1. Assembly Line Balance
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2. 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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3. The Assembly Chart
The assembly chart of a toolbox
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4. Time Standards Are Required for Every Task
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5. 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.
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6. Plant Rate and Conveyor Speed
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7. 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).
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8. 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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9. Assembly line balancing
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. SA = 240 pieces per hour and hour each. If balanced, the standard would be 180 pieces per hour and hour each.
.0057 balanced cost
by itself cost
savings hour per unit
X 500,000 units per year
700 hours per year
@ $ per hour
= $10, per year savings
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10. 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.
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11. 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 percent station limits the output of the whole assembly line.
(b) The total hours required to assemble one finished toolbox is hour. The average hourly wage rate times hour per unit gives us the assembly and packout labor cost. Again, the lower this cost is the better the line balance is.
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12. Assembly line balancing
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13. Assembly line balancing
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14. 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.
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15. Step-by-step procedure for completing the assembly line balancing form
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16. 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
12. Average cycle time
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17. 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.
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18. 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.
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19. Step-by-step procedure for completing the assembly line balancing form
14. Hours per unit:
Example: Hours per unit of the toolbox assembly line balance
The .167 time standard is for one person, if considering the people number, the hour per unit will be:
Two people = hour per unit
Three people = hour per unit
Four people = hour per unit
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20. 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 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 minutes divided by 60 minutes per hour equals hour per unit.
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21. Efficiency of the assembly lineor
For our example:
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22. Analysis of single model assembly lines
Production Rate is given by
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.
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23. Analysis of single model assembly lines
The cycle time can be determined as
where Tc = cycle time of the line, min./cycle;
Rp = production rate, units/hr;
E = line efficiency;
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24. Analysis of single model assembly lines
The cycle rate can be determined as
where Rc = cycle rate, cycles/hr;
Tc is in min./cycle;
Line efficiency E therefore defined as:
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25. Analysis of single model assembly lines
The number of workers on the line can be determined as
where w = number of workers on the line;
WL = workload to be accomplished in a given time period.
AT = available time in the period.
TWc = work content time, min/piece.
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26. Analysis of single model assembly lines
Using the previous equation, we also have
The available time in the period, AT.
AT = 60E
Substitute these terms for WL and AT into w equation, we can state:
If we assume one worker per station, then this ratio also gives the theoretical minimum number of workstations on the line.
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27. 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.
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28. Analysis of single model assembly lines
Example
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29. Analysis of single model assembly lines
Example
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30. Analysis of single model assembly lines
Solution:
The total work content time is:
Twc = 4.0 min.
(b) The production rate is:
(c) The cycle time Tc with an uptime efficiency of 96% is:
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31. Analysis of single model assembly lines
Solution:
(d) The theoretical minimum number of workers is given by:
(e) The average service time against which the line must be balanced is:
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32. 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
subject to:
and
(2) all precedence requirements are obeyed.
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33. Analysis of single model assembly lines
The algorithms are:
Largest Candidate Rule
Kilbridge and Wester method
Ranked positional weights
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34. 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.
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35. Largest Candidate Rule
Work elements sorted in descending order
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36. 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:
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37. Largest Candidate Rule
Work elements assigned to stations by LCR
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38. Analysis of single model assembly lines
Example
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39. Analysis of single model assembly lines
Kilbridge and Wester method
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40. Analysis of single model assembly lines
Ranked positional weights
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41. Analysis of single model assembly lines
Ranked positional weights
Largest Candidate Rule
Kilbridge and Wester method
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42. 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
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