Aux 1 Unit 11 Q8

Propulsion shaft intermediate bearings of the plain bearing type are designed with certain features to accommodate some degree of shaft misalignment that can arise from various vessel conditions. Here’s how they achieve this:

1. Bearing Clearance:

• Built-in Clearance: Plain bearings are designed with a small intentional clearance (gap) between the shaft journal (the part of the shaft supported by the bearing) and the bearing surface. This clearance allows for slight misalignment without causing excessive friction or binding.
• Oil Film: During operation, a hydrodynamic oil film is formed between the rotating shaft and the bearing surface. This oil film further helps to accommodate minor misalignment and ensures smooth rotation.

2. Bearing Material and Design:

• Soft, Conformable Material: The bearing shells or bushings are typically made of a soft, malleable material like white metal (babbitt). This allows the bearing surface to conform slightly to the shaft’s position, accommodating minor misalignments.
• Split Bearing Design: Many plain bearings are split into two halves. This design allows for some adjustment during installation to compensate for initial misalignment and facilitates easier maintenance and replacement.

3. Flexible Couplings:

• Strategic Placement: Flexible couplings are strategically placed in the shafting system, often adjacent to the plain bearings. These couplings can accommodate a certain degree of angular and axial misalignment, reducing the stress transmitted to the bearings.
• Various Types: Several types of flexible couplings exist, such as rubber element couplings, gear couplings, or diaphragm couplings, each with its own misalignment capabilities.

4. Limited Self-Adjustment:

• Oil Film Dynamics: Under ideal operating conditions, the hydrodynamic oil film can help to self-center the shaft within the bearing, to some extent, compensating for minor misalignment.
• Bearing Wear: However, it’s important to note that excessive or prolonged misalignment can lead to uneven wear on the bearing surface, which might compromise its ability to self-adjust and lead to further problems.

Limitations and Considerations:

• Misalignment Tolerance: While plain bearings can handle some misalignment, they have a limited tolerance compared to other bearing types, such as tilting pad bearings.
• Regular Alignment Checks: It’s essential to regularly check and adjust the shaft alignment, especially after significant changes in the vessel’s loading condition, maintenance work on the propulsion system, or if there are any signs of bearing distress.

In essence, the combination of bearing clearance, soft bearing material, split bearing design, and the use of flexible couplings allows plain bearings to accommodate some degree of shaft misalignment that may occur due to changes in vessel condition, such as loading, thermal expansion, or minor structural deformations. However, it’s crucial to remember that excessive or prolonged misalignment can lead to bearing problems, so regular alignment checks and maintenance are vital.

In a propulsion system with plain intermediate shaft bearings, the shaft needs to be able to move longitudinally (axially) due to a couple of key reasons:

1. Thermal Expansion and Contraction

• Temperature Variations: The propulsion shaft, especially in larger vessels, experiences significant temperature changes during operation. This is due to factors like:
  • Friction: Friction within the bearings themselves generates heat.   1. Thermal equilibrium – Rolling bearings – SKF www.skf.com
  • Engine/Gearbox Heat: Heat from the engine or gearbox can be conducted along the shaft.
  • Ambient Temperature: Changes in the surrounding environment, especially when moving between different water temperatures, can affect the shaft’s temperature.
• Expansion and Contraction: As the shaft’s temperature changes, it undergoes thermal expansion (when heated) and contraction (when cooled). This can lead to noticeable changes in its overall length.
• Preventing Binding: If the shaft is rigidly fixed at both ends and cannot move axially, thermal expansion or contraction can create significant axial forces and stresses. These forces could cause the shaft to bind within the bearings, leading to increased friction, overheating, and potential damage.

2. Shaft Deflection and Misalignment

• Operational Loads: The propulsion shaft experiences various loads and forces during operation, such as:
  • Thrust: The force generated by the propeller pushing the vessel forward.
  • Torque: The twisting force transmitted from the engine to the propeller.
  • Bending Moments: Caused by the vessel’s motion in waves or during maneuvering.
• Shaft Deflection: These loads can cause the shaft to deflect or bend slightly along its length. The amount of deflection depends on the shaft’s material, diameter, length, and the magnitude of the applied loads.
• Misalignment Compensation: Even with careful alignment during installation, minor misalignments can occur over time due to wear, vibrations, or changes in the vessel’s structure. The ability of the shaft to move longitudinally allows for some compensation for these misalignments, preventing excessive loads on the bearings and ensuring smooth operation.

How Longitudinal Movement is Facilitated

• Bearing Design: Plain bearings themselves have a small clearance between the shaft journal and the bearing surface, allowing for some axial movement.
• Thrust Bearings: Dedicated thrust bearings are installed at strategic points in the shafting system to handle the axial thrust from the propeller and control the shaft’s longitudinal position.
• Flexible Couplings: Flexible couplings are often used to connect different shaft sections. These couplings can accommodate some degree of axial movement, further allowing the shaft to expand or contract and adjust to minor misalignments.

In summary:

The ability of the propulsion shaft to move longitudinally is crucial for accommodating thermal expansion, shaft deflection, and minor misalignments. This flexibility prevents excessive stresses, minimizes wear and tear on the bearings, and ensures the smooth and reliable operation of the propulsion system, even under varying operating conditions and potential structural changes in the vessel.

Let’s explore the rationale behind the design difference between the aftmost and other intermediate shaft bearings in a propulsion system using plain bearings.

Aftmost Bearing:

• Highest Load: This bearing, being closest to the propeller, experiences the highest loads in the system. It bears a significant portion of the propeller shaft’s weight and also absorbs a degree of thrust load, especially during maneuvering or in rough seas.
• Complete Bush for 360-degree Support: To handle these combined radial and thrust loads effectively, a complete bush (a full cylindrical sleeve lining the bearing housing) is necessary. This provides full circumferential support to the shaft, ensuring stability and adequate load-carrying capacity under all operating conditions.

Other Intermediate Bearings:

• Lesser Load: Intermediate bearings situated further forward along the shaft experience lower loads compared to the aftmost bearing. Their primary role is to guide and support the shaft, with less emphasis on carrying significant weight or thrust.
• Lower Half Bush for Normal Operation: Under typical operating conditions, the majority of the load on these bearings is directed downwards due to gravity. Thus, a bush in the lower half of the bearing is often sufficient to provide adequate support and lubrication.
• Upper Half Clearance: The upper half of the bearing may have a larger clearance or even no bush at all. This design allows for some degree of shaft deflection and thermal expansion without causing excessive friction or binding.
• Oil Film Support: The hydrodynamic oil film generated during operation helps to further support the shaft in the upper half, even with a larger clearance.

Benefits of this Design Approach:

• Cost-Effectiveness: Using a partial bush in some bearings reduces material and manufacturing costs compared to full bushes in all bearings.
• Reduced Friction: The larger clearance in the upper half of some bearings can lead to lower friction and heat generation under normal operating conditions, improving efficiency.
• Accommodation of Misalignment: The combination of bearing clearance and the use of flexible couplings allows for some degree of shaft misalignment, enhancing the system’s adaptability to varying loads and operational conditions.

Key Considerations:

• Bearing Load Analysis: The specific bearing design (full or partial bush) is determined through careful analysis of the expected loads and operating conditions at each bearing location.
• Maintenance and Inspection: Regular inspections and maintenance are critical to ensure the bearings remain in good condition and the clearances are within acceptable tolerances. Excessive wear or persistent misalignment can lead to increased friction, overheating, and potential bearing failure.

In essence, the use of a complete bush in the aftmost bearing and partial bushes in other intermediate bearings reflects a balance between load-carrying requirements, alignment considerations, and cost-effectiveness. It optimizes the design for efficient and reliable operation of the propulsion shaft system while minimizing unnecessary complexity and expense.