Drum Buffer Rope (DBR)
Is it possible to really get a step ahead from current methods of production management? The answer is yes! The Theory of Constraints based approach called Drum- Buffer-Rope is such a step ahead. But when speaking about a dramatic improvement, we should first describe the current state. There are two basic approaches to production management nowadays:
The “push” approach:
Material or parts are released to manufacturing based on a production plan. A production model enables the prediction of the flow of materials or parts through the production process, when it will arrive to a specific machine, when finished products will be ready for shipment. This methodology labeled as the MRP II /Manufacturing Resource Planning (now it is part of the ERP concept) form a base for a huge part of production management software products.
The “push“ system is based on the idea that everything happening in production can be put into a program which calculates an optimal solution. In reality this idea is quite unrealistic. Computer systems usually lives its own life, while operations are managed intuitively by the person in charge
The “push” system:
The initial impulse for releasing material moves from the beginning to the end of the process, from where the impulse spreads “against the flow” (e.g. by means of the Kanban Cards) up to the material and parts resources. Therefore nothing currently unneeded is not produced, stocks drop and lead times shorten. The core idea of JIT systems is seductively logical: The production process is not planned in detail, though reality cannot contrast with the plan. However, responsibility is delegated to operators who do not see the requirements of the whole system.
TOC world accepts the existence of the notorious trouble maker “Murphy” who causes something goes wrong quite often. Even if Murphy is not in action for a while, production process is influenced by variability of many factors. Thus such a process could be called Stochastic by an expert. MRP II does not register variability and relies on IT possibilities, JIT is robust against variability, so it does not take it into account.
TOC looks at any process as a chain of events, none of which is resistant to variability or Murphy’s attacks. It is however possible to estimate the length of individual tasks, even so no chain is stronger than its weakest link. Every product must go through this link, a production facility with the lowest capacity and the longest operating time.
When we say the weakest it implies there must be only one such constraint. If at least one part of the final product must go through this constraint, we cannot produce more of this product than the constraint allows. It makes little sense to release more or less material than the constraint can process. Thus the constraint sets the rhythm of production as the drum sets the pace of marching soldiers. That is why this function of the constraint is called the DRUM.
The constraint limits the throughput of the whole system, thus also its ability to generate profit. There is no other production facility with this quality. This facility identified as a constraint must be 100 % utilized. Every minute lost on the constraint is irreplaceable. Every hour saved in any other place of the production is of zero economic value. The constraint must be protected against “starvation” by a suitably dimensioned BUFFER.
Let’s say the constraint is placed somewhere in the middle of the production process. Downstream operations have higher capacity and products easily flow through them even if some obstacles occur. But let’s look upstream. It is important to be able to estimate (based on our experience), how long it will take to the material to get to the constraint. This period will determine the release of material. In the TOC world this time is called ROPE. This Rope must be long enough to ensure the constraint Buffer gets neither too full nor too empty.
The Theory of Constraint application is not a question of installing new computer software. It is a question of change in thinking of people who, in some cases, have been in charge of production management for a long time. That is why Dr. Goldratt stopped developing new software products and started concentrating on the most effective methods of gaining consensus of the key players in a company. Those, who are capable of planning, enforcing and implementing the change. The “Operations” application provided by Goldratt CZ is available to facilitate this goal.
Benefits that can be expected from the TOC application in production:
• Significant reduction in stock
• Throughput increase
• Production lead time shortening
• Easier planning than in the MRP II and a higher level of control than in the JIT
• Better predictability of the production process
• A possibility to direct process improvement initiatives (SPC, TPM, DOE, Poka –yoke, SMED) to the places, where it really matters.
• A possibility to direct investment in production system where it will bring real results.
The whole concept of the TOC production is nicely described in a gripping novel The Goal by E. M. Goldratt.