Assembly Lines – Reliable Serial Systems

Active Learning Module 1

Dr. César O. Malavé

Texas A&M University

Background Material

Modeling and Analysis of Manufacturing Systemsby Ronald G. Askin , Charles R. Standridge, John Wiley & Sons, 1993, Chapter 2.

Manufacturing Systems Engineering by Stanley B. Gershwin, Prentice – Hall,1994, Chapter 2.

Any good manufacturing systems textbook which has detailed explanation on reliable serial systems.

Lecture Objectives

At the end of this module, the students should be able to Explain the fundamentals of assembly lines.

Explain the basics of problem formulation of

line – balancing problems.

Formulate the problem and solve them

Time Management

3Assignment

15Problem Formulation

5Team Exercise

50 MinsTotal Time

5Spot Exercise

12Assembly Lines - Introduction

5Readiness Assessment Test (RAT)

5 Introduction

Readiness Assessment Test (RAT)

Assume that there is a proposal for developing new car. Enumerate the various and basic stages in the development of this new product.

At the end, each team should turn in the solutions and the instructor may ask a group to discuss with the class.

RAT – Solution

Customers

Product Features

Functions

Product Design

Part Design

Process Planning

Fabricate

Assemble

Introduction

Assembly Line – Set of sequential workstations, connected by a continuous material handling system.

Each Assembly activity divided into productive work elements, adds value to product.

Group of such elements are assigned to each workstation.

Assembly Lines rely on Principle of Interchangeability and Division of Labor.

Principle of Interchangeability – Individual Components that make up the final product must be interchangeable

Division of Labor – Work Simplification, Standardization and Specialization.

Introduction – Cont…

Advantages of Assembly Lines Ability to keep direct labor or machines busy doing work Minimal setup requirements as products are repeated. Less space required, lower inventory costs and shorter

throughput time.

Many items don’t justify assembly lines. So Mixed lines are used.

Mixed Lines – Several products on the line in different workstations at the same time.

Single or Multiple Assembly Lines depends on various factors like economics, labor psychology etc.

Spot Exercise

Discuss the advantages & disadvantages of multiple parallel lines

Advantages Disadvantages

Easier to balance work load between stations

Higher setup costs

Increased scheduling flexibility Higher equipment costs

Job enrichment Higher skill requirements

Work Independence Slower Learning

Increased accountability More complex supervision

Introduction – Cont…

Use of buffers increase productivity and flexibility.

Buffers provide the “Cushion Effect” in production.

Paced Lines – Each workstation given same amount of time to operate on each unit of product.

Unpaced Lines – Each workstation removes a new unit from the material handling system as soon as it completes the previous unit.

Flexible Flow Lines – Product units routed thru workstations based on task requirements & input buffers. Also facilitates job enrichment & cycle time balancing.

Problem Formulation

Objective is to minimize unit assembly cost.

Assembly Cost = Labor Cost + Idle Time Cost.

Assume P Production rate

M Number of Parallel Lines

Cycle Time = m/p

No worker assigned with tasks exceeding the cycle time.

Set IP shows the ordering constraints IP = {(u, v): task u must precede v}

Problem Formulation – Cont…

Zoning Restrictions – Which tasks must be and must not be assigned to the same workstation.

ZS Set of tasks to be assigned ZD Set of tasks not to be assigned

Binary indicators used as decision variables

To minimize idle time, we try to force tasks into the lowest numbered stations.

Unused stations will be discarded.

Otherwise

kstationtoassignedisitaskifikx

,0

,1

The formulation becomes

Problem Formulation – Cont…

ZDvuandKkXX

ZSvuXX

IPvuandKhXX

NiX

KkCXt

toSubject

Xc

vhuh

K

kvkuk

h

jujvh

K

kik

N

iiki

N

i

K

kikik

),(,...,11

),(1

),(,...,1

,...,11

,...,1

min

1

1

1

1 1

Constraint ensures that the sum of task times for the set of tasks assigned to each workstation doesn’t exceed the cycle time.

Constraint ensures that the task is assigned to exactly one station

Constraint forces the adherence to precedence restrictions

Zoning Constraint : Marriage Type

Zoning Constraint : Divorce Type

Objective Function – Advantageous to fill up lower numbered stations before opening new station.Let K* Number of station (workers) required by the solution.Balance Delay D, measure for comparing solutions, proportion of idle time.

Objective function fails to recognize a secondary objective of allocating the idle time equally to all the workstations.

CK

tCKD

N

ii

*

*1

Problem Formulation – Cont…

Team Exercise

Develop a complete binary integer programming formulation for the line balancing problem. Let C = 100.

Task Time Immediate Predecessor

a 40 -

b 75 a

c 50 a

d 35 c

e 80 d

Team Exercise – Solution

1008035507540 s.t.

4

1

ekdkckbkak

e

ai kikik

XXXXX

XCMinimise

k = 1,…,4

43214

3213

212

11

4

1

,,,,1

aaaab

aaab

aab

ab

kik

XXXXX

XXXX

XXX

XX

edcbaiforX

Likewise for (a, c), (c, d) and (d, e)

All Xik 0 or 1

Ci1 = 1; Ci2 = 20;

Ci3 = 400; Ci4 = 8000

We Choose K = 4 as a start since

ti / C = 2.8

Assignment

A manufacturer of communications equipment is constructing a line to assemble several similar models of speaker phones. An industrial engineer had divided assembly of each model in to elemental tasks. Phones require about 30 operations. Task times vary from 5 to 36 seconds. Determine the appropriate cycle time if demand requires producing 750 phones per shift. Each shift has 8 productive hours.