Pull
The terms “pull” or “pull system” are often used interchangeably with flow. It
should be understood that, like flow, pull is a concept, and the two are linked,
but not the same. Flow defines that state of material as it moves from process to
process. Pull dictates when material is moved and who (the customer) deter-
mines that it is to be moved.
Many people are confused about the difference between the “push” method
and the “pull” method. Some erroneously think they are “pulling” because the
material continues to move or flow. It is possible to flow without having pull.
There are three primary elements of pull that distinguish it from push:
- Defined. A defined agreement with specified limits pertaining to volume
of product, model mix, and the sequence of model mix between the two
parties (supplier and customer). - Dedicated. Items that are shared between the two parties must be dedi-
cated to them. This includes resources, locations, storage, containers, and
so forth, and a common reference time (takt time). - Controlled.Simple control methods, which are visually apparent and
physically constraining, maintain the defined agreement.
In a push system there is no defined agreement between the supplier and the
customer regarding the quantity of work to be supplied and when. The supplier
works at his own pace and completes work according to his own schedule. This
material is then delivered to the customer whether the customer requested it or
not. Locations are not defined and dedicated, and material is placed where there
is an opening. Since there is no definition, or dedication, there is no clear way
to understand what to control or how to control it.
Of course, some element of control does happen through expediting, chang-
ing the schedule, and moving people, but this only leads to additional waste
and variation. It could be argued as well that the agreement is defined based on
the schedule. All processes are working to the “same” schedule. In fact they
may be on the same schedule, but they are not on the same page.
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0 .5 seconds over takt. The next step would be to reduce other waste
and simplify the operation so it can be completed in 10 seconds or less,
resulting in a net time per piece below takt time (5 seconds).
In this example, the creation of flow actually reduced performance by
33 percent (three operations rather than two). Also, in the scope of
the entire value stream, this operation was a very small portion of the
total material flow. There were much greater opportunities to create
flow and reduce the throughput time in other areas by connecting
operations utilizing the pull methods described below.