Maintenance planning and total productive maintenance

Breakdown, preventive, predictive and reliability-centred maintenance, planning and scheduling with MTBF, MTTR and availability, preventive-versus-breakdown cost comparison, TPM pillars, the six big losses and OEE.

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Why it matters

In a typical Indian machine shop, unplanned breakdowns, long set-ups and minor stoppages quietly eat a third or more of the available machine time. Maintenance planning decides when to service equipment so that it is available when production needs it, at the lowest total cost. Total Productive Maintenance (TPM) extends this to the whole workforce and measures the result with Overall Equipment Effectiveness (OEE), a number plant managers track daily.

Key ideas

Types of maintenance

  • Breakdown (corrective, run-to-failure): repair after failure. Cheapest planning effort, but failures come at random, cause secondary damage and stop production. Sensible only for cheap, non-critical items or items with constant failure rate.
  • Preventive (time- or usage-based): service or replace at fixed intervals (hours, cycles, kilometres) whether or not the item shows wear. Effective only when the failure rate rises with age (wear-out); for items with a constant failure rate, replacing a working part does not reduce failures.
  • Predictive (condition-based): monitor the condition and intervene when a measured indicator shows deterioration: vibration analysis (bearings, unbalance, misalignment), thermography (electrical joints, bearings), oil and wear-debris analysis (gearboxes, engines), ultrasonic and acoustic emission, motor current analysis. It uses most of the useful life while avoiding most failures.
  • Proactive / reliability-centred maintenance (RCM): analyses each failure mode and its consequences (often with FMEA) and chooses the best policy per mode, including redesign to remove the root cause.
  • Opportunistic maintenance: uses planned or unplanned stoppages to do other due work.

Maintenance planning and scheduling. Planning decides what work is needed and how (job plans, skills, spares, tools, safety isolation, estimated time). Scheduling decides when, in agreement with production. A computerised maintenance management system (CMMS) holds the equipment register, history, work orders and spares; criticality ranking (effect of failure on safety, quality, output and cost) sets priorities. Key indicators: MTBF (reliability), MTTR (maintainability), availability, percentage of planned work, and maintenance cost per unit output.

The economics. As preventive effort increases, breakdown cost falls but preventive cost rises; total cost has a minimum at some PM interval. The optimum is found by comparing the expected cost per period for each interval, including the breakdowns expected between services.

Total Productive Maintenance (TPM). Developed in Japan (Nakajima, JIPM) for production equipment, with the goals of zero breakdowns, zero defects and zero accidents through participation of everyone from top management to operators. The usual eight pillars are: autonomous maintenance (jishu hozen: operators clean, inspect, lubricate and tighten their own machines), focused improvement (kobetsu kaizen), planned maintenance, quality maintenance, early equipment management, education and training, safety-health-environment, and TPM in offices. TPM is built on a 5S foundation.

Six big losses and OEE. TPM groups equipment losses into six:

  • Availability losses: (1) breakdowns, (2) set-up and adjustment.
  • Performance losses: (3) idling and minor stoppages, (4) reduced speed.
  • Quality losses: (5) process defects and rework, (6) start-up (yield) losses. OEE multiplies the three rates. A widely quoted world-class benchmark is about 85 % (availability 90 %, performance 95 %, quality 99.9 %); many plants run nearer 50–60 %.

Formulas

MTBF = Total operating time / Number of failures, MTTR = Total repair time / Number of repairs

  • Both in hours.

A = MTBF / (MTBF + MTTR)

  • Inherent availability (dimensionless).

Availability = Operating time / Planned production time

  • Operating time = planned production time − downtime (breakdowns, set-ups); planned production time excludes planned breaks.

Performance = (Ideal cycle time × Total count) / Operating time

  • Ideal cycle time in min per part; total count includes defectives.

Quality = Good count / Total count

OEE = Availability × Performance × Quality = (Good count × Ideal cycle time) / Planned production time

Bₙ = N(p₁ + p₂ + … + pₙ) + B₁pₙ₋₁ + B₂pₙ₋₂ + … + Bₙ₋₁p₁

  • Expected number of breakdowns in n periods after a group preventive service of N machines; pᵢ = probability that a machine fails in period i after servicing.

Cost per period (PM every n periods) = (N·c_pm + c_bd·ΣBᵢ) / n; breakdown-only policy: N·c_bd / (Σ i·pᵢ)

  • c_pm = preventive cost per machine; c_bd = breakdown repair cost per failure (₹).

Worked examples

Example 1 (standard): OEE Given: an 8 h shift (480 min) with 30 min of planned breaks; 30 min breakdown and 15 min set-up; ideal cycle time 0.5 min per part; 720 parts made, of which 36 are rejected.

  1. Planned production time = 480 − 30 = 450 min; operating time = 450 − 45 = 405 min.
  2. Availability = 405 / 450 = 0.900.
  3. Performance = 0.5 × 720 / 405 = 360 / 405 = 0.889.
  4. Quality = (720 − 36) / 720 = 684 / 720 = 0.950.
  5. OEE = 0.900 × 0.889 × 0.950 = 0.760. Check: 684 × 0.5 / 450 = 0.760.

OEE = 76 %. The largest loss is performance (minor stoppages and slow running), not breakdowns.

Example 2 (GATE level): preventive versus breakdown maintenance Given: 10 identical machines. After a service, the probability that a machine fails in month 1, 2, 3, 4 is 0.2, 0.1, 0.3, 0.4. Breakdown repair costs ₹1000; preventive service costs ₹100 per machine. Find the best policy.

  1. Breakdown only: expected life = 1(0.2) + 2(0.1) + 3(0.3) + 4(0.4) = 2.9 months. Breakdowns per month = 10/2.9 = 3.448; cost = 3.448 × 1000 = ₹3448 per month.
  2. Expected breakdowns: B₁ = 10 × 0.2 = 2.0; B₂ = 10(0.3) + 2.0 × 0.2 = 3.4; B₃ = 10(0.6) + 2.0 × 0.1 + 3.4 × 0.2 = 6.88.
  3. PM every month: (10 × 100 + 1000 × 2.0)/1 = ₹3000 per month.
  4. PM every 2 months: (1000 + 1000 × (2.0 + 3.4))/2 = 6400/2 = ₹3200 per month.
  5. PM every 3 months: (1000 + 1000 × 12.28)/3 = ₹4427 per month.

Best policy: group preventive service every month at ₹3000 per month, saving ₹448 per month over breakdown maintenance.

Common mistakes

  • Using calendar time instead of planned production time as the base for availability.
  • Counting only good parts in the performance rate; performance uses the total count.
  • Applying time-based replacement to items with a constant failure rate; it adds cost without reducing failures.
  • In the Bₙ recursion, forgetting the second-generation failures of machines repaired earlier in the cycle.
  • Treating TPM as a maintenance-department programme; its core is operator ownership (autonomous maintenance).

For GATE PI

Expect MTBF, MTTR and availability numericals, OEE from shift data, classification of maintenance types and of the six big losses, TPM pillars, and the group preventive-versus-breakdown cost comparison using expected breakdowns. Practise setting up the Bₙ recursion carefully.

Quick check

  1. MTBF = 100 h, MTTR = 2 h. What is the availability?
  2. Availability 0.9, performance 0.9, quality 0.95: OEE?
  3. Which TPM pillar makes operators responsible for basic care of their machines?
  4. Which maintenance type uses vibration analysis to decide when to act?

Answers: 1. 0.980. 2. 0.770 (77 %). 3. Autonomous maintenance. 4. Predictive (condition-based).

Interview questions

All Metrology, Quality and Reliability interview questions

Try answering each one aloud before you open it.

  1. 1.What is maintenance planning in the context of industrial engineering?Concept

    Maintenance planning involves scheduling and organizing maintenance activities to ensure that equipment and machinery operate efficiently and reliably. It includes determining the necessary resources, such as labor, tools, and spare parts, and setting timelines for preventive and corrective maintenance tasks.

  2. 2.Explain the concept of Total Productive Maintenance (TPM).Concept

    Total Productive Maintenance (TPM) is a holistic approach to equipment maintenance that aims to achieve perfect production with no breakdowns, defects, or accidents. It involves all employees, from top management to shop-floor workers, and focuses on proactive and preventive maintenance to improve equipment reliability and efficiency.

  3. 3.Why is preventive maintenance important in industrial settings?Application

    Preventive maintenance is important because it helps to avoid unexpected equipment failures, reduces downtime, and extends the lifespan of machinery. By regularly inspecting and servicing equipment, potential issues can be identified and addressed before they lead to costly breakdowns.

  4. 4.What are the key differences between corrective maintenance and preventive maintenance?Concept

    Corrective maintenance is performed after a failure has occurred to restore equipment to its operational state, while preventive maintenance is carried out regularly to prevent failures before they happen. Corrective maintenance is reactive, whereas preventive maintenance is proactive.

  5. 5.How does Total Productive Maintenance (TPM) contribute to quality improvement?Application

    TPM contributes to quality improvement by ensuring that equipment is always in optimal working condition, which reduces the likelihood of defects and errors in production. By involving all employees in maintenance activities, TPM fosters a culture of continuous improvement and accountability, leading to higher quality standards.

  6. 6.What could happen if a company neglects maintenance planning?Application

    Neglecting maintenance planning can lead to frequent equipment breakdowns, increased downtime, higher repair costs, and reduced production efficiency. It can also compromise safety and lead to lower product quality, ultimately affecting the company's profitability and reputation.

  7. 7.Explain the role of predictive maintenance in modern industries.Concept

    Predictive maintenance uses data analysis and monitoring technologies to predict when equipment is likely to fail, allowing maintenance to be performed just in time. This approach minimizes downtime and maintenance costs by addressing issues before they lead to failures, improving overall equipment effectiveness.

  8. 8.What is Overall Equipment Effectiveness (OEE) and how is it calculated?Concept

    OEE measures how much of the planned production time is turned into good parts at the ideal rate. OEE = Availability × Performance × Quality, where Availability = operating time / planned production time, Performance = (ideal cycle time × total count) / operating time, and Quality = good count / total count. Equivalently OEE = good count × ideal cycle time / planned production time. The three factors map onto TPM's six big losses; about 85 % is often quoted as world class.

  9. 9.A machine is planned for 16 h a day and has 1 h of downtime. In the 15 h it runs it could make 1000 units at its ideal rate; it actually makes 900 units, of which 50 are defective. Calculate the OEE.Numerical

    Availability = 15/16 = 0.9375. Performance = 900/1000 = 0.90. Quality = (900 − 50)/900 = 0.9444. OEE = 0.9375 × 0.90 × 0.9444 = 0.797, i.e. about 79.7 %.

  10. 10.If a company wants to improve its OEE from 75% to 85%, what strategies could it implement?Application

    To improve OEE, a company could reduce downtime through better maintenance planning, increase performance by optimizing production processes, and enhance quality by implementing stricter quality control measures. Employee training and adopting new technologies for predictive maintenance can also contribute to higher OEE.

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