Choose the right PM task frequency

There are two main components of an effective maintenance plan: the type of maintenance task and the task interval.

12/23/2014


Having a background in mechanical engineering, I have always appreciated the level of analysis necessary for many conventional engineering tasks. For example, when asked to consider a bearing carrying a particular load at a particular speed, the engineer’s job is not over after determining in which direction the load is being applied.

An engineer should determine the amount of applied load and make the calculations to determine the size and type of bearing needed. In this sense, would it be enough for a reliability engineer to look at an asset’s failure history and simply point out that time-based maintenance should be performed? Would it not be better for the engineer to recommend condition-based monitoring or calculate the optimum replacement interval if wear-out is the predominant failure mode?

Unfortunately, I encounter many organizations that ignore, fail to consider, or simply don’t record the failure history of their equipment and are forced into defaulting to time-based replacements and guessing at the maintenance interval. What would happen if electrical engineers didn’t measure or inquire about the incoming voltage and simply specified a 120 Vac motor? What if a structural engineer never asked where the building was going to be built and as a result failed to consider seismic or wind loads of the particular area? As reliability and maintenance engineers, we need to embrace the scientific aspects of our profession and treat our daily activities with the same regimen as our other engineering peers.

To this point, I’d like to focus on the determination of a maintenance plan. A maintenance plan is the actions taken to mitigate failure modes and ensure asset reliability. Although building a maintenance plan is only one part of the reliability and maintenance engineer’s job requirements, it is an important one, and a strategic maintenance plan can deliver immediate results.

There are two main components of an effective maintenance plan: the type of maintenance task and the task interval. I will use three main maintenance task types. For readers who are familiar with RCM, you will recognize these types as On-Condition Tasks, Hard-Time Tasks, and Failure-Finding Tasks.

On-Condition Tasks

On-Condition Tasks are periodic tasks, such as inspections, checks, or tests, designed to detect a potential failure and allow correction before it becomes a functional failure. This means periodically looking at the component and comparing it to a standard that ensures the component can still fulfill its function.

If you are familiar with the P-F curve, an On-Condition Task is one that looks for the P point on the curve with the intent that the detection gives adequate time to have the failure corrected before the F point on the curve. Constructing a P-F curve requires recording the results of the check and plotting the result versus the elapsed time from installation at which the check was conducted.

If enough measurements are taken, a fairly consistent curve can be developed for each failure mode. Making sure that the data is gathered carefully and consistently will aid in increasing the quality of the P-F curve.

Picking the proper frequency to make the check or inspection can be a little tricky. The basis for the frequency is some fraction of the P-F interval. In most situations, picking a frequency that is 1/3 of the P-F interval will be adequate, but in more critical situations using 1/5 of the P-F interval will give better confidence that the check will be made and the negative condition found and corrected.

On-Condition Tasks are preferred over any other task type. They allow for most, if not all, of the useful life of a component to be used while still avoiding the failure. This category includes the traditional predictive maintenance (PdM) tasks. Keep in mind that PdM does not always have to involve sophisticated detection equipment. Using your human senses, gauges, or simple tools can also detect conditions that indicate a potential failure has occurred.

Hard-Time Tasks

Hard-Time Tasks are periodic tasks, such as removal, rebuilding, or reworking, that are designed to prevent a functional failure. This means that regardless of the condition, the equipment will be worked on when a calendar date, number of miles, or number of cycles has been reached. The intention is to remove, rebuild, or repair the equipment just before the probability of failure (hazard) reaches an unacceptable level and return the equipment to its original condition. Constructing a hazard profile for the equipment involves collecting life data (times to failure) for the failure mode in question, as well as calculating the probability of failure for potential replacement frequencies, and plotting the probabilities versus time. Remember that Hard-Time Tasks are only applicable to failure modes that result in an increasing hazard profile at the end of the equipment’s life.

Selection of the task frequency will depend on the shape of the hazard profile and at what point the curve begins to rise. Picking a point that is directly before the rise in slope will result in the most useful life, but may result in occasional failure due to slight differences between curve profiles on each installation.

In contrast, picking a point far away from the rising point of the curve will result in fewer unplanned failures, but will shorten the useful life of the equipment. It is preferred to pick a point that is close to the rising point of the curve, but how close will depend on your confidence in the consistency of the curve.

Hard-Time Tasks have the dubious honor of being the most selected maintenance task type, but probably also the most misunderstood. A number of studies have shown that the majority of equipment does not exhibit an increasing failure rate at the end of life. This would mean that a lot of the time-based maintenance replacements and rework are potentially being done to mitigate failure modes that are not suitable for this type of maintenance task.

Failure-Finding Tasks

Failure-Finding Tasks are periodic tasks, redundant equipment checks, or emergency system checks that are designed to detect a functional failure that has occurred, but is not evident. Evident failures are ones that will be noticed by operations during the normal operation of the equipment, while hidden failures are ones that would not be noticed. Examples of hidden failures are seizing of stand-by pumps and sensor fouling on smoke alarms. The intention of these tasks is to check on equipment to uncover functional failures before a need for the equipment arises and only then is the failure detected. These checks can be anything from test-running the equipment to observe the operation to simulating a hazardous condition to ensure the safety alert equipment is operational.

The primary reason for selecting the appropriate frequency to perform Failure-Finding Tasks is to ensure the availability requirements are met. If the frequency of inspection is too great, the functional failures that are occurring to the equipment are discovered when operated versus during a check. Consideration must be given to the mean time to failure (MTTF) of the equipment as well. For equipment with very large MTTFs, the inspection frequency can be extended, while equipment with small MTTFs will need to be checked more frequently.

I encourage anyone responsible for the creation and upkeep of a preventive maintenance program to embrace the engineering aspects of this work and give it the attention that it deserves.

Bill Barto is a reliability engineering subject matter expert with Life Cycle Engineering (LCE). He can be reached at bbarto@LCE.com.    



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.
June 2018
2018 Lubrication Guide, Motor and maintenance management, Control system migration
May 2018
Electrical standards, robots and Lean manufacturing, and how an aluminum packaging plant is helping community growth.
April 2018
2017 Product of the Year winners, retrofitting a press, IMTS and Hannover Messe preview, natural refrigerants, testing steam traps
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
February 2018
Focus on power systems, process safety, electrical and power systems, edge computing in the oil & gas industry
Spring 2018
Burners for heat-treating furnaces, CHP, dryers, gas humidification, and more
April 2018
Implementing a DCS, stepper motors, intelligent motion control, remote monitoring of irrigation systems
February 2018
Setting internal automation standards

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.
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.
Maintenance & Safety
The maintenance journey has been a long, slow trek for most manufacturers and has gone from preventive maintenance to predictive maintenance.
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.
click me