Facilities Planning - Unit
Balancing Production Lines
06
Factors Affecting Capacity
External Factors
Internal Factors
 Government regulations
 Union agreements
 Supplier capabilities
 Technology
 Product and service design
 Personnel & jobs
 Equipment Capabilities
 Materials management
 Q.C. Systems
 Management capabilities
 Plant layout & flow
Balancing Production Lines - 2
Process Selection and System Design
Forecasting
Capacity
Planning
Product and
Service Design
Technological
Change
Facilities and
Equipment
Layout
Process
Selection
Work
Design
Capacity is significantly impacted by process selection and facility layout.
Balancing Production Lines - 3
Basic Layout Types
 Product layout
 Process layouts
 Fixed-Position layout
 Group Technology / Combination layouts
Balancing Production Lines - 4
Product Layout
 Facility organized around product
 Design minimizes line imbalance
 Delay between work stations
 Types: Fabrication line; assembly
line
M1
M6
M2
M7
M3
Offices
M5
M4
Balancing Production Lines - 5
Product Layout
Fabrication Line
Assembly Line










Builds components
Uses series of machines
Repetitive process
Machine paced
Balanced by physical redesign
Assembles fabricated parts
Uses workstation
Repetitive process
Paced by tasks
Balanced by moving tasks
Balancing Production Lines - 6
Product Type Layout - Requirements




Standardized product
High production volume
Stable production quantities
Uniform quality of raw materials & components
Balancing Production Lines - 7
Product Layout - Flow Diagram Showing the
Production Process for Harley Davidson, York, PA.
Balancing Production Lines - 8
Product Layout
Raw materials
or customer
Material
and/or
labor
Station
1
Material
and/or
labor
Station
2
Material
and/or
labor
Station
3
Station
4
Finished
item
Material
and/or
labor
Used for Repetitive or Continuous Processing
Balancing Production Lines - 9
Product Layout – More Examples
Balancing Production Lines - 10
Line Balancing Problem
 Work stations are arranged so that the output of one is an
input to the next, i.e., a series connection
 Layout design involves assigning one or more of the
tasks required to make a product to work stations
 The objective is to assign tasks to minimize the workers’
idle time, therefore idle time costs, and meet the required
production rate for the line
Balancing Production Lines - 11
Line Balancing Problem
 In a perfectly balanced line, all workers would
complete their assigned tasks at the same time
(assuming they start their work simultaneously)
 This would result in no idle time
 Unfortunately there are a number of conditions that
prevent the achievement of a perfectly balanced line
 The estimated times for tasks
 The precedence relationships for the tasks
 The combinatorial nature of the problem
Balancing Production Lines - 12
Assembly Line Balancing Steps
1.
2.
3.
4.
5.
6.
7.
8.
Determine tasks (operations)
Determine sequence
Draw precedence diagram
Estimate task times
Calculate cycle time
Calculate number of work stations
Assign tasks
Calculate efficiency
Balancing Production Lines - 13
Assembly Lines Balancing Concepts
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?
Minutes
per Unit
Station 1
Station 2
Station 3
6
7
3
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.
Balancing Production Lines - 14
Assembly Lines Balancing – Example 1
You’ve just been assigned the job a setting up an electric
fan assembly line with the following tasks:
Task
A
B
C
D
E
F
G
H
Time (Mins)
2
1
3.25
1.2
0.5
1
1
1.4
Description
Assemble frame
Mount switch
Assemble motor housing
Mount motor housing in frame
Attach blade
Assemble and attach safety grill
Attach cord
Test
Predecessors
None
A
None
A, C
D
E
B
F, G
Total Task Times = 11.35 Minutes
Balancing Production Lines - 15
Assembly Lines Balancing – Example 1
Structuring the Precedence Diagram
Question: Which process step defines the maximum rate of
production?
2
A
1
B
1
G
C
D
E
F
3.25
1.2
.5
1
1.4
H
Answer: Task C is the cycle time of the line and therefore, the
maximum rate of production.
Balancing Production Lines - 16
Assembly Lines Balancing – Example 1
Cycle Time Determination
Question: Suppose we want to assemble 100 fans per day. What
would our cycle time have to be?
Answer:
Required Cycle Time, C =
Production time per period
Required output per period
420 mins / day
C=
= 4.2 mins / unit
100 units / day
Balancing Production Lines - 18
Assembly Lines Balancing – Example 1
Theoretical Minimum Number of Workstations
Question: What is the theoretical minimum number of workstations
for this problem?
Answer:
Theoretical Min. Number of Workstations, N t
Sum of task times (T)
Nt =
Cycle time (C)
11.35 mins / unit
Nt =
= 2.702, or 3
4.2 mins / unit
Balancing Production Lines - 19
Assembly Lines Balancing – Example 1
Workstations Tasks Assignment
2
A
1
B
1
G
C
D
E
F
3.25
1.2
.5
1
Station 1
A (4.2 – 2 = 2.2)
B (2.2 – 1 = 1.2)
1.4
H
Task
A
C
D
B
E
F
G
H
Station 2
Followers
6
4
3
2
2
1
1
0
Time minutes
2
3.25
1.2
1
0.5
1
1
1.4
Station 3
C (4.2 - 3.25) = 0.95 D (4.2 - 1.2) = 3
E (3 - 0.5) = 2.5
G (1.2 -1 = 0.2)
F (2.5- 1) = 1.5
H (1.5 - 1.4) = 0.1
Idle = 0.2
Idle = 0.95
Idle = 0.1
Balancing Production Lines - 20
Assembly Lines Balancing – Example 1
Efficiency of Assembly Line
Sum of task times (T)
Efficiency =
Actual number of workstations (Na) x Cycle time (C)
11.35 mins / unit
Efficiency =
=.901
(3)(4.2mins / unit)
Balancing Production Lines - 21
Assembly Line Balancing
Layout Heuristics for Assigning Tasks
Basic rules
Rule 1: Pick the task with the longest required time.
Save shorter tasks for fine-tuning
Rule 2: Select the task with the largest number of
followers. This preserves options for
subsequent. stations and tends to reduce idle
time.
Common Practice: Use rule 2. When ties exist use rule 1
Balancing Production Lines - 22
Assembly Lines Balancing – Example 2
Vicki’s
Pizzeria
and
Diagram
Example 10.4
Vicki's
Pizzeria
andthe
thePrecedence
Precedence Diagram
Immediate
Task Time
Work Element Task Description
Predecessor
(seconds
A
B
C
D
E
F
G
H
I
Roll dough
Place on cardboard backing
Sprinkle cheese
Spread Sauce
Add pepperoni
Add sausage
Add mushrooms
Shrinkwrap pizza
Pack in box
None
A
B
C
D
D
D
E,F,G
H
Total task time
50
5
25
15
12
10
15
18
15
165
Balancing Production Lines - 23
Assembly Lines Balancing – Example 2
Determine output rate
 Vicki needs to produce 60 pizzas per hour
Determine cycle time
 The amount of time each workstation is allowed to complete its tasks
C ycletime(se c./unit
)

availabletimese c./day 60 min/hrx 60 se c/min

 60 se c./unit
de sire doutputunits/hr
60 units/hr
Limited by the bottleneck task (the longest task in a process):
Maximum output 
available time
3600 sec./hr.

 72 units/hr, or pizzas per hour
bottleneck task time
50 sec./unit
Balancing Production Lines - 24
Assembly Lines Balancing – Example 2
Compute the theoretical minimum number of stations
 Number of stations needed to achieve 100% efficiency (every
second is used)
Number of Stations 


 task times  
cycle time
165 seconds
60 sec/station
 2.75, or 3 stations
Always round up (no partial workstations)
Serves as a lower bound for our analysis
Balancing Production Lines - 25
Assembly Lines Balancing – Example 2
Assign tasks to workstations
 Start at the first station & choose the longest eligible task following
precedence relationships
 Continue adding the longest eligible task that fits without going over the
desired cycle time
 When no additional tasks can be added within the desired cycle time,
begin assigning tasks to the next workstation until finished
Workstation
1
2
3
Eligible task
A
B
C
D
E, F, G
E, F
F
H
I
Task Selected
A
B
C
D
G
E
F
H
I
Task time
50
5
25
15
15
12
10
18
15
Idle time
10
5
35
20
5
48
38
20
5
Balancing Production Lines - 26
Assembly Lines Balancing – Example 2
Compute efficiency and balance delay
 Efficiency (%) is the ratio of total productive time divided
by total time
Efficiency (%) 

t 
NC
165sec.
100  91.7%
3 stations x 60 sec.
Balance delay (%) is the amount by which the line falls
short of 100%
Balancede lay 100% 91.7% 8.3%
Balancing Production Lines - 27
Assembly Lines Balancing – Example 3
Total Work Content, ∑t = 306 decimal min.
Need 450 dolls/day, one shift per day
Available time = 420 minutes
a.
b.
c.
d.
e.
Precedence diagram
Maximum cycle time (c) = 420 min x 100 / 450 dolls ≈ 93
Minimum number of stations (n) = 306/93 = 3.3 → 4
Assigning work elements to workstations (next page)
Balance delay = (nc-∑t)/nc = [(4 x 91) – 306] / (4 x 91) = 0.159
or 15.9% idleness among the four stations
Balancing Production Lines - 28
Assembly Lines Balancing – Example 3
Left
sock
9
Left 26
Shoe
G
C
22
10
Undershorts
A
Pocket
items
J
Slacks
E
Belt
I
9
Right
sock
D
Undershirt
B
Right
Shoe
H
42
11
Shirt
F
Coat
L
6
Hat
M
Tie
K
20
30
26
32
6
3
Balancing Production Lines - 29
Assembly Lines Balancing – Example 3
Element
Assigned
Element
Time
Start
Station 1
B
F
L
M
11
42
32
6
Start
Station 2
K
A
C
D
63
10
9
9
Start
Station 3
E
I
G
22
30
26
Start
Station 4
H
J
26
20
Remaining
Unassigned
Time
Permissible
Remaining
Elements
Task With
Greatest
Element
Time
B
93
A, B, C, D
82
40
8
2
93
A, C, D, F, M
A, C, D, L, M
M
None
A, C, D, K
30
20
11
2
93
A, C, D
C, D, E
D
None
E
A
C or D
D
71
41
15
G, H, I, J
G, H, J
None
I
G or H
93
H, J
H
67
47
J
None
J
F
L
M
K
E
Balancing Production Lines - 30
Assembly Lines Balancing – Example 3
Station 3
Station 2
G
9
C
A10
B11B11
K
J20
H 26
63
F 42
26
I30
E 22
9
D
Station 4
Station 1
32
L
M6
Balancing Production Lines - 31
Assembly Lines Balancing – Example 4
Precedence Graph for Credit Applications
Balancing Production Lines - 32
Assembly Lines Balancing
Balancing the Work Through Better Work Flow Design
Balancing Production Lines - 33
Assembly Lines Balancing - Layout Design
U-Shaped or horseshoe arrangements
better enable workers
communications and task sharing
In
1
2
3
E
S
S
4
E
5
Workers
6
Out
10
9
8
S
S
E
7
S
S
Connections of subassembly islands S feed
parts/components to end-assembly E islands each island functions as a separate team
Balancing Production Lines - 34
Line Balancing Procedure Summarized
1.
2.
3.
Determine the cycle time and the minimum number of
workstations.
Make assignments to workstations in order, beginning with
Station 1. Tasks are assigned to workstations moving from left to
right through the precedence diagram.
Before each assignment, use the following criteria to determine
which tasks are eligible to be assigned to a workstation:
a. All preceding tasks in the sequence have been assigned.
b. The task time does not exceed the time remaining at the
workstation.
If no tasks are eligible, move on to the next workstation
Balancing Production Lines - 35
Line Balancing Procedure Summarized
4.
5.
6.
7.
After each task assignment, determine the time remaining at the
current workstation by subtracting the sum of times for tasks
already assigned to it from the cycle time
Break ties that occur using one of these rules:
a. Assign the task with the longest task time.
b. Assign the task with greatest number of followings.
Continue until all tasks have been assigned to workstations.
Compute appropriate measures (e.g., percent idle time, efficiency)
for the set of assignments.
Balancing Production Lines - 36