Assembly-Line Balancing

Assembly-Line Balancing:   An effective  tool for improving Productivity

1. What is assembly-line balancing?

l  to a workstation within an assembly line in order to meet the required production rate and to achieve a minimum amount of idle time.

l  Line balancing is the procedure in which tasks along Assigning each task the assembly line are assigned to work station so each has approximately same amount of work.

2.  Unbalance Line and Its effect

l  High work load in some stages (Overburden)

l  Maximizes wastes (over-processing, inventory, waiting, rework, transportation, motion)

l  High variation in output

l  Restrict one piece flow

l  Maximizes Idle time

l  Poor efficiency

3.Balanced Line and its effect

l  Promotes one piece flow

l  Avoids excessive work load in some stages (overburden)

l  Minimizes wastes (over-processing, inventory, waiting, rework, transportation, motion)

l  Reduces variation

l  Increased Efficiency

l  Minimizes Idle time

4.  How Can Assembly-Line Balancing   Help Organization ?

l  Increased efficiency

l  Increased productivity

l  Potential increase in profits and decrease in costs

 5. Steps in Balancing an Assembly Line

1. List the sequential relationships among tasks and then draw a precedence diagram.
2. Calculate the required workstation cycle time.
3. Calculate the theoretical minimum number of workstations.
4. Choose a primary rule that will determine how tasks are to be assigned to workstations.
5.  Beginning with the first workstation, assign each task, one at a time, until the sum of the task times is equal to the workstation cycle time or until no other tasks can be assigned due to sequence or time restrictions.
6.  Repeat step 5 for the remaining workstations until all the tasks have been assigned to a workstation.
7. Evaluate the efficiency of the line balance.
8. Rebalance if necessary.

Example of Assembly-Line Balancing

Problem: The Model Z Bicycle is assembled in an assembly line. Four hundred and twenty  bicycles are required each day. Production time per day is 420 minutes.

            Find the balance that minimizes the number of workstations, that stays within the workstation cycle time limitation, and that complies with task precedent constraints. 

Example of Assembly-Line Balancing Cont.
1. Building the Model Z Bicycle: Assembly Steps and Times

Task	Task Time (in seconds)	Task Description	Tasks that must precede
A	50	Connect the front tire to the bicycle frame.	—
B	16	Insert the handle bar.	A
C	14	Tighten handle bar with two screws and nuts.	B
D	55	Connect the rear tire to the bicycle frame.	—
E	20	Position chain mechanism to the frame.	D
F	17	Attach right hand brake to handle bar.	C
G	17	Attach left hand brake to handle bar.	C
H	17	Attach right side pedal.	E
I	17	Attach left side pedal.	E
J	13	Position chain onto chain mechanism.	F,G,H,I
K	14	Attach seat post.	J
	250

STEP 1. List the sequential relationships among tasks and then draw a precedence diagram

STEP 2.  . Calculate the required workstation cycle time

Convert minutes to seconds because task times are in seconds.

CYCLE TIME  = (PRODUCTION TIME PER DAY) /(OUTPUT PER DAY)

                        = (60 sec. X  420 min.)  / ( 420 bicycles)  

                         = 25,200 / 420   =  60  sec.

STEP 3. Calculate the theoretical minimum number of workstations.

NUMBER OF WORK STATIONS  = ( SUM OF TOTAL TASK TIMES) / (CYCLE TIME)

=  250sec’s  /  60 sec’s

                                                        =   3.97  = 4  (rounded)

Task	Number of Following Tasks
A	6
B or D	5
C or E	4
F, G, H, or I	2
J	1
K	0

STEP 4.Choose a primary rule that will determine how tasks are to be assigned to workstations

l  For this example, our primary rule is to prioritize tasks based on the largest number of following tasks.

l  If there is a tie, our secondary rule is to prioritize tasks in the order of the longest task time.

l  In general, select rules that prioritize tasks according to the largest number of followers or based on length of time.

STEP 5. Beginning with the first workstation, assign each  task, one at a time, until the sum of the task times is equal to the workstation   cycle time or until no other tasks can be assigned due to sequence or time restrictions.

STEP 6. Repeat step 5 for the remaining workstations until all the tasks have been assigned to a workstation

STEP 7.Evaluate the efficiency of the line balance.

EFFICIENCY   =   (SUM OF ALL TASK TIMES))/(ACTUAL NO OF WORKSTATIONS)X(CYCLE TIME)

=  (250)  / (5) X (60)  

=  0.83  OR  83 %

STEPS  5& 6. Balance made according to the Largest-Number-of-Following Tasks Rule

Stations	Task	Task Time (in seconds)	Number of Following Tasks	Remaining Unassigned Time	Feasible Remaining Tasks	Task with Most Followers	Task with Longest Operating Time
Station 1	A	50	6	10 idle	None
Station 2	D	55	5	5 idle	None
Station 3	B E C	16 20 14	5 4 4	44 24 10 idle	C, E C, F,G, H, I,J None	C, E C	E
Station 4	F G H	17 17 17	2 2 2	43 26 9 idle	G, H, I H, I, J None	G, H, I H, I	G, H, or I H or I
Station 5	I J K	17 13 14	2 1 0	43 30 16 idle	J K None