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遼寧科技大學(xué)本科生畢業(yè)設(shè)計(jì) 第6頁(yè)
Identification and Countermeasures to Resolve Hot Strip Mill Chatter
1 Introduction
Quality and productivity are major requirements for any rolling mill. Physical and metallurgical properties and surface appearance of the finished product are goals shared both by the operating and maintenance personnel. All equipment must function properly to achieve these goals.
In both hot and cold rolling processes, mechanical vibrations called chatter can be observed under certain condition, in most cases audibly. As a result of this chatter, transverse marks covering the strip width are impressed at the exit of the considered mill stand. The occurrence of this phenomenon in the last stands of cold rolling mills has been reported by several researchers, but information is not as widespread foe hot mills. The problem is highly undesirable, as it not only changes strip surface appearance, but creates gauge chatter in sever case.
Siderar’s 66-inch hot strip mill consist of four reheating furnaces, a condition roughing mill, a descaling mum speed of 640 meter per minute, Product ranges from 1.6 to 12.5 mm in thickness and 560 to 1525 mm in width. A schematic of the hot strip mill facility is shown in Fig. During the processing of light gauge and narrow tinplate product, a repetitive vibration problem occurred. Consequently, work roll wear increased, affecting mill productivity. Reject rates also increased. This problem was only when rolling strip with rolling speeds close to 120meters per minute in stand F2. In general, the vibrations were accompanied by a low-frequency audible humming of variable intensity.
In Aug. 1997, Siderar decided to carry out an intensive campaign of measurements and data correlation with calculation models to detect chatter origin. From previous work, a series of measurements was made to relate rolling speed and number of spindle teeth with observed vibration characteristics.
To improve understanding of the phenomenon at Siderar’s rolling mill, an additional set of measurements and a mode analysis study were performed. On Feb a second set of measurements was carried out for achieving a spectral characterization of the vibrations in the whole stand, as well as the existing correlation among different points, including an analysis of the kinematic system and rolling for determination of the assembly’s natural frequencies and validation of calculation models. In the latter case, a modeling of the stand by means of finite element (FEM) was developed, along with simplified models for the calculation of torsion frequencies in the drive system and vertical roll vibrations.
In May 2000, Anther set of measurements was carried out on stands F2 and F3, during rolling of tinplate bands.
Results obtained from calculations and their comparison with measured values, as well as an explanation of the observed phenomenon observed on these results, are presented.
2 Description of the problem
The chatter problem in the finishing mill at Siderar dates back to the mill’s start-up, and experienced a sizeable increase due to two factors:
(1) Modifications were introduced in the F2 and F3 stands-housing windows were machined for installation of mobile blocks for roll shifting and roll bending.
(2) Increased demand for surface critical product, free of chatter defects. The conditions of maximum vibration appeared when light gauge and thin material was being rolled, material for tinplate, or in material rolled late in the rolling schedule, such as 1020 mm wide and 1.60 mm thick.
Appearance of the phenomenon could be sudden, in some cases during the first strips of the rolling schedule, and in other case after a gradual evolution, reaching its maximum level in the middle of the rolling. As the chatter affected the fist stands, subsequent passes in the F4, F5 and F6 stands corrected surface variations. In the most unfavorable cases, a partial roll change was required based on observed wear on finished product samples.
The removed rolls showed chatter on the whole surface width, with varying intensity and number of marks related to the vibration frequency. Similar to others, chatter marks were observed on the upper surface of the strip at exit of stands F2 and F3, but the roll most affected by irregular wear was he lower roll.
3 Measurements performed
Three sets of measurements were performed: Aug 1997, Feb. 1998 and July 1998. The first two characterized the vibratory behavior of the F2 stand via spectral analysis of signals measured with accelerometers in the locations indicate in Fig.3.
In the first data set only the linear spectra of each signal were determined, as the intention was determine if the primary cause of the chatter was spindle wear. In this case, analysis of the measured spectra was based on correlation of the characteristic frequencies of the kinematic system with the natural frequencies calculated. A characteristic spectrum is illustrated in Fig.
One observed frequency was approximately 40 to 41 Hz, which corresponds to the spindle teeth meshing during operation around 60 rpm. When thicker strips were rolled, the F2 speed is around 25% lower and no chatter was observed. Thus, it can be concluded that resonance was excited by a periodic force originating in worn spindle teeth. A higher vibration in the lower roll than the upper roll was due to a higher stiffness of the lower part of the housing compared with the upper part.
Additionally, the high stiffness of the spindle more efficiently transmitted vibrations to the work roll.
Actions taken on elements of the F2 kinematic system and process variables resulting from the first set of measurements are presented in Table.
TO validate the results, a second and more complete set of measurements was performed, including wave shape determination, spectral analysis and correlation of vibration from at different measurement points.
Measurement were obtained with accelerometers and proximity sensors positioned between the upper work roll and the mobile shifting blocks, together with a Hewlett Packard 3560A two-channel analyzer.
The measurements performed in July 1998 had the purpose of determining the natural frequencies of the F2 stand utilizing the impact technique.
Methodology for determining natural frequencies consisted of analyzing the real and imaginary parts of the estimated transfer function in each of the tests Fig shows the magnitude of the function for a vertical measurement on each stand. Resonance frequencies of 60.75 and 72.75 Hz are evident. In Fig, the magnitude of the transfer function, take in the vertical direction in presence of a horizontal excitation, is shown. In this case, the main resonances were within the range of 10 to 90 Hz and the one corresponding to approximately 39.5 HZ was excited.
In May 2000, another verification was performed under normal process conditions on stands F2 and F3, utilizing the same measurement points to evaluate the influence of the action items taken in Table. Results indicated a substantial improvement.
4 Results of the calculation model
To interpret the measurements, a modal characterization of stand F2 was carried out with different models. The main objective was to determine which part of the kinematic system enters into resonance during chatter, as spectra characteristics indicated a resonance condition. Furthermore, chatter marks observed on the rolled strip indicated that only vertical movements of the roll/stand assembly could produce them, as they covered the entire strip width. The predominant hypothesis was to search for a form of torsional vibration amplification in the spindle, due to its being the frequent cause of vibration in hot rolling. The most complex part of the calculations was determining normal frequencies and vibration modes of the whole stand structure utilizing FEM. The COSMOS structural analysis program was used, dividing the main part of the stand into finite three-dimensional elements of a hexahedron type, and simulating the rotating structural components with beam type element.
The model includes the spindle, upper separators and mass of the cylinder supports. Table shows the most important calculated frequencies. For example, the third mode of vibration at 35 Hz.
For estimation of the torsional frequencies of the drive assembly, from the work and backup roll through the kinematic system up to the motor, the drive assembly was modeled, dividing the assembly into approximately 1100 elements, the greatest number being for the spindle. The Holzer method was used to solve the problem. The frequencies of interest are indicated in Table. A sensitivity study was performed, changing component masses and moments of inertia. Little influence on calculation results was observed, indicating that for changing the natural frequencies it would be necessary to make major modifications to the assembly, the vertical vibration frequencies of the stacked assembly of the four rolls. This was based on a scheme similar to that utilized in reference. Main results are shown in Table.
5 Interpretation of the measurement results
Field measurements taken during rolling of noncritical material revealed predominant frequencies of 13, 17, 32 and 55Hz, none of which excites any of the characteristic frequencies of the stand.
During narrow strip rolling, a predominant frequency of 35.7Hz occurs. On the other hand, among the natural frequencies measured via the impact technique, those of interest are: 39.5 and 60.5Hz, representing a vertical movement of the stand; 84.5Hz,correspond to the vertical movement of rolls; and approximately 41Hz, resulting from tangential impact on the coupling between the work roll and the pinion stand, using a sensor tangential to the shaft.
Vibrations result when friction condition in the arc of contact are unstable (i.e, where the friction coefficient decreases as speed increases). When a disturbance occurs, torque is increased torque, producing a self-sustained vibration. This result relates to high reductions and conditions of friction with slip.
Focus generally falls on torsional frequencies of 40 to 41 Hz and vertical vibration of the stand at 35 Hz. These values, when associated with the 37.5 Hz frequency measured during chatter when rolling material for tinplate, allow for a simplified description based on the following hypothesis for chatter characteristics at Siderar.
Vibration is incited when strip enters the stand and is probably roduced by inadequate lubrication (friction coefficient), yielding torsional vibrations of the spindle. These torsional spindle vibrations excite a vertical vibration mode of the stand, which actuates as a movement amplifier, likely corresponding to vibration mode 3 in Table calculated by the finite element model. This movement starts making the roll. Once this occurs, the effect is perpetuated by vibration of chattered work roll.
6 Couclusions
Successfully suppressing vibration in hot rolling starts with perfect identification of the source. Analytical tolls and field measurements performed allowed Siderar to converge on the obtained result.
Chatter is a peculiar self-sustained oscillatory movement that appears during the operation of rolling mills, due to interaction between the dynamic structure of the equipment assembly and the rolling process itself.
Sensitivity analyses revealed the practical impossibility of modifying the moments of inertia of the most compromised element.
The most efficient countermeasures were lubricated rolling, decrease of the clearance of the mobile assemblies and modification to the frequency of spindle changes. As a result of these corrective action, hot strip mill vibration decreased considerably.
When spindles are in phase, a correlation among reduction, speed and strip temperature develops.