Push vs. Pull: The Fundamental Choice
In a push system, production is driven by a forecast. MRP calculates what should be produced when, and releases work orders to the floor. The problem: forecasts are always wrong. When demand is lower than forecast, inventory accumulates. When demand is higher, you are short. And because work is released on schedule rather than on signal, WIP builds throughout the factory with no mechanism to limit it.
In a pull system, production is driven by actual consumption. When the downstream process uses a part, a signal goes upstream: “I consumed one — make another.” Nothing is produced without this signal. This creates a natural WIP limit: the total number of pull signals in the system equals the maximum WIP. Little’s Law guarantees that limiting WIP reduces lead time.
Three Types of Pull Signals
| Signal Type | How It Works | Best For |
|---|---|---|
| Kanban Card | A physical or electronic card that authorizes production of a specific part in a specific quantity. When the downstream process consumes a container, the card returns upstream as a production signal. | Repetitive parts with stable demand. Standard fasteners, brackets, common machined components. |
| FIFO Lane | A physical lane (rack, conveyor, or marked floor area) with a fixed capacity. Upstream produces until the lane is full, then stops. Downstream consumes from the lane. | Custom or variable parts flowing between two specific operations. No need to specify part number — just “next in sequence.” |
| CONWIP | A system-wide WIP limit. When one unit exits the entire system, one unit is authorized to enter. See CONWIP guide. | Job shops with high mix, make-to-order, or long-cycle assembly. Where part-level kanban is impractical due to variety. |
Kanban Sizing
Part: Aluminum bracket, used at 20/day on the assembly line.
Replenishment lead time: Machine shop takes 2 days to produce a batch after receiving the kanban signal.
Safety factor: 30% (machine shop has some variability).
Container size: 10 parts per bin.
Kanban formula: K = D × L × (1 + S) ÷ C
K = 20 × 2 × (1 + 0.30) ÷ 10 = 52 ÷ 10 = 5.2 → 6 kanban cards
Total inventory in system: 6 bins × 10 parts = 60 parts maximum.
How it works: The supermarket holds 6 bins. When the assembly line takes a bin, the kanban card goes back to the machine shop. The machine shop produces one bin of 10 and sends it to the supermarket. The system self-regulates: if demand drops, cards circulate slower; if demand rises, cards circulate faster.
💡 Start with More Kanban, Then Reduce
When first implementing kanban, err on the side of too many cards (more inventory). This provides a safety buffer while you learn the system. Then systematically remove cards one at a time. Each card you remove lowers inventory and tightens the system. When removing a card causes a stockout, you have found a problem — solve it (reduce lead time, improve reliability), then remove another card.
Supermarket Design
A kanban supermarket is the physical location where pull inventory is stored. It is not a warehouse — it is a designed, visual, managed buffer with specific rules:
| Rule | Why |
|---|---|
| Every part has a labeled, fixed location | No searching — anyone can find any part |
| Maximum quantity marked visually | Overproduction is immediately visible (bins overflowing their marked area) |
| FIFO within each lane | Oldest parts are consumed first — prevents shelf-life and revision issues |
| Located between the producing and consuming processes | Minimizes transport; the consuming process is the “customer” |
| Replenishment triggered only by kanban card/signal | Prevents push-driven overstock |
Integrating Pull with MRP
Pull systems and MRP are not mutually exclusive. In most aerospace factories, both coexist:
| MRP Does | Pull Does |
|---|---|
| Long-range capacity planning (months out) | Daily execution on the floor |
| Material procurement (supplier lead times) | Production release timing |
| Determines WHAT and HOW MANY to produce | Determines WHEN to release to the floor |
| Runs weekly or daily batches | Signals in real-time based on consumption |
MRP plans. Pull executes. MRP says “we need 200 brackets this month.” Pull says “don’t release more until the supermarket signals need.” The combination prevents the overproduction that pure MRP-push creates while maintaining the planning visibility that pure pull cannot provide for long-lead procurement.
🎯 The Bottom Line
Pull systems prevent overproduction by linking production directly to consumption. Kanban cards work for repetitive parts, FIFO lanes for sequential flow, and CONWIP for high-mix environments. Size kanban conservatively at first, then systematically tighten. Integrate pull with MRP: let MRP plan, let pull execute. The result is lower WIP, shorter lead time, and a self-regulating production system. Next: Heijunka — leveling the demand signal so pull systems do not get overwhelmed by volume and mix variation.
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