Push System

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 push systems.

Generally Lean systems attempt to move towards pull systems, in which no work can move without a request.

The basic problem with push systems is that they are disconnected 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.

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. By contrast, 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.)

Words of Warning About Push Systems

• Push systems work reasonably well when demand is level and the mix doesn’t vary. You may not see a crisis in this situation, especially if you don’t have a lot of inventory on hand. Don’t get complacent, though. Demand in the modern world can change in an instant.
• Push systems work poorly with custom products. If a part is linked to a custom item early in the production process, lead time goes up. If it is linked late in the process, it limits what can be customized or adds inventory. Pull systems have similar problems, but with less inventory in the value stream, it is not amplified as much.
• Push systems can be improved. But don’t let improvement of an inherently inefficient system lead to complacency.
• Push systems are used for seemingly valid reasons. Don’t let those reasons stop progress, though. Identify the root cause of the need for push and eliminate that barrier.
• Don’t just jump into a transition to pull. Make sure that the underlying problems are addressed first. These will likely include long setup times, poor equipment maintenance, quality issues, and the like.
• Don’t assume pull equals one-piece flow. You might have fairly high kanban quantities until you start reducing problems. You should, though, continually work to reduce inventory to improve flow.

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 there’s an opportunity 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 generally lowers inventory levels.

How does less inventory result in fewer stockouts? While a pull system does tend to reduce the overall inventory, it also makes sure that there is enough of the right parts at the right time.

Employee Perceptions

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. It is important that you don’t just dismiss problems that team members bring up. Your job will be to make extensive use of the PDCA cycle to tear down any obstacles that come up.

Key Points About Push Systems

• Pull systems aren’t magic. It takes a lot of work to eliminate the problems that make push systems appealing. Don’t try switching to pull before the problems are reduced.
• Push systems have little communication between processes, limiting the ability to solve problems quickly.
• Push systems are most commonly run by schedules. Pull systems generally make use of kanban cards.

If you are transitioning from push to pull, start off by identifying a work area with relatively few problems and low risk to start learning about pull. Do a trial run to develop confidence in whatever methods you establish.

Use the trial run to identify problems, and then work on eliminating those issues before going live.

Consider creating a safety stockpile of products to draw from when the inevitable problems occur, but then make sure you consume them once the new system is fully established.

Focus on building systems before rolling pull out company wide. You’ll find that early pull systems need quite a bit of refinement. You’ll save time if you roll out a finished process rather than a beta version. You are better off creating a strong infrastructure before you remove the safety nets.