Tuesday, 12 June 2012

Holding down arrangements,

The engine must be firmly in position in the ship in order to preserve alignement between the crankshaft and the propeller shaft.

Vibration, rolling and pitching will tend to move the engine from its original set position.

The bedplate is aligned and individual chocks are used to maintain that alignement. Holding down studs which are preferred to bolts because they are cheaper are then used to hold the engine firmly to the tank top.

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The bedplate must be as rigid as possible in order to keep the crankshaft straight. The ships structure, which includes the tank tops will distort as the ship rolls and pitches. Modern bulk carriers are constructed of high tensile steel because thinner plates may be used, thus reducing the deadweight and allowing more cargo to be carried. Thinner plates are more flexible and this increases the relative movement between tank top and bedplate.

This relative movement between the rigid bedplate and flexing tanktop leads to bending of the stud and possible fatigue failure.

One solution is to use spherical washes but these are expensive. A better solution is to use longer studs. When these bend the radius of curvature is grater and so reducing the bending moment for the same relative movement. A large radius of curvature means a lower stress and the risk of fatigue failure is reduced.

Graph showing effect of changing length on bending stress

M /I = s/y = E/R

M=Bending moment
I=2nd moment of area of the cross section
s =Stress
y=distance from the axis of bending to the outer face

E= modulus of elasticity
R-radius of curvature of the bending.

This can be arranged into

s = E . y /R

or s = k. 1 /R

where k is a constant

that is, stress is inversely proportional to the radius of curvature.

Holding down bolt
Side Chocking
side chocking

End Chock (aft end of the engine only)
front chocking
Resin Chocking
Steel chocking has the disadvantages that each block must be individually fitted, a time consuming process, and after fitting are susceptible to fretting and wear. Resin chocks are poured and therefore are much quicker to apply. They form into the shape of the clearance and key into surface imperfections. This much reduces damage due to fretting and removes bending momemts on the holding down bolts.

The disadvantage is that the resin creation must be precise and that it is less straight forward to replace in the event of damage of misaligenement.

The material used for the rsin chocking is Class tested to ensure minimum standards.

    A sample cured in the correct way is tested for the following;
        The impact resistance
        Compressive strength (stress at maximum load) and modulus of elasticity.
        Water absorption.
        Oil absorption.
        Heat deflection temperature.
        Compressive creep
        Curing linear shrinkage.

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