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.

   

different types of marine diesel engines( slow, medium, fast, 2 stroke, four stroke)

Slow Speed Engines :

Slow Speed engines are engines in the range of 50 – 300 rpm at rated power. These are mostly two stroke, crosshead engines with between 4 to 12 cylinders in line and turbocharged.

The stroke – bore ratio of low speed engines varies between 2.5:1 to 4.2:1. These engines are called Super long stroke engines, and are well suited to operate on low quality fuels and have proven to be economically advantageous.

These engines are more fuel efficient and lub oil consumption is less when compared to medium and high speed engines, though the initial cost is considerably higher.

Low speed engines are generally intended for direct drive, hence connected to the propeller directly.

Medium and High Speed Engines :

Medium Speed engines are in the range of 300 – 800 rpm whereas high speed engines are above 800 rpm, though there are no clear physical features that enable the distinction to be made. These are usually four stroke, trunk piston engines which have up to 10 cylinders inline or up to 18-24 cylinders in V configuration, and mostly turbocharged.

Stroke to bore ratios are not very high, normally up to 1.5:1, because of the forces created by high speeds. These engines are lighter, more compact, and lower in acquisition cost than low speed engines.

These engines have advantage of high power to weight ratio and used extensively in passenger / cruise ships due to less head room required.

Though medium speed engines have proved capable of running on fuels of poor quality, high speed engines are normally restricted to distillate fuels.

The maintenance required on these engines is more and these are more noisy as compared to slow speed engines.

Medium and high speed engines, because of their higher rpm, drive propellers through speed reduction gears.

2 STROKE AND 4 STROKE ENGINES

TWO STROKE ENGINES :

Advantages :

* Able to develop much higher Power at lower rotational speed.

* Cheaper quality fuels can be used

* Slow speed so less wear and tear of moving parts

* Turning movement is more uniform, lighter flywheel can be used.

* Cost of lubricants is less.

* Crosshead bearings capable of sustaining high loads.

* High stroke bore ratio of about 4.2:1

Disadvantages :

* Requires more space due to super long strokes.

* Expensive

FOUR STROKE ENGINES :

Advantages

* High Power to weight ratio.

* Requires less head room

* Working parts are fewer and less costly.

Disadvantages :

* Gudgeon pin Bearing assembly not suited for higher loads

* Cost of lubricant is more

* Medium speed or high speed, more wear and tear of moving parts.

* Power developed is less.

* Due to 1 power stroke is every 2 revolutions the turning movement is not

so uniform. Heavier flywheel is required.

* Difficulty in burning cheaper quality fuels.

* Maintenance required is more