push vs pull

Topics: Kanban, Trigraph, Push–pull strategy Pages: 15 (1578 words) Published: April 20, 2014
Push and Pull Production Systems

You say yes.
I say no.
You say stop.
and I say go, go, go!
– The Beatles

© Wallace J. Hopp, Mark L. Spearman, 1996, 2000

1

http://factory-physics.com

The Key Difference Between Push and Pull
Push Systems: schedule work

Pull Systems: authorize work

releases based on demand.
• inherently due-date driven
• control release rate, observe WIP
level

© Wallace J. Hopp, Mark L. Spearman, 1996, 2000

releases based on system status.
• inherently rate driven
• control WIP level, observe
throughput

http://factory-physics.com

2

1

Push vs. Pull Mechanics
PUSH

PULL

(Exogenous)
Schedule

(Endogenous)
Status

Production Process

Job

Job

Push systems are inherently
make-to-order.
© Wallace J. Hopp, Mark L. Spearman, 1996, 2000

Production Process

Pull systems are inherently
make-to-stock.
3

http://factory-physics.com

Pulling with Kanban
Outbound
stockpoint

Production
cards

Completed parts with cards
enter outbound stockpoint.

When stock is
removed, place
production card
in hold box.

© Wallace J. Hopp, Mark L. Spearman, 1996, 2000

http://factory-physics.com

Outbound
stockpoint

Production
card authorizes
start of work.

4

2

Push and Pull Line Schematics
Pure Push (MRP)

Stock
Point

Stock
Point

...

Pure Pull (Kanban)

Stock
Point

Stock
Point

...


Stock
Point

CONWIP

Authorization Signals
© Wallace J. Hopp, Mark L. Spearman, 1996, 2000

Stock
Point

...

Full Containers

http://factory-physics.com

5

Push/Pull Interface
Eliminate: entire portion of cycle time by building to stock. Requirements:
• Level demand.
• Relatively few distinct parts.
• Relatively constant product mix.

Implementation:
• kanban
• late customization (postponement)

© Wallace J. Hopp, Mark L. Spearman, 1996, 2000

http://factory-physics.com

6

3

Example - Custom Taco Production Line
Push/Pull Interface
Pull

Push

Refrigerator

Cooking

Assembly

Packaging

Sales

Customer

© Wallace J. Hopp, Mark L. Spearman, 1996, 2000

7

http://factory-physics.com

Example - Quick Taco Production Line

Pull

Refrigerator

Cooking

Push/Pull Interface

Assembly

Packaging

Warming
Table

© Wallace J. Hopp, Mark L. Spearman, 1996, 2000

http://factory-physics.com

Push

Sales

Customer

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4

The Magic of Pull
Pulling Everywhere?
You don’t never make nothin’ and send it no place. Somebody has to come get it.

– Hall 1983
No! It’s the WIP Cap:
•Kanban – WIP cannot exceed number of cards
•“WIP explosions” are impossible

© Wallace J. Hopp, Mark L. Spearman, 1996, 2000

WIP

http://factory-physics.com

9

Advantages of Pull Systems
Low Unit Cost:

Good Customer Service:

• high throughput
• low inventory
• little rework

• short cycle times
• steady, predictable output stream

Flexibility:
High External Quality:
• high internal quality
• pressure for good quality
• promotion of good quality (e.g.,
defect detection)

© Wallace J. Hopp, Mark L. Spearman, 1996, 2000

• avoids committing jobs too early
• tolerates mix changes (within
limits)
• encourages floating capacity

http://factory-physics.com

10

5

Pull Benefits Achieved by WIP Cap
Reduces Manufacturing Costs:

Improves Quality:

• prevents WIP explosions
• reduces average WIP
• reduces engineering changes

Reduces Variability:

• pressure for higher quality
• improved defect detection
• improved communication

Maintains Flexibility:

• reduces cycle time variability
• pressure to reduce sources of
process time variability (e.g., long
repair times)
• promotes improved customer
service

© Wallace J. Hopp, Mark L. Spearman, 1996, 2000

• accommodates engineering
changes
• less direct congestion
• less reliance on forecasts
• air traffic control analogy

11...
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