Bed Plate – Functions And Structure of IC engine

What is a Bedplate of Diesel Engine ?

The bed plate forms the foundation of the engine (or constructions )on which the other structural component such as A-frame, all column,and Guides are mounted and it can withstand heavy fluctuating stresses from working parts.

Bedplate supports the Load of the Engine and it’s other Constructional parts and provide a base for the large mass.

bed plate

What are the Functions of a Bed Plate ?

1. It is required to transmit engine load including propeller thrust,to the ship Structure and distribute these over necessary area and ensure alignment of the crankshaft.

2. The bed plate collects the crankcase lube oil and returns it to the drain tank for recirculation.

3. It is bolted at the tank top which in turn is reinforced with thicker plate and girders.

4. The bed plate acts as a base and provides support to the engine structure.

5. It holds the Static weight of the whole Engine and it’s parts.

6. For support the dynamic load of running gear of the engine.

7. The crankshaft is so fitted in the Bedplate in the main journal Bearings.Itsupport the crankshaft and hold it in perfect alignment.

What are the fundamental requirements of Bed plate ?

  1. Strength
  2. Lightness
  3. Toughness
  4. Simple design
  5. Seal
  6. Access
  7. Dimenstions
  8. Rigidity

Read About : Difference between two Cross head and Trunk type piston engine


1. For a small engine one piece cast iron bed plates are common. Generally, for small engines it is of single casting of cast iron.

2. For large engines fabricated mild steel, longitudinal girder with cast steel transverse girders supporting the crankshaft bearing is constructed.

One important thing to be considered in the construction of Bedplate of Large Marine Diesel engine is to use materials of carbon content only upto 0.23% like Low carbon steel.

On the other hand the small Bedplate can be construed with cast iron which is required to have internal vibration damping feature to smoothen and dampen the vibration .

What are the forces act on bed plate

The forces are :-

  • Firing pressure (Cylinder gas pressure )
  • Forces of inertia of moving masses
  • Side thrust due to movement of connecting rod.
  • Whole weight of engine act on the bed plate.
  • Torque due to rotation of propeller.
  • Hull deflection.
  • Vibrations forces (It is generated due to fluctuations in torque,shock loading and thermal stress )
  • Rough weather (Forces due to ship’s movement in heavy seas).

Materials use

◆ Cast iron

Hybrid: fabricated mild steel for longitudinal Cast steel for transverse.

Maintenance / What are the checks to be carried on the bed plate during inspections

Since Bedplate is the base of the Engine it should be in proper order for the proper functioning of the Engine. And for that it should be regularly checked and maintenance should be carried out.

The inspection should be carried on following :

1. Cracks :-

Cracks is the most common problem on the structure of the bedplate. which may occur due to long working and loading .

The following areas need to be carefully checked for cracks:

  • At the welding portions that join transverse girders to longitudinal girders.
  • Under the bearing pockets, cracks can tend to be radial or follow the pocket line that holds the bearing.
  • Radially At tie bolt and at holes frame bolt.
  • Around the lightening holes which provided in the bedplates and girders.
  • At the base of main bearing keeps.

2. Welding :-

Generally, Welding joint and also if any recent welding work has been carried out.

3. Faulty Casting :-

The newly delivered engine with casting construction should be tested.

4. Corrosion :-

Being at the bottom most part of the Engine it is susceptible to corrosion also. It should be inspected for that.

5. Loose frame :-

Loose Frames may happen because of excessive loading or if issue with the tie rods.

The bedplate is fixed together by means of a tie rod with the A frame and the engine entablature. Check that the tie rod is tightened and that the frame and bedplate do not have a loose section.

6. Faulty Holding Down Bolts :-

The Holding down Bolt should be inspected for tightness as it hold the Bedplate with the bottom structure of the ship.

7. Oil leakages :-

The bedplate is also responsible for storing and transferring the lube oil to the motor sump. Test bedplate or the joint between the bedplate and the frame for any oil leakage.

What are the cause of breaking of bed plate ?

  1. Due to Over loading.
  2. Uneven loading on engine.
  3. Due to faulty techniques of maneuvering.
  4. Loosen tie bolts.

Faults in Bedplate

  1. Cracks
  2. Oil leak
  3. Loose chocks

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