Adequate lubrication is required for the rotation of the propeller shaft inside the stern tube. For this reason, various types of lubrication systems are used. This article speaks about most commonly arrangement of stern tube bearing.
Stern tube is located at the aft end of the ship and is that part which connects the engine inside of the ship to the propeller outside of the ship. The propeller shaft from the engine passes through the stern tube and is connected with the propeller. A critical part of the ship, the stern tube can be a soft area for the sea water to seep inside the ship. In order to prevent the water ingress stern tube bearings are used inside the stern tube. We will learn about stern bearing housing and its arrangement in this article
Thus, the two main purposes of a stern bearing are:
- To prevent the entry of sea water into the machinery spaces
- To support the tail shaft and a considerable proportion of propeller weight.
Stern Tube Bearings
The stern tube gland should be such that it allows a free movement of the propeller shaft without causing any harm. In the ancient days, lignum vitae - a dense form of timber, was used as bearing material. The timber was continuously lubricated by sea water. However, with the advent of higher tonnage steel ships, the size and weight of propellers increased and timber bearings became outdated.In most of the modern ships, an oil lubrication system with a white metal lined stern tube bearing is used.
Construction and Working of Stern Tube Bearing
The arrangement of a stern tube with oil lubrication system has been shown in the figure. The bush of the white metal bearing has grooves made into it. Oil is pumped in though this external axial groves into the bush of the bearing. The oil then passes through the internal axial passages through the holes made on each side. A return line, at the end of the bush facilitates the flow of the oil back to the pump. A pump attached with a cooler, continuously pumps oil into the bearings to maintain the required pressure. In case the pump fails,of the two header tanks under gravitational pressure, provide the required oil with a back pressure to the bearing system. Thus, it is imperative that the level of hydraulic oil is well maintained in each header tank. Moreover, both the tanks should also be fitted with low level alarms.
The pressure in the system should be maintained at a level higher than that of the static sea water head to prevent the water from seeping inside in-case of the seal failure. A low pressure alarm is also provided to the system to prevent any kind of accident.