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1、1,Why Kanban Systems Fail and What You Can Do About it Saifallah Benjaafar Center for Manufacturing Logistics Department of Mechanical Engineering University of Minnesota Minneapolis, MN 55455,2,Kanban,A “kanban” is a sign-board or card in Japanese and is the name of the flow control system develope
2、d by Toyota.,3,Outline,1. The JIT context 2. Basic mechanics 3. Advantages 4. Limitations 5. Alternatives to Kanban 6. CONWIP systems 7. PFB systems 8. MTO/MYS systems 9. Conclusions,4,Role,Kanban is a tool for realizing just-in-time. For this tool to work fairly well, the production process must be
3、 managed to flow as much as possible. This is really the basic condition. Other important conditions are leveling production as much as possible and always working in accordance with standard work methods. - Ohno 1988,5,Just-in -Time (JIT),A collection of principles aimed at reducing inventory, incr
4、easing throughput, and minimizing manufacturing lead times.,6,Origins of JIT,Japans attempt to become competitive with US and European manufacturers after WW II The Toyota Production System,7,JIT principles,Reduce setup times reduce batch sizes Reduce variability Reduce material handling Reduce defe
5、cts and rework Reduce breakdowns Increase capacity Smooth production schedules,8,JIT principles (cont.),Maintain constant WIP (limit WIP buffer sizes) Limit finished goods inventory and raw materials Synchronize operations within the factory Synchronize material delivery with suppliers and customers
6、 Improve worker flexibility and empower worker to make improvements Simplify workflow,9,The Seven Zeros,Zero (excess) lot sizes Zero setups Zero breakdowns Zero lead times Zero surging Zero material handling Zero defects,10,Kanban Systems,A shop floor control strategy aimed at (1) reducing inventory
7、, (2) simplifying workflow, (3) increasing throughput, (4) reducing cycle time, (5) improving customer lead times, and (5) improving quality.,11,Mechanics of Kanban,Push vs. Pull: Kanban is a “pull system” Push systems schedule releases Pull systems authorize releases Synchronous manufacturing: Pull
8、 signals ensure tight coupling between operations throughout the system,12,Mechanics of Kanban (cont.),One card systems Two card systems,13,Mechanics of Kanban (cont.),Work Center,Buffer,Card Flow,14,One-Card Kanban,15,Two-Card Kanban,16,MRP versus Kanban,Lover Level Inven-tory,MRP,Kanban,Kanban Sig
9、nals,Full Containers,Lover Level Inven-tory,Assem-bly,Assem-bly,17,Signaling,Cards Lights & sounds Electronic messages Automation,18,The Key Issue,How many Kanbans should we have at each stage of the process and for each product?,19,Tradeoffs,Too many Kanbans Too much WIP and long cycle times Too fe
10、w Kanbans Lower throughput and vulnerability to demand and process variability,20,Littles Law,Cycle time = WIP/Throughput WIP = (Cycle time)(Throughput),21,Example,Four identical tools in series. Each takes 2 hours per piece. No variability. Constant WIP.,22,The Penny Fab,23,Throughput and Cycle Tim
11、e vs. WIP,24,Throughput and Cycle Time vs. WIP (cont.),Cycle time (Hours),0 2 4 6 8 10 12 14,0,8,16,12,20,4,WIP (Jobs),25,The Case of an Unbalanced Line,The maximum feasible throughput rate is the processing rate of the bottleneck Critical WIP = (Bottleneck rate)(Total processing time),26,The Impact
12、 of Variability,Processing times are subject to variability, material handling is not instantaneous, processes are subject to breakdowns, demand is subject to fluctuation Longer cycle times and lower throughput,27,Number of Kanbans at Toyota,Number of cards = WIP Number of cards = D/QL(1 + a) D: Dem
13、and Q: Container L: Raw processing time a: safety (variability) factor,28,Guidelines for allocating Kanbans,Allocate Kanbans evenly in a balanced system Allocate more Kanbans to slower processes or processes with higher workloads Always protect the bottleneck,29,Guidelines for allocating Kanbans (co
14、nt.),Start with current averages Gradually reduce Kanbans at stations that are always full Increase Kanbans at stations that are always empty,30,Advantages of Kanban,Simplifies workflow Synchronizes manufacturing Reduces WIP accumulation at all processes stages Improves performance predictability an
15、d consistency Fosters communication between neighboring processes Reduces defects and enables 100% inspection,31,Advantages of Kanban (cont.),Encourages line balancing and process variability reduction Encourages focused and cellular manufacturing Y2K robust,32,Limitations of Kanban Systems,Ideal fo
16、r high volume and low variety manufacturing Vulnerable to fluctuations in demand volume and product mix Vulnerable to process variability and machine breakdowns Inefficient in handling infrequent orders or expediting special orders Vulnerability to raw material shortages and variability in supplier lead times,33,Alternatives to Kanban,Constant Work-in-Process (CONWIP) Pull from the Bottleneck (PFB) Hy
