SPECIAL TECHNIQUES – ORBIT ANALYSIS

By aligning the phase marker transducer and the phase marker key way or projection,one can read off the high spot of the shaft.Below the first critical,this high spot indicates the position of unbalance and trial weight needs to be added at 180º.Above the first critical this high spot indicates the position at which the trial weight should be added.When two eddy probes are looking at a stationary shaftat 90 degrees to each other and displayed on the X and Y plates of an oscilloscope,an illuminated dot will appear on the CRT.This dot represents the centre-line of the shaft and will be seen to move in relation to the positionof the shaft.At slow roll the shaft centre-line will appearas a discrete dot but as the speed is increased,thecentre-line of the shaft is represented as a continuous circle known as a Lissajous figure.This is the actual shaftmovement within the bearings.If this Lissajous figure is projected onto the screen with a phase reference on theZ axis input then it is known as an orbit.

Shaft Pre-load (Misalignment -Aerodynamic Forces - Elliptical Bearings)

A pre-load is defined as a directional load or forceapplied to the rotating shaft.The immediate result of pre- load is to force the shaft into one sector of a bearing and results in non-linear restraint.This is where the spring constant is much higher in the opposite direction to the pre-load than in the perpendicular direction to the pre- load.This produces the classical twice per rev frequency associated with misalignment.This can be seen below:- Shaft-Unbalance Oil Whirl Shaft Pre-load Oil Whip Shaft Bow Shaft Rubbing The orbit and its resultant shape under variousconditions can be used to augment machinery surveillance and analysis.Certain machine conditions produce certain orbit shapes,thus a knowledge of the orbit can lead to detection of machine conditions.As the orbit is a direct display of the output from eddy probes any run-out be it electrical or mechanical will also be displayed.For meaningful orbit displays,then run-out should be minimised.The conditions that cause classical orbit shapes are as follows:-
Orbit from machine with misalignment Orbit showing oil whirlShaft unbalance orbit showingposition of high spot

Oil Whirl

The oil film within a sleeve bearing normally flows around with the journal surface to lubricate and cool the bearing. Film flow occurs because of oil shear viscosity.The speed of this oil film flow is slightly less than half the speed of the journal surface.During normal stable rotor conditions,the oil film separates at 180º from the minimum oil thickness.However,when Oil Whirl occurs there is an oil filmaround the 360º of the bearing.The orbit in the case of Oil Whirl is characterised below.The phase markers are approximately 180º apart and because it is slightly less than 180º the orbit is seen to slowly rotate in the direction opposite to that of rotation.This counter rotation is due to the sub-synchronous frequency of the Whirl at 43% to 46% of rotational speed.

Shaft Unbalance

An orbit which is essentially circular is usually generated by an unbalanced condition.It is the phase reference marker that indicates the high spot at any particular time and the dimensions of the orbit on the display that give an indication of the magnitude of unbalance.

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