Vibration and Measurement

In the past, there have been ships that have been discarded for years due to unacceptable level of vibrations, rendering them unsafe for operation. So, over the years, with research by classification societies, a lot of improvement has been achieved in terms of marine vibrations. Design techniques have undergone changes to incorporate factors related to reduction of vibration levels during the entire lifespan of a ship. With the increase of ship size and speed, shipboard vibration becomes a great concern in the design and construction of the vessels. Excessive ship vibration is to be avoided for passenger comfort and crew habitability. In addition to undesired effects on humans, excessive ship vibration may result in the fatigue failure of local structural members or malfunction of machinery and equipment.

Machinery vibration-The vibrations that originate due to the operation of machinery can be listed under Machinery Vibrations. Any machinery that have parts moving at a certain frequency induce vibrations. So, main engines, propulsion shafts, gearboxes, propellers, pumps, diesel generators- all machinery transmit vibrations

The role of a ship designer is to first understand how each of these machinery causes vibrations, and then device methods to keep them within safe levels. Machinery Vibrations can again be categorised into three types, depending on the nature of the vibrations:

1. Torsional Vibration is angular vibration of an object commonly a shaft along its axis of rotation. Torsional vibration is often a concern in power transmission systems using rotating shafts or couplings where it can cause failures if not controlled. A second effect of torsional vibrations applies to passenger cars.

The main propulsion system of a ship consists of the main engine, which is connected to a propeller by a shaft. The shaft is again, not a single component. Usually, a marine shaft consists of an intermediate shaft and a propeller shaft, which are connected by means of coupling flanges. The presence of connections, like coupling flanges, thrust block, engine connection flange, and the cylinder piston system in the main diesel engine creates torsion in the rotating shaft system. In other words, the rotatory motion of the diesel engine creates an ‘excitation’. So, the entire propulsion system can be simplified, for vibrational analysis.

 Longitudinal vibration- occurs as a result of guide forces resulting from transverse reaction forces on the cross heads. The transverse reaction forces are generated when connecting rod and crankshaft mechanism converts reciprocating motion in to rotary motion. Such vibrations move the engine top athwart ship causing rocking or twisting. Axial vibration is a kind of longitudinal shafting vibration which occurs in the crankshaft because of the radial as well as tangential force.

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  • Raj Janorkar

    Vibration and Measurement

    2 years ago