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15.
Foundation
Engines can be either rigidly mounted on chocks, or resiliently mounted on rubber elements.
If resilient mounting is considered, Wärtsilä must be informed about existing excitations such
as propeller blade passing frequency. Dynamic forces caused by the engine are shown in the
chapter Vibration and noise.
15.1
Steel structure design
The system oil tank should not extend under the generator, if the oil tank is located beneath
the engine foundation. The oil tank must also be symmetrically located in transverse direction
under the engine.
The foundation and the double bottom should be as stiff as possible in all directions to absorb
the dynamic forces caused by the engine and the generator.
The foundation should be dimensioned and designed so that harmful deformations are avoided.
The foundation of the generator should be integrated with the engine foundation.
15.2
Engine mounting
The engine can be either rigidly or resiliently mounted. The generator is rigidly mounted and
connected to the engine with a flexible coupling.
15.2.1
Rigid mounting
Engines can be rigidly mounted to the foundation either on steel chocks or resin chocks.
The holding down bolts are usually through-bolts with a lock nut at the lower end and a
hydraulically tightened nut at the upper end.
Bolts number two and three from the flywheel end on each side of the engine are to be Ø46
H7/n6 fitted bolts. The rest of the holding down bolts are clearance bolts.
A distance sleeve should be used together with the fitted bolts. The distance sleeve must be
mounted between the seating top plate and the lower nut in order to provide a sufficient
guiding length for the fitted bolt in the seating top plate. The guiding length in the seating top
plate should be at least equal to the bolt diameter.
The design of the various holding down bolts appear from the foundation drawing. It is
recommended that the bolts are made from a high-strength steel, e.g. 42CrMo4 or similar,
but the bolts are designed to allow the use of St 52-3 steel quality, if necessary. A high strength
material makes it possible to use a higher bolt tension, which results in a larger bolt elongation
(strain). A large bolt elongation improves the safety against loosening of the nuts.
To avoid a gradual reduction of tightening tension due to unevenness in threads, the threads
should be machined to a finer tolerance than the normal threads. The bolt thread must fulfil
tolerance 6G and the nut thread must fulfil tolerance 6H.
In order to avoid bending stress in the bolts and to ensure proper fastening, the contact face
of the nut underneath the seating top plate should be counterbored.
The tensile stress in the bolts is allowed to be max. 80% of the material yield strength. It is
however permissible to exceed this value during installation in order to compensate for setting
of the bolt connection, but it must be verified that this does not make the bolts yield. Bolts
made from St 52-3 are to be tightened to 80% of the material yield strength. It is however
sufficient to tighten bolts that are made from a high strength steel, e.g. 42CrMo4 or similar,
to about 60-70% of the material yield strength.
Wärtsilä 50DF Product Guide - a16 - 9 September 2016
15-1
15. Foundation
Wärtsilä 50DF Product Guide
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