Exam Question

(a) Top bracings are used to control the vibration of large crosshead engines onboard ships. When the engine is running, longitudinal vibration from the piston movement is transmitted to the crosshead guides and then to the engine structure. To protect against the twisting forces generated in the crosshead guides, braces are fitted on the topmost part of the engine to provide support. The braces are intended to be fitted in pairs (or three for the large engines) to one side of the engine, usually the exhaust side. By introducing top bracing, the stiffness of the engine is increased to the ship attachment, thus increasing the natural frequency of the engine and the ship structure. Hence resonance of the engine structure will not occur within the normal operating range of the engine.
(b) The hydraulic top bracing consists of a single-acting, self-adjusting unit.
In practice, the oil pressure will increase rapidly when the engine starts to vibrate. The forces are transferred through the top bracing cylinders, which will act as rigid connections and thereby detune the natural frequencies.
An oil pressure gauge and a pressure transmitter are placed in the pipe branch connected to the accumulator to monitor the pressure of the pre-charged hydraulic oil. The measuring range of the transmitter is 0-10 bar and the AMS should give an alarm when the pressure of pre-charged hydraulic oil becomes lower than 6.3 bar.