The vibrations set up in a diesel engine are most complex, as both the magnitude and direction of the forces creating the vibration vary throughout one revolution. A mathematical approach is required, but the results of vibration can easily be understood by the watchkeeping engineer and should always be aware of the potential problems that continued vibration can bring. There are not only different magnitudes of vibration, but also great variations in the frequency of the vibration.\r\n
The firing forces in a slow running engine create large, low frequency (relatively speaking) vibrations, whereas one could imagine that the blades in the turbocharger rotating at several thousand rev/min are being subjected to a very high frequency of vibration albeit of fairly low magnitude. Each in their turn can lead to component failure, although the design engineer will have done his best to provide scantlings and materials that should, with proper maintenance, provide long service without failure. The failure that results from vibration is almost universally ‘fatigue failure’, which accounts for the greatest proportion of material failures in engineering (normally accepted as being in excess of 65% of all failures).
Vibrations can nominally be separated into one of two forms. One is natural vibration which is a function of the material itself and its resistance, or lack of it, to movement. The classic example of this is a tuning fork where the legs of the instrument vibrate, once struck, quite freely and for some time until the internal resistance of the metal gradually dampens down the movement. All components will have, to a greater or lesser extent, a natural frequency of vibration, and the greater the mass involved the greater the natural resistance to vibration and the slower the vibrational frequency. The other form of vibration is forced vibration which the applied force occurs. For example, a 4-cylinder engine rotating at 100 rev/min will have a forcing frequency of 4x100=400 Hz.
The main problem arises when the natural and forced vibrational frequency coincide. Resonance is then said to occur. The forcing frequency, acting at the same time and in the same direction, tends to magnify the natural frequency substantially to such an extent that the strength of the material may no longer be able to withstand the stressing. Ultimately, Fatigue failure occurs with the cracks passing through the material until insufficient area is left to carry the load and complete failure takes place.
Most large bore engines have a ‘critical speed’ which is one at which resonance occurs. The particular range of revolutions will be marked on a plate adjacent to the controls and the engine should always be taken through the range as quickly as possible. Bridge controlled engines have an automatic block over the range to prevent inadvertent operation at that speed. The so called critical speed is that at which the torsional forces created by the firing impulses and the reactions from the propeller synchronise with the natural frequency of the shafting system. Balance weights may be fitted to change the natural frequency of the shaft as well as to counter some of the rotational, out of balance forces generated by the crank throw. Detuners, usually in the form of a floating mass in the shaft system, are particularly useful in dampening down the vibrations generated at critical speeds. This is achieved by changing the natural frequency of the shaft as the floating mass puts drive back into the shaft as it hesitates over a critical speed.
The complexity of the vibration can be understood by considering the piston and crosshead as a reciprocating mass. (It is also normal to include the top two-thirds of the connecting rod in the reciprocating masses, the lower third being considered as a rotating element.) The reciprocating masses are forced down the cylinder during the expansion stroke by the expanding pressures of the combustion gases. The shape of the power card shows the pattern of pressure distribution above the piston and gives some idea of the change in applied force over the expansion stroke.