Why Do I Need to Use Time Waveform Analysis?

January 16, 2018

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!

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Ultrasound Finds Worn and Misaligned Couplings

January 9, 2018

Simon is a condition monitoring specialist from a local oil refinery. He contacted my office for advice about predicting flexible coupling failures. Currently, they perform basic vibration analysis on their pumps and motors using an overall meter. They have some success predicting bearing failures but the same cannot be said for couplings. Several unexpected failures shut them down this year.

Within the facility they identified 58 pump systems considered “A Critical”, meaning if they go down, the plant goes down. I suggested ultrasound as a fast, safe, and affordable solution. Specifically, the SDT270DU offered him best value. Not only could Simon use it to monitor couplings with ultrasound; it also takes vibration measurements, thus eliminating the need for Simon to carry two data collectors.

With SDT270 Simon trends bearings with vibration, replenishes lubrication with ultrasound, and finds worn and misaligned couplings.

 

 

 

 

 

 

 

 

 

 

By placing an airborne sensor near to the coupling Simon can quickly trend an evolving defect. The SDT270DU gives Simon the choice to either spot check for defects – good – or integrate all 58 couplings into his established bearing routes – best.

Find worn and misaligned couplings with SDT’s Flexible Sensor.

I explained to Simon how several clients already trend couplings using the Flexible Wand. The SDT270 collects a STATIC ultrasound measurement that gives four indicators of condition. The first two – Overall RMS and Max RMS – indicate the level of friction produced by the defect. When these indicators rise, maintenance may consider corrective alignment during a planned shutdown. The second two – Peak and Crest Factor – identify the emergence of impacting. Together, all four indicators establish a life cycle trend for each coupling.

Once impacting appears, the Peak indicator increases in step with Overall RMS. Crest Factor (CF) is a comparative ratio between Overall RMS and Peak. As CF trends higher it warns that the window for simple maintenance has narrowed. Inspectors may choose to collect a DYNAMIC measurement when CF alarms are triggered. The DYNAMIC measurement provides a visual representation of friction and impacting severity. For both STATIC and DYNAMIC measurements it’s important to define the signal acquisition time.

 

Without the ability to define user time, pulling the measurement trigger (M) misses key events

Instead, set the signal acquisition time to suit the asset. This slow speed example used 20sec to capture all defect data. Why use an ultrasound instrument that overlooks the importance of user defined acquisition time?

User defined signal acquisition time, available exclusively on SDT instruments, is a luxury that lends ultrasound technicians the highest level of precision. Without the ability to set the sample time, inspectors must guess when to pull the measurement trigger, and question the validity of their data. Simon explained that all 58 pumps turn at speeds above 1800 RPM. Accordingly, he should set his SDT270’s signal acquisition time to between one and three seconds. One to three seconds at 1800 RPM samples the coupling for 30-90 revolutions.

Shaft couplings are guarded for safety. Any ultrasound inspector working around rotating equipment must be required to demonstrate an understanding of company safety policies. Safety considerations are engineered into SDT sensors. The Flexible Wand’s 10mm diameter sensor is designed to access the coupling with the safety guard in place (see figure 2). The 21” long sensor sports a comfortable, ergonomic grip that allows an inspector to collect danger-free data.

Simon seemed convinced but wanted to Hear More. Since this solution was already working well at a nearby paper mill, I introduced Simon to the plant manager, Sunil, and invited them both to lunch. Sunil and Simon connected on so many common reliability issues that afternoon. He confirmed the affordability of this solution based on coupling failures alone but went on to explain how their mill was rolling out ultrasound for acoustic lubrication, steam trap monitoring, electrical inspection, and air leak management. Simon and Sunil continued their conversation well into the afternoon. They agreed that ultrasound, with its 8 primary applications for reliability, represented a fast, safe, and affordable technology with the potential to revolutionize reliability culture. I sat back, happily watching two impassioned specialists strategize about reliability culture. I love my job!

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Sheave Alignment is not All! Remember Belt Tension!

January 2, 2018

Belt alignment is extremely important, and we recommend you do it with a good laser alignment system such as the Easy-Laser XT190 or the DotLine Laser system.

Laser Pulley Alignment System

But, once the alignment is done, equally important is set the belts to the proper tension. Typically, the correct tension is one that allows the belt to be deflected on its tight side by a specified amount of force to an amount of 1/64th inch per inch of span length. The span length is the distance between the nearest points of contact of the belts on their sheaves. If this distance is not known, you can use the center distance between the pulleys; that’ll be close enough.

Belt Deflection when force is applied

To do this, you use a spring tension gauge, which is a device that measures the amount of force that you apply to something when you push against it or pull on it.

Spring Tension Gauges

 

 

 

 

 

 

 

 

 

 

 

 

So, say the span length of a given belt drive is 36 inches. You should deflect the belt (in a group of belts, usually the center one, but measure the two outside ones as well) by 36/64″, (or 9/16″) which is 1/64″ of deflection for every inch of span length, and measure how many pounds or newtons of force it takes to do that. This force should not be less (too loose) or more (too tight) than what the manufacturer of the belts recommends for that drive or for that set of belts. Also, you perform this test by pressing down with your gauge upon the belts in the middle of their span length on the “tight side” of the belts. The tight side of the belts is the side that is stretched as the drive turns andthe driver pulley applies rotational force to the driven pulley. The return side is slack side of the belts.

The recommended belt tension deflection forces are usually supplied in a table that takes into account the size, length and type of belts, the number of belts in the drive, the anticipated application loads and drive ratios of the sheaves. Move the driver until the recommended force specification is met for the desired deflection, being careful not to mess up the alignment while doing so!

Download 5-Step Sheave/Pulley Alignment Procedure

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I use a Laser Alignment System, so I am Performing Precision Alignment, Right?

December 26, 2017

Condition Monitoring Expert Tip #10 by Mobius Institute

No, sadly, that is not right. Unless the person has been properly trained, and unless the company has specified precision alignment tolerances, and unless the training is followed and the tolerances are achieved, then you are not performing precision alignment.

We see this as a very common problem. Laser alignment systems can achieve terrific results. And a precision aligned machine is far more reliable than a machine that has been “roughly” aligned with an alignment system, and far superior to a machine aligned with a straight edge. But if your maintenance technicians do not appreciate why that last shim should be installed, and why the motor must be moved such a small amount to the left or right, then those corrections will not be made – and yes, it does matter.

Research by Tedric A. Harris, in the book “Rolling Element Bearing Analysis” (John Wiley & Sons) showed that just 5 minutes (5/60 of a degree) of angular misalignment can reduce the life of a bearing by half. Yes, precision matters!

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Why Do I Need to Use Phase Analysis?

December 19, 2017

Condition Monitoring Expert Tip #6 by Mobius Institute

The vibration spectrum can provide clear indication of certain fault conditions, but when you see a large peak at the running speed (1X) what will your diagnosis be? What if you also see peaks at 2X and 3X? Now, if you are monitoring a large fan with a history of the building up on the fan blades, then you may reasonably conclude that the high 1X peak indicates unbalance. But in the more general case, how do you distinguish between unbalance, bent shaft, looseness, resonance, eccentricity, misalignment, cocked bearing, and other fault conditions? This is where phase analysis is your friend.

Once upon a time phase analysis was difficult to perform because most people owned single channel vibration analyzers. But with a two-channel analyzer, and two vibration sensors, it is very easy to perform phase analysis. By simply placing one sensor vertically on the bearing and one sensor horizontally you can determine if unbalance exists. By comparing the vibration from one end of the machine to the other (in the same axis) you can confirm the unbalance diagnosis and assess whether it requires single-plane balance or two-plane. Comparing phase axially across a coupling, and radially across the coupling can help you diagnose and confirm misalignment.

We could go on and on, but phase analysis the best tool for distinguishing between all of the listed fault conditions and more.

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