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Vishwadeep Khatri

Drum Buffer Rope approach

Drum Buffer Rope approach

 

Drum Buffer Rope is a production management technique from Theory of Constraints (TOC). It helps plan and schedule the production in an organization. Drum is the constraint (step with the lowest throughput) and it sets the pace of the entire process. Buffer is the inventory (or safety stock) that is maintained just before the constraint to ensure consistent production. Rope is the signal generated when the buffer falls below a threshold value. It is a signal for the previous step to replenish the inventory

 

An application-oriented question on the topic along with responses can be seen below. The best answer was provided by Mohamed Asif and Bhavesh Wakde.

 

Applause for the respondents- Abdallah Madaoo, Raksha Doli, Bhavesh Wakde, Mohamed Asif, Somrita Chatterjee, Ram Rajagopalan & Sreyash Sangam

 

Also review the answer provided by Mr Venugopal R, Benchmark Six Sigma's in-house expert.

Question

Q. 185  Explain Drum, Buffer and Rope approach and describe how it relates to Kanban.

 

Note for website visitors - Two questions are asked every week on this platform. One on Tuesday and the other on Friday.

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Drum Buffer Rope commonly known as DBR is an application from Theory of Constraints (ToC) used during planning and scheduling. Drum as constraint, Buffer as Inventory, and Rope for scheduling. 

 

DBR was developed by Eliyahu Goldratt. He also spearheaded Optimized production techniques, ToC and Critical Chain Project Management (CCPM).

 

His famous writings include “The Goal” and “The Race”.


In “The Race” he describes about Logistical system based on metaphor developed in “The Goal”

DBR is similar to Kanban or CONWIP (Constant Work in Progress)

 

DBR is similar to Kanban or CONWIP (Constant Work in Progress)

 

Primary Objective of DBR is to ensure Throughput Expectations are met at the same time managing the operating Expense and the Inventory.

 

In an assembly line, typically manufacturing setup, the constraint is referred as Drum (Slowest step/activity).

 

This constraint acts as a Drumbeat to set the speed of the system.

 

The process of releasing new work into the system is referred as Rope. 
Drum and Rope work together, as drum completes load, rope allows new work orders to be released.
 

To ensure Drum is always engaged with load (Work @ full capacity), buffer is created in front of Drum.

System Throughput is dependent on Drum’s working.

 

Considering the below production line,


863459028_WhatsAppImage2019-08-15at4_14_39PM.jpeg.ea33a08067e6ff83e3b4285838b340d4.jpeg

 

Here A3 is the constraint, as it is used as Drumbeat, pace is set based on A3. A3 can process only 35 pieces per hour but A1 and A2 produces more than 35 pieces per hour.


Rope monitors work load and pulls work from supplier and keeps buffer ready.

 

Referring to 5 focusing steps in Theory of Constraint (ToC)
1) Identify the Constraint
2) Exploit 
3) Subordinate all decisions to exploit
4) Elevate the constraint 
5) Avoid inertia
 

Drum: (High throughput as the constraint churns out work @ max capacity)

  • Identifies constraints in the system
  • Set’s pace
  • Schedule Capacity

Buffer: (High throughput as constant buffer is maintained before constraint)

  • Exploits the constraint 
  • Shields the constraint (Ensuring buffer is created)

Rope:

  • Rope subordinates work to the constraint 
  • It Alert’s/Signal’s to release work to constraint 

 

Maximum output and by effectively using DBR we get less idle time

Quote

"Let Idle people play rather than do unnecessary work" - Taiichi

:huh:

 

In general, Objective of DBR in manufacturing setup to have minimum inventory, maximum output and to have delivery dates obvious or predictable. Processes which are predictable can be well planned and continuously improved. 
 

Both DBR and Kanban are systems to control production, pull based and is very similar to each other. However there are few difference.  
DBR assumes on single constraint in the production system whereas it need not be the case for Kanban. Kanban controls inventory level at each production stage, however DBR does it only at constraint. 

 

Kanban can be simply improved by applying DBR (ToC), combining lean & JIT manufacturing along with planning tool to have improved throughput.  Thus preventing over production and having an highly efficient system.

 

Kanban with Super Market before constraint is similar to DBR.

 

Kanban can have more than 1 rope for effective processing steps and hence has an upperhand over DBR. 

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Drum-Buffer-Rope (DBR) is a method derived from the concept Theory of constrain(TOC) from the book written by Eliyahu Goldratt . As book indicated that slowest scout of mountaineering team decided the pace of climbing on mountain of the troupe. Troupe  had identified that scout was loaded heavily (constrain) which decides the pace of team. After load of the scout was distributed to the team & same scout had been put up to lead the team. Overall pace of the troupe got increased and scout boy (Drum) become the frontier to lead the troupe & decide the pace which others follows easily .

Similar pattern is being co-related with the methodology of DBR. Terminology are :

Drum : The pace setting resource/Production pace(Constrain)

Buffer : The amount of protection in front of bottleneck resource

Rope : The scheduled staggered release of material to be in the line with the Drum’s schedule.

 

 DBR_Pic.jpg.d5d5d21565b57de87ce77805c411488a.jpgAs shown in the picture(Source: https://www.marris-consulting.com/en/training-news/training/training-theory-of-constraints-in-production) considering the industrial scenario set of processes are involved to make the output from the system. In which one of the process will be considered as bottleneck which is having higher cycle time or lesser throughout , compare to other processes is being considered as constrain(Drum).This bottleneck decides the throughput or rate of production of entire cycle/system.

                                     To avoid shortages at bottleneck process buffer is getting planned ,as an hour lost for the sake of inputs bottleneck process will affect the entire output of the system .Once the buffer depleted in front of bottleneck process  signal (though rope) is getting pass on to non- bottleneck process to initiate to feed up to bottleneck process. This systematic approach protects weakest link in production system against process variation and dependency, which maximize system’s overall production effectiveness.

                                     The Drum is a schedule for the bottleneck process in entire system and the Rope is a schedule for releasing raw materials to the shop floor and derived according to the Drum and Buffers. The Rope ensure the proper subordination of non-bottlenecks.

Any operational plan needs to be productive, reliable, realistic and robust. 

- If it aligns with market requirment, we consider it as productive. 
- If it is aligned with our resource (capacity and add-on capability), we consider it reliable as well as robust.

in case of  inevitable disturbances or disruptions that will hit it; “realistic” means that it is capable of being done with the available resources, including material supply, maintenance etc.

In Toyota Production System. Rope (signal) concept is similar to KANBAN (signal-card) which provides inventory details at different process & refurbishing the stock at operational levels . Both the concepts provide the system for the proper planning of inventory management except the buffer description in DBR is taken up w.r.t. time whereas in KANBAN relevant to parts quantities. Since KANBAN provides better trace-ability which consider physical count of parts it gives faster and better result than Drum-Buffer-Rope method.

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Benchmark Six Sigma Expert View by Venugopal R

Drum, Buffer and Rope is a phase used in ‘Theory of Constraints’. It refers one of the methods for ‘synchronous’ flow in a production line. The Drum refers to a workstation that could be a constraint or bottleneck. For instance if we have a production line, where we have 3 workstations, A, B and C of which 2 of them (say A and C) are capable of producing at the rate of 120 units per hour and one workstation (say B), can produce only 100 units per hour, then the overall production rate for the line is 100 units per hour. Here, the production rate of the entire line is determined by workstation B, which is the constraint, or known as ‘DRUM’. We can afford some slackness for workstations A and C without impacting the overall production rate, but no slackness is affordable for station B as it will directly impact the production rate of the production line.

 

Since the workstation B is a constraint, it is important to place some ‘Buffer’ stock of the material being fed into it, so that there is no chance of this workstation to run out of stock. This precautionary inventory before the DRUM is know as the BUFFER. The buffer does not mean that more material will be produced than required, but it is an inventory in terms of time. This means that the buffer stock is an advance quantity of input material that will be made available to workstation B, even as it starts producing the first unit.

 

If the buffer inventory level decreases or increases, the communication that is provided to the previous process(es) is know as the ‘ROPE’. The ROPE ensures that the system is always maintained with a required level of inventory, neither excess not short.

 

image.png.8b7980d354b7468174c9ab1b7288d711.png

 

The DBR approach helps to plan the production schedules based on the output requirements. The quantity to be produced is determined by the market / client requirement. The maximum production rate that the production line can produce is determined by the DRUM workstation. The starting time for each workstation can be worked backward from the output requirement, taking into account the production rate of each workstation and the buffer requirements.

 

The representation shown above is a simple one for purpose of easy understanding. We can also apply the DBR on multiple lines leading to a final assembly line. Once we address a DRUM workstation with buffer, it is quite possible that we may have the possibility of another potential constraint to emerge. This would call for buffer requirements for other workstations as well. The entire network of the process flow can be mapped with this concept and the timings and buffer requirements be worked out.

 

The DBR approach helps as a planning tool and provides inputs to Kanban dashboard for regulating the production flow, scheduling the timing for workstations and to prevent inventory build-up.

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I need to obtain a finished product that product had to go through 3 process 
Process 1
Process 2
Process 3

Process 1 produce 50 product per hour
Process 2 produce 50 product per hour
But process 3 produce 40 product per hour... So the pace of our final product is 40 product per hour which is set by process 3...

So that makes process 3 a constraint.. Process 3 is our drum

 

Now what does a making process 3 our drum means?

It means that it is producing 40 product per hour so we cant let it go down.. if there is some problem in process 1-2 and their output reduce ultimately the output of process 3 will reduce...

 

How to avoid that?

We place some already processed product from process 2 as an inventory to process 3.. this is called as Buffer

 

What does buffer do..?

When there is a reduce in input to process 3 the buffer provide an extra input so as to keep the process on its pace

 

What does rope do?


Rope is a signal to maintain the buffer level... Suppose i have 5 product in my buffer and when the buffer product reduce to 3 a mechanism will send the signal which will fill my buffer inventory back to 5

 

The same way we use Cards Kanban to ensure the inventory is upto the mark and the process didnt lack because of it

Edited by Abdallah Madaoo
Missed the Kanban part

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Drum is the  Constant which we  in the process that is always the whole process is nearly dependent (main factor ) ,buffer is the back up (inventory )Which we create to manage the drum ,rope ensures  us that the buffer is always maintained   ( ensure that inventory is never out of limit ).

Kanban (pronounced as KAAN BAAN) is a process to maintain inventory


If I m working in a night shift and some inventory is out of stock I will write it down on the card and in morning when other person take over he will first read the card and than fill the inventory.

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Drum-Buffer-Rope Approach

 

Goldratt introduces the concept of Drum-Buffer-Rope (DBR) approach in this book “The Goal”. This is one of the TOC concepts which are used to manage production systems. Widely used in supply-chain teams and manufacturing systems. TOC concepts are very useful with combination of Agile, Lean in production floor and provides huge developments.

How it works:

For a general example:

We have got a line of kids who are up for a cycle ride in the woods. In the middle we have got the kid D who is the slowest or the bottleneck. Everyone else wants to go faster than D, now if that happens all the kids will be lost. Now how do we keep the line from spreading?

Suggestions: Tie the rope for the kids so the speed is constant for all the kids

                          Keep kid D at the front of the line.

Industry processes:

The drum is the slowest process in a system. It can be the limited resource, the bottleneck or any constraint. The first step in a process is to identify the drum in the process. Once identified, the drum tells us the upstream system to identify when to more items to the system and stopping maximum progress at once or unnecessary inventory at the start of the system. The rhythm of the drum also gives the maximum delivery rate because any system can deliver maximum value as its slowest part. Once the drum is identified, the next thing is to make sure the drum is beating faster as fast as possible. This is where the “rope” comes into picture and ensures the drum doesn’t exhaust out or disappear with the velocity of the factory production.

Many practitioners also believe in Drum-Rope only. Ideally, the drum will beat at a steady rate, a consistent amount of materials will be delivered to the drum and downstream steps will be receiving the drum’s output and delivering to customer. But in reality input to drum might be disrupted so buffer is needed. Adding a buffer before the drum gives the flexibility to the drum to pace up with the slowness and adding buffer later will add value to the downstream process.

Business Approach-

In an agile environment of multiple sprint teams been into production system of delivering business models to the Business annually, the bottleneck or the Scrum Sprint Team which is delivering less process models is the drum. Driving the velocity of the sprint teams equally can be identified as the rope. The Buffer is the additional time given at the start of the sprint when the project is in scope and timelines are being set.Limiting work-in-progress is an effective rope as it stops parts of the Sprint Team shooting ahead.

1) Identifying the Scrum Masters and shuffling the Scrum masters and teams for a better pace in delivering models can be a possible solution.

2) Also No Scrum Team should maximize delivering models in any of the sprints at the start of the cycle.DBR.PNG.446258dd0dd40a763036a3edd907cd86.PNG

Disadvantages:

DBR follows a fixed drum approach. If the bottleneck differs in a process which usually happens, then the DBR becomes rigid in the process. The Kanban methodology becomes more effective in this scenario and acts as a better solution.

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Drum Buffer Rope is a concept popularized by Eliyahu Goldratt in his book Goal.

 

It was used in analogy to a  Boy Scout team, to ensure that the team doesn’t get dispersed wide due to the variations in speed of the fast movers vs the slower ones. A synchronized approach ensured that the team is in step. Once the rope is stretched to the max, the fast movers have to slow down or stop.

 

The same concept can be applied to manufacturing, where speed is controlled by the drum, buffer adjustments by a rope with a goal of increasing throughput. The drum relays the information of the bottleneck to the group. The rope controls the speed of the set of processes ahead of the bottleneck from running away and creating excess WIP. Rope is like a signal to the front of the line to start producing at the front as consumption is happening in the bottleneck. The limitation of DBR approach is due to shifting bottlenecks.

 

Kanban is a Lean Management technique to achieve Just In Time. Kanban controls the upper limit of inventory, thus avoiding over capacity. It is achieved by going from the consumer of the process, taking demand from the end consumer up thorough the chain, with the rate of demand controlling the rate of production. Conceptually, for DBR, this is like keeping the drum at the last person and making it to a demand driven process.

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Drum, Buffer, Rope is a supply chain technique to define the flow of processes based in  pull system of production. Drum is synonymous with the first link of the supply chain which defines the pace of flow or pace of manufacturing which in order is followed by rest of the processes. This is in line with the concept of Theory of constraints which states the business performance or supply chain effectiveness is controlled by one or few critical factors which are actually bottleneck in the entire pipeline.By identifying that area of constraint, effort is made to improve that so that Throughput and effectiveness of the line can be improved.

In Drum, Buffer, Rope system, Just like Theory of constraint, pace is set by the bottleneck operation i.e., the process or operation with least cycle time or efficiency is placed in the front and other processes / operations follows the front line. This helps to make sure that the preceding operations work in tandem with the first operation and hence maintains the optimum inventory in result. The speed of preceding operations is increased based on the improvement in the effectiveness of the front bottleneck process. 

 

Thus Drum refers to the process operation which sets the pace and stage for assembly line or supply chain pipeline. Buffer refers to the number of amount if inventories in between the operations. These intermediate inventories are nothing but the business cash entrapped in between the processes. Rope is an imaginary line which connects the entire set of operation in the pipeline.

 

Toyota introduced the functioning of Kanban which resembles the Drum, Rope and Buffer. Kanban set the pace of the pull system. It acts as the tool to develop the pace for manufacturing and regulates. 

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The chosen best answers are that of Bhavesh Wakde and Mohamed Asif. Both answers use simple language to explain the concept and graphics to support the answer.

 

Benchmark Expert View has been provided by Venugopal R.

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