Describe stern tube design with oil lubricated bearings and it's features.
Oil lubricated stern tube requires a pipeline to supply oil and gravity tank to keep oil hydrostatic pressure inside higher than outboard water hydrostatic pressure. Two seals: FWD and AFT retain oil inside the stern tube and prevent water ingress (if higher hydrostatic oil pressure maintained). A liner is fitted in way of the aft seal bolted on to the propeller boss. It excludes sea water contact with the main shaft and provides an easily replaceable rubbing surface for the seal. Lined with white metal are to have a bearing length so as not to exceed a bearing pressure from the weight of the shaft and propeller of 5 kg/cm2. The limitations of a bearing are the load it can withstand without metal cracking or squeezing out and the temperature it can withstand without melting. Length of bearing not less than 2 Diameter . Cast iron and bronze bearings must have a bearing length not less than 4 Diameters. Lubrication system must be capable of maintaining oil tightness despite varying temperature. Gravity tanks fitted with low level alarms. Aft peak to be filled with water to provide sufficient cooling.
Top half of white metal bearing is usually machined to give a left hand and right hand helix, this gives a small pumping pressure forward to aft to provide lubrication and to assist in maintaining oil tightness of the oil seals. When large propellers are fitted the heavy overhanging weight greatly increases the load at the after end of the stern tube breaking down the hydrostatic lubrication causing metal to metal contact and seizure towards the aft end of bearing. To obviate this it is usual to angle the shaft downwards for about 8mm over 100m length thus attempting to ensure than the weight of the bearing is taken on the full length of the bearing. It is good practice to leave the oil tank valve open to the stern tube when in port with machinery stopped, this prevents sea water leaking into the system. However, water has been known to contaminate lubricating oil systems causing rusting of tail shaft particularly when shaft is stopped for periods long enough for water to settle in bottom of bearing. In ships with large changes in draught it is usual to fit two gravity tanks. The upper tank is used when fully loaded or there is water leaking in.
Periodically oil sample to be collected from sample cock and analysed for water and metal particles content.
