Bedplate of a Large Two Stroke Marine Engine

Bedplate:

A bedplate is the lowermost portion of a marine engine (2 and 4 stroke), which supports the engine structure and is also one of the most loaded constructional parts of the engine. For large engines, the bedplate is fabricated in parts with flat bottom type construction having high surface finish.

Construction of Bedplate:

The bedplate acts as the main strength member, maintains correct alignment and supports the weight of the components. it must be capable of withstanding the fluctuating forces created during operation and transmit them to the ships structure. In addition it may also collect lubricating oil.

In slow speed engine design, it consists of a deep longitudinal box section with stiffening in the form of members and webs.
Transverse members are fitted between each throw of the crankshaft. These support the main bearing saddles and Tie -rod connection. They are attached to the structure by substantial butt welds.To reduce the engine height the sump of the bedplate may be sunken allowing it to fitted into a recess in the ships structure.
Plate and weld preparation is required with welds of the double butt type if possible. Regular internal inspection of the parts especially the transverse girder is required for fatigue cracking. Tie bolts should be checked for tightness.

The advent of the small bore slow speed has seen the use of single side bedplates. A box section is then created by using a box section crankcase structure rather than the more traditional A-frame.This has the advantages of reducing width as well as weight and increasing the amount of fabrication so reducing assembly times.

Due to the weight penalty, the use of cast iron is generally limited to smaller units where fabrication becomes impractical. However, cast iron has internal resilience allowing it to dampen down vibrations, this has led to its usage on some medium speed installations, especially passenger carriers, where noise and vibration suppression is important.

The most highly loaded pat of a bedplate is the transverse girder. Classification societies require that residual stress is removed after construction.
The transverse girder acts as a simple beam with the forces of combustion acting on the piston passing down through the bearing. The forces acting on the head are passed through the Tie rods.

It can be seen that to reduce the bending moment the tie rods have to be brought closer to the crankshaft. The limit to this is the securing arrangement required for the main bearing keep. One method is to use two instead of one bolts which can be made of smaller diameter. Sulzer use an alternative and very successful method in the form of jacking bolts. These jack against the bottom of the A-frame.

The important functions of bedplate are:

1. To support the static load of stationary engine frame and blocks
2. To support the dynamic load of the running gear
3. To support the crankshaft and hold it in perfect alignment
4. To distribute the static and dynamic load generated by running engine onto the ship structure
5. To collect the crankcase lube oil and transfer it to the sump tank from where the lube oil pump can take suction
6. To fasten the engine to the tank top transmitting propeller thrust to the hull structure
7. To contribute to the hull strength of the ship at engine room bottom structure

Checks to be Carried on the Bedplate:

It is important to keep a regular check on the bedplate as it’s the foundation of the marine engine. The inspection of bedplate is also included in the planned management system of engine.

Following checks to be carried on the bedplate during inspection:

Cracks: Cracks is the most common problem that occurs on the bedplate structure. Following areas to be carefully checked for cracks:
Welding portion which joins the transverse girders to the longitudinal girders
Under the bearing pockets where cracks can emerge to be radial or follow the line of the pocket which holds the bearing
Radially at tie bolt and frame bolt holes
Around lightening holes provided in the bedplates and girders
At the base of main bearing keeps



Faulty welding: It is to be checked on newly delivered engine or if any welding repairs are carried out in the recent past


Faulty casting – It is to be checked on newly delivered engine with casting construction


Corrosion: As the bedplate is the bottom structure fitted in the bilge section of the engine room, it comes in contact with various fluids such as oil, water etc. and therefore is prone to corrosion. A close check should be made for identifying corrosion.


Loose Frames: The bedplate is held together with A frame and entablature of the engine by means of tie rod. Check the tie rod is tightened and there is no loose portion between the frame and bedplate.
Faulty Holding Down Bolts: The holding down bolts keeps the bedplate in position with the bottom structure of the ship. Check for loose holding down bolts and tighten as per the manual if found loose. Also, check for shearing and fretting on the holding down bolts.


Oil leakages: The bedplate is also responsible to collect the lube oil and transfer it to engine sump. Check for any oil leakage from the bedplate or the joint between the bedplate and the frame.


Ship’s engineers are responsible to ensure that marine engine bedplate is inspected at regular intervals of time and all faults are identified and repaired at the earliest. Failing to do so will result in heavy vibrations, misalignment of crankshaft, and reduction in engine efficiency and failure of engine components.

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Reference:

www.marineinsight.com

Marine Engineers' World (Facebook)

www.marineengineering.org.uk

Bangladesh Marine Academy