English Version / TPM Concepts and Literature Review / Concept / Planned Maintenance

Planned Maintenance Pillar (PM)

The objective of Planned Maintenance is to “establish and maintain optimal equipment and process conditions”. (Suzuki 1994 p. 145) As defined by JIPM, “Devising a planned maintenance system means raising output (no failures, no defects) and improving the quality of maintenance technicians by increasing plant availability (machine availability). Implementing these activities efficiently can reduce input to maintenance activities and build a fluid integrated system, which includes:

Regular preventive maintenance to stop failures (Periodic maintenance,predictive maintenance).
Corrective maintenance and daily MP [maintenance prevention] to lower the risk of failure.
Breakdown maintenance to restore machines to working order as soon aspossible after failure.
Guidance and assistance in ‘Jishu-Hozen’ [Autonomous Maintenance].” (Japan_Institute_of_Plant_Maintenance 1996 p. 119)

Like Focused Improvement, Planned Maintenance supports the concept of zero failures. “Planned maintenance activities put a priority on the realization of zero failures. The aim of TPM activities is to reinforce corporate structures by eliminating all losses through the attainment of zero defects, zero failures, and zero accidents. Of these, the attainment of zero failures is of the greatest significance, because failures directly lead to defective products and a lower equipment operation ratio, which in turn becomes a major factor for accidents.” (Shirose 1996 p. 309) Maintenance activity can be viewed as a continuum of regimes as illustrated in Figure 18.*

Figure 18- Maintenance Regimes

Breakdown Maintenance (BM): Breakdown Maintenance refers to maintenance activity where repair is performed following equipment failure/stoppage or upon a hazardous decline in equipment performance.** (Japan_Institute_of_Plant_Maintenance 1996)

Time-Based Maintenance: Time-Based Maintenance*** refers to preventive maintenance activity that is scheduled based on an interval of time (for instance daily, weekly, monthly, etc.) “Preventive maintenance … keeps equipment functioning by controlling equipment components, assemblies, subassemblies, accessories, attachments, and so on. It also maintains the performance of structural materials and prevents corrosion, fatigue, and other forms of deterioration from weakening them.” (Suzuki 1994 p. 149)

Usage-Based Maintenance: Usage-Based Maintenance refers to preventive maintenance activity that is scheduled based on some measure of equipment usage (for example number of units processed, number of production cycles, operatinghours, etc.) Usage-Based Maintenance is significantly different from Time-Based Maintenance in that it is scheduled based on the stress and deterioration that production activity places on equipment rather than just a period of time. Since equipment may run different levels of production from one time period to another,

Usage-Based Maintenance: allows preventive maintenance to be aligned with the actual workload placed on the equipment.

Condition-Based Maintenance: Condition-Based Maintenance is a form of preventive maintenance that is scheduled by actual variation or degradation that is measured on the equipment. Condition-Based Maintenance expands on the concept of Usage-Based Maintenance by scheduling maintenance based on observed (ormeasured) wear, variation, or degradation caused by the stress of production one quipment. Examples of monitored equipment parameters include vibration analysis, ultrasonic inspection, wear particle analysis, infrared thermography, video imaging, water quality analysis, motor-condition analysis, jigs/fixtures/test gauges, and continuous condition monitoring. (Leflar 2001) To execute Condition-Based Maintenance, the user must determine observation points or parameters to be measured that accurately predict impending loss of functionality for equipment. Observations and measurements are taken during scheduled inspection cycles. Visual controls play a role in Condition-Based Maintenance by providing graphic indicationsfor out-of-specification measurements or conditions.

Leflar identifies two types of equipment degradation that should be considered when developing the site Planned Maintenance TPM pillar. (Leflar 2001)

Graceful Deterioration: Degradation is gradual and the thresholds of acceptable performance can be learned and failures projected within scheduled inspection cycles (see Figure 19). Since the deterioration progresses slowly, the pre-failure degradation is identifiable within the scheduled Condition-Based Maintenance inspection cycles.
Non-graceful Deterioration: Deterioration progresses rapidly (from normal measurement to failure in less than the inspection cycle) and maynot be detected within the inspection cycle of Condition-Based Maintenance (see Figure 20). Non-graceful deterioration may be learned, which allows the life expectancy of the component or function to be projected. In this case, Calendar-Maintenance Maintenance or Usage-Based Maintenance preventive maintenance scheduling will be effective.

Figure 19 - Graceful Deterioration

Figure 20 - Non-Graceful Deterioration

Predictive Maintenance: Predictive Maintenance takes Condition-Based Maintenance to the next level by providing real-time monitors for equipment parameters (for example voltages, currents, clearances, flows, etc.). “The objective of predictive maintenance is to prevent the function of equipment from stopping. This isdone by monitoring the function or loss of performance of the parts and units of which equipment is composed, to maintain normal operation.” (Nishinaga 1999 p. 13) Predictive Maintenance can be considered the ‘crystal ball’ of Planned Maintenance. (Steinbacher and Steinbacher 1993)

Predictive Maintenance “measures physical parameters against a known engineering limit in order to detect, analyze, and correct equipment problems before capacity reductions or losses occur…The key to the predictive method is finding the physical parameter that will trend the failure of the equipment.” (Wireman 1991 p.87) Takeuchi refers to this as understanding the “pre-disease” (optimal operating) condition of the equipment so that variation from optimal can be identified. (Takeuchi 2001) Preventive maintenance is then scheduled when a monitored parameter is measured out-of-specification. Nhsinaga notes that the flow of predictive maintenance is divided into three broad elements, 1) establishment of diagnostic technologies (monitoring techniques), 2) diagnosis (comparing actual totarget readings), and 3) maintenance action (responding to variation). (Nishinaga1999) Where Condition-Based Maintenance occurs as the result of scheduled inspections, Predictive Maintenance identifies variation or degradation as it occursand initiates maintenance activity.

Closed-Loop Automation: Simple Closed-Loop Automation describes an advanced automation capability in which equipment performance variation or degradation is monitored real-time and automated corrective input is made to the equipment (when possible within acceptable performance conditions) to adjust for the variation or degradation and continue normal in-specification processing (see Figure 21).

Figure 21 - Simple Closed-Loop Automation

Advanced Closed-Loop Automation**** looks beyond just the equipment performance and monitors production flow as well as equipment, including the following functionality (see Figure 22). (Vadas and Walker 2003)

1. Sense changes.
2. Execute real-time decision logic acting on all data available to factory automation.
   a. Work in Progress (WIP).
   b. Maintenance Repair Operations (MRO).
   c. Production inventory.
   d. Resource capacity.
3. Issue work directives according to enterprise goals.
4. Coordinate equipment and material processing.
5. Continuously monitor and report status of equipment, material, and other factory resources.

Figure 22 - Advanced Closed-Loop Automation

Corrective Maintenance: Corrective Maintenance is planned maintenance that makes permanent continuous improvement changes (versus repair activity) to equipment.***** Within the TPM framework, identification of desirable corrective action activity occurs within the Focused Improvement, Autonomous Maintenance, and Planned Maintenance TPM pillar activity. Corrective Maintenance may reduce/eliminate failure modes, improve variation/degradation identification (visual controls), or simplify scheduled or unscheduled maintenance activity.

The key to effective Planned Maintenance is to have a PM plan for every tool. The PM plan is based on the history and analysis of failure modes to determine preventive practices. The PM plan consists of five elements. (Leflar 1999)

1. A set of checklists for PM execution.
2. A schedule for every PM cycle.
3. Specifications and part numbers for every checklist item.
4. Procedures for every checklist item.
5. Maintenance and parts log (equipment maintenance history) for everymachine.

The PM plan is then executed with precision; meaning that is implemented 100% of the time, completed 100% as specified, and implemented without variation by knowledgeable people. Leflar estimates that only 40% of all Planned Maintenance is accomplished with the expected degree of precision. “The precision level that will beachieved on any given maintenance task is 80% determined before a maintenance technician even begins the work! Management must supply the basic tools that create precision maintenance within their own organization.” (Leflar 2000 p. 8) The PM plan is continually improved to make it easier, faster, and better. Equipment failures suggest the need for further improvement of the PM plan. To this end, two questions must be answered for every equipment failure post-mortem. (Leflar 2001)

1. Why did we not see the failure coming?
2. Why did the PM plan not prevent the failure?

Thomas presents a process for PM optimization that supports Leflar’s conceptof continuous PM process improvement (Figure 23). (Thomas 2003)

Figure 23 - PM Optimization Process

[Original: Total Productive Maintenance (TPM) Concepts and Literature Review by Thomas R. Pomorski, Principal Consulting Engineer, Brooks Automation, Inc.]

* The maintenance regimes are color coded to represent the SEMI E10 equipment states. SEMI(2001). SEMI E10 Standard for Definition and Measurement of Equipment Reliability, Availability, and Maintainability (RAM). San Jose, CA, Semiconductor Equipment and Materials International.

** Classic TPM strives for zero equipment failures, and thus considers any event that requires breakdown maintenance to be a continuous improvement opportunity. Nakajima, S. (1984). Introduction to TPM: Total Productive Maintenance. Cambridge, MA, Productivity Press. Shirose, K., Ed. (1996). TPM-Total Productive Maintenance: New Implementation Program in Fabrication and Assembly Industries. Tokyo, Japan, Japan Institute of Plant Maintenance. Reliability-Centered Maintenance (RCM), on the other hand, allows equipment to run-to-failure (RTF) for some failure modes based on the economic and safety impact of the failure. Nowlan, S. F. and H. F. Heap(1978). Reliability Centered Maintenance. San Francisco, United Air Lines. Moubray, J. (1991). Reliability Centered Maintenance. Oxford, England, Butterworth-Heinemann. Asubsequent paper on Reliability-Centered Maintenance will examine this concept in more detail.

*** Time-Based Maintenance is also known as Periodic Maintenance.

**** A subsequent paper on Automation Engineering will examine Advanced Closed-Loop Automationin further detail.

***** In advanced automated manufacturing environments the concept of equipment maintenance is expanded to include supporting automation and automated material handling systems. No assumption is made as to actual internal organizational maintenance responsibilities.