Aux 1 Unit 11 Q9

Intermediate shaft bearings of the roller type, particularly spherical roller bearings, are specifically designed to accommodate some degree of angular misalignment of the shaft. This capability is crucial in marine propulsion systems where slight misalignments can occur due to factors such as:

• Vessel flexing: The ship’s structure can flex and bend under various loads and sea conditions, causing the shaft’s alignment to deviate slightly from its ideal position.
• Thermal expansion: Temperature changes can cause components in the propulsion system to expand or contract at different rates, potentially leading to misalignment.
• Bearing wear: Over time, wear and tear on bearings and other components can result in slight changes in alignment.

How Spherical Roller Bearings Accommodate Angular Misalignment:

• Spherical Outer Race: The key feature of a spherical roller bearing is its spherical (curved) outer race. This allows the inner ring and roller assembly to tilt or swivel within the outer race, accommodating angular misalignment between the shaft and the bearing housing.
• Self-Aligning Capability: This self-aligning feature ensures that even if the shaft is not perfectly aligned with the bearing housing, the rollers can still maintain proper contact with the raceways, preventing excessive stress and wear.
• Load Distribution: The spherical design also helps distribute the load evenly across the rollers, even in the presence of misalignment, further reducing the risk of localized stress and premature wear.

Limitations and Considerations:

• Misalignment Limits: While spherical roller bearings can accommodate angular misalignment, they have specific limits. Exceeding these limits can lead to increased friction, wear, and reduced bearing life.
• Axial Movement: Spherical roller bearings primarily accommodate angular misalignment. They might have limited ability to handle axial movement (along the shaft’s length). Thrust bearings or other mechanisms are typically used to control axial movement in the shafting system.
• Lubrication: Proper lubrication is crucial for the effective operation and longevity of spherical roller bearings, even with their self-aligning capability.

Benefits:

• Reduced Maintenance: The self-aligning capability of spherical roller bearings reduces the need for frequent alignment adjustments, leading to less downtime and maintenance costs.
• Improved Reliability: By accommodating misalignment, they minimize the risk of bearing failure due to excessive stress or uneven loading.
• Enhanced Efficiency: Reduced friction and wear due to proper alignment contribute to improved efficiency in the propulsion system.

In summary, spherical roller bearings, with their self-aligning capability due to the spherical outer race, effectively handle angular misalignment in propulsion shaft intermediate bearings. This feature is vital in marine applications where shaft alignment can change due to various factors.

1. Bearing Design and Internal Clearances

• Roller Bearings: Roller bearings, especially cylindrical roller bearings, inherently allow for some degree of axial movement due to the clearance between the rollers and the inner and outer races. This clearance is designed to accommodate thermal expansion and contraction of the shaft, as well as minor axial displacements due to operational loads.
• Needle Roller Bearings: Similarly, needle roller bearings, with their numerous small-diameter rollers, also provide some allowance for axial movement within the bearing.

2. Thrust Bearings:

• Dedicated Thrust Handling: While roller bearings can handle some axial movement, the primary responsibility for accommodating significant longitudinal movement and absorbing axial thrust from the propeller lies with dedicated thrust bearings.
• Location: Thrust bearings are typically located at one end of the shafting system, often near the gearbox or engine, to handle the thrust forces effectively.
• Design: Thrust bearings come in various designs, such as:
  • Tilting Pad Thrust Bearings: Use multiple pads that pivot to accommodate misalignment and maintain an optimal oil film.
  • Michell Thrust Bearings: Employ a similar principle with tilting pads but have a more compact design.
  • Tapered Land Thrust Bearings: Rely on tapered surfaces to generate a hydrodynamic oil film for thrust load support.

3. Flexible Couplings:

• Axial Flexibility: Flexible couplings, installed between shaft sections, can also contribute to accommodating longitudinal movement. Some types of flexible couplings, like disc couplings or gear couplings, have a certain degree of axial flexibility built into their design.

Importance of Longitudinal Movement Accommodation:

• Thermal Expansion: As the shaft heats up during operation, it expands longitudinally. The bearings and couplings need to allow for this expansion to prevent excessive axial forces that could damage the system.
• Shaft Deflection: Operational loads, such as thrust from the propeller, can cause the shaft to deflect slightly. The bearings and couplings need to accommodate this deflection without binding or causing excessive friction.
• Maintenance and Assembly: Longitudinal movement is also necessary for ease of assembly and disassembly of the shafting system during maintenance or repairs.

In summary:

Intermediate shaft bearings of the roller type, in conjunction with dedicated thrust bearings and flexible couplings, can effectively accommodate the longitudinal movement of the propulsion shaft. This flexibility is crucial to handle thermal expansion, shaft deflection, and facilitate maintenance, ultimately ensuring the reliable and efficient operation of the propulsion system.