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The following correspondence resulted from from "Diagnosis Unkown" article by John Piotrowski which appeared in March 2007 issue of Uptime.
A PDF is attached or you can access the interactive digital edition here This is an extensive discussion and is posted here at the request of both parties and Jeff Shuler - Editor in Chief at Uptime Magazine. Please feel free to jump in to the discussion. Terry O This message has been edited. Last edited by: Terrence O'Hanlon, Prec_Maint_March_2007.pdf (2,787 Kb, 135 downloads) Diagnosis Unkown Article in PDF |
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date received: 3/29/2007
Mr. Piotrowski, Interesting article. This complex test could have been done in a much quicker and easier way, with better results. Thermal growth is easily determined by installing dial indicators at strategic positions on the machine train and measuring the growth relative to a fixed point off machine directly upon startup and temperature stabilization. The fixed point can be selected from something nearby if you're lucky or it can be created quite simply by tack welding some plate to a beam or some other object not connected to the machine train. After the initial realignment is performed based on these thermal growth offsets, an Operating Deflection Shape analysis would be performed to determine how the machine is moving and where to focus any further efforts. The ODS can even be done prior to the thermal growth measurements for comparison purposes later. All this can be done in an 8 hour shift with some preparation and it would yield far superior results in a much more cost effective way. The rule of thumb for all troubleshooting efforts is to start with the simplest tests and solutions and work towards the more complex. On another note, OEM provided thermal growth offsets are NEVER correct and should always be verified and never taken for granted. Regards, Brian Roy Reliability Specialist Prince George, BC |
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date of reply: 4/4/2007
Brian: Thank you for reading and responding to the article in the March 2007 issue of Uptime magazine on the off line to running machinery movement survey on the motor - fluid drive - pump drive system. I would like to respond to each of the points in your correspondence. In point no. 1 you said:
Your suggestion on using dial indicators to measure thermal growth is most intriguing. Have you ever tried this? If so, what were your results? Curious that you mentioned this as I have made two attempts to use dial indicators to measure as you say "thermal growth". I've never been very fond of the term "thermal growth" as it implies that all the movement that occurs in rotating machinery is due to temperature changes in the machine case and that machines will always grow or move upward. Neither of which is always true. We live in a three dimensional world so machinery will move in all three directions, not just upwards. Some machine will actually "thermally shrink", for example, refrigeration compressors. The term "thermal growth" also does take into account lateral or axial movement of machinery. Around 1977, while working at a chemical plant, we were having some problems with a 32,000 hp steam turbine driving three barrel compressors. We suspected that the drive system was running in a misalignment condition and suspected that the shafts were moving after the unit was started up and running at full load. We had just finished replacing one of the compressors that had a bearing failure and I wanted to see if the steam turbine and the compressors were moving so I asked if I could weld a few pieces of angle iron to the soleplates near each bearing so I could place a dial indicator on top of each bearing to observe how far, how fast, and how much the bearings were moving upwards. I was not allowed to weld anything because the production department wanted to get the plant back up and running and I was unable to get the necessary paperwork done in time for the welding permits. So I welded a 1/2" thick plate to the bottom of each piece of angle iron and C-clamped each plate to the soleplates, and then set up the indicators on each bearing. Good enough for a quick test I thought. It took about 24 hours to get the process up to full load conditions and this drive system was one of four pivotal drive systems in the process link. I remember standing outside on the compressor deck when the steam turbine started rotating the three compressors, eagerly running back an forth watching all eight indicators for any movement. Not much happened during the slow roll tests used so production could make sure the refurbished compressor was not going into tilt mode from a vibration or temperature standpoint. After a 30 minute hold at 500 rpm, with all systems looking good, production starting opening the steam valve on its way to a final operating speed of 11,000 rpm. Fifteen minutes went by and none of the indicators moved, then after about 30 minutes, each indicator started moving. Most showed an upwards movement but two of them were moving downward. What? I decided to keep calm and just watch the experiment unfold and figure out what happens later. Now at 2500 rpm, production had a problem at another part of the process so they put everything in a temporary hold basis with the compressor and turbine drive steady at 2500 rpm. I noticed that the bearing housings were starting to vibrate a little and the indicator needles were swinging back and forth. No problem, I could just visually watch the sweeping needle and get an average position. Another hour passed and the speed of the compressor drive was still at 2500 rpm as I looked at the reed tachometer on the steam turbine. Cheez, I thought, what's going on? So I went down to the control room at ground level to get some first hand info on when we were going to get rolling up in speed. Two more hours went by and the problem at the other end of the process involved a stuck control valve that just got freed up and now seemed to be working properly. So production decided to continue increasing the flow rate through the system which meant that the compressor I was watching was on its way up in speed. I walked back up to the compressor deck and was crestfallen to discover that the needles on four of the eight dial indicators had vibrated off and were now useless. Thank goodness the indicators were still working on both turbine bearings and both bearings on the recently rebuilt compressor! I decided I should begin recording what the working dial indicators were measuring so I got my notepad and one by one looked at the dial indicators. The needle on the dial indicator at the exhaust end of the steam turbine was flapping back an forth 4-6 mils but the average sweep appeared to be 2 mils higher than when I started. The needle on the dial indicator at the supply end of the steam turbine (the hotter end) was also flapping back and forth 4-6 mils and much to my surprise showed that bearing had dropped 15 mils! The average dial indicator positions on the inboard and outboard ends of the compressor seemed to have an average reading of 4 and 6 mils respectively. As the speed of the drive system began to increase, so too did the flapping of the dial indicator needles. After 8 hours, every needle vibrated off but I still recorded the average dial indicator positions with the most surprising measurement of 28 mils of downward movement at the supple end bearing of the steam turbine. Since all of the needles had fallen off by this time, I decided to remove the angle iron supports. I still remember what happened when I removed the angle iron support at the supply end of the steam turbine. I reached over and took hold of the angle iron about half way up and burnt the palm of my hand. The heat from the turbine transferred to the angle iron over the eight hour test. Couldn't see the heat, but I sure felt it. My guess is that it was about 180 degrees F when I foolishly grabbed it. The next day I discussed the results of my experiment with my engineering manager who noticed the blisters on my hand. Embarrassed, I told him what happened. He didn't laugh but just thought for a minute and then he said "How do you know that what you measured wasn't the thermal growth of the angle iron itself?" At that moment, I realized that my reference position (i.e. the angle iron or any other object like a building column) wasn't really a thermally stable "fixed" reference point. The only way something like this would work would be to insure that the holding platform for a measurement sensor had to be thermally stable. Charlie Jackson of Monsanto also realized this which is why his reference stands are water cooled as I'm sure you are aware. I also realized that a dial indicator had a limited life span when subjected to vibration. A much more reliable sensor should be used, one that could still measure distance but not have to physically make contact with the object it is observing. Something like a proximity probe maybe? Water cooled reference stands with proximity probes is discussed in Chapter 16 of the third edition of the Shaft Alignment Handbook. I would be most interested in hearing how you insure thermal stability in "The fixed point that can be selected from something nearby if you're lucky or it can be created quite simply by tack welding some plate to a beam or some other object not connected to the machine train". Also, with your field experience, how do you keep the needles on the dial indicators from vibrating off? Incidentally, the second time I tried this was for another gentleman who suggested that this method would work. I told him of my experiment but he convinced me that putting a piece of rubber between the tip of the indicator and the bearing housing would take care of the vibrating needle problem. The results ended up the same way except this time I wore some thick leather gloves. In point no. 2 you said:
Um, I'm really confused here. I thought ODS was the process of measuring vibration and phase at several points on a vibrating object (e.g. a drive system) then inputting the amplitude and phase angle data into a software program where one draws a "wire frame" simulation of the the objects being measured. The software program then takes this data and runs an animation of the vibratory movement of the object showing how far and which way each point in the wire framed model moves. What does this have to do with a positional change of the shafts from off line to running conditions? If you observe that the inboard bearing of a pump moves upward 25 mils, are you implying that it is vibrating up and down 25 mils? Are you implying that shaft misalignment will always show a 180 degree phase shift across the coupling? Are you saying that in an eight hour shift, you can: 1. Select several measurement points on a drive system to develop a meaningful wire frame model for the ODS software program. 2. Operate the drive system and collect the ODS data. 3. Input the data into the software to observe the running misalignment ODS simulation. 4. Shut the unit down, weld fixed references on beams or some other objects not connected to the machine train, and set up several dial indicators. 5. Start the unit up again and capture the measurements from the dial indicator to get the "thermal growth offsets". 6. Shut the unit down and align the drive train based on the "thermal growth offsets". 7. Start the unit up again and capture the same measurement points on the drive system now with the corrected alignment situation and input these points in the ODS software program and compare the before and after results. I will have to witness this to believe it. Also, if the vibration increases after the alignment has been corrected, where are you going to "focus any further efforts"? In your final point you said:
I agree with one caveat. That assumes that someone who works for the OEM actually knows what "thermal growth" means and they have actually conducted several accurate off line to running machinery movement surveys on their equipment and what it is connected to. Good luck finding them. The primary reason why the Essinger Bar system was used for the study was because the plant had one and wanted to use it. They also wanted a secondary technique to verify the results of the Essinger Bar system so the BRTC system was selected since they had spare proximity probes and power supplies. Again, I appreciate your reply and would seriously like you to answer the above questions I've asked. Also, once our correspondence is completed, would you allow me to send the entire correspondence to UPTIME so they could publish it in a future publication for the benefit of their readers? If you would prefer, I could indicate that your name be withheld by request. Sincerely John Piotrowski |
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date received: 4/4/2007
Hi John, Thanks for the comments, very interesting. Let me clarify some of my points and that may help with some of the confusion. Let's start with the Operating Deflection Shape analysis. What you described is correct except that you didn't mention the amplitude component of the ODS that is very important to consider or you're only looking at one part of the information. Misalignment can be diagnosed in several ways. On the run, you can take a vibration spectra and typically you will see the 2nd and 3rd harmonics of turning speed. This the TYPICAL spectral signature but doesn't apply to every case of misalignment. Another way is to acquire waveforms at points closest to both ends of the coupling. The waveform should show 5 or 6 rotation cycles. Misalignment usually shows up very clearly in time based analysis. A more powerful way to diagnose this is by taking phase readings in all directions on either end of the coupling. Since key phasors are rarely present unless you can stop the machine and apply one, cross-channel phase is the best option. This is where relative phase is measured between 2 accelerometers. Misalignment always causes a phase shift across the coupling, it is rarely 180 degrees but it is significant. In the same way, an ODS will show that phase shift in a graphical way. The amplitudes are grossly exaggerated and that allows you to visualize the movement of the machine at the animation frequency. Bearing clearances have nothing whatsoever to do with this. Even though the amplitudes are exaggerated, they are exact in relative terms. You will see the flexure at the point in the model where the two machines meet based on the extrapolation of nearby phase readings. Also, there could be other problems than simple misalignment. The ODS can give you powerful hints as to what could be the problem especially if you're good at visualizing dynamic movements. I've used this method for years with great success. Now for the dial indicator method for measuring thermal growth. By the way, thermal growth is the accepted term and it refers to either an increase in dimension caused by an input of energy in the form of heat or a decrease in dimensions caused by the removal of energy from the cold. Blowers for example typically run cold on one end and hot on the other. Thermal growth considerations are critical in these applications. Dial indicators are a widely used and accepted method. In my 23 years of consulting, I must have used this method successfully dozens of times, if not more. As you said, we live in a 3D world and that has to be taken into consideration on any test you perform. The dials have to be installed in all 3 directions (horizontal, vertical and axial) at either end of the machine being measured. They must then be zeroed at the MIDPOINT of the indicators travel. This way you can see the true movement no matter what the direction. If the machine is vibrating too much to get proper readings from the dial indicators, then you have a significant problem with your machine that has to be addressed first. In such a case, I would shut the machine down, if that's possible, and perform an initial laser alignment to get it closer and hopefully this will allow the installation of the dials. At the same time, I'd perform a bump test, since the vibration was that significant, to ensure it is not in a resonant condition. If you can't shut the machine down, that's when the ODS will give good value as an initial test as it will help you determine, in conjunction with basic vibration data, what the issue is that's causing this excessive vibration. There will be a phase shift across the coupling if there is misalignment, it is rarely exactly 180 degrees but is usually significant, and it is easily seen in the ODS. The amplitudes are exaggerated for visual purposes but are very exact in relative terms. The ODS shows that phase component in a graphical way that's easy to interpret. An ODS on a motor and pump takes me about two or three hours to perform and display in the software. It's a simple test, the longest part is drawing the diagram in the software and you get quicker as you use it a few times. All this can be done in an 8 to 12 hour shift but that obviously depends on the tradespeople you're provided and the surrounding conditions. Finally, the purpose of my response was simply to provide another method, which I believe is simpler and more effective. Just for background, I have been a reliability consultant for 23 years now. I have consulted worldwide and have solved many complex problems. I've also taught courses throughout North America. Everything I've talked about here, is field proven many times over. As for publication, I don't mind but I would request that the company name be withheld as these are my opinions and it would be presumptuous on my part to say that I am representing the company's views in this particular matter. Anyway, I hope this helps to clarify my thoughts in this matter and good luck to you in your future endeavors. Regards, Brian Roy Reliability Specialist Prince George, BC |
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date of reply: 4/6/2007
Brian: Thank you so much for responding to my correspondence. What a pleasure it is to know that someone is actually trying to measure off line to running machinery movement besides myself and trying to relate the dynamic behavior of misaligned equipment to the vibration patterns that are exhibited when rotating machinery is forced to operate in this destructive mode. I think this correspondence is very important because I know a lot of people who are confused about diagnosing shaft misalignment using vibration analysis or ODS or any other type of NDT technique and somebody needs to clarify this ataxia once and for all. Again, I would like to respond to each of the sections in your most recent correspondence. In the first part of your first paragraph you said:
I thought I did mention that both amplitude and phase data was inputted into the ODS database. In the second part of your first paragraph you said:
I agree with some caveats. I have taken vibration spectral data on misaligned machines. Sometimes there are running speed components only, sometimes there are twice running speed components only, sometimes there are 3X, 4X, and higher multiples of running speed, and sometimes there is a mixture of all of the above. Question 1. Why is misalignment vibration signatures so varied? Why doesn't misalignment always show up at 1X, 2X, and 3X? Question 2. If you were to measure the vibration on a well aligned machine, shut it down and purposely misalign it, start it back up and measure the vibration again, what would happen to the amount (i.e. overall amplitude) of vibration that you would see? Would the overall vibration go up, go down, or stay the same? If possible, explain why it would go up, go down, or stay the same. In the third part of your first paragraph you said:
Question 3. The "points closest to both ends of the coupling" would be taken where ... on the inboard bearings in the radial direction or axial direction? Question 4: How does misalignment usually show up very clearly in the time domain? What does the time domain signal look like? In the fourth part of your first paragraph you said:
In this case I'm assuming that a once per rev timing device is already installed on the machine that you are monitoring (e.g. a piece of reflective tape and a phototach or a proximity probe observing a keyway or a notch). If so, I'm not clear on your statement "...taking phase readings in all directions on either end of the coupling". Question 5: Would I place a vibration sensor (I assume a seismometer or accelerometer) on both inboard bearing housings (i.e. on the bearings on both sides of the flexible coupling) in the vertical, lateral, and axial directions? In the fifth part of your first paragraph you said:
I'm not totally clear on your statement "...where relative phase is measured between 2 accelerometers". Question 6: Where are the accelerometers placed? In the axial direction, 180 degrees apart (i.e. 3 and 9 o'clock) on one bearing on one of the machines then again in the axial direction, 180 degrees apart (i.e. 3 and 9 o'clock) on the other bearing on the other machine? Or is it in the axial direction, 180 degrees apart (i.e. 12 and 6 o'clock) on one bearing on one of the machines then again in the axial direction, 180 degrees apart (i.e. 12 and 6 o'clock) on the other bearing on the other machine? Or is it in the radial direction (i.e. 3 and 9 o'clock), 180 degrees apart on one bearing on one of the machines then again in the radial direction (i.e. 3 and 9 o'clock), 180 degrees apart on the other bearing on the other machine? Or is it in the radial direction (i.e. 12 and 6 o'clock), 180 degrees apart on one bearing on one of the machines then again in the radial direction (i.e. 12 and 6 o'clock), 180 degrees apart on the other bearing on the other machine? Or is it none of the above? In the sixth part of your first paragraph you said:
I'm not totally clear on your statement "Misalignment always causes a phase shift across the coupling, it is rarely 180 degrees but it is significant." In this case, I again assume that there is some sort of once per rev signal being inputted to a vibration analyzer. I also assume that the analyst would place the sensor on one machine, get the amplitude and phase angle then put the sensor on the other machine and get the amplitude and phase angle there. My understanding of phase is that it is a once per rev timing reference. I'm not sure but I think the way phase works is as follows: 1. The vibration analyzer watches and remembers the period of time between each once per rev pulse. 2. The vibration analyzer then divides this time period into 360 parts (i.e. degrees of rotation). 3. The vibration analyzer then looks at the vibration signal itself (i.e. the waveform) and looks for the voltage to cross from a positive to a negative voltage (or vice versa). 4. The vibration analyzer then measures the period of time from when it sees the once per rev pulse to when the voltage goes from positive to negative, divides that time period by the period between the once per rev pulses and multiples by 360. For example, if a shaft is rotating at 3600 rpm, the time period for one revolution is 16.6 milliseconds. If the period of time from when it sees the once per rev pulse to when the voltage goes from positive to negative is 8.3 milliseconds, the phase angle would be 180 degrees. Question 7: Where do you place the vibration sensor on each machine? In the axial or radial direction and on which bearing(s) or position on the housing does the sensor get placed? Question 8: If vibration from shaft misalignment shows up at 2 or 3 times running speed, what does phase mean if it's related to a once per rev signal? In the seventh part of your first paragraph you said:
Question 9: I do not understand what you mean by "...the flexure at the point in the model where the two machines meet". Would you explain this more clearly? In the first part of your second paragraph you said:
I realize that thermal growth is the generally accepted term. I'm trying to get people to stop using it and begin using a more accurate term ... off line to running machinery movement (often abbreviated as OL2R). The mechanisms for OL2R movement is not always thermal and the direction for the movement is not always growth. I sincerely hope that dial indicators are not widely used and the generally accepted method for measuring OL2R movement. Every time I've tried it, it has failed miserably. Remember what I said in my first correspondence:
Question 10: Do you still have the results from the dozens of times you used this method? If so, would you be willing to share the results of some, if not all, of your data? Question 11: Your statement "The dials have to be installed in all 3 directions (horizontal, vertical and axial) at either end of the machine being measured." Do you mean both ends or just one end. If so, which end do the indicators get put on, the inboard or outboard end? Question 12: When you used dial indicators and reference platforms, how do you know that what you measured wasn't the thermal growth of the reference platform itself? In the second part of your second paragraph you said:
This is really interesting. I would have done just the opposite, that is, I would have used dial indicators to do the alignment and laser to do the OL2R survey. Question 13: If you are using dial indicators to do OL2R surveys why aren't you using them to do the off line alignment? I think that we all need to take our measurements in a consistent pattern so our results agree. If I'm taking measurement one way and you are taking them another way then we will never agree. Please forgive me for these thirteen questions but I believe this is very important because if I'm confused, I believe there is a possibility that other people may also be confused. In fact I think that hundreds of vibration analysts are confused. Case in point. In the same issue of UPTIME, Jason Trantler talks about misalignment and phase (page 43). What he describes seems to be different from what you are describing. Another case in point, in the February 2007 issue of Pumps&Systems magazine, Dr. Lev Nelik submitted an article entitled "Pump-to-Motor Alignment: Why 0.002-in and Not 0.020-in?". In it he talks about asking a group of sixty engineers who attend a presentation he gave at a Vibration Institute session the following question: "Will a dial indicator aligned pump last 10 times longer than a straight edge aligned one? Will its vibration be 10 times lower? 5 times lower?" No one in the audience could answer the question. Later in the article he describes an experiment he did where he purposely misaligned a piece of machinery and measured the vibration before and after. The results he saw were similar to what I have seen when I have done several similar experiments. If you would, send me your answer to question 2 before you see what happened to him. I will await your answers. Please let me know if you would agree to sending this to UPTIME and see if they would be interested in publishing our correspondence. Sincerely John Piotrowski |
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date received: 4/10/2007
date of reply (imbedded in response): 4/12/2007 Great discussion points John, and I totally agree with your statements on misalignment and how it's diagnosed and sometimes misunderstood. Your questions are very relevant so I'll just go through them one at a time. Question 1. Why is misalignment vibration signatures so varied? Why doesn't misalignment always show up at 1X, 2X, and 3X? Brian Roy: The reason that it doesn't always show up in the typical way has to do with the fact that the misalignment is not necessarily the only vibration source in the system. This is the exact reason that I always caution students against blindly using those diagnostics charts (the main one being the Technical Associates of Charlotte one by Jim Berry) to determine faults. These charts are based on the premise that there is only one fault component in the system at a time, which is rarely the case. Another thing that can influence the spectral signature is the machine construction itself. Ambient conditions, mass, stiffness and damping, while components of natural frequencies can also affect how vibration propagates and is dissipated through the system. Coupling issues can also change the spectral signature, as can base conditions. What if the base the motor is sitting on has got broken anchor bolts that aren't visible? The base looseness may show up as 1x, 2x, 3x turning speed which would tend to make a person think this was misalignment. However, phase readings across the coupling would eliminate misalignment as the true culprit as there would not be any significant phase shift in this case. Also, the noise floor underneath the turning speed multiples in the spectrum would tend to have a raised noise floor in the case of looseness. The waveform would also look different, more random. Induced vibration from nearby machines can also muddle up your spectrum. There are many reasons why misalignment doesn't always show up in the typical manner, that's one of our main challenges as machinery diagnostic specialists. We have to bring more to the table and use whatever tests we can think of to get more information to help us make the best determination. John Piotrowski: I agree that there can be more than one problem occurring at the same time on a drive system. I don't believe that Jim Berry from Technical Associates of Charlotte implied that his charts were to be interpreted based on the premise that only one problem can exist on a machine. Jim has done an exemplary job with his charts and I see them everywhere. The first attempt at vibration diagnostic charts I remember seeing was composed by John Sohre several decades ago. Jim Berry expanded on that idea and I think it has been very helpful to me and to a lot of people. Jim will attest to the fact that he and I do not see eye to eye about his misalignment vibration patterns. As I, and most other experienced vibration analysts understand is that other problems may be present that could account for the multiples of running speed such as looseness but the general consensus of most vibration analysts that I know is that shaft misalignment will appear in the spectrum as running speed or multiples thereof. You then went on to reiterate that phase analysis is the best indicator of misalignment but you never really answered why misalignment shows up at running speed or multiples thereof. In Chapter 2 of the 3rd edition of the Shaft Alignment Handbook entitled "Detecting Misalignment on Rotating Machinery", in section 2.2.10 there are several examples of known vibration signatures of misaligned rotating machinery with different flexible couplings as shown in figures 2.34 through 2.39. The vibration spectral patterns shown in these figures were taken from case histories and information from other people who had done misalignment studies (Jerry Lorence, Daniel Nower, David Dewell, and Steve Chancey). Although the author of the Shaft Alignment Handbook didn't explain why the vibration spectral patterns were different, here is my take on this... The vibration that occurs on misaligned rotating machinery is quite possibly caused by the mechanical action that is occurring in the flexible coupling as it attempts to accommodate the misalignment condition. Since different flexible couplings are designed to accommodate misalignment differently, the dynamic forces that occur will be distinctive producing the different responses that are seen. In all the years that I have been studying vibration analysis, very few people have ever mentioned anything about the dynamic forces that occur in a flexible coupling trying to accommodate a misalignment condition. Yet I continually hear that unbalance is caused by a dynamic force that is produced by a heavy spot as it whirls around once every revolution. I also hear about the dynamic forces of gear teeth as they mesh or when a ball rolls over a spall in a bearing raceway producing a pulse / force as it drops into the hole and comes bouncing back out. I have asked the same question to hundreds of other people over the years and they, as well as you, have yet to give me a scientific explanation of why this happens. Forget about phase, answer the question someone, anyone, please. The answer ... "I heard it from a vibration expert or at a vibration course." doesn't count. I know this as a fact of physics: Vibration can only be caused by dynamic forces. A machine that is not rotating does not produce vibration despite the fact that it may be out of balance and have damaged gears or bearings. Not until rotation starts will the dynamic forces become present. Then, and only then, will the vibration occur. I also know this as a fact of physics: There a two basic types of forces, dynamic forces and static forces, both of which are present in every drive system. Some of the forces that act on our bearings and rotors are dynamic and some are static. The total amount of force on our bearings and rotors is a combination of both static and dynamic forces. If I see a vibration signal where a three times running speed component is present, to me, for that to happen, three forces must occur every time the shaft rotates around one revolution. For example, a jaw type coupling (e.g. a Lovejoy) typically manifests a misalignment condition with a strong three and six times running speed components as shown in figure 2.34. Isn't it curious that there are three "fingers" on each of the two coupling hubs. Under a misalignment condition, each time the shaft rotates around once, each "finger" on each coupling hub produces a pulse that carries through the shafts to the machine casing and the bearing housings where we measure the vibration with a sensor. Three fingers, three pulses, three times running speed. Is this just a coincidence? But why is there a six times running speed component? I guess because there are six total "fingers" (three on one hub and three more on the other hub) where each alternating finger is spaced 60 degrees apart. Six times sixty equals 360, degrees that is, in one rotation. Other types of flexible couplings (e.g. gear, rubber tire, flexible disk) exhibit different vibration spectral patterns and I could go on about how I think each design emanates the dynamic forces that cause the vibration patterns but this dialog is going to be long enough so I'll stop here. Question 2. If you were to measure the vibration on a well aligned machine, shut it down and purposely misalign it, start it back up and measure the vibration again, what would happen to the amount (i.e. overall amplitude) of vibration that you would see? Would the overall vibration go up, go down, or stay the same? If possible, explain why it would go up, go down, or stay the same. Brian Roy: If the machine was running well and you induced a misalignment, then I would tend to think that the overall vibration would go up as you are generating running speed multiples that were not previously present and causing forces to be exerted at the coupling. It is absolutely impossible to guess that by misaligning the machine that the overall vibration will go up by X% or that the 2nd multiple will appear but not the third etc. There are modeling techniques and known case studies that could help you take a guess but you can never anticipate exactly what will happen. If the same thing happened in the same way every time a fault is induced in a machine, we would be out of business as an on-line system could do our job more efficiently that we could. John Piotrowski: I ask this question in every one of my shaft alignment training courses and have also asked it at several presentations at conferences where I have been asked to speak about this subject. Although I do not have an accurate count, my sense is that 70% of the people believe that the vibration will go up on a drive system if you misalign it. Here is my answer to that question: If you misalign a drive system, the vibration may go up, go down, or stay the same but in the majority of cases the vibration levels will go down. Yup, exactly the opposite of what the majority of people say will happen. In Chapter 2 of the Shaft Alignment Handbook, sections 2.2.7 and 2.2.8 show examples of controlled tests where vibration data was taken under misalignment conditions and then again after the alignment was corrected to acceptable tolerance levels. These tests show that the vibration will go down if you misalign a machine. I am in the process of publishing an electronic book tentatively entitled the "Turvac Field Service Files" where I have 12 examples of actual case histories where this is true. Right now there are 50 case histories in the book taken from about 60% of my field service reports dating from 2000 through 2007 and the current page count is just shy of 800 pages. If I go back to 1979 and put all of my field service reports in this book, it may tip the scales at 3000+ pages. I literally have at least a hundred case histories where the vibration amplitude levels were lower on a drive system running under a misalignment condition, then after shutting the unit down, correcting the misalignment, starting the machine back up and measuring the vibration again, the vibration went up (making it look like I did the job wrong). I found it extremely hard to believe that I was the only person seeing this until I realized that the majority of people who measure vibration on rotating machinery are not the same people who align the equipment. Talk about a disconnect in communication, there's a big one. Several top notch mechanics I know think that their vibration analysts are fruitcakes for this and many other reasons. Wait a minute here, why would vibration decrease with increasing misalignment? The author has given me permission to reprint a section from Chapter 2, to explain this phenomena:
Get it? If you take a balance training demonstrator and set it on top of a table and start it up with a huge amount of imbalance in it, the demonstrator could actually start bouncing up and down on the table. Before it vibrates off the table and crashes on the floor, if you grab a hold of the unit and push down hard enough, the unit will stop bouncing around. If, during this experiment, a vibration sensor happened to be taking a measurement in the vertical direction on one of the bearings, the vibration amplitude would drop the instant you push down preventing it from falling off the table. Why did the vibration go down? Because the dynamic forces from unbalance went away? Absolutely not. But by applying a static force to the drive system, you diminished it's capacity to move freely. If you think I'm lying, try the experiment and let me know what happens. I know this as a fact of physics: Vibration sensors measure motion, not force. We are totally incapable of measuring the amount of force generated in a bearing using any type of vibration sensor. The best electronic sensor I know of to directly measure force is a strain gauge. Do you know anybody who has strain gauges buried inside their machine at or near the bearings to measure force? I do. Ask Dr. Wes Hines at the University of Tennessee who directed an experiment 1997 where they misaligned two different drive systems using for different types of flexible couplings subjecting each to fifteen different misalignment conditions. A landmark study that was not received very well by vibration analyzer manufacturers and some laser alignment system distributors. So much for telling the truth. Here's something to ponder, if the total amount of force in a bearing comes from both static and dynamic forces, for all rotating machinery on our planet, what average percentage of the total force is due to dynamic forces and what percentage is due to static forces? Is it 50% dynamic and 50% static, or 20% static and 80% dynamic, or is it 80% static and 20% dynamic? I don't think anyone can answer that question, but in my humble opinion, if it's the third choice, then we're looking at the wrong thing if we are using vibration as the sole determiner for detecting misalignment or any other destructive mechanism that is damaging our rotating machinery. Question 3. The "points closest to both ends of the coupling" would be taken where ... on the inboard bearings in the radial direction or axial direction? Brian Roy: Exactly. At the inboard bearings in both radial and axial directions. When doing any diagnostics on a machine, I'm a strong believer in taking all data in all directions whenever possible. There's no such thing as too much data as long as the time constraints are respected. John Piotrowski: OK, that clarifies it, I think. Question 4: How does misalignment usually show up very clearly in the time domain? What does the time domain signal look like? Brian Roy: When you focus in on 5 or 6 cycles of turning speed in the time domain, you are indirectly filtering out the higher frequency components which allows you to see the turning speed vibrations more easily. a well aligned machine will show up as an almost perfect sine wave as there is only the turning speed component in the waveform. As you generate 2nd and 3rd multiples of turning speed due to misalignment, you are superimposing one or two other waveforms (one sine wave for each multiple) into the overall waveform and you end up with a waveform that is now slightly more complex, something like this: (not the best example, but all I could find at short notice) Note the clear repeatable pattern, which still shows the dominant sine wave with the waveform from turning speed multiples added to it. Looseness will be more random, much less repeatable. Question 5 asks where I would put the sensors and yes, I would put them, or at least take readings, in all 3 directions on the inboards. John Piotrowski: Well, I, uh ... never mind. I'm going to have to read this several times to get your point.
Brian Roy: This is really question 6. Cross channel phase measurements require a true 2-channel analyzer and the appropriate capability (CSI analyzers have this functionality). When you require phase readings on a machine, and you don't have, or can't install a reflective tape in order to use a tachometer as your reference, then cross channel phase is the best approach. I actually prefer it in all instances. As you know, phase is simply defined as the relationship between two signals based on a common reference. Basically, to get the phase across a coupling using this technique, you simply place an accelerometer on each side of the coupling (machine inboards) in the same direction. In the radial directions, it's important that the accelerometers be oriented in the same direction. So both vertical or both horizontal. As soon as you press the acquire button on your analyzer, the machine triggers a phase measurement between the two accelerometers and gives you a very exact phase reading. The reference is internal to the machine, no tachs required. Caution must be taken in the orientation though, if you have a vertical on one side pointing up, and on the other side the vertical is pointing down, then you have to add 180 degrees to the reading you obtained on the analyzer. In a similar fashion, if you measure the axials on either inboard, your accelerometers will be oriented 180 degrees away from each other and that must be factored in. Other than the direction caution, this is a very powerful tool for measuring phase. It is also much more flexible than using a tach reference as you can get phase at any frequency within the FMAX you define! Another benefit of this tool, is that you can get cross-channel coherence. Coherence is basically output divided by input. Here's a quick example of how to use this. Let's say you have a pump with several frequencies in the spectrum and there is one frequency that doesn't seem to apply to anything or maybe you suspect that this frequency is coming from another source. You install one accelerometer on the machine you're measuring, and then you take readings simultaneously in the same direction on other nearby sources of vibration. If the machine you are reading with your roaming sensor is not influencing the machine of interest, your coherence will be very low (below 30% or so), once you find the real source of the induced vibration, your coherence will go to nearly 100%. I also use this technique for doing bump tests when testing for natural frequencies. As you know, a true bump test is a 2 channel measurement that has to be done with the machine shut down. With cross-channel coherence, you can do your bump test while the machine is still running and the coherence will confirm which peaks in the resulting spectrum are due to the impact of the modal hammer and which ones are due to other vibration sources. Very powerful. John Piotrowski: I understand the second part of your statement above on cross-channel coherence but I'm still not clear on the first part. I'm going to have to read this several times to get your point here too. I've read it twice and I'm still struggling. Please forgive me I'm just a little slow learning complex things. Question 9: I do not understand what you mean by "...the flexure at the point in the model where the two machines meet". Would you explain this more clearly? Brian Roy: This flexure point is the coupling, that's what I meant by "the flexure at the point in the model where the two machines meet", the connection between the driver and the driven. When you do an ODS, you draw a model and assign measurement points. The software extrapolates data in areas that can't be measured, like a rotating coupling, to build the visual model you see in the animation. John Piotrowski: OK, so you mean that the flexible coupling is the same thing as the "flexure at the point in the model where the two machines meet." Do you mean that you will see the two machines pivoting at the coupling? Or maybe another way of saying that is that the ODS model will show the coupling as a nodal point? I'm still not clear on what someone would see in the ODS model. (referring to John P statement in correspondence dated 4/6/2007): I realize that thermal growth is the generally accepted term. I'm trying to get people to stop using it and begin using a more accurate term ... off line to running machinery movement (often abbreviated as OL2R). The mechanisms for OL2R movement is not always thermal and the direction for the movement is not always growth. I sincerely hope that dial indicators are not widely used and the generally accepted method for measuring OL2R movement. Every time I've tried it, it has failed miserably. Brian Roy: I have succeeded many times using dial indicators and have found them to work just about every time except if there is no way to install them or if the vibration is too severe to get a proper reading. That takes a lot of vibration though. I really like the term OL2R, I agree it is a much better term than thermal growth. Question 10: Do you still have the results from the dozens of times you used this method? If so, would you be willing to share the results of some, if not all, of your data? Brian Roy:No reply Question 11: Your statement "The dials have to be installed in all 3 directions (horizontal, vertical and axial) at either end of the machine being measured." Do you mean both ends or just one end. If so, which end do the indicators get put on, the inboard or outboard end? Brian Roy:No reply Question 12: When you used dial indicators and reference platforms, how do you know that what you measured wasn't the thermal growth of the reference platform itself? Brian Roy: Interesting question but I can answer it. The reference platform, whatever you use, is only subject to ambient temperature as it is static. It will not heat up any more once the machine in question is started up. The reason the machine heats up or cools down is due to the dynamics of that machine and process conditions which may affect it's temperature. Care must be taken to let the reference platform reach ambient temperature before doing the measurement. If your ambient temperature swings a lot, then this won't work well, but neither will any alignment you perform as the machine will grow or contract with these ambient swings. John Piotrowski: I totally disagree that "... it will not heat up any more once the machine in question is started up." Apparently my burnt hand wasn't enough to convince you that the type of reference platform you are talking about isn't a good reference position. There are two ways to get a pretty decent reference platform however. Both are discussed in Chapter 16 of the 3rd edition of the Shaft Alignment Handbook. |
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Question 13: If you are using dial indicators to do OL2R surveys why aren't you using them to do the off line alignment?
Brian Roy: Several reasons. It has been proven that laser alignment systems are much quicker and MUCH MUCH easier to use than reverse dial or straight edge alignments. Aligning with dials is a nightmare and if you haven't done them often, are quite complicated. This would be a colossal waste of time. Alignment is one of the most important factors, along with proper lubrication, in ensuring adequate bearing life. Laser alignment systems are cheap, easy to use, and very precise. Why would anyone use anything else? I have come across cases where the physical size of the shaft to be aligned was such that the laser gear could not be installed, in this case we used optical alignment tools. Not that many machines are subject to OL2R, but when they are, the biggest challenge is determining the thermal offsets to input into the laser alignment system, after that, it's just a simple alignment. John Piotrowski: I agree with your statement that alignment and lubrication are extremely important in ensuring bearing life but I totally disagree with just about everything else you said. Laser systems are cheap? Really? I though they cost anywhere from $3000.00 to $45,000.00 depending on what system you bought and what features you wanted. Everything is "cheap" when someone else is paying for it. That's what my children thought when they drove automobiles I let them use when they were living at home. They are out on their own with spouses and children and I'll let you ask them if they think vehicles are cheap now that they have to pay for them. My one grandson, who just started driving a few months ago will tell you that the vehicle provide to him by his parents was cheap, it the gas that's expensive since his parents make him pay for that. It sounds like you have used a lot of different vibration analysis equipment, laser alignment systems, computers, ODS and vibration software in your career. May I ask how many of them you paid for our of your own wallet? When you take your early retirement five or so years from now and go out and start doing vibration analysis and shaft alignment as a leading industrial consultant forking over $60K to $100K from your savings account to buy the "new hotness" in data collectors, the associated software that comes with it, a "just have to have" laser alignment system with a 1080i screen with software that animates the corrective moves that wirelessly uploads its data to your laptop computer with 100 Gb of RAM and a 15 terabyte hard drive running Windows Vista version 7.0, six accelerometers, power supplies, cables, dial indicators, bracketing for your OL2R surveys, and an optical Jig Transit, stand, scale targets, magnetic bases, and Invar rod set just in case you laser system "could not be installed" ... let me know if you think this stuff is cheap then.
Really? Here are five actual events that I know of that may alter your sheltered view. Case History 1 "¢ A maintenance technician at a chemical plant was asked to align a motor and a pump with a newly purchased laser shaft alignment system. Shaft position measurements were captured with the instrument and the alignment corrections required to align the motor (assigned as the movable machine) to the pump indicated that the outboard end of the motor had to be lowered 85 mils and the inboard end of the motor had to be lowered 37 mils and there was no shim stock under any of the motor feet. After completely removing the motor, the technician began grinding the baseplate away. The motor was placed back on the base and shaft position measurements were captured again. The technician then added shims under the motor since too much metal had been ground away and several side to side moves were made to bring the equipment into alignment. If this had been an article in a daily newspaper, the title may have read "Machinery doctor accidentally removes legs and installs prosthetic appendages hoping no one will notice". Case History 2 "¢ A paper company was having a vibration problem with one of their motor driven boiler feedwater pumps in their power house. After two new motors were installed using a laser alignment system it was discovered that the pump shaft had 45 mils of runout and was permanently bent. The plant maintenance personnel and the service personnel for the motor companies were unaware that a laser alignment system is incapable of measuring runout enabling one to align a machine with a bent shaft and never know it. If this had been an article in a daily newspaper, the title may have read "Curved shafts still rotate on a straight centerline of rotation." Case History 3 "¢ The right angled gearbox on a cooling tower fan drive system had failed and was replaced. A new gearbox was installed and a laser alignment system was used to align the motor to the input shaft of the gearbox which were spaced 20 feet apart. Upon attempting to capture a set of measurements, the laser beam would move outside of the detector target after the shafts were rotated through less than 5 degrees of arc. The laser system luckily had a feature that allowed the user to rotate backwards until the system saw the beam, aim the laser to the center of the target again, then continue taking measurements. However, the beam wandered out of the target area after only rotating less than 5 degrees. The technicians repeatedly rotated back, reset the beam and continue on. After an hour and a half, the team had repositioned the beam 36 times after rotating halfway around. Luckily the laser system had a feature that was able to calculate the required moves with less than a full rotation so shims were added under the gearbox and a lateral move was attempted. The laser system was then positioned back at 12 o'clock and another set of measurements were taken to determine if the alignment corrections worked. Much to the dismay of the workers, the laser once again went outside of the detector target area but this time it happened with slightly over 10 degrees of rotation. Feeling a sense of accomplishment, the technicians continue on and after repositioning the beam 12 times with a half rotation, the required moves were calculated again. Some shims were added and some were removed and another sideways move was performed. Measurements were taken again and after repositioning the beam 6 times with a half rotation, the required moves were calculated again. After three more measurements with three more moves, eight hours had elapsed and the beam finally stayed inside the detector target area through a half rotation. If this had been an article in a daily newspaper, the title may have read "Studies conclude that all our electronic gadgets have actually decreased productivity" Case History 4 "¢ An electric utility company had replaced a 1500 hp motor whose windings had failed. The motor was flexibly connected to an induced draft fan and the rewound motor needed to be aligned to the fan. Two maintenance mechanics grabbed the new laser alignment system from the tool crib and proceeded out to the ID fan. The bolts were installed in the coupling, the laser system was mounted onto the shafts, and the beam was zeroed in the 12 o'clock position ready for the shafts to be rotated. One of the mechanics noticed that the fan inspection cover was left off and a block of wood was wedged in between the fan blades. Unbeknownst to the mechanic, the intake dampers wouldn't close all the way and a fairly strong breeze of air was flowing through the fan housing. The previous team of mechanics, who removed the motor, jammed the wood into the fan to stop it from rotating due to the air flowing through the fan. The lead mechanic, responsible for operating the laser, indicated that he was ready for his helper to begin rotating the shaft to capture a set of readings reminding him that he wanted to stop the shaft after 90 degrees of rotation so he could push the enter button on the keypad to capture the measurements at each quarter turn. The helper removed the block of wood and viola! the fan started rotating. "OK" yelled the lead mechanic "Stop there". The helper grabbed one of the blades on the fan wheel to stop the rotation but there was too much inertia in the 30 foot wheel and his hand lost it's grip on the blade. "What happened?" yelled the lead mechanic. "Uh sorry", said the helper. "I lost my grip". OK, OK, it's coming around to the bottom so stop it there", said the lead mechanic. The helper tried grabbing the fan wheel to stop the rotation but there was still too much inertia and again his hand lost it's grip on the blade. "Hey, what's going on?", yelled the lead mechanic. "I can't stop the fan from rotating. It's too big!" replied the helper. The lead mechanic decide to set the operator keypad on the ground and leave his perch to find out what the problem was. "Hand me that strap wrench over there and I'll wrap it around the exposed fan shaft at the outboard bearing" the lead mechanic said to the helper. Much to his chagrin, the strap wrench just kept slipping on the shaft. By now, the fan had slowly picked up some speed and the helper said "Hey, what's that clunking noise up at the coupling?" The two of them walked up to see that the cable, wired from the keypad to the laser, had wrapped around the shaft several times and the operator keypad was now banging against the ground at each rotation. "Holy cow!" yelled the lead mechanic. "Quick, run over to the other side and let's try to unwind the cable!" When the helper got to the other side of the coupling, the lead mechanic grabbed the keypad, handed it to the helper over the top of the shaft who then handed it back to the lead mechanic under the shaft. Over, under, over, under, over, under they went and the shaft just kept rotating faster and faster. Realizing that this was futile, the lead mechanic got a good grip on the keypad and yanked it loose from its mooring, put the keypad down, threw the block of wood back into the fan blades and everything came to a screeching halt. The lead mechanic then got his bracket and dial indicator, disconnected the coupling, performed the face-rim method, and finished aligning the motor to the fan shaft. If this had been an article in a daily newspaper, the title may have read "Maintenance manager discovers frowny face permanent burned on the liquid crystal display of their new laser system with the words "I'm dizzy" along the bottom of the screen." Case History 5 "¢ An aggregate company overhauled the drive system on a dredge replacing several water bearings along the length of a 180 foot long drive shaft along with the pump at the end of the shaft. As some of you know, a dredge is a large, U shaped pontoon boat with an "arm" (also know as a ladder) that is lowered into a lake where a rotating auger churns up the sand, pea gravel, and stones at the bottom of the lake. A pump, located right behind the auger, vacuums the stones and sand from the bottom and delivers the material through a long pipe onto the shore so it can be separated. The centrifugal pump, which is submerged during operation, is driven by a long shaft supported in several bearings. At the back of the dredge a diesel engine or an electric motor is flexibly connected to the other end of the drive shaft. A contractor was hired to align the entire drive system after the pump was rebuilt and new water bearings were installed on the drive shaft. To get to the dredge, which was out in the middle of the lake, the maintenance workers at the facility transported the alignment consultant out to the dredge on a small work boat. When they arrived at the dredge, the contractor handed his laser alignment system to one of the workers who was standing on the side of the dredge. The laser alignment system slipped out of his hands, fell into the lake and sank to the bottom before the panic stricken consultant could grab it. The worker, who dropped the unit, felt so bad that he dove into the lake, swan to the bottom, and retrieved the box from the abyss. When the alignment consultant inspected the contents now safely on the dredge, he discovered that the printed circuit board on the operator keypad and display unit shorted out ruining the $20,000 unit. The manager at the aggregate plant was contacted and came out to the dredge so he could explain that they were not responsible for any damage that occurred to the contractors tools and asked him if he could still align the drive system. The contractor, who did not know how to do alignment any other way left the facility in a bad mood. The plant needed to get the dredge running as quickly as possible so another contractor was contacted. The maintenance workers at the facility transported the alignment consultant out to the dredge on their small work boat. When they arrived at the dredge, the contractor handed his dial indicator based alignment system to one of the workers who was standing on the side of the dredge. As luck would have it, the dial indicator based alignment system slipped out of his hands, fell into the lake and sank to the bottom. The same worker, who dropped the tools the second time, felt so bad that he dove into the lake, swan to the bottom, and retrieved the box from the abyss. When the alignment consultant inspected the contents now safely on the dredge, he dried off the dial indicators and the brackets with a rag and finished the alignment on the drive system by the end of the day. If this had been an article in a daily newspaper, the title may have read "Accidental tests discover that light not only bends but also breaks in water" Over the course of the years, apparently based on some of my "sound bites", several people have remarked to me ... "You don't like laser systems, do you?". Here is my answer to that question ... "Laser systems are just fine, but they are a tool and like any other tool and they have their limitations." To blindly trust a tool and not understand how the tool works will eventually cause you some grief. I firmly believe that the number one reason laser systems are popular is because it encourages the dismissal of thinking. What would cause a rational individual to stop thinking about their job? I believe it is due to several reasons but at the top of the list I've surmised that industry does not feel responsible for training individuals inside their organization. I'm pretty sure high schools, trades schools, colleges and universities do not teach shaft alignment. Where are people supposed to learn how to do this? Where did you learn how to do shaft alignment? A colleague of mine recently made a very profound statement ... "I see, from my small window at ground level, a tendency in upper management to value technology over talent." It was a response to a sentence I said to him which was "Lasers don't work everywhere, but a persons knowledge does." In your statement "Aligning with dials is a nightmare and if you haven't done them often, are quite complicated. This would be a colossal waste of time." Aligning with dials is a nightmare because people have not been trained on how to use them and how to interpret what the dial indicators have measured. As an obvious reader of UPTIME magazine, I'm sure you read an article that I wrote for UPTIME that appeared in the September 2006 issue entitled "Is anyone listening?". In it, I discuss the results of a survey I give out in my training courses. One of the questions asked what tools the attendees needed to do alignment correctly. Over 40% of the attendees said that they need brackets and dial indicators. Less than 10% thought they needed a laser alignment system. Why? Because at the end of the course they knew, for the first time, how to do alignment without one and knew the advantages and disadvantages of all the methods and tools available to do alignment accurately and quickly. A few days after the above mentioned article appeared, I got a phone call from an engineer in British Columbia in a plant north of you (Kitimat). He was blown away by the article and the results of the survey. He indicated that they had a laser alignment system but he felt the maintenance personnel really did not understand the process of aligning rotating machinery. He had been begging his management to do some formal alignment training, training that wasn't a sales pitch about some system someone was trying to sell them, a real, honest to goodness course that taught alignment methodology, not which buttons to push and the robotic response to to a software program that leads a human being through some step by step process. He asked if I could give him a quote to do some training. I'm still waiting to hear from him. If I was a betting man, his request for training funds were rejected and I won't hear from him again. I can just hear the maintenance manager grilling him with "What? Shaft alignment training? We just bought a $20,000 laser alignment system that the sales guy said could be used with no training. So forget about asking for $6000 for training. We got money for tools but no budget for understanding how to use them." Imagine how he feels. After a while, even the most conscientious individual will give up with a mentality like that. "Will a dial indicator aligned pump last 10 times longer than a straight edge aligned one? Will its vibration be 10 times lower? 5 times lower?" Brian Roy: You don't need 60 engineers to answer the real crux of this question, but as it's stated, it is impossible to answer. As soon as you put numbers like "10 times longer" or "5 times lower" you make the answer impossible because that is simply impossible to predict, at least in a practical sense as there are too many variables. I'll answer the real question, and I'm very confident in my answer: the question is not "can a dial indicator or straight edge aligned pump last as long or longer", the question is why would you even consider such a thing? That's like mailing your beer to the north pole to get it chilled instead of putting it in a refrigerator that's 4 feet away. The beer will get cold in both instances but one method is just not practical. Another important consideration is time, and time is money. I guarantee I can perform at least 2 or 3 laser alignments in the time it takes an experienced tradesman to do a reverse dial alignment which requires a lot of time if you want the same precision you'll get from a laser, and as for a straight edge, you may as well eyeball it for all the precision you will get. Straight edge is only good for a rough alignment and even there the results will be marginal. There should be no debate whatsoever about the preferred alignment method in industry in the 21st century. Laser alignments only, end of story. John Piotrowski: Your metaphor on beer may be the source of this linguistic incoherence. According to recent studies, the magnetic north pole is drifting slowly southward so you may not have to wait that long to get rid of your refrigerator. I am very confident that a knowledgeable individual using dial indicators will beat the pants off a simpleton using a black box who's being told what to do by software whose programmer probably never did an alignment job in their life. The alignment job done by the knowledgeable person will be quicker, just as accurate, and they probably won't have to remove metal from the baseplate, cut bolt shanks down, or open holes in machinery feet to get the machinery within acceptable alignment tolerances like I continually see people doing with laser alignment systems. This same person with a $90 dial indicator and a bracket they made themselves will walk away from the job knowing that it was their intellect and skill that proved successful, not a $20K electronic gadget that whose batteries could fail any minute, or a cable short out, or suppress a panic attack when their screen goes blank. I'm all for technical advancement. I have a laser system, I obviously own a computer and several vibration analyzers, I own dozens of outstanding software programs and have actually written a few programs myself. I however, have no intentions of allowing technological advancements to replace my capacity and desire to think. On our web site in the Technical Info section, there is a reprinted article I wrote a while back on "How Qualified Are You In Shaft Alignment". It discusses the reasons why people should become certified in shaft alignment of rotating machinery. For several years now, people are capable of being certified in vibration analysis and infrared thermography. This year, the Vibration Institute is beginning the process of certification in shaft alignment. I do not believe that laser alignment systems will be part of the certification process but the five alignment measurement methods probably will be. Assuming that people will have the courage to admit that they may not know what they are doing, attend a reputable course to learn about vibration analysis, infrared thermography, or shaft alignment, take the certification exams and pass, then and only then can we be assured that we are competent to work in the 21st century. Brian Roy: I hope this answers the majority of your questions. It has been very interesting for me to discuss this, it makes me miss my days as a consultant. I do have data in various places but bringing it all together is a different story. My plan is/was to use this historical data to help develop and teach a training course. When I take early retirement in 5 years or so, I intend to teach and do some diagnostic consulting. Right now, I'm more involved in Root Cause Failure analysis, RCM studies, PM and PdM development, KPI development etc. within a plant environment. Interesting but not as fun. Talk to you later. Regards, Brian Roy Reliability Specialist Prince George, BC |
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