The importance of an equipment bill of materials

Doug Wallace

The main purpose of the materials management organization is to provide the "right parts in the right quantities at the right time." But where do those material requirements come from? Whether or not demand is predictable, whether the materials are for production or maintenance, the requirements are usually generated from a bill of material (BOM).

Without a complete and accurate bill of materials, decisions regarding material planning and replenishment are often made in a vacuum, resulting in excess inventory, stockouts, expediting charges and expensive downtime.

For manufactured items, the bill of materials lists all the raw materials, pieceparts and other components required to complete a single unit of an end item. An equipment bill of material (EBOM) lists all of the components of an asset, including its assemblies and subassemblies. With a reliable equipment bill of materials, a planner can determine exactly what parts are needed. And in an emergency situation, the EBOM provides valuable information to craftsmen and others to ensure that the right parts are identified and procured.

This article provides guidelines for ensuring availability and accuracy of the equipment bill of materials data required to support work order planning and execution. This information provides materials management with one of the tools required to assure an adequate supply of materials while maintaining optimum inventory levels.


This tree shows a product structure for "Product 1".


Figure 1 shows a product structure for a generic item called "Product 1." As shown in the graphic, Product 1 has several assemblies, subassemblies and components, with different quantities of each required. This structure could easily represent a typical manufactured item, or in this particular case, even an equipment asset.

The associated equipment bill of materials could take several different forms, but the most common are single-level (a.k.a. "spare parts list") and multi-level ("indented product structure"). Examples of each are shown on Page 31.

Single-level EBOM: Figure 2 represents a simple single-level EBOM for Product 1. In this type of BOM, each individual part that makes up the asset is shown only once, with the total quantity required to complete the entire asset. This BOM type is convenient for exploding material requirements for manufactured items, where production requirements can be calculated from a master production schedule.

For equipment, though, this structure does not show the actual parent/child relationships of assemblies, subassemblies and components. This might lead a maintenance planner to erroneously conclude that any time this asset needed to be repaired or replaced, all the items on the BOM might be required. This is clearly not the case if only a particular part of the asset needs repair or replacement.

Multi-level EBOM: Figure 3 shows a multi-level BOM that provides a better level of detail for equipment assets. In this BOM type, the same total material requirements are shown, but the product structure is indented to show parent/child relationships between the components, subassemblies and assemblies. Because of these relationships, an indented structure takes a little more effort to create and enter into the computerized maintenance management system (CMMS).

However, it provides a greater level of specificity when determining which materials are required to repair or replace a particular part of the asset. If the CMMS supports a multi-level BOM, the indented product structure is preferable.

Figure 2: Single-level BOM for “Product 1”.

Figure 3: Multi-level BOM for “Product 1”.


EBOM creation is itself a process that should be followed in a disciplined fashion, and consists of several steps:

Determine which assets require an equipment bill of materials: Over time, all assets should have an EBOM. However, base the order of priority for EBOM creation on the criticality ranking of the assets. Every critical asset must have an EBOM. All other assets should have one. Develop them in order of priority based on the criticality ranking as time, resources and availability of data allow.

Determine what items should be on the equipment bill of materials: In general, any spare parts or components for which there is a reasonable expectation of being repaired or replaced as a result of a preventive maintenance (PM) or corrective maintenance (CM) work order should be included on the EBOM.

At a minimum, the EBOM should reflect any item identified as a critical spare. In addition, the EBOM should include any other materials used directly to repair the asset, or any item for which it is necessary or beneficial to have usage history tied to the asset in the CMMS.

Items that typically don't need to appear in the EBOM are specialty tools, free-issue material (primarily small hardware), expense materials (rags, towels, etc.) and personal protection equipment.

Determine what data should be on the EBOM: Some of the EBOM data will be dictated by the CMMS based on the fields available. Most systems support the following elements that should be considered the minimum required:

  • CMMS part number

  • Description (properly formatted and in sufficient detail to specify what is required)

  • Quantity per

  • Unit of measurement

  • Manufacturer

  • Manufacturer's part number

In addition, the following data provides valuable information that should be included in the equipment bill of materials if available and supported by the CMMS:

  • Preferred supplier

  • Supplier's part number

  • Authorized substitutes

  • Revision number/date and/or effective dates

  • Estimated price

Gather the required data: This step often leads to two interesting questions:

  1. Who provides the data?
  2. Where does the data come from?

The answer to the first question is fairly simple. The data may come from a variety of sources, but it is usually coordinated by a reliability engineer (RE). It doesn't matter who provides the data, as long as it's accurate. Before creating the actual EBOM, however, the RE should review it.

Where the data comes from is much more involved. The best source of equipment bill of materials data is the original equipment manufacturer (OEM). The most effective means of capturing EBOM information, especially for new equipment, is to request the equipment manual from the OEM at the time of purchase. Part numbers can be assigned to components, and the data can be reviewed and entered into the CMMS even before the equipment enters the facility.

In lieu of data from the OEM, the equipment supplier may also be able to provide assistance in obtaining such documentation. Other sources of EBOM data may include nameplate information; engineering change notices (ECNs) or other redesign documentation; work order history; or even "tribal knowledge" from craftsmen, engineers, operators or other experts familiar with the equipment.

If all else fails, it may be worthwhile to track down other users of the same or similar equipment to see what information they may have. The most important thing is not where the data comes from, but whether it is accurate.

Enter the data in the CMMS: It is usually at this point that the inevitable question arises: "Who is responsible for entering the data into the CMMS?" The correct answer is "Anyone with adequate knowledge of the system and the skills required to enter the data accurately into the required fields."

More often than not, the default answer is "materials management," which isn't necessarily the worst thing. The materials group is most familiar with the basic structure of the part master from entering inventory items. They are also usually familiar with the structure of the EBOM, or at least the fields that need to be populated and the data that should be entered.

Depending on the CMMS, there may be an option to automatically add parts to an EBOM when they are issued against a work order for a specific asset. This can be a useful way to build and/or maintain EBOMs based on actual material usage, but it can also lead to degradation of EBOM data integrity if not managed properly.

Several types of events can necessitate equipment bill of materials modifications. Once an EBOM is created, it must be updated as necessary and reviewed periodically to ensure its continued accuracy. Consider the following:

Decommissioning: Whenever an asset is decommissioned, the associated EBOM should be deleted as well. As part of that process, each item linked to the asset should be reviewed to see if the item can be deleted and/or if any remaining inventory can be used elsewhere, sold or scrapped.

Design changes: Whether initiated by the manufacturer or user, if equipment is redesigned or modified, there is the possibility that spare parts and components have been upgraded or changed. Reflect these changes in the EBOM to assure that the correct materials are used.

Any item changed on the EBOM should go through similar scrutiny as if the asset were decommissioned to determine if existing inventory can be used up or if the item should be deleted. This process helps avoid inventory of unusable parts, which is often a major contributing factor to excessive inventories.

Part substitution/standardization: As a result of material availability issues, or as part of engineering/purchasing best practices, there may be a need to authorize parts that are suitable substitutes for an EBOM item. There may also be opportunities to standardize similar parts across multiple assets or plants within an enterprise. In these cases, update the EBOM to reflect the currently acceptable parts that can be used on the asset, including effective start and stop dates if the system supports them.

EBOM review: The EBOM is one of the principal tools used by the planner to properly plan work orders. As a result, the planner is the primary user of the EBOM, and, therefore, is in the best position to monitor data accuracy. This can most effectively be accomplished in one of two ways.

The easiest way is to make a cursory review when an equipment bill of materials is accessed during the planning process. This is a relatively quick and painless way to identify obvious errors in the EBOM based on the types of materials listed or specific knowledge that the planner has about the EBOM items. In these cases, the planner will generally get blanket authority to document and make necessary corrections in the CMMS.

A more disciplined approach is to perform periodic reviews of each EBOM in much the same way as a storekeeper will cycle-count his inventory. Specific EBOMs are chosen, and typically the review is done in a more formal fashion by a review board - a group of people knowledgeable about the particular asset (they may, in fact, be the same individuals who provided the initial input to the EBOM). The collective wisdom of the board often provides insights that the individuals may not have themselves about any of the previously mentioned issues.

There are two primary metrics for measuring EBOM effectiveness. With a robust process in place, the data is easy to capture.

EBOM completion: This is no more than a measure of whether an EBOM exists in the CMMS for each asset. The goal should be 100 percent for all critical assets and 95 percent for others.

EBOM accuracy: Calculating EBOM accuracy is a bit more complicated. It usually reflects data captured during periodic reviews, and the data often must be maintained manually outside the CMMS.

The accuracy can be calculated on a line-item basis or on the entire EBOM. In either case, the target for critical assets is 100 percent (i.e. zero errors in the EBOM); the target for other assets is 98 percent (i.e. no more than one error in a sample of 50).

The direct and indirect benefits of accurate EBOMs can be difficult to quantify, but aren't difficult to delineate.

Fewer incorrect material purchases: By utilizing the information contained in the EBOM to generate purchase requisitions, there is less of an opportunity for guesswork, variation or errors in the transmission of material requirement data to suppliers.

Faster execution of planned work: Accurate EBOMs reduce the amount of time spent researching required materials. This helps to streamline the planning and procurement processes, which in turn reduces the length of time required to obtain the necessary parts to complete the job. This allows faster execution of equipment PMs or CMs and gets equipment back on line sooner.

Faster execution of unplanned work: An effective EBOM provides craftsmen with quick access to accurate part requirements and descriptions in an emergency situation. Combined with a reliable inventory control system, craftsmen can quickly determine the on-hand quantity and location of available parts in stock. Should insufficient inventory be available, the EBOM can also provide valuable manufacturer and supplier details to facilitate expedited procurement.

Disposition of inactive inventory: Use EBOMs to determine whether a non-moving inventory item is required for an active asset.

More effective reliability engineering: Use EBOMs to identify similar items or equipment where individual materials can be standardized or substituted.

In many cases, the CMMS will provide the capability to do an inverse examination of EBOM data. Rather than querying an asset to see what parts are listed on the EBOM, the system allows a query on a part and displays all the EBOMs where it is listed. This "where used" capability provides a tool for determining the impact of decommissioning and design changes on the continued need for specific material items.

Also a feature of a robust CMMS, the EBOM (or in some cases, another field in the part master) will provide a flag to identify critical spares. These are items that must be readily available to avoid significant production losses due to downtime.

As a rule, the RE is primarily responsible for providing initial EBOM information and all engineering-driven changes. The planner is responsible for ensuring equipment bill of materials accuracy. But the key to overall EBOM effectiveness is to recognize that data creation and maintenance is a collaborative process that requires teamwork and communication.

Doug Wallace is a materials management expert for Life Cycle Engineering. To learn more, e-mail dwallace@LCE.com, call 800-556-9589 or visit www.LCE.com.

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About the Author

Doug Wallace, CPIM, has more than 30 years of combined experience in supply chain operations and management consulting, specializing in the areas of global enterprise planning, production and in...