grand scheme of predictive maintenance, The

InTech, Jul 2005 by Blaisdell, Jonathan

Tracking key indicators over time calculates when equipment needs repair.

Predictive maintenance programs come in all shapes and sizes, depending on a facility's size, equipment, regulations, and productivity goals.

Take a look at these aspects of the practice:

* Outlines of some of the most common predictive maintenance methods

* An explanation of how to determine the potential cost savings of maintenance improvements

* A walk through of a predictive maintenance process

Maintenance methods

Reactive Maintenance: Run-to-failure approach-letting a system run until something breaks. Maximum cost in terms of revenue lost and equipment replacement.

Preventive Maintenance (PM): Maintenance repairs performed on a regular schedule to minimize component degradation and extend the life of equipment. Preventive maintenance takes place after a set amount of elapsed calendar time or machine run time, regardless of whether the repair is necessary. While more cost-effective than reactive maintenance, preventive maintenance still requires substantial human resources and replacement parts inventories.

Predictive Maintenance (PdM): Tracking key indicators over time to predict when equipment needs repair. Predictive maintenance programs measure equipment on a regular basis, track the measurements over time, and take corrective action when measurements are about to go outside the equipment operating limits. Repairing equipment as needed requires fewer personhours and parts than preventive maintenance. However, tracking the measurements requires new tools, training, and software to collect and analyze the data and predict repair cycles.

Reliability-Centered Maintenance (RCM): Prioritizing maintenance efforts based on equipment's importance to operations, its downtime cost in revenue and customer loss, its impact on safety, and its cost of repair. Reliability maintenance depends on the same measurements used in predictive maintenance but saves additional maintenance resources by spending less effort on less important machinery. RCM also requires more training and software than PdM.

Maintenance software, CMMS and AMS: Most facilities practicing predictive maintenance purchase or develop a specialized database, commonly referred to as an asset management system (AMS) or a computer managed maintenance system (CMMS). To track trends, a database system should be able to store:

* List of critical equipment

* Maintenance and measurement procedures for each type of equipment

* Maintenance schedule

* History for each measurement

* Limits for each measurement (maintenance alarm trigger)

Many systems also track warranty status, depreciation records, and purchasing information and can generate works orders, manage schedules, and track employee training histories and related skills.

Investment in infrastructure

There is plenty of evidence that careful, well-planned maintenance prolongs the life of equipment and prevents costly downtime. Insurance data indicates that roughly half of the damages associated with electrical failures alone are preventable through regular maintenance.

To determine the investments to make in your system, you need to know two things: probability of a failure and cost of the failure. By multiplying these two figures, you can estimate a level of investment in your infrastructure, including maintenance.

Probability of failure: The IEEE 493 standard contains useful data on the failure rates of electrical equipment and techniques for determining the probability of downtime for any given load. For each facility, also incorporate operator knowledge, maintenance history, and manufacturer's specifications, as well as the failure analysis provided by PdM software tools.

Failure Modes and Effects Analysis (FMEA): Used in reliability-centered maintenance, FMEA is a method for analyzing how a system can fail, the impact of the failure, the frequency of failure, and the probability of a hidden failure. The FMEA method assigns risk priority to assemblies based on:

* Severity of impact

* Probability of occurrence

* Probability a failure will remain hidden

For example, for a critical three-phase motor, overcurrent trip due to phase loss would be failure mode. The probability of phase loss remaining hidden can be high, since the motor may continue to run. To bring hidden failures to the surface, take measurements that closely correlate to the failure mode. In the motor example, current monitoring will quickly uncover loss of a phase on a three-phase motor.

Cost of failure: Unplanned downtime cost variables are:

* Lost revenue during downtime, especially critical if the plant is running at or near capacity, or in highly competitive markets. This measurement is in dollars per hour.

* Lost revenue due to loss of customer confidence-how many customers will leave you.

* Replacement cost of damaged electrical or production equipment.

* Repair costs, especially labor.

* Cost of scrap.

* Cost to clean and restart production.

* Insurance premium reductions.

To build a case for preventive maintenance, estimate the cost of failure and compare that to the cost of a maintenance program.