ch 7 assembly line balance by HC12110410446

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									Assembly Line Balance




    Balance         1
Assembly analysis

                   Assembly Chart
    It shows the sequence of operations in putting
the product together. Using the exploded drawing
and the parts list, the layout designer will diagram
the assembly process.

    The sequence of assembly may have several
alternatives.

    Time standards are required to decide which
sequence is best. This process is known as assembly
line balancing.
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The Assembly Chart



The assembly chart
of a toolbox




                     Balance   3
Time Standards Are Required for Every Task




                     Balance                 4
Plant Rate and Conveyor Speed


 Conveyor speed is dependent on the number and
 units needed per minute, the size of the unit, the
 space between units. Conveyor belt speed is
 recorded in feet per minute.


     Example:
 Charcoal grill are in cartons 30X30X24 inches
 high. A total of 2,400 grills are required every
 day.


                          Balance                     5
Plant Rate and Conveyor Speed




                     Balance    6
Assembly line balancing
   The purpose of the assembly line balancing technique is:

   1.    To equalize the work load among the assemblers
   2.    To identify the bottleneck operation
   3.    To establish the speed of the assembly line
   4.    To determine the number of workstations
   5.    To determine the labor cost of assembly and packout
   6.    To establish the percentage workload of each operator
   7.    To assist in plant layout
   8.    To reduce production cost

    The assembly line balancing technique builds on:
         The assembly chart;
         Time standards;
         Takt time (minutes/piece)     (Plant rate, R value,
 Pieces/minutes).
                                   Balance                       7
Initial assembly line balancing of toolbox




 Takt time (for 2,000 units per shift, considering 10%
 downtime and 80% efficiency) = .173 minutes per unit.
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Assembly line balancing

 1. Cost of balancing
    Subassemblies that cost too high can be taken off the
 line.

    SA3 could be taken off the assembly line and handled
 completely separate from the main line and we can save money.
 SA3 .250 = 240 pieces per hour and .00417 hour each. If balanced,
 the standard would be 180 pieces per hour and .00557 hour each.
         .0057 balanced cost
    -    .00417 by itself cost
         .00140 savings hour per unit
    X    500,000 units per year
                700       hours per year
    @ $15.00 per hour
    =    $10,500.00 per year savings
                                 Balance                             9
Assembly line balancing

   Subassemblies that can be taken off the line must be:

   1.   Poorly loaded. The less percent that is loaded.
        For example, a 60 percent load on the assembly
        line balance would indicate 40 percent lost time.
        If we take this job off the assembly line (not tied
        to the other operators), we could save 40 percent
        of the cost.
   2.   Small parts that are easily stacked and stored.
   3.   Easily moved. The cost of transportation and the
        inventory cost will go up, but because of better
        labor utilization, total cost must go down.

                              Balance                         10
Assembly line balancing

   2. Improvement of assembly line
      Improve the busiest (100 percent) workstation first.
  (a) The busiest workstation is P.O. It has .167 minute of work to do
      per packer. The next closest station is A1 with .155 minute of
      work. As soon as we identify the busiest workstation, we identify
      it as the 100 percent station, and communicate that this time
      standard is the only time standard used on this line from now
      on. Every other workstation is limited to 360 pieces per hour.
      Even though other workstations could work faster, the 100
      percent station limits the output of the whole assembly line.
  (b) The total hours required to assemble one finished toolbox is
      .06960 hour. The average hourly wage rate times .06960 hour
      per unit gives us the assembly and packout labor cost. Again,
      the lower this cost is the better the line balance is.


                                   Balance                            11
Assembly line balancing




                      Balance   12
Assembly line balancing




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Assembly line balancing

    2. Improvement of assembly line
       Improve the busiest (100 percent) workstation first.
  Look at the 100 percent station (P.O.).

  If we add a fourth packer, we will eliminate the 100 percent station
  at P.O.

  Now the new 100 percent (bottleneck station) is A1 (93 percent).
  By adding this person, we will save 7 percent of 25 people or 1.75
  people and increase the percent load of everyone on the
  assembly line (except P.O.). We might now combine A1 and A2,
  and further reduce the 100 percent.

  The best answer to an assembly line balance problem is the
  lowest total number of hours per unit. If we add an additional
  person, that person’s time is in the total hours.
                                     Balance                             14
Step-by-step procedure for completing the
assembly line balancing form




                      Balance               15
Step-by-step procedure for completing the
assembly line balancing form

 9. R value
    The R value goes behind each operation. The plant rate is the
    goal of each workstation, and by putting the R value on each
    line (operation), one keeps that goal clearly in focus.

 10. Cycle time
     The time standard.

 11. Number of stations
                                       cycle time
                  number of stations 
                                           R
 12. Average cycle time
                                             cycle time
                          ave. cycle time 
                                            # of stations
                                 Balance                            16
Step-by-step procedure for completing the
assembly line balancing form

 13. Percentage load:
        The percentage load tells how busy each workstation is
     compared to the busiest workstation.
        The highest number in the average cycle time column 12 is
     the busiest workstation and, therefore, is called the 100 percent
     station.
        Now every other station is compared to this 100 percent
     station by dividing the 100 percent average station time into
     every other average station time. The percent load is an
     indication of where more work is needed or where cost
     reduction efforts will be most fruitful. if the 100 percent station
     can be reduced by 1 percent, then we will save 1 percent for
     every workstation on the line.

                                    Balance                                17
Step-by-step procedure for completing the
assembly line balancing form

 13. Percentage load:

 Example: percent load of the toolbox assembly line balance

  In Figure 4-11, the average cycle times reveals that .167 is the
largest number and is designated the 100 percent workstation.
  The percentage load of every other workstation is determined by
dividing .167 into every other average cycle time:

       Operation SSSA1 = .153 / .167 = 92 percent
                  SSA1 = .146 / .167 = 87 percent
                  SSA2 = .130 / .167 = 78 percent

               and so on.
                                  Balance                            18
Step-by-step procedure for completing the
assembly line balancing form

 14. Hours per unit:
                  100 % average cycle time
         h.p.u. 
                    60 minutes per hour
  Example: Hours per unit of the toolbox assembly line balance

                   .167
          h.p.u.        .00278 hour per unit
                    60
 The .167 time standard is for one person, if considering the people
number, the hour per unit will be:
       Two people = .00557 hour per unit
       Three people = .00835 hour per unit
       Four people = .01113 hour per unit

                                   Balance                             19
Step-by-step procedure for completing the
assembly line balancing form

 15. Piece per hour:
    Inversion of hours per unit.

 16. Total hours per unit
      Sum of the elements in column 14. For this example is .0696
      hour.

 17. Average hourly wage rate, say $15 per hour

 18. Labor cost per unit
       Total hours X average hourly wage

 19. Total cycle time
      It tells us what a perfect line balance would be.
      Our example 3.494 minutes divided by 60 minutes per hour
      equals .05823 hour per unit. Balance                     20
 Efficiency of the assembly line


                       Sum of hours per 1000
Line efficiency                                      100%
                  Sum of hours per 1000 line balance

or
                       Sum of hours per unit
Line efficiency                                      100%
                  Sum of hours per unit line balance

 For our example:
                         0.05823
       Line efficiency           100%  84%
                         0.06960
                             Balance                      21
Analysis of single model assembly lines

Production Rate is given by

                            Da
                    Rp 
                         50 S w H sh

 where Rp = average hourly production rate, units/hr;
 Da = annual demand, units/year;
 Sw = number of shifts/week;
 Hsh = hrs/shift.
 This equation assume 50 weeks per year.

                               Balance                  22
Analysis of single model assembly lines

The cycle time can be determined as

                         60 E
                    Tc 
                          Rp

 where Tc = cycle time of the line, min./cycle;
 Rp = production rate, units/hr;
 E = line efficiency;




                             Balance              23
Analysis of single model assembly lines

The cycle rate can be determined as

                         60
                    Rc 
                         Tc

 where Rc = cycle rate, cycles/hr;
 Tc is in min./cycle;
 Line efficiency E therefore defined as:
                        Rp
                        Tc
                 E   
                    Rc Tp
                              Balance      24
Analysis of single model assembly lines

The number of workers on the line can be
determined as
                      WL
                   w
                      AT
 where w = number of workers on the line;
 WL = workload to be accomplished in a given time period.
 AT = available time in the period.


    WL  R pTwc      TWc = work content time, min/piece.

                             Balance                        25
Analysis of single model assembly lines

 Using the previous equation, we also have

                        60 ETwc
                   WL 
                           Tc
  The available time in the period, AT.   AT = 60E

Substitute these terms for WL and AT into w
equation, we can state:
                                   Twc
             w  minimun integer 
               *

                                   Tc
If we assume one worker per station, then this ratio also
gives the theoretical minimum number of workstations on
the line.
                              Balance                       26
Analysis of single model assembly lines

Example
A small electrical appliance is to be produced on a single
model assembly line. The work content of assembling the
product has been reduced to the work elements listed in
table below along with other information. The line is to be
balanced for an annual demand of 100,000 units per year.
The line will be operated 50 weeks/yr, 5 shifts/wk, and 7.5
hrs/shift. Manning level will be one worker per station.
Previous experience suggests that the uptime efficiency for
the line will be 96%, and repositioning time lost per cycle
will be 0.08 min. Determine (a) total work content time
Twc, (b) required hourly production rate Rp to achieve the
annual demand, (c) Cycle time, and (e) service time Ts to
which the line must be balanced.
                             Balance                          27
Analysis of single model assembly lines

Example




                     Balance              28
Analysis of single model assembly lines

Example




                     Balance              29
Analysis of single model assembly lines

Solution:
 (a) The total work content time is:
    Twc = 4.0 min.

 (b) The production rate is:
            100 ,000
      Rp                53 .33 units/hr
           50 (5)( 7.5)
 (c) The cycle time Tc with an uptime efficiency of 96% is:

                 60(0.96)
            TC            1.08 min .
                  53.33
                               Balance                    30
Analysis of single model assembly lines

Solution:
(d) The theoretical minimum number of workers is given by:

                       Twc
        w*  min int       3.7  4
                       Tc


 (e) The average service time against which the line must
 be balanced is:

      Ts  Tc  TR  1.08  0.08  1.00 min .

                            Balance                         31
Analysis of single model assembly lines

The objective in line balancing is to distribute
the total workload on the assembly line as
evenly as possible among the workers
                                              w
 minimize ( wTs  Twc ) or minimize            (T  T
                                              i 1
                                                     s   si   )
 subject to:

                  (1)   T
                        k i
                               ek    Ts
                   and

                   (2) all precedence requirements are
                   obeyed.
                                    Balance                       32
Analysis of single model assembly lines




The algorithms are:
1) Largest Candidate Rule
2) Kilbridge and Wester method
3) Ranked positional weights




                       Balance            33
  Largest Candidate Rule

Step 1: Rank the Teks in the descending order.
Step 2: Assign the elements to the worker at first station
by starting at the top of the list and selecting the first
element that satisfies precedence requirements and does
not cause the total sum of Tek at that station to exceed the
allowable Ts; when an element is selected for assignment
to the station, start back at the top of the list for
subsequent assignments.
Step 3: when no more element can be assigned without
exceeding Ts, then proceed to the next station.
Step 4: repeat steps 2 and 3 for as many additional
stations as necessary until all elements have been
assigned.
                               Balance                         34
Largest Candidate Rule

Work elements sorted in descending order




                       Balance             35
Largest Candidate Rule

Solution:
The largest candidate algorithm is carried out as presented
in table below. 5 workers and stations are required in the
solution. Balance efficiency is computed as:


               Twc   4.0
            E            0.8
               wTs 5(1.0)




                             Balance                          36
Largest Candidate Rule
Work elements assigned to stations by LCR




                        Balance             37
Analysis of single model assembly lines

Example




                     Balance              38
Analysis of single model assembly lines

 Kilbridge and Wester method




                      Balance             39
Analysis of single model assembly lines

 Ranked positional weights




                       Balance            40
 Analysis of single model assembly lines




                                      Ranked positional
Largest Candidate   Kilbridge and
                                         weights
   Rule             Wester method


                            Balance                       41
    Analysis of single model assembly lines


     Automation, Production Systems, and
      Computer-Integrated Manufacturing, By
      Mikell P. Groover, 3rd edition, c2008.
     Manufacturing Facilities Design and Material
      Handling, By F. E. Meyers and M. P. Stephens,
      4th Edition, Prentice-Hall, Inc., 2010




                           Balance                    42

								
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