The torsional vibration of rotating shafts contributes significantly to machinery vibration
and noise but is notoriously difficult to study experimentally. New developments are
reported which address the need for appropriate measurement tools, through improved
understanding of the laser torsional vibrometer (LTV) and the application of modal
The LTV was developed previously for non-contact measurement of torsional oscillation.
This thesis describes comprehensive theory to account for the sensitivity of its
measurements to shaft motion in all degrees of freedom. The significance of this sensitivity
is compared with the instrument noise floor and typical torsional and lateral vibration
levels. Optimum instrument alignments are thereby specified to ensure immunity to all
lateral motion. A new technique is proposed permitting unambiguous measurement in
situations where conventional use of an LTV shows unavoidable lateral vibration
sensitivity. Simultaneously, a previously unattained measurement of shaft bending vibration
is derived. Practical application is demonstrated with measurements from an engine
crankshaft, with identification of the first bending mode and estimation of its bending
Experimental torsional modal analysis on rotating systems has had limited progression due
to the absence of a versatile means to apply an instrumented torque. A novel device has
been developed to provide a controllable and measurable torsional excitation, based on the
principle of eddy current braking. Together with an LTV to measure response, estimation
of the torque input permits frequency response functions to be obtained without
modification to the system under test. This system achieves full modal analysis from a
rotating shaft using conventional techniques for data processing, with derivation of natural
frequencies, mode shapes and damping factors. Results from simple shaft systems consider
the variation of modal parameters under rotating conditions.
Application of this technology is clearly demonstrated in studying the behaviour of a
centrifugal pendulum vibration absorber (CPV A) used to control the resonant modes of a
shaft system. Accurate measurement of each individual pendulum tuning is achieved. with
examination of other effects related to successful absorber design. These results are
complemented by novel use of the LTV to study the actual pendulum motion. The depth
of information obtained underlines the analysis potential made possible by these advances
in torsional vibration measurement.
A Doctoral Thesis. Submitted in partial fulfillment of the requirements for the award of Doctor of Philosophy of Loughborough University.