TOC uses a mechanism to diagram the interdependence of elements within a chain, allowing the building of a resource planning system.
Introduction to Planning
The TOC developed by Goldratt postulates that any business has a purpose, and is limited to reach it by its weakest link, constraint1.
The concept of planning according to the ToC: the DBR and the CCPM
TOC thus offers a method of scheduling based on real demand and complying with the company’s constraint: The Drum-Buffer-Rope (DBR) for the world of Production, and the Critical Chain project Management (CCPM) for that of Project2 .
The methods of Ohno and Goldratt are based on four concepts:
- Allow to thin the flow and thus guarantee the Lead time (total Production time) sold to the customer.
- Have a mechanism to prevent overproduction.
- Consider the overall efficiency of the system and not the local efficiencies.
- Have a process to target where continuous improvement actions are carried out.
TOC Planning: The DBR method
It is the constraint, the drum, that gives the rhythm of the system. This can be all types of constraint: regulation, human… In the case where the constraint is the capacity of a resource (machine throughput…), it is then referred to as the “capacity Constrain resource”3.
From this drum, the stress ratio is deduced. It is the key indicator of TOC and gives us a vision of our ability to meet customer needs.
Constraint Ratio = constraint throughput/client need
As described in step 2 of the ToC, it is necessary to protect the DRUM to avoid its inactivity. For this, E. GOLDRATT proposes the setting up of Buffer. Rather than protecting each operation, the DRUM is protected with a time Buffer:
- The customer is guaranteed to be delivered by a ” Shipping Buffer ” Downstream of the constraint
- The constraint is protected with a ” constrain Buffer ” upstream.
All originality is therefore based on the fact that we are talking here of Buffer of time and not of stock of parts of outstanding for example, and of the fact that they are positioned upstream and downstream of the constraint and not disseminated on the whole system.
The interest is based on overcapacity of the upstream and downstream processes of the constraint and therefore, in the event of hazards being still able to catch up with the delay. The management of these buffers is therefore paramount4. This allows a risk management by exception, with a real proactivity, a clear definition of priorities in the Gemba, help in the analysis of the special causes for continuous improvement and monitor the stability of the system 5.
The rope, which is the mechanism that triggers, from the constraint, the process of upstream production, the production order being launched by the constraint. Thus, by pulling the rope from the date of customer delivery to the exit of the constraint, one gets the work that the constraint must perform on the time window studied.
The most important thing is therefore to respect the sequence to produce the constraint and not the respect of the dates to all the operations.
Of the author of TOC, the application of the DBR method can only be done if the cumulative operating time (in the case of a productive environment) is less than 10% of Lead time6.
It is also necessary to adapt the method according to your context and your stakes.
TOC and JIT
Here is simply a chart that shows the difference in tool between what the ToC and the JIT propose to set up a system called today’s widely-used flow.
|Drum = Neck||PaceMaker = Takt Time|
|Time Buffer||Stock In Progress|
1 – E.M. Goldratt (1990)-Theory of constraints
2 – E. Schragenheim, B. Ronen (1990)-Drum-Buffer-Rope Shop Floor Control
3 – M.M. Umble, M.L. Srikanth (1995)-Synchronous Manufacturing: Principles for world-class excellence
4-E. Schragenheim, B. Ronen (1991)-Buffer Managment-a Diagnostic Tool for Production Control
5 – R. Stratton, A. Knight (2009)-using buffer management to manage patient flow.
6 – E.M. Goldratt (2009)-standing on the shoulders of giants – production concepts versus production applications The Hitachi Tool Engineering Example
E.M. Goldratt, R.E. Fox (1986) – The Race
M. Spearman, D. Woodruf, W. Hopp (1990)-CONWIP: A pull alternative to kanban.
J. Orlicky, G.W. Plossl (1994)-Orlicky ʼ s Material requirements Planning
T. Ohno (1988)-Toyota production System: Beyond Large-Scale production