Question

With reference to behaviour of fabricated bed plates and frames in services:

(a) Identify various forces imposed simultaneously upon them. (6)

(b) Explain how engine structure withstands these forces. (5)

(c) State how these forces are transferred to ship's structure. (5)

Answer 1

Part (a)

Static weight of components – The combined weight of piston, connecting rod, bearings, crank webs, piston rod, rings, liner, etc.
Gas forces – High cyclic combustion and exhaust pressures impose alternating tensile and compressive loads on the structure.
Inertia forces of moving parts – Caused by acceleration and deceleration of piston and connecting rod, varying throughout the cycle.
Centrifugal forces – Produced by the rotating crank webs of the crankshaft.
Oscillating guide forces – Crosshead and connecting rod impose lateral forces on guides and engine frames.
Hull stresses – Ship’s hogging and sagging induce bending moments on bedplate and frames.
Propeller thrust and shafting forces – Transmitted to the bedplate via the thrust bearing.

Part (b)

Bedplate, frame, and cylinder jackets are held in compression by tie rods/bolts, tightened under pre-tension (hydraulic tightening preferred for accuracy).
The bedplate is firmly secured to the tank top using foundation bolts.
Gas pressure is contained within the cylinder head; resultant combustion forces on the piston are partly opposed by inertia forces and absorbed by main bearings on either side of the working cylinder.
At BDC, only inertia forces act on the main bearings.
Tie rods transmit gas loads to the bedplate; at standstill they remain in pre-tension, and during operation, inertia forces dominate.
Bedplate and frame are constructed of cast steel with longitudinal and transverse box girders, giving high strength and rigidity, minimizing deformation and twisting.
Main bearings absorb inertia and centrifugal forces of reciprocating and rotating masses.
Crosshead guide forces are resisted by bracing and frame strengthening.
Bedplate is designed to resist bending due to hogging and sagging, preventing structural failure during ship motion.
Unbalanced loads are minimized by careful pretension and structural reinforcement.
Thus, the majority of forces are effectively transmitted as power to the propeller, while vibrations and stresses are absorbed by the engine structure.

Part (c)

All forces are transmitted first to the bedplate.
From the bedplate, loads are transferred to the ship’s tank top (double bottom structure) through resin chocks and holding-down bolts.
Holding-down bolts, fitted around the periphery of the bedplate, pass through the bedplate, resin chock, and tank top, ensuring firm securing.
Resin chocks provide uniform surface contact, prevent fretting, absorb cyclic stresses, and add slight damping against vibration.
This ensures smooth transfer of forces from the engine to the ship’s double bottom, distributing them evenly across the hull framework and allowing the structure to withstand combined engine loads and sea-induced stresses.

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