Pull System

In a pull system, the flow of production signals moves backward — from customer to shipping to final assembly to sub-assembly to fabrication to raw materials. When a customer buys a product, a replenishment signal flows upstream through each process step, authorizing production of exactly what was consumed. No upstream process produces anything until it receives a signal from its downstream customer. The signal can take many forms: a physical kanban card returned to the upstream process, an empty container sent back for refilling, an electronic message in a digital scheduling system, or even a visual signal like an empty square on the floor. The critical characteristic is that the signal originates from actual consumption, not from a forecast or schedule. This simple principle has profound implications: WIP is bounded (because only authorized work can exist in the system), overproduction is prevented (because production stops when demand stops), and lead time becomes predictable (because WIP limits directly determine lead time via Little's Law).

Pull systems come in several variants, each suited to different manufacturing environments. Replenishment pull (supermarket pull) maintains a small inventory buffer (supermarket) between processes, replenished by kanban signals when items are withdrawn. This is the most common type, ideal for repetitive, make-to-stock production. Sequential pull produces each item in a defined sequence without maintaining inventory between stages — FIFO lanes connect processes, and each downstream pull triggers the next upstream operation. This works well for make-to-order environments with short, predictable lead times. Mixed pull combines supermarket pull for common components with sequential pull for custom or variable elements. CONWIP (Constant Work-In-Progress) is a simplified pull system that limits total WIP in the system without managing individual part types — when any item exits the system, a new item is authorized to enter. CONWIP is easier to implement than part-specific kanban and works well for high-mix, low-volume production.

Pull systems fundamentally change how production is scheduled. In a traditional push environment, the scheduler creates a detailed plan for every workstation, and the schedule drives production. In a pull environment, the schedule typically exists only at a single point — the pacemaker process — and all other processes are controlled by pull signals. The pacemaker is usually the process closest to the customer that runs continuously (often final assembly or packaging). The scheduler creates the sequence and pace for the pacemaker, and pull signals propagate this sequence upstream. This dramatically simplifies scheduling: instead of creating and coordinating schedules for every workstation, the planner manages only the pacemaker schedule and trusts pull mechanisms to synchronize everything else. Tools like LinePlanner can serve as the pacemaker scheduling interface, defining what to produce on each production line at each shift, while kanban signals handle the internal flow of components between upstream operations.