Assembly Line Assembly lines are a special case of

product layout. In general sense , the term assembly line refers to progressive assembly linked by some material handling devices .

The classic example is Henry Ford’s auto chassis line. Before the “moving assembly line” was introduced in

1913, each chassis was assembled by one worker and required 12.5 hours.

Once the new technology was installed, this time was reduced to 93 minutes

Characteristics of Assembly Line Important differences among line types :Material handling device – Belt , roller

conveyer overhead craneLine configuration –U-shape , straight ,

branchingPacing – Mechanical , humanProduct mix- One product or multiple

products.Workstations characteristics – Workers

may sit , stand ,walk with the line, etc.Length of the line : Few or many workers.

Key Concept The most common assembly line is a

moving conveyer that passes a series of workstations in a uniform time interval called the cycle time (which is also the time between successive units coming off the end of the line).

The work performed at each station is made up of many bits of work , termed task, elements or work units.

Contd.The total work to be performed at a workstation

at a workstation is equal to the sum of the tasks assigned to workstations. The assembly line balancing problem is one of assigning all tasks to a series of workstations so that each workstations has no more than can be done in their cycle time. The unassigned time (idle) across all workstations is minimised .The problem is complicated by the relationships among tasks imposed by product design and process technologies . This is called PRECEDENCE RELATIONSHIP which specifies the order in which the tasks must be performed in the assembly process.

Line balancing:

Precedence Requirement :Physical restriction on the order in which operations are performed on the assembly line.Work element : Jobs that are broken down into their smallest portions, called work elements. Normally work elements are designed such that only one worker can perform the task at one work station Cycle time : cycle time is the time between completed items rolling off the assembly line

Assembly Line Balancing

Steps for Assembly Line Balancing

1. Draw the precedence diagram

2. Determine the required cycle time (c)

C=(Production time/day) /Output per day (in units)

3. Determine the theoretical no of work stations

Nt = Sum of the task times/cycle time

4. Select a rule which tasks are to be assigned to work stations and a secondary rule to break the ties.

5. Efficiency = Sum of the tasks times/[ actual no of workstations x Cycle time ]

6. Balance delay =1-Efficiency

Precedence diagram: Tool used in line balancing to display elemental tasks and sequence requirements

A Simple Precedence

Diagrama b

c d e

0.1 min.

0.7 min.

1.0 min.

0.5 min.

0.2 min.

Figure 6.10

Precedence Diagram

Example 1: Assembly Line Balancing

Arrange tasks shown in Figure 6.10 into three workstations.Use a cycle time of 1.0 minuteAssign tasks in order of the most number of

followers

Example 1 Solution

WorkstationTimeRemaining Eligible

AssignTask

RevisedTime Remaining

StationIdle Time

1 1.0

0.9

0.2

a, c

c

none

a

c

-

0.9

0.2

0.2

2 1.0 b b 0.0 0.0

3 1.0

0.5

0.3

d

e

-

d

e

-

0.5

0.3 0.3

0.5

Line Balancing

Chapter 5: Transformation System Design10

number of theoretical workstations, task times / cycle timeTN

efficiency = output

input

total task time

N stations) cycle timeA

(

demand

work timeavailable timeCycle

Line Balancing Example

Task Time Required PrecedesA 2.2 min. B, C, DB 3.4 EC 1.7 ED 4.1 FE 2.7 FF 3.3 GG 2.6 --

Line Balancing Example continued

Chapter 5: Transformation System Design12

Company operates one shift per dayAvailable time per shift is 450 minutesDemand is 100 units/day

Precedence Diagram

Chapter 5: Transformation System Design13

A

B

C

D

E

F G

Calculations

cycle time = 450/75 = 6 minutes/part

NT = 20/6 = 3.33 = 4 stations

Task Assignment

StationTimeAvail.

Elig.Tasks

WillFit?

TaskAssign.

IdleTime

1 6.0 A A

3.8 B,C,D B,C B

0.4 C,D -- -- 0.4

2 6.0 C,D C,D D

1.9 C C C

0.2 E -- -- 0.2

Task Assignment continued

StationTimeAvail.

Elig.Tasks

WillFit?

TaskAssign.

IdleTime

3 6.0 E E E

3.3 F F F 0.0

4 6.0 G G G 3.4

Line Balancing Solution

A

B

C

D

E

F G

Station 1

Station 2

Station 3

Station 4

Efficiencyefficiency = 20/(4*6) = 83.3%

Assembly for contact breakerNo. Task assigned Prede

cessorTask time /unit (Hrs)

1 A: Contact Breaker Assembly; take molding half and clean burrs etc.

None 0.010

2 B;install contacts A 0.020

3 C: Install springs B 0.020

4 D: install plastic levers on molding half

A,C 0.040

5 E:Installcontacts A 0.020

6 F: Install springs B 0.020

7 G : Install plastic levers etc on molding Half

A,C 0.040

8 H: Close with other molding half

G 0.050

9 I : Assemble all the above components

H 0.008

10 J : Insert rivets I 0.040

11 K : Rivet the sandwich units J 0.098

12 L : Switching test under load E 0.050

13 M : Pack contact Breaker unit F 0.020

Total