Answer EKM Question 45

With reference to behavior of fabricated bed plates and frames in services:
A. Identify various forces imposed simultaneously upon them;
B. Explain how engine structure withstands these forces;
C. State how these forces are transferred to ship’s structure.
Answer:A. Identify various forces imposed simultaneously upon them;
(a) The Bedplate is the foundation on which the 2 stroke engine is built. It must be rigid enough to support the weight of the rest of the engine, and maintain the crankshaft, which sits in the bearing housings in the transverse girders, in alignment. At the same time it must be flexible enough to hog and sag with the foundation plate to which it is attached and which forms part of the ships structure.
(b) If the bedplate was too rigid, then as the hull flexed, the holding down bolts, which secure the engine into the ship would be likely to break, and there would be a danger of the bedplate cracking.
Various forces acting on the Bedplates are;
i. Gas force due to combustion loads.
ii. Guide forces created by the oscillating cross head.
iii. Inertia forces of the rotating masses.
iv. Hogging & sagging forces of the ship's hull.
v Bending moments and twisting forces due to loading, Unloading
vi. Forces due to weather.
vii. Thrust forces by the propeller and the shafting.

B. Explain how engine structure withstands these forces;
1. Effects of the gas loads:
(a) Alternate units firing either side of bearing pocket cause a rocking effect on the supporting transverse girder.
(b) The firing load, transmitting down through the running gear to the crankshaft, pushes the crankshaft into the transverse girder, at the same time the tension in the tie bolts increases as the firing load is transmitted back into the bedplate. The distance of the centre of the tie bolts from the crank journal centre gives a bending moment which tends to distort the bedplate. Bending due to the firing loads will be exacerbated if the tie bolts are loose or broken.

 

(c) Fretted chocks and distorted bedplates will induce bending stresses to be set up in the transverse girder.
Ways to handle gas loads:
(a) To handle all the above causes of gas loads, the bedplate must be strong and rigid enough to support the engine for which it forms the foundation and hold the crankshaft in alignment.
However, it must be flexible enough to move with the hull as it hogs and sags in a seaway.
(b) The bedplate consists of two longitudinal members fabricated by welding from steel sections and plate with a max carbon content of 0.23%. The longitudinal members are welded to the cast steel transverse members which are situated between the crank throws and either side of the thrust collar.
(c) The transverse members contain the tie bolt sockets and the main bearing housing, which can be supported by cast in webs. After fabrication the welds are crack detected and the whole assembly stress relieved by heating in a furnace to 600 deg C, soaking at that temperature and cooling slowly. The landing surfaces for the A frame and the chocks are machined and the bearing pockets line bored.
(d) The A frames are erected over each transverse member. They support the entablature and with the side plating form the space for the running gear (the crankcase). They carry the crosshead guides in which the crosshead slippers reciprocate.
(e) Fabricated from a low carbon steel (max 0.23%) they can be of single plate construction with stiffening webs and stringers to support the crosshead guides or of a double plate box construction.
(f) Sharp changes of section must be avoided to prevent stress concentrations and cracking. The guides can he integral with the A frame or manufactured separately from cast iron and bolted on.
(g) Alter manufacture, crack detecting and stress relieving, the landing faces are machined and the frames are then erected on the bedplate, aligned and located using fitted bolts.
2. Guide forces

(a) The force on the crosshead guides varies during the stroke of the piston. At Top Dead Centre when the gas load on the piston acts vertically downwards, there is no side component of thrust. However as the piston starts to move downwards and the angle the connecting rod makes with the vertical increases, so the component of side thrust increases.

 

(b) Top bracing is fitted to some 2 stroke crosshead engines. Sideways vibration or rocking due to reaction forces at the crosshead and main bearings may cause damage to turbochargers and attached pipework as well as causing vibration in the engine room and through the ships structure.
3. Inertia forces
(a) The inertia forces originating from the unbalanced rotating and reciprocating masses of the engine create unbalanced external moments although the external forces are zero. Of these moments, only the Pt order (producing one cycle per revolution) and the 2nd order (two cycles per revolution) need to be considered, and then only for engines with a low number of cylinders.
(b) The inertia forces on engines with more than six cylinders tend, more or less, to neutralize themselves. Countermeasures have to be taken if hull resonance occurs in the operating speed range, and if the vibration level leads to higher accelerations and/or velocities than the guidance values given by international standards.
(c) Inertia forces are calculated at manufacturing stages and handled by fly wheel design and by attaching counter weights to the crankshaft.
C. State how these forces are transferred to ship's structure
(a) The engine is mounted on resin or cast iron chocks and bolted to the hull using holding down bolts.
(b) The bedplate is mounted on chocks and is securely bolted to the engine foundation plate on which it sits and which forms part of the structure of the hull.
(c) Tie rod takes the most of the loads and transmit to the bed plate which passes it on to the ship hull through chocks.
(d) Top Bracing are used to handle the side thrust of the engine.
(e) Holding down bolts should be checked regularly for tightness. If they are allowed to come loose, then the mating surfaces will rub against each other and wear away in a process known as fretting. If this continues and the bolts are subsequently tightened down, the bedplate (and main bearings) will be pulled out of alignment.

 

(f) Side chocks are fitted to prevent the engine from moving sideways due to the movement of the vessel or because of the sideways component of thrust from the reciprocating and rotating parts. The chock is welded to the foundation plate.