Throughput and constraint management - Drum buffer rope conc...

Learning Outcomes

After reading this article, you will be able to explain the Drum-Buffer-Rope (DBR) methodology and how it addresses production constraints. You will understand throughput accounting, identify bottlenecks, describe the DBR process flow, and apply these concepts to performance management scenarios. You will also be able to analyse the impact of DBR on operational decisions and KPIs, and advise on improvements to maximise organisational performance.

ACCA Advanced Performance Management (APM) Syllabus

For ACCA Advanced Performance Management (APM), you are required to understand the role of bottlenecks and constraint management in improving performance, especially in manufacturing or service environments using throughput accounting. Focus your revision on:

• The concept of throughput as a key performance metric
• Identification and management of bottlenecks (constraints) within operational systems
• The principles and operation of the Drum-Buffer-Rope (DBR) scheduling method
• How DBR influences resource utilisation and overall organisational performance
• The information needs and performance measures under a constraint-based production environment

Test Your Knowledge

Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, remember to look more closely at that area during your revision.

1. Which element in the Drum-Buffer-Rope system represents the production constraint?
   1. Rope
   2. Drum
   3. Buffer
   4. All equally

2. True or false? Throughput accounting focuses mainly on minimising operating expenses and inventory rather than maximising sales through the constraint.

3. A factory identifies that its painting station is the slowest process. According to DBR, how should production flow be organised?

4. Explain how a protective buffer works in DBR and its role in protecting throughput.

5. Calculate the throughput per unit if sales price is $100, direct material is $25, and all other costs are classified as operating expenses.

Introduction

Constraint management is a central theme in throughput accounting. Many organisations have at least one process stage, resource, or department that restricts the total output—the bottleneck or constraint. Failing to recognise and manage this area can result in suboptimal performance, wasted capacity elsewhere, and misaligned KPIs. The Drum-Buffer-Rope (DBR) concept, arising from the Theory of Constraints, provides a rigorous method for scheduling production, protecting throughput, and managing constraints to maximise value.

Key Term: throughput accounting
A management accounting approach focusing on maximising the rate at which a system generates money through sales, by identifying and managing process constraints.

DRUM-BUFFER-ROPE (DBR): AN OVERVIEW

Drum-Buffer-Rope is a process scheduling system designed to optimise performance in environments with bottlenecks. It sets the “drumbeat” of the system at the rate of the constraint, uses “buffers” to protect flow, and “rope” as synchronisation to avoid overproduction upstream.

The Drum

The “drum” is the system’s constraint (bottleneck). It sets the pace for the whole process, as through this point all product must pass. The organisation must synchronise all activity to the output rate of the drum; producing more simply creates excess work-in-progress (WIP) and hides inefficiencies.

Key Term: constraint
Any resource whose capacity is less than or equal to the demand placed upon it, limiting the system’s output.

Key Term: bottleneck
The process or resource with the lowest capacity, restricting the total flow of production.

The Buffer

A buffer is a controlled amount of inventory placed immediately before the bottleneck to ensure it never stands idle due to shortages upstream or unpredictable disruptions. Buffer size is carefully set; too small and the drum stops, too large and inventory costs rise.

Key Term: buffer
Stock or time deliberately held before a constraint resource to prevent it running out of work—thus protecting throughput.

The Rope

The “rope” is a signalling mechanism that ties release of material or work orders from the start of the process to the drum’s pace. It prevents upstream operations from overproducing and building unnecessary inventory.

Key Term: rope
The information or communication flow that ensures production release is synchronised with the drum rate.

IMPLEMENTING DBR: STEPWISE APPROACH

1. Identify the system’s constraint
   Locate the bottleneck or resource that limits output.
2. Exploit the constraint
   Maximise output from the bottleneck—minimise downtime, prioritise high-throughput products.
3. Subordinate other activities to the constraint
   All other departments align to the drum rate; stop producing excess WIP.
4. Upgrade the constraint
   Consider investing in additional capacity for the bottleneck.
5. Repeat—address new bottlenecks as they emerge.

DBR is iterative—eliminating one constraint may reveal another elsewhere in the system.

Worked Example 1.1

BrightPrint Ltd makes custom brochures using three operations: printing, binding, and packaging. Each hour, printing produces 100 units, binding 80, and packaging 120. Where should the buffer be positioned and what is the correct release rate under DBR?

Answer:
Binding is the bottleneck at 80 units per hour. The buffer should be placed before binding to ensure it never runs out of work. All process steps should be synchronised to release a maximum of 80 units per hour into production—the capacity of the drum. Upstream (printing) and downstream (packaging) rates must not exceed this, or inventory and waiting will result.

THROUGHPUT ACCOUNTING AND PERFORMANCE MEASUREMENT

Unlike traditional accounting which focuses on maximising utilisation of all resources and minimising overheads, throughput accounting puts the spotlight on maximising the flow of money via sales, constrained by the bottleneck.

Key Measures:

• Throughput (per unit): Sales price minus direct material cost.
• Return per factory hour: Throughput per unit / time on bottleneck
• Product ranking: Products are prioritised by throughput per minute/hour of constraint resource.

Key Term: throughput
The rate at which the system generates money through sales, calculated as sales revenue minus direct material costs.

ALIGNMENT AND CONTROL USING DBR

DBR helps avoid two common performance management issues:

• Overproduction upstream: Without the rope, departments ahead of the bottleneck may create excess WIP.
• Idle time at bottleneck: Without a buffer, any upstream disruption halts the constraint, lowering throughput.

KPIs must reflect constraint-aware logic—utilisation rates at non-bottleneck operations are irrelevant, and traditional absorption of overheads elsewhere can lead to dysfunctional decisions.

Worked Example 1.2

SwiftCables produces three cable types. The wire-insulation stage (insulate) is the bottleneck. A manager suggests running the packing section overtime to catch up on customer orders, even if insulation cannot produce any faster. Critically assess this proposal under DBR logic.

Answer:
Running the packing section overtime is wasteful, because the system’s overall output is limited by the capacity of “insulate.” Increasing effort in packing will simply result in idle time or backlog with no improvement in customer delivery performance. A better solution would be to ensure insulation is fully utilised and to add a buffer before it to prevent stockouts. Only if insulation capacity can be increased (by elevating the constraint) will overtime elsewhere deliver increased throughput.

Exam Warning

In APM scenarios, do not recommend attempting to maximise output from every process. Focus only on the system’s bottleneck—other departments should be subordinated to the drum rate. Irrelevant local targets (such as maximising machine running time outside the constraint) can lead to overproduction, excess WIP, and poor financial performance.

DBR, BUFFER MANAGEMENT, AND REAL-WORLD CHALLENGES

The buffer size is essential: a small buffer risks lost throughput in case of disruption; a large buffer increases inventory cost and hides process inefficiencies. Monitoring buffer consumption can alert management to process variability or reliability issues.

Worked Example 1.3

A toy assembler has set the bottleneck pace to 200 units/hour, with a buffer before the painting station. Past data shows frequent consumption of the entire buffer, causing bottleneck stoppages. What does this indicate, and what management response is best?

Answer:
Frequent depletion of the buffer suggests unreliability or poor upstream management, resulting in the bottleneck standing idle. Management should investigate the causes—machine breakdowns, stockouts, uneven work release—and either improve reliability or, if justified, increase buffer size. The goal is to protect the bottleneck so it operates as continuously as possible.

Summary

Drum-Buffer-Rope provides a framework for aligning the entire workflow to the system’s constraint. By synchronising production release (the rope) to the bottleneck’s pace (the drum) and protecting it from disruptions (via a buffer), organisations can maximise total throughput. Performance should be measured by throughput accounting metrics. Non-bottleneck resource utilisation is secondary—focus all efforts on ensuring the continuous operation of the constraint.

Key Point Checklist

This article has covered the following key knowledge points:

• Describe the purpose and components of Drum-Buffer-Rope: drum, buffer, rope
• Identify bottlenecks and apply throughput accounting concepts
• Explain how DBR prevents overproduction and idle time at the constraint
• Analyse buffer management and its impact on throughput protection
• Critically assess performance measures relevant to DBR environments
• Advise on appropriate organisational responses to changing constraints

Key Terms and Concepts

• throughput accounting
• constraint
• bottleneck
• buffer
• rope
• throughput