Guest Post by Bob Dunn from I&E Central, Inc.
I had the opportunity use the Easy-Laser XT440 to assist a customer aligning a machine that had perpetually given them problems, with bearings always running hot. They had recently aligned the machine with dial indicators, but when we checked, it was off by .007, and this on a 3600 RPM motor. We removed their old shims and did a soft foot check indicating .032 under one of the feet. Further inspection showed an angular gap under one foot. It turns out that when new, someone had ground down the feet on the motor to better align to the pump – obviously not a precision job. We step-shimmed to fill the angular gap, then aligned the machine in a single move. Several of the techniques we used were unfamiliar to these mechanics.
- Do your pre-alignment homework to detect and correct foundation issues.
- Be sure mechanics are really trained in alignment – not just how to push the buttons. By the way, Ludeca Inc. and I&E Central provide excellent training.
- The Easy-Laser XT-Series is a fast, accurate, and incredibly easy-to-use tool for coupling alignment and more. If you use something else, you should see what you are missing!
by Ana Maria Delgado, CRL
Condition Monitoring Expert Tip #7 by Mobius Institute
Spectrum analysis provides a great deal of information about the health of rotating machinery. But you should consider the spectrum as a summary of the vibration within the machine.
The Fast Fourier Transform takes the time waveform and computes how much of each frequency is present and displays that as a line in the spectrum (grossly summarized, but that is basically the case). Therefore, if the vibration from the machine is generated by smooth periodic motion, then the spectrum provides a very good representation of what is happening inside the machine. But as damaged gears mesh together, and rolling elements pass over damaged areas on the raceway of the bearing, and as the pump vanes push through the fluid causing turbulence or cavitation, the vibration generated is not smooth and periodic. And there are a lot of other fault conditions that likewise do not generate smooth and periodic vibration. Thus, the only way to really understand what is happening inside the machine is to study the time waveform.
The time waveform is a record of exactly what happened from moment to moment as the shaft turns, the gears mesh, the vanes pass through fluid, and the rolling elements roll around the bearing. Each minute change that results from impacts, rubs, scrapes, rattles, surges, and so much more is recorded in the time waveform and then summarized in the spectrum. Therefore, it is critical to record the time waveform correctly and analyze it when you have any suspicion that a fault condition exists.
Special thanks to Mobius Institute for letting us share this condition monitoring expert tip with you!
by Ana Maria Delgado, CRL
To appropriately determine monitoring intervals a couple of things must be known. First, the point in time (P) that the potential failure becomes detectible must be known (detected with vibration monitoring, etc.). Second, the time (F) at which the potential failure would degrade to a functional failure must be known. This difference in time (P-F Interval) is the window to take corrective action and avoid the negative consequences of the failure. This difference in time will determine how often conditional tasks such as vibration monitoring must be done to detect potential failures such as bearing issues, etc. Typically, the monitoring interval would be set to half of the P-F interval. This allows enough time for the technology to detect the problem and corrective action to be taken. However, in certain circumstances it may be necessary to collect data at shorter intervals than half of the P-F interval.
by Trent Phillips
MYTH: A well-trained technician can predict within a window of a few hours when a machine will fail.
TRUTH: This myth is common throughout industry and poses a danger to machinery. The myth often leads to operating machinery with known defects for periods longer than is safe for the equipment. Let’s explore the elements that brought about this myth and why it remains so prevalent.
One reason for the birth of this myth is the curve fitting plots found in many PDM software programs. Figure 1 shows such plot. Those not trained in the use of the software may think that the time projected for the condition to reach the alarm level indicates “time-to-failure”. In reality, the projected time is “time-to-alarm”. The machine may run quite sometime after exceeding an alarm. These plots are very valuable if used as intended.
Another reason for this pervasive myth is that published PF curves are almost always shown as downward exponential curves. Figure 2 shows a typical curve. If PF curves really are this shape, finding the “time-to-failure” would simply be a mathematical process. However, PF curves are seldom this shape because many variables may influence the shape of the curve. Figure 3 shows how a real curve may look and also presents some reasons why the shape is erratic. PF curves may have hundreds or thousands of shapes.
Even the best-trained technicians, using the best tools can’t possibly know all variables that may lead to component or machine failure. Load, speed, temperature, and environment are only a few of the many variables that may affect “time-to-failure” for a given defect. A well trained technician can detect defects and may even be able to state with a fair amount of confidence that a machine will operate or will not operate until the very first opportunity to take it out of service for repairs. Doing otherwise and operating with known defects is tantamount to rolling the dice and gambling with your machinery.
by Bill Hillman CMRP