asly line balancing 1

8/8/2019 Asly Line Balancing 1

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Assembly Line Balancing


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What will be the

production rate ?

Will grouping of tasks

help in achieving a

required production

rate ?

How will you group

them ?

What criteria will you

use for grouping ?

Can an algorithm be

used ?

Task Time Predecessor

A 1.5 -

B 2.0 A

C1 3.2 B

C2 3.2 B

C3 3.2 B

C4 3.2 B

D1 1.5 C1

D2 1.5 C2

D3 1.5 C3

D4 1.5 C4

E 3.7 -

F 2.6 E

G 3.2 F

H 2.0 G

I 3.1 -

J 3.7 I

K 3.2 J

L 2.0 K


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Cycle time is the maximum time

allowed at each workstation tocomplete its set of tasks on a unit.


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The classic definition of the line balancing problem

Given a set of tasks of various durations, a set of precedence

constraints among the tasks, and a set of workstations, assign each

task to exactly one workstation in such a way that no precedence

constraint is violated and the assignment is optimal.

The optimality criterion gives rise to two variants of the problem:

either a cycle time is given that cannot be exceeded by the sum of

durations of all tasks assigned to any workstation and the number of

workstations is to be minimized, or the number of workstations is

fixed and the line cycle time, equal to the largest sum of durations of

task assigned to a workstation, is to be minimized.


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Assembly Line Balancing

Assembly line balancing is the process of assigning tasks to

workstations so that idle time is minimized. This will,

distribute the total workload on the assembly line as evenly

as possible

ensure a smooth flow of products through the layout, with allresources used as fully as possible

Why i s this important?

Consider the following process, with cycle times given:

Process cycle time is determined by the work station taking the

longest time. Here, stationsA and C will experience idle time.

A 6 minutes B 7 minutes C 3 minutes


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Required production = 350 engines

Available hours = 160

Cycle time = 27.43 min/engine

If we make it 28 min, we will fall short by 8 engines

If we make it 27 min, we end up with 6 engines in excess

How effective are small improvements in

cycle time ?


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Assembly Line Balancing Procedure

1. Specify task precedence relationships (flow diagram)2. Determine required workstation cycle time:

3. Determine minimum number of workstations needed:

4. Assign tasks to workstations, one at a time, until the sum of thetask times is equal to the workstation cycle time

Da i ly Production TimeC =

RequiredDa ily Ouput

t

Sum of TaskTimes TN =

Cycle Time C


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Assembly Line Balancing Example

Consider a fan assembly line with the tasks below. 100 fans are

to be assembled per day (production time of 420 minutes). How

would you set up the workstations?

Task

Time

(minutes) Description Predecessors

A 2 Assemble frame None

B 1 Mount switch A

C 3.25 Assemble motor housing None

D 1.2 Mount motor housing in frame A, CE 0.5 Attach blade D

F 1 Assemble and attach safety grill E

G 1 Attach cord B

H 1.4 Test F, G


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AA

CC

BB

DD EE FF

GG

HH

2

3.25

1

1.2 0.5

1

1.4

1

Flow diagram

Cycle time of the line cannot be smaller than the time for task C, the

maximum possible production is:

T o talTi 420 i Max r o d uctio n = = = 129 u n its

Bo tt l n eck ate 3 .25 in / u n it


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Production time per period mins/dayC = = = .2 mins/unit

Required output per period 1 unit s/day

t

Sum of tasktimes(T) 11. 5mins

N = = = 2.702 (round up to 3)Cycle time (C) .2mins

Theoretical minimum number of workstations

(for the above cycle time):

Required cycle time:


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AA BB

EE

2

3.25

1

1.2 0.5

1

1.

1

Tas Time

A 2

C 3.25

D 1.2

B 1

E 0.5

F 1

G 1

H 1.

Assign tas s to wor stations such that,

No. of wor stations = 3

ycle time 4.2


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I came up with this, what about you ?

AA

CC

BB

DD EE FF

GG

HH

2

3.25

1

1.2 0.5

1

1.4

1

Station 1 Station 2 Station 3

A 2 minB 1 min

G 1 min

2 min 3.25 min 1.2 min3 min4 min

C 3.25 min D 1.2 minE 0.5 min

F 1 min

H 1.4 min

1.7 min2.7 min4.1 min

Tas Time

A 2

C 3.25

D 1.2

B 1

E 0.5

F 1

G 1

H 1.4


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Assembly Line Balancing: Evaluating the performance

The new bottleneck is workstation 3, at 4.1 minutes:

TotalTi e 420 inMax roduction = = = 102unitsBottleneck ate 4.1 in/unit


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Task Time Predecessor

A 1.5 -

B 2.0 A

C1 3.2 B

C2 3.2 B

C3 3.2 B

C4 3.2 B

D1 1.5 C1

D2 1.5 C2D3 1.5 C3

D4 1.5 C4

E 3.7 -

F 2.6 E

G 3.2 F

H 2.0 G

I 3.1 -

J 3.7 I

K 3.2 J

L 2.0 K

Balance the

assembly linefor a cycle time

of 10


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A B

C1

C2

C3

C4

D1

D2

D3

D4

E F G H

I J K L

Precedence

Diagram


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A B

C1

C2

C3

C4

D1

D2

D3

D4

E F G H

I J K L

9.8

10

9.6

7.2

9.2


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Assembly Line Balancing Using Rules

AA

CC

BB

DD EE FF

GG

HH

2

3.25

1

1.2 0.5

1

1.4

1

as ime

A 2

3.25

D 1.2

1

0.5

1

G 1

H 1.4

Largest Candidate Rule

ListtasksindescendingorderoftasktimesStartingwithfirststation,assignthefirst jobinthelist.Assignonlyif

workstationisavailableandthetaskisfeasible.

reateanewstationifnoneofthestationsareavailable.


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Assembly Line Balancing : Another rule

How would you set the workstations if you wished to produce400 units a day (8 hr/day) of the product with the following

activities? Assign tasks by the greatest number of following

tasks.

Task

Time

(minutes) PredecessorsA 0.2 None

B 0.2 A

C 0.8 None

D 0.6 CE 0.3 B

F 1.0 D, E

G 0.4 F

H 0.3 G


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If the cycle time is less than the amount of time to perform

a specific task, then there is a problem.

In such cases, we can perform what is termed task

splitting.

In reality, it is actually workstation duplication.


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Workstation Duplication

D E

1 min 1 min

A B

C1 min 1 min

2 min

C

2 min60/hr

30/hr 30/hr

60/hr

30/hr 30/hr

60/hr

D E

1 min 1 min

A B C

1 min 1 min 2 min

30/hr 30/hr 30/hr 30/hr 30/h

r


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Answer

We can have the task performed by three people (not

necessary) at three machines independent of one another.

The effect is that three units will be done every 5 minutes

which is equivalent to one unit every 1.33 minutes which is

smaller than the cycle time (2 minutes).

Workstation Duplication


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Task Time

1 0.70

2 0.33

3 0.25

4 0.75

5 0.15

6 0.42

7 0.52

8 0.40

9 0.88

Desired cycle time = 0.95min

Rank Position Weight Method


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Use RPW method

0.70

0.33

0.25

0.75

0.15

0.42 0.40

0.52

0.88

1

2

3

4

5

6

7

8 9


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Task Time RPW

1 0.70 4.40

2 0.33 3.45

3 0.25 3.22

4 0.75 2.55

5 0.15 1.95

6 0.42 2.22

7 0.52 1.80

8 0.40 1.289 0.88 0.88

RPWmethod

asly line balancing 1

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