What is a Pull System?
A pull system essentially means requesting things as needed on the floor. This is the basis for the Kanban system. The theory is that you won’t build up excess inventory or work in progress (WIP) by only requesting things when you need them.
A pull system gives you more flexibility, helps you reduce the amount of work in progress, and helps reduce inventory, potentially.
The disadvantages are that it can be difficult to implement, and once it is implemented, can create a lot of chaos because it’s exposing other problems you have. The concept of chaos is detailed in The Toyota Way, and basically, if you use a pull system, you have to be willing to deal with those problems and fix them in order to get the benefits.
The biggest potential disadvantage is that it may not work for you in your particular environment. For example, if you’re a job shop, and everything is different every time, it makes it difficult to enact a pull system. What are you pulling? What are you requesting? These “pulls” can vary every single time.
What is a Push System?
A push system is more similar to what you’d see in an ERP system. This ERP looks at the demand and then tells you what you should make, when, how, and the date to be completed by. The schedule or the activities you need to do are being “pushed” down from an ERP to the production floor. It’s a very rigid process.
An advantage of the push system is that you are pretty much guaranteed to have product on hand to complete customer orders, though it does increase the probability of building up excess inventories if you don’t have good visibility into the existing inventory you have.
Another disadvantage of the push system is that it lacks flexibility because it’s planned in advance.
Work in Progress in Push vs. Pull Systems
The kicker is that you always have work in progress (WIP) in both, but how that works is different depending on if you’re using a push or pull system.
In a pull or Kanban system, you typically have buffers in between that place limits on work in progress so nothing is made unless there is demand. So, let’s say you have 10 coffee cup lids in a bin, and when you’re making coffee cups on the line, you use 5 of those lids. When you remove 5 from the bin, a pull system would trigger a work order to make 5 more lids so you refill the bin to 10.
You aren’t just running and running and running the line, producing things you may not need. Instead, you’re only producing more when there are no longer 10 coffee cups in the bin.
In a push system, there is no limit placed on WIP. You can produce as many coffee cup lids as you want, in preparation for a work order to come in the future. Maybe, a line operator has run out of things to do and they start making coffee cup lids even if there is no work order. It’s just to occupy their time. Or, there is an order, but the forecast may not be 100% accurate so an excess inventory of coffee cup lids is built up to compensate for the variability.
There are some instances where push manufacturing is needed, like in a job shop environment which we will discuss in greater detail below.
Is Lean Manufacturing a Push or Pull System?
Typically in Lean manufacturing, you try to implement a pull system. But, this goes back to the idea that it may not work in every case, and just because you don’t do a pull system doesn’t mean that you’re doing lean manufacturing. And vice versa. Just because you’re doing Lean manufacturing doesn’t mean you’re using a pull system.
The pull, the Kanban system, they’re merely just tools. It’s how you do it, how you implement it, how it works for you that is going to be unique to your situation. Again, this goes back to The Toyota Way. One of the reasons they’ve called their system The Toyota Production System is because it’s unique to their company, Toyota. Sure, they use a lot of elements of the different manufacturing systems like Lean, but Toyota implements each of those in their own fashion.
The system (or systems) you use should also utilize tools that will work for your company and how you need to do things.
Real-Life Example: Push vs. Pull Strategy in Grocery Stores
In order to think about the push vs. pull system in a way that most of us relate to, think about a grocery store. Any grocery store, whether it’s Walmart or your local neighborhood store down the street.
On the shelf of that grocery store, there are bags of Doritos. In the backroom, there is an extra stock of those Doritos. The distributor also carries stock of Doritos. The manufacturer makes the Doritos.
There are multiple parts to this equation as you can see. (And, no we aren’t endorsed by Doritos, but we can dream.)
Let’s say you’re browsing the shelves of your grocery store and pick up a couple of bags of Doritos for a party and proceed to the checkout line. A worker then notices the Doritos are running low and goes back to retrieve 5 more to restock the shelves. The worker used the pull system to add more stock to the shelves. But, eventually, the backroom will be become depleted to a certain point that they will need to get more Doritos from the distributor.
This is where we will demonstrate the use of both the push and pull systems to show how the grocery store will receive more Doritos.
With the pull system, the grocery store let’s the distributor know when they’ve run low and need more stock. The same applies to the distributor when they’ve run low and need to let the manufacturer know. Then, the manufacturer produces more and distributes it down the ladder.
In the push system, the grocery store gets a regular stock of Doritos every week, regardless if they need it or not. The distributor also does the same and every week receives a pallet of Doritos from the manufacturer.
Both systems can work, but there are two disadvantages to a push system. If the grocery store faces high demand, they aren’t able to keep Doritos in stock on the shelves and may not receive a new shipment for another 5 days. On the other hand, if they or the distributor faces very low demand, they run the risk of having too much inventory that isn’t being consumed and could expire without being sold.
In the case of the pull system, the grocery store is sending signals that more Doritos are needed in stock rather than relying on a schedule set by the manufacturer.
Push vs. Pull in Manufacturing: What Should I Be Using?
At the end of the day, what type of system you use, pull vs. push, all depends on what is going to fit your business.
Whatever system you choose, can you make it work for your company? If pull works for you, great. If it doesn’t, that’s ok, too. In fact, there may be an opportunity to use a push system in one area and a pull system in another.
What you’re trying to figure out is the right system for your company.
Again, a job shop is a great example to drive this point home. Let’s say you’re a job shop that is contracted to make an airline part for Southwest, but you only make 2 of those parts a year. Is a pull system really going to work for a part that you only make 2 of a year? Probably not. You’re going to want to use a push system in this case.
However, in your plant, you have a vending machine that provides tools, gloves, goggles, all kinds of things, to employees. When the vending machine provider comes in once a month, he refills the slots of the vending machine that are low. This reflects a pull system.
In your job shop, you’re using both systems, just for different processes.
So, think about what will work for your business. Do you need to use one or the other? Or can you use both a push or pull system in different areas of your plant?
At the end of the day, what will work for your company?