Water lubricated stern tube arrangement

      Water Lubricated Stern Tube

 

•  With the increase in size of VLCC's shipping companies required a stern tube bearing capable of operating with high degrees of water contamination

•       The alloys in white metal tend to oxidize and the clearance is removed leading to seizure.

•  In addition as shaft revs reduced in search of improved propeller efficiency the hydrodynamic forces available become limited for oil film generation.

•        For this reason Railco WA80H bearings where developed.

•      These contained a phenolic resin impregnated asbestos yarn.

•      The next generation contained non-asbestos material.

•    This material tended to be tainted due a series of overheating problems. (later found due to the    combination of stiff high power transfer shafts and flexible hull design)

•    The modern material is called Stern Safe and comprises an inner bearing surface with an over wound outer layer.

•   This has greater tolerance to overheating and reduced swell in the event of water contamination.

•    The latter allows for reduced running clearance and thereby greater control of the shaft   position      reducing oil loss, seal damage and water ingress.

•      A wear gauge is incorporated into the bearing as our temperature sensors. 

   

       

  Unlike for the water lubricated stern tube a shaft liner is unnecessary. Generally a small one is fitted in way of the aft seal bolted on to the propeller boss. In this way 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 pi D in any case. Cast iron and bronze bearings must have a bearing length not less than 4D.

  
  

   Lubrication system must be capable of maintaining oil tightness despite varying temperature.

  
  

  Gravity tanks fitted with low level alarms, Usual for aft peak to be filled with water to provide cooling low suction valve to be fitted to be locked shut.

  
  

  Wear down for the white metal should not exceed 2mm to avoid hammering out and the period for inspection is 6 years.

  
  

  A highly resilient reinforced plastic may be used in place of the white metal.

   It is claimed to have greater load carrying capacity, high resistance to fatigue and shock loading, with good lubrication properties. Ceramic liners can also be used