Heat-Flow-Induced Vibration of Rotating Shafts: The Newkirk Effect
Packing rub effect is the unstable vibration induced by friction forces and generated heat on a rotor when it rubs on a clearance annulus. Its main controlling factors are friction characteristics at the sliding contact and the dynamic response of the rot
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Heat-Flow-Induced Vibration of Rotating Shafts: The Newkirk Effect
Abstract Packing rub effect is the unstable vibration induced by friction forces and generated heat on a rotor when it rubs on a clearance annulus. Its main controlling factors are friction characteristics at the sliding contact and the dynamic response of the rotor, bearings, supports and foundation system. In Chapter 5 rotor instabilities are investigated, resulting from friction heating, a phenomenon known as the ’Newkirk effect’. The interaction between vibration characteristics and heat generation leads to a non-linear feedback system exhibiting either stable or unstable behavior.
5.1 Introduction Around rotating shafts there exist stationary components such as packings, seals and oil deflectors, used to separate two fluids and to ensure mechanical limitations. When rotating parts come into contact with stationary elements, heat is generated which deforms the parts, and finally imbalance and vibration are produced [1–4]. On other occasions, due to normal or accidental presence of fluids in the gaps between rotating and stationary components, heat may be generated due to an abnormal flow of these fluids, or the abnormal energy dissipation within their bulk, leading to the same results as above. The usual form of seals, subject to rotor rubbing phenomena, is of the labyrinth type used in turbines. With these seals the designer has to choose between materials which wear quickly and with a low coefficient of friction, or harder materials resisting wear but with a high coefficient of friction. The materials selected must meet additional requirements, such as corrosion and erosion resistance, high thermal conductivity, and strength at high temperatures. At the places of contact between
A. D. Dimarogonas et al., Analytical Methods in Rotor Dynamics, Mechanisms and Machine Science 9, DOI: 10.1007/978-94-007-5905-3_5, Ó Springer Science+Business Media Dordrecht 2013
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5 Heat-Flow-Induced Vibration of Rotating Shafts
stationary and rotating parts, considerable forces may appear as friction forces or shock forces in case of violent interaction [5]. The combined effect of heating imbalance, and direct forces, can be either amplifying or a stabilising. In case of amplification, a continuous increase in amplitude and phase angle usually appears and a polar presentation of these quantities exhibits an outward spiral curve [6]. The rotating parts may also be surrounded by a fluid environment, which can exert considerable influence. For example, the phenomenon of fluid-induced instability in bearings has been observed with high-speed machinery, as well as with other cases in which the rotors are working in a fluid environment such as steam and gas turbines, pumps, etc. Under certain circumstances, vibration can be induced by fluids trapped within the annuli between stationary and rotating parts [7] (see Chap. 4). Vibration can also be initiated or amplified by structural instabilities arising from transmitted forces (buckling), or torques (torsion
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