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MONITORING OF SHAFT CRACKS

Specifications subject to10 - gb_monitoring_shaft_cracks_V2 Rv2_24/04/2006 change

without notice

Risks related to the occurrence of cracks on lines of shafting are very important. Unfortunately theyare quite frequent. They develop rapidly as far as their size is concerned (a few days) and can lead tothe destruction of the machine.

There are many reasons for the development of cracks that affect rotors. The first reason is related tothe static deflection of the rotor (1

steigen bending pulse). Fatigue cracks can originate at the bottom of

turbomachine vanes. They can affect hoop fittings, keying, etc.

A crack generates stiffness anisotropy within the motor (smaller stiffness along the crack axis).

Let us assume that forces

r

r

Fz

Fyare exerted on the rotor and that there is no (yr, zr) coupling, then:

=

r

r

z

y

r

r

z

y

k0

0k

Fz

Fy

In a fixed coordinate system (observation reference point, bearing), the stiffness matrix becomes:

+

+

=

tcosktsink2

t2sin)k(k

2

t2sin)k(ktsinktcosk

K2

z2

yy-z

z-y2

z2

y

zy,

In this simple model, we can see components at 2. In a more realistic model (cracked rotor with a

disk), components also exist at 3.

yr

kz

With k

C

Fy

Fz

ky

zr

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Notes

Cracks have little effect on eigen pulses. Eigen frequencies should be controlled (verticallysuspended rotor) for different azimuths. Eigen frequency deviations are very small (unlikedeviations resulting from hooping problems).

It is useful to know the eigen pulses of the rotor on its bearings for speeds ranging from 0 to 3.5

max since they can be excited for harmonics 2 and 3 of the rotation frequency on a run-up or acoast-down phase.

Typical Bode diagrams of vibrations for a cracked rotor are as follows:

represents the angular position of the crack with respect to unbalance.

o is the first eigen pulse on an uncracked rotor on isotropic rigid linkings.

Caution!

Vibrations at 2 and 3 can also have various origins:

- non linearity- misalignment- rotor distortion- rotor-stator contact- vane breakage blading defect

Here are simple diagnosis rules:

IF vibrations (amplitudes and phases) at 2

and 3

are in constant evolution AND : vibrations at are stable average shaft positions (GAP) are stable

THEN : crack problem is likely

IF vibrations at , 2 and 3 develop rapidly (a few days) :THEN: crack problem is likely

IF vibrations at 2 and 3 are in constant evolutionAND:

they are important compared to those observed at

(precession) orbits at 2 and 3 are close to circumferences

isotropic bearings, e.g., 4 oscillating padsTHEN : crack problem.

Uncracked shaft

Cracked shaft =

Cracked shaft =/2

Cracked shaft =0

0/3 0/2 0 Rotation speed

E