Plant Maintenance

Plant maintenance is the work of keeping machines, buildings and services in working condition so that production continues with the required quality, safety and economy. Its objectives are to maximise availability, extend equipment life, reduce breakdown cost and ensure safety.

Breakdown (corrective) maintenance

Repair is done only after the machine fails.

• Merits: no planning cost; suited for non-critical, cheap or standby equipment.
• Demerits: unplanned stoppage, production loss, rush repair at high cost, possible damage to other parts and safety risk.

Preventive maintenance

Planned inspection, lubrication, adjustment and replacement carried out to find and correct faults before failure.

• Merits: fewer breakdowns, longer life, better quality and safety, planned downtime.
• Demerits: cost of inspection and spares; over-maintenance may waste money; needs records.

Scheduled maintenance

Maintenance carried out at fixed intervals (time, running hours or output) according to a schedule, such as overhauls and periodic shutdowns of boilers and turbines. It is a form of preventive maintenance.

• Merits: easy planning of men, spares and downtime.
• Demerits: parts may be replaced while still usable; interval may be badly chosen.

(a) TPM

Total Productive Maintenance (developed in Japan by Nippondenso and promoted by Seiichi Nakajima) is a company-wide approach in which all employees, from top management to operators, take part in keeping equipment in the best condition. Its aim is zero breakdowns, zero defects and zero accidents. The operator performs routine cleaning, lubricating and inspection (autonomous maintenance), while the maintenance staff do specialised work.

Main pillars (8 pillars)

1. Autonomous maintenance (Jishu Hozen): operator cleans, inspects, lubricates.
2. Planned maintenance: scheduled and predictive maintenance.
3. Focused improvement (Kobetsu Kaizen): remove losses by small team projects.
4. Quality maintenance: keep equipment conditions that give defect-free product.
5. Early equipment management: new equipment designed for easy maintenance.
6. Training and education.
7. Safety, health and environment.
8. Office TPM: remove waste in administration.

The base is 5S. The six big losses are breakdown, set-up and adjustment, idling and minor stops, reduced speed, process defects and start-up losses.

(b) Overall equipment effectiveness

Planned production time $= 480 - 30 = 450$ min. Operating time $= 450 - 45 = 405$ min.

$$\text{Availability} = \frac{405}{450} = 0.900$$ $$\text{Performance} = \frac{\text{ideal cycle time} \times \text{total units}}{\text{operating time}} = \frac{1 \times 340}{405} = 0.8395$$ $$\text{Quality} = \frac{340 - 12}{340} = 0.9647$$ $$OEE = 0.900 \times 0.8395 \times 0.9647 = 0.7289$$

Answer: Availability 90.0%, performance 84.0%, quality 96.5%, OEE = 72.9%.

Total Planned Quality Maintenance (TPQM) is an approach that joins planned maintenance with quality management. It is based on the idea that product quality is the result of equipment condition, so the machine conditions that produce good quality are identified, measured and kept under control through planned maintenance. The aim is to prevent defects at the source instead of detecting them by inspection after production.

Basic idea

Defects are caused by the "4Ms" - machine, method, material and man. TPQM controls the machine factor by:

1. Finding the relation between machine conditions (wear, vibration, temperature, alignment, tool condition) and each quality characteristic.
2. Setting standards for these conditions.
3. Checking them regularly and correcting before they drift out of limits.
4. Recording results and improving the standards.

 TPM (equipment effectiveness)  +  TQM (quality of product)
              \                      /
               +---- TPQM ----------+
   planned maintenance aimed at zero defects

Steps

• Make a quality-defect analysis and link each defect to a machine condition.
• Set quality-related check points in the maintenance schedule.
• Train operators to monitor and adjust them (autonomous maintenance).
• Use control charts and Pareto analysis to review.

Relation to TPM and TQM

TPM works for zero breakdown and TQM for customer satisfaction through quality. TPQM is the meeting point: it applies the planned maintenance pillars of TPM (and its quality maintenance pillar) so that maintenance contributes directly to product quality.

Benefits

Fewer defects and rework, lower scrap and inspection cost, longer equipment life, better delivery.