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The most critical time for any bearing with regards to lubrication

  •  Most of the bearings used on the engines work on the principle of hydrodynamic lubrication. 
  • During hydrodynamic lubrication, there is no metal-to-metal contact between the bearing and the shaft. The bearing and shaft is separated by lubricating oil film. 
  • This oil film is formed due to the rotation of the shaft inside the bearing. As the speed of the shaft increases, more oil is drawn between the shaft and the bearing and thicker lubricating oil film is formed. 
  • During starting of the engine, the speed is very low. Hence it is difficult to form a hydrodynamic lubrication between the shaft and the bearing. 
  • During this time, for a short period, there is metal-to-metal contact between the shaft and the bearing surface. 
  • Hence this is the most crucial period as the characteristics of the lubricating oil (oiliness) comes into use and this is time when more wiping out of the bearing takes place. 
  • Hence it is necessary to coat the bearings with anti-friction material to reduce the friction between the bearing and the shaft at the time of starting because once the engine is started and the engine has achieved the speed, the white metal does not play an important role because there is no metal-to-metal contact and it is pure hydrodynamic lubrication. 
  • Hence white soft coating is useful for the boundary lubrication which is existing at the time of starting the engine.


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