There are many approaches and methods that focus on improving flow. Material Requirements Planning (MRP), Drum-Buffer-Rope (DBR) or Simplified-DBR (SDBR), Demand-Driven MRP (DDMRP), Quick Response Manufacturing (QRM) and of course Lean’s comprehensive Just-in-Time (JIT) approach share the same objective. In comparing these approaches, however, discussions often concentrate on their inner workings. But they often lack an operationalization of the concept of “flow” which they intend to improve. This series of three post tries to develop two measures – flow velocity and flow smoothness – as the way to evaluate the level of and the progress towards flow.



What is Flow?

We all have an intuitive understanding about the concept of flow. Flow is commonly defined as to move steadily and continuously in a current or stream. This definition involves multiple other concepts like to move, a current and a stream as well as steady and continuous. Let’s first explore these basic terms that define flow. To move is about changing position, place or state in a specified, particular direction. It implies progress and advancement. A current is an ordered, directional movement. Similar to a current, a stream is to continuously run into a specified direction. Steady means it is regular and even, with a constant unvarying frequency or intensity. Continuous indicates it is without interruption.

The picture of flow that emerges in our context is one of products that advance towards their completion and value-added use by a customer or consumer in an uninterrupted way and with a constant, unvarying speed. I prefer advance instead of move as moving product around in itself does not necessarily imply progress. Advancing or progressing implies the product is coming closer to the state in which it actually provides value for the customer. As seen in the common definition already, flow is in a particular and specified direction, otherwise it cannot be called flow.

The Flow of Work, not Product

With respect to ‘direction’, it is also interesting to highlight what Toyota sees as flow. Toyota specifically speaks of “the flow of work”, not necessarily “the flow of product”. If a conveyor belt is used to transport things, and no further improvement is made, then work is said to be only “floating” and not “flowing” (Lu, 1985). As Shingo also wrote, it is work that actually advances a product and adds value while merely moving quickly and efficiently may not accomplish anything (Shingo, 1981).

Furthermore, definitions of flow also often indicate that (for perfect flow), progress should be effortless and free. Toyota itself sees effortless flow as a core element of its famed Toyota Production System. It employs the words “smooth” and “continuous” when it states that “the Toyota Production System provides for arranging all the processes from raw materials to finished products in a single, smooth flow” and that “the Toyota Production System works by establishing a smooth, continuous flow through the entire production sequence” (TMC OMCD, 1998).

Obstacles to Flow

In order for an electric current to flow from one point to another (with a voltage difference), the two points need to be connected to form a circuit and resistance needs to be limited. When the circuit is interrupted or when the resistance is sufficiently high, there will be no flow or at least it will be severely constrained. This is also the case for a water flow. A floodgate or a dam will interrupt the flow of water in a river, and the more friction a river has against its banks and its bed, the more it will slow down.

These laws also apply to the flow of products. When the customer of an operation does not require the operation’s product right now for whatever reason, flow will be interrupted. When the downstream operation is slower than the preceding one, flow will be slowed down. And whenever we see obstacles in the way of creating an uninterrupted flow of product, most often we tend to try and avoid these by batching. In that way we will have, for instance, less material loss during startup, less machine time loss as a result of changeovers and also less manual effort required. Or we will be able to have full truck or container loads to reduce the otherwise inefficient use of transport space. And when we are not willing to invest a lot of time and money in overcoming these obstacles for each and every individual product (comparable to a creating and maintaining a high voltage difference), these obstacles (like circuit breakers or resistance in an electrical circuit) will decelerate or even temporarily interrupt the flow. Unless, of course, we can eliminate these obstacles, reduce the friction, and maintain a certain flow with less effort. This is also the main point in the thinking of Toyota is that if the production flow is smooth then products low in cost and high in quality will emerge naturally (Ohno and Mito, 1986).

The above makes clear that for perfect flow to exist, there should be no interruptions and no obstacles. Products should advance continuously, steadily, smoothly and effortlessly towards the state in which they actually provide value for the customer.

So, how can we determine a certain level of flow, or measure our progress towards better flow? What should be the check item for flow?

The Problem of Flow Rate as a Measure

Typically, in physics, flow is measured by using so called volumetric or mass flow rates, defined as the volume that passes through a specific surface during a given period.

In an industrial context, typical flow rate (also referred to as “running rate”) examples are the number of pieces (pcs), or the amount of metric tons (mt), cubic meters (m3) or hectoliters (hl) produced or delivered per day, week, month or year. Typically, we refer to these types of flow rate indicators as the throughput rate.

Throughput rate indicators are often used in organizations. But as may already be clear from the description of perfect flow, they do not necessarily indicate whether we are actually moving towards perfect flow. A flow rate of 10,000 pcs/day or 1,000 mt/day based upon the actuals over the last 10 days may come from producing 10,000 pcs or 1,000 mt of the specific product in one singular day or from producing 1,000 pcs or 10 mt per day during each of the ten days. The latter obviously is far closer to true flow as defined before. And when moving towards the single unit level — so speaking of 1 piece every 25 seconds or 1 mt every 42 minutes instead of 1,000 pcs or 10 mt per day — we would really be approaching a level where we can almost see individual units of product flowing continuously and steadily through our value stream.

Being able to produce 10,000 pcs or 1,000 mt a day does not necessarily imply a better flow. Flow is not about peak capacity or even average flow rate. It is also not about how many units can pass a certain point in the flow in a given period. Flow is about every single unit of product progressing continuously and steadily with the same speed.

Therefore, we will need another check item than flow rate to be able to determine the level of flow that exists in a value stream. A check item that will enable us to set objectives and to check upon our progress against those objectives on the road towards perfect flow.

As a first check item, I will therefore introduce the concept of ‘flow velocity’ in the next and second post of this series of three posts.