. Introduction
For a long time all kinds of aspects of conveyor-belt oscillations have been studied. Recently, it has been
pointed out in Refs. [1–3] that it is not always mathematically correct to truncate the infinite moderepresentation
for the oscillations to a single or a few modes of oscillations. Usually partial differential
equations are used to describe these oscillations. For non-resonant problems the truncation method can
usually be applied successfully. For a resonant problem, however, it has been explicitly shown in Refs. [1–3]
that the truncation method (and so, a finite mode representation for the solution of the problem) can lead in
certain cases to results which describe wrong internal mode-interactions, and which give rise to wrong
resonance frequencies. Since the (in)stability of a system is usually determined by the internal modeinteractions
and the resonance frequencies it follows for instance from Refs. [1–3] that one has to be careful in
applying the truncation method.
On the other hand, there exist a lot of engineering approaches (see the list of references in Ref. [2,3]) that
only use a single or a few modes to describe these oscillations. The aim of this paper is to contribute to filling
the gap between the above mentioned approaches, that is, the gap between an infinite mode-representation or
a finite one. Asymptotic techniques (as for instance described in Ref. [4]) will be used to illustrate how this gap
most likely can be filled, and a new approach will be proposed.
In the new approach the applicability of the string model, the stretched beam model, and the beam model
will be taken into account. That is, for the lower frequencies (and oscillation mode numbers) a perturbed