A push system is one in which an upstream process sends work to a downstream process prior to the downstream process requesting it. Batch and queue systems in which large lots are sent to the next workstation to wait in line are commonly push systems.
The basic problem with push systems is the disconnect from the customer. In most cases, schedulers give the upstream processes lists of what to produce based on forecasts. By the time those products make it downstream, they are unlikely to match true customer demand.
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There are three basic problems with push systems.
The first is that they tend to increase inventory. Because of the lack of direct coordination between what the downstream process needs and what the upstream process is delivering, the wrong quantity or product is often sent.
Unfortunately, despite a steady stream of product deliveries, the downstream process is often starved for the correct products. This can lead to large piles of the wrong parts sitting throughout the system. So, despite having a lot of money tied up in inventory, it is the wrong inventory. This leads to the second basic problem.
When the right parts are not available, the team has to work outside the normal process to get the correct materials. This means expedites, special orders, and a variety of hidden factories. Whatever it is called, it creates waste and adds substantial cost to a system.
Finally, the third problem is a lack of teamwork. When two processes are linked, people can shift around to help out if there is a problem. There is a much more immediate feedback loop for finding a resolution. There is a great deal more process improvement when people are in closer communication.
With push, all that communication is nonexistent. There is no established communication system. For example, in a pull system, if the downstream process runs into trouble and stops sending kanban cards, the upstream process immediately goes to check what is happening. If the upstream process misses a delivery, the downstream process immediately sends help to get parts flowing again.
One of the most common forms of pull systems is the kanban card. It is simply a small card that is a signal to produce, transport, or buy more materials. (See our pull systems entry for more information on other methods.)
Push systems are sneaky. On the surface, they seem efficient and effective when looked at locally. They keep the operator working rather than waiting for signals. Because the batch sizes are often large in push, some machines can be run at full speed and churn out parts quickly. Unfortunately though, frontline operators often only have this local view of a process, and can’t see the system-wide effects of their choices.
If you are operating in a push mode, and are being asked to go to pull, some of the requirements will sound illogical. The biggest of these will be stopping work when there is no pull signal. In push, you probably get a daily schedule that keeps you fully engaged. With pull, you may on occasion get ahead of the downstream operators and there will be no demand on your process.
The truth is that it’s good news when you run out of work. It either identifies a problem you can work on, or it exposes a mismatch in workload. Busy work, or non-productive work gets replaced by more important work.
If done right, though, you should never get overloaded and overworked. That exposure of problems I mentioned above works both ways. If your pull signals come at you too fast, your boss will have to get you help or the system will starve.
Transitioning to pull will be difficult, but the biggest challenge is often believing that switching to a pull system will improve the operation. Often, leaders are so used to push systems that the idea of changing is difficult. It also may make little sense that stock-outs can be resolved by using a system that actually lowers inventory levels. Of course, it reduces the overall inventory, just not the inventory of the right parts at the right time.
Also be prepared for a substantial pushback from employees. While a pull system may sound good on paper or when simulated in a Lean Lego® exercise, it can be a different story in real life. There will invariably be hiccups. Your job will be to make extensive use of the PDCA cycle to tear down any obstacles that come up.
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