are, without endangering reliability at work, some mechanical calculations
were done. During the process of starting and stopping at the end of the
cogs a certain difference is appearing in the tension
Δ
σ
= 700
. . .
760
MPa
in radial direction (yield, strain border
>
700
MPa, resistance to tearing
>
800
MPa). This calculation has shown that with taking into consideration
small radii at the bottom of the groove, some cracks may appear and their
sizes are measured by tenth of millimeter. At the critical area, during the
work at 3000 min
−
1
, there is radial tension of the values
σ
r
= 85
±
15
MPa
at zero load and
σ
r
= 125
±
15
MPa at maximum load. This calculation
with some of the assumptions has shown that zero load crack is growing
when its depth is
0
.
7
mm; at full load this value is
0
.
12
mm. During the first
inspection, after 55 days, first cracks could have been noticed. After fourth
stopping, cracks existed on 10 cogs. Depth of the cracks was between 24
and 42 mm (Fig. 8). After that, this rotor was put out of work because two
other were repaired.
In Fig. 8 you can see the depth enlargement of the cracks as the function
of work duration (time). Therefore we consider that the crack growth was
very little during the work lasting 12000 hours. The measured crack depth
was 52. . . 58 mm. The vibration behaviour of the turbogenerator was good,
the influence of normal changes of the drive parameters onto the vibration
behaviour was known.
Figure 9 presents the state of vibrations at the measuring places where
the biggest changes have been recorded. They are the vertical vibrations
on the exciting side and the found changes could be normally considered
as non-substantial, but taking into consideration the situation, it has been
decided that the rotor should be put out of oreration. The vibrations during
the machine halting slightly differed from those of the previous cases,
among them of course there were differences too.
The checking after the disconnection however showed that some cracks
very fast expanded during the last working period. Once it has been found
that some of them have the depth up to 230 mm (see Fig. 8), this rotor is
thrown away as an inapplicable one.
The alarm trend of vibrations.
The cogs cracks in the rotor 1 were
removed when its active part shortened and after that the rotor was again
switched on. The vibration behaviour of the turbogenerator was controlled
with the same attention as earlier when we had worked with the rotor 3.
During the next months the turbogenerator worked under full loading.
It has been noted that the generator vibrations constantly change in
one direction, in spite of the constant working conditions. It is important
especially for vertical vibrations on the turbine side (Figs. 10 and 11).
The same tendencies have also the other vibration values of the generator.
Without taking into consideration the fact that the vibration level was
not high, the changes of the amplitude and phase angles were extreme
and decision was made to put turbogenerator out of work. Its behaviour
during the process of stopping confirmed this decision, because the machine
was vibrating while it was passing through different phases of number of
ISSN 0236-3941. Вестник МГТУ им. Н.Э. Баумана. Сер. “Машиностроение”. 2009. № 3 115
