Line Balancing and TOC

Topics Part 1 – Line Balancing1)Line Balancing concepts and definitions2)Line Balancing Procedure3)Line Balancing Heuristics

Line Balancing• Applies to Product Assembly Layout• Primary focus is on the analysis of production

lines• The goal of the production line analysis is to:

– Determine how many workstations to have– Determine which tasks to assign to which workstation– Minimize the number of workers & machines used– Provide the required amount of capacity

• Line balancing is a key part of the analysis

Line Balancing - Workstation

• Workstation: Where work is performed on a product by adding a part or completing an assembly

• Work: Tasks performed at the workstation• Workstation Cycle Time/ Takt time: Time spent at a

workstation to complete the work (also the time between successive parts coming off the assembly line)

• Precedence Relationship: Order in which tasks must be performed in the assembly process

• Assigned Work at Workstation: No. of tasks at the workstation such that the sum of the time of assigned tasks is less than or equal to cycle time

Line Balancing ProcedureProcedure

1. Determine the tasks involved in completing 1 unit2. Determine the order in which tasks must be done3. Draw a precedence diagram4. Estimate total task times (T = Sum of the times of all

the tasks5. Calculate the cycle time (C = Production time per day/

Required output in units per day)6. Calculate the theoretical minimum number of

workstations (Nt= T/C)7. Use heuristics to assign tasks to workstations. This

may increase actual Workstations (Na)8. Efficiency = T/(Na * C)

Line Balancing Heuristics

• Heuristic methods, based on simple short-cut rules, have been developed to provide good (not optimal) solutions to line balancing problems

• Heuristic methods include:– Incremental utilization (IU) method– Longest-task-time (LTT) method– … and many others

Incremental Utilization Method

• Add tasks to a workstation in order of task precedence one at a time until utilization is 100% or is observed to fall

• Then the above procedure is repeated at the next workstation for the remaining tasks

• Pro – Appropriate when one or more task times is equal to the cycle time (NOTE: If any task time exceeds cycle time, it is a bottleneck)

• Con – Might create the need for extra equipment

Longest-Task-Time Method

• Adds tasks to a workstation one at a time in the order of task precedence.

• If two or more tasks tie for order of precedence, the one with the longest task time is added first

Theory of Constraints

Topics Part 2 – TOC1)Concepts2)System Links3)Variability4)Kanban5)DBR

The Reality

WIP 3 0 10 8 4 Total 25

RM

FG

Process A B C D E

CapabilityParts 7 9 5 8 6per Day

MarketRequest

5

CONSTRAINT – C with capacity of 5

WHAT IS TOC?A. TOC is a management philosophy that

emphasizes constraint identification and management as the keys for achieving an organization’s goals.

B. GOALS, SYSTEMS, CONSTRAINTS, FLUCTUATIONS “Goal” – Make Money

“Systems” – Interconnected Activities to achieve the Goal

“Constraints” – Limiting activities of a System “Fluctuations” – Unreliability of ActivitiesC. NOTE: A constraint becomes a bottleneck when

demand exceeds its capacity

WHAT IS TOC? (Cont’d)

The Chain Analogy

System – interconnected linksConstraint - The weakest linkThe Constraint determines the Strength of the

Chain (the output of the line)

BUSINESS Input Output

Input MATLS Output Input LOGIST OutputInput MFG Output

Understanding The System

Input SUPPLY CHAIN Output

Systems Concepts• Organizations / Systems exist for a purpose

• That purpose is better achieved by cooperation of multiple, independent elements linked together

• Each Inter-linked event depends in some detail upon the other links.

• An event not linked to the purpose is not a part of the System and hence a waste

• The system owner determines purpose which is adopted by the sub-systems

What gives rise to the weakest link

• Different link capabilities, normal variation and changing workload make it impossible to balance everything.

• One element of the system is more limited than another.

• When the whole system is dependent upon the cooperation of all elements, the weakest link determines the strength of the chain.

• Is there a chain with equal strength links?100

The “Chain” in Reality

• A simple chain over-simplifies reality• A Real Chain Supplier (1) Pur (2) Stores (3) MRP (4) Mfg (5) Dispatch (6)

Warehouse (7) Retail (8) Customer (9)

• Link 1 has a relationship (inv) with Link 5 • Link 5 has a different relationship (qual) with 1

1 2 3 4 5 6 7 8 9

Traditional Approach:Divide and Conquer

• Division of Labor breaks down linkages complex systems into manageable chunks.

• Which is harder to manage? Left or Right?• Which is the correct one to manage? Left or Right?

Left Right

CustQual

Inv Mfg

Sch DemInv

Mfg

QualRej

WHseR&D

Cust

The Issue of Local Efficiency/Optima

• Work flows from left to right through processes with capacity shown.

Process A B C D E

RM

FG

CapabilityParts 7 9 5 8 6per Day

Excellent Efficiency--Near 100%Chronic Complainer

Too Much Overtime

MarketRequest

i) 11

ii) 5

iii) 4

Long Term Reality...

Process A B C D E

PotentialP/D 7 9 5 8 6

Reality 5 5 5 5 5

• Processes A and B won’t produce more than Process C for long. Processes D and E cannot produce more than Process C for a long time

RM

FG

Then Variability Sets In

• Processing times are just AVERAGE Estimates

Process A B C D E

Variability Reality 5+/-2 5+/-2 5+/-2 5+/-2 5+/-2

RM

FG

What’s an Average? 50%

Process A B C D E

Reality 5+/-2 5+/-2 5+/-2 5+/-2 5+/-2Probability 0.5 0.5 0.5 0.5 0.5

• Half the time there are 5 or more per day at each process--Half the time less

Two at a time: 0.25 0.25

Over all: 3% Chance of 5 per day

RM

FG

Previous Solution: Inventory

WIP 5 5 5 5 5 Total 25Process A B C D E

Variable 5+/-2 5+/-2 5+/-2 5+/-2 5+/-2ProcessInventory (WIP) quickly shifts position.Inventory manager/expediter tries to smooth it out.Distribution problems result.Costs go up.

• Put a day of inventory (WIP) at each process!

RM

FG

System Variability Takes Over--Chaos

WIP 3 0 10 8 4 Total 25

Variable 5+/-2 5+/-2 5+/-2 5+/-2 5+/-2ProcessShifting work-in-process creates large queues at somelocations. This makes work wait longer to be processed.Other workstations can be starved for work. The work theycould be doing is delayed because it is not there. They can’t take advantage of their extra capability. So...

RM

FG

Process A B C D E

System Variability Takes Over--Chaos

WIP 3 5 10 8 4 Total 25

Variable 5+/-2 5+/-2 5+/-2 5+/-2 5+/-2ProcessSo… Management Helps! Management puts in more work(More Inventory) to give everyone something to do!

RM

FG

Process A B C D E

30

Attempts to Control WIP

WIP 5 5 5 5 5 Total 25Process A B C D E

Variable 5+/-2 5+/-2 5+/-2 5+/-2 5+/-2ProcessJust-In-Time uses Kanban Cards to limit the queuesbuilding in the system. No more than 5 parts are allowed at any station.Looks good, but is it?

• Put a lid on it-Use Kanban Cards-JIT

RM

FG

Effects of Inventory Limits on Production – Excess Capacity

WIP 5 5 5 5 5 Total 25Process A B C D E

Variable 5+/-2 5+/-2 5+/-2 5+/-2 5+/-2Process

• What does a Kanban card of 5 Mean?

RM

FG

5+/-2Average = 5

BeforeKanban

5+/-2Average = 3.5

Can’t exceed

5

AfterKanban

TOC: 5 Focus Steps to

Continuous Improvement

Step 0. Identify the Goal of the System/OrganizationStep 1. IdentifyIdentify the system’s constraint.

Step 2. Exploit Exploit the system’s constraint.

Step 3. SubordinateSubordinate everything else to the above decision.

Step 4. ElevateElevate the system’s constraint.Step 5. If a constraint is broken, go back

to Step 1. (continuous improvement)

Don’t allow inertiainertia to become a constraint.

Drun-Buffer-Rope5 Steps Applied to Flow

Operations

A B C D E

7 9 5 8 6

WIP Total

Step 3. Subordinate Everything Else 5(Rope)

Step 4. Elevate the Constraint (7?)Step 5. If the Constraint Moves, Start Over with E (Continuous Improvement)

7

Five Focusing Steps

RM

Step 1. Identify the Constraint 5 (The Drum)

FG

Step 2. Exploit the Constraint 12 (Buffer the Drum) against variability)

12 12

Additional Buffers

• Constraint Buffer (as we discussed)– Protects the Constraint from running out of work

• Finished Goods Buffer– Protects customer delivery from Constraint variation

• Raw Material Buffer– Protects the Release of material from suppliers

• Assembly Buffer– Facilitates speedy flow of products

More Systems Study

• Most Systems are not physical production and operations

• We can manage any system using TOC Concepts

• US Passport Office

Managing Systems Through (Introduced)

ConstraintsImpede Flow• Tirupathi• Police Traffic Control with Blockades • CA Exams• Dam

Are there any examples of introduced constraints for tangibles