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Preventive and predictive maintenance tools: or why not to fix the degreaser's filters with a thread and needle.

Over the past two decades there have been clear advances in the management of the maintenance of production equipment. Maintenance is no longer considered just another cost of doing business. First, let's look at fundamental reasons why maintenance is critical. The elimination of unscheduled breakdowns is in keeping with the Japanese management concept of Muda (1-2), the elimination of waste, and is vital both for just-in-time manufacturing environments, total quality management and for dealing with cost-conscious customers. Unscheduled breakdowns introduce the waste of time into a production environment. This waste of time has two effects:

1. Since it is unknown when production equipment will fail and the amount of time that will elapse before the equipment and the associated production processes are once again fully operational, additional inventory must be kept on hand. By reducing the frequency of expected failure through improved maintenance the amount of inventory that must be kept on hand just in case equipment fails can be reduced and eventually eliminated.

2. A reduction in unexpected down time also increases profitability. Equipment can only produce products and value for the firm if the equipment is operational. While this is important for any firm, it is highly critical for processes that are commodity-like in nature or if a customer has an understanding of what the cost of manufacture is. In a highly competitive industry like electronics, a manufacturer that has no unexpected breakdowns will have a noticeably lower cost of production than a firm with many unexpected breakdowns of equipment. Now let's turn attention to preventive and predictive maintenance.

Preventive maintenance. Preventive maintenance assists in avoiding unexpected breakdowns through the replacement of parts, before parts are expected to fail. By replacing parts well before they are expected to fail, unexpected machine downtime can be reduced (in some cases practically eliminated). The result is a replacement of more parts than would be required if the equipment was allowed to fail, but the cost of maintenance labor is lower per unit since maintenance can be scheduled to times that are convenient. Furthermore, the collateral damage that can occur when one part of a machine is allowed to fail can be avoided.

Preventive maintenance is typically managed through the specification of either testing or part replacement after a certain period of time has elapsed. Manufacturers frequently provide preventive maintenance schedules for equipment, so the problem with preventive maintenance programs is typically not misspecification, but failure to pursue the specified program. In some facilities preventive maintenance programs are either not followed at all or only partially followed, resulting in equipment operating at below-expected standards or failing in an unexpected fashion. To illustrate these sorts of situations two anecdotes are offered, with associated lessons.

A degreaser was not operating properly. There was a disagreement over why the degreaser was not cleaning product to the extent anticipated. Review of the preventative maintenance schedules showed that filter replacement and other actions were being taken care of on a weekly basis, as scheduled. However, when inquiries were made regarding the parts that were being replaced, according to the preventive maintenance schedule, the personnel at non-production stores reported that replacement filters had not been ordered for the degreaser in over six months. A meeting was called between engineering, production and maintenance personnel for all three shifts. The late-night maintenance personnel advised that the lack of filter replacement was not indicative of a lack of maintenance. About one year earlier non-production stores had ran out of the micron filters, so maintenance personnel scrubbed the existing filters with soap and water to ensure that the degreaser was equipped with clean filters. After awhile, one maintenance mechanic explained, the filters either developed large holes or the fabric tore. These flaws were rectified by the maintenance mechanic's wife--she mended the filters with a sewing needle and thread. Upon recognizing this novel maintenance practice it was explained to the mechanics that the size of the mesh in the filters was so small that additional wear rendered the filters useless long before a tear or a repair with thread occurred. Having identified this problem, non-production stores started ordering filters again. The maintenance mechanics started replacing filters again and the quality problems disappeared. Besides offering a happy ending this anecdote offers a lesson: An indirect way to check if a preventive maintenance program is being followed is to check if non-production stores are ordering parts and that the maintenance personnel are using the parts required by the preventive maintenance program.

In another facility, the maintenance mechanics needed a lubricant for parts in a piece of equipment. The maintenance mechanics ran out of the regular lubricant, but found an excellent substitute: the hand soap used in the employee restroom. This unanticipated use of hand soap worked well and was continued for a long period of time. One day, however, the hand soap in the restroom was replaced with a soap designed especially for removing oil and grit from employees' hands. The new hand soap had rough little balls embedded in it. The soap would assist in removing oil and grit from the hands of employees. The maintenance personnel were so much in the habit of using the soap as a lubricant that they without hesitation or thought applied the new soap to the machine parts. The hard inclusions in the soap were useful for removal of oil and grit from hands, but were fatal to the use of the soap as a machine parts lubricant. The use of hand soap as a lubricant for machine parts was discontinued and replaced with an actual machine part lubricant. This anecdote, like the previous one, serves as a good reminder that maintenance mechanics are innovative and will find a way to get the job done, even when short of specified equipment of materials. And the only time that the mechanics' innovations will be noticed is when they have unintended consequences that are undesirable, such as causing an unanticipated machine failure or substantially degrading the performance of the equipment.

Having considered some of the challenges and situations to be wary of with preventive maintenance programs, predictive maintenance is now considered.

Predictive maintenance. Predictive maintenance has been pursued by many firms as a next step after the implementation of a preventive maintenance program. Predictive maintenance involves identifying changes in equipment that warn of either a pending degradation in performance or imminent failure of the monitored equipment. The continuing improvements in sensor quality coupled with declining cost and size has made predictive maintenance feasible for an increasingly large number of applications. Changes in equipment or equipment subsystems are detected using sensors to determine the onset of failure due to wear or other causes. For electrical equipment predictive maintenance is frequently conducted through the monitoring of the equipment's thermal signature. As a piece of equipment is approaching failure, there is often an increase in its operating temperature. (Thermal signatures can also be used to identify impending failure on other equipment, such as dies that are in contact with molten metal.) In the case of mechanical equipment such as bearings, noise or vibration is frequently monitored. A change in the noise level or in the vibration is indicative of the need for lubrication or of accumulated wear requiring replacement of parts.

Discussion of predictive maintenance quickly becomes highly specific due to the wide range of sensors available. There are a large number of suppliers of services and equipment for predictive maintenance programs. Consequently, it is recommended that the OEM be contacted regarding recommended methods of conducting predictive maintenance on equipment.

Concluding Notes

The use of SPC in combination with preventive and predictive maintenance is worth considering. Key questions include:

1. Does maintenance result in a decline in the range (volatility) of processes or a favorable shift in the center line of the process? If so, one should consider whether maintenance should be conducted earlier.

2. If there is no indication of declining quality or increased range (volatility) in critical characteristics, then it is possible that maintenance is being conducted more frequently than needed? However, one should be careful when delaying maintenance since the cost of unexpected breakdowns is usually higher than it appears, due to increased competition and moves toward JIT across entire supply chains.

3. After maintenance is there an increase in out-of-control conditions, process range (volatility), or a shift of the process in an undesirable direction? Such a condition is indicative of flaws in the servicing of the equipment. In some facilities, people joke that once the maintenance department works on the equipment, the performance of the equipment actually declines (according to the SPC charts) until someone goes and fixes it. A decline in performance identified by SPC charts or otherwise is an opportunity to improve the maintenance procedures and the overall production process. A failure to take advantage of this opportunity is unfortunate.


1. Jeffrey Liker, The Toyota Way: 14 Management Principles from the World's Greatest Manufacturer, McGraw-Hill, 2004.

2. Taiichi Ohno, Toyota Production System: Beyond Large-Scale Production, Productivity Press, 1988.

Jonathan Linton, is the Paul Desmarais Professor of the Management of Technological Enterprises at the School of Management at the University of Ottawa and editor of Technovation: the International Journal of Technological Innovation, Entrepreneurship, and Technology Management;

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Title Annotation:Focus on Business
Author:Linton, Jonathan
Publication:Circuits Assembly
Date:Oct 1, 2006
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