VIBRATION DIAGNOSTICS OF METALLURGICAL EQUIPMENT

G.N. Subbotin, A. Zaglubotskij
“SSM (Severstalmash – engineering center),”

A.Yu. Azovtsev
Association VAST, Ltd.,

Presented on the COMADEM 2003 international conference, Vaxjo, Sweden.


Copyright (ñ) 2003 G.N. Subbotin, A. Zaglubotskij, A.Yu. Azovtsev, all rights reserved

ABSTRACT

On the metallurgical plants heavy-loaded slowly rotating rolling element bearings are used. Their limited service resources lead to frequent maintenance and repair of the equipment. To make a long term bearing condition forecast it is necessary to detect and identify the incipient faults in the first half of the bearing's service life.

In this case the most efficient method of early fault detection is the high frequency vibration analysis as its parameters are changed most quickly in the beginning stage of the defect development. The shock pulse and acoustic emission methods are not recommended for the diagnostics of slowly rotating high loaded bearings as during slow rotation even in the bearings without faults the film of lubrication fluid is disrupted and hence many shock pulses occur.

The “Severstal” analysts used for early defect detection the methods of envelope spectrum analysis of the high frequency vibration, excited by the groups of shock pulses, using the most popular and highly recommended in Russia systems designed by VAST, Ltd. The most loaded elements of the mould are the bearings of the tilting mechanism’s eccentric shafts. Before 1998 these bearings were replaced each quarter because of frequent failures while the calculated service life of the bearings was one year (50 million revolutions).

The methods enabled to detect all the mounting and wear defects. The diagnostics was made after mounting and each two weeks during operation. Last years the number of emergency work stoppages decreased by 80%, turnaround time increased in three times. A significant part of the bearings is now replaced after 10,000 castings (108*106 revolutions) that in eight times exceeds the service life before diagnostics and twice the calculated service life. A system for continuous- diagnostics of the casting mould’s rolling element bearings is discussed.

 

Low speed rotation rolling element bearings are often used in the metallurgical industry equipment. They are working with heavy static load and thus have limited service life. The diagnostics of such bearings is one of the main goals of diagnostic department on “Severstal” plant.

While choosing the methods for diagnostics of such bearings we had to abandon the use of such well known methods as the temperature control, shock pulse method, acoustical emission detection because of the possibility to omit dangerous defects and high possibility of false alarms. The most number of false alarms occurred when the shock pulses were detected and analyzed as when the rotation speed is less than 1-2 Hz then the lubrication film in a loaded bearing becomes unstable and the shock pulses appear even in no faulted bearings.

The autospectra of the equipment’s low frequency vibration could not be used for diagnostics as on the same foundation usually are mounted several machines with very close rotation frequencies and in each measurement point there are many harmonic vibration components including those that are excited by different bearings. So the low frequency vibration spectra were measured and analyzed only to estimate the danger of operation of the bearings with severe defects.

The best results in rolling element bearing diagnostics were gained by using envelope spectra of random vibration with the frequencies about 3-5 kHz. The vibration power, excited by shock pulses, when the lubrication film is unstable, depends on the frequency of the appearance of these pulses. During the contacts of defective parts of the rolling surfaces this frequency increases. So the easiest way to detect the rolling surfaces with increased wear or cracks even on the earliest part of their development is to analyze the periodical increase of random vibration power.

Below are described as an example the results of periodic measurements of envelope spectra of the rolling element bearing vibration. The bearings belong to the continuous-casting mould’s tilting mechanism (MTM). The diagram of (MTM) is shown on the Fig. 1. It consists of a welded cage 1 that is mounted on a parallel link that includes two shoulder arm 2, and kinematical arms 5, 6. The two shoulder arm makes the swinging movements around the axis 7 on the cage of the replacement unit. This movement is transferred to the two shoulder arm from the electric drive with eccentric shaft 4 via the link 3.
 
Fig. 1. Kinematical diagram of the mould’s tilting mechanism (MTM)

The MTM eccentric shafts’ rotation frequency is 120 RPM.

Each line of continuous-casting has 2 MTM and 7 more of them are in reserve or in repair. Emergency shutdown of the continuous-casting lines most often occur because of the MTM eccentric rolling element bearings’ failures. So though the calculated service life of the bearings is about 1000 castings with the duration of each of them 90 minutes, to eliminate the emergency shutdowns, before the diagnostics was implemented, the predictive maintenances with the replacement of bearings were made each three months, i.e. each 1000 castings.

The MTM bearings’ vibration diagnostics was implemented in 1998. During the turnaround time such diagnostics was made about ten times with the periodicity 7-14 days. For each bearing a vibration autospectra in the frequency range 0-50 Hz and 0-800 Hz and envelope spectrum in third octave frequency band with the center frequency 2500 were measured. The diagnostics of all 17 MTM took about 30% of the working time of one diagnost (expert in vibration diagnostics).

The experience in continuous monitoring and diagnostics of rolling element bearings’ condition of MTM has shown that the bearing’s service life is in a wide range (from 1000 to 15000 castings) and depends on many factors, particularly on the place and quality of its mounting.

For example one of the shortest service life had the eccentric shaft’s bearing of the MTM No 6 that operated only 4 months from 01.05.98 till 05.20.98. Fig. 2 shows the envelope spectrum of the bearing’s unit where the symptoms of rolling elements’ faults were detected already on the 03.27.98. In April 1998 the automatic diagnostic system gave the recommendation to replace the bearing and in the period from 05.20.98 till 06.12 98 the maintenance of the MTM was made with the replacement of the bearing.
 
Fig. 2. Envelope spectrum with the symptoms of rolling elements’ defects of the MTM No 6.

The visual inspection showed that one of the rolling elements was splited in 3 pieces (See Fig. 3).
 
Fig. 3. The MTM’s No 6 rolling element of the bearing is split in tree pieces.

One more example concerns a mistake in bearing mounting. The bearing was mounted on MTM No 6 and worked only 129 castings (from 12.11.99 till 12.22.99). Just after mounting the bearing the envelope spectrum showed a defect of its cage. After six days of operation the symptom of this defect increased significantly and additionally increased the high frequency vibration – the symptom of bad lubrication. (See Fig. 4)
 
Fig. 4. Envelope spectrum. A combined defect of the bearing on MTM No.6, a defect of the cage (FTF) and poor lubrication (the increase of the high frequency vibration level – increase of the envelope spectrum background)

After the bearing was replaced several defects were visible including the spallings on rolling elements and inner race, spallings on the cage and cavities on the rolling elements. (See Fig. 5).
 
Fig. 5. The bearing which spectra are
shown on Fig. 4. The spallings
and cavities on the rolling elements,
cage and inner race are seen

A special commission, called to discuss this case, decided that the cause of such early failure was a too small radial clearance that was only 0.014 mm.

The diagnostics of MTM rolling element bearings enable not only to detect the faults of the bearing itself but also to detect the defects of other units that apply alternating loads on these bearings. For example Fig.6 shows the bearings’ spectra of two MTM. In the first of them there are no symptoms of faults, but in the second one a defect of joint coupling was detected by the results of diagnostics. It was confirmed by visual inspection.
 
Fig. 6. Two spectra of MTM’s bearing. The first one belongs to a MTM without failure and the second one shows a defect of shaft coupling.

The symptom of a defect of the shaft coupling differs from the similar symptom of cavity on inner race of the bearing. Such defect type is detected often too, see for example an envelope spectrum of the rolling element bearing of the MTM’s eccentric shaft No 12 (Fig. 7.)
 
Fig. 7. An envelope spectrum of the bearing of MTM No.12 with a cavity on the inner race

The implementation of vibration diagnostic system (data collector DC-11 and software for automatic diagnostics DREAM®) and periodical rolling element bearing condition monitoring of MTMs’ bearings enables to eliminate the emergency shutdowns of the MTMs because of bearings’ failures. The number of periodic maintenances decreased from 33 in 1997 to 10 in 2000-2002.

Nowadays most bearings are replaced not after 1000 castings but after 10000 castings. This exceeds the calculated service life in more than two times. All the maintenances of the eccentric shafts are made according to their real condition estimated by the results of diagnostics.

The implementation of the diagnostics means gave a significant economic effect. The savings only on the MTM maintenance (not taking into account the consequences of the emergency shutdowns) paid off the diagnostic system after one year of operation.


Copyright (ñ) 2003 G.N. Subbotin, A. Zaglubotskij, A.Yu. Azovtsev, all rights reserved

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