By Mike Hastings, Brüel & Kjær Vibro
Chatter vibration can affect both the quality and production capacity of cold-rolled steel if it is not properly monitored and reduced. Third-order octave chatter is a self-generating resonance caused by the inter-reaction of the strip and mill, and can quickly reach destructive proportions if the speed is not reduced quickly enough. Fifth-order chatter is also a resonant condition, but it is most often excited by machine defects that can result in visual barring of the strip if the source of the excitation is not removed. Arcelor's (now Liberty Steel) cold-rolling mill in Tilleur, Belgium, has a five-stand tandem mill that is prone to third-order chatter and a four-stand tandem mill that is subject to fifth-order chatter. Brüel & Kjær Vibro’s vibration monitoring system was successfully used at the Tilleur location for both machine condition monitoring and product quality control of their tandem mills.
BACKGROUND
Global steelmakers are compelled to become more competitive. Although the overall world steel consumption is steadily increasing, individual mills are producing smaller lot sizes with more diversified products. This, plus faster production order processing and increasing quality requirements, put special demands on production machine uptime and maintenance. The speed of production and the condition of the cold-roll tandem machine components can limit the capacity and quality of production. The cold-rolling mill in Tilleur had set out to meet these challenges by implementing a comprehensive and integrated monitoring strategy.
THE COMPANY
The Tilleur cold-rolling mill is one of several plants involved in cold rolling and finishing processes in the Liége area. The Tilleur plant imports hot-rolled coils from mills in France and Luxemburg, and produces cold-rolled, full-hard mild carbon steel coils that are exported to nearby plants for further processing (galvanizing, annealing, tinplating and polymer coating). The coils are used in the packaging, appliance and automotive industries. As the only cold-rolling mill in the Tilleur area, the other finishing and coating companies are completely dependent on it. The four-stand, four-high tandem mill is used for making a single product for the automotive industry, but the five-stand mill is used for a range of different thin-gauge strip products for the packaging and appliance industry (mostly tinplate).
SURFACE CONDITION
Thin, cold-rolled steel is a surface-critical product with little tolerance for surface defects. There are two primary sources for steel product surface defects: Surface roll defects and chatter vibration. Defects on the rolls will directly transfer to the strip. The distance of the marks will be constant, no matter what speed the strip is moving. Roll defects could be caused by such factors as grinding faults in the roll itself, or deformation caused by the hydraulic clamping system.
The steel strip surface can also be visually or physically affected by roll vibrations called chatter. These vibrations are speed related, meaning the vibration changes as the strip speed changes. Chatter vibration is the result of a backup or work-roll resonance in one or more stands of the cold mill, which is excited by the elastic properties of the steel itself and/or by external forces from the machine.
There are four primary vertical vibration modes of a roll stand with four rolls. Only the second and third vibration modes can affect the surface quality of the steel, also known as the third- and fifth-octave vibration modes.
THIRD OCTAVE
The third-octave chatter vibration occurs at 110-170 Hz, which is the natural frequency of the backup rolls that resonate out of phase. It is a self-excited vibration instability condition that is created by the interaction of forces between the elastic strip together with the resonant characteristics of the mill structure in the form of a positive feedback loop. It is a function of the inter-stand properties of the steel product such as the strip tension, thickness, strip speed and other factors such as the type of steel, strip width, amount of gauge reduction, friction factor, roll force, roll bite, and lubrication.
The build-up of vibration at the natural frequency of the mill is regenerative and therefore can reach destructive proportions in less than 5 seconds. It can result in unacceptable thickness variations of the strip and even strip rupture.
The primary method of control is reducing the strip speed, but the vibration levels can also change when other process changes are made such as the roll bending forces. Reducing strip speed has a negative effect on productivity, but it saves money by reducing the number of coils that are rejected, and by reducing downtime associated with strip ruptures and damaged rolls.
FIFTH OCTAVE
The fifth-octave vibration chatter occurs at 550-650 Hz, which is the natural frequency of the work rolls that resonate in phase. It generally occurs in the highest speed stands. If the vibration level is not reduced, it can spread to the other stands via the strip. There are many potential causes for exciting the fifth-octave vibration. It is sometimes necessary to perform modal analysis with an instrumented hammer to find the external forces that are exciting the resonance.
Often, the external forces that excite the fifth-octave mode resonance are related to problems with the rolls, motor, shafts, bearings and gearbox. This could be grinding the rolls with small imperfections, or faults in the roll machine itself such as spindle and liner clearances, work roll bearing defects, hydraulic pressure system on rolls, vibration from worn pinion gears, gear mesh frequencies, passage of key ways in backup roll necks through the load zone, and unbalance.
UNWANTED ROLL MAKING
Over time, the backup rolls become marked by chatter vibration where the continued presence of the resonant vibration slowly wears transverse marks into the roll surface. The process is exacerbated if the frequency of the forced vibration excites mill resonances in the fifth-octave range. If the backup rolls are not changed at this time, the chatter-marked rolls will transfer these to the work roll and to the strip itself as transverse bands that can be seen on both sides of the strip without thickness variation. Chatter frequency can be calculated by dividing the strip speed by the banding bar spacing in like units.
Controlling this chatter vibration level can be done superficially by changing speed or roll pressure, but it is more a question of identifying and then isolating or removing the external vibration source that is exciting the resonance frequency. Corrections must be made if it is caused by excessive journal bearing clearance or a fault in the motor, gearbox or drive assembly.
POTENTIAL SOLUTIONS
If the external frequency excitation is caused by work-roll bearing defects, there are a couple of solutions. If the bearings are exhibiting a brinelling-type fault (many points of impacts on the race), the bearing can be put on a different stand where there is no fifth-octave mode vibration until it can be replaced at a convenient time.
If the fault is a spall (a single impact flaw on the race), this can be rotated out of the load zone. If the backup rolls have been subjected to high levels of vibration over a period of time and are marked, these will have to be replaced (this is sometimes done several times per shift). The backup rolls could also be subjected to mechanical fatigue due to prolonged operation within its resonance frequency. To prevent this, one or both rolls can be replaced to change the mass and reduce or increase the natural frequency so it will no longer be excited by the external forced vibrations.
Condition and product quality monitoring strategy
An on-line monitoring system was used for the following purposes:
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Automatic condition monitoring – Early detection of developing faults in motors, bearings and gearboxes of the rolling mills. This is not only for minimizing machine downtime and maintenance costs, but also for minimizing the source of external vibration that could excite chatter vibration, a part of the product quality monitoring function.
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Diagnosis and analysis – Evaluate the nature and severity of the developing fault so maintenance can be planned ahead of time.
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Product quality monitoring – Identify 1x and chatter vibration early before it can affect the strip quality, and evaluate the accumulated severity of the vibration over time to determine when the rolls should be changed.
Product-quality monitoring is the primary objective for using a monitoring system on a cold-roll mill. It is important to monitor chatter vibration in order to (1) optimize steel production speed; (2) reduce strip thickness variations and visual markings; and (3) reduce strip breakage and strip waste
MONITORING CONFIGURATION
Forty transducers were installed on the five-stand mill, and 80 sensors were placed on the four-stand mill to detect and diagnose developing faults. Most of these are used for both monitoring the condition of the motors, bearings, shaft, gearboxes and rolls and also for identifying component fault frequencies that could excite fifth-octave chatter vibration.
For detecting chatter vibration frequencies, a single accelerometer is mounted on top of the backup roll pressure cylinder for those stands where there is a potential problem. The sensor would ideally be installed directly on the bearing chocks, but there is a risk that the sensors are damaged when rolls are replaced. In any case, the sensor mounted atop the stand has demonstrated that there is sufficient sensitivity to detect the relevant signals.
FAULT DETECTION, DIAGNOSIS AND CONTROL
The four-stand mill is running the same product at a constant speed and constant process conditions, and the strip thickness is greater than the products rolled by the five-stand mill. There is little risk for third-octave mode chatter vibration to occur on this mill, but it is, however, prone to fifth-octave mode vibration chatter. The frequency of interest is around 650 Hz. The main problem for analysis is understanding the mode of vibration conditions that lead to the problem. Modal analysis identified the roll resonance frequency and any external vibration frequencies that can excite the resonance frequency.
A 0.5-1000 Hz FFT spectrum and various FFT zoom spectra are used for diagnosis, determining the severity of chatter vibration and for identifying the new resonant frequency after a roll change. It can also be used to identify component fault frequencies that could excite the chatter vibration. FFTs with other frequency ranges are used for diagnosing faults of other machine components.
A 400-800 Hz bandpass measurement is used for automatic chatter detection and trending. It is useful for helping to determine when to change rolls. A 1x bandpass is also monitored to determine vibration caused by the defects in the roll itself.
The five-stand cold rolling mill produces a number of products that are all thinner than what is produced in the four-stand mill, and consequently more susceptible to third-octave mode vibration chatter. The frequency of interest is around 170 Hz. The fifth-octave mode vibration is not considered to be a problem for this rolling mill.
The vibration levels are different for different types of steel, so the monitoring system uses different monitoring classes for the basic steel products. When a particular type of coil is being rolled, the monitoring system is informed of this by a digital signal, which will automatically monitor the vibration levels to alarm limits for that particular steel strip. This provides early alarm detection.
AUTOMATION
The automatic process control system uses several proprietary process parameters that detect the onset of the chatter vibration quickly, and reduces the strip speed accordingly. It is possible to hear the chatter vibration after it has started. Speed reduction is only a few percentage points and is required for only a few minutes at a time, so there is no significant loss of production. The chatter vibration from the monitoring system is not directly connected to the speed control system, but both the alarm information and signal are automatically sent to the operators as verification. The operator can use the system to judge the maximum “safe” speed for running surface-critical material. It is sometimes enough to change rolls, thus changing the mass of the system.
As with the fifth-octave mode vibration, there is a bandpass measurement for automatic fault detection (80-170 Hz), a 1x bandpass for detecting roll faults and a FFT spectrum for diagnosis (0.5-200 Hz).
CONCLUSION
The Tilleur location’s primary function is to cold-roll steel coils, and the finishing plants in the area depend on the quality and reliability of this production. A machine condition monitoring system can be used for both condition and product quality monitoring of cold roll mills to meet this demand. Third-order chatter vibration can seriously affect the quality of the rolled steel and should be monitored in real time so action can be taken quickly to avoid this. The monitoring system is not interconnected to the DCS for automatically controlling strip speed to reduce third-order chatter vibration, but alarm relays and plots are used to confirm speed reduction. Early detection is important, and the monitoring system provides automatic alarming functions that can be customized for the particular product being rolled.
Tandem mill components should be continuously monitored to detect component defects and to identify potential sources of excitation for fifth order chatter vibration. These trends also provide a means for determining when back-up rolls should be changed.
About the author: Mike Hastings is a mechanical engineer who has worked as a design and field engineer across various industries throughout the United States and abroad, before moving to Europe. He has worked with Brüel & Kjær Vibro for 31 years and has written numerous articles and papers on condition monitoring.