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Module 1 : Overview on Rotor Dynamics History and RecentTrends

Lecture 1 : A Brief History of Rotor dynamics

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A brief history of rotor dynamics eld has been documented in the present review. It

reviews early development of simple rotor models startin from the Ran!ine to"e#cott rotor models and physical interpretations of various !inds of instabilities in

rotor$bearin systems. It also reviews developments of analysis methods for the

continuous and multi$derees$of freedom systems to allow practicin enineers to

apply these methods to real turbo$machineries. It also summarises wor! on

conditionin monitorin and the recent trends in the area of rotor dynamics.

Before startin discussion of the sub%ect of rotor dynamics it would be pertinent to

as! why this sub%ect is re&uired to be studied and how it di#ers from the

conventional structural dynamics' (ince rotatin machinery is omnipresent rihtfrom house$hold machinery) automotives) marines to space applications and hence

understandin the dynamic behaviour of rotatin machinery is crucial for lon life of

the machinery and safety of humans. Rotor dynamics deals with these aspects and

hence its study is very important for desiners and practicin enineers. As

compared to structural vibrations the rotor dynamics di#ers in several ways: *i+

rotatin machineries have inherent forces and moments due to dynamics of various

machine elements or faults in occur them) *ii+ yroscopic e#ects which is

predominant at hiher speeds ma!es natural fre&uency speed dependent) *iii+

bearins and seals also ma!es natural fre&uency of the rotor system speeddependent) moreover) it also ma!es system unstable) *iv+ the asymmetry in rotors

due to operational re&uirements *such as !eyways or slots in rotors+ causes the rotor

instability) *v+ the internal dampin *hysteretic or friction between two matin parts

in rotors+ ma!es the system unstable) and *vi+ there are several other reasons for the

instability due to wor!in ,uid interactions with rotor components *e..) blades+.

Aforementioned reasons ma!e the rotor dynamics more challenin as compared to

the structural dynamics.

Rotatin machinery have applications with varied speed of operations: -$ rpm for

cement factory !ilns) -/// rpm for steam turbine$enerator) 0/)/// rpm for %et$

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enines for aero planes) /) /// rpm for cryoenic$fuel pumps in roc!ets) to 1) //)

/// rpm for vacuum pumps for centrifues. (imilarly) these rotatin machineries

have varied power capacities: /.$- 2 for household appliances) / 32 for %et$

enines) 4//$10// 32 for steam turbines. 5ependin on the application the lenth

of the rotor could be as lon as / m *for steam turbine$enerator+) 0 m for %et

enines) /. m for cryoenic pumps and few centimeters for helicopters.

A rotoris a body suspended throuh a set of cylindrical hines or bearins that allow

it to rotate freely about an a6is 6ed in space *7is. 1.1 and 1.0+. Rotors can often be

represented as a sinle beam or series of beam elements and riid discs. 8he beam

is fre&uently considered as ,e6ible. Riid discs are mounted on the ,e6ile beam

either by shrin!$t or by other mechanical means. 9ractically) a riid disc model

represents ,ywheels) blades) cran!s) rotary wins) couplin) disc bra!es) impellers)

rollin bearins) etc. nineerin components concerned with the sub%ect of rotor

dynamics are rotatin components of machines) especially of turbines) enerators)

motors) compressors) blowers and the li!e. 8he parts of the machine that do not

rotate are referred to with eneral denition of thestator. 8he machine element that

allows relative motion of the rotor relative to the stator is called thebearing. Rotors of

machines have) while in operation) a reat deal of rotational enery) and a small

amount of vibrational enery. It is very evident from the fact that a relatively small

as turbine propels a hue aircraft. 8he purpose of rotor dynamics as a sub%ect is to

!eep the vibrational enery as small as possible. In operation rotors underoes the

transverse *lateral or bendin+) lonitudinal *a6ial+) and torsional *or twistin+

vibrations; individually or in combination.

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7iure 1.1*b+ A close view of a rotor consistin of two discs mounted on a ,e6ible

shaft

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