When NOT to Pull!

Pull production is a highly useful tool in manufacturing, logistics, services, and other industries. However, there are instances where pull may be not the best option. These instances are rare, but they do exist. In this blog post I will list different cases when pull may not be the best option.

Recap: Pull Is a Limit on the WIP

Pull systems overall are very robust and stable, well suited for pretty much any production system. In fact, they can also be used outside normal industry (e.g., in healthcare, military, call centers, banking, data processing, and other service industries). See my post Why Pull Is So Great! for more. However, many people define pull incorrectly. For me, pull is a fixed limit on the number of parts or jobs in the system. I wrote about it extensively in my post on The (True) Difference Between Push and Pull.

Don’t Pull If You Have No Control on the Number of Parts Arriving

Pull limits the number of parts in your system. Hence, one key requirement for doing pull is to control the number of parts entering your system. If you can’t do that, then you simply can’t pull.

In manufacturing, you usually have control over the number of parts arriving. Parts arrive only when you explicitly order or produce them. Without a purchase or production order, you won’t get any parts. Thus, you can limit the maximum inventory simply by not ordering or producing more when you reach that limit.

A counterexample would be retail, a repair store, or a key-copying location. A typical store usually has no influence on when a customer shows up. They usually cannot really limit the number of customers in the store unless they want to get bad reviews. Instead, if multiple customers arrive, the customer has to wait. If very few customers arrive, the staff has to wait. There is no defined limit on the upper number of customers.

Of course, at one point you will hit a limit where you can’t physically fit any more customers in the store. But this is usually way past any sensible workload or even way past any fire regulations, although this happens annually on Black Friday.

Don’t Pull If It Is Very Expensive to Turn Off Your Process

Another case against pull is if it is very expensive or impossible to turn off the process. Even if the customer purchases less, you may have to produce and build up inventory to avoid the even larger expense of shutting down your process.

This may happen in blast furnaces for steel smelting. Conventional wisdom is to turn on the furnace and then run it for twenty years without a break, until the inner lining is getting too thin and needs to be replaced. Stopping and restarting this process is very expensive, and companies try to avoid this.

Another example may be oil platforms, where the main cost is setting up the platform and drilling a hole. Once the platform is running, it often makes economic sense to keep operating even if the demand and the oil price goes down.

Don’t Pull If You Can Outperform a Pull System

Finally, pull may be inferior if you have a very high level of control of your system and an excellent knowledge of upcoming fluctuations (i.e., you are close to all-knowing and all-seeing in your production system). Even then, pull would still work.

However, it is imaginable to have a push-type system that outperforms a pull system if you have such an excellent understanding of your system. A pull system reacts immediately whenever a part is consumed or completed. A push system controlled by humans, computer logic, or artificial intelligence has to be able to outperform pull systems, giving either a better availability (for make-to-stock) or better utilization (for make-to-order) for the same inventory, or having a lower inventory for the same availability or utilization, or a combination of those.

Hence, if the knowledge of the human, computer logic, or AI of future fluctuations and disruptions is exceptional, they may be able to push a production system better than pull production. Yet, I believe such companies are as rare as unicorns. While you can occasionally meet a manager who believes his organization is all-knowing (usually in the higher ranks far removed from the shop floor), it is not true.

Other than that, I am always hard-pressed to find good examples where pull is not superior to push. In literature, you can find many scientific articles that claim push is better in some cases, but if you dig deeper, you will find that they just did not understand what pull actually is. Pull is much better at handling uncertainty and faster at maintaining your inventory buffer. Hence, the only case where push may be better is a highly stable system where you have an excellent understanding of the processes or of the expected fluctuations.

The common approach is usually to let pull do the day-to-day control, and human workers adjust the pull system if they know of an upcoming fluctuation. These could be seasonal demand changes, or your ship with goods just sunk, or all your products failed the safety test and need to be remade, or there is a craze for your products just because Beyoncé said she loves them, or your products sit like lead on the shelves just because Beyoncé said she hates them, or … or … or … I am sure you are familiar with many such examples in your industry.

Summary

Overall, pull is, in my view, still the best solution for almost all cases, providing an easy-to-use system that requires little intervention except for the occasional updates. There are very few exceptions where pull is inferior, as mentioned above. Now, go out, see where you can pull and where not, and organize your industry!