A bed-plate 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. The bedplate is later arranged together during installation in large engines, whereas for smaller engines the bedplate is fully casted.
For large bedplates the material used can be cast or prefabricated steel or a mixture of both. The important requirement for large engine bedplate is to use material with low carbon steel with maximum carbon content of 0.23%. For small bedplate, cast iron with internal vibration damping characteristics is used, which reduces the frequency of cracks in the bedplate. The main components of any bedplate are longitudinal and cross girders.
The important functions of bedplate are:
To support the static load of stationary engine frame and blocks
To support the dynamic load of the running gear
To support the crankshaft and hold it in perfect alignment
To distribute the static and dynamic load generated by running engine onto the ship structure
To collect the crankcase lube oil and transfer it to the sump tank from where the lube oil pump can take suction
To fasten the engine to the tank top transmitting propeller thrust to the hull structure
To contribute to the hull strength of the ship at engine room bottom structure
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 bed-plate 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.