Preventive Maintenance

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Preventive Maintenance (PM)







It is 1st in the hierarchy of planned maintenance.
All actions carried out on a planned, periodic, and specific schedule to keep an
item/equipment in stated working condition through the process of checking and
reconditioning. These actions are precautionary steps undertaken to forestall (to prevent
something from happening by acting first) or lower the probability of failures or an
unacceptable level of degradation in the latter service, rather than correcting them after
they occur.
Its purpose is to minimize breakdowns and excessive depreciation.
In simplest form PM is compared to the service schedule for an automobile.
It should focus on cleaning, lubrication and correcting deficiencies found through testing
and inspections.

Reasons for PM




Increased automation in industry
Minimize energy consumption in maintenance
Business loss due to production delays

Necessity of PM




Reduced production downtime, resulting in fewer machine breakdowns.
Better conservation of assets and increased life expectancy of assets, thereby eliminating
premature replacement of machinery and equipment
Timely routine repairs circumvent fewer large scale repairs

Traditional PM was based on concept of bathtub curve (i.e.) new parts went through three stages
1. Infant mortality stage
2. Fairly long run stage
3. Wear out stage
The PM concept was to replace those parts before they entered the wear out phase. The research
found that very few nonstructural components exhibit wear out characteristics, they also found
that only 11% of all components exhibit wear out characteristics but 72% of components do
exhibit infant mortality characteristics.

E.g.: servicing of a car

Predictive Maintenance





It goes one step further to PM E.g.: If a flicker lamp or hooter is provided to indicate the
oil level in the engine
The use of modern measurement and signal processing methods to accurately diagnose
item/equipment condition during operation
There are various technologies of predictive maintenance
1. Vibration measurements
2. Thermography for measurement of heat generated as well for alignment checking
3. Oil and lubricant monitoring
4. Ultrasonic methods to detect leaks and corrosion levels

Proactive Maintenance
E.g.: if a disease is pinpointed and eliminated long before any symptoms occur in your body, it
would save you money in doctor bills and keep you out of the hospital in the long run



It commissions corrective actions aimed at the sources of failure
It is designed to extend the life of mechanical machinery as against
1. Making repairs when often nothing is broken
2. Accommodating failure as routine and normal
3. Pre empting crisis failure maintenance

Total production time = (up time + down time)
Mean Time between Failures = (Total up time) / (number of breakdowns)
Mean Time to Repair = (Total down time) / (number of breakdowns)
Availability = Uptime / (Downtime + Uptime)
Availability (Intrinsic) Ai = MTBF / (MTBF + MTTR)
Availability (Operational) Ao = MTBM / (MTBM+MDT)
MTBM = Mean time between maintenance.
MDT = Mean Down Time
MTBF = Mean time between failures
MTTR = Mean time to repair / Mean time to replace.

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