Corrective Versus Preventive Maintenance: What is the Difference and Where is the Value?

Corrective maintenance can be defined as a maintenance task performed to identify, isolate, and rectify a fault so that the failed equipment, machine, or asset can be restored to an operational condition within the tolerances or limits established for in-service operations.

Corrective maintenance is maintenance which is carried out after failure detection and is aimed at restoring an asset to a condition in which it can perform its intended function.

Preventive maintenance (PM) has the following meanings:

1. The care and servicing by qualified personnel for the purpose of maintaining equipment and facilities in satisfactory operating condition by providing for systematic inspection, detection, and correction of incipient failures either before they occur or before they develop into major defects.
2. Maintenance, including tests, measurements, adjustments, and parts replacement, performed specifically to prevent faults from occurring.

Difference between Preventive and Predictive Maintenance

Predictive maintenance tends to include direct measurement of the item. Example, an infrared picture of a circuit board to determine hot spots.

While Preventive Maintenance includes the evaluation of particles in suspension in a lubricant, sound and vibration analysis of a machine.

The Value of Preventive and Predictive Maintenance

The main value of Predictive Maintenance is to allow convenient scheduling of corrective maintenance, and to prevent unexpected equipment failures. The key is “the right information in the right time”. By knowing which equipment needs maintenance, work can be better planned (spare parts, people etc.) What would have been “unplanned stops” are transformed to shorter and fewer “planned stops”, thus increasing plant availability. Other advantages include increased equipment lifetime, increased plant safety, and optimized spare parts handling.

Moreover, the planned maintenance of equipment will help improve equipment life and avoid any unplanned maintenance activity. A successful preventive maintenance program is dependent on the cooperation of all the parties involved. Maintenance managers must rely on the knowledge, ideas, and contributions of all the maintenance personnel at the property.

The implementation of a preventive maintenance program can be time consuming and costly. This creates constant debate as to whether a preventive maintenance program is worth utilizing.
Will the man hours and money invested in the program outweigh emergency repairs? From our experience, we believe that when a program is properly operated the benefits exceed the costs.

Here are other important benefits of a properly operated preventive maintenance program:

• Production capacity is increased and the number of repairs are reduced
• Better conservation of assets and increased life expectancy of assets, thereby eliminating premature replacement of machinery and equipment
• Reduced overtime costs and more economical use of maintenance workers due to working on a scheduled basis instead of an emergency basis to repair breakdowns
• Timely, routine repairs circumvent fewer large-scale repairs
• Improved safety and quality conditions for everyone

Maintenance evaluation technologies and techniques

To evaluate equipment condition, predictive and preventive maintenance utilize nondestructive testing technologies such as infrared, acoustic (partial discharge and airborne ultrasonic), vibration analysis, sound level measurements, oil analysis, and other specific online tests.

• Vibration analysis is most productive on high-speed rotating equipment. Vibration analysis, when properly done, allows the user to evaluate the condition of equipment and avoid failures. The latest generation of vibration analyzers comprises more capabilities and automated functions than its predecessors. Many units display the full vibration spectrum of three axes simultaneously, providing a snapshot of what is going on with a particular machine.
• Acoustical analysis can be done on a sonic or ultrasonic level. New ultrasonic techniques for condition monitoring make it possible to “hear” friction and stress in rotating machinery, which can predict deterioration earlier than conventional techniques. Ultrasonic technology is sensitive to high-frequency sounds that are inaudible to the human ear and distinguishes them from lower-frequency sounds and mechanical vibration. Machine friction and stress waves produce distinctive sounds in the upper ultrasonic range. Changes in these friction and stress waves can suggest deteriorating conditions much earlier than technologies such as vibration or oil analysis.
• Sonic monitoring equipment is less expensive, but it also has fewer uses than ultrasonic technologies. Sonic technology is useful only on mechanical equipment, while ultrasonic equipment can detect electrical problems and is more flexible and reliable in detecting mechanical problems.
• Infrared monitoring and analysis has the widest range of application (from high- to low-speed equipment), and it can be effective for spotting both mechanical and electrical failures; some consider it to currently be the most cost-effective technology.
• Oil analysis is a long-term program that, where relevant, can eventually be more predictive than any of the other technologies. It can take years for a plant’s oil program to reach this level of sophistication and effectiveness. Analytical techniques performed on oil samples can be classified in two categories: used oil analysis and wear particle analysis.

  Used oil analysis determines the condition of the lubricant itself, determines the quality of the lubricant, and checks its suitability for continued use. Wear particle analysis determines the mechanical condition of machine components that are lubricated. Through wear particle analysis, you can identify the composition of the solid material present and evaluate particle type, size, concentration, distribution, and morphology.