Assembly Line Balancing

• Assigning all tasks to a series of workstations so that each workstation is limited by the cycle time and so that the unassigned or idle time across all workstations is minimized. Workload equalization is another important objective of the balance.

• Product interval time, time between products being completed at a workstation, also called takt time and cycle time.

• Task: is the smallest element of work; it can not be subdivided further.

• Workstation: is a place of work with workers, equipment and material to perform one or more tasks.

• Line: is made up of one or more workstations.

• Cycle Time (throughput time): the overall time to complete an individual product also the time it takes to assemble one unit or the time elapsed between two successive units coming off the line.

• Precedence Chart: a chart that specifies the order in which the tasks must be performed in the assembly process.

Line Balancing Steps1. Specify the sequential relationships among tasks using a precedence diagram.

2. Calculate cycle time (C), using the following formula:

Productive time per day (P)

3. Calculate the minimum theoretical number of workstations (Nt), using the following formula:

T, sum of task times

4. Select a primary rule by which tasks are to be assigned to workstations and a secondary rule to break up ties should they occur. Assign tasks to workstations according to specified rules.

5. Evaluate efficiency (Eff.) and balance delay (BD) as follows:

T Sum of idle time

Rebalance if eff. is low using a different primary rule.

C =Unit output per day (D)

CNt =

Eff. =Na * C Na * C

, BD = = 1 – Eff.

Designing Line-Flow LayoutsDesigning Line-Flow Layouts

Line Balancing: Assignment of work to stations in a line so as to achieve the desired output rate with smallest number of workstations

There are two constraints in line balancing :

1. Precedence requirements– Physical restriction, order of operations

2. Cycle time restrictions– Max. operating time allowed for each workstation

– Depend on demand rate, production time

Line Balancing ProcessLine Balancing Process1. Draw and label a precedence diagram.

2. Calculate the desired cycle time required for the line.

3. Calculate the theoretical minimum number of workstations.

4. Group elements into workstations, recognizing cycle time and

precedence constraints.

5. Calculate the efficiency of the line.

6. Stop if theoretical minimum number of workstations on an

acceptable efficiency level reached. If not, go back to step 4.

Station 1

Minutes per Unit 6

Station 2

7

Station 3

3

Assembly Lines Balancing ConceptsQuestion: Suppose you load work into the three work stations below such that each will take the corresponding number of minutes as shown. What is the cycle time of this line?

Assembly Line Balancing Concept

Station 1

Minutes per Unit 6

Station 2

7

Station 3

3

Question: Suppose you load work into the three work stations below such that each will take the corresponding number of minutes as shown. What is the cycle time of this line?

Answer: The cycle time of the line is always determined by the work station taking the longest time. In this problem, the cycle time of the line is 7 minutes. There is also going to be idle time at the other two work stations.

Example of Line Balancing• You’ve just been assigned the job a setting up

an electric fan asbly line with following tasks:

Task Time (Mins) Description PredecessorsA 2 Assemble frame NoneB 1 Mount switch AC 3.25 Assemble motor housing NoneD 1.2 Mount motor housing in frame A, CE 0.5 Attach blade DF 1 Assemble and attach safety grill EG 1 Attach cord BH 1.4 Test F, G

Example of Line Balancing: Precedence Diagram

Task PredecessorsA None

A

B A

B

C None

C

D A, C

D

Task PredecessorsE D

E

F E

F

G B

G

H E, G

H

Example of Line Balancing: Precedence Diagram

A

C

B

D E F

GH

2

3.25

1

1.2 .5

11.4

1

Question: Which process step defines the maximum rate of production?

Example of Line Balancing: Cycle Time

Question: Suppose we want to assemble 100 fans per day. What would our cycle time have to be?

Example of Line Balancing: Number of Workstations

Question: What is the theoretical minimum number of workstations for this problem?

Example2 : Line BalancingExample2 : Line Balancing

A company must produce 600 unit output in a 40-hour week. Given the following conditions, balance the assembly line

Work Element Precedence Performance Time (min)

ABCDEF

-AB

A, E-

C, D

122434

Example3: Line BalancingExample3: Line BalancingAdditional ExampleAdditional Example

work elements, precedence reqmts and time reqmts to assemble a picture frame are shown here.

1. Construct a precedence diagram of the process and label task times.2. Set up an assembly line capable of producing 1,600 frames per 40-hour week.3. Calculate the efficiency and balance delay of the line.4. Calculate the maximum number of frames that can be assembled each week.5. Rebalance the line for maximum production. Indicate the composition of each station.6. Assume the company can sell as many frames as can be produced. If workers are paid

Rs8 an hour and the profit per frame is Rs5, should the production quota be set to the maximum? Assume one worker per station.

Elements Description Precedence

Time (min)

A

B

C

D

E

F

G

H

I

Attach left frame side to top of frame

Attach right frame side to bottom of frame

Attach left and right frame subassemblies

Cut 8-inch by 10-inch glass

Cut 8-inch by 10-inch cardboard

Place glass into frame

Place cardboard into frame

Secure cardboard and glass

Apply descriptive label to glass

-

-

A, B

-

-

C, D

E, F

F, G

D

0.35

0.35

0.70

0.50

0.50

0.20

0.20

0.50

0.10

Toy Wagon Assembly Line Balance Example

Task Performance Time (sec.) Tasks that must precede

A 45 ---

B 11 A

C 9 B

D 50 ---

E 15 D

F 12 C

G 12 C

H 12 E

I 12 E

J 8 F,G,H,I

K 9 J

Example4: Line BalancingExample4: Line Balancing

Precedence Diagram

A

D

B C F

G

J K

EH

I

45 sec.

50

11

15

9

12

12

12

8 9 12

Balance the Line Using Most Followers Rule

Task time(sec) time Rem. Tasks most followers longest opr. time

St. 1 A 45 5.4 idle none

Task Remaining Feasible Task @ Task@

Balance the Line Using Most Followers Rule

Task time(sec) time Rem. Tasks most followers longest opr. time

St. 1 A 45 5.4 idle none

St. 2 D 50 0.4 idle none

Task Remaining Feasible Task @ Task@

Balance the Line Using Most Followers Rule

Task time(sec) time Rem. Tasks most followers longest opr. time

St. 1 A 45 5.4 idle none

St. 2 D 50 0.4 idle none

St. 3 B 11 39.4 C, E C, E E

Task Remaining Feasible Task @ Task@

E 15 24.4 C,H,I C

C 9 15.4 F,G,H,I F,G,H,I F,G,H,I

F* 12 3.4 idle none

Balance the Line Using Most Followers Rule

Task time(sec) time Rem. Tasks most followers longest opr. time

St. 1 A 45 5.4 idle none

St. 2 D 50 0.4 idle none

St. 3 B 11 39.4 C, E C, E E

St.4 G* 12 38.4 H, I H, I H, I

Task Remaining Feasible Task @ Task@

E 15 24.4 C,H,I C

C 9 15.4 F,G,H,I F,G,H,I F,G,H,I

F* 12 3.4 idle none

H 12 26.4 I

I 12 14.4 J

J 8 6.4 idle none

Balance the Line Using Most Followers Rule

Task time(sec) time Rem. Tasks most followers longest opr. time

St. 1 A 45 5.4 idle none

St. 2 D 50 0.4 idle none

St. 3 B 11 39.4 C, E C, E E

St. 4 G* 12 38.4 H, I H, I H, I

Task Remaining Feasible Task @ Task@

E 15 24.4 C,H,I C

C 9 15.4 F,G,H,I F,G,H,I F,G,H,I

F* 12 3.4 idle none

H 12 26.4 I

I 12 14.4 J

J 8 6.4 idle none

St. 5 K 9 41.4 idle none

Efficiency and Balance Delay Calculations

Efficiency =

Balance Delay =

Check, Total = 100%

Notice that BD = 1.0 – Eff.

195

(5) 50.4)= 0.77 or 77%

57

(5) (50.4) = 0.23 or 23%