Soft foot poses a challenge for plant operations and maintenance personnel.

The common term for machine frame distortion, soft foot is caused when one or more feet of a machine are shorter, longer or angled some way different than the rest of the feet. This non-uniformity causes stress on the machinery when the foot is forced into place by tightening the hold-down bolt. Missing shims under a foot, a bent foot, or a deteriorating base plate or foundation can cause this condition. In addition, pipe stress can cause machine frame distortion and, therefore, also is considered a soft foot condition.

When soft foot is present, the intended machine design and clearances are compromised. It can cause high vibration levels because the machinery is unduly exposed to excessive wear with each shaft revolution.

Deflection and Damage
Often, soft foot is not seen as a critical problem, but let’s investigate the damage it can cause. When machine frame distortion exists, the bearing housings are misaligned with respect to one another. This offset, as well as angular misalignment, creates a load on the rotating shaft that results in shaft deflection. When the shaft turns, this results in vibration since the shaft must deflect by double the amount of the deflection at rest, at twice the speed of rotation. For example, at 1,800 revolutions per minute (RPM) running speed, a soft foot distortion provokes 5.184 million deflection cycles every 24 hours (Figure 1).

Figure 1. An example of preload on the bearings.
At 1,800 RPM, which equals 60 stress-reversal
cycles every second (reverses twice in each full
rotation), this is nearly 5.2 million back-and-forth
deflection cycles every 24 hours.

It is, of course, the bearings that directly transmit the forces required to do this to the shafts. Therefore, soft foot greatly increases the load on the bearings, results in increased power consumption and provokes metal fatigue of the shaft over time. Bearings and seals suffer premature wear and are prone to premature failure.

Vibration data can reveal signs of many machinery health problems, among them soft foot. How do you determine if soft foot is the culprit? A machine is determined to have a soft foot if the calculated value of movement of a machine foot when tightened or loosened is at least 0.002 to 0.003 inches while the remaining feet are bolted tight. This condition often (but not always) causes an increase in overall machine vibration.

If the overall vibration level is too high, the next step is to determine why. A common practice is to take a spectrum on the bearing housing of the equipment. If safety is not compromised, take measurements while the machinery is operating. Take a spectrum in live mode. This is a feature of some data collectors that allows the user to continuously view the spectrum while the machinery is running. This is handy if you are trying to view a transient event.

To help determine a soft foot issue, loosen each foot one at a time, always keeping the others tight, and then retighten while the spectrum is still measuring. Any notable reduction in the one-times (1x) energy during the loosening process can be a strong indication of the relief of machine frame distortion. The graphs in Figure 3 display typical before and after conditions.

Figure 3a. Before the loosening sequence:
All four feet are tightened; the 1X amplitude
is 0.127 inches/second. Radial measurements
were taken in the vertical direction.

Figure 3b. After the loosening sequence:
One of the four feet was loose; the 1X
amplitude is now 0.048 inches/second.

This is a great indication of which foot has the greatest impact on relief of soft foot distortion, but you will need to further investigate with a good laser alignment tool, dial indicator or feeler gauges to properly measure and correct the problem.

Often, the soft foot signature or pattern is hard to detect. Commonly, soft foot appears at 1x the running speed of the equipment, both axially and radially, but is occasionally present at two and three times RPM.

Since soft foot can also affect alignment, misalignment characteristics may appear. The best indication of soft foot is an elevated 1x RPM vibration value. This is typically in the neighborhood of 0.3 inches/second or higher. Very often, there is a significant 2x alternate current line frequency peak. The presence of this peak helps differentiate soft foot from imbalance and misalignment.

Using Phase Analysis
The signature of soft foot sometimes appears similar to looseness because the foot may, in fact, be loose. Foot looseness may emerge due to weakness in the base plate or foundation, deterioration of the grouting, loose hold-down bolts, or a cracked or broken foot. All of these situations are part of machine frame distortion. Phase analysis will help reveal this.

Phase analysis documents the relative motion between two different points. You can use it to determine continuity between two adjacent pieces of machinery (i.e., machine frame and supporting foundation). Phase analysis may show a 90- to 180-degree phase difference between vertical measurements on the bolt, machine foot, base plate or base (Figure 2).

Figure 2. Phase relationship can show differences
between vertical measurements on the bolt,
machine foot, base plate or base.

A quick way to acquire this data is with a data collector that offers cross-channel phase measurements. This allows the user to take vibration readings with an accelerometer and ascertain the phase information without the use of a tachometer. This saves a considerable amount of time due to the skipped setup time that is necessary when a tachometer is used to collect phase data.

Irene Hamernick is an applications and sales engineer for Ludeca Inc. For more information on vibration analysis and soft foot conditions, contact her at 305-591-8935 or irene@ludeca.com. Also, visit the Ludeca Web site at www.ludeca.com.

"Noise and vibration testing is an increasingly important topic in many industries, including on-road and off-road transportation, aerospace, utilities, and manufacturing," said Percy Wang, Ph.D., manager of the Noise and Vibration group at Southern Research. "This workshop will provide both a basic and more in-depth understanding of noise and vibration testing, and how it can be used to better understand and resolve engineering problems across a wide range of industries."

"Applications for Noise and Vibration Testing" is a free, one-day workshop being offered at the Southern Research Engineering Research Center, 757 Tom Martin Drive, in Birmingham, Ala.

  The workshop will consist of two sessions:
   - Morning Session: 90 minutes of interactive lecture covering the basics
     of noise and vibration testing
   - Afternoon Session: Hands-on laboratory activities to reinforce concepts
     learned during the morning session
   - The workshop will also include a brief tour of the Southern Research
     Engineering Research Center.

  After completing this workshop, participants will understand:
   - The language of sound (noise) and vibration testing
   - The scientific principles behind noise and vibration testing
   - How noise and vibration engineering can be applied to practical, day-
     to-day engineering challenges as well as complex, advanced engineering
     research
   - How to better assess the need for noise and vibration testing on the
     job

Because of the individual, hands-on participation in this program, space is limited to 20 participants. Registration will close April 10. For further information, call 205-581-2532 or visit the workshop Web site at http://www.southernresearch.org/engineering-workshop.html.

Southern Research is a not-for-profit organization that conducts scientific research at facilities in Alabama, Florida, Maryland and North Carolina. Southern Research provides contract research in the fields of advanced engineering, environmental and energy-related research, and preclinical drug discovery and drug development. For more information, see www.southernresearch.org.

A Reliable Plant reader, a maintenance planner at a major U.S. utility, recently submitted this case study. It offers invaluable information to all of you who work on pumps.

During monthly vibration data collection rounds, a power station’s 500-horsepower vertical turbine circulating water pump experienced a large increase in vibration compared to the previous month readings.

The running speed vibration trend showed the November 2004 reading went from a normal 0.257 inches/second velocity to 0.468 inches/second in February 2005, and then increased again to 0.637 inches/second in March. Analyst review of the data found the largest response at the top of the motor, which is normal for most vertical pumps as they tend to pivot at the pump discharge head mounting to the floor. Resonance is common with this design of pump, however, the natural frequency of this pump/motor system was found to be well above the 500 rpm operating speed of the pump through the use of a simple bump test. The waveform pattern showing a predominant 1x (running speed) cycle supported an imbalance/wear condition.

Knowing this, shaft displacement readings were taken in an effort to judge the severity of the problem. After using a strobe light on the exposed pump shaft below the coupling to ensure no keyway or protrusion would interfere, a shaft stick (fish tail) with the vibration sensor mounted on it was safely held against the shaft. In the past, the shaft displacement had generally been around three to five mils. Now the vibration exceeded 16 mils, all at running speed.

A spring outage was previously planned in three weeks for this unit. Due to the rapid change in vibration and the severity, recommendations were made to pull the pump during this outage. The pump was monitored on a bi-weekly basis until the outage to ensure it would not catastrophically fail. 

Preparations were made for impeller replacement as this was likely the source of the high running speed vibration due to imbalance, possibly from a broken vane due to foreign object damage or excessive wear ring clearances. This type of damage was suspected because this pump is in a pit fed by a nearby river and has a history of damage due to debris. River water is screened into the intake house, however, during the spring, high river levels tend to bypass debris and repetitive failures of the screen wash system have allowed debris past the screens which travels to the pit at the plant. 

However, plant management still needed convincing that this was the problem, so prior to the outage, divers were brought in to inspect the circulating water pit for debris and to look at the pump impeller. The pit was surprisingly clean, but the diver found the pump impeller tips damaged and an obvious offset to the fit of the impeller to the bowl. This likely meant that the impeller wear ring was worn excessively and/or the lower shaft was bent.

Armed with this information, the pump was then scheduled for repair during the outage. Upon inspection, the impeller wear ring was found worn, blade/vane tips were damaged/bent and the shaft was bent. Parts were available for repair, but additional time was required to order a new shaft. The condenser was found with excessive debris, so the hypothesis is that the pump had passed large debris that likely bent the shaft. Repairs were completed within the timeframe of the scheduled outage and the pump was successfully returned to service. 

After the outage, an inspection of the remotely located river water intake house found damaged intake screens and inefficient screen washing. In an effort to prevent any future failures to the circulating water pumps, repairs were made to the screen and a new screen wash pump with a stand-by unit were installed. A new control system was added to sense differential pressure on the screen improving the overall efficiency of this system. Regular preventive maintenance inspections of the intake house system were developed and scheduled in the computerized maintenance management system to ensure this often neglected system is kept in good condition. To date, less debris has been found in the condenser water boxes and the circulating water pumps have been running fine, but we are still fighting the high water levels during the spring rains.

In the basement of the engineering building on the University of Missouri-Columbia campus, P. Frank Pai is trying to find a quick, convenient and inexpensive method of detecting cracks in airplane parts.

Pai is trying to develop a portable system that can test a part while it's on the aircraft. He has come up with a computer program that analyzes data collected from parts that are vibrated and simultaneously scanned with a laser beam.

Read the full article by clicking on the link below:

http://www.asminternational.org/Content/NavigationMenu/News/HeadlineNews/HeadlineNewsArticle.htm?SMContentIndex=1&SMContentSet=0

Qualmark Corporation, a world leader in designing, manufacturing and marketing HALT (Highly Accelerated Life Testing), HASS (Highly Accelerated Stress Screening) and electrodynamic systems, on April 10 announced the asset acquisition of Derritron Inc., an electrodynamic vibration system repair and rebuild company. Terms of the deal were not disclosed.

“The acquisition of Derritron strengthens our product offerings, accelerates innovation and meets customer demand more rapidly. Derritron’s core business complements Qualmark’s electrodynamic technology, through its subsidiary Ling Electronics, to provide an overall customer reliability solution,” stated Charles D. Johnston, president and CEO of Qualmark Corporation. “We believe this is a strategic acquisition that will expand and create new sales opportunities.”

Derritron manufactures a line of electrodynamic shakers, controllers and amplifiers.

“This acquisition continues Qualmark's strategy to be the world leader in vibration testing,” said Johnston. “With Qualmark's acquisition of ACG Dynamics and Ling Electronics, leaders in service, repair, rebuild and new electrodynamic shakers, we continue to offer a full complement of service and electrodynamic shaker equipment.”

The Reliability in Action department features case studies submitted by our readers. To have your case study considered for inclusion in an upcoming issue, e-mail it to parnold@noria.com or mail it to Reliable Plant, P.O. Box 87, Fort Atkinson, WI 53538. If we publish your case study, we'll send you an official Reliable Plant T-shirt. This issue's case study was written by Brian Miska, technical analyst at Consumers Energy's J.C. Weadock plant in Hampton Township, Mich.:

Figure 1 displays readings before the exhauster bearing housing replacement.

Vibration analysis is conducted monthly on four Combustion Engineering Raymond Bowl coal pulverizers (Model 673), which supply fuel to a 1.028 million-pounds-per-hour boiler, which supplies steam to a turbine and 165-megawatt generator.

Figure 2 is after the exhauster bearing housing replacement.

During routine vibration data collection on the Unit 8, No. 4 pulverizer exhauster, high vibration was discovered on the outboard end of the bearing housing, which supports a 6-foot-diameter overhung fan. The overall vibration level in the spectrum was .38 inches per second (IPS) velocity in the vertical and horizontal direction. Normal bearing vibration should be less than .10 IPS velocity. Vibration data collection and monitoring was conducted over several days and it was found that the vibration level would change from one startup to the next. Vibration analysis determined the vibration was caused by mechanical looseness, which allowed the bearing to spin in the housing.

The bearing in question is located in the center of this picture, to the right of the exhauster wheel.

A maintenance work order was written to replace the exhauster bearing housing during the next pulverizer outage. After the replacement of the bearing housing, the pulverizer was returned to service and the vibration level was reduced to .08 inches per second. Inspection of the bearing housing found evidence of the bearing spinning in the housing, which was caused by excessive clearance between the bearing and the bearing housing sleeve. The plant maintenance department determined that the machine shop, which installed the new sleeve for the bearing housing, used the incorrect bearing outer race measurement.

An inspection of our spare exhauster bearing housings found two additional bearing housings with the incorrect bearing sleeve size. The identified bearing housings were returned for the proper repair, and all newly rebuilt bearing housings will be measured to ensure dimensional accuracy upon return from the local machine shop.

Identifying and correcting the exhauster bearing problem during a scheduled equipment outage prevented a production loss of 30 megawatt hours, with a potential cost of $58,000. Damage to the pulverizer gearbox also could have occurred, which would have resulted in extra downtime and repair costs.

Top-quality electric vibration equipment is an engineered solution that must be installed and operated properly (according to the manufacturer's instructions) in order to obtain maximum benefits.

In continuing efforts to ensure that users get the most out of its products, VIBCO Vibration Equipment shares with you some of the most frequent installation missteps and, of course, what you can do to fix it if any of these issues apply to your electric vibration application.

The top five most common electric vibration equipment performance issues are:

1. The electric vibration equipment is mounted directly to the bin or hopper rather than to a mounting plate and mounting channel.

Mounting directly to the bin or hopper can cause the bin or hopper to crack, can cause damage to the vibration equipment, and can cause a high amperage draw. It is critical for vibration equipment to be mounted to a plate and channel.

2. The electric vibration equipment is mounted to just a plate or just a channel.

It is extremely important to mount vibration equipment in the manner recommended by the manufacturer. Vibration, while an effective material flow aid, can be a destructive force if it is not isolated properly. Using a mounting plate alone, or a channel alone, may not be sufficient to secure and isolate the vibration force exerted by the equipment. For best results, use a channel iron and mounting plate together.

3. The electric vibration equipment has been mounted in an incorrect position.

Electric units should be mounted perpendicular to the channel iron, and the mounting plate should be checked for warping and shimmed, if necessary, to achieve a tight seal. Any looseness or "give" will prevent the vibration force from being properly isolated. A sufficient length of channel should be used to distribute the force throughout the bin or hopper.

4. The welding, shimming and/or bolting of the vibration mount is problematic.

Mounting channel should be stitch-welded to the hopper or bin, with no welds on the corners of the channel. Welding on the corners of the channel can lead to bin damage and unit failure. All bolts should be tightened down securely to prevent slippage and all shims should be positioned to keep the mounting plate flat.

Pneumatic vibration equipment is an engineered solution that must be installed and operated properly (according to the manufacturer’s instructions) in order to obtain maximum benefits. To that end, VIBCO shares some “lessons learned” regarding the installation and operation of pneumatic vibration equipment.

Its top five pneumatic vibration equipment lessons learned include:

1. Correct installation and mounting is critical to performance.
2. The condition, size and configuration of your air line is important.
3. Correct air pressure (PSI) and air flow (CFM) are essential.
4. Periodic checks for loose mounting fixtures can extend the life of your pneumatic vibration equipment.
5. If your unit requires lubrication (piston and ball style), make sure that your lubricator is functioning correctly and is no more than 5 feet away from the unit.

The best way to ensure that you receive the maximum benefits and life from pneumatic vibration equipment is to follow the manufacturer’s installation and operation instructions.

(VIBCO instruction manuals are online for your convenience.)

1. Correct installation and mounting is critical to performance.

Make sure to follow all of the mounting and installation steps exactly as they are provided in the manual. One of the ways you can ensure a good mount is to select the correct mounting plate and channel for your bin. The following charts are good “rule of thumb” guides:

2. The condition, size and configuration of your air line is important.

Kinks, holes and leaks in your air line can easily degrade the performance of your pneumatic vibration equipment, causing it to run and then stop unexpectedly. Pneumatic equipment performance can also be degraded if your air line is split between units (“T” junction) or used to run multiple pneumatic units. Best practice is to run separate, correctly sized lines for each unit in operation, and to perform periodic checks for kinks, leaks and holes.

3. Correct air pressure (PSI) and air flow (CFM) are essential.

Make sure that you are providing your pneumatic unit(s) with correct PSI and CFM. Pneumatic units should have the recommended PSI and CFM indicated on the housing or on the shipping box. A general rule of thumb is that PSI should be 40 to 80, and you should consult your manual for proper CFM. Insufficient or too high of air pressure flow can cause unit failures.

(Click here for a handy Air Flow Pressure calculator.)

4. Periodic checks for loose mounting fixtures can extend the life of your pneumatic vibration equipment.

Most pneumatic vibration equipment is virtually maintenance-free. You can increase the life of your equipment by performing periodic checks of the mounting assembly and hardware. Check to ensure that all mounting bolts are tight and hardware is securely welded to the bin.

5. If your unit requires lubrication (piston and ball style), make sure that your lubricator is functioning correctly and is no more than 5 feet away from the vibration unit.

Too much distance between the lubricator and vibration equipment can cause too little or no lubrication to reach the unit. Your lubricator should be no more than 5 feet away from the unit. For best results, you should use using air tool oil. You may also use SAE 10 or lighter machine tool oil, Marvel Mystery oil or automatic transmission oil.

About the author:

VIBCO is a manufacturer and supplier of pneumatic vibration equipment. You may contact the company’s technical support staff 24 hours per day, seven days per week at 800-633-0032. For more information, visit www.vibco.com.

Roper Industries has acquired Hardy Instruments, a California-based provider of custom solutions for process weighing, tension control and vibration monitoring.

Hardy Instruments has provided superior, innovative products and excellent customer service for more than 85 years. Its process weighing technology provides new opportunities to companies under Roper’s Energy Systems and Controls group. Roper’s excellence in market connectivity and international presence will aid Hardy’s global reach and future growth.

Metrix Instrument Co., a customer-driven provider of condition monitoring instruments, has been a Roper company since 1995. Metrix will gain from Hardy’s engineering and technical expertise as Hardy’s vibration monitoring products are integrated into the Metrix line.

“With Hardy’s addition, we can now offer a fully compliant API-670 protection system to our extensive global markets,” said Chris Krieps, president of Metrix.

Hardy will continue to drive the process weighing and tension control segments of their business out of San Diego.

The Datastick SiteConnex VSA-2215 instantly connects a technician in the field with vibration analysis assistance, maintenance coordination and written work authorizations from the main plant, office or anywhere.

• Smartphone- and e-mail-enabled vibration analyzer allows technicians to capture vibration data in the field with very little training, and instantly send the data to a vibration expert for immediate help via phone and email — all on the same instrument
• Instantly notify management of critical machine reliability issues, and provide supportng data for complete documented accountability
• Authorize a repair in writing via text message or email
• Minimize downtime by getting technical assistance NOW — even when you're miles away from help
• Confirm new machine acceptance or a completed repair while you're still on site

The Datastick SiteConnex VSA-2215 Vibration Spectrum Analyzer opens the door to more and better use of vibration analysis in predictive maintenance for facilities of all sizes, especially those with remote locations. It saves time and money, eliminates guesswork and reduces return trips to remote locations. It cuts training requirements for field personnel. It allows consultants and clients to work more efficiently and effectively. That means increased machine reliability and uptime.

Vibration analysis is the cornerstone of any condition monitoring program. The venerable Art Crawford (founder of IRD) said, "No single measurement can provide as much information about a machine as the vibration signature." Gross defects such as severe shaft misalignment to subtle conditions such as differences in grease types can be detected with vibration analysis. As powerful as this technology is, it's often the most misapplied of the condition monitoring tools. This article will focus on the proper philosophy behind the technology's use inside an overall condition monitoring program. While many maintenance and reliability initiatives incorporate vibration analysis, few people have a sound philosophical base for how it is to be utilized.

The proper use of a quality vibration analysis program can provide focus to the crafts personnel. The analysis of the vibration signature, when performed in a timely manner, pinpoints the precise nature and location of the problem. This empowers the planning and scheduling process by allowing the crafts to go straight to the heart of the problem and not waste time on troubleshooting. Vibration analysis performed too late in the propagation of a defect won't help the crafts as much. Waiting too late actually allows the defect to propagate beyond the physical part it initiated in and create even more work for technicians. An example is that, detected early enough, the shaft misalignment can be corrected before the bearing defect has even begun. Wait too long and not only are the shafts misaligned, but the bearings are defective and the bearing housings are damaged to the point that the bearings are loose in the housings. At this point, it will be difficult to get a technician to extol the benefits of vibration analysis, as it has not resulted in any more efficient work or easier troubleshooting. The correct use of vibration analysis enables the crafts to focus their resources on the issues that make the greatest difference.

Another powerful benefit of vibration analysis is that it buys the planning and scheduling effort a substantial amount of time to actually work at a normal pace. While any job can be expedited and hurried, jobs that don't require expediting are cheaper and easier by virtue of the fact that additional fees aren't paid for expediting parts, and less effort is expended in the reallocation of labor and other resources. These activities lead to higher costs and higher stress levels within the planning effort. Having parts drop-shipped and air-freighted always cost more money. Having maintenance work teams working on machinery that hasn't already failed makes their jobs much easier. Having a planning group and a crafts group able to take their time and not be rushed creates a less stressful environment and, thus, increases the conditional probability of correct execution of both the job plan and the job itself. Vibration analysis identifies defects early enough to allow the planning and scheduling process the maximum amount of time to effectively and efficiently deal with the situation.

The single-largest error people make is to use vibration analysis to simply find yet another bearing fault, fix the fault and then celebrate the fact that a bearing defect was detected and corrected. This represents an incomplete understanding of the purpose of condition monitoring. This attitude is referred to as "optimizing your run-to-failure maintenance strategy". By being satisfied with finding the defect and merely correcting it, you haven't improved your processes.

The only way lasting improvement can be made is to identify the latent root of the systemic problems that are creating these defects. Vibration analysis is a powerful tool for identifying the physical cause of the defect or the physical effect of the condition. This information empowers the root cause analysis effort.

Vibration analysis, when used incorrectly, does nothing more than optimize an organization's run-to-failure maintenance strategy. At this point, it is nothing more than a cost-added activity and not a value-added activity. Using vibration analysis in this manner drives up costs, and direct benefits are difficult to quantify. However, used correctly, it provides some distinct benefits to maintenance and reliability efforts. It enables the crafts to focus their resources on the issues that make the most difference. It empowers the planning and scheduling process to be able to achieve more in a shorter period of time and with less effort. And, it enables an RCA process that will allow for a much quicker and thorough identification and elimination of the root cause of the defect.

Now is the time to ask yourself, why are you performing vibration analysis?

Andy Page is the director of Allied Reliability's training group, which provides education in reliability engineering topics such as root cause analysis, Reliability-Centered Maintenance and integrated condition monitoring. He has spent 15 years in the maintenance and reliability field, holding key positions at Noranda Aluminum (maintenance engineer) and Martin Marietta Aggregates (asset reliability manager). Andy has an engineering degree from Tennessee Tech and is a Certified Maintenance and Reliability Professional (CMRP) through the Society for Maintenance and Reliability Professionals (SMRP). Contact him at pagea@alliedreliability.com.

Fifty years of ongoing development has given SKF a bearing vibration analyzer that can probe deep into rotating bearings while they are under load. Initially used as a production line tester, it’s now proving invaluable during the maintenance and refurbishment of gearboxes and other critical units in demanding applications such as aerospace.

SKF vibration testing equipment is released by Rolls-Royce for noise testing of bearings during gearbox servicing. Release became necessary when Rolls-Royce Deutschland decided to transfer service work on gearboxes for secondary power systems to the sub-contractor Vector Aerospace in Almondbank, Scotland. Overhaul of these gearboxes requires detailed inspection of more than 3,000 components, some of which require testing with special equipment. This includes the various types of rolling bearings used in the gearbox that require stringent noise and vibration testing and measurement of the radial and axial clearances.

Vector Aerospace selected SKF as the best suitable supplier of test equipment for the bearing tests, but before putting the equipment to use, it was necessary to run a release process with Rolls-Royce Deutschland. A detailed test and acceptance program was developed by Rolls-Royce Deutschland using reference and series bearings of different manufacturers to confirm the usability of the noise-test equipment after its installation at Vector Aerospace site.

Development of SKF vibration testing equipment
A good first step toward solving any problem is to get the facts. That’s exactly what a group of senior engineers at SKF did in the early 1950s.

SKF already had a proven reputation as a leading rolling bearings manufacturer. Its bearings were used successfully in countless applications all around the world. They were made from quality materials, using the most up-to-date production processes and carefully checked before packaging. So, what was the problem?

It was said that collectively the group members knew everything there was to know about bearing materials, bearing design, bearing manufacturing and bearing applications. But still, the SKF engineers had a problem. They were concerned about the causes of failure that bearings from all manufacturers were exhibiting. Minute exterior examination of bearing components and bearing assemblies could only take them so far. They wanted more facts. They wanted to probe deep inside a bearing while it was rotating and while it was under loads similar to those it would experience in service.

Intuitively, they knew that vibrations caused by the rotating and flexible parts in a bearing caused noise and could be a source of wear and bearing damage. But they also felt that understanding the vibrations better could tell them even more – perhaps even tell them what caused the vibration in the first place and how to design and manufacture in order to reduce the vibrations and the corresponding wear. And so, SKF set off on a path to develop an understanding of vibration phenomena in bearings as well as a method of measuring vibrations and connecting this back to distinct causes, as specific as rolling element-induced vibration or damaged ring-induced vibration, etc.

By the mid-1950s, the group had successfully created its first vibration testing equipment for analyzing structure-borne noise and vibration in bearings. They went on to develop equipment that would reveal problems hidden within a bearing such as dirt particles, cage noise or form deviations in a component. Their search had revealed the factors that could lead to bearing failure and customer dissatisfaction.

Early milestone
Early versions of the equipment were introduced into production line testing, but the group continued to probe further until, in 1965, one member of the group, E. Yhland, reached a milestone in the understanding of the quasi-static problem of bearing vibration.

His work became a contribution to establishing national standards such as AFBMA 13-1987 and DIN 5426 (draft). These standards define and specify the physical quantities to be measured and the test conditions to be applied. The equipment became of extreme importance for high-quality bearing production.

Today, SKF equipment such as the MVH vibration tester is used to analyze precisely the structure-borne noise and vibration of deep-groove ball bearings, angular contact ball bearings, self-aligning ball bearings and spherical roller bearings. The MVH is also used as the SKF reference equipment for these measurements.

Operation of the MVH 90C/200C vibration tester is semi-automatic, so when a bearing is to be tested, the only things to be done by hand are: the loading of the bearing on the test spindle; the pressing of the two-handed start; and removal of the bearing after the test.

The extremely precise sliding bearing spindle drives the inner ring of the bearing at a constant set speed while loading is provided by an adjustable, pneumatic axial loading unit. When the automatic test cycle begins, the axial loading unit applies an axial load to the outer ring and moves the bearing against the testing spindle.

A pickup is applied to the stationary outer ring of the bearing and any bearing noise is measured, analyzed and displayed. After a predetermined time, the axial loading unit returns to its rest position and the machine is ready for the next test cycle. Resetting for another type of bearing can be done quickly and simply.

The tip of the pickup rests against the outer ring and converts the radial vibration of the bearing into an electrical signal that is proportional to the velocity of the pickup tip. When the signal is amplified and analyzed, the vibration level is measured in three frequency bands. The result identifies one or more of a possible number of defect types as well as detecting dirt particles, cage noise and form deviations.

Reducing costs and ensuring reliability
The great success of vibration measuring equipment in bearing refurbishment for the aerospace industry has lead to increasing interest from other areas of industry. There is also increasing awareness of the savings to be made from the reuse of bearings that have been tested and meet quality and reliability standards.

Other examples of the vibration tester’s versatility are its use in manufacturing companies where the equipment can be used to inspect incoming components, and its use in research departments to support R&D activities or act as a test rig for gears, motors or steering units, etc. The equipment is equally useful for grease noise testing.

Continued development
Although the bearing vibration analyzer has come a long way since the 1950s, its development continues at SKF, where the search is on for even more accurate measurements and analysis. At the present time, research is focusing on three interesting areas: the application of different sensor technologies; the use of a detailed noise map; and the introduction of an expert system.

For more information on SKF vibration analyzers, visit www.skf.com .

http://www.youtube.com/watch?v=PmCxQfoYX28

Mechanical Solutions Inc. was called on behalf of a customer who had observed a large increase in the vibration levels of an LM500 gas turbine generator set that was being used to supply power to a large pharmaceutical facility. The increase in the vibration levels was serious enough to warrant shutdown. 

This power facility was used to meet much of the electrical power needs of the entire complex. Any loss of generating capacity translated directly into increased electrical operating costs because of the need to buy power from the grid at a time of the year when such purchases were particularly expensive. The plant wished to continue operation for another three months, when a planned shutdown was to occur. 

The OEM had not been able to find the reason for the difficulty. Therefore, MSI was called in to help diagnose and fix the excessive vibration. The customer had noticed that the large increase in the overall vibration was particularly troublesome in between the gas turbine and the generator. They suspected that the problem was a torsional natural frequency of the gearbox. MSI performed testing using generator shaft strain gauges and sophisticated radio telemetry. Because of MSI’s experience conducting such tests, the generator was shut down for only one hour in order to install the telemetry system. 

The testing was to focus on torsional oscillation but showed that the torsion did not cause the high gear box vibration. MSI continued testing and was able to quantify the vibration levels with increasing load.  Based on the shape of the vibration spectrum, the strength of the vibration appeared to be associated with a natural frequency resonance. However, impulse testing on exposed components detected no strong natural frequencies near 1X or 2x gear mesh. It also was noted that the vibration increase was sensitive to load level.  

In light of this, the sudden increase in vibration suggested a possible shaft or casing crack which opens up under increasing load. This crack could open up further as the load transmitted through the gearbox was increased. Stationary “bump” tests did not indicate that this crack was present, but vibration measurements at many locations and at various loads made it clearly apparent. 

In order to be completely certain that this was the most likely cause, MSI also tested the electrical generator to confirm that the problem was not generator or electrical system based. This testing revealed nothing unusual in the phase-to-phase characteristics of the system, and left a crack as the most likely cause of the problem. 

On the recommendation of MSI, die penetrant testing of the gearbox was conducted. The die penetrant testing revealed that a crack had formed on the foot of the gearbox casing. The crack was welded shut and, subsequently, the vibration of the gearbox was reduced to the earlier acceptable levels. This work allowed the end-user to continue power generation during a critical portion of production. 

In addition to knowledge of the LM500, the staff of Mechanical Solutions Inc. has background and experience in the analysis and testing of aeroderivative gas turbines, such as the popular FT-4, FT-8, LM2500, LM6000 PC, and Frame 7FA units. One of MSI’s principals was one of the design analysis team members for the original ground-based and marine-based conversions of the FT-4. Generally, problems with these units are installation or system related. MSI’s ability to separate and evaluate rotordynamic, casing and support structure effects allows such problems as well as any warranty issues to be quickly identified, such that responsibilities can be determined, and appropriate fixes can be developed and implemented. 

About the authors
William Marscher is the president and technical director and Eric Olson is the director of marketing for
Mechanical Solutions Inc. (MSI), a business based in Whippany, N.J. For more information, visit www.mechsol.com.

Many vibration programs fail because they become too complicated. Too much data can sometimes become more confusing than too little data. Many potential machinery problems can be eliminated with the analysis if you keep in mind several simple concepts.

1. Vibration units such as acceleration are more sensitive to high frequencies than low frequencies.
2. Vibration units such as displacement are more sensitive to low frequencies that high.
3. Velocity units are evenly sensitive, on average, between 60 CPM to 60,000 CPM.
4. High-frequency vibration does not travel far and degrades rapidly through metal seams.
5. In general, the closer your measurement is to the source of the vibration, the higher the amplitude will be.

These differences can be used to zero in on machine faults.

For example, take a generic 100-horsepower motor. If an outboard rolling element bearing begins to fail because of a lack of lubrication, the first indicator is high-frequency ringing from the bearing. This is characterized by a large increase in acceleration amplitude and small to no increase in velocity or displacement. Now you have identified that there is a high-frequency problem and not a low-frequency mechanical problem. You can eliminate low-frequency sources such as looseness, balance or misalignment. What is the most likely source of high-frequency vibration on the back end of a motor? It’s probably a bearing or shaft or rotor rub.

Now, you can apply a simple test. Grease the bearing and see if the acceleration returns to normal. If it does, you have nailed the problem without knowing the bearing frequencies or even taking a spectrum. Come back the next day and see if the acceleration is back up. If it is, you either have a lubrication problem with contamination or a loss of grease, a damaged bearing or both.

While not perfect, understanding the behavior of vibration units combined with a mechanical understanding of machinery can help you quickly identify machinery problems.

Joe Van Dyke, Professional Engineer and creator of the Azima DLI ExpertALERT Automated Diagnostics System, describes the latest technology, Machinery Vibration Baseline Synthesizer.

Access this 17-minute, two-part video series by clicking on the link below.

Sciemetric Instruments, an industry leader in manufacturing quality control technology, announced the newest addition to the sigPOD family of products.

sigPOD PSV is an out-of-the-box, user-configurable solution that can be used to test or monitor virtually any operation during manufacturing, including press, torque, vibration, dispense and functional test. An intuitive setup interface makes it easy to leverage the expansive library of processing and analysis tools available, and can be used on up to eight channels to allow for greater output without sacrificing quality.

sigPOD PSV is equipped with Sciemetric's robust Process Signature Verification software, and in addition to its straightforward user interface features: robust data collection options, powerful processing speed, thorough analysis, built in signal conditioning, comprehensive data management and reporting features, and unparalleled connectivity.

About Sciemetric Instruments
Founded in 1981, Sciemetric Instruments provides its manufacturing intelligence software and test platforms to companies around the world to detect and analyze manufacturing defects as they occur, improve quality, increase productivity and decrease costs across the entire production lifecycle. Headquartered in Ottawa, Canada, the company has its North American sales office in the U.S., European offices in the United Kingdom, and sales and support at its new office in India.

This 2-minute, 30-second video provides an introduction to the nature of vibration, its causes, and steps to improve compressor operation, reliability, and performance. Vibration is the leading cause of maintenance headaches, on high and low speed reciprocating compressors. Primarily directed to owners, operators, packagers, and engineering companies, mechanical engineering students will also find this topic of interest. This is the first of three videos in the series from Beta Machinery Analysis.

Key concepts about vibration problems on reciprocating compressors are introduced in this training video, which runs approximately 8 minutes. Learn about forces on compression equipment. This is the second in a series of training videos from Beta Machinery Analysis.

The Hydraulic Institute (HI) has published a new American National Standard for Rotodynamic Pumps for Vibration Measurements and Allowable Values (ANSI/HI 9.6.4 - 2009).

HI’s Rotodynamic Pumps for Vibration Measurements and Allowable Values standard applies to the evaluation of vibration on rotodynamic pump applications. It pertains to evaluation of vibration when the vibration measurements are made on non-rotating parts (bearing housing vibration). The general evaluation criteria are included for acceptance tests in field environments or at the manufacturer’s test facility, as appropriate and as defined in the standard.

“This new normative vibration standard builds upon the trends begun in the previously published standard, with significant improvements that should facilitate the use of the document, allow it to find broad acceptance, and benefit the pump industry,” said Jack Claxton, vice president of engineering for Patterson Pump Company and chairman of the Hydraulic Institute Vibration Committee.

This document will be supplemented in the future by a new ground-breaking guideline document, Dynamics of Pumping Machinery, currently being drafted by the committee.

This standard is based on experiences from pump users and manufacturers as well as vibration measurements by many companies. Vibration data from both factory test and field test environments have been incorporated into the maximum allowable vibration values. Values are applicable when the pump is installed per Hydraulic Institute or the manufacturer’s specifications.  

"The newly restated HI Vibration Standard comes as a result of substantial research and brings together collective experiences of pump users and manufacturers from a variety of industry segments,” stated Mick Cropper, global product development manager for Sulzer Pumps US Inc. and vice chairman of HI’s Vibration Committee.

Copies of Rotodynamic Pumps for Vibration Measurements and Allowable Valueswill be available for $115 by visiting HI’s e-Store atwww.eStore.Pumps.orgor by calling the purchasing line at 973-267-9700, ext. 118.

The Hydraulic Institute (HI) is the largest association of pump producers and suppliers to the pump industry in North America and is a global authority on pumps and pumping systems. HI Pump Standards are reliable, widely accepted references for anyone involved in pumps, including users, consultants, contractors, construction firms, manufacturers of pumps, seals, motors, instrumentation, controls, and pump software developers and systems integrators. HI periodically introduces new Standards based on industry needs. For more information about the Hydraulic Institute, its member companies and Standards Partners, visit www.Pumps.org and www.PumpLearning.org.