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CLT10: Flow, pull systems, and just-in-time

CLT10 · Work through your exam, one topic at a time.

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Flow, pull systems, and just-in-time

Flow, pull systems, and just-in-time (JIT) tests whether you can design material flow that responds to actual consumption, size buffers with logic, and avoid turning pull into a disguised push schedule.

Core rules

  • Continuous flow means one-piece flow where possible; when processes cannot be linked, use pull systems such as supermarkets and kanban, FIFO lanes, or CONWIP to cap WIP.
  • A kanban authorizes replenishment; it is not a production forecast. A two-bin system is a simple pull signal. The signal must be visible, controlled, and tied to a replenishment rule.
  • JIT means producing the right quantity at the right time, not zero inventory. Buffer inventory should be sized for replenishment lead time, demand variation, and safety, then reduced as capability improves.
  • Heijunka levels production by volume and mix so the pull system does not amplify demand swings. Takt sets the pace; line balance distributes work to takt.
  • Little's Law, lead time = WIP / throughput, explains why capping WIP shortens cycle time when throughput is stable.

Worked example

Demand is 800 units/day. Replenishment lead time is 0.5 day, and the team wants 10% safety. Demand during lead time = 800 × 0.5 = 400 units. Safety = 400 × 0.10 = 40 units. Total required = 440 units. With a container quantity of 50, kanban count = 440 / 50 = 8.8, rounded up to 9 kanban. The reasoning: round up because a fractional kanban cannot exist; then verify container size, lead time, and signal discipline. If lead time falls, recalculate rather than keeping nine cards because that is how we have always done it.

Traps

Do not use kanban as a schedule from planning. Do not size kanban with annual demand or ignore replenishment lead time. Do not add safety stock without a stated variation or risk basis. Do not claim flow is always possible; pull is a fallback where flow is not practical.

Key points

  • Pull authorizes replenishment from consumption; kanban is not a forecast.
  • Size kanban from demand during replenishment plus safety, divided by container quantity, rounded up.
  • JIT is right item, time, and quantity; not zero inventory.
  • Heijunka and WIP caps stabilize pull.