With the introduction of laser alignment tools, it has become standard operating procedure to perform precision shaft-to-shaft alignment when machines are installed or reinstalled after a rebuild. Before the introduction of laser alignment tools, harmful soft foot conditions were often not addressed.
There are two reasons for this. One was that before laser tools, the soft foot condition was more difficult to measure precisely, and secondly, many did not fully understand the consequences of an uncorrected soft foot condition.
Today, the standard tolerance calls for correcting any soft foot in excess of .002 inches. Since this is smaller than the diameter of a human hair, it is difficult to realize how such a small error could cause a problem. Suppose that a soft foot of .004 inches (in terms of shaft movement as extrapolated to foot movement) is present in a motor foot. This is two times the standard, and if we tighten the motor foot down without correcting the soft foot, the motor foot will not be damaged. In fact, if we were to loosen the foot after a year, it would return to its original position because .004 inches deflection will not exceed the elasticity of the foot material. So, why is the standard for soft foot a maximum of .002 inches? We have learned that the damage is not usually to the machine foot but to the interior of the machine. Power is induced from the stationary part of a motor across an air gap into the rotating part in order to produce torque on the rotating part. The air gap is kept as small as practical in order to make the motor as efficient as possible. The induced forces are magnetic (electromagnetic and electrostatic), and any variance in the air gap causes changes in the magnetic forces. If the distance between two magnets is reduced by half, the forces are quadrupled. In other words, the magnetic forces become four times greater. Only a slight variance in the air gap can result in substantial differences in magnetic forces and current flow in the area of the variance. The uncorrected soft foot may distort the motor housing which, in turn, may distort the stator iron, resulting in reducing the air gap. The stronger magnetic forces in the area of the smaller air gap produce an increase in localized currents that generate heat. For every 10 degrees centigrade temperature rise in a motor, insulation life is cut in half. Infrared technology may be used to detect the heat resulting from an uncorrected soft foot condition. Figure 1 displays an exaggeration of this condition.
When laser alignment tools were first introduced, they were used only as corrective tools. Meaning that machines were precision aligned when installed, and the alignment was not considered again until problems arose or until vibration measurements indicated misalignment. However, we soon learned that the laser tools could also be used as mechanical detectives. Their speed and ease of use made it possible for us to write procedures for performing alignment checks on machinery. Once aligned, a machine should remain in alignment and these checks help to ensure that precision alignments endure. If a scheduled check indicates misalignment, we have detected other problems that were previously hidden. A fault is present, or the machine would have remained in alignment. Causes for the machine becoming misaligned must be determined and corrected. The alignment check can also be used to verify vibration measurements that may confuse misalignment with other problems. The combined technologies provide definitive answers.
Ralph Buscarello, founder of Update International, deserves credit as the father of precision in industrial maintenance. Mr. Buscarello advocated precision in machine balancing and parts fitting before most of us fully understood the true value of precision. We were slow to embrace precision concepts because we had difficulty measuring the benefits of precision.
With regard to precision, let’s discuss “the point of no return.” This is where the effort or investment in achieving a better level of precision costs more than the value gained from the greater precision. This point will always exist, but knowing when we reach this point can be difficult to determine. However, new advances in condition monitoring technologies have helped us push “the point of no return” toward higher precision. PdM technologies provide us with the ability to better measure the value of precision and make greater precision easier to obtain. Before these tools came onto the market many of the available tools for detecting and correcting machine faults were either difficult to use, required too much time or training, or provided minimal results.
Early vibration measuring tools detected only overall vibration amplitudes and were very limited in diagnostic capabilities. Even their detecting abilities were limited to finding faults or potential failures in advanced stages. Today’s technologies make it possible to detect sub-surface flaws in bearings — flaws not even visible in a magnified view of the bearing surface. How is it possible to detect bearing faults that aren’t visible even when magnified? Let’s first consider one possible way that bearings fail. When a ball or roller passes through the load zone of a bearing raceway, the raceway deflects under the pressure of the rolling element. After millions of such cycles, the crystalline material of the metal race will begin to separate due to fatigue wear. When the metal molecules separate, they rub against one another, producing small stress waves. This happens each time the rolling elements pass over the area of separation. These stress waves can be detected with vibration transducers. Signals from the transducers are used for generating plots or displays that are essential for analyzing various types of faults.
Why would we even want to detect faults in equipment that are invisible to the eye or undetectable by our other senses? Because the earlier a fault or potential failure can be detected, the more time one has to take action to prevent the fault from progressing to a failure, which may result in loss of production or collateral damage. Fault detection is only worthwhile if there is time to take action before the fault results in failure. Figure 2 shows a typical failure curve. The “P” point on the curve represents the time when the fault or potential failure is first detected. The “F” point represents the time of failure. Precision maintenance practices delay the onset of the fault and hence move both the “P” and “F” points farther out in time. The effect of higher precision in fault detection is to put more distance between the “P” and “F” points because the “P” point is detected earlier in time. This provides more time to take action after a fault is detected. Combining PdM or condition monitoring technologies can greatly enhance the likelihood of earlier fault detection.
Problems with low speed machines were very difficult to detect, and even more difficult to analyze, when the available vibration tools measured only overall vibration amplitude levels. Through the use of high frequency technologies, it is now possible to detect problems in rotors that have fractional rotation speeds. Detecting problems buried deep inside complex gear units is also made much easier by these same technologies.
Modern PdM technologies have also greatly improved our ability to establish root cause. Before the advent of modern PdM technologies, most machine faults were undetected, resulting in complete and sometimes catastrophic failure. Finding the cause of these failures was often very difficult because much of the evidence was destroyed during the failure. An example would be bearing failures. Bearings that are removed from service in an early stage of failure due to a PdM alarm can easily be inspected to determine fault causes. If bearing faults are allowed to progress to stages where the metal is plasticized by heat and the rolling elements are deformed or welded to the races, the cause of failure is much more difficult to determine because much of the evidence of the early fault has been obliterated. Establishing root cause is critical to preventing repeat failures, and early fault detection preserves the data on which we base our preventive maintenance decisions.
Lubrication is another area where PdM technologies improve the precision of the process. The science of lubricants is well-founded in many years of experiment that have helped to continually improve products. We have developed lubrication procedures that use formulas to precisely measure how much lubricant to inject into various bearings. But, the science of applying lubricants to bearings has remained mostly an art because we rarely know how much lubricant is already in bearings when we lubricate. Many factors can enter into the calculations for how much lubricant to apply to a specific bearing, including, bearing size, speed, load, environment, bearing type, vibration levels, and temperature. The calculations can also be quite confusing. After making calculations, we apply what we hope is the proper amount. Yet without knowing how much lubricant is already present in the bearing, we are never quite sure, so we give the bearing what we think it needs. But, now by using modern ultrasonic (sound above human hearing range) lubrication techniques, the bearing now tells us how much lubricant it needs. Different lubricant film conditions emit different frequencies in the ultrasonic range. Ultrasonic probes process the ultrasonic frequency into frequencies that are audible. They also provide indications of the signals’ power levels. By applying lubricant as we listen to the bearing’s ultrasonic emissions, we let the bearing tell us when it has received the proper amount of lubricant. Ultrasonic lubrication changes lubrication from a time-based task to a condition-based task.
We have explored some of the authentic and positive benefits of modern PdM technologies, as well as some of the less apparent benefits. Learning about these benefits results in increased maintenance knowledge leading to improved machine reliability. Increased knowledge and greater reliability produce increased plant profitability.
Bill Hillman is a technical contributor for LUDECA, INC., vendor of alignment, vibration analysis and balancing equipment. He can be reached at 903-927-1962 or billcmrp@yahoo.com
