Capacity and constraints

Most manufacturing companies have a deep hierarchy of planning and scheduling processes that start with market forecasts and business plans, and end with machine and unit schedules. The higher levels are the responsibility of logistics, marketing, sales, and executive management, while the lower levels of the hierarchy are typically the responsibility of manufacturing operations.


Most manufacturing companies have a deep hierarchy of planning and scheduling processes that start with market forecasts and business plans, and end with machine and unit schedules. The higher levels are the responsibility of logistics, marketing, sales, and executive management, while the lower levels of the hierarchy are typically the responsibility of manufacturing operations. Manufacturing operations will become involved when actual production capacity is used in a finite capacity schedule. A finite capacity schedule takes into account limited resources and determines a schedule that does not exceed the resource limitations. The limiting resources are often equipment, such as a maximum throughput. However, limiting resources could also be raw material availability, storage space, or personnel availability.

Manufacturing IT’s responsibility is to maintain and export the predicted production capacity so that it can be used to generate a finite capacity schedule. In many production facilities, the capacity is constrained by a single resource for each production line, such as a machine. Such “bottleneck machines” are typical in discrete manufacturing and are usually product-independent. In these situations, the predicted capacity can be easily represented in a table of capacity per bottleneck for fixed time periods. The finite schedule time period, called the time bucket, can be hours, shifts, days, weeks, or months.

Time buckets

Consumable products may have time buckets of hours or shifts, while other goods usually run time buckets of days and weeks. The time bucket will often be a compromise between production personnel, who want long periods of steady production for high efficiency, and supply chain planning personnel, who want small buckets for maximum flexibility. Finite capacity scheduling systems often use theory of constraint (TOC) models and the drum-buffer-rope (DBR) method for fixed bottleneck problems. These are explained in an easy to read series of books by Dr. Eliyahu Goldratt ( ) and should be required reading for any manufacturing IT professional.

Process manufacturing often has “floating bottlenecks.” This means that the bottleneck resource can change based on the current product or product mix. For example, a single line may generate materials that flow into several downstream lines. In these situations, a more complicated scheduling method called process flow scheduling (PFS) is usually used. PFS uses a model of the physical process and may be optimized for minimum material inventory or for economic manufacturing run lengths, depending on the company’s business needs. Representing the predicted capacity in a PFS scheduled system can be complex and complicated. Because there is no single bottleneck machine, capacities must be maintained for each bottleneck machine in each part of a production line. Capacities must also be defined for different product mixes. This complexity usually requires a database or a set of tables, one table per product mix.

In both discrete and process manufacturing, the capacity information is also useful for operations management, providing a quick snapshot of committed capacity (the part of total capacity that is already committed to previously accepted production), unattainable capacity (the part of total capacity that is unavailable due to product mix, maintenance, or other reasons), and the available capacity that can be used for future production requests. Capacity information may also contain a confidence factor. For example, it may specify what production will be available at 95% confidence and what additional capacity may be available at 50% confidence. A confidence factor allows plant management to decide on the risk to take in accepting additional production requests.

Providing accurate and timely capacity information to business scheduling systems should be a goal of every production facility. Plants need to maintain a database of capacity information so that they can receive accurate and implementable schedules.

Author Information

Dennis Brandl is president of BR&L Consulting, Cary, NC, which is focused on manufacturing IT solutions. He is also chairman of the ISA88 committee. Reach him at .

No comments
The Top Plant program honors outstanding manufacturing facilities in North America. View the 2013 Top Plant.
The Product of the Year program recognizes products newly released in the manufacturing industries.
The Engineering Leaders Under 40 program identifies and gives recognition to young engineers who...
The true cost of lubrication: Three keys to consider when evaluating oils; Plant Engineering Lubrication Guide; 11 ways to protect bearing assets; Is lubrication part of your KPIs?
Contract maintenance: 5 ways to keep things humming while keeping an eye on costs; Pneumatic systems; Energy monitoring; The sixth 'S' is safety
Transport your data: Supply chain information critical to operational excellence; High-voltage faults; Portable cooling; Safety automation isn't automatic
Case Study Database

Case Study Database

Get more exposure for your case study by uploading it to the Plant Engineering case study database, where end-users can identify relevant solutions and explore what the experts are doing to effectively implement a variety of technology and productivity related projects.

These case studies provide examples of how knowledgeable solution providers have used technology, processes and people to create effective and successful implementations in real-world situations. Case studies can be completed by filling out a simple online form where you can outline the project title, abstract, and full story in 1500 words or less; upload photos, videos and a logo.

Click here to visit the Case Study Database and upload your case study.

Maintaining low data center PUE; Using eco mode in UPS systems; Commissioning electrical and power systems; Exploring dc power distribution alternatives
Synchronizing industrial Ethernet networks; Selecting protocol conversion gateways; Integrating HMIs with PLCs and PACs
Why manufacturers need to see energy in a different light: Current approaches to energy management yield quick savings, but leave plant managers searching for ways of improving on those early gains.

Annual Salary Survey

Participate in the 2013 Salary Survey

In a year when manufacturing continued to lead the economic rebound, it makes sense that plant manager bonuses rebounded. Plant Engineering’s annual Salary Survey shows both wages and bonuses rose in 2012 after a retreat the year before.

Average salary across all job titles for plant floor management rose 3.5% to $95,446, and bonus compensation jumped to $15,162, a 4.2% increase from the 2010 level and double the 2011 total, which showed a sharp drop in bonus.

2012 Salary Survey Analysis

2012 Salary Survey Results

Maintenance and reliability tips and best practices from the maintenance and reliability coaches at Allied Reliability Group.
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 Society for Maintenance and Reliability Professionals an organization devoted...
Join this ongoing discussion of machine guarding topics, including solutions assessments, regulatory compliance, gap analysis...
IMS Research, recently acquired by IHS Inc., is a leading independent supplier of market research and consultancy to the global electronics industry.
Maintenance is not optional in manufacturing. It’s a profit center, driving productivity and uptime while reducing overall repair costs.
The Lachance on CMMS blog is about current maintenance topics. Blogger Paul Lachance is president and chief technology officer for Smartware Group.