structure. This precise assembly guarantees that the bearing can effectively handle loads and transmit rotational motion without issue. The primary reasons for a bearing rotating on a shaft, along with their technical implications, are detailed below:

1. Insufficient Interference Fit (Loose Tolerance): The diametrical clearance between the bearing’s inner ring and the shaft is critical for preventing slippage. If the shaft diameter is too small relative to the bearing’s inner diameter, or if the bearing’s inner diameter is too large for the shaft, an adequate interference fit cannot be achieved. This can result from manufacturing tolerance deviations in the shaft or bearing, wear, or improper mounting techniques (e.g., insufficient heating during press-fitting). International standards (ISO, ABMA) define specific shaft and housing tolerances (e.g., h6, g6, js6, k6, m6, n6). While looser fits (like g6 or h6) might be used for light loads or ease of assembly/disassembly, tighter fits (k6, m6, n6) are generally recommended to prevent bearing rotation on the shaft.
2. Improper Mounting Techniques: Bearing installation requires precision. Applying excessive force when mounting a bearing onto a shaft can damage the inner ring, deforming it and reducing its effective diameter, thus compromising the interference fit. Incorrect use of hydraulic or heating methods can also lead to inadequate fits. For instance, when using induction heaters, the bearing must be heated to the correct temperature (typically 80-120°C) to allow it to slide easily onto the shaft, after which it contracts upon cooling, creating the necessary tight fit.
3. Wear on Shaft or Bearing Housing: Prolonged operation, inadequate lubrication, or excessive loads can cause wear on the shaft’s bearing journal surface, reducing its diameter and eliminating the interference fit. The bearing’s inner ring itself can also wear or deform. This condition can lead to creep, a phenomenon where the bearing’s inner ring slowly slips on the shaft, eventually leading to full rotation. Signs of fretting corrosion may be visible on the worn surfaces, indicating micro-movement under vibration.
4. Excessive Vibration and Dynamic Loads: High vibration levels or sudden shock loads can momentarily reduce the friction between the bearing and the shaft, causing the bearing to slip. Imbalanced rotating components, misaligned shafts, or structural resonance can generate vibrations that initiate bearing creep. This occurs when the static friction coefficient is overcome, leading to slippage.
5. Incorrect Bearing Selection: Choosing a bearing that is not suited for the application’s load, speed, and temperature requirements increases the risk of premature failure. For example, a bearing with insufficient radial or axial load capacity may deform, leading to loosening on the shaft. Thermal expansion differences between the shaft and bearing materials must also be considered in high-temperature applications.
6. Inadequate Lubrication: Lubrication reduces friction between rolling elements but also influences the friction at the shaft-bearing interface. Insufficient lubrication can increase friction, leading to overheating and wear. Conversely, over-lubrication or the use of incorrect lubricant types can sometimes contribute to slippage.