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Part Three
Planning and control
output. A process or stage can be activated in the sense that it is working, but it may only
be creating stock or performing other non-value-added activity.
4 An hour lost (not used) at a bottleneck is an hour lost for ever out of the entire system.
The bottleneck limits the output from the entire process or operation, therefore the
under-utilization of a bottleneck affects the entire process or operation.
5 An hour saved at a non-bottleneck is a mirage. Non-bottlenecks have spare capacity
anyway. Why bother making them even less utilized?
6 Bottlenecks govern both throughput and inventory in the system. If bottlenecks govern
flow, then they govern throughput time, which in turn governs inventory.
7 You do not have to transfer batches in the same quantities as you produce them. Flow
will probably be improved by dividing large production batches into smaller ones for
moving through a process.
8 The size of the process batch should be variable, not fixed. Again, from the EBQ model,
the circumstances that control batch size may vary between different products.
9 Fluctuations in connected and sequence-dependent processes add to each other rather
than averaging out. So, if two parallel processes or stages are capable of a particular
average output rate, in parallel, they will never be able to achieve the same average output
rate.
10 Schedules should be established by looking at all constraints simultaneously. Because of
bottlenecks and constraints within complex systems, it is difficult to work out schedules
according to a simple system of rules. Rather, all constraints need to be considered together.
OPT uses the terminology of ‘drum, buffer, rope’ to explain its planning and control
approach. We explained this idea in Chapter 10. The bottleneck work centre becomes a ‘drum’,
beating the pace for the rest of the factory. This ‘drum beat’ determines the schedules in
non-bottleneck areas, pulling through work (the rope) in line with the bottleneck capacity,
not the capacity of the work centre. A bottleneck should never be allowed to be working at
less than full capacity; therefore, inventory buffers should be placed before it to ensure that
it never runs out of work.
The five steps of the theory of constraints
As a practical method of synchronizing flow, TOC emphasizes the following five steps. 11
1 Identify the system constraint – the part of a system that constitutes its weakest link; it
could be a physical constraint or even a decision-making or policy constraint.
2 Decide how to exploit the constraint – obtain as much capability as possible from the
constraint, preferably without expensive changes. For example, reduce or eliminate any
non-productive time at the bottleneck.
3 Subordinate everything to the constraint – the non-constraint elements of the process are
adjusted to a level so that the constraint can operate at maximum effectiveness. After this,
the overall process is evaluated to determine if the constraint has shifted elsewhere in the
process. If the constraint has been eliminated, go to step 5.
4 Elevate the constraint – ‘elevating’ the constraint means eliminating it. This step is only
considered if steps 2 and 3 have not been successful. Major changes to the existing system
are considered at this step.
5 Start again from step 1.
Table 15.2 shows some of the differences between the theory of constraints and lean
synchronization. Arguably, the main contribution of TOC to smooth, synchronized flow is
its inclusion of the idea that the effects of bottleneck constraints (a) must be prioritized,
and (b) can ‘excuse’ inventory, if it means maximizing the utilization of the bottleneck. Nor
(unlike ERP / MRP, for example) does it necessarily require large investment in new information
technology. Further, because it attempts to improve the flow of items through a process,
it can release inventory that in turn releases invested capital. Claims of the financial payback
from OPT are often based on this release of capital and fast throughput.