marine i.c engine :: bedplate :: crankshaft

Bedplate

The bedplate is the foundation on which 2 stroke engine is built. It must be rigid enough to support the weight of the engine components and maintain the alignment of the crankshaft. At the same time, it must be flexible enough to be capable of withstanding the fluctuating forces created during operation and transmit them to the ships structure.

The bedplate consists of two longitudinal girders which run the length of the engine. Connecting these longitudinal girders are transverse girders which are positioned between each crank throw, and either side of thrust collar. Built onto these transverse girders are cast steel bearing supports. The crankshaft sits in the bearing housing in the transverse girders. The aftmost transverse girder is designed with ample stiffness to transmit the variable thrust from the thrust collar to the engine seating.

On smaller bore engines, bedplate can be made from cast iron as a simple casting. Cast iron has internal resilience allowing it to dampen down vibrations, this has led to its usage on some medium speed installations.

The bedplate of the modern engines is constructed from fabricated longitudinal girders with cast steel transverse sections welded into place, containing the bearing pocket and holes for tie bolts.

The bedplate should be inspected regularly for evidence of cracking. This can occur in the welds joining the transverse girders to the longitudinal and under the bearing pockets. Inspection should be carried out especially after heavy weather or damage has occurred.

Causes for cracks can be

• faulty manufacturing techniques
• uneven loading / overloading of the engine units
• loose tie bolts and loose holding down bolts.
• Excessive vibrations
  
  
  

  CRANKSHAFT

  
  

  Crankshaft is the backbone of diesel engines and transmits power from the cylinders to the propeller shaft. Strength and rigidity are essential features of any crankshaft design. Crankshaft is subjected to fluctuating bending, torsion and shear stresses.

  
  

  The material used must have

  • high strength and rigid to resist bending
  • high endurance limit to fatigue failure
  • form good bearing surface.
  • Must be hardened to resist wear of journal and crank pin.

  
  

  Material used for manufacture of crankshaft is

  Carbon 0.2%

  Silicon 0.32%

  Manganese 0.7%

  Phosphorus 0.02%

  Sulphur 0.02%

  
  

  Types of crankshafts :

   
  

  Types of crankshafts :

  
  

  01.One piece construction : Either cast or forged is usually restricted to smaller medium and high speed engines. After casting or forging operation crankshaft is machined and oil passages are drilled.

  Advantages :

  • Better fatigue resistance,
  • lesser stresses,
  • smaller and lighter shaft,
  • continuous grain flow throughout shaft, no need for shrunk fits and balanced counter weights can be fitted.

   
  02. Fully built up :

   

  Webs are shrunk onto journals and crankpins.

  These types of crankshafts were mainly used on older engines. They consist of forging separate webs, crankpins and journals. The crankpins and journals are machined and matching holes bored in webs of slightly smaller diameters. The webs are heated up and cooled slowly to shrunk fit crankpin and journals.

   
  Semi built up : Webs and crankpins as one unit shrunk onto journals.

  In this method of construction the crankshaft throws consisting of two webs and crankpin are made from single forging of 0.4% Carbon steel. The webs are bored to take separately forged and machined journals which are shrunk fit. The shrunk fit allowance is between 1/570 and 1/660 of the diameter.

  
  

  The advantage of this method is that the grain flow in the steel follows the web round into the crankpin and back down the other web.

  • It has better fatigue resistance,
  • less shrink fits,
  • smaller webs and lighter shaft weight,
  • larger pin diameters can be used.