Aft Seal Arrangement for Oil-Lubricated Stern Tube Bearing
The aft seal in an oil-lubricated stern tube bearing plays a critical role in preventing seawater ingress and oil leakage. Here’s a breakdown of a typical arrangement:
Components:
- Housing: A robust housing is securely attached to the stern tube bossing of the vessel. This housing provides a secure enclosure for the sealing elements.
- Liner: This is a wear ring made from a low-friction material (often white metal or a suitable plastic) that is fixed onto the propeller hub. The sealing elements make contact with the liner to create a tight seal.
- Primary Seal: This is the first line of defense against seawater ingress. It can be one of several designs, such as:
- Lip seal: A spring-loaded lip makes contact with the liner to create a sealing effect.
- Mechanical seal: This utilizes rotating faces and a lubricating film to achieve a tight seal.
- Secondary Seal: This provides additional protection against seawater ingress. It can be similar in design to the primary seal or might utilize a different sealing mechanism like a labyrinth seal with a series of grooves and clearances to impede water ingress.
- Oil Buffer Chamber: This is a chamber within the aft seal housing located between the primary and secondary seals. The chamber is filled with oil at a pressure slightly higher than the seawater pressure outside. This creates a pressure barrier that helps prevent seawater from entering the stern tube.
- Drain Line: A drain line allows any leakage from the primary or secondary seals to be collected and returned to the oil lubrication system. This prevents oil accumulation within the aft seal housing.
- Ventilation: In some designs, a ventilation system might be incorporated to remove any moisture or vapors that could accumulate within the aft seal housing.
Operation:
As the propeller shaft rotates, the liner rotates with it. The primary and secondary seals make contact with the liner, creating a barrier against seawater. The oil buffer chamber maintains a slight positive pressure to further prevent seawater intrusion. Any leakage past the primary seal is collected and drained back to the oil system.
Importance of Maintenance:
Regular inspection and maintenance of the aft seal arrangement are crucial. Monitoring oil pressure in the buffer chamber, checking for leaks, and inspecting the condition of the liner and seals are essential to ensure continued functionality and prevent potential seawater contamination of the lubricating oil or oil leakage into the environment.