JEDAN PRISTUP PRORAČUNU ELASTIČNOG TEMELJENJA BRODSKIH STROJEVA | An Approach to Calculation of Marine Machinery Flexible Seating

Brodski porivni i pomoćni strojevi temelje se kruto (na metalnim, ili podlogama od smole) ili elastično (na elastičnim podlogama s gumenim dijelovima). Osnovna je svrha elastičnog temeljenja smanjiti nepovoljan utjecaj vibracija i neposredan prijenos buke izazvane radom strojeva na brodsku konstrukciju. Elastično se temeljenje zbog toga često rabi na putničkim brodovima. Cilj je u ovom radu prikazati pojednostavnjeni postupak proračuna elastičnog temeljenja brodskih strojeva kako bi se olakšao rad projektantima brodskih pogonskih i pomoćnih sustava. Pojednostavnjeni se proračun u osnovi svodi na određivanje statičkih sila u elastičnim podlogama, a zatim i prirodnih frekvencija sustava sa šest stupnjeva slobode, iz kojih se dobivaju kritične brzine vrtnje. Proračun je prikazan matrično (analogno metodi konačnih elemenata za deformabilne sustave) s pojedinostima potrebnim projektantu za samostalan rad. Načelno je prikazana i metodologija proračuna prisilnih prigušenih vibracija, to jest uzbudnih sila i posljedičnih dinamičkih odziva sustava.Doprinos je rada u matrično sažetoj metodologiji proračuna, te u prijedlogu izbjegavanja izračuna uzbudnih sila. Naime, u izvjesnim uvjetima, dostatno je poznavati samo ritam (frekvenciju) uzbude izazvanu radom motora s unutarnjim izgaranjem i brodskoga vijka, a ne i njezinu veličinu ili fazu, te u Campbellovu dijagramu usporediti tu frekvenciju s izračunatim prirodnim frekvencijama sustava. Tako se odlučuje je li složeniji pristup potreban ili ne. Prikazan je primjer rezultata proračuna za porivni sustav više putničkih brodova izgrađenih u domaćem brodogradilištu, uz usporedbu s rezultatima mjerenja iz literature na prvom brodu. Zaključuje se da proračun elastičnog temeljenja ne treba izbjegavati u praksi, što je često, nego primjenom predloženoga jednostavnog postupka provesti potrebne statičke i dinamičke analize. Tako se dovoljno rano u projektnoj fazi mogu predvidjeti i riješiti poteškoće koje je poslije bilo teško otklanjati.

Marine propulsion and auxiliary machinery are seated on stiff seating (metal supports or resin chocks) or flexible seating (elastic supports with rubber parts). The basic purpose of flexible seating is to reduce detrimental influence of vibrations and direct transfer of noise to the ship structure. Consequentially, flexible seating is usually implemented in passenger ships. The aim of this paper is to present a simplified procedure for calculation of flexible seating for marine machinery, in order to ease the work of designers of marine propulsion and auxiliary machinery. The simplified calculation basically contains of evaluation of static forces in the flexible supports, as well as the natural frequencies for the six degree-of-freedom system, from which critical rotational speeds can be determined. The calculation is presented in matrix form (similar to the conventional FEM calculations of linear elastic systems) with all the necessary details enabling a designer’s autonomous work. Generally, the methodology of calculation of system dynamic response to forced damped vibrations is also presented. The contribution of the paper is in the calculation methodology presented in matrix form, as well as in the proposal to avoid evaluation of excitation forces. Under certain conditions it is enough to know only the frequency of the excitations, caused by internal combustion engine cylinders and propeller in operation, but not their magnitude or phase, as well as to compare that frequency in Campbell diagram with the calculated natural frequencies to make the decision about the necessity of more complex approach. An example of calculation results for the flexibly mounted propulsion system of a series of newbuildings, built in a domestic shipyard, has been presented, together with the comparison with the measurement results from the literature on the first ship. It is concluded that the calculation of flexible seating should not be avoided in practice, what is a common situation. Contrary, by implementing the proposed simplified calculation procedure the necessary static and dynamic analyses should be performed. This is the way to avoid difficulties during the design phase early enough, which are not easy to deal with later on.