- Buyer's Guide
Required machine reliability is placing increasingly higher demands on maintenance budgets. The common answer to this is to implement condition monitoring in some form. Traditionally this has been a vibration-based program that includes some other measurement parameter or technology such as oil analysis as a secondary consideration. This has brought successful results; practitioners of this simple form of condition monitoring have demonstrated financial benefits, proving the value of investment.
When machinery problems are resolved using vibration-based condition monitoring principles, there is universal agreement that unbalance, misalignment, looseness and bearing damage are the easiest to detect. In fact, many condition-monitoring systems are set-up to look only for bearing problems because of the lack of time and resources allocated to the program.
When setting-up a condition-based predictive maintenance system, the most common approach is to attempt to find what is wrong with the machine (troubleshooting) or look for the weak points in the plant. The advantage of this approach is that money spent on condition-monitoring is easily justified because it can be proven that it found a problem. However, more can be done to ensure that savings are realized as a result of using the information that has been meticulously gathered. Too often the program is used to detect a problem or change a component with no further investigation. However, if a bearing failed, one should ask the following questions: “Why did it fail?” and “How can such failure be prevented in the future?”
The most effective way to make use of the tools available is not to perform machine condition-monitoring at all, but to work toward component condition-monitoring instead. Although the concentrated effort is often applied to the data and the software, the most important issue is the knowledge of the machine, its components, how it works and how it fails. This is proactive maintenance - recognizing the root cause of a problem and fixing it before the machines break.
Consider the following case of an unbalance in a shaft supported by two antifriction rolling element bearings lubricated by oil. Traditional programs would use high frequency acceleration enveloping (vibration analysis) to detect when the bearing starts to fail; and would use velocity to look for unbalance, looseness and misalignment. The proactive approach is to use the velocity information to determine the cause of the failure rather than waiting for the effect to appear in bearing or enveloped data.
at the Forces of Unbalance
By looking at the forces generated by an unbalance, one can understand why this simple engineering problem needs to be controlled. In fact, if the amount of unbalance weight in ounces or grams was known, along with the distance from the shaft centerline, the actual amount of unbalance force generated could be easily calculated.
Table 1 demonstrates what effects the applied weight has on the force on the shaft. For example, a 2-ounce weight on the shaft will generate an applied force of 826 lbs. at a speed of 3,600 rpm. Doubling that weight to 4 ounces will double the force to 1,652 lbs. again at 3,600 rpm.
Consider also what happens with increased speed. Because the force is proportional to the square of the speed, increasing the speed from 3,600 rpm to 7,200 rpm increases the unbalance force by a factor of four to nearly 3,304 lbs. Although actually calculating the force generated by an unbalance is not typically done for routine vibration measurement and analysis work, this example illustrates how a relatively small unbalanced weight can produce a significant amount of force at machine operating speeds.
The concern is: “Where is the shaft-force dissipated to in the outside world?” Consider what happens to the oil, because it is the oil that must transmit the shaft-generated forces out to the bearing components. Increased force often generates increased heat. If the amount of heat generated through the increased unbalance is greater than the oil was designed to take, then the additives and the base oil can break down, leading to premature lubricant degradation. This may ultimately prevent the oil from supporting the shaft in the bearing, which would cause mechanical wear and begin the countdown to failure. Taking a proactive look at this machine by monitoring the 1X vibration amplitude and phase can easily avoid this problem.
Effects on the Lubricant
By monitoring the viscosity regularly to evaluate lube condition, in conjunction with the particle count to monitor increases in wear debris, early indicators of the onset of unbalance can be detected. If the unbalance is caused by problems such as a bent shaft or defects to the rotor outside of the bearing, then wear debris will not help.
It is more likely to see an increase in temperature resulting in an initial decrease in the viscosity at operating conditions. Without on-board sensing this initial drop in viscosity would be impossible to detect; however, its effect may reduce the oil film thickness, resulting in increased friction, increased wear and a rise in temperature. Long term, the viscosity would be expected to steadily rise due to oil oxidation and the formation of sludge and varnish.
Wear debris analysis would also show characteristic particles in accordance with the scuffing taking place. These particles would be expected to show some bluing of the material due to the high temperatures. The resultant wear debris generated at the point of failure is going to add to the metal presence in the oil which will deplete the additive package again by attracting the metal-wetting additives such as the antiwear and rust inhibitors. The dispersants (if used) that keep the debris in suspension will be prematurely depleted because of the increased loading. Also, the catalytic effect of the wear metal debris in conjunction with the increased heat will accelerate the rate of oil oxidation. Aside from the fact that this debris will continue to damage other surfaces and components, it also contributes to the depletion of additives and premature oil degradation.
The proactive approach to this problem is to recognize the root cause, in this case unbalance, and correct it.
Effects of Misalignment
As with unbalance, misalignment is a fact of life in machinery installations. Even when hot alignment techniques have been correctly applied, a certain amount of misalignment is inevitable and acceptable. There are many ways to nonintrusively detect and monitor changes in alignment, the two most effective are vibration and temperature monitoring. Examples of these methods are as follows:
1. A change in amplitude in a specific direction may indicate misalignment. Vibration is normally measured in a radial direction on all bearing housings, and at least one reading is taken per shaft in the axial direction. The golden rule applied by vibration analysts when looking for misalignment: “When the amplitude in the axial direction is more than 50 percent of the highest radial amplitude, then misalignment is suspected.” Misalignment will also be detected by looking for the 2X and 3X components of the vibration spectrum (Figure 2).
2. Confirmation of misalignment is best performed by using phase analysis. The amplitude and phase of the 1X vibration can be trended. When a phase change is detected, alarms can be set-up to indicate a developing problem.
Infrared thermography is another method of detecting misalignment. This technique involves measuring the infrared emissions from machine surfaces allowing surface temperatures to serve as a useful analytical tool when high, localized heat is present.
|a) This image was taken with the machine correctly aligned. The area of the seal is the hottest, but that is to be expected because this type of seal relies on surface contact. The bearing housing is still quite cool at about 82°F.|
|b) The motor was then misaligned by 0.001 inches in an angular direction. This image shows that the temperature of the bearing has started to increase. The entire end cover of the motor is also starting to increase in temperature and even the shaft coupling is affected.|
|c) The alignment was adjusted so that there was a 0.010-inch angular and 0.030-inch parallel misalignment. Notice that the temperature of the entire end casing of the motor has increased to 105°F.|
The images shown in Figure 4 were taken from a motor and pump installation joined by a flexible coupling. As the misalignment develops, the temperature increases due to increasing friction. Left unchecked, this problem will first cause the bearing to wear, followed by the seal and finally the coupling.
The motor pump set used in this demonstration was on low output load. On higher loads, the temperatures would probably be even higher. This example was created to illustrate how much heat energy is generated when misalignment occurs. All of this heat energy passes into the bearing and subsequently into the lubricant. The resulting head and forces accelerate wear and oil degradation in a way similar to the unbalance condition. Again, the proactive approach is to use condition monitoring to determine the root cause of the problem before lubricant degradation and machine wear occur.
The examples shown here present simple problems, that if corrected, will help to prolong machine life through the application of proactive maintenance strategies. In particular, the removal of the problems that cause lubricant deterioration ensures that the bearing is allowed to perform within its design parameters, while ensuring that the life of both the lubricant and the equipment are maximized. Such a proactive strategy to equipment maintenance requires a balanced approach of several different technologies, including vibration analysis, thermography and oil analysis, all of which play a vital and indispensable role in any condition-monitoring program. When using these condition-monitoring tools, it is important that they proactively address the root cause of problems rather than the observable symptoms.