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 2015 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...
2015 Top Plant: Phoenix Contact, Middletown, Pa.; 2015 Best Practices: Automation, Electrical Safety, Electrical Systems, Pneumatics, Material Handling, Mechanical Systems
A cool solution: Collaboration, chemistry leads to foundry coat product development; See the 2015 Product of the Year Finalists
Raising the standard: What's new with NFPA 70E; A global view of manufacturing; Maintenance data; Fit bearings properly
Special report: U.S. natural gas; LNG transport technologies evolve to meet market demand; Understanding new methane regulations; Predictive maintenance for gas pipeline compressors
Cyber security cost-efficient for industrial control systems; Extracting full value from operational data; Managing cyber security risks
Drilling for Big Data: Managing the flow of information; Big data drilldown series: Challenge and opportunity; OT to IT: Creating a circle of improvement; Industry loses best workers, again
Migrating industrial networks; Tracking HMI advances; Making the right automation changes
Understanding transfer switch operation; Coordinating protective devices; Analyzing NEC 2014 changes; Cooling data centers
Upgrading secondary control systems; Keeping enclosures conditioned; Diagnostics increase equipment uptime; Mechatronics simplifies machine design

Annual Salary Survey

After almost a decade of uncertainty, the confidence of plant floor managers is soaring. Even with a number of challenges and while implementing new technologies, there is a renewed sense of optimism among plant managers about their business and their future.

The respondents to the 2014 Plant Engineering Salary Survey come from throughout the U.S. and serve a variety of industries, but they are uniform in their optimism about manufacturing. This year’s survey found 79% consider manufacturing a secure career. That’s up from 75% in 2013 and significantly higher than the 63% figure when Plant Engineering first started asking that question a decade ago.

Read more: 2014 Salary Survey: Confidence rises amid the challenges

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.
This article collection contains several articles on the vital role that compressed air plays in manufacturing plants.