What's a maintenance unit?

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An MU is the level that provides the best efficiency for tracking and recording work for your company.  Though the equipment is the same across different industries, the MU definition is not always the same.  When setting up MUs, it’s necessary to have knowledge about your entire business, plant and process.  You must understand how you utilize equipment and how critical it is to a site’s overall operation.

Different industries typically view their equipment differently.  In chemical plants, each pump is treated as an MU, but steel plants could generally care less about the individual pump and track whole systems (i.e., hydraulics) as an MU.  And, of course, the opposite breakdown is true for ducting in a chemical plant; it’s generally considered part of the fan, while the ducting for a fan on a steel mill rotary furnace is generally broken down into several MUs.  You have to understand the industry and sites.  This also means that facility and production groups work together when determining their structures.

Here are general criteria for defining what a maintenance unit should encompass or exclude.  Again, each criterion must be judged with the different needs for recordkeeping and accountability for your industry.

Value as a single entity for reporting purposes: If a unit deserves, or requires, data to be kept for that unit alone, it should be a separate MU.  This is probably the most important concept in determining an MU.

How the equipment is currently considered in the maintenance department: It always helps to define MUs as they are currently broken down in the mind of the maintenance department.  Unfortunately, this isn’t always possible or desirable from the point of view of CMMS effectiveness.

Change in functionality or manufacturer: In production lines or machine centers, a change in function or equipment manufacturer generally indicates a division point between MUs.

Criticality of the unit and effect on downtime: A unit that can cause considerable downtime, is hard to replace or repair, has a long parts lead time, or has a significant effect on the productivity of other machines should be a separate MU.

Regulatory requirements: The government or manufacturers require separate tracking by regulation or warranty.  For some industries, this is the driving, critical force behind MU identification.

Physical size: In theory, the entire plant could be one MU.  Generally, an MU should entail no more than can be viewed from one location.  Most of the time, if it is bigger than a car, it is probably more than one MU.  There are always exceptions, though — building areas, for example.

Control loops: Controls and loops are separate MUs, if they are not attached directly to the unit.  For example, a pressure gauge at the pump is not a separate MU if it is not tracked separately for calibration or moved often.

Mobility and interchangeability of the unit: If the equipment moves to different locations, it should be a separate MU.  A large, portable hydraulic unit or a large motor used interchangeably with other installed motors should also be separate units.

Cost: If the unit is extremely expensive, it should be defined as one MU or more.  Total cost of ownership analysis requires historical data be kept separately so as not to confuse performance by including information from other units.

Relationships to asset definition: When possible, establish a one-to-one relationship between the CMMS and the plant’s asset numbers.  Generally, however, assets are defined under accounting criteria, not engineering criteria; so what is considered one asset, may or may not be one MU.

Constant definition across the company: The same type of equipment should be similarly defined in each area of the plant or in multiple plants to maintain consistent reporting, task development, executions, etc.  This is especially critical is setting up enterprise systems.

Number of preventive maintenance (PM) work orders that will be generated: This is based on experience.  The more MUs established, the more work orders generated.  You must be able to handle the paperwork.  A general rule of thumb is for every 100 MUs, a CMMS will generate 80 work orders per week.

Number of PM tasks: This is also based on experience.  If the equipment requires more than 60 unique PM tasks, it’s probably more than one MU.

Operation of the equipment: Units that operate on an infrequent schedule or are available as backups should be evaluated for blocking.  Remember, even blocked equipment can have individual identification in the PM tasking.

Support equipment: Maintenance support equipment (lathes, drills, etc.) and small exhaust fans are good candidates for blocking by areas.  Their maintenance requirement is usually low and their operation sporadic.

Routes: Sometimes, the logical flow of work requires established routes.  Some CMMS products require them to be set up this way.  A route example is the lubrication of bearings on a production line.  For estimate purposes, each route for 30 pieces of equipment should be set up as at least one MU.

Building areas: Generally, each building should be set up to track costs associated with the structure’s areas and grounds.

Administrative MUs: Don’t forget standard administrative MUs set up on all accounts (miscellaneous equipment not on your system, maintenance administration, etc.).

To help you determine your structure, review any system you currently own or those of an acquaintance in the same industry.  Understand the how/why of their area, equipment and MU structures.

Finally, when you think you have the right structure, run it past your end-users — people utilizing your CMMS daily, ordering parts, recording problems and completing work.  If they agree with your structure, chances for a successful system are vastly improved.

Arne Oas is the practice area manager for Computerized Facility Integration. He can be reached at .

This article appeared in the December 2000/January 2001 issue of MRO Today magazine. Copyright, 2000.

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