Difference between Preventive Maintenance and Predictive Maintenance

    IATF16949 (and ISO/TS16949) requires preventive maintenance and predictive maintenance for the machines. This article will have a discussion about the difference between these two types of maintenance activities.

    First, let’s take a look at their definitions in IATF16949:

• Preventive maintenance: planned activities at regular intervals (time-based, periodic inspection, and overhaul) to eliminate causes of equipment failure and unscheduled interruptions to production, as an output of the manufacturing process design；
• Predictive maintenance: an approach and set of techniques to evaluate the condition of in-service equipment by performing periodic or continuous monitoring of equipment conditions, in order to predict when maintenance should be performed.

    Based on their definitions, the most critical difference between preventive maintenance and predictive maintenance is that the former is time-based and it is known when to perform the maintenance, while the latter is machine condition based and it is unknown in advance when to perform the maintenance.

    There are some analogies in our daily life for preventive maintenance and predictive maintenance. One analogy for preventive maintenance is the maintenance on your vehicle according to the period suggested by the manufacturer. And one analogy for predictive maintenance is the gas filling in your vehicle tiers: the tire pressure is monitored. Once the tire pressure drops down to a threshold, a light on your vehicle dashboard turns on to notify the driver that it’s time to fill gas into your tires.

    For preventive maintenance, one needs to define what should be done to the machines and what is the frequency for such actions. While for predictive maintenance, there’s an extra step: it should be defined first what conditions of the machine should be monitored.

    Below is a list of some typical examples of preventive maintenance and predictive maintenance.

Type of Maintenance	Typical Examples
Preventive maintenance	-         Replace a spare part of a machine periodically, if the life time of the spare part is fixed in days, months or years; -         Clean the machine weekly; -         Apply lubricant to a machine quarterly; -         Replace the cooling water for a machine weekly; -         Fill oil into the machine bi-weekly;
Predictive maintenance	-         Count the number of times a cutter is used and replace the cutter when it reaches the expected life time, if the life time of the cutter is determined by the times it is used. -         Monitor the product surface roughness and sharpen the cutting tool when the surface roughness goes beyond the control limit; -         Monitor the vibration of the machine and tighten the bolts and nuts when the vibration reaches a threshold; -         Monitor the gas pressure of a gas tank and replace the gas tank when the pressure goes below a threshold;