Predictive Maintenance / Preventive Maintenance

The basics of setting up an effective PdM program

Here’s what you need to do (and why) when it comes to developing and implementing a predictive strategy.

By Timothy Thomas, Hibbs ElectroMechanical

In this article:

In the past half-century, numerous strategies and ideas have been developed and tested regarding the best ways to maintain a safe and efficient operating environment. Asset management (AM), reliability-centered maintenance (RCM) and operator-driven reliability (ODR) are just a few of the dozens of concepts being touted as best practices.

However, there are basically three broad strategies when it comes to maintaining motors and machinery in any facility: corrective, preventive, and predictive. Each has its own benefits and disadvantages, and the correct choice for a given situation requires consideration of many vital factors. In some cases, a combination of all three strategies is the right choice, but most often a predictive approach with some preventive activity yields the best results.

Maintenance strategy: Corrective maintenance


Corrective maintenance is essentially “run-to-failure” maintenance, or “fix-it-when-it-breaks” maintenance. This concept requires no preplanning other than having some spares on hand and is usually employed only in very small manufacturing operations, where the motors are cheap and easy to replace and where downtime is not a consideration.

As a maintenance plan, corrective maintenance is by far the most expensive way to operate. Disadvantages to this approach include the costs to keep spare motors and parts in inventory, the interruption of processes that occurs when a repair or replacement is needed, and the expense of emergency repairs. Motors will almost always choose a very inopportune time to fail, and they seldom provide indicators of their deterioration if they are not being properly monitored.

One major aspect that should be considered before choosing to let a machine system run to failure is worker safety. In some cases, machinery failure can result in a hazardous situation for the operator or other individuals nearby. Another consideration is the environmental impact of employing a corrective maintenance approach. Before choosing corrective maintenance as your strategy, it is necessary to consider all possible effects and consequences of doing so.

Maintenance strategy: Preventive maintenance


Preventive maintenance is a great deal more practical than corrective maintenance, but it has its share of costly drawbacks, too. This is a time-based plan that calls for making repairs or replacements based entirely on an asset’s time in service without any consideration of the asset’s condition. Preventive maintenance is often referred to as “fix it before it breaks” or “fix it even if it isn’t broken.” It is commonly known as periodic maintenance, or calendar-based maintenance. The concept assumes that all machines will fail in time, which is realistic, but when will they fail, and why?

John Moubray, in the book “Reliability Centered Maintenance II,” writes, “There is often little or no relationship between how long an asset has been in service and how likely it is to fail.”

One major disadvantage of a preventive maintenance plan is the possibility that some machines will fail before their planned shutdown. Other drawbacks include the prospect of removing perfectly good machines from service and overrepairing them, as the reconditioning process itself may introduce new faults. And the procedure will require unnecessary downtime, lost production, and needless repair costs.

Some industries – including, for the most part, the marine industry – continue to operate on a preventive maintenance plan. Cargo ships, fuel oil carriers, freighters of all kinds, and even cruise liners are bound by regulations imposed and/or enforced by various entities, including the U.S. Coast Guard, the American Bureau of Shipping, and various insurance agencies. These organizations have stringent rules mostly centered on safety issues. Depending on their size, their cargo, and their destinations, ships may be required to have proof that all of their assets are reconditioned periodically, without consideration of current condition. In this situation, it may not make economic sense to do any condition-based testing or verification and take only minor preventive measures.

Other industries utilize a combination of time-based and condition-based maintenance. Most major theme parks, for example, operate in this manner. They cannot risk unplanned failures, and the alternative leads them to overmaintaining their assets. Their maintenance costs are considerably higher than in most other industries, but downtime could be extremely costly, and the public safety issues at play merit extreme precautions.

However, even in those cases, some preventive activities are necessary. These include addressing lubrication issues and performing belt tensioning and brush maintenance. A successful PdM program will generally include some site-specific preventive maintenance procedures.

Maintenance strategy: Predictive maintenance


Predictive maintenance is defined as the routine monitoring and trending of data related to the overall health of an asset. Using as many tools as practical will allow the technician to accurately predict imminent failures before they become catastrophic. A well-designed and well-managed predictive maintenance program will more than pay for itself by limiting unplanned downtime and allowing for minor repairs in place of incurring the costs associated with emergency shutdowns.

The idea of predicting failures has gained worldwide acceptance, and PdM is replacing many preventive maintenance activities. Better diagnostic equipment and new technologies are being released constantly, and the tools with which to make accurate predictions are rapidly becoming “smarter.” This makes keeping up with technology advancements imperative for a PdM program to be successful.

John Moubray defined predictive maintenance as “the process of monitoring the condition of an asset and taking an action to avoid the consequences of a failure.” Basically, “if it ain’t broke, don’t fix it.”

Predictive maintenance is most often referred to as condition-based maintenance. The concept, when applied properly, allows operators to plan their downtime to take place at some convenient time, eliminating the costs incurred when emergency repairs are required. It is known that some motors will fail very quickly – a concept known as infant mortality – but once a motor has passed that period, its life expectancy can be many years.

Routine monitoring provides information that will lead to indicators of imminent failure. The more tools a technician has, the better his or her chances of locating problem areas and defining weaknesses that lead to asset failure. Properly managed predictive maintenance programs will ensure the safe and continued operation of all assets, increased productivity, and lower overall operating costs. The outcome will benefit both the production level and the bottom line.

Establishing a predictive maintenance program


There are some essential facts that you must consider and evaluate when establishing a predictive maintenance program. The first, and maybe the most important, is this: Who is going to do the testing and monitor the program? Do you have the personnel, and do they have the time? Available labor hours can be a huge issue, and often in setting up a new PdM program, the hiring of several new technicians is deemed necessary. In some cases, it may be more realistic to consider outsourcing some or all of your predictive maintenance technologies. New technologies may require extensive and varied training. Are you willing to hire new people and spend the time and money necessary to train them? Undertrained technicians are a major problem throughout industry, partially due to the “hidden” cost associated with state-of-the-art technologies. For instance, vibration analysis training requires years of practical hands-on use and several levels of competency before a technician can be fully capable of making accurate calls.

Besides considering personnel matters, another important step is to define the issues that are important to your operation. Identify problem areas by reviewing past issues, with emphasis on conducting root-cause analyses. Research all technologies and determine which would best help resolve those issues.

Establish criticality of equipment and classify equipment by level of importance to the operation. Again, worker safety along with production goals must be considered when establishing priorities.

Define your long-range goals and needs. Purchasing several new technologies all at once is usually not a good idea. Besides the cost associated with purchasing, there are many other factors to consider, including the acquisition of personnel and training and the integration of diagnostic data into a useful and beneficial format. Purchase one or two technologies and allow time for them to become incorporated into your facility’s operation and to begin functioning properly. Approximately 75% of issues that cause motor problems are mechanical in nature. Mechanical problems include looseness, imbalance and misalignment. Because mechanical issues are the leading cause of deterioration and failure in all rotating equipment, the logical first choice for a new technology is vibration analysis. Continue to add new technologies as your awareness level increases and the technical staff becomes adept at using the new tools. As stated earlier, motor insulation is the heart of the motor, so static electrical testing would be a good second addition.

When considering long-range goals, determine how many motors are of major concern. Are there other mechanical problem areas – perhaps issues pertaining to gearboxes, conveyor belts and other pieces of machinery? Their numbers may affect which technologies you acquire first.

Once you have defined a plan to build your predictive maintenance program, begin acquiring equipment. Again, research all available equipment within each technology. Learn the differences between manufacturers and within manufacturers’ product lines. Cheaper equipment may identify the faults you want to find, but these pieces will probably not have the features you want. Less-expensive equipment usually doesn’t have saving or reporting capabilities, and those features are essential to a predictive maintenance program. Trending depends on historical data, so it is vital that the equipment you purchase has the ability to save and compare data.

Top-of-the-line and state-of-the-art equipment may have more bells and whistles than you need, but this level of equipment will almost always make your life easier. Generally, state-of-the-art equipment is computer-controlled, which allows collected data to be easily transferred to a desktop computer for evaluation and reporting. Safe testing of assets is also a major concern. Cheaper, manually operated equipment may allow an unskilled user to raise test voltages to an unsafe level, which can introduce faults. Generally, top-of-the-line equipment will have safeguards built in to prohibit unsafe levels of testing.

One major pitfall with predictive maintenance programs is the complacency of managerial level personnel and their emphasis on production. Once a good predictive maintenance program is established and operating efficiently, upper level managers tend to place less importance of keeping up with the very predictive technologies that made their facilities more productive and less prone to expensive unplanned down-time. And the vicious circle begins. The best way to insure that a predictive maintenance program, that has been established and performing properly, continues to receive the funding it requires is to document findings. When your program identifies and rectifies a potential problem, document the findings and make sure management is aware of your successes.

Another important and often overlooked issue is motor responsibility. In many situations, mechanical and electrical departments are separated, and although each may be collecting good data, there may be no meaningful communication between the departments. A good predictive maintenance program requires a “motor owner” – someone who will review all of the collected data and who will be responsible for making asset decisions.

Requirements for a successful PdM program

 

  1. Regular routine maintenance activities are completed on a scheduled basis.
  2. Predictive maintenance activities have been established based on a process to identify critical equipment, common modes of failure, and primary failure indicators.
  3. Actual repair findings are compared with the asset’s predicted condition to validate and evaluate diagnostic applications.
  4. Predictive maintenance results are continually tracked and evaluated to ensure proper application.
  5. Many PM activities are replaced with PdM activities.
  6. The frequency and type of predictive and PM activities are adjusted based on operating experience, results of reliability analyses, changes in operating conditions and environment, and vendor recommendations.
  7. Maintenance histories are analyzed to determine equipment failure trends that can be attributed to aging.
  8. Corrective actions are taken before condition specifications are exceeded.
  9. Systems and methodologies for inspection are developed by a group of experts to ensure that the inspection plan is comprehensive.
  10. New baseline performance measurements are documented whenever equipment has been repaired.
  11. There is maximum use of see-through panels, inspection ports, and other visual indicators.
  12. Regular, routine, PdM activities occur on a scheduled basis. PdM monitoring is given priority for scheduling purposes and is executed on a timely basis.
  13. Establish a “motor owner” who has all collected data and is responsible for making decisions regarding each asset.

In summary, research all relevant technologies; analyze your specific needs; consider the pros and cons of different types of equipment; and be willing to invest and keep up with enhancements. Consider outsourcing as a viable alternative.

Conclusion


Predictive maintenance programs have proved to be the most cost-effective method of maintaining a safe and efficient operation. The vast array of monitoring tools available makes it easier to identify potential problem areas and to take steps to remedy an emerging issue before it becomes a major catastrophe. The cost savings from predicting imminent failures in time to avoid them more than offsets the cost of implementing and administering a predictive maintenance program.

A successful program will necessitate state-of-the-art equipment, qualified technicians, routine monitoring, vigilance and continuous adherence to details. A successful program also will require motor ownership, where one person or a small group of people sees all collected data and is allowed or obligated to make decisions regarding an asset’s future.