An introduction to predictive maintenance - part 8 pot

This extremely important factor can be used to evaluate many of the failure modes of continuous process lines. For example, the vibration profile resulting from the trans- mission of strip tension to the roll and its bearings can be used to determine proper roll alignment, strip tracking, and proper strip tension. Alignment Process rolls must be properly aligned. The perception that they can be misaligned without causing poor quality, reduced capacity, and premature roll failure is incorrect. In the case of single rolls (e.g., bridle and furnace rolls), they must be perpendicular to the pass line and have the same elevation on both the operator and drive sides. Roll pairs such as scrubber/backup rolls must be parallel to each other. 314 An Introduction to Predictive Maintenance Figure 14–25 Load from narrow strip concentrated in center. Figure 14–26 Roll loading. Failure-Mode Analysis 315 Figure 14–27 Typical vibration profile with uneven loading. Single Rolls. With the exception of steering rolls, all single rolls in a continuous- process line must be perpendicular to the pass line and have the same elevation on both the operator and drive sides. Any horizontal or vertical misalignment influences the tracking of the strip and the vibration profile of the roll. Figure 14–28 illustrates a roll that does not have the same elevation on both sides (i.e., vertical misalignment). With this type of misalignment, the strip has greater tension on the side of the roll with the higher elevation, which forces it to move toward the lower end. In effect, the roll becomes a steering roll, forcing the strip to one side of the centerline. The vibration profile of a vertically misaligned roll is not uniform. Because the strip tension is greater on the high side of the roll, the vibration profile on the high-side bearing has lower broadband energy. This is the result of damping caused by the strip tension. Dominant frequencies in this vibration profile are roll speed (1¥) and outer- Figure 14–28 Vertically misaligned roll. race defects. The low end of the roll has higher broadband vibration energy, and dominant frequencies include roll speed (1¥) and multiple harmonics (i.e., the same as mechanical looseness). Paired Rolls. Rolls that are designed to work in pairs (e.g., damming or scrubber rolls) also must be perpendicular to the pass line. In addition, they must be parallel to each other. Figure 14–29 illustrates a paired set of scrubber rolls. The strip is captured between the two rolls, and the counter-rotating brush roll cleans the strip surface. Because of the designs of both the damming and scrubber roll sets, it is difficult to keep the rolls parallel. Most of these roll sets use a single pivot point to fix one end of the roll and a pneumatic cylinder to set the opposite end. Other designs use two cylinders, one attached to each end of the roll. In these designs, the two cylinders are not mechanically linked and, therefore, the rolls do not main- tain their parallel relationship. The result of nonparallel operation of these paired rolls is evident in roll life. For example, the scrubber/backup roll set should provide extended service life; however, in actual practice, the brush rolls have a service life of only a few weeks. After this short time in use, the brush rolls will have a conical shape, much like a bottle brush (see Figure 14–30). This wear pattern is visual confirmation that the brush roll and its mating rubber-coated backup roll are not parallel. Vibration profiles can be used to determine if the roll pairs are parallel and, in this instance, the rules for parallel misalignment apply. If the rolls are misaligned, the vibration signatures exhibit a pronounced fundamental (1¥) and second harmonic (2¥) of roll speed. Multiple Pairs of Rolls. Because the strip transmits the vibration profile associated with roll misalignment, it is difficult to isolate misalignment for a continuous-process line by evaluating one single or two paired rolls. The only way to isolate such mis- 316 An Introduction to Predictive Maintenance Figure 14–29 Scrubber roll set. alignment is to analyze a series of rolls rather than individual (or a single pair of) rolls. This approach is consistent with good diagnostic practices and provides the means to isolate misaligned rolls and to verify strip tracking. Strip tracking. Figure 14–31 illustrates two sets of rolls in series. The bottom set of rolls is properly aligned and has good strip tracking. In this case, the vibration profiles acquired from the operator- and drive-side bearing caps are nearly identical. Failure-Mode Analysis 317 Figure 14–30 Result of misalignment or nonparallel operation on brush rolls. Figure 14–31 Rolls in series. Unless there is a damaged bearing, all of the profiles contain low-level roll frequen- cies (1¥) and bearing rotational frequencies. The top roll set is also properly aligned, but the strip tracks to the bottom of the roll face. In this case, the vibration profile from all of the bottom bearing caps contain much lower-level broadband energy, and the top bearing caps have clear indications of mechanical looseness (i.e., multiple harmonics of rotating speed). The key to this type of analysis is the comparison of multiple rolls in the order that the strip connects them. This requires comparison of both top and bottom rolls in the order of strip pass. With proper tracking, all bearing caps should be nearly identical. If the strip tracks to one side of the roll face, all bearing caps on that side of the line will have similar profiles, but they will have radically different profiles compared to those on the opposite side. Roll misalignment. Roll misalignment can be detected and isolated using this same method. A misaligned roll in the series being evaluated causes a change in the strip track at the offending roll. The vibration profiles of rolls upstream of the misaligned roll will be identical on both the operator and drive sides of the rolls; however, the profiles from the bearings of the misaligned roll will show a change. In most cases, they will show traditional misalignment (i.e., 1¥ and 2¥ components) but will also indicate a change in the uniform loading of the roll face. In other words, the overall or broadband vibration levels will be greater on one side than the other. The lower readings will be on the side with the higher strip tension, and the higher readings will be on the side with less tension. The rolls following the misalignment also show a change in vibration pattern. Because the misaligned roll acts as a steering roll, the loading patterns on the subsequent rolls show different vibration levels when the operator and drive sides are compared. If the strip track was normal before the misaligned roll, the subsequent rolls will indicate off-center tracking. In those cases where the strip was already tracking off-center, a misaligned roll either improves or amplifies the tracking problem. If the misaligned roll forces the strip toward the centerline, tracking improves and the vibration profiles are more uniform on both sides. If the misaligned roll forces the strip farther off-center, the nonuniform vibration profiles will become even less uniform. 14.2.7 Shaft A bent shaft creates an imbalance or a misaligned condition within a machine-train. Normally, this condition excites the fundamental (1¥) and secondary (2¥) running- speed components in the signature; however, it is difficult to determine the difference between a bent shaft, misalignment, and imbalance without a visual inspection. Figures 14–32 and 14–33 illustrate the normal types of bent shafts and the force pro- files that result. 14.2.8 V-Belts V-belt drives generate a series of dynamic forces, and vibrations result from these forces. Frequency components of such a drive can be attributed to belts and sheaves. 318 An Introduction to Predictive Maintenance Failure-Mode Analysis 319 Figure 14–32 Bends that change shaft length generate axial thrust. Figure 14–33 Bends that do not change shaft length generate radial forces only. Figure 14–34 Eccentric sheaves. 320 An Introduction to Predictive Maintenance Figure 14–35 Light and heavy spots on an unbalanced sheave. The elastic nature of belts can either amplify or damp vibrations that are generated by the attached machine-train components. Sheaves Even new sheaves are not perfect and may be the source of abnormal forces and vibra- tion. The primary sources of induced vibration resulting from sheaves are eccentric- ity, imbalance, misalignment, and wear. Eccentricity. Vibration caused by sheave eccentricity manifests itself as changes in load and rotational speed. As an eccentric drive sheave passes through its normal rotation, variations in the pitch diameter cause variations in the linear belt speed. An eccentric driven sheave causes variations in load to the drive. The rate at which such variations occur helps determine which is eccentric. An eccentric sheave may also appear to be unbalanced; however, performing a balancing operation will not correct the eccentricity. Imbalance. Sheave imbalance may be caused by several factors, one of which may be that it was never balanced to begin with. The easiest problem to detect is an actual imbalance of the sheave itself. A less obvious cause of imbalance is damage that has resulted in loss of sheave material. Imbalance caused by material loss can be deter- mined easily by visual inspection, either by removing the equipment from service or by using a strobe light while the equipment is running. Figure 14–35 illustrates light and heavy spots that result in sheave imbalance. Misalignment. Sheave misalignment most often produces axial vibration at the shaft rotational frequency (1¥) and radial vibration at one and two times the shaft rotational frequency (1¥ and 2¥). This vibration profile is similar to coupling misalignment. Figure 14–36 illustrates angular sheave misalignment, and Figure 14–37 illustrates parallel misalignment. Wear. Worn sheaves may also increase vibration at certain rotational frequencies; however, sheave wear is more often indicated by increased slippage and drive wear. Figure 14–38 illustrates both normal and worn sheave grooves. Failure-Mode Analysis 321 Figure 14–36 Angular sheave misalignment. Figure 14–37 Parallel sheave misalignment. Figure 14–38 Normal and worn sheave grooves. Belts V-belt drives typically consist of multiple belts mated with sheaves to form a means of transmitting motive power. Individual belts, or an entire set of belts, can generate abnormal dynamic forces and vibration. The dominant sources of belt-induced vibra- tions are defects, imbalance, resonance, tension, and wear. 322 An Introduction to Predictive Maintenance Figure 14–39 Typical spectral plot (i.e., vibration profile) of a defective belt. Figure 14–40 Spectral plot of shaft rotational and belt defect (i.e., imbalance) frequencies. Figure 14–41 Spectral plot of resonance excited by belt-defect frequency. Failure-Mode Analysis 323 Defects. Belt defects appear in the vibration signature as subsynchronous peaks, often with harmonics. Figure 14–39 shows a typical spectral plot (i.e., vibration profile) for a defective belt. Imbalance. An imbalanced belt produces vibration at its rotational frequency. If a belt’s performance is initially acceptable and later develops an imbalance, the belt has Figure 14–42 Examples of mode resonance in a belt span. [...]... goals A well-planned 3 28 An Introduction to Predictive Maintenance program should not be structured so that all machines and equipment in the plant receive the same scrutiny Typical predictive maintenance programs monitor from 50 to 500 machine-trains in a given plant Some of the machine-trains are more critical to the continued, efficient operation of the plant than others The predictive maintenance program... vibrationmonitoring capability and provides process parameter, visual inspection, and pointof-use thermography 3 38 An Introduction to Predictive Maintenance Operating Cost The real cost of implementing and maintaining a predictive maintenance program is not the initial system cost Rather, it is the annual labor and overhead costs associated with acquiring, storing, trending, and analyzing the data required to. .. have a built-in prejudice against the maintenance organization Many are convinced that maintenance is the root-cause of the plant’s poor performance If your justification package and program plan are defined in maintenance terms or you limit improvements to traditional maintenance issues, your chances for approval will be severely limited Division Management Total, absolute support of division managers is... plant Most plants can be cost-effectively monitored using a microprocessor-based system designed to use vibration, process parameters, visual inspection, and limited infrared temperature monitoring Plants with large populations of heat transfer systems and electrical equipment will need to add a full thermal imaging system in order to meet the total-plant requirements for a full predictive maintenance. .. supply these means as part of your plan Plant Management To a lesser degree, plant executives are driven by the same stimuli as those at corporate level Although they tend to have a broader view of plant operations, plant-level managers want to see justification couched in terms of total plant One other factor is critical to success at this level Most plant executives do not have a maintenance background... program and upper management of the plant A predictive maintenance program is not an excuse to buy sophisticated, expensive equipment Neither is the purpose of the program to keep people busy measuring and reviewing data from the various machines, equipment, and systems within the plant The purpose of predictive maintenance is to minimize unscheduled equipment failures, maintenance costs, and lost... the true cost of maintenance, this may be the most difficult part of establishing a predictive maintenance program At a minimum, your baseline data set should include the staffing, overhead, overtime premiums, and other payroll costs of the maintenance department It should also include all maintenance- related contract services, excluding janitorial, and the total costs of spare parts inventories The baseline... a total-plant predictive maintenance program is expensive After the initial capital cost of instrumentation and systems, a substantial annual labor cost is required to maintain the program To be successful, a predictive maintenance program must be able to quantify the cost–benefit generated by the program This goal can be achieved if the program is properly established, uses the proper predictive maintenance. .. 336 An Introduction to Predictive Maintenance vibration, process, and other data that will provide a viable predictive maintenance database Therefore, the system must be able to automatically select and set monitoring parameters without user input The ideal system would limit user input to a single operation, but this is not totally possible with today’s technology Automated Data Management and Trending... of data required to support a total-plant predictive maintenance program is massive and will continue to increase over the life of the program The system must be able to store, trend, and recall the data in multiple formats that will enable the user to monitor, trend, and analyze the condition of all plant equipment included in the program The system should be able to provide long-term trend data for . accompanied by noise and smoke, causing belts to overheat and be glazed in appearance. It is important to replace worn belts. 324 An Introduction to Predictive Maintenance The decision to establish. can generate abnormal dynamic forces and vibration. The dominant sources of belt-induced vibra- tions are defects, imbalance, resonance, tension, and wear. 322 An Introduction to Predictive Maintenance Figure. components of such a drive can be attributed to belts and sheaves. 3 18 An Introduction to Predictive Maintenance Failure-Mode Analysis 319 Figure 14–32 Bends that change shaft length generate