Aux 1 Unit 12 Q4

The transmission of propeller thrust to a vessel’s hull involves a series of interconnected components designed to efficiently transfer the force generated by the propeller to the ship’s structure, ultimately propelling the vessel forward. Here’s the breakdown of this process:  

1. Ship Propulsion System Lecture 4

1. Propeller:

• Thrust Generation: The rotating propeller blades generate thrust by pushing water backward, creating an equal and opposite reaction force that propels the vessel forward.   1. F.1513 Propellers – Book : The Contact Patch the-contact-patch.com
• Shaft Connection: The thrust is transmitted to the propeller shaft, which is connected to the propeller hub.

2. Propeller Shaft:

• Torque and Thrust Transmission: The propeller shaft not only transmits the rotational torque from the engine to the propeller, but it also carries the axial thrust force generated by the propeller.

3. Stern Tube Bearings:

• Shaft Support and Guidance: The propeller shaft passes through the stern tube, which houses bearings that support and guide the shaft as it rotates.   1. WHAT IS A STERN TUBE? – Lagersmit www.lagersmit.com
• Aft Seal: The aft end of the stern tube has a seal to prevent water ingress and oil leakage.   1. Sealing up the stern: water kept out, oil kept in – Riviera Maritime Media www.rivieramm.com

4. Thrust Bearing:

• Thrust Absorption: The thrust bearing is the critical component that absorbs the axial thrust force from the propeller shaft.   1. Thrust bearing – Wärtsilä www.wartsila.com
• Location: It’s usually located near the engine or gearbox, within the thrust block or integrated into the engine itself.
• Design: Thrust bearings come in various designs, such as tilting pad bearings or Michell bearings, which use oil films or rolling elements to support the thrust load.

5. Thrust Block:

• Force Transmission: The thrust bearing is mounted within the thrust block, a sturdy structure bolted to the ship’s structure.   1. Thrust block – Wärtsilä www.wartsila.com
• Foundation: The thrust block provides a solid foundation for the thrust bearing to transfer the thrust force to the hull.

6. Ship’s Structure:

• Force Distribution: The thrust force is ultimately transmitted through the thrust block and its foundation into the ship’s structure, specifically the hull and internal framing.   1. Ship Engines: Here is How They Function – Sea News seanews.co.uk
• Propulsion: This force acts against the resistance of the water, propelling the vessel forward.

Key Points:

• Efficient Transmission: The entire system is designed to minimize energy losses and ensure efficient transmission of thrust from the propeller to the hull.
• Alignment: Proper alignment of the shafting system, including the propeller, bearings, and couplings, is crucial for efficient thrust transmission and to prevent vibrations and premature wear.
• Maintenance: Regular inspection and maintenance of the stern tube bearings, thrust bearing, and other components are essential to ensure their proper function and prevent failures that could lead to loss of propulsion or other safety hazards.

In summary, propeller thrust is transmitted to the vessel’s hull through a series of components, including the propeller shaft, stern tube bearings, thrust bearing, and thrust block, which ultimately transfer the force to the ship’s structure and propel it through the water.  

1. Ship Propulsion System Lecture 4

The mounting arrangement of a thrust block to the hull of a vessel is critical to ensure the safe and efficient transfer of propeller thrust forces into the ship’s structure. There are two main types of mounting arrangements for thrust blocks:

1. Independent Thrust Block:

• Separate Structure: An independent thrust block is a self-contained unit, separate from the main engine or gearbox. It’s typically a massive, rigid structure made of cast or fabricated steel.
• Mounting: It is securely bolted or welded to the ship’s structure, usually on a specially reinforced foundation or seating designed to handle the thrust loads.
• Location:
  • Close to the Engine/Gearbox: To minimize the effects of shaft deflection and thermal expansion.
  • Aft of the Engine/Gearbox: The thrust is transmitted forward, pushing against the thrust block and into the ship’s structure.
• Chocking: The thrust block might also be supported by chocks or other structural elements to prevent any movement or vibration.

2. Integrated Thrust Block:

• Built into Engine/Gearbox: In some designs, especially with large slow-speed diesel engines or certain gearboxes, the thrust bearing is integrated directly into the engine or gearbox casing.
• Advantages:
  • Space Saving: Eliminates the need for a separate thrust block, saving space and weight.
  • Reduced Alignment Issues: Minimizes potential misalignment between the engine/gearbox and the thrust bearing.
• Disadvantages:
  • Maintenance Access: Accessing the thrust bearing for inspection or maintenance might require opening the engine or gearbox casing, which can be more complex and time-consuming.
  • Limited Flexibility: The integrated design might offer less flexibility for adjusting or replacing the thrust bearing if needed.

Common Features of Thrust Block Mountings:

• Robust Foundation: The foundation or seating for the thrust block (whether independent or integrated) is typically heavily reinforced to handle the significant thrust loads.
• Multiple Attachment Points: The thrust block is securely bolted or welded to the foundation at multiple points to distribute the load and prevent any movement or vibration.
• Access for Alignment: Provisions are made for adjusting the alignment of the thrust bearing and shafting system during installation and maintenance.
• Insulation (if applicable): In some cases, insulation might be applied around the thrust block to reduce noise and vibration transmission.

Importance of Proper Mounting:

• Efficient Thrust Transfer: A secure and rigid mounting ensures efficient transfer of thrust forces from the propeller to the ship’s structure, preventing damage to the propulsion system and ensuring smooth operation.
• Vibration Control: Proper mounting helps to minimize vibrations generated by the propeller and transmitted through the shafting system, enhancing crew comfort and reducing stress on the vessel’s structure.
• Safety: A secure and well-maintained thrust block mounting is crucial for the safety of the vessel and its crew, preventing potential shaft or bearing failures that could lead to loss of propulsion or other hazards.

In summary, the mounting arrangements of a thrust block to the hull are critical for ensuring the safe, efficient, and reliable operation of the propulsion system. Whether it’s an independent unit or integrated into the engine/gearbox, the thrust block needs to be securely attached to a robust foundation that can handle the significant thrust loads and minimize vibrations, contributing to the overall structural integrity and performance of the vessel.

The clearance between the thrust block pads and the collar on a propulsion shaft is critical for several reasons that directly affect the bearing’s performance, longevity, and the overall efficiency of the propulsion system. Let’s delve into these reasons:

1. Thrust Block :- – AlfiDelfi

1. Formation of Hydrodynamic Oil Film:

• Wedge-Shaped Gap: The clearance creates a wedge-shaped gap between the thrust collar (attached to the rotating shaft) and the stationary thrust pads.   1. Thrust Block :- – AlfiDelfi alfidelfi.com
• Hydrodynamic Lubrication: As the shaft rotates, it drags oil into this wedge-shaped gap. The oil gets pressurized, forming a hydrodynamic oil film that separates the collar from the pads.   1. Thrust Block :- – AlfiDelfi alfidelfi.com
• Load Support: This pressurized oil film is responsible for supporting the axial thrust load from the propeller, preventing metal-to-metal contact and minimizing friction and wear.
• Optimal Clearance: The clearance must be within a specific range to ensure the formation of a stable and effective oil film.

2. Heat Dissipation:

• Friction and Heat: Even with proper lubrication, some friction occurs between the oil film and the bearing surfaces, generating heat.
• Coolant Role of Oil: The oil film also acts as a coolant, carrying away the heat generated by friction.
• Adequate Clearance: Sufficient clearance is necessary to allow for adequate oil flow and heat dissipation, preventing the bearing from overheating.

3. Thermal Expansion:

• Temperature Changes: The shaft and thrust block experience temperature changes during operation due to friction and heat transfer from the engine or gearbox.
• Expansion: These temperature changes cause the components to expand and contract.
• Clearance Accommodation: The clearance between the pads and collar needs to be large enough to accommodate this thermal expansion and prevent the components from binding or seizing.

4. Prevention of Excessive Wear:

• Metal-to-Metal Contact: If the clearance is too small, there’s a risk of metal-to-metal contact between the collar and pads, leading to excessive wear, friction, and potential damage.
• Oil Film Breakdown: Insufficient clearance can also prevent the formation of a proper oil film, increasing friction and wear.

Consequences of Incorrect Clearance:

• Too Small Clearance:
  • High bearing temperatures due to inadequate oil flow and heat dissipation.
  • Increased friction and power losses.
  • Premature wear and potential bearing failure.
  • Risk of seizure or binding.
• Too Large Clearance:
  • Unstable oil film, leading to increased vibration and noise.
  • Reduced load-carrying capacity of the bearing.
  • Potential for oil leakage and contamination.

Maintaining Correct Clearance:

• Regular Inspection and Measurement: The clearance should be checked periodically during maintenance using feeler gauges or other appropriate tools.
• Adjustment: If the clearance is outside the manufacturer’s specified tolerance, adjustments can be made by shimming the thrust pads or machining the collar.

In Conclusion:

The clearance between the thrust block pads and collar is a critical parameter that directly influences the formation of the hydrodynamic oil film, heat dissipation, and overall performance of the thrust bearing. Maintaining the correct clearance is vital for ensuring efficient power transmission, preventing wear and damage, and maximizing the lifespan of the propulsion shaft system.