At this time I would like to delve a little deeper into the use of Ohno’s river system to model the inner workings of Lean/TPS. I will start by going back to the TPS House.
(If you have not already done so, I suggest you read the three part “What is Lean” series at the beginning of this website).
Here is the basic TPS House once again. Familiarize yourself with the various components of the roof, pillars, floor and base. I think we will discover that these various components fit very well within a river system model.
Well, to start off, you have to admit that a river system is driven by a “pull system” – the pull of the gravitational field on the mass of the river water. And this pull, of course, will result in a “continuous flow” of water all along the river path. These two components of the JIT pillar fit very nicely within our river model.
And this “continuous flow” will sustain itself – unless “abnormalities” in the river path disrupt that flow. And this flow disruption would be very visible within the river structure – i.e., jidoka. And these “abnormalities” would constitute some form of muri and/or mura (muda/waste) which must be eliminated to restore “continuous flow” – i.e., kaizen. So the jidoka and kaizen components are important to the maintenance of a healthy river system.
It is probably important to stop here for a moment and reflect on the relationship between the “pull system” and “continuous flow”. Which comes first? Which is most important? There is frequently a lot of confusion around this point when trying to implement a Lean system.
What happens in an actual river system when, say, a boulder falls into the river’s path? The flow is restricted and flow to the downstream portion of the river is inhibited, or halted completely (waiting/mura). And upstream of the boulder a lake is formed (inventory/mura) or the river is diverted to a new route (transportation/over processing/muri). The pull system is still pulling but the flow is not flowing. Muda has been inserted into the system. The pull system is not working.
Ohno discussed this in his book Toyota Production System:
“Kanban [pull system] is a tool for realizing just-in-time. For this tool to work fairly well, the production process must be managed to flow as much as possible. This is really the basic condition. Other important conditions are leveling production as much as possible and always working in accordance with standard work methods.” [Emphasis mine]
“Flow” is the “basic condition” for a “pull system” to work. Therefore the first priority is to tackle the muda that exists in the river system to allow flow to occur. Jidoka is the mechanism that alerts us that flow is not flowing and kaizen is the method to find and rid ourselves of the muda that is inhibiting flow. Only when this is under reasonable control will a “pull system” work (i.e., just-in-time).
Now back to the river system and the TPS house. Let’s look again at the opening remarks in the first quote by Ohno in the previous post (Part 1):
“I had this idea of a fast, even-flowing river in which there were no dams that slowed the flow or rapids that sped it up.”
His reference to “even-flowing” obviously refers to “heijunka”. And the fact that he doesn’t want the river to slow down or speed up would imply “takt time”. And since “takt time” sets the river flow rate, his reference to “fast” must mean “shortest lead time”.
At this point our river system model has done a good job of encompassing most of the components of the roof, the two pillars and floor of the TPS House. All that is missing is “standardized work” (I will ignore human and machine work in this analysis). But “standardized work” defines the flow path of the river. If the river constantly changes course, you will eventually get lost. “Standardized work” and “heijunka” create an even, predictable flow.
And “standardized work” and “kaizen” are the means by which we will remove the “abnormalities” that may alter or interrupt the river flow path and reinstate “continuous flow”. Jidoka identifies structural defects – it notifies us when flow is disrupted. Kaizen redesigns the structure and the new structure is “standardized” to sustain the improvements – to sustain the “continuous flow” that allows us to achieve the “highest quality, lowest cost and shortest lead time”.
We build “stability” into the river system structure with these ongoing, continuous actions.
Thus, all aspects of the TPS House can be illustrated in our river system. I will cover “lowest cost” in more detail in the next post where I construct a cost analysis around the river system.
I don’t know about you, but I think this river thing just might just work out pretty dam well (excuse the pun).