Predictive vs. preventive: The debate—and the future

Though they have similarities, preventive maintenance (PM) and predictive maintenance (PdM) significantly differ in their execution.


Figure 1: Example of a Potential-Failure (P-F) curve. Courtesy: T.A. Cook Consultants, Inc.The primary objective of an organization's maintenance function is to maximize the availability and efficiency of the organization's assets throughout their expected lifecycles at the lowest possible cost. Maintenance generally falls under two categories: reactive and proactive.

Reactive maintenance, also known as run-to-failure maintenance, focuses on repairing an asset after it has failed. Proactive maintenance focuses on avoiding repairs and failures through preventive or predictive methods.

Run-to-failure is the oldest and simplest maintenance strategy and implies the intention to deliberately continue running an asset until the end of its life before corrective maintenance is performed. This strategy should only apply to non-critical assets, such as general-purpose light bulbs, or redundant assets, such as standby pumps, whose failure poses no safety risk and has a minimal impact on productivity.

Most successful manufacturers have transitioned to an overall proactive maintenance strategy which extends the life of assets, increases productivity, improves overall efficiency and reduces maintenance costs. Though they have similarities, preventive maintenance (PM) and predictive maintenance (PdM) significantly differ in their execution. Does one offer a competitive advantage over the other?

The type and frequency of each PM activity is primarily predetermined by the OEM for each piece of equipment. Preventive maintenance is predominantly classified as exploratory, planned component replacements or planned shutdowns. The frequency of PM activities is usually time or meter based, regardless of the current condition of the component.

Typically, machines are removed from production so the technicians can perform the required PM activities. As a result, not only should these interventions be planned and scheduled, but the affected operators should be reassigned to other machines. This requires strong collaboration efforts between an organization's maintenance, production and scheduling departments to ensure machines are released at the appropriate, periodic basis for preventive maintenance activities to be completed.

A PdM strategy relies on the use of technology and tools such as vibration analysis, oil analysis and thermal imaging to constantly monitor an asset's efficiency and wear to provide advanced warning of an impending failure or loss of function. The advantage of this methodology is that this monitoring occurs while the asset is running. There is no need to impede productivity to identify an impending failure. This methodology provides the most realistic, up-to-the-minute data on an asset's condition.

However, this information can only be interpreted properly by highly trained maintenance technicians. Nevertheless, this approach offers flexibility to schedule repairs and restorations at the company's convenience as long as the work is planned and scheduled before a failure takes place. Compared to PM, PdM offers the more proactive maintenance approach with the least impact on productivity.

Potential Failure (P-F) curve

In the context of a preventive versus predictive maintenance methodology comparison, it's important to review and understand the Potential Failure (P-F) curve. An asset's failure is often thought of as an event rather than a process. However, most malfunctions occur over time. The P-F Curve (Fig. 1) illustrates an asset's deterioration over time. Essentially, the P-F Curve offers tremendous value as a forecasting or early warning tool. This concept applies to each component in a machine.

The P-F Curve's horizontal (X) axis represents time while the vertical (Y) axis represents condition. As an asset gradually deteriorates, the potential failure (P) is the point in this process when it is first possible to detect that a bug or loss of function is imminent. If a flaw remains undetected and unmitigated, the deterioration process accelerates until the asset experiences functional failure (F). The time range between P and F is referred to as the P-F Interval, which could range from days to weeks to months, depending upon the specific component.

Maintenance leaders can use the P-F Curve to challenge and update the OEM's recommended PM activities. The quantity of these PM activities can be reduced by efficiently scheduling these tasks based on the historical data provided by the P-F Curve. Unfortunately, many organizations fail to recognize the value of the P-F Curve or put forth the required effort to collect and analyze the historical data. Thus, the PM activities remain status quo.

PdM's real-time, condition-monitoring process provides the best opportunity to take advantage of the P-F Curve's "crystal ball" data and avoid functional failure, provided the condition-monitoring equipment data is properly interpreted. As a result, the replacement or restoration of an asset's component(s) can be properly planned and scheduled before a total functional failure occurs at the earliest convenience. Additionally, PdM requires fewer productivity disruptions than the intervention process required by PM.

Reliability-centered maintenance

Rather than debating whether one proactive maintenance strategy has an advantage over the other, maintenance organizations should focus on the future maintenance trend: reliability-centered maintenance (RCM). The RCM process seeks to maximize asset performance by applying the right activity to the right asset at the right stage in its lifecycle while simultaneously optimizing productivity. Rather than simply focusing on asset performance, this approach focuses on system function.

A successful RCM implementation requires incorporating an optimum combination of all other maintenance strategies which directly corresponds to the asset's criticality and the costs associated in the event of its failure. Before they begin toward RCM, the manufacturing leadership should conduct a thorough analysis of each asset and phase in the production process. The next step is to designate the criticality, safety concerns, potential costs, opportunities for failures and risks. The final step is to align each asset to its best maintenance strategy.

An effective RCM strategy also should include the integration of a risk-based maintenance and inspection program. By considering the probability and consequences of failure, maintenance resources are focused and optimized on assets whose failures pose the greatest risk to safety, the environment and productivity. The primary objective is to minimize the capital, operational and maintenance costs without comprising safety, environmental standards, reliability and productivity.

Looking forward

Preventive maintenance had long been the dominant maintenance strategy. Maintenance organizations have slowly transitioned to the PdM strategy—the comparatively more proactive approach. Manufacturers must recognize that maintenance is a process. A PdM strategy cannot be adopted if a PM strategy does not currently exist. An organization must have a PM approach before it can utilize a PdM approach. From there, it can continue to advance to an RCM approach.

In today's highly competitive environment, asset-intensive organizations are under increasing pressure to reduce costs, improve operational uptime and enhance employee safety. Applying RCM, the future generation of maintenance professionals will align an organization's asset management strategy with its business and asset infrastructure. When implemented and performed correctly, RCM maximizes asset availability while simultaneously minimizing costs.

Mark C. Munion is a consultant for T.A. Cook Consultants, Inc.

ONLINE extra

See additional stories from the Plant Engineering May 2017 cover story below.

Top Plant
The Top Plant program honors outstanding manufacturing facilities in North America.
Product of the Year
The Product of the Year program recognizes products newly released in the manufacturing industries.
System Integrator of the Year
Each year, a panel of Control Engineering and Plant Engineering editors and industry expert judges select the System Integrator of the Year Award winners in three categories.
September 2018
2018 Engineering Leaders under 40, Women in Engineering, Six ways to reduce waste in manufacturing, and Four robot implementation challenges.
GAMS preview, 2018 Mid-Year Report, EAM and Safety
June 2018
2018 Lubrication Guide, Motor and maintenance management, Control system migration
August 2018
SCADA standardization, capital expenditures, data-driven drilling and execution
June 2018
Machine learning, produced water benefits, programming cavity pumps
April 2018
ROVs, rigs, and the real time; wellsite valve manifolds; AI on a chip; analytics use for pipelines
Spring 2018
Burners for heat-treating furnaces, CHP, dryers, gas humidification, and more
August 2018
Choosing an automation controller, Lean manufacturing
September 2018
Effective process analytics; Four reasons why LTE networks are not IIoT ready

Annual Salary Survey

After two years of economic concerns, manufacturing leaders once again have homed in on the single biggest issue facing their operations:

It's the workers—or more specifically, the lack of workers.

The 2017 Plant Engineering Salary Survey looks at not just what plant managers make, but what they think. As they look across their plants today, plant managers say they don’t have the operational depth to take on the new technologies and new challenges of global manufacturing.

Read more: 2017 Salary Survey

The Maintenance and Reliability Coach's blog
Maintenance and reliability tips and best practices from the maintenance and reliability coaches at Allied Reliability Group.
One Voice for Manufacturing
The One Voice for Manufacturing blog reports on federal public policy issues impacting the manufacturing sector. One Voice is a joint effort by the National Tooling and Machining...
The Maintenance and Reliability Professionals Blog
The Society for Maintenance and Reliability Professionals an organization devoted...
Machine Safety
Join this ongoing discussion of machine guarding topics, including solutions assessments, regulatory compliance, gap analysis...
Research Analyst Blog
IMS Research, recently acquired by IHS Inc., is a leading independent supplier of market research and consultancy to the global electronics industry.
Marshall on Maintenance
Maintenance is not optional in manufacturing. It’s a profit center, driving productivity and uptime while reducing overall repair costs.
Lachance on CMMS
The Lachance on CMMS blog is about current maintenance topics. Blogger Paul Lachance is president and chief technology officer for Smartware Group.
Material Handling
This digital report explains how everything from conveyors and robots to automatic picking systems and digital orders have evolved to keep pace with the speed of change in the supply chain.
Electrical Safety Update
This digital report explains how plant engineers need to take greater care when it comes to electrical safety incidents on the plant floor.
IIoT: Machines, Equipment, & Asset Management
Articles in this digital report highlight technologies that enable Industrial Internet of Things, IIoT-related products and strategies.
Randy Steele
Maintenance Manager; California Oils Corp.
Matthew J. Woo, PE, RCDD, LEED AP BD+C
Associate, Electrical Engineering; Wood Harbinger
Randy Oliver
Control Systems Engineer; Robert Bosch Corp.
Data Centers: Impacts of Climate and Cooling Technology
This course focuses on climate analysis, appropriateness of cooling system selection, and combining cooling systems.
Safety First: Arc Flash 101
This course will help identify and reveal electrical hazards and identify the solutions to implementing and maintaining a safe work environment.
Critical Power: Hospital Electrical Systems
This course explains how maintaining power and communication systems through emergency power-generation systems is critical.
Design of Safe and Reliable Hydraulic Systems for Subsea Applications
This eGuide explains how the operation of hydraulic systems for subsea applications requires the user to consider additional aspects because of the unique conditions that apply to the setting
click me