Antivibration Mountings

Over the past 25 years the use of elastomer-based mounting systems to suppress or attenuate noise and vibration in ships has advanced both in technical sophistication and growth of application. Rubber-to-metal bonded systems are the most commonly applied anti-vibration mountings, and also deliver the highest performance rating for a given size. Shipboard installations mainly benefit propulsion engines, gensets and diverse ancillary machinery, such as ventilation fans, compressors, pumps, sewage treatment units, refrigeration plant and instrument panels.

Any item of machinery with moving parts is subject to out-of-balance forces and couples created by the interaction of those parts. The magnitude of the forces and couples will vary considerably, depending on the configuration of the machine design. In theory, some machines will be totally balanced and there will be no external forces or couples

(for example, an electric motor or a six-cylinder four-stroke engine). In practice, however, manufacturing tolerances may create out-of-balance excitations that can become significant in large elements of the machinery. The weights of pistons and connecting rods in large diesel engines may vary substantially, although to some extent this is allowed for by selective assembly and balancing. Uneven firing in large engines, due to variation in combustion between the cylinders, can also cause significant excitation.

The various excitations in a machinery element will either have discrete frequencies, based on a constant running speed, or there will be a frequency range to contend with (as in the case of a variable speed machine). In practical terms, it may not be possible or cost effective to eliminate vibration at source, and the role of anti-vibration mountings is therefore to reduce the level of transmitted vibration.

In an active isolation system the use of anti-vibration mountings beneath a machine will isolate the surrounding structure from the vibrating machine and equipment; in a passive system the mountings may be used to protect an item of sensitive equipment—such as an instrument panel—from external sources of vibration or shock. The amplitude of movement of the equipment installed on a flexible mounting system can be reduced by increasing the inertia of the vibrating machine, typically by adding mass in the form of an inertia block. Unfortunately, adding mass is not generally acceptable for marine and offshore applications because of the penalties of weight and cost.

A number of standard ranges of rubber/metal-bonded anti-vibration and suspension components are offered by specialist companies, but most installations differ and may call for custom-designed solutions. Computer analysis is exploited to study individual applications, predict the behaviour of various isolation systems and recommend an optimum solution. It is usually possible to design an anti-vibration system of at least 85 per cent efficiency against the worst possible condition, reports Sweden-based specialist Trelleborg.

LOW SPEED ENGINE VIBRATION: CHARACTERISTICS AND CURES

Ship machinery installations have two principal sources of excitation: the main engine(s) and the propeller(s). The two components are essentially linked by elastic shaft systems and may also embrace gearboxes and elastic couplings. The whole system is supported in flexible hull structures, and the forms of vibration possible are therefore diverse.

Problems in analysing ship machinery vibration have been compounded in recent years by the introduction of new fuel-efficient engine designs exploiting lower running speeds, longer stroke/bore ratios and higher combustion pressures. Another factor is the wider use of more complex machinery arrangements including, for example, power take-offs, shaft alternators, exhaust gas power turbines and multiple geared engines. The wider popularity of four- and five-cylinder low speed two-stroke engines for propulsion plant—reflecting attractive installation and operating costs—has also stimulated efforts by designers to counteract adverse vibration characteristics.

The greater complexity of vibration problems dictates a larger number of calculations to ensure satisfactory vibration levels from projected installations. For optimum results the vibration performance of the plant has to be investigated for all anticipated operational modes. In one case cited by Sulzer, in which two unequal-sized low speed engines were geared to a single controllable pitch propeller with a pair of PTO-driven generators, some eleven different operational configurations were possible.

Sophisticated tools such as computer software and measuring systems are now available to yield more detailed and accurate analysis of complex vibration problems. Even so, inaccuracies in determining shaft stiffness, damping effects, coupling performance and hull structure response make it advisable in certain borderline cases to allow, at the design stage, for suitable countermeasures to be applied after vibration measurements have been taken during sea trials.

Both MAN B&W Diesel and Wartsila (Sulzer) stress that proper consideration should be given to the vibration aspects of a projected installation at the earliest possible stage in the ship design process. The available countermeasures provide a good safety margin against potential vibration problems. Close collaboration is desirable between naval architects, machinery installation designers, enginebuilders and specialist component suppliers.

MAN B&W Diesel emphasises that the key factor is the interaction with the ship and not the mere magnitude of the excitation source. Excitations generated by the engine can be divided into two categories:

• Primary excitations: forces and moments originating from the combustion pressure and the inertia forces of the rotating and reciprocating masses. These are characteristics of the given engine which can be calculated in advance and stated as part of the engine specification with reference to a certain speed and power.

• Secondary excitations: stemming from a forced vibratory response in a ship sub-structure. The vibration characteristics of substructures are almost independent of the remaining ship structure.

Examples of secondary excitation sources from sub-structures could be anything from transverse vibration of the engine structure to longitudinal vibration of a radar or light mast on top of the deckhouse. Such sub-structures of the complete ship might have resonance or be close to resonance conditions, resulting in considerable dynamically magnified reaction forces at their interface with the rest of the ship. Secondary excitation sources cannot be directly quantified for a certain engine type but must be calculated at the design stage of the specific propulsion plant.

The vibration characteristics of low speed two-stroke engines, for practical purposes, can be split into four categories that may influence the hull (Figures 1.11(a) and 1.11(b)):

• External unbalanced moments: these can be classified as unbalanced 1st and 2nd order external moments which need to be considered only for engines with certain cylinder numbers.

Figure 1.11(a) Forces and moments of a multi-cylinder low speed two-stroke engine. The firing order will determine the vectorial sum of the forces and moments from the individual cylinders. A distinction should be made between external forces and moments, and internal forces and moments. The external forces and moments will act as resultants on the engine and thereby also on the ship through the foundation and top bracing of the engine. The internal forces and moments will tend to deflect the engine as such (MAN B&W Diesel)

Figure 1.11(a) Forces and moments of a multi-cylinder low speed two-stroke engine. The firing order will determine the vectorial sum of the forces and moments from the individual cylinders. A distinction should be made between external forces and moments, and internal forces and moments. The external forces and moments will act as resultants on the engine and thereby also on the ship through the foundation and top bracing of the engine. The internal forces and moments will tend to deflect the engine as such (MAN B&W Diesel)

Figure 1.11(b) Free couples of mass forces and the torque variation about the centre lines of the engine and crankshaft (New Sulzer Diesel): Mv is the 1st order couple having a vertical component. Mjh is the 1st order couple having a horizontal component. M2V is the 2nd order couple having a vertical component. DM is the reaction to variations in the nominal torque. Reducing the 1st order couples is achieved by counterweights installed at both ends of the crankshaft

Figure 1.11(b) Free couples of mass forces and the torque variation about the centre lines of the engine and crankshaft (New Sulzer Diesel): Mv is the 1st order couple having a vertical component. Mjh is the 1st order couple having a horizontal component. M2V is the 2nd order couple having a vertical component. DM is the reaction to variations in the nominal torque. Reducing the 1st order couples is achieved by counterweights installed at both ends of the crankshaft

• Guide force moments.

• Axial vibrations in the shaft system.

• Torsional vibrations in the shaft system.

The influence of the excitation forces can be minimized or fully compensated if adequate countermeasures are considered from the early project stage. The firing angles can be customized to the specific project for nine-, ten-, eleven- and twelve-cylinder engines.

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