Archive | 2004

164

2:05 pm
December 1, 2004
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Clutch Assembly Aids in Condition Based Monitoring

The installation of a new clutch assembly using a torque monitoring device has saved a General Motors manufacturing facility in Spring Hill, TN, more than $1 million. The first prototype of the clutch was introduced in 2000 and savings are predicted to grow as more units are installed. Currently, equipment in the vehicle systems area of the plant is being upgraded to include the “smart” clutch.

The clutch is being used on drive units that power a vertical lift and transfer system in the assembly plant where the Saturn VUE small sport utility vehicle and the Saturn ION sedan and quad coupe are built. This assembly is part of a condition monitoring program that is being used to optimize maintenance at the facility.

Avoiding downtime
Significant downtime has been attributed to the vertical lifts and transfers which move the vehicles through the assembly process. These enormous drive units carry large amounts of weight which causes massive torque loads and sizable vibrations. These drives can fail for a variety of reasons: gearbox failures, motor failures, bearings, sprockets, etc.

If one of these drives fails, the line stops. Although most units are equipped with a backup drive, it can take maintenance personnel anywhere from 45 min to 1 hr to manually switch to the backup. A projected loss of up to $3500/min makes this downtime extremely expensive.

Prototype installed
A prototype clutch, designed jointly by Autogard Corp., Rockford, IL, and GM Spring Hill, was installed on selected lifts. The clutch quickly switches between drives and can be coupled with a torque ring and telemetry system for continuous monitoring of the torque and velocity of the drive shaft.

Using the torque ring outputs, the primary drive can be switched automatically to the secondary drive in 1 min rather than the 45 min at minimum that it takes for a manual changeover. Because this switch occurs automatically, the operations continue to run smoothly, allowing the maintenance personnel to redirect their work to other priority areas.

1204saturnfig1

Fig. 1. Right side angle shot with lifting carriage in full down position

In 2000, the first clutch and torque monitoring system was added to a vertical transfer that delivers the trunk lid from the upper to the lower chain conveyor for assembly. It has two identical drives but operates only one at a time. If one drive has a problem during production, employees quickly switch to the other drive by disassembling the flex steel coupling on one side and re-assembling the coupling on the other side (Fig. 1). Adding the smart clutch reduced the changeover time drastically and saved $157,000 every time a changeover was required during production.

Reduced interruptions in the flow of parts to the line and the stream of vehicles through the plant results in improved uptime and lower operating costs. The end result is that the maintenance department and operations management see improved efficiencies that favorably impact the bottom line.

Monitoring identifies problems
The data ring monitoring system provides key running load information on the condition of the vertical transfer. In one instance, information from the new unit identified a problem caused by a gearbox failure during the first 5 min of operation. The limit switches and spring preloads on the vertical transfer were adjusted incorrectly resulting in overtorque of the gearboxes by a factor of four times the maximum rating of the gearbox.

The monitoring system was used to aid in the proper adjustment of the limit switches and preload springs to bring the elevator back to its designed parameters. The system facilitates the detection of developing problems and allows the scheduling of maintenance based on conditions requiring attention before an impact to production occurs.

The result has been improved preventive maintenance scheduling, more efficient use of maintenance personnel, and early detection of mechanical problems through continual monitoring of the equipment.

Once the torque ring receiver is attached to a programmable logic controller, a computer program is then able to monitor the torque and velocity readings. If the threshold is reached, the system warns the maintenance team that something is about to fail. The monitor can be used to determine how much counterweight is needed.

Lessons learned
While considering the implementation of this project throughout the plant, several key lessons were learned:

  • If designed correctly, the clutch can be an extremely cost effective, efficient, and useful way to switch to the backup drives. In the current situation, it paid for itself the first time that it was used.
  • The torque monitor is not always required for brake solutions.
  • The torque monitor is not required for clutch implementations but without the data ring, the clutch cannot switch automatically between the primary and backup drive. With the torque monitor, the clutch switches to the backup during a failure, and provides an indication that the primary has failed. The data ring also can warn of imminent failure, allowing repair or replacement of the primary drive during scheduled downtime.
  • If a condition based monitoring system is in place, it is not imperative to have a backup. However, it should be evaluated on a case-by-case basis, depending on the criticality of the equipment and the efficiency of the maintenance organization. If planned downtime and occasional unplanned downtime can be tolerated, then the data ring should be sufficient.

Understanding each unique environment will help to ascertain which components and strategies need to be implemented. MT


Thomas A. Rogers, Ph.D, is vehicle systems engineer at General Motors’ General Motors Spring Hill Manufacturing, 100 Saturn Parkway , P.O. Box 1500, Spring Hill, TN 37174-1500. He can be reached at (931) 486-6782

How the System Works

The predictive part of the clutch and data ring assembly is the monitoring system. It is composed of a torque ring, telemetry receiver, and a torque monitor (Fig. 2).

1204saturnfig2

Fig. 2. The system is composed of a torque ring, telemetry receiver, and the torque monitor.

The torque ring is the heart of the system. It measures and transmits real-time torque data and can be installed virtually anywhere in the drivetrain. The torque ring consists of a battery power supply, strain-gauge bridge assembly, microprocessor-based system to interpret the strain-gauge data, and an electronic data transmitter. It is mounted in a 1-in.-thick aluminum or stainless steel ring. The data from the ring is transmitted as a 10-bit digital signal using an FM radio signal.

Angular and axial loads are isolated to ensure accurate torque measurements, and the torque can range up to 500,000 lb-ft. The ring cannot monitor high-frequency vibrations, but it can handle low-frequency signals less than 10 Hz. The torque ring is selected based on the drive location, torque requirements, and the drive train component to which it is to be adapted, such as a coupling, gear, or sprocket. The torque ring should be placed as close as possible to the part being analyzed.

The first step in installation is deciding which part of the machine/drive is to be analyzed or used as the basis of control. This position also must be the best position for measuring the torque directly rather than through a gear reduction. In some instances, by positioning the ring closer to the prime mover, the torque data can indicate changes in the drive train performance or gearbox efficiency. According to Autogard, this system is accurate to ±5 percent full scale.

The telemetry receiver picks up the radio signal containing the torque data transmitted from the torque ring aerial. It is enclosed in a small plastic box positioned approximately 5 mm from the aerial. A 6 m shielded cable connects the receiver to the torque monitor. The torque monitor is the control and display unit for the system. It is wired to the receiver, which provides the torque monitor with the true torque data from the equipment. Torque is displayed in the most relevant form for the environment. Trip points, relays, analog outputs, and an optional serial communications link for process or production control also are available. The trip points are extremely important in this sort of application.

The system allows for three programmable trip points, which can be set at different load levels to provide a variety of warning or control signals. The first trip is generally configured as an underload so that it will activate when the load falls below a set value. The second trip is typically configured as an overload so that it will activate when the load climbs above a set value. The third trip is typically configured as an overload and set above the second trip level to signal the end of a process/production cycle or to protect the equipment by shutting down the motor. The monitor can be set to hold this peak value, which can be useful when running the equipment again.

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1560

3:37 pm
November 19, 2004
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Instituting a Zero-Based Maintenance Budget Based on Equipment Requirements

Process also establishes staffing needs.

Currently many maintenance budgets are developed based on previous maintenance budgets or a percentage of the replacement asset value (RAV) of maintained equipment. Sometimes maintenance budgets and staffing decisions are based on negotiations between management and the maintenance department. Often an exacting method of determining the specific number of maintenance technicians and their required skill set is not in place.

This article presents a method of establishing a defendable zero-based maintenance budget and staffing requirements based on equipment requirements and company goals that are supported by documentation. Decisions regarding budgeting and staffing are based on facts instead of emotions or educated guesses. Key areas for optimizing equipment expenditures and staffing requirements are easily identified.

Step 1. Determine the maturity of your maintenance and reliability program
The ability to establish, control, and predict a maintenance budget is directly related to the maturity of a maintenance and reliability program. Maintenance and reliability professionals often describe the maturity of a program in distinct phases, levels, or steps. It is important to understand the level of maturity of the maintenance program in a plant prior to attempting to develop a budget.

Typical phases in the maturity continuum of a maintenance and reliability program can be seen in the accompanying text “Maturity Continuum of a Maintenance and Reliability Program.”

Maintenance and reliability practitioners often debate the actual number of phases and what programs or systems should be included in each phase. The phases and the programs or activities associated with each phase are listed as an example of a typical progression of a maintenance program.

A maintenance and reliability program must be built in phases. It is important to have the Phase 1 programs and systems in place prior to developing Phase 2 programs and activities. Phase 2 programs and systems must be in place prior to effectively developing Phase 3 programs, etc.

A maintenance budget and staffing needs are best controlled when the maturity of a maintenance and reliability program has progressed to Phase 3. The more the maintenance and reliability program has progressed, the easier it is to control the maintenance budget.

Step 2. Determine how your maintenance budget and staffing levels compare to those of your competitors
It is important to determine how your maintenance budget and staffing requirements compare with those of your competitors. If you are spending too much on maintenance, you cannot be competitive. If you are not spending enough on maintenance you will be under-maintaining your assets and your equipment will start to deteriorate.

A method of determining how your maintenance budget compares with those of your competitors is to compare your maintenance expenditures and staffing levels to the RAV of your plant. The RAV is the amount of current dollars that would be required to replace the assets in a plant. Benchmarks for the maintenance budget as a percent of RAV and the number of maintenance technicians compared to the RAV are available for most industries.

The more advanced a maintenance program becomes; the less money will be spent on maintaining equipment. Fig. 1 shows typical RAV maintenance costs for a few generalized industries.

Step 3. Develop an equipment hierarchy
To properly manage a maintenance and reliability program, and a maintenance budget, an equipment hierarchy must be established. Equipment hierarchies are not “standardized” in the maintenance and reliability community. Typically, the methodology for developing a hierarchy for a specific company is standardized across a company.

See accompanying text “Typical Equipment Hierarchy for a hypothetical chemical company. This hierarchy has been developed for use in this article.

Step 4. Understand methods used by your corporation and local plant to track and control maintenance expenditures and develop a maintenance budget for each asset

Corporate maintenance metrics are typically developed to reflect the performance of assets identified in Level 1 through Level 3 of the hierarchy. An associated metric might be the total cost to produce a thousand pounds of polymer per maintenance dollars spent. It is difficult to control maintenance expenditures at Levels 1 through 3.

Individual plant maintenance metrics are typically tracked for each department and each production area (Levels 4 and 5). Many plants can easily track production costs for each department and each production area of the plant. At most plants, an attempt is made to control the maintenance budget at these levels. Although the costs can be easily tracked, it is difficult to control maintenance expenditures at these levels.

Costs to maintain individual assets can be effectively developed and maintained only at the asset level (Level 7).

Consider the following:
•Observation made at company level (Level 1): We are not competitive. A key reason is that maintenance costs are too high.
• Observation made at the business unit (Level 2): The maintenance costs at the chemical plant in Mobile, AL, are too high. The costs of maintenance at the chemical plant in Macon, GA, are at an acceptable level.
•Observation made at the production unit level (Level 6): The cost of maintenance for Reactor #3 is too high. (Note: Although the source of the high maintenance costs, Reactor #3, has been identified, the reasons for the high maintenance costs have not been identified and cannot be controlled.)
• Observation made at the individual asset level (Level 7): The cost of maintenance is high primarily due to failures of the reactor vessel. The vessel continues to develop leaks. It is often necessary to shut down the vessel and build scaffolding inside the vessel in order to repair the leak. There have been four serious leaks in the reactor vessel within the past 8 months. The costs are generated at this level. This is where the costs must be controlled.

Typically maintenance budgets are developed and managed at levels above the asset level (Level 7), but maintenance budgets can best be developed and managed at the asset level. Each asset should have a budget that includes material and labor. Once a budget is established for each asset, the budget for the department and plant can be determined. See accompanying text Budget Example for a Specific Asset.”

Step 5. Control maintenance costs using the maintenance budget
A budget developed at the asset level can be used as a tool to reduce and control costs, determine manpower requirements, identify training needs, and develop business cases.

•Reduce and control costs. Review the Budget Example for a Specific Asset. All the tasks shown are preventive maintenance, time based, tasks. Costs can be reduced by performing predictive maintenance tasks.

For example, vibration monitoring can be performed on the pump and the bearings can be replaced when they start to fail. The mean time between failures can be predicted by determining the B10 life of the bearing. The budget then would be modified as shown in “Modified Budget Example for a Specific Asset.

The costs for maintaining the equipment based on the initial maintenance plan can be compared to the costs for maintaining the equipment as shown in the “Modified Budget Example for a Specific Asset.” Any cost savings are easily identified.

On a regular basis, the actual cost of maintaining each asset should be compared to the budgeted costs of maintaining the asset. Any over- or underexpenditure should be addressed on an asset-by-asset basis. By controlling the expenditures on each asset the overall maintenance budget is effectively managed.

•Determine manpower requirements. Manpower requirements can easily be determined by using the individual budget for each asset. In the example, the specific manpower requirements are identified by craft. The overall manpower requirements for each craft can be developed by combining the manpower requirements required to maintain each asset.

If more resources are needed to maintain equipment, the information required to justify an increase in staffing is available. If a reduction in manpower is required, the maintenance manager can work with plant supervision on an asset-by-asset basis to determine which maintenance tasks will no longer be performed.

• Develop business cases. Information from the zero-based budget can be used to create business plans for improving the maintenance of the plant. How many times have you attempted to improve maintenance at your plant but could not convince management to support the effort? It is much easier to convince management to support an effort if an effective business plan is developed to support your case.

In the example above, vibration analysis was used to extend the life of the bearings. To start a vibration analysis program at a plant, one could modify the budget for each piece of equipment that has the potential to be monitored. The costs savings then can be identified.

The increase in manpower due to the addition of a vibration analysis program to your existing maintenance program can be developed. The decrease in manpower brought about by replacing bearings based on the bearing’s condition as opposed to the time the bearing had been in service can be calculated. Once the cost of developing the vibration analysis program is determined, the payback for implementing a vibration monitoring program can be calculated.

• Develop a budget, staffing, and training plan for each asset. If a budget is developed for each asset the following items can easily be developed:

1. A maintenance staffing plan that identifies and supports the number of technicians, by craft, required to maintain the plant.

2. A specific training plan because all tasks that will be performed are identified. Technicians can be trained to perform the specific tasks identified in the budget.

3. An overall maintenance budget that can be defended.

4. A justification for increasing or decreasing the maintenance budget when pieces of equipment are installed or removed.

5. A justification for increasing the maintenance budget if an asset is utilized more than it has been in the past. If an asset is utilized more, the specific budget for the asset must be modified to reflect any increases in maintenance expenditures required to ensure that the equipment can be operated reliably.

6. A basis for a business plan that will support maintenance improvements.

•Utilize zero-based budgeting to develop life cycle costing. The process that is used to develop budgets for individual assets can be utilized in a life cycle cost analysis. Life cycle costing is part of a world class, reliability centered maintenance–advanced reliability program.

For the purpose of this article, life cycle costs are listed as a Phase 5 activity in the development of a maintenance and reliability program. The total cost of maintaining an asset along with the manpower needed to maintain the asset should and can be considered during the project delivery phase of the project. Various alternatives with various life cycle costs can be evaluated.

Call to action
It is important to understand the current methods that are used to establish maintenance budgets within your organization. Is there a well-thought-out method of developing a maintenance budget? Or is last year’s budget simply increased or decreased by an arbitrary amount to develop this year’s budget?

Determine the maturity of your maintenance program and benchmark your maintenance costs with those of your competitors. The cost of maintaining equipment vs the RAV for various types of manufacturing plants is available. The information on the cost of maintaining a plant based on the maturity level of the plant’s maintenance program is also available. By gathering this information, you can answer such questions as how much your plant could benefit from maintenance improvements and whether you are spending too much or not enough on maintenance.

Either develop an equipment hierarchy for your facility or validate the existing hierarchy. An equipment hierarchy is a vital element of any maintenance program.

Control your maintenance budget. Do not let your maintenance budget control you. By controlling maintenance expenditures at the asset level, the overall maintenance budget can be managed effectively.

Determine your specific staffing requirements and training needs. An asset-based equipment budget will provide this type of detailed information.

Consider incorporating asset life cycle costing into your capital deliver program. MT


Michael Eisenbise PE, CMRP, CPE, is director of performance technology and site services at Fluor Corp., 100 Flour Daniel, C301J, Greenville, SC 29607-2770; (864) 281-8625

Maturity Continuum of A
Maintenance and Reliability Program

Typical phases in the maturity continuum of a maintenance and reliability program are as follows:

Phase 1: Reactive Maintenance/Firefighting
Lack of formalized maintenance program

Phase 2: Basic Maintenance
Equipment hierarchy
Computerized maintenance management system (CMMS)
Work order system
Planning and scheduling
Preventive maintenance program
Reliability metrics
Basic maintenance skills program

Phase 3: Proactive Maintenance
Predictive maintenance program
Equipment history documentation
Root cause failure analysis
Advanced maintenance skills program

Phase 4: Advanced Maintenance
Autonomous maintenance (maintenance performed by operators)
Shutdown, turnaround, outage optimization
Maintenance craft flexibility development
Optimizing asset performance

Phase 5: World Class, Reliability Centered Maintenance,
Advanced Reliability Maintenance

Reliability centered maintenance (RCM)
Life cycle costing
Reliability analysis of existing assets
Standardization of equipment

 

 

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Maintenance Budget Based on RAV

1104eisenbise

Fig. 1. This shows typical replacement asset value (RAV) maintenance costs
for a few generalized industries. The more advanced a maintenance program
becomes; the less money will be spent on maintaining equipment.

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typical equipment hierarchy

Here is a typical equipment hierarchy for a hypothetical chemical company. The hierarchy has been developed for use in this article.

1. Company, ACME Chemicals Level 1

Level 1

1.1
1.2
1.3

Business Unit, Plastics
Business Unit, Solvents
Business Unit, Specialty Chemicals

Level 2

1.3.1
1.3.2

Polymer Plant, Macon GA
Polymer Plant, Mobile, AL

Level 3

1.3.2.1
1.3.2.2
1.3.2.3
1.3.2.4

Department, Raw Materials/Receiving
Department, Shipping/Packaging
Department, Utilities
Department, Production

Level 4

1.3.2.4.1
1.3.2.4.2
1.3.2.4.3

Production Area, Formulation
Production Area, Filtrantion/Blending
Production Area, Polymers

Level 5

1.3.2.4.3.1
1.3.2.4.3.2
1.3.2.4.3.3

Production Unit, Reactor #1
Production Unit, Reactor #2
Production Unit, Reactor #3

Level 6

1.3.2.4.3.3.1
1.3.2.4.3.3.2
1.3.2.4.3.3.3

Asset, Polymer Reactor
Asset, Reactor Heat Exchamger
Asset, Circulating Pump

Level 7

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Budget Example for a Specific Asset

Equipment Name: Circulating Pump, Reactor #3

Labor Required

Task

Frequency

Mat’l Cost

Mech

I&E

Lubr.

Helper

Adjust packing

Twice/wk

$0

 

 

 

0.25 hr

Replace packing

Twice/yr

$22

1 hr

 

 

1 hr

Change oil

Once/mo

$15

 

 

0.45 hr

 

Replace bearings

Once/2 yr

$853

8 hr

0.5 hr

 

8 hr

Meg motor

Once/yr

$0

 

1 hr

 

 

Modified Budget Example for a Specific Asset

Equipment Name: Circulating Pump, Reactor #3

Labor Required

Task

Frequency

Mat’l Cost

Mech

I&E

Lubr.

Helper

Adjust packing

Twice/wk

$0

 

 

 

0.25 hr

Replace packing

Twice/yr

$22

1 hr

 

 

1 hr

Change oil

Once/mo

$15

 

 

0.45 hr

 

Preform vibration monitoring

Once/mo

 

 

0.5 hr

Replace bearings

Estimated every 6 yr

$853

8 hr

0.5 hr

 

8 hr

Meg motor

Once/yr

$0

 

1 hr

 

 

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135

12:44 pm
November 11, 2004
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Error 404 - Page not found!

The page you are trying to reach does not exist, or has been moved. Please use the menus or the search box to find what you are looking for.

289

2:17 am
November 2, 2004
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Web-Based Skills Assessment Tool Aids Maintenance Staff

I get bombarded with maintenance product news and seldom get excited about the “innovations” in the maintenance marketplace. This changed the other day when Universal Technologies Interactive sent me news of an online “Skills Accelerator.”

OK, I will admit that anything related to the Web and maintenance gets my immediate attention—and this looked very interesting indeed. In brief, the Web-based Skills Accelerator allows maintenance managers and supervisors to determine what their employees do not know about their jobs and then identify resources and tools to develop these skills within their work groups.

Although there are off-the-shelf industrial skills assessment programs out there, Universal Technologies Interactive grew out of a maintenance training company and a skills assessment technology company combining resources to create a specialized and detailed offering.

To use the Skills Accelerator the maintenance supervisor logs into the secure Web site, defines specific jobs, and assigns discipline and job tasks. This list can be edited and updated at any point in the future. Once the system is set up, job and tasks analysis (JTA) defined processes are used to identify specific job classifications such as mechanical, electrical, and operations. Common tasks are identified for various skill areas as well as specific skills required.

An employee starts the assessment process by logging into the Web site and answering questions. The assessment can be taken in stages or completed in one sitting. The evaluation is sent to the supervisor immediately; the employee does not have any direct access to the results.

The knowledge and skills assessments are designed to identify “employee readiness” to perform tasks in accordance with identified best practices, and develop strategies to overcome identified gaps. The key to developing an individual development program is to assess each individual’s knowledge and skills for each element of each assigned task.

Gaps between knowledge and skills possessed vs those that are needed are part of the Skills Accelerator. The system also identifies areas of opportunity for future employee development. This allows the supervisor to select from a wide variety of training resources that are aligned with the company’s business priorities and budget. This method not only identifies individual skill gaps but can be used to spot skill deficiencies within certain employee groups as well.

Once the supervisor selects the appropriate curriculum, an employee’s development plan is generated. With the implementation of the development plans, companies can help each employee to become world class.

Eventually the system will even rate the effectiveness of the various training resources, including live instructor led, distance learning, and computer-based products, as the use of the system grows. Training resource companies are invited to send an e-mail to support@utinteractive.com with a brief explanation of the maintenance training offered to be added to the resource index.

To generate a valid result, managers and supervisors must communicate the positive aspects of employee and career development and avoid using the system for the “blame game.”

Individuals can log on for less than $75 and corporate pricing plans are also available for volume users.

There is even a patent pending on the Skills Accelerator. It sure is exciting to see an innovative leading-edge technology applied to improving maintenance skills which we all know make our industries more competitive in world markets. MT

Internet Tip: Update, Update

Be sure and visit the Microsoft Windows Update site to read about possible incompatibility issues of certain programs (like firewalls and automatic updaters) with new Windows XP Service Pack 2. There is a list of known issues and you will do well to address them before you select the update. This update is a good one. It includes a firewall to beef up XP security, although some new flaws have already been identified with SP2.

Blogging, anyone?

Blogs or Web logs are becoming very popular. They are simply daily or weekly writings of everyday people who have something to say. A Blog is really an ongoing conversation between the author and the readers.

There is one maintenance and reliability blog site at www. maintenancetalkcom and a Blog 101 explanation will give you a good overview of blogging and whether it is for you or not. If you want your own blog, you can easily set up one at Blogger.com

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362

1:06 am
November 2, 2004
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Reduce Maintenance for Spray Systems

Major savings in time and money can be achieved through an aggressive spray system optimization program.

Spray nozzles are vital components in many production facilities. Their accuracy, durability, and interchangeability are absolutely essential to maximum uptime.

If a spray system is not working optimally, it can drain staggering amounts of money. The cost of wasted water alone can approach $100,000 annually even in a system with relatively minor performance problems.

Factor in all the related expenses—the cost of excess chemicals, wasted energy, extra scrap caused by quality problems, unscheduled production downtime, and additional labor—and the true total can quickly mount to hundreds of thousands of dollars per year.

Once the magnitude of the issue is appreciated, it is time to begin the process of optimizing a spray system. Start by learning about the typical sources of spray problems.

Spray nozzle troubles
They may look simple enough, but in reality spray nozzles are highly engineered precision components that can wear over time, or suffer damage during normal operations or cleaning. The most common problems that cause substandard spray performance include:

• Erosion/wear. Gradual removal of metal causes the spray nozzle orifice and internal flow passages to enlarge and/or become distorted. As a result, flow usually increases, pressure may decrease, the spray pattern becomes irregular, and liquid drops become larger.

• Corrosion. Spray nozzle material can break down due to the chemical qualities of the sprayed material or the environment. The effect is similar to that caused by erosion and wear, with possible additional damage to the outside surfaces of the spray nozzle.

• High temperature. Certain liquids must be sprayed at elevated temperatures or in high-temperature environments. The spray nozzle may soften and break down unless special temperature-resistant materials are used.

• Caking/bearding. Buildup of material on the inside, on the outer edges, or near the orifice is caused by liquid evaporation. A layer of dried solids remains and obstructs the orifice or internal flow passages.

• Clogging. Unwanted solid particles can block the inside of the orifice. Flow is restricted and spray pattern uniformity disturbed.

• Improper reassembly. Some spray nozzles require careful reassembly after cleaning so that internal components, such as gaskets, o-rings, and valves, are properly aligned. Improper reassembly causes leaking and inefficient spray performance.

• Accidental damage. Damage can occur if a spray nozzle is dropped or scratched during installation, operation, or cleaning.

Detecting worn nozzles
This is more difficult than it sounds. The human eye is a remarkable instrument, but it simply cannot provide the true story when it comes to actual spray nozzle wear.

In the photos, the spray tip on the left is new, and sprays properly. The spray tip on the right is worn, and sprays 30 percent over capacity. The difference is undetectable with the naked eye—but there are other tip-offs that something is amiss.

Watch for these clues:

• Quality control issues and increased scrap. Worn, clogged, and damaged spray nozzles will not perform to specification, and can result in uneven coating, cooling, cleaning, humidifying, and drying.

• Increased maintenance time. Unscheduled spray system downtime, or an increase in cleaning frequency, is an indicator of spray nozzle wear.

• Flow rate change. The flow rate of a spray nozzle will increase as the surfaces of the orifice and/or the internal core begin to deteriorate. In applications using positive displacement pumps, the spraying pressure will decrease as the spray nozzle orifice enlarges. Even small changes in flow rate can have a negative impact on quality, so routine monitoring can reveal potential problems. But in some instances, the spray pattern will look fine so it will be necessary to actually collect and measure the spray fluid output to reveal wear.

• Deterioration of spray pattern quality. When orifice wear occurs in hollow cone spray nozzles, spray pattern uniformity is destroyed. Streaks develop and the pattern becomes heavy or light in the circular ring of fluid. In full cone spray nozzles, the pattern distribution typically deteriorates as more liquid flows into the center of the pattern. In flat fan sprays, streaks and heavier flows will be visible in the center of the pattern and the effective spray angle coverage will decrease.

• Spray drop size increase. Liquid flow will increase, or spraying pressure will decrease, as nozzles wear. The result is larger drops and less total liquid surface area. This is difficult to detect visually, so if a problem is suspected, arrange for drop size testing.

• Lowered spray impact. Worn spray nozzles operate at lower pressure, generally resulting in lower spray impact. (Ironically, in applications with centrifugal-type pumps, impact may actually increase because of increased flow through the spray nozzle.) Special testing may be required.

Preventing and solving problems
A comprehensive spray nozzle maintenance program will help ensure fewer headaches. By setting a regular schedule, key issues can be addressed before they cripple a production line.

The checklist that follows should become the foundation of a spray nozzle maintenance program. Consistent evaluation of these factors will enable early wear detection and appropriate action. Specific applications will determine how often each factor should be checked. The proper frequency could range from the end of every shift to every few months.

By implementing a nozzle maintenance program and documenting its procedures, the best nozzle maintenance and replacement strategy for achieving optimal performance can be determined.

Flow rate. For centrifugal pumps, monitor flow meter readings to detect increases. Or collect and measure the spray from the spray nozzle for a given period of time at a specific pressure. Compare these readings to the flow rates listed in the manufacturer’s catalog or compare them to flow rate readings from new, unused spray nozzles.

For positive displacement pumps, monitor the liquid line pressure for decreases; the flow rate will remain constant.

Spray pressure (in nozzle manifold). For centrifugal pumps, monitor for increases in liquid volume sprayed. The spraying pressure is likely to remain the same.

For positive displacement pumps, monitor the pressure gauge for decreases in pressure and reduction in impact on sprayed surfaces. The liquid volume sprayed is likely to remain the same. Also, monitor for increases in pressure due to clogged spray nozzles.

Spray pattern. Visually inspect the spray pattern for changes. Check the spray angle with a protractor. Measure the width of the spray pattern on the sprayed surface. If the spray nozzle orifice is wearing gradually, changes may not be detected until there is a significant increase in flow rate. If uniform spray coverage is critical to the application, request special testing from the spray nozzle manufacturer.

Drop size. Drop size increases cannot be visually detected in most applications. An increase in flow rate or decrease in spraying pressure will affect drop size.

Nozzle alignment. Check uniformity of spray coverage of flat spray nozzles on a manifold. Spray patterns should be parallel to each other. Spray tips should be rotated 5-10 deg from the manifold centerline.

Product quality/application results. Check for uneven coating, cooling, drying, cleaning, and changes in temperature, dust content, and humidity.

If, after implementing a spray nozzle maintenance program, it is determined that current nozzles are not performing as well as they should, it is time to replace them.

Extending spray nozzle life
There are some proven techniques to prolong the useful life of your spray nozzles.

Improve cleaning procedures. Nozzles are precision instruments. Cleaning should be done regularly but carefully, with materials that are much softer than the nozzle orifice surface. Use plastic bristle brushes, wooden probes, or plastic probes. Never use wire brushes, pocket knives, or welder’s tip cleaning rasps. It is easy to damage the critical orifice shape or size and end up with distorted spray patterns or excess flow. If faced with a stubborn clogging problem, try soaking the orifice in a noncorrosive cleaning chemical to soften or dissolve the clogging substance.

Add line strainers, or change to spray nozzles with built-in strainers. Orifice deterioration and clogging is typically caused by solid dirt particles in the sprayed liquid and is particularly common in systems using continuous spray water recirculation. Strainers, or spray nozzles with built-in strainers, are recommended—with a screen mesh size chosen to trap larger particles and prevent debris from entering the spray nozzle orifice or vane.

Decrease spraying pressure. Although it is not always possible to implement, decreasing the pressure—which will slow the liquid velocity through the orifice—may help reduce the wear and corrosion rate.

Reduce the quantity of abrasive particles or concentration of corrosive chemicals. In some applications, it is possible to reduce the amount of abrasive particles in the feed liquid, and/or change the size and shape of the particles to reduce wear effects. Also, the corrosive activity of a solution can occasionally be reduced by using different concentrations or temperatures, depending on the specific chemicals involved.

Consider durability and resistance issues. It is important to keep in mind that replacing old spray nozzles with the very same type (for example, replacing an aluminum nozzle with an aluminum nozzle) may not be the best option. Obviously a new spray nozzle is superior to a worn nozzle, but the situation may call for replacing current spray nozzles with nozzles that are much better suited to handle the types of liquids and chemicals that are routinely used.

Spray nozzles made of stronger material generally provide longer wear life. Predictably, stainless steel has a greater abrasion resistance ratio than aluminum, while carbides provide far greater abrasion resistance than stainless steel. To determine whether a different material should be considered for nozzles, spray tips, or orifice inserts, consult the chartApproximate Abrasion Resistance Ratios.”

In addition to abrasion resistance, corrosion resistance may be necessary. The rate of chemical corrosion on a spray nozzle depends on several factors, including the corrosive properties of the liquid being sprayed, its concentration in the solution, its temperature, and the properties of the nozzle material.

Explore special nozzle types. New types of spray nozzles feature extremely convenient, nonslip extensions that are easy to grip and twist even in wet or sticky conditions involving lubricants, oils, or other viscous materials.

Also, look for single and double pipe clamps that enable a worker to quickly change entire nozzle mounts whenever necessary.

Fortunately, many modern nozzles can be installed and replaced without the use of any tools. This makes the whole process faster, easier, and more reliable than ever.

Get expert assistance. A spray nozzle manufacturer should have the capacity to test and evaluate spray nozzles to help establish baseline performance measures that will guide cleaning, maintenance, and repair schedules. This can minimize downtime significantly, and help avoid quality control issues through timely spray nozzle replacement.

A fast and convenient calculator is available online to help you figure out the actual costs of sub-par spray nozzle performance in your own application. MT


Jon Barber is a specialist at Spraying Systems Co., P. O. Box 7900, Wheaton, IL 60189-7900; (630) 665-5000

1104sprayfig1 1104sprayfig2
1104sprayfig1a 1104sprayfig2a

Worn nozzles cannot be determined just by a visual examination. Differences can be seen in a new nozzle (left) and a worn one (right) in a magnified view, though.

 

 

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Approximate Abrasion Resistance Ratios

Spray Nozzle Material

Resistance Ratio

Aluminum

1

Brass

1

Polypropylene

1-2

Steel

1.5-2

Monel

2-3

Stainless steel

4-6

Hastelloy

4-6

Hardened stainless steel

10-15

Stellite

10-15

Silicon carbide (nitride bonded)

90-130

Ceramics

90-200

Carbides

180-250

Synthetic ruby or sapphire

600-2000

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4418

8:58 pm
November 1, 2004
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Detecting Broken Rotor Bars Prevents Catastrophic Damage

With advancements in digital electronics and reduced component costs in recent years, monitoring instruments for use in condition-based maintenance programs have become more cost-effective and dependable. Machinery does not need to be taken out of service as many tests are done online, and in many cases very little expertise is required for testing and data interpretation. This enables the user to make well-informed decisions for planning maintenance and repairs, which ultimately leads to increased productivity.

This article concentrates on one technology that has been developed to reliably detect broken rotor bars, abnormal levels of air gap eccentricity, and other problems in squirrel cage induction motors and driven components using motor current signature analysis (MCSA).

Consequences of broken rotor bars
Rotor windings in squirrel cage induction motors are manufactured from aluminum alloy, copper, or copper alloy. Larger motors generally have rotors and end-rings fabricated out of these whereas motors with ratings less than a few hundred horsepower generally have die-cast aluminum alloy rotor cages.

1104iris

Fig. 1. A 1700 hp motor with broken rotor bar

Broken rotor bars (Fig. 1) rarely cause immediate failures, especially in large multi-pole (slow-speed) motors. However, if there are enough broken rotor bars, the motor may not start as it may not be able to develop sufficient accelerating torque. Regardless, the presence of broken rotor bars precipitates deterioration in other components that can result in time-consuming and expensive fixes.

Replacement of the rotor core in larger motors is costly; therefore, by detecting broken rotor bars early, such secondary deterioration can be avoided. The rotor can be repaired at a fraction of the cost of rotor replacement, not to mention averting production revenue losses due to unplanned downtime.

Some of the more common secondary effects of broken rotor bars are:

• Broken bars can cause sparking, a serious concern in hazardous areas.

• If one or more rotor bars are broken, the healthy bars are forced to carry additional current leading to rotor core damage from persistent elevated temperatures in the vicinity of the broken bars and current passing through the core from broken to healthy bars.

• Broken bars cause torque and speed oscillations in the rotor, provoking premature wear of bearings and other driven components.

• Large air pockets in die-cast aluminum alloy rotor windings can cause nonuniform bar expansion leading to rotor bending and imbalance that causes high vibration levels from premature bearing wear.

• As the rotor rotates at high radial speed, broken rotor bars can lift out of the slot due to centrifugal force and strike against the stator winding causing a catastrophic motor failure.

• Rotor asymmetry (the rotor rotating off-center), both static and dynamic, could cause the rotor to rub against the stator winding leading to rotor core damage and even a catastrophic fault.

MCSA technology
Motor current signature analysis technology has existed for many years to help diagnose problems in induction motors related to broken rotor bars, air gap eccentricity, drive-train wear analysis, and shaft misalignment. The technology relies on the fact that each of these problems produces recognizable frequency patterns in the motor load current that can be predicted by using empirical formulae and measured. These problems give rise to magnetic asymmetry in the rotor air gap that produces current components at specific frequencies in the load current.

A trace of the motor supply current is obtained by using a clamp-on current probe either from one of the main phase leads to the motor or from the secondary side of a motor CT. A Fast Fourier Transform is performed on the time-domain data to obtain a frequency spectrum. Depending on the device used, this can be done either by the datalogger itself or by computer software.

Once the frequency spectrum is obtained and stored, empirical formulae are used to look for frequency signatures in the spectrum within various frequency ranges depending on the problem to be diagnosed. For example, broken rotor bar frequencies (also called sidebands or pole-passing frequencies) usually can be found within ±5 Hz of the motor supply frequency; for air gap eccentricity a wider range is required for the search, from a few hundred Hz up to a few kHz. If the predicted frequency patterns are present in the spectrum, a positive diagnosis is returned.

In all cases, accurate estimate of the operating slip of the motor is a prerequisite to reliable diagnosis as the predictor equations require operating slip as one of the input parameters. In an induction motor, slip is dependent on the load and increases with increased load. In most cases, the only knowledge a tester would have regarding slip is that at full load; the motor nameplate data contains the rated speed at rated horsepower and the slip can therefore be easily derived when the motor is running at full rated load. However, as motors rarely operate at exactly full load, determining the operating slip becomes a challenge.

There are several ways to determine operating slip—a stroboscope or axial flux measurement are two examples. However, between the time the speed is determined using these techniques and the current measurement taken the load can change, leading to an inaccurate slip estimate. Not to mention the fact that these methods are cumbersome and time consuming.

Much work has been done in recent years to make MCSA technology reliable and user-friendly by calculating the slip based on motor nameplate parameters and measured load current. Depending on the MCSA instrument vendor, several algorithms may be employed to calculate slip. Some algorithms rely on deriving slip from the torque and some from operating current. Such algorithms do not need an external speed input.

Advances in pattern-recognition technology have now made it possible that systems rely less on expert knowledge, thereby making these systems useable by nonexperts who may not have in-depth knowledge of current signature analysis.

Detection of broken rotor bars
The location of the frequency components of the current due to broken rotor bars in the frequency spectrum is given by the formula:

fsb = f1(1±2s) Hz

where:

fsb = frequency components of the current due to broken rotor bars, also known as sidebands

f1 = power supply frequency (Hz)

s = operating slip (per unit)

Figure 2 illustrates the current spectrum from a 13.8 kV primary air fan motor with broken rotor bars operating in a fossil power station. The motor supply frequency is 60 Hz. Frequencies due to broken rotor bars are clearly visible.

Frequency spectrum from motor
with broken rotor bars

1104iris2

Fig. 2. Frequencies due to broken rotor bars are clearly visible, as is the influence of load changes during data acquisition.

The influence of load
Figure 2 also illustrates the influence of load changes during the data acquisition process. Note the skirting effect at the base of the 60 Hz spike. Keeping in mind that the slip is dependent on load one would, in fact, expect such a skirting effect as the current components are recorded in multiple positions on the x-axis.

The influence of gearboxes
Speed-reducing gearboxes or belt drives connected to the motor also may induce frequency components of the current in the spectrum and also have been a cause of false alarms. The position of such components depends on the rotational frequency of the individual gearbox shafts. Often the frequencies of these components are very close to positions that are expected from broken rotor bars.

Take the case of a coal-mill motor for which the current spectrum is shown in Fig. 3. This motor is rated at 300 hp, 575 V, 295 A, 885 rpm, and is connected to a 3-stage gearbox for which the output shaft rotates at 19.39 rpm (0.32 Hz) at full load (nameplate data). Speeds of the individual shafts internal to the motor are 52.8 rpm (0.88 Hz) and 141.69 rpm (2.36 Hz), respectively. Table 1 depicts the location of the frequency components of the current due to each shaft rotational speed at full load.

In addition to fundamental speeds of shaft rotation, harmonics also can produce frequency components that occur at locations in the spectrum where broken rotor bars are expected (see Table 2). It can be seen from Table 2 that gearbox shaft rotation, especially the rotational harmonics from the 2nd and 3rd stages, induces frequency components of the current at locations very close to where components from broken rotor bars are expected to occur. Keep in mind that Table 2 depicts conditions at full load.

Spectrum from a motor connected to a gearbox

1104iris3

Fig. 3. Several current components are present in the spectrum. The question is which ones are due to broken rotor bars.

In this case study, the motor was operating at less than full load with a current of 250 A and therefore at lower slip (higher speed). Even at this load, the harmonics from shaft rotation may lead a user to raise a false alarm of broken rotor bars if not correctly identified as such. Whereas frequency components due to the gearbox are expected to remain at almost the same location for full load (295 A) as well as reduced load (250 A), components due to broken rotor bars move “inwards” at reduced load, i.e., toward the fundamental 60 Hz component. As a corollary, if it is possible to collect data at two different loads, chances of misdiagnosis can almost be eliminated as this would help identify twice-slip-frequency components from mechanical components. In fact, the motor in this case study did not have broken rotor bars.

Problems due to gearbox interference are easily circumvented by embedding intelligence in the instrument that enables it to predict such interfering frequencies. This requires that the reduction stage ratios are known and fed in prior to processing the data for diagnosis.

The importance of high resolution
This case also highlights the necessity of using high resolution in data acquisition and spectrum analysis. A resolution of 10 MHz would generally be sufficient to discriminate between distinct sidebands and therefore enable reliable diagnosis. High resolution is particularly important when testing low-slip and/or low-speed motors where the sidebands do not move as much as high-slip or high-speed applications and therefore could make frequency discrimination difficult.

One of the problems encountered when acquiring high-resolution data is the acquisition and processing time. However, with modern processors and digital technology this problem has largely been overcome due to high-speed sampling and processing capabilities.

Motor current signature analysis technology can reliably be used to detect problems in induction motors. Advancements in technology have made devices intelligent enough to minimize false alarms while at the same time minimizing need for expert interpretation and reducing time for testing and diagnosis. MT


Information supplied by Hasnain Jivajee, product specialist, and Ian Culbert, rotating machines specialist, at Iris Power Engineering Inc., 1 Westside Dr., Unit 2, Toronto M9C 1B2, ON; (416) 620-5600.

Table 1. Expected Frequency Positions from
Broken Rotor Bars and Gearbox at Full Load

Broken rotor bars at

58 and 62 Hz

1st stage

60 ± 2.36 Hz = 57.64 and 62.36 Hz

2nd stage

60 ± 0.88 Hz = 59.12 and 60.88 Hz

3rd stage

60 ± 0.32 Hz = 59.68 and 60.32 Hz

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Table 2. Expected Frequency Positions of
Gearbox Harmonics at Full Load

1st stage, fundamental

60 ± 2.36 Hz = 57.64 and 62.36 Hz

2nd stage, 2nd harmonic

60 ± 2×0.88 Hz = 58.24 and 61.76 Hz

3rd stage, 6th harmonic

60 ± 6×0.32 Hz = 58.1 and 61.9 Hz

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