Engine Rocking and Use of Bracing Units

When highly powered marine diesel engines run, they produce vibrations which may resonate with vibrations of other machinery and cause heavy damage to ship structure as well as machinery.

When the engine is running it experiences longitudinal(athwartship) as well as transverse(fore-aft) vibrations. 

Bracing or Rocking is particularly concerned with the longitudinal or athwartship vibration of the engine.

Cause of Longitudinal vibrations

When the piston of engine is at TDC or BDC, the alignment of piston rod and connecting rod is in a straight line. Hence all of the gas force will be only transmitted to the crankshaft without any side force on the slippers of the crosshead. But, when the piston is anywhere between TDC and BDC, then only a component of gas force is transmitted entirely to the crankshaft. The other component acts on the crosshead slippers from where it is transmitted to crosshead guide and eventually to the engine structure.

Consider this diagram,

The gas force P acting on the crosshead has two components:
A, along the connecting rod, and
B, perpendicular to connecting rod.

The component perpendicular to connecting rod (B) acts on the crosshead guide, thus exerting a force to turn the engine structure athwartship. This causes longitudinal vibration of engine structure.
To prevent this, bracing units are attached on top of engine structure. 

The bracing unit is normally installed on the exhaust side of the engine but may also be installed on the manoueuvring side, or even on both side.

Two types of bracing units can be fitted:
1. Mechanical bracing units, or
2. Hydraulic bracing units.

The relative location of bracing units can be seen in the figure above. The construction of bracing units will be discussed in another post. 

For more information on engine rocking and bracing refer "Reciprocating Machinery Dynamics" by A.S. Rangwala
The link for above:

https://books.google.co.in/books?id=dXGF22w-jkUC&pg=PA19&lpg=PA19&dq=forces+acting+on+crosshead&source=bl&ots=UZbCI_Vrzj&sig=KgOKNpKtS9kFvh-1--NjMuYRo34&hl=en&sa=X&redir_esc=y#v=onepage&q&f=true

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