Sometimes unbalance can be caused by a shaft key being too long. When a shaft assembly goes to the balance machine, the assembly is normally balanced with a half key installed. The coupling and key have been removed and a half key installed into the keyway on the shaft. The straight portion of the shaft keyway is filled with a piece of steel; however its height is ground down so that it closely matches the outer radius of the shaft. In other words, it doesn’t stick out!
Electrical tape is then wrapped around the half key and shaft so that the half key doesn’t come flying out during the balancing operation. After the rotating assembly has been successfully balanced to within tolerance, the rotating assembly is returned to the technician for final reassembly.
In the following example, let’s assume that the actual length of the keyway in the shaft is 8 inches long and 3/8″ deep. The length of the coupling hub keyway is 4 inches long and it is also 3/8″ deep. If the technician installed a key that measured 8″ long × ¾” × ¾” and then mounted the coupling onto the shaft it would result in an excessive key length sticking out past the back edge of the coupling hub. The extra 4 inches × 3/8″ high key stock sticking out behind the coupling could be enough mass to cause the imbalance to exceed ISO balance tolerances.
The following method should be used to calculate the proper key length:
Source: Practical Solutions to Machinery and Maintenance Vibration Problems, Chapter 5, Unbalance, Section 16, Unbalance Due to Assembly Errors – Key Length Considerations by Update International
Our advanced field balancers can help you identify, correct and avoid the unwanted consequences of equipment unbalance. For more information, visit our website.
by Ana Maria Delgado, CRL
Vibration is everywhere! Vibration is a “back and forth” movement of a structure or component. Vibration can also be referred to as a “cyclical” movement. It can be inherent in a piece of equipment or can be induced by another form of energy. The real question is whether the vibration is detrimental to the equipment and its internal components.
Vibration is typically monitored through some form of analyzer, either online or offline such as the VIBWORKS analyzer.
What causes vibration? Here are just a few causes, but there are so more which can lead to elevated vibration levels. More importantly, if caught early enough, they can be corrected and thereby maximize the life of our equipment:
- Installation of the machines
- An improperly mounted bearing can cause severe vibration. This can lead to damage of the bearing as well as other components within the machine.
- Operation of the machine
- Pushing our machine beyond its recommended maximum output. Our machines respond by vibrating more than the recommended allowable limits and will eventually fail.
Watch our video ‘What’s Misalignment’ to learn more about the causes and effects of having misalignment in your rotating equipment
Some common machine problems that generate mechanical vibration:
- Misalignment is one of the most common issues that leads to high vibration and eventually failure of the machine. It can be easily detected and corrected. Take the time to laser align machines properly to the recommended tolerance.
- Unbalance is another easily missed problem that causes severe damage to our equipment. It can also lead to cracks of the housing itself. If not detected and corrected soon enough it can lead to dangerous catastrophic failure. Unbalance can be easily detected and corrected extending the life of equipment.
We never have enough time to do things right the first time but always find time to do them again.”
These few issues can be easily detected with properly set-up software. Often, the setup is incorrect and inaccurate. Invalid data is captured in the FFT. Please consult an expert to make certain you are utilizing your condition monitoring software to its fullest potential. Remember… If it’s Critical and Rotates it should be Aligned, Balanced and Monitored.
by Ana Maria Delgado, CRL
Just what does it take to be successful at balancing? Let’s start with some basics. First you need to have an understanding of the balancing process, next nomenclature: is it unbalance, imbalance, out of balance, or what? Use a consistent description and stick with it. Next, think about what the source of unbalance could be: is it uneven wear on parts? Voids within castings? Damage from impacting? Material buildup? Even though buildup is not usually a problem, when it begins to come off it rarely does so evenly thus creating an unbalance. In other words, unbalance is simply the uneven distribution of mass.
Simply review or collect data to ensure that the undesirable vibration is from unbalance and not some other issue such as a belt problem, misalignment, electrical issue, etc. Once you’ve determined that the vibration is indeed unbalance you need to inspect the object to be balanced. If it is not clean, clean it. Look for damaged or broken parts. On belt driven equipment inspect the belts as their frequency can be very close to running speed and can hinder the balance job. Make sure you have the proper tools for doing the balancing job, such as a balancing instrument capable of reading the vibration that is produced at running speed or what is commonly referred to as 1×or 1 times and capable of indicating the phase angle at 1×. This could involve utilizing an optical tachometer, laser tachometer, magnetic pickup or even a stroboscope. Some tachometers will require a piece of reflective tape on the shaft for the tachometer to read from and this might require stopping the machine if still in service.
Tip: I try to place the tape horizontally, or along the axis of the shaft, with the leading edge of the tape on the trailing edge of the key way. This can be helpful if you ever have to return for another balancing job on the same machine. You need to determine if you will be adding or removing material in order to balance the rotating component. If removing material, how will you determine how much you’ve removed;if adding weight, you need to make sure the weight you are adding is of a material that is compatible with the service the machine is exposed to. If adding material “weights”, how will they be attached? With set screws? Bolts? Clamps? Welded on? All this should be considered. One last tip: if after two runs you’re not there or almost there yet, you might need to stop and examine your process to ensure no mistakes have been made.
Download LUDECA’s 5-Step Balancing Procedure.
by Gary James CRL
As Published by Maintenance Technology Magazine September 2017 issue
If greater reliability and uptime are of any concern to you, then precision maintenance is a key component in achieving it. This means having clear and simple, yet meaningful, procedures in place for the different tasks involved. Two such tasks are precision alignment and balancing. LUDECA’s 5-Step Procedures will help guide your facility and maintenance staff to achieving precision maintenance.
The alignment and balancing procedures lay out the basic steps required to align and balance machines safely, reducing risk of injury and increasing likelihood of a quality outcome. Checklists simplify the workflow and serve to remind employees of the processes required to consistently and safely perform the precision maintenance task.
Well-aligned and balanced machines run more reliably, with a greatly reduced probability of failure. This allows for better maintenance planning, greatly reduced repair and maintenance expenses, increased uptime and more profits.
A good alignment procedure ensure that machines are aligned to the proper tolerances for the running condition of the machines, taking into account such things as thermal growth and anticipated positional changes. This ensures that the greatest efficiency is achieved in your running machinery, prolonging their health and reducing power consumption. Studies have shown that well-aligned machines result in a 3% to 10% reduction in power consumption. Noise and heat generation is reduced, producing a safer work environment.
- Production Quality
Good alignment and balancing result in better product quality since vibration is minimized, resulting in a more uniform and higher product quality. Unexpected breakdowns in production machinery may lead to costly waste from scrappage and high restart costs for the production line.
- Training & Procedural Consistency
Once implemented, a procedure ensures all employees involved in the activity face clear and consistent expectations and processes, leading to a better understanding between all staff in the facility. Training expense can be reduced since often only refresher training is required to update understanding of the technology utilized as updates are rolled out. Records should be kept that document employee training.
The next step in precision maintenance and reliability is the Implementation of formal specifications that detail every step in a task from safety to activity process to documentation, to ensure that anyone involved can follow the procedures and guidelines without confusion, and reach the desired outcome for all machinery types in the plant. Such specifications typically take from two to three months to develop and a further two to three months to roll out and fully implement. LUDECA has written a number of these specifications for customers worldwide. Let us help you as well.
by Alan Luedeking CRL CMRP
Guest Post by Bob Dunn from I&E Central, Inc.
A customer was having difficulty balancing the rotor shown above. They had made multiple corrections, some contradictory, and were worse than when they started. In that this is on a shop stand and controlled conditions, something was not right. Looking at the photo, I saw a couple of likely issues.
- The shaft is pretty reflective itself, it is doubtful that they were getting a good or consistent phase reading. I recommended they put a ring of black tape on the shaft, with the reflective tape on the black.
- The tach sensor is pointing at the shaft at about a 90 degree angle. Optical sensors and reflective tape works better if the sensor is aimed at an angle – 30 degrees or so.
- The tach sensor is pretty close to the rotor. In this case it is not too close, but you can be too close. A sensor like this will work from several feet away, if you are having problems, try moving the speed sensor further away.
The customer applied the tape and adjusted the tach position. The rotor was balanced in a single run.
by Yolanda Lopez
Proper equipment function requires a properly aligned and balanced machine. Allowing a machine to operate with an unbalance condition can result in bearing damage, cracks, loose components and many other costly maintenance issues. Loose debris can dislodge and impact the balance quality of a machine. Debris buildup on the impellers/blades, and other rotating parts can create unbalance conditions. Before balancing the machine it is very important that the rotating surfaces (blades, etc.) are cleaned of any debris. Removing buildup will help ensure that the machine can be properly balanced and remains in a balanced condition.
by Trent Phillips CRL CMRP - Novelis
The practice of reliability has many tools, processes and methodologies that can and should be implemented within a facility. Try as we may, it is usually not possible to implement and sustain all of them. So the challenge quickly becomes deciding which aspects of reliability to implement and in what order!
Implementation and enforcement of standardized work procedures is a very critical aspect of reliability and should be at the very top of your list of required reliability tools! Standard procedures focusing on fundamentals such as proper torquing, alignment, balancing, bearing installation, and equipment installation, should be in place. In addition, standard procedures for work request, work approval, planning, scheduling and work execution should be implemented as well.
Make sure that standard procedures are in place to execute the reliability methodologies at your facility. Otherwise, your site may always find it difficult to achieve sustainable and best practice maintenance and reliability.
Why? Unfortunately, people are usually the biggest obstacle we face in our jobs. People do not like to change, forget or misunderstand what needs to be done. Standard procedures will help ensure that reliability processes are routinely followed and things do not fall backwards to the unreliable way they have always been done. Additionally, it will provide the ability to track how well your facility or company is doing at implementing, executing and maintaining the reliability practices desired.
by Trent Phillips CRL CMRP - Novelis
- Right safety procedures before you balance.
- Right machines to balance.
- Right balancing procedure.
- Right balancing tool.
- Right balancing tolerances ISO or API.
- Right data collection
- Right weights.
- Right weights locations.
- Right corrections.
- Right balancing report.
Download [Infographic] 5-Step Balancing Procedure
by Ana Maria Delgado, CRL
Precision balancing is an essential part of a proactive reliability program as it can eliminate many machine failures and defects. This Infographic outlines an easy and effective way to balance your rotating equipment.
by Ana Maria Delgado, CRL
In today’s world, video platform is the way to accomplish effective visual knowledge and a learning mechanism in many organizations. With the use of video, one not only is able to promote products and services but one can also strengthen a culture and demonstrate how-to scenarios easily and quickly.
LUDECA believes in communicating visually to help customers educate and train their personnel on precision skills. For this reason, we are pleased to announce the release of our new microsite www.LudecaVideos.com, which features a Shaft Alignment Know-How series plus a Know-How series for Vibration Analysis and Balancing. The video site features basic terminology, fundamental concepts, advanced measurements as well as product demonstrations. The videos are indexed by category but also searchable by keyword.
We felt there was a need to go back to basics and help educate on precision skills and related technology to improve asset reliability. Following the Uptime Elements™ holistic approach to reliability, alignment and balancing are key components of your asset condition management (ACM) program. We are happy to offer these videos to our customers for their personnel to access and for use in their training programs. We hope this content assists them and others in either improving their reliability program or in getting one started and leads to world-class reliability programs,” —Frank Seidenthal, president of LUDECA.
We encourage you to visit www.LudecaVideos.com and see for yourself the value behind each video.
by Yolanda Lopez
The following blog relates to those who field balance using a photo or laser tach and reflective tape.
By far the most common pitfall to field balancing is a problematic tach signal. When one balances a rotor using one’s field balancing unit (VIBXPERT II, VIBXPERT or VIBSCANNER) the equipment is recording the energy displayed at the frequency of the signal from the tachometer. To help visualize the importance of a clear tachometer signal that is exactly 1 pulse per revolution, look at figure 1.
What amplitude will your equipment record if the tach pulses:
1. 1,195 times per minute?
2. 2,002 times per minute?
3. 2,006 times per minute?
4. 2,011 times per minute?
5. 2,013 times per minute?
We often start a balance job by haphazardly placing our tach and tape. Because both the tach and tape are well engineered, we may go on without a problem. But just a little attention to some of the common tach signal problems is usually all it takes to avoid having to restart a botched attempt at field balancing. What should be avoided when setting up a tachometer?
1. Don’t place your tach too close to the rotor. Most tachometers used in the field work by sending some type of light out and bouncing it back, so they have a sending function and a receiving function. The wavelength of the light is such that not just any light will be accepted by the receiver, but only that wavelength of light sent out by the sending unit. So the receiver counts a pulse every time that wavelength of light appears (or disappears, depending on whether you are triggering by leading or trailing edge). The receiver is no smarter than that, we must supply the rest of the intelligence. When we put the receiver too close to the rotor, even a poor reflector may be able to bounce back enough of the light signal to create a pulse. The balancing technician should determine the distance from the rotor to set up their tach with the understanding that they want a good signal bounced back from their chosen reflector, AND ONLY THEIR CHOSEN REFLECTOR! Most often, a 6 inch space is sufficient.
2. Don’t place your tach pointing perpendicular to the rotor. Earlier we stated that “both the tach and tape are well engineered”. One thing most of us field balancers take for granted is the reflective tape. This tape is actually a well-engineered tool. Reflective tape is faceted in such a way that light can strike it at an acute angle, and still be reflected right back along the axis from which it came. This allows the tach to be staged at such an angle that light will strike the rotor, even a rotor that is itself a good reflector, and be reflected off and away from the receiver UNTIL the tape comes into the line of the light, and then with its special faceting, it will bounce the light back to the receiver. This gives one clean pulse every time the tape comes around, and only when the tape comes around.
3. Don’t use old reflective tape that may not be in proper working condition. Make sure the tape is clean and in good shape. Reflective tape works very well when it is clean and in like-new condition, but can get dirty or even deteriorate if conditions are right. Replacing a small piece of tape is most often very quick, easy, and cheap compared to extra balancing runs or possibly even worse.
4. Don’t use a tach with dirty lenses. Make sure the tach lenses are clean and in good shape. When your lens is dirty, it forces you to do things (in order to get a strong enough signal to go through the dirty lens) that aren’t conducive to a clean, clear, once per revolution pulse; like move the tach too close to the rotor, or place it at a 90° angle to the rotor.
Doing everything we have suggested here could take all of 5 minutes (if you work slowly) at the beginning of a field balance job, but it could save a lot!
by Mike Fitch CRL
Recently I visited a customer’s facility to provide onsite training for the VibXpert vibration data analyzer they had recently purchased. Before we could get started collecting data, we needed to build the equipment hierarchy and measurement templates required. Once the database was created, we loaded routes into the VibXpert and proceeded to collect vibration data.
The first room we entered had two large belt driven overhung fans. At first glance it was obvious that one of the fans was running extremely rough. We collected vibration data on both fans and paused to review the results. We noticed that the 1× amplitude on the rough fan was over 1.0 inches per second peek. The local CM technician immediately commented that the fan should be balanced and his observation was correct when simply looking at the vibration data.
The room was full of clues which explained the cause of the fan unbalance. This facility processed and manufactured wood products. Large amounts of wood dust are produced and these fans were designed to ventilate a high dust area. Everything in the room was covered with wood particles and dust. The only question was how much had accumulated on the inside of these fans.
I asked if the fan could be stopped for a short period and the inspection door opened. My request was honored and the fan was shut down and locked out. Our examination revealed the fan blades had amassed substantial amounts of wood particles. The fan blades were cleaned and resulting in a pile of wood chips weighing about 5 lbs. The fan was placed back into operation and allowed to run for several minutes. Vibration data was recollected on the fan and the 1× amplitude had reduced to 0.1 inches per second.
Fans require corrective action to eliminate unbalance conditions from time to time. However, the cause of unbalance may simply be buildup of foreign matter on the blades. This fan was being allowed to beat itself to death due to product buildup. This facility learned a few lessons from the experience. First, inspections utilizing the human senses (touch, hearing, etc) could have been used to determine that this fan was in need of attention. Second, periodic vibration monitoring would have identified a need for maintenance on this fan. Third, if a fan is properly balanced, simply cleaning foreign matter buildup may reduce the vibration, prevent equipment damage and maintain the reliability of the equipment. Make sure that you utilize these three steps during your daily maintenance efforts on equipment.
by Dave Leach CRL CMRT CMRP
I have been using the VIBXPERT II Balancer for a year. I have balanced fans ranging in size from 10, 000 to 1 HP, mostly within 3 shots. My last customer had a contractor quoting a fan rotor replacement. Using the VIBXPERT II, the fan was balanced to 2 mils from 25 mils in 3 shots. The customer cancelled the fan replacement. The ability to acquire and view spectrum, review cost down data, and perform resonance testing are features not found in other balancers. —Victor Galanto, Fan Services Associates
by Ana Maria Delgado, CRL
If you want to find the secrets of the universe, think in terms of energy, frequency and vibration.” ? Nikola Tesla
Could Tesla’s secret be the energy wasted due to vibration at a frequency equal to shaft speed all caused by rotor unbalance?
Balanced rotors are critical for achieving production and profit goals. Unbalance creates high vibration, which leads to other faults resulting in decreased machine life, wasted energy and reduced efficiency. Smooth-running machines are required for producing products that meet customer specifications. The IOSR Journal of Mechanical and Civil Engineering states that rotor unbalance is the major cause of vibration problems. A good balancing process is essential for successful physical asset management.
Read my entire article Field Balancing Rigid Rotors at Reliable Plant.
by Bill Hillman CMRP
As more and more wind turbines are coming out of warranty, the industry is focusing on making sure that the assets they have in place are in proper running form. If it is the gearbox, the generator, and/or the rotor blades, they need to be inspected and/or checked.
In the case of the rotor blades, if they have had any sort of work performed on them such as repairs after lightning strikes, moisture in the blade, removal and/or addition of coating on the blades, etc. It is good practice to check the mass unbalance. By checking mass unbalance first and then performing the balance job, this will constrain the vibration levels to acceptable tolerances such as the VDI 3934. It will also help reduce the amount of wear on the rolling elements and gears.
by Alex Nino CRL
Is your equipment considered a slow running speed machine? If so, what speed do you consider slow? Is it 30 RPM? 60 RPM? 100? 200? 600?
No matter what you consider slow speed, the two most critical points to consider for slow running equipment are:
1) Does your vibration sensor (accelerometer) have the appropriate frequency range to measure low frequencies?
2) Does your vibration analyzer and/or online monitoring system measure down to those frequencies?
Unfortunately, some vibration analysis devices on the market are not truly capable of measuring slow speed equipment and providing a true mechanical diagnostic analysis. These devices can actually create a reactive maintenance result that the device was supposed to prevent.
For example, a motor shop in South Texas had completed a rebuild of a 100 HP motor. The motor is used in the oil and gas industry. It has an average running speed of 30 RPM. The customer tested the motor on their motor test stand. As it was in its test cycle, vibration was measured using a self-diagnostic vibration analyzer. The results and diagnostics the analyzer provided to the customer was “please replace bearing”. After several further tests running the motor on the test stand, the customer refused to accept those results and retested the motor using a VIBXPERT II analyzer with machine templates designed for slow running machinery and a VIB 6.147 low frequency accelerometer.
The final analysis revealed a high unbalance condition on the motor (11 mils peak-peak). The motor shop followed up with a balancing job (single plane) on the motor. The balancing was performed with the VIBXPERT II as well. Subsequent tests showed that the unacceptable low frequency amplitude that had been observed (11 mils pk-pk) prior to performing the balance job had now disappeared. Final mechanical diagnostics showed no problems and the bearings were in proper condition. A balance report was printed and the motor was ready to leave the shop.
If you want to increase your uptime and availability and reach your financial goals, a proper investment in Condition Monitoring and reliability will provide a positive return on your investment.
by Alex Nino CRL
We recently ran a poll to find out what the Top Machine Faults are for the attendees of the IMC-2012 International Maintenance Conference. Here are the results, which came from maintenance and reliability professionals who attended our Learning Lab:
Bearing Failure: 31%
The good news is that all our lab participants were acquainted with our Condition-based Maintenance tools which can help them detect, prevent and correct all these problems.
It is essential to understand how equipment performs in a facility and to be able to identify these common machine reliability issues before they result in functional failures in your equipment. Payback technologies like vibration analysis, alignment and balancing when part of a comprehensive condition monitoring program can improve your equipment performance, reduce equipment downtime and minimize risk.
by Ana Maria Delgado, CRL
Vibration analysis is the best all-around technology for diagnosing and predicting problems in rotating machinery. Over the years I have seen time and time again where adopters of this technology have saved themselves and their companies countless man hours and thousands of dollars by getting to the root cause of a problem early on. By analyzing the data, they are able to schedule their valuable time on the right problem on the right machine long before the problem escalates into a major outage or emergency. But too many companies have not adopted vibration analysis. While it is true that one could spend many years learning the skills of the multiple levels of the vibration analysis disciplines, it is also true that even a basic understanding of the relationship of the time waveform and the spectrum can yield huge benefits and savings to a new user.
For example, the root cause of most roller bearing/seal failures is either shaft misalignment or rotor imbalance, which can take months to develop. It is also the most common problem analyzed within most facilities in the first two years of vibration analysis implementation. The good news is that misalignment and rotor imbalance are the easiest problems to diagnose by observing a high amplitude 1× running speed frequency in the spectrum. After that, a phase analysis with your analyzer can easily differentiate between misalignment or an imbalance problem, and quickly completed without shutting down the machine.
We all know that Rome wasn’t built in a day but we all must start somewhere and just a few days in an analysis class could yield major benefits to new companies.
Thanks to Jay Gensheimer with Solute LLC for this valuable post.
by Ana Maria Delgado, CRL
Thank you for joining us for our Webinar The Field Balancing Mine Field by Greg Lee. We hope you found the presentation to be valuable and very informative. If you missed our Webinar, you can view the recorded version at any time. Watch now!
Here are Greg’s answers to your questions:
Q: Do you have experience balancing cooling towers?
A: Yes. Cooling towers are interesting because there are a number of causes for vibration. One very dangerous condition that can look like unbalance is a cracked hub. This can lead to a catastrophic failure of the hub, allowing the blades to break free and wreak havoc on anything near. I once saw the result of a hub failure that caused the gearbox to break through the wood mounting frame and fall into the water tank. The motor was still running with a 12-inch piece of jack shaft flailing around.
With cooling towers, it is especially important to run a complete vibration analysis before attempting to balance. The customer in the example above had another cooling tower cell with the same cracked hub problem. We caught that one before the failure using the VIBXPERT II vibration analyzer. It was showing a high 1× radial vibration as one would expect from unbalance. In addition it was showing a high 1× axially, as large as the radial. The spectrums also showed high 5×, 20× and 25× frequencies as the blades bobbed up and down as they passed the 4 main gearbox supports and the jack shaft. This is derived from 5 blades times 4 supports for 20× and 5 blades times 5 (4 supports and 1 jack shaft) for 25×.
Q: What about a multiplane, multipickup balance? i.e. Nuclear rotor train, 4 rotors, 8 bearings?
A: I am not sure what your exact question is, but, multiple rotors in a single train can be complex to balance. If the cross effect from plane to plane is large, the complexity grows exponentially. I worked with a field engineer that balanced a long train of 4 generators and 2 steam turbines using the trial weight field balancing method. It took him a week to balance this system of rotors and bearings.
For something as complex and expensive as you describe, I would bring in the OEM or a company that specializes in Nuclear Turbine applications. Because of the number of bearings, they would most likely use a 16- or 32-channel dynamic data recorder versus trying to use a typical two-channel field balancer.
Q: To select a trial weight, is there a ratio between the machine weight to trial weight to get the correct change in phase or amplitude?
A: Many of the companies that produce field balancing equipment have developed proprietary formulas to calculate how much trial weight to use and where to place the weight. The intent of these formulas is to obtain a 30% change in amplitude and/or a 30% change in phase angle. It should be understood that these derived trial weights are guides, not an absolute. In most cases, these formulas take into account rotor weight, speed and the amount of initial unbalance. The instrument then calculates the suggested trial weight and its position.
There are a number of “Trial Weight Formulas” used. For example the United States Department of the Interior Bureau of Reclamation recommends that the “trial weight should be approximately equal to the weight of the rotating parts divided by 10,000.” Most of the time in field balancing the weight of the rotor is not known or is at best a rough guess. In these cases it is advisable to look at the correction weights previously placed on the rotor and use these as a guide.
Q: How do you determine if it is hydraulic imbalance instead of mechanical?
A: By hydraulic unbalance, I take it that you are referring to internal hydraulic forces in a pump. Fans can experience similar interference from wind. Unbalance will manifest itself at exactly 1 times the running speed. The unbalance vibration amplitude will be exhibited primarily in the radial direction. If you see a lot of axial vibration (50% or more of the radial) then you likely have additional problems that are not balance related.
For hydraulic problems, look for an additional frequency equal to the number of vanes times the running speed. Hydraulic instability in a pump is often seen in spectrums as low frequency broad-banded vibration below the running speed. Often, hydraulic problems are accompanied by cavitation. There are specific Shock Pulse measurements which will help you identify cavitation.
Q: On a balance stand, ideally you would want to be at running speed. Most of the time this is not possible due to size/mass etc. How much difference does it make if you can only run at slower speeds such as 30% of operating RPM?
A: First, it is important to understand that balanced is balanced at any speed. For an object to be balanced, the rotational centerline and the mass centerline must be the same. This will hold true at any speed. Because of this, it is not necessarily true that the best stand balance is at running speed. For clarification, please refer to the first few slides of the presentation.
Now, to the answer: In general, as long as one is away from the rotor’s critical resonance speeds, it is fine to balance a rotor at a speed lower than running speed. No percentage rule is necessary. Just stay away from critical speeds. With that said, there are some differences between balancing machines. There are two primary types of balancing machines; a Soft Bearing Machine and a Hard Bearing Machine. Both types have advantages and disadvantages.
- Soft Bearing Balancer
A soft bearing balancer allows the rotor ends to move freely in the horizontal direction in the balancing stand. This type of balancer allows the rotor to turn at much slower speeds than the rotor’s operational speed. The balancing procedure is almost identical to field balancing and a calibration or trial weight is used to test the response of the rotor. In this way each rotor balance is in essence self-calibrated. Like field balancing, multiple runs are required and the correction and trim weights are applied until the rotor meets the acceptable criteria. As long as the speed is above the resonance of the soft work supports, and not at the rotor’s critical speed, the response will be linear and very accurate. Some of the largest steam turbines in the country have been balanced using soft bearing work supports resting on rail road tracks. These rotors are balanced at speeds around 30 RPM. If one is concerned about the number of runs in a stand, then a hard bearing machine might be preferred.
- Hard Bearing Balancer
A hard bearing machine fixes the rotor ends to the balancing stand pedestals. This system only requires one run to determine unbalance and correction weights. A hard bearing stand measures force, rather than motion like the soft bearing machine. If one knows the force and angle of the unbalance plus the weight of the rotor, a correction can be calculated. The advantage is that only one run is required to determine correction weights. However, because the hard bearing machine measures force directly, the accuracy is sensitive to speed. If the speed of a rotor doubles, the force increases by a factor of 4. Thus the higher the speed, the higher the measurable force and the better the accuracy of the balancing stand. One may be nervous about running rotors such as fans at higher speeds due to wind forces. In this case, a soft bearing machine would be better.
Q: What do you suggest if site balancing requires disassembling the pumps to get access to the impeller, Isn’t it worth doing in a balancing machine in the workshop?
A: Of course this depends on a lot of factors. If one has to disassemble the pump to add or remove weight, it is probably preferable to remove the pump rotor impeller assembly to a balancing machine.
Q: For which machine sizes is site balancing more effective—small and medium machines or heavy duty machines?
A: In general terms, the larger a machine, the more expensive and difficult it is to move. Thus the strongest case for field balancing is for larger machines. However, machines like fans can be quite small and easy to access. Field balancing is not limited to large expensive machinery. It really depends on the application and the access to insert correction weights.
Q: Is site balancing a valuable condition to ask for during engineering and procurement stages?
A: I would recommend that any piece of new or used equipment being purchased have vibration and unbalance limits included in the specification. I would refer you to the International Standards Organization (ISO) balancing and vibration standards for an internationally recognized reference for vibration standards. If you are in the petrochemical industry, I would recommend looking at the American Petroleum Institute (API) specifications for vibration.
Q: Isn’t a coast down needed to find out if the machine is operating above or below critical speed to get a correct balance solution?
A: Yes. It is highly recommended that one identify the resonances of a machine before attempting to balance. Field balancing at or near the critical speed can cause issues with amplitude and phase measurements. As a general rule, one should stay approximately 20% away from a shaft resonance when balancing. Because field balancing is basically a vector ratio problem, the field balancing technique will work fine for rotors running above or below the critical.
Capturing phase and amplitude during coast-down or startup is one of the best ways to identify the resonant frequencies of a rotor. In the majority of situations, it is preferable to capture a coast down, as the data will not be influenced by the motor torque like it is during a startup. With phase and amplitude data, one can view Bode and Nyquist plots which graphically identify the resonant frequencies.
Another method is to capture a cascade plot of spectrums as the machine starts up or coasts down. Once again, this provides a particularly graphic method to identify a rotor’s resonant frequency.
Finally, perhaps the most common method of identifying resonance is a bump test. This method can be used while the machine is off. If your analyzer supports negative averaging, one can perform a bump test on a running machine. The result shows a frequency spectrum where the peaks represent the resonant frequencies of the object being bump tested.
Q: For what size, speed, and HP machine would you recommend the installation of an external balance disc on the rotor to make field balancing and adding weights easier?
A: If it is the type of machine that would go out of balance often, is expensive to remove from service, does not have an easy way to add or remove weight, or is difficult to move to a balancing stand, I would recommend installing balancing disks.
Q: Have you experienced balancing long shafts where maybe 2 planes are not enough?
A: Absolutely. If a shaft is long and flexible, additional planes may be necessary. There is no hard and fast rule that states if a shaft is 10 times longer than its diameter, additional planes will be required. Often, shafts will be supported by more than 2 bearings. This would generally lead one to balance in more than 2 planes.
Q: I have heard that vibration due to misalignment conditions can be minimized through balancing but that seemed contrary to a remark made during the presentation. Can balancing be effective in reducing machine response due to misalignment? Thanks.
A: The first field job I did was for balancing a high speed, direct drive fan. When I got there, an analysis revealed that there was a high amount of fan unbalance, a large amount of misalignment, and a very loose cork base. The unbalance contributed to the looseness and the looseness caused the base to flex and all of these contributed to the misalignment. The looseness contributed to the unit’s ability to vibrate at 1× the unbalance frequency and flex in the frame allowed additional misalignment. The misalignment also contributed to the base looseness and the amplitude of the unbalance. Any machine is a system, and, in this case, each condition made the other conditions worse. They fed each other, but each condition must be corrected to fix the machine as a unit. For example, if the looseness was corrected first, it would have zero effect on the balance. By clamping down the base, more of the unbalance force is transferred to the bearings. If the imbalance is left uncorrected, the bearings will fail early. The unit still needs to be balanced and balancing will not correct the looseness or misalignment. Since the balance is in essence a forcing frequency, the looseness may go down in amplitude but the machine is still loose.
In this case, the first problem to fix was the cork base. It was removed and the fan grouted in, thereby eliminating the looseness. If this were all we did, we would still have unbalance and misalignment. So next we aligned the motor to the fan shaft. Once this was done, the unit was started and the fan balanced.
In a pinch, we could have balanced the fan first. It is likely that the looseness and misalignment would have been reduced, but would still have been present and feeding each other. So I would say that balancing might reduce the symptoms of misalignment but not correct the misalignment. The inverse would be true for correcting the misalignment.
Q: What is the difference between field balancing and using a balancing stand in a motor shop?
A: If the balancing stand is a soft bearing type, very little. The process and math are the same. In the field, there is less response linearly in the structures when compared to a soft bearing stand. Thus, in the field, you can expect to see a little less unbalance reduction when placing correction weights. This effect in the field is minimal.
If the balancing stand is a hard bearing type, then the shop process is a little different. A hard bearing system measures force directly. Knowing the weight of the rotor, the RPM and the force of unbalance, one can calculate the correction. The results are nearly identical to a soft bearing machine.
Q: We have problems in balancing fans at full operating speed due to operational factors. What percentage of operating RPM should we try to balance at and what problems could we look for not balancing at full operating speed?
A: There is no specific percentage of running speed that will yield better results. If you can reduce speed, then make sure you are not near a resonance where phase angle and amplitude shift. A Bode or Nyquist plot taken during a startup or coast down is best for identifying the resonant frequencies of the fan. Refer to the first few slides of the presentation. When the mass and rotating centerline are the same the rotor is balanced regardless of the RPM.
It is also important to make sure your fan is truly out of balance. On a belt driven fan, check for sheave eccentricity where the sheave is off center or out-of-round. This can cause vibration that looks like unbalance. For instance, look for other influences such as air turbulence, unequal blade pitch, and looseness, to mention just a few.
Q: How do you calculate system lag? And will it change based on RPM?
A: By system lag I assume that you are referring to the balancing system. In the old days, when we commonly used strobe lights to determine phase angle, there was significant lag in the electronics. By knowing this, we were able to shortcut the balancing procedure and determine the heavy spot of a rotor. Often this lag was about 40 degrees between when the heavy spot passed the transducer and the strobe triggered. With the digital equipment we use today, electronic lag is virtually eliminated. For example, I was balancing a spindle turning 40,000 RPM and was seeing less than 5 degrees of lag.
The easiest way to determine your instrument’s lag is to get a rotor that is balanced, place a weight at a known position, and see your instrument’s result. By using the field balancing procedure built into today’s modern balancing instruments, lag is automatically compensated for in the balancing procedure.
Q: How to distinguish couple unbalance and quasi static unbalance?
A: Look at the phase angle of each plane. If they are the same, it is purely static. If they are 180 degrees opposite from each other, it is pure uncouple unbalance.
Q: What is the maximum level of vibration at which we can perform in-situ Balancing?
A: There is no set amount of unbalance where we cannot perform a field balance. Of course one must apply a little logic. If the vibration is so bad that it is causing damage, then it might be wiser to pull the rotor and place it on a balancing stand.
Q: Is there any procedure to perform single-shot balancing rather than 4-run method?
A: For a single plane balance, it requires 2 runs to secure a solution and an additional run to verify the result. With a two-plane balance it takes 3 runs to secure a solution and a 4th run to verify the result. Normally, in the field this is the best approach.
On a journal bearing machine where the masses are known and the heavy spot is verified, one can calculate how much weight is needed to reduce the vibration. This would only take an initial measurement to determine. However it is rare that we know the precise weight at each bearing, and even this process often takes multiple runs.
Q: Must the 30 -30 rule be followed for on-site balancing?
A: In the words of Captain Barbosa “…the code is more what you’d call guidelines than actual rules.” The 30-30 rule is under ideal conditions. I have balanced where I got the phase exactly right so the trial weight change was more like 0 degrees and 15% amplitude change.
Q: If we change angle of blades of cooling towers, will it have any effect on balanced impeller (During balancing let’s say we have 11 degree angle of blade, and then we have to change angle to 7 degrees because of process requirement)?
A: If one blade’s pitch is off relative to the other blades, it will look like unbalance. However, this condition would be accompanied by a lot of axial force at a frequency of 1× because of the unequal blade pitch. If it were pure unbalance, then the axial force would be steady and not have a large 1× frequency component. So, in your example, as long as the blades pitch the same amount and the aerodynamic lift changes equally on all blades, the rotor will still be in balance.
Q: Do quasi static unbalance and couple unbalance have the same solution or something else?
A: You seem to be mixing terms. When using a two plane balancing technique, the program takes into account the cross effect between planes A and B. Separating the static and couple balance is possible, but with the accuracy of today’s instruments, it is rarely done.
Q: Is there any effect, if we put the trial weight at 75% RPM and then correction weight at 85% RPM?
A: The context of your question is not clear to me. I think you are saying, if the speed changes during the initial measurement, trial weight measurement and the correction measurement, will this have an adverse effect. Yes, changing speeds can create problems with the field balancing vector solution. Sometimes it is impossible to take measurements at the same speed, and, the more this speed varies, the less accurate the balancing solution will be.
Q: Could you please go over the “no phase balancing” variation that you talked about in more detail?
A: To review the process would require an article. This process is primarily used on single plane problems. There are 4 steps required to calculate a solution.
- Take an initial amplitude measurement.
- Place a known amount of weight at zero degrees and take a second amplitude reading.
- Remove that weight and place it at 120 degrees. Take the third amplitude reading.
- Remove that weight and place it at 270 degrees. Take the third amplitude reading.
- This data can be plotted on polar paper to determine a solution.
There is more information on this process available on the Internet.
Q: Rather than try balancing at near critical resonance speed, would it be beneficial to try and stiffen structure to move resonance away from balancing and operating speed?
A: Yes this can be, and is done. Many times it takes a lot of stiffening or adding mass to significantly shift a resonance. Changing speed is easier, if possible.
I once balanced a large vertical fan in a 40 foot high tower. The tower was resonating and causing problems. We loosened the guidewires going to the top of the structure to decrease the stiffness. This lowered the resonant frequency and helped us achieve a good balance.
I hope these answers were beneficial to all of you. If you have any additional questions, please feel free to contact Greg Lee, the presenter, directly.
by Yolanda Lopez
Accumulation of dust and dirt on all rotor blades lead to a 1x vibration peak in the velocity spectrum. The sine waveform and phase analysis confirmed the results. A static unbalance was the reason for the increased vibration.
The balancing procedure was successfully performed on-site during the next shutdown phase of the plant. VIBXPERT II and OMNITREND software were used for the balancing runs. The static unbalance requires only a one-plane balancing procedure which was ideal for those fans. The accelerometers were attached to the non-drive end (NDE) bearing in horizontal direction. VIBXPERT offers a “second plane control feature” where the second accelerometer contorls any negative influence on the NDE bearing during trim runs. This ensures that the vibration on both bearing will be equally reduced and balanced. The target quality grade of 6.3 according DIN ISO 1940 was easily reached.
The plant went back online a few days later and the customer was extremely satisfied with the result of this service, who later stated:
“In the last 6 years, I have never seen those fans run so smooth.”
Special thanks to PRUEFTECHNIK Canada for sharing this successful story with us.
by Yolanda Lopez