Replenishment Pull

Replenishment pull is the most widely used flavor of pull in lean manufacturing, and the one most people picture when they hear the word kanban. The mechanism is simple: stock a small inventory between two processes, refill it when the downstream takes some, repeat forever. The discipline is in the signal mechanism, the sizing, and the rule that only consumption triggers production. Get those right and replenishment pull runs itself for years.

"Take what you need. The supplier refills what got taken. No schedule. Just refill."

How replenishment pull works

The setup centers on a supermarket between two processes. The supermarket holds sized inventory of each variant in labeled bins, each bin with a kanban card or other signal attached. The consumer process (downstream) takes bins from the supermarket as needed. Each bin consumed generates a signal (the card detaches, an empty bin returns) that travels back to the supplier process (upstream).

The supplier produces refills based on the signals coming back. The signal is the order to produce; nothing else triggers production. The supplier makes one bin's worth, attaches the card to the refilled bin, and sends it to the supermarket. The card stays with the bin until the next time the consumer takes it. Then the cycle repeats. Every card is always in one of three states: traveling back to the supplier as a signal, attached to a bin in the supplier queue waiting to be produced, or attached to a stocked bin in the supermarket.

Sizing the supermarket is sizing each bin and deciding how many bins per variant. Bin size is consumption rate during supplier lead time plus a defined cushion. Number of bins is set so the supermarket never stocks out under normal demand variation. The math is straightforward and the result has to be revisited periodically as consumption and supplier responsiveness drift.

Replenishment pull only works for parts that justify stocking. The right candidates are runners and repeaters in runner-repeater-stranger classification. Strangers do not belong in a supermarket; they would just create dead stock. For strangers and one-offs, sequenced pull is the right mechanism instead. Most shops run both forms of pull in parallel: replenishment for the steady-demand variants, sequenced for the rest.

Where replenishment pull fits on the shop floor

Picture a small contract manufacturer running 30 part variants for two customers. Twelve of the variants are clearly runners: ordered weekly, predictable volume. The shop runs an MRP-driven schedule for everything, which gets rerun weekly when orders shift. Lead time on the runners is about two weeks, mostly queue.

The shop converts the twelve runners to replenishment pull. Each runner gets a supermarket of three bins (sized to two days of consumption each) between the production cell and the shipping area. Each bin has a kanban card. When shipping takes a bin to fulfill an order, the card returns to the cell via a daily mizusumashi route. The cell produces a refill batch and sends the bin back to the supermarket. The MRP schedule no longer covers these twelve variants; the cell runs entirely from cards.

Within a quarter, lead time on the twelve runners drops from two weeks to two days. Total inventory of those variants is actually lower than before, because the supermarket is sized rather than padded. The remaining eighteen variants (mostly repeaters and strangers) continue on the MRP schedule, but with less competing for attention because the runners no longer need expediting. The shop runs two pull mechanisms in parallel: replenishment for the twelve, traditional scheduling for the rest, until repeaters and strangers eventually get their own treatment.

Common mistakes with replenishment pull

• Stocking strangers. Replenishment pull is for runners and repeaters. Strangers should run on sequenced pull, not in a supermarket.
• Sizing bins by feel. Each bin should be sized from consumption rate, supplier lead time, and a defined cushion. Anything else is a stockroom.
• Losing signals. A card or empty bin that does not return is a refill that never happens. Define an explicit return mechanism.
• Keeping MRP scheduling alongside. Hybrid schedules drift back to MRP under pressure. If a part is on replenishment pull, the cards are the only schedule.
• Never revisiting sizing. Consumption shifts and supplier lead times drift. Bin sizes should be checked quarterly and adjusted.

Replenishment pull and related Lean tools

Replenishment pull is one of the two main forms of pull system; the other is sequenced pull. It uses supermarkets as the stocked store and kanban cards as the refill signal. The simplest physical implementation is a two-bin system, where the empty bin itself is the signal and no separate card is needed. Replenishment pull is best applied to runners and repeaters identified through runner-repeater-stranger classification.