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it results in information inventories that serve to confuse flow through the process. For
example, an Australian tax office used to receive applications by mail, open the mail and
send it through to the relevant department which, after processing it, sent it to the next
department. This led to piles of unprocessed applications building up within its processes,
causing problems in tracing applications, and losing them, sorting through and prioritizing
applications, and worst of all, long throughput times. Now they only open mail when
the stages in front can process it. Each department requests more work only when they have
processed previous work.
Pull control
The exact matching of supply and demand is often best served by using ‘pull control’
wherever possible (discussed in Chapter 10). At its simplest, consider how some fast-food
restaurants cook and assemble food and place it in the warm area only when the customerfacing
server has sold an item. Production is being triggered only by real customer demand.
Similarly supermarkets usually replenish their shelves only when customers have taken sufficient
products off the shelf. The movement of goods from the ‘back-office’ store to the shelf
is triggered only by the ‘empty-shelf ’ demand signal. Some construction companies make it
a rule to call for material deliveries to their sites, only the day before those items are actually
needed. This not only reduces clutter and the chances of theft, it speeds up throughput time
and reduces confusion and inventories. The essence of pull control is to let the downstream
stage in a process, operation, or supply network, pull items through the system rather than
have them ‘pushed’ to them by the supplying stage. As Richard Hall, an authority on lean
operations put it, ‘Don’t send nothing nowhere, make ’em come and get it.’ 6
Kanbans
The use of kanbans is one method of operationalizing pull control. Kanban is the Japanese
for card or signal. It is sometimes called the ‘invisible conveyor’ that controls the transfer of
items between the stages of an operation. In its simplest form, it is a card used by a customer
stage to instruct its supplier stage to send more items. Kanbans can also take other forms. In
some Japanese companies, they are solid plastic markers or even coloured ping-pong balls.
Whichever kind of kanban is being used, the principle is always the same: the receipt of a
kanban triggers the movement, production or supply of one unit or a standard container
of units. If two kanbans are received, this triggers the movement, production or supply of
two units or standard containers of units, and so on. Kanbans are the only means by which
movement, production or supply can be authorized. Some companies use ‘kanban squares’.
These are marked spaces on the shop floor or bench that are drawn to fit one or more work
pieces or containers. Only the existence of an empty square triggers production at the stage
that supplies the square. As one would expect, at Toyota the key control tool is its kanban
system. The kanban is seen as serving three purposes:
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It is an instruction for the preceding process to send more.
It is a visual control tool to show up areas of over-production and lack of synchronization.
It is a tool for kaizen (continuous improvement). Toyota’s rules state that ‘the number of
kanbans should be reduced over time’.
The single-card system
There are a number of methods of using kanbans, of which the ‘single-card system’ is most
often used because it is by far the simplest system to operate. Figure 15.7 shows the operation
of a single-card kanban system. At each stage (only two stages are shown, A and B) there is a
work centre and an area for holding inventory. All production and inventory are contained
in standard containers, all of which contain exactly the same number of parts. When stage B
requires some more parts to work on, it withdraws a standard container from the output
stock point of stage A. After work centre B has used the parts in the container, it places the
move kanban in a holding area and sends the empty container to the work centre at stage A.