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 Introduction
Services & Analysis
Dynamic Balancing
Client List
Case Study
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DYNAMIC BALANCING OF PAPER MACHINE DRYER CANS BY IMPACT METHOD
Jan E. Borhaug, Ph.D.
The topic is specialized technical service, which has been commercially available since 1979. The program incorporates state-of-the-art structural dynamic technology with the widely accepted dynamic balancing by influence coefficient method. The resulting technique has been applied to 800-4,000 FPM paper machines with very favorable results.
1.0 GENERAL BACKGROUND
The impetus for undertaking development of an in-place dryer balancing program originated from observance of rapidly increasing vibration levels and decreased mechanical reliability in instances where significant machine speed-ups in the range between 800 and 4,000 FPM were made. During the initial investigation, continued reliance on statically balanced dryers was shown to be insufficient and specifically caused drastic increases in overall frame vibration levels accompanied by disproportional reductions in bearing lives, mechanical reliability and dynamic foundation loads throughout the machine. Conversely, vast improvements in bearing lives, gear stresses and overall run ability of the machine resulted from proper dynamic balancing of the dryers.
2.0 OBJECTIVE
The two techniques traditionally employed for dynamic dryer balancing are both costly and time consuming. The first method requires the removal, balancing and reinstallation of the dryer cans. This is very time consuming and can result in minimal reduction of the residual unbalance dependent on the balancing machine utilized for the process. The second method is an in-place balancing technique which requires the removal of the drive gear section and installation of an independent drive to conduct the balancing process. Once again, this can result in minimal reduction of the dryer residual unbalance dependent on the influence the drive gears add to the overall operating balance of the dryer cans. This technique also requires substantial amounts of downtime and manpower.
The Dynamic Balancing of Paper Machine Dryer Cans by Impact Method was developed to 1) subject all the elements of the dryer can, i.e., gear and dryer can, to a dynamic balancing procedure and 2) reduce the downtime to conduct the testing to with in normally scheduled maintenance periods. All testing is conducted using state-of-the-art signal sensors, signal conditioning and analytical testing procedures.
The final result is a two-plane balance correction, which renders dynamic residual balances within predetermined tolerances. Our present criterion is the ISO G6.3 standard, which is favored industry wide.
In more plain terms, this standard corresponds to a maximum residual CG unbalance of approximately 15 pounds in a single plane or 7.5 pounds in two planes (the residual CG unbalance is dependent of the weight of the dryer can and gear assembly) for a typical 60 inch diameter dryer can.
3.0 METHODOLOGY
A balancing project of this kind encompasses three distinct phases of testing as follows.
- Paper Machine Baseline Vibration Audit and Initial Unbalance Profile for all Dryer Cans.
- Complex Transfer Function Testing and Cross-Machine structural Response Dynamic by Sample Trial Weight Application.
- Supervision of Correction Weight installation.
The objectives of Phase I are two fold. First, a Baseline Vibration Audit of the paper machine is conducted to determine the overall mechanical reliability of the dryer, gears and felt rolls. This data will indicate any deficiencies in the rotating elements and their associated bearing, which would influence the dynamic response of the unbalance coefficient of the dryer cans.
Secondly, the Unbalance Profile is simple to determine the level of unbalance response for each dryer roll in the machine and identify the specific dryers whose unbalance response will have detrimental long-term effects upon gear mesh quality, bearing performance and frame vibration at increased operating speeds. The data is collected at fixed or variable speeds depending on test technique chosen and yields essentially the two degrees of freedom mode shape for each dryer roll at any operating speed. The results are presented in both graphical and tabular form in terms of radial acceleration, amplitude and phase response, and form the basis for selection of dryer rolls to be balanced.
Once the selection of subject dryers has been made, the project proceeds to Phase II in order to determine the unit response vectors for correction weights calculations. This requires establishing the Complex Transfer Functions for each dryer scheduled for balancing, and determining the cross machine structural dynamics influence coefficients via the trial weight application method.
Trial Weight Testing is generally conducted on two separate dryer sections; (defined from the Baseline Analysis) to confirm the cross-machine structural response dynamics indicated in the complex transfer function testing. The testing requires two separate test runs for a bottom and top dryer roll by placement of temporary weight attached to the dryer can. Therefore, a total of four separate runs for each section are necessary for this program. Further testing may be required on additional dry sections if the Baseline Analysis indicates variances in the global response matrix between independent dryer sections.
The Impact Test program is performed during a machine shutdown and can be completed with all dryers secured. During this procedure, each dryer is separately and externally excited by calibrated impacting simulating the input force caused by a rotating unbalance.
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The excitation spectrum and response are measured and analyzed simultaneously, and a set of Transfer Function spectra and Phase diagrams are produced. The procedure is repeated for each dryer roll scheduled to be balanced.
Following determination of the cross-machine structural influence coefficients and correlation between this data and the complex transfer function, the final correction weights can now be calculated for each dryer.
Installation of correction weights (Phase III) can be completed by any number of suitable schedules. In a long-term shutdown situation, all corrections may be made at once, or if continuous times are not available, corrections can be made during successive, normal shutdowns.
An increasingly more important feature of the methodology used in this approach is to identify global structural resonance's in machine frames and support structures. As machine speedups are becoming more aggressive (often increasing 25% to 50% above historical and design speeds) consideration of cross-machine, machine direction and vertical vibration modes becomes extremely important.
Several past cases have shown dynamic behavior of frame sections and their support structures that preclude satisfactory operation even under virtually perfect balance conditions. Such problems are caused by global mechanical resonance's with very high amplification factors and accompanying extreme vibration over narrow ranges of operating speed. When serious resonanceçs of these kinds are present, it is normally recommended that additional frame and structural analysis be conducted (Finite Element Analysis) and appropriate reinforcements are made before correction weights are specified and installed. This is by far the most practical long-term approach since with a resonant amplification factor of say, 20, the effective accuracy of residual unbalance required to meet ISO G6.3 is .375 pounds per balance plane at resonance speed. This is certainly obtainable but practically impossible to maintain long-term due to roll sag, eccentric condensate loads and a host of other changing factors.
It should be noted that structural analysis and retrofit design are not part of the balance quotation and are handled separately as a Phase II-1 if required.
The Phase II-1 testing program is a cursory investigation to determine the possible presence of a support structure resonance. The test program is a "Run-up Test" where two sections of the machine are monitored simultaneously at both the sole plate and upper frame structure. The paper machine is then operated at 200-FPM intervals until maximum present operating speed is reached. This data is then analyzed and graphic presentations of the dryer response as well as felt responses are documented as a dynamic modal deflection diagram. This test can be conducted during Phase I or II schedules and is normally quoted separately as requested or indicated.
4.0 TEST SCHEDULES
In summary of the above, it is estimated that a minimum of two separate field trips are required for test purposes. The first (Phase I) requires two to three days on on-site testing and tape recording followed by a three to five week laboratory effort for its completion depending on the size of the machine. Phase II is conducted during a long-term shutdown situation (16-24 hours) and would require about one hour of testing per each dryer scheduled to be balanced.
Though certain immediate results would be available from Phase II, computations in this phase are handled by a laboratory computer, which correlates the data from Phase I, and II and determines the proper size and location of the correction weights.
5.0 INSTALLATION OF CORRECTION WEIGHTS
The procedure for attaching internal correction weights is normally established during Phase II. We assist the client in their design, fabrication, and final installation of the initial few weights. The actual placement is very straightforward and can easily handled by plant maintenance personnel under Pretech supervision on the most suitable schedule. Both internal and external correction weight placement in available.
6.0 POST-REBUILD ANALYSIS
Post-Rebuild Analysis and Balance Verification is often required by our clients in order to document performance under new machine speed. These services are provided as Phase IV, as a separate quote to the regular program and is an abbreviated version of tests performed in Phase I. This kind of analysis is, in our opinion, imperative before and after major rebuilds to insure that all dynamic characteristics of the machine during post-rebuild operation are as expected.
A Post-Balance Profile is recommended to verify and if necessary provide trim-balance data to further improve the operating balance of the dryer cans.
7.0 CONCLUSION
Serious thought should be given to this unique Pretech analytical and correctional program for reasons of the significant reductions in downtime and the optimum results it can deliver. All of which leads to more effective overall plant performance.
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