Wednesday, May 13, 2020

Top bracing

 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. Two modes of vibration known as H or X modes can exist. Mode and amplitude of vibration will depend on number of cylinders and size and stiffness of engine structure. Fitted between the upper gallery of the main engine and a very stiff location on the ships side, the bracings act as detuners, increasing the natural frequency of the system, so that resonance occurs above the engine running speed.

It relies on the friction between the pads to brace the engine at the top so that the resonances with critical orders are above the speed range of the engine.
Because it relies on the frictional grip to work correctly, the tension on the hydraulic bolts must be regularly checked. Also inspect the structure for evidence of cracking especially around the welds.

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