The old model of searching for root causes relied on checking the time when a Holdup/Idle state began and then checking another machine if it was not in the Failure/Lack of Components/Stopped by Operator state in the same period of time, regardless of the moment in which that state began.
All the other states are “transparent”. There is no “bouncing” the other way: if we are looking for the cause of a holdup, we never look at the machines before the Base Availability machine even if one of the downstream machines is in the Idle state.
If there was no breakdown at this point of time, the Holdup state would not find any root cause.
Setting the buffer time allowed you to “target” a different spot. The buffer time was actually the “reaction time”. If your machines always work with transporters that are full or they work synchronously, the buffer time equals 0s despite the fact that it takes some time for the product to be moved between machines.
Upon detecting Holdup/Idle state, we appropriately move in time before we start looking for a cause in the next machine:
This is particularly important when modules stop and start moving again as buffer times help spot actual causes of stopping:
On the other hand, it may also create a problem if a state is slightly misaligned in relation to a specific buffer. The farther from the source, the bigger the probability of “mistargeting” if the specified buffer time between machines is different from what it actually is (for example, that time can be greater due to slower work or shorter because of the bigger dimensions of products).
In order to take into account full buffers / shorter reaction times, you take into consideration the entire period starting from the maximum reaction time (x) and ending in the moment when a stoppage began - or even a while after the stoppage started ( y ) which would let you spot a situation when a driver did not “report” a breakdown as fast as it should have:
The event which was the first one in this period of time takes precedence:
In a string of many machines, it is vital to observe the relative delay of stoppages from the beginning of the observed reaction time (marked in pink +1s/+5s). Using that relative time, it is possible to indicate the stoppage that began first as the root cause because a stoppage that occurred relatively later could be a consequence of the problem rather than its cause.
The time that has passed starting from the set “reaction time” is more important than the order of machines: