What is pull – and where does it fit in Lean Thinking?
Womack and Jones (1996)1 identify Pull as one of the fundamental principles of lean thinking — in addition to Value, Value Stream, Flow, and Perfection. Pull “in its simplest terms means that no one upstream should produce a good or service until the customer downstream asks for it” (p. 67). This is in contrast to our long term mass production “Push” paradigm of producing parts to send downstream based on Master Production Schedules and MRP, while striving to keep the backlog of raw material/parts waiting for our process to a minimum (if its waiting to be processed, process it).
Pull, when implemented in the physical and informational relationship of process, is compatible with the Value Stream as defined in the Business Efficiency Consulting blog entitled “Supply Chain or Value Chain – What’s the Big Deal?” which includes a quotation from Feller, Shunk and Callarman (2006)2 “In common parlance, a supply chain and a value chain are complementary views of an extended enterprise with integrated business processes enabling the flows of products and services in one direction, and of value as represented by demand and cash flow in the other” (p. 4). In pull systems we physically manifest and embed that concept into the process – demand information as expressed by signals (kanban) of required product from downstream is passed up the value stream to initiate production of materials to flow downstream only as needed!
What are the advantages of Pull?
One of the advantages of using pull systems is that inventory is minimized through the use of kanban supermarkets where minimizing inventory levels between decoupled processes is the goal, though as noted by Liker and Meir (2006)3 it is preferable to use flow (by coupling processes – another lean principle) over kanban inventory points as indicated on the following graphic (p. 108):
[click on image for clear view]
Laksham (2006)4 provides an excellent description of the pull process and its value as follows (para. 7):
Because a series of well-timed interconnected loops develops between the preceding and succeeding processes, buffer inventory, usually maintained at tier-transaction points, can be substantially reduced. Material replenishment levels and continuous information flow provides visibility for suppliers and customers. Demand signals flow continuously through the supply chain and are delayed at each tier only as long as it takes to consume or ship material. Due to kanban’s high visibility and predictability, intermediate tiers and buffer stock in customer and manufacturer supply chains can be eliminated.
The end result of using a pull system is: Reduced cost, Reduced floor space requirement; Reduced inventory and WIP; Reduced rework and scrap; and Reduced waste, ensuring that we have what we need, when we need it, and in the amounts it is needed!
1Womack, J., Jones, D. (1996). Lean Thinking: Banish Waste and Create Wealth in Your Corporation. New York: Simon & Schuster.
2Feller, A., Shunk, D., & Callarman, T. (2006). Value Chains versus Supply Chains. BPTrends, 1-7. Retrieved from: http://www.bptrends.com/publicationfiles/03-06-ART-ValueChains-SupplyChains-Feller.pdf
3Liker, J., & Meier, D. (2006). The Toyota Way Fieldbook: A Practical Guide for Implementing Toyota’s 4Ps. New York: The McGraw-Hill Companies, Inc.
4Laksham, N, (2006). When Push Comes To Pull, Kanban Wins. Manufacturing.Net. Retrieved from: http://www.manufacturing.net/news/2006/07/when-push-comes-to-pull-kanban-wins