Aux 1 Unit 11 Q12

The aft seal, also known as the outboard seal, of an oil-lubricated stern tube bearing is a critical component located at the aft (rear) end of the stern tube, where the propeller shaft exits the vessel’s hull. Its primary function is to create a barrier between the seawater outside and the lubricating oil inside the stern tube, preventing:  

1. Sterntube seals, propeller shaft seals – Wärtsilä

1. Seawater Ingress: The aft seal stops seawater from entering the stern tube. Seawater contamination would degrade the lubricating oil, leading to increased friction and wear on the bearings, potentially causing damage to the propeller shaft and stern tube.   1. Stern Tube Seals – Lagersmit www.lagersmit.com
2. Oil Leakage: It also prevents the lubricating oil from leaking out of the stern tube into the marine environment, minimizing pollution and complying with environmental regulations.   1. Stern Tube Seals – Lagersmit www.lagersmit.com

Construction and Types:

Aft seals come in various designs, but the most common types for oil-lubricated stern tubes include:

• Lip Seals:
  • Simple and cost-effective, using flexible lip elements that create a sealing contact with the rotating shaft.   1. Exploring the Applications and Benefits of Rotary Shaft Lip Seals in Various Industries advanced-emc.com
  • Typically used in combination with multiple lips to provide a more effective seal.   1. Stern Tube Seals – Lagersmit www.lagersmit.com
  • Can be susceptible to wear and tear, requiring periodic replacement.
• Face Seals (Mechanical Seals):
  • More complex but offer superior sealing performance and longer service life.
  • Consist of rotating and stationary sealing rings that create a contact seal under pressure.   1. End-face mechanical seal – Wikipedia en.wikipedia.org
  • Require careful installation and alignment.
• Labyrinth Seals:
  • Utilize a series of grooves and clearances to create a tortuous path for fluid, making it difficult for seawater or oil to pass through.
  • Often used in combination with other seal types for added protection.

Importance:

• Bearing Protection: The aft seal protects the stern tube bearings from seawater contamination, ensuring proper lubrication and preventing premature wear or damage.   1. Stern Tube Seals – Lagersmit www.lagersmit.com
• Environmental Protection: It prevents oil leakage into the sea, minimizing pollution and complying with environmental regulations.   1. Stern Tube Seals – Lagersmit www.lagersmit.com
• Operational Reliability: A well-functioning aft seal contributes to the overall reliability of the propulsion system by maintaining the integrity of the stern tube bearings and preventing water ingress.

Maintenance:

• Regular Inspections: The aft seal should be inspected regularly for signs of wear, leakage, or damage.
• Monitoring Oil Levels: The oil level in the stern tube should be monitored to ensure adequate lubrication and detect any potential leaks.
• Seal Replacement: Seals should be replaced according to manufacturer recommendations or when signs of wear or leakage are observed.

In conclusion, the aft seal of an oil-lubricated stern tube bearing is a critical component that prevents seawater ingress and oil leakage, protecting the bearings and the environment while ensuring the reliable operation of the vessel’s propulsion system.

Main Components to Include:

1. Stern Tube:

• Tube Wall: The cylindrical structure of the stern tube, typically made of steel or cast iron.
• Shaft: The propeller shaft passing through the stern tube.
• Liner (Bearing Surface): A low-friction material (often white metal or synthetic) fitted onto the shaft within the stern tube, providing the bearing surface.

2. Aft Seal Housing:

• Housing: A robust housing attached to the aft end of the stern tube, providing a secure enclosure for the sealing elements.
• Oil Buffer Chamber: A chamber within the housing, located between the primary and secondary seals, filled with lubricating oil at a slightly higher pressure than the seawater outside. This creates a pressure barrier to help prevent seawater ingress.

3. Sealing Elements:

• Primary Seal: The first line of defense against seawater ingress. Two main types are common:
  • Lip Seal: A spring-loaded lip made of elastomer or similar material that makes contact with the rotating liner to create a sealing effect.
  • Mechanical Seal: Utilizes rotating faces and a lubricating film to achieve a tighter seal, often preferred in larger vessels or demanding applications.
• Secondary Seal: Provides additional protection against seawater ingress. Can be of the same type as the primary seal or use a different mechanism, like a labyrinth seal with a series of grooves and clearances to impede water ingress.

4. Other Components:

• Drain Line: A pipe or tube connected to the aft seal housing to collect any leakage from the seals and return it to the oil lubrication system or a designated collection tank.
• Vent (Optional): Some designs may incorporate a vent to release any trapped air or vapors from the seal housing.

Additional Considerations for your Sketch:

• Clear Labels: Label each component clearly for easy identification
• Material Indication: If possible, indicate the materials used for key components (e.g., bronze housing, stainless steel shaft, white metal liner).
• Arrows: Use arrows to show the direction of shaft rotation and the potential paths of seawater ingress and oil leakage.
• Pressure Differences: You could use shading or annotations to indicate the higher oil pressure in the buffer chamber compared to the seawater outside.

Remember:

• Simplicity: While it’s important to include the key components, keep the sketch relatively simple and focused on the essential features of the aft seal.
• Reference: Refer to any available diagrams or technical drawings of the specific aft seal design for accuracy.

By incorporating these elements into your sketch, you’ll be able to effectively illustrate the layout and key components of an aft seal for an oil-lubricated stern tube bearing.