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
1
Kanban
A “kanban” is a sign-board or card in Japanese and is
the name of the flow control system developed by Toyota.
2
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
3
Role
Kanban is a tool for realizing just-in-time. For
this tool to work fairly well, the production
process must be 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
4
Just-in -Time (JIT)
A collection of principles aimed at reducing inventory,
increasing throughput, and minimizing manufacturing
lead times.
5
Origins of JIT
•
Japan’s attempt to become competitive with US and
European manufacturers after WW II
•
The Toyota Production System
6
JIT principles
•
Reduce setup times
•
reduce batch sizes
•
Reduce variability
•
Reduce material handling
•
Reduce defects and rework
•
Reduce breakdowns
•
Increase capacity
•
Smooth production schedules
7
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
•
Improve worker flexibility and empower worker to make
improvements
•
Simplify workflow
8
The Seven Zeros
•
•
•
•
•
•
•
Zero (excess) lot sizes
Zero setups
Zero breakdowns
Zero lead times
Zero surging
Zero material handling
Zero defects
9
Kanban Systems
A shop floor control strategy aimed at (1) reducing
inventory, (2) simplifying workflow, (3) increasing
throughput, (4) reducing cycle time, (5) improving
customer lead times, and (5) improving quality.
10
Mechanics of Kanban
Push vs. Pull: Kanban is a “pull system”
• Push systems schedule releases
• Pull systems authorize releases
Synchronous manufacturing: Pull signals ensure tight
coupling between operations throughout the system
11
Mechanics of Kanban (cont.)
• One card systems
• Two card systems
12
Mechanics of Kanban (cont.)
Work Center
Buffer
Card Flow
13
One-Card Kanban
Outbound
stockpoint
Production
cards
Completed parts with cards
enter outbound stockpoint.
When stock is
removed, place
production card
in hold box.
Outbound
stockpoint
Production
card authorizes
start of work.
14
Two-Card Kanban
Inbound
stockpoint
Outbound
stockpoint
Move stock to
inbound stock point.
Move card
authorizes
pickup of parts.
When stock is
Remove move
removed, place
card and place
production card
in hold box.
Production
Production in hold box.
Move
card authorizes
cards
cards
start of work.
15
MRP versus Kanban
MRP
Lover
Level
Inventory
…
Assembly
…
Assembly
Kanban
Lover
Level
Inventory
…
Kanban Signals
Full Containers
16
Signaling
• Cards
• Lights & sounds
• Electronic messages
• Automation
17
The Key Issue
How many Kanbans should we have at each stage of the
process and for each product?
18
Tradeoffs
• Too many Kanbans
cycle times
Too much WIP and long
• Too few Kanbans
Lower throughput and
vulnerability to demand and process variability
19
Little’s Law
Cycle time = WIP/Throughput
WIP = (Cycle time)(Throughput)
20
Example
• Four identical tools in series.
• Each takes 2 hours per piece.
• No variability.
• Constant WIP.
21
The Penny Fab
WIP
1
2
3
4
5
6
7
8
9
TH
CT
TH x CT
22
Throughput and Cycle Time vs. WIP
Throughput (Jobs/hr)
.5
.4
.3
.2
.1
0
0
2
4
6
8
10
12
14
WIP (Jobs)
23
Throughput and Cycle Time vs. WIP (cont.)
Cycle time (Hours)
20
16
12
8
4
0
0
2
4
6
8
10
12
14
WIP (Jobs)
24
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)
25
The Impact 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
26
Number of Kanbans at Toyota
• Number of cards = WIP
• Number of cards = D/QL(1 + a)
• D: Demand
• Q: Container
• L: Raw processing time
• a: safety (variability) factor
27
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
28
Guidelines for allocating Kanbans (cont.)
• Start with current averages
• Gradually reduce Kanbans at stations that are always full
• Increase Kanbans at stations that are always empty
29
Advantages of Kanban
• Simplifies workflow
• Synchronizes manufacturing
• Reduces WIP accumulation at all processes stages
• Improves performance predictability and consistency
• Fosters communication between neighboring processes
• Reduces defects and enables 100% inspection
30
Advantages of Kanban (cont.)
• Encourages line balancing and process variability reduction
• Encourages focused and cellular manufacturing
• Y2K robust
31
Limitations of Kanban Systems
• Ideal for 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
32
Alternatives to Kanban
• Constant Work-in-Process (CONWIP)
• Pull from the Bottleneck (PFB)
• Hybrid Make-to-stock/Make-to-order system
33
CONWIP
CONWIP Cards
Production Line
Inbound
Stock
Outbound
Stock
34
Basic Configurations
•
Constant work-in-process
•
Input/output control
•
Asynchronous operation
•
Dispatching list
•
Example: Flow lines
35
CONWIP Control
Dispatching list
DD PN Quant
–—
–—
–––––
–—
–—
–––––
–—
–—
–––––
–—
–—
–––––
–—
–—
–––––
–—
–—
–––––
–—
–—
–––––
–—
–—
–––––
–—
–—
–––––
–—
–—
–––––
–—
–—
–––––
–—
–—
–––––
–—
–—
–— –— –––––
–––––
LAN
R G
PC
PC
...
Work Centers
36
CONWIP Configurations
Basic CONWIP
Multi-Loop CONWIP
Kanban
Work Center
Buffer
Card Flow
37
CONWIP Mechanics
•
A new job is introduced whenever one completes
•
The next job is selected from a dispatching list based on
current demand
•
The mix of jobs is not fixed
•
Priorities can be assigned to jobs in the dispatching list
•
WIP level can be dynamically reduced
38
Advantages of CONWIP-based Control
• Accommodates multiple products and low production volumes
• Allows expediting and infrequent orders
• Less vulnerable to demand and process variability
• Less vulnerable to breakdowns
• Protects throughput and prevents bottleneck starvation
• Simpler to implement and manage
39
Coupled and Uncoupled CONWIP Loops
Bottleneck
CONWIP Loop
CONWIP Card
Buffer
Material Flow
Job
Card Flow
40
Splitting Loops at Shared Resource
Routing A
Routing A
Routing B
Routing B
CONWIP Loop
Card Flow
Buffer
Material Flow
41
Modifications of Basic CONWIP
Multiple Product Families:
• Capacity-adjusted WIP
• CONWIP Controller
• Running a card deficit
Assembly Systems:
• CONWIP achieves synchronization naturally
• WIP levels must be sensitive to “length” of fabrication lines
42
Card Deficits
Jobs without Cards
Jobs with Cards
B
Bottleneck Process
Failed Machine
43
CONWIP Assembly
Processing Times
for Line A
2
1
4
1
Processing Times
for Line B
3
Buffer
3
2
Card Flow
3
Assembly
Material Flow
44
Pull From the Bottleneck
Problems with CONWIP/Kanban:
• Bottleneck starvation due to downstream failures
• Premature releases due to CONWIP requirements
PFB Remedies:
• PFB ignores WIP downstream of bottleneck
• PFB launches orders when bottleneck can accommodate them
PFB Problem:
• Floating bottlenecks
45
Simple Pull From the Bottleneck
B
Material Flow
Card Flow
46
Make-to-order/Make-to-stock Configurations
• Build components and subassemblies to stock
• Build final assemblies to order
• Pull system for MTS
• Push System for MTO
47
Conclusion
• There are Pros and Cons to everything
• Pull systems are one piece of the puzzle
• Change throughout the organization is essential
• An integrated supply chain strategy is critical
48
References and Additional Reading
•
Factory Physics, W. J. Hopp and M. Spearman, Irwin,
1996 (and teaching notes)
•
The Race, E. M. Goldratt and R. E. Fox, North-River
Press, 1986
•
QRM Revisited: Don't Push or Pull - POLCA, APICS
Magazine, Vol. 8, No. 11, 1998.
49
Additional Resources at U of M
•
Industrial Engineering Graduate Program
•
Master of Science in Manufacturing Systems
•
Graduate Student Internships
•
Center for Manufacturing Logistics
50