Production levelling
with heijunka

Once upon a time

In Ohno's (TOYOTA Production System thinker) teaching, there is the tale of "the slower but consistent tortoise causes less waste and is much more desirable than the speedy hare that races ahead and then stops occasionally to doze.

This tale is an illustration of the leveling principle in which the workload is leveled for the sake of continuity (consistency of tortoise's pace) regardless to orders variations (the dashing of the hare).

Reality

Customer's orders may be considered relatively constant when looked upon a long time period. (averaging effect), or erratic and unpredictable if considered in a shorter period and apart from any negotiated program.

• In the first case, production variations occur because of process itself (planning, scheduling, batch size policy, breakdowns and stoppages, opportunistic choices...)

• In the second case, it is a literal application of "one piece flow" or batch size reduction to size = 1 driving industrials to try to stick instantly to market requirements, hence having to cope with all the variability of orders.

However, efficient work supposes:

• constant flow
• rhythm
• "standardized" work (regular and invariant)

In order to produce with less waste (overtime, inventories, express supplies and shipments, rework, stress...) while satisfying market demand.
Precisely what is sought through production leveling.

Principle

Production levelling is based on orders analysis (products and volumes mix) in a given time span (one month, for example) in order to find out a pattern fitting in a smaller time span (daily, for example). This pattern is to be repeated until the whole demand (of the month) is satisfied.