Defective Gear Profiles If the gear set develops problems, the amplitude of the gear-mesh frequency increases and the symmetry of the sidebands changes. The pattern illustrated in Figure 14–18 is typical of a defective gear set, where overall energy is the broadband, or total, energy. Note the asymmetrical relationship of the sidebands. Excessive Wear. Figure 14–19 is the vibration profile of a worn gear set. Note that the spacing between the sidebands is erratic and is no longer evenly spaced by the input shaft speed frequency. The sidebands for a worn gear set tend to occur between the input and output speeds and are not evenly spaced. Cracked or Broken Teeth. Figure 14–20 illustrates the profile of a gear set with a broken tooth. As the gear rotates, the space left by the chipped or broken tooth increases the mechanical clearance between the pinion and bullgear. The result is a low-amplitude sideband to the left of the actual gear-mesh frequency. When the next (i.e., undamaged) teeth mesh, the added clearance results in a higher-energy impact. The sideband to the right of the mesh frequency has much higher amplitude. As a result, the paired sidebands have nonsymmetrical amplitude, which is caused by the disproportional clearance and impact energy. Failure-Mode Analysis 307 Figure 14–17 Sidebands are paired and equal. 308 An Introduction to Predictive Maintenance Figure 14–18 Typical defective gear-mesh signature. Figure 14–19 Wear or excessive clearance changes the sideband spacing. Improper Shaft Spacing In addition to gear-tooth wear, variations in the center-to-center distance between shafts create erratic spacing and amplitude in a vibration signature. If the shafts are too close together, the sideband spacing tends to be at input shaft speed, but the amplitude is significantly reduced. This condition causes the gears to be deeply meshed (i.e., below the normal pitch line), so the teeth maintain contact through the entire mesh. This loss of clearance results in lower amplitudes, but it exaggerates any tooth-profile defects that may be present. If the shafts are too far apart, the teeth mesh above the pitch line, which increases the clearance between teeth and amplifies the energy of the actual gear-mesh frequency and all of its sidebands. In addition, the load-bearing characteristics of the gear teeth are greatly reduced. Because the force is focused on the tip of each tooth where there is less cross-section, the stress in each tooth is greatly increased. The potential for tooth failure increases in direct proportion to the amount of excess clearance between the shafts. Load Changes The energy and vibration profiles of gear sets change with load. When the gear is fully loaded, the profiles exhibit the amplitudes discussed previously. When the gear is unloaded, the same profiles are present, but the amplitude increases dramati- cally. The reason for this change is gear-tooth roughness. In normal practice, the back- side of the gear tooth is not finished to the same smoothness as the power, or drive, side. Therefore, more looseness is present on the nonpower, or back, side of the gear. Figure 14–21 illustrates the relative change between a loaded and unloaded gear profile. 14.2.5 Jackshafts and Spindles Another form of intermediate drive consists of a shaft with some form of universal connection on each end that directly links the prime mover to a driven unit (see Figures 14–22 and 14–23). Jackshafts and spindles are typically used in applications where the driver and driven unit are misaligned. Most of the failure modes associated with jackshafts and spindles are the result of lubrication problems or fatigue failure resulting from overloading; however, the actual failure mode generally depends on the configuration of the flexible drive. Failure-Mode Analysis 309 Figure 14–20 A broken tooth will produce an asymmetrical sideband profile. 310 An Introduction to Predictive Maintenance Figure 14–21 Unloaded gear has much higher vibration levels. Figure 14–22 Typical gear-type spindles. Lubrication Problems Proper lubrication is essential for all jackshafts and spindles. A critical failure point for spindles (see Figure 14–22) is in the mounting pod that provides the connection between the driver and driven machine components. Mounting pods generally use either a spade-and-slipper or a splined mechanical connector. In both cases, regular application of suitable grease is essential for prolonged operation. Without proper lubrication, the mating points between the spindle’s mounting pod and the machine- train components impact each time the torsional power varies between the primary driver and driven component of the machine-train. The resulting mechanical damage can cause these critical drive components to fail. In universal-type jackshafts like the one illustrated in Figure 14–23, improper lubri- cation results in nonuniform power transmission. The absence of a uniform grease film causes the pivot points within the universal joints to bind and restrict smooth power transmission. The typical result of poor lubrication, which results in an increase in mechanical loose- ness, is an increase of those vibration frequencies associated with the rotational speed. In the case of gear-type spindles (Figure 14–22), both the fundamental (1¥) and second harmonic (2¥) will increase. Because the resulting forces generated by the spindle are similar to angular misalignment, the axial energy generated by the spindle will also increase significantly. The universal-coupling configuration used by jackshafts (Figure 14–23) generates an elevated vibration frequency at the fourth (4¥) harmonic of its true rotational speed. The binding that occurs as the double pivot points move through a complete rotation causes this failure mode. Fatigue Spindles and jackshafts are designed to transmit torsional power between a driver and driven unit that are not in the same plane or that have a radical variation in torsional power. Typically, both conditions are present when these flexible drives are used. Both the jackshaft and spindle are designed to absorb transient increases or decreases in torsional power caused by twisting. In effect, the shaft or tube used in these designs Failure-Mode Analysis 311 Figure 14–23 Typical universal-type jackshaft. winds, much like a spring, as the torsional power increases. Normally, this torque and the resultant twist of the spindle are maintained until the torsional load is reduced. At that point, the spindle unwinds, releasing the stored energy that was generated by the initial transient. Repeated twisting of the spindle’s tube or the solid shaft used in jackshafts results in a reduction in the flexible drive’s stiffness. When this occurs, the drive loses some of its ability to absorb torsional transients. As a result, the driven unit may be damaged. Unfortunately, the limits of single-channel, frequency-domain data acquisition prevent accurate measurement of this failure mode. Most of the abnormal vibration that results from fatigue occurs in the relatively brief time interval associated with startup, when radical speed changes occur, or during shutdown of the machine-train. As a result, this type of data acquisition and analysis cannot adequately capture these transients; however, the loss of stiffness caused by fatigue increases the apparent mechanical looseness observed in the steady-state, frequency-domain vibration signature. In most cases, this is similar to the mechanical looseness. 14.2.6 Process Rolls Process rolls commonly encounter problems or fail because of being subjected to induced (variable) loads and from misalignment. Induced (Variable) Loads Process rolls are subjected to variable loads that are induced by strip tension, track- ing, and other process variables. In most cases, these loads are directional. They not only influence the vibration profile but also determine the location and orientation of data acquisition. Strip Tension or Wrap. Figure 14–24 illustrates the wrap of the strip as it passes over a series of rolls in a continuous-process line. The orientation and contact area of this wrap determines the load zone on each roll. In this example, the strip wrap is limited to one-quarter of the roll circumference. The load zone, or vector, on the two top rolls is on a 45-degree angle to the pass line. Therefore, the best location for the primary radial measurement is at 45 degrees opposite to the load vector. The secondary radial measurement should be 90 de- grees to the primary. On the top-left roll, the secondary measurement point should be to the top left of the bearing cap; on the top-right roll, it should be at the top-right position. The wrap on the bottom roll encompasses one-half of the roll circumferences. As a result, the load vector is directly upward, or 90 degrees, to the pass line. The best loca- tion for the primary radial-measurement point is in the vertical-downward position. The secondary radial measurement should be taken at 90 degrees to the primary. 312 An Introduction to Predictive Maintenance Because the strip tension is slightly forward (i.e., in the direction of strip movement), the secondary measurement should be taken on the recoiler side of the bearing cap. Because strip tension loads the bearings in the direction of the force vector, it also tends to dampen the vibration levels in the opposite direction, or 180 degrees, of the force vector. In effect, the strip acts like a rubberband. Tension inhibits movement and vibration in the direction opposite the force vector and amplifies the movement in the direction of the force vector. Therefore, the recommended measurement-point loca- tions provide the best representation of the roll’s dynamics. In normal operation, the force or load induced by the strip is uniform across the roll’s entire face or body. As a result, the vibration profile in both the operator- and drive- side bearings should be nearly identical. Strip Width and Tracking. Strip width has a direct effect on roll loading and how the load is transmitted to the roll and its bearing-support structures. Figure 14–25 illus- trates a narrow strip that is tracking properly. Note that the load is concentrated on the center of the roll and is not uniform across the entire roll face. The concentration of strip tension or load in the center of the roll tends to bend the roll. The degree of deflection depends on the following: roll diameter, roll con- struction, and strip tension. Regardless of these three factors, however, the vibration profile is modified. Roll bending, or deflection, increases the fundamental (1¥) frequency component. The amount of increase is determined by the amount of deflection. As long as the strip remains at the true centerline of the roll face, the vibration profile in both the operator- and drive-side bearing caps should remain nearly identical. The only exceptions are bearing rotational and defect frequencies. Figures 14–26 and 14–27 illustrate uneven loading and the resulting different vibration profiles of the operator- and drive-side bearing caps. Failure-Mode Analysis 313 Figure 14–24 Load zones determined by wrap. 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. [...]... important to replace worn belts 15 ESTABLISHING A PREDICTIVE MAINTENANCE PROGRAM The decision to establish a predictive maintenance program is the first step toward controlling maintenance costs and improving process efficiency in your plant Now what do you do? Numerous predictive maintenance programs can serve as models for implementing a successful predictive maintenance program Unfortunately, many programs... investment Unless your program can definitely pay for itself, it should not be implemented 334 An Introduction to Predictive Maintenance Frankly, most maintenance improvement programs will not pay for themselves Traditional applications of predictive maintenance, reliability-centered maintenance, total productive maintenance, and a myriad of others are not capable of generating enough return to justify implementation... generation, and analysis capabilities of the predictive maintenance program Therefore, care should be exercised during the selection process This is especially true if multiple technologies will be used within the predictive maintenance program Each predictive maintenance system will have a unique host computer specification that will include Establishing a Predictive Maintenance Program 341 hardware configuration,... achieved 15 .2. 3 Efficient Data Collection and Analysis Procedures Efficient procedures can be established if adequate instrumentation is available and the monitoring tasks are structured to emphasize program goals A well-planned 328 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. .. 330 An Introduction to Predictive Maintenance Because of the almost unlimited numbers and types of machinery and systems used in industry, it is impossible to cover every one in this book; however, Chapter 7 provides a cross-section that illustrates the process used to identify the monitoring parameters for plant equipment 15.3 SELLING PREDICTIVE MAINTENANCE PROGRAMS Justification of a predictive maintenance. .. 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 production It is also intended to improve the pro 325 326 An Introduction to Predictive Maintenance duction efficiency and product quality in the plant This is accomplished by regular monitoring of the mechanical... provide the basis for a long-term predictive maintenance program Automated Data Acquisition The object of using microprocessor-based systems is to remove any potential for human error, reduce staffing, and automate as much as possible the acquisition of 336 An Introduction to Predictive Maintenance vibration, process, and other data that will provide a viable predictive maintenance database Therefore,... thermography 338 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 determine the operating condition of plant equipment This is the area where predictive maintenance. .. the existing maintenance costs and other parameters that will establish a reference or baseline data set Because most plants do not track 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... situation would be to have the predictive systems vendor establish a viable database as part of the initial capital equipment purchase This service is offered by a few of the systems vendors Unfortunately, many predictive maintenance programs have failed because these important first critical steps were omitted or ignored There are a variety of beneficial technologies and predictive maintenance systems How . 309 Figure 14 20 A broken tooth will produce an asymmetrical sideband profile. 310 An Introduction to Predictive Maintenance Figure 14 21 Unloaded gear has much higher vibration levels. Figure 14 22 Typical. important to replace worn belts. 324 An Introduction to Predictive Maintenance The decision to establish a predictive maintenance program is the first step toward controlling maintenance costs and improving. 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