Predictive and preventive maintenance

An individual bought an incandescent light bulb. The manufacturing company
mentioned that the life span of the bulb is 3 years. Just before the 3 years, the
individual decided to replace the bulb with a new one. This is called preventive
maintenance.

On the other hand, the individual has the opportunity to observe the bulb operation
daily. After two years, the bulb starts flickering. The individual predicts at that time
that the bulb is going to fail very soon and decides to change it for a new one. This is
called predictive maintenance.

The individual ignores the flickering bulb and only goes out to buy another
replacement light bulb when the current one fails. This is called corrective
maintenance.
Preventive maintenance is changing the oil of your car every 5000 miles : You do not
know if your car would have broken if you did not change the oil, but you know it will
prevent it to fail.
- Predictive maintenance : Analysing the harmonics of a bearing periodically to check if it
is going to fail, you will change only if the analysis show that the risk of failing has
increased.
In the case of electrical powered machines, the increase in power consumption (amp.
meter reading) is usually a sign that the motor is working harder than normal – symptom
that something is going to give way soon.
I would go further and contend that predictive maintenance is a part of preventive
maintenance – actually a technique used in preventive maintenance. All are done to
prevent unforeseen shutdowns and maximize the working life of machines.
Predictive maintenance tends to include direct measurement of the item. Example, an
infrared picture of a circuit board to determine hot spots.
Preventive tends to be done on time schedules (as you have.) Predictive tends to be
direct measurement of the item to determine if it needs maintenance.
Other examples of Predictive Maintenance - evaluation of particles in suspension in a
lubricant, sound and vibration analysis of a machine.
. Bearings
 Bearings are designed to reduce the friction between moving parts – usually
rotational movement of a shaft within a housing.

Classification:
1. Air bearings – the rotating parts are kept
apart by a thin film of air pumped under
pressure between the journal and the rotating
shaft. In some systems the air pressure is
applied before the shaft starts to rotate and
friction rates can be very low.
2. Journal (Plain) Bearings – the shaft rotates
within a bush usually supplied with oil under pressure.
3. Roller/Ball Bearings – sometimes called Low Friction
bearings. Use is made of balls or rollers running between inner and outer races.
Lubricated with oil or grease. Ball bearings are usually used for light to medium
loads while roller bearings are better able to cope with heavier loads.
Fluid film thrust bearings:
Used to create a clearance between the moving and stationary elements.