Preventive Maintenance – two kinds


If “Preventive Maintenance” (PM) is defined as any maintenance activity that prevents breakdowns, it would logically include the following three kinds of maintenance.


1.a. Any maintenance activity that extends the life of equipment components. This traditionally includes:

– Cleaning

– Lubrication

– Balancing

– Alignment

– Adjustments and calibration

– Fixed-time component replacement


1.b. The following activities also affect the life of components. They are normally excluded from the PM definition but are just as important as “PM” and should get the same attention:

– Operating procedures

– Equipment rebuild procedures

– Storage and handling of components


  1. The detection of failures before they become breakdowns, through any kind of inspection, either during operation or while equipment is shut down. These activities include:

– Checks for vibration, with instruments or by hand.

– Checks for abnormal noise, temperature, smells or other changes (e.g. discoloration, smoke, etc)

– Thickness measurements

– Crack tests

– Infra-red thermography of electrical and mechanical systems

– Wear measurements (e.g. drive or conveyor chain)

– Backlash measurements (in gears and couplings)

– and many other options


  1. Logically, the repair work that results from the early detection of failures using the inspection techniques listed in 2 above, should be considered “preventive maintenance” because, as long as they are executed soon enough, they do prevent breakdowns. However, the normal practice is to call this work “corrective maintenance” because it is correcting a known fault.

So not by logic, but by convention, the two kinds of preventive maintenance are:

– Early detection of failures, or “condition monitoring” and

– “Life extending” activities listed in 1.a. above.

It has some value to record the amount of corrective maintenance work that results from condition monitoring, but this requires considerable discipline, as the same failure may be identified by different people who may each submit work requests of which only one will be used. With a good maintenance computer system and following good database principles, a work order field for “How identified” is the best way to track this information, which has value for assessing the effectiveness of the PM process (see “Work Order Coding“).



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Don Armstrong, P.Eng, President
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