Push and Pull Production Systems

Distinction between Push & Pull

• Fundamental feature of flow control system – mechanism that triggers the movement of work.

• MRP – trigger is actual and forecasted demand.

• Kanban system – trigger is inventory voids signaled by cards.

Distinction between Push & Pull

Production Process

Production Process

Exogenous (Stock Void)

Exogenous (Schedule)

Unlimited WIP

LimitedWIP

Definition of Pull

• A pull system establishes an a priori limit on the WIP, while a push system does not.

• Push system doesn’t have a feedback loop that communicates the WIP status.

• While in Kanban, the card limits the WIP.• Push-Pull Hybrid system.

Pull Systems

• Limit on the maximum inventory in the system.• This is WIP cap.• Reduced Manufacturing Costs.• Reducing Variability.• Improving Quality.• Maintaining Flexibility.• Facilitating Work Ahead.

Reduced Manufacturing Costs

• Push systems – WIP explosion happens when there is a disruption in the line.

• Pull systems – releases are choked off before the system gets overloaded.

• This provides flexibility to change engineering designs and scheduling priorities relatively easy.

Reducing Variability

• Cycle time variability• Kanban achieves less variations in the cycle time as

the variation in WIP is less.• Little’s law CT = WIP * TH• Analogy of 2 m/cs: M/c 1 feeding m/c 2 with m/c 2

as bottleneck. • If High levels of throughput with low WIP levels

(and short cycle times) needs to be achieved, then the disruptive sources of variability (failures, setups, recycle etc.) need to be reduced.

Improving Quality

• Low WIP allows the system to be more effective on quality.

• Inspection by downstream operator.• Push systems – High WIP makes the job of QA

inspector difficult.