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					Assembly Lines – Reliable
Serial Systems

Active Learning
Module 1



                   Dr. César O. Malavé
                  Texas A&M University
Background Material
  Modeling and Analysis of Manufacturing Systems
  by 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

  Introduction                        5
  Readiness Assessment Test (RAT)     5
  Assembly Lines - Introduction       12
  Spot Exercise                       5
  Problem Formulation                 15
  Team Exercise                       5
  Assignment                          3
  Total Time                        50 Mins
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
                                   Higher setup costs
between stations
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
           1, if task i is assigned to station k 
       x                                        
        ik 0, Otherwise                          

  To minimize idle time, we try to force tasks into the
  lowest numbered stations.
  Unused stations will be discarded.
Problem Formulation – Cont…
The formulation becomes
            N         K
                                                               Constraint ensures that the sum of
 min  cik X ik                                               task times for the set of tasks
            i 1 k 1                                          assigned to each workstation
 Subject to                                                    doesn’t exceed the cycle time.
 N

 t X
 i
        i        ik   C       k  1,...,K                     Constraint ensures that the task is
                                                               assigned to exactly one station
 K

 X
 k 1
            ik    1         i  1,...,N                       Constraint forces the adherence to
                                                               precedence restrictions
                  h
 X vh   X uj                  h  1,...,K     and   (u , v)  IP
                 j 1
                                                                Zoning Constraint : Marriage Type
 K

 X
 k 1
            uk    X vk  1       (u , v)  ZS                   Zoning Constraint : Divorce Type
 X uh  X vh  1                k  1,...,K     and   (u , v) ZD
Problem Formulation – Cont…
  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.
                              N
                      K * C   ti
                 D           i 1
                         K *C
  Objective function fails to recognize a secondary
  objective of allocating the idle time equally to all the
  workstations.
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
                   e     4
 Minimise          Cik X ik
                  i  a k 1
                                                                     Ci1 = 1; Ci2 = 20;
                                                                   Ci3 = 400; Ci4 = 8000
 s.t.40 X ak  75 X bk  50 X ck  35 X dk  80 X ek  100
                                                                    We Choose K = 4
                                                     k = 1,…,4
                                                                     as a start since
  4
                                                                       ti / C = 2.8
 X
 k 1
        ik   1        for i  a, b, c, d , e

 X b1  X a1                          
 X b 2  X a1  X a 2                      Likewise for (a, c),
                                      
                                              (c, d) and (d, e)
 X b 3  X a1  X a 2  X a 3                  All Xik   0 or 1
 X b 4  X a1  X a 2  X a 3  X a 4 
                                      
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.

				
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