lecture 4-unit 1 lesson 4
TRANSCRIPT
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Unit 1: Lesson 4
PRODUCT LAYOUT DESIGN TECHNIQUES – LINE BALANCING
This lesson will develop understanding of the ‘product’ layout, by considering its detailed design. The technique we will be using is called line balancing and its objective is to design the most efficient process possible which allows the expected volumes to be met. Therefore upon completion of this lesson you should be able to:
• Describe the line balancing technique as a method for product layout design and analysis.
• Apply the technique to design and analyse a typical product layout.
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Detailed design of product layout
Product layout concerned about ‘WHAT TO PLACE WHERE”. Rather than “WHERE TO PLACE WHAT”Locations are frequently decided upon and then work
tasks are allocated to each location. For example, it may have been decided that four stations are needed to make
computer cases. The decision then is which of the tasks that go into making the cases should be allocated to each station.
The main product layout decisions are as follows: What cycle time is needed? How many stages are needed? How should the task-time variation be dealt with? How should the layout be balanced?
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1. Cycle Time
“Cycle time” is used to express the total manual work involved in a process, or between completed part of a process.
Cycle time is a vital factor in the design of product layouts and has a significant influence on most of the other detaileddesign decisions.
It is calculated by considering the likely demand for the products or servicesover a period and the amount of production time available in that period.
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1. Cycle Time: Work example
Suppose the regional back-office operation of a large bank is designing an operation which will process its mortgage applications. The number of application to be processed is 160 per week and the time available to process the applications is 40 hours per week.
Layout Cycle Time =
processed be number toavailable time
hours25.016040
So, the bank’s layout must be capable of processing a completed application once every 15 minutes.
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2. Number of stages
The total amount of work required to produce a unit of output.
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2. Number of stages
Number of stages depends on the cycle time and total quantity of work involved (total work content).
The larger the total work content and the smaller the required cycle time, the more stages will be necessary.
Normal bakery production stages
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Worked example (number of stages)
Suppose the bank in the previous example calculated that the average total work content of processing a mortgage application is 60 minutes. The number of stages needed to produce a processed application every 15 minutes can be calculated as follows:
Number of stages=
timecycle required
content work total minutes 15minutes 60
stages 4
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Task-time variation
In practice, the flow (Task time) may not be regular
Variation in time of processes is a general characteristic of repetitive processes
Variation contributes to irregularity into the flow along the line, periodic queues at the stages, and lost processing time
It is necessary to introduce more resources into the operation to compensate for the loss of efficiency
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Balancing work time allocation
Line balancing is one of the most important design decision in product layout.
Line balancing work contents are allocated equally.It is nearly impossible to achieve line balancing in practice.
Inevitably it will increase the effective cycle time of the line.
The effectiveness of the line-balancing activity is measured by balancing loss time wasted through the unequal allocation of work as a percentage of the total time invested in processing the product or service.
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Balancing techniques (Precedence Diagram)
Precedence diagram This is a representation of the ordering of the elements which compose the total work content of the product or service.
Rules in applying diagram:1. Circles are drawn as far to the left as possible2. None of the arrow should be vertical.
a b c d
e
f g
h
i0.12 mins
0.30 mins
0.36 mins
0.25 mins 0.05 mins
0.17 mins
0.10 mins
0.08 mins
0.25 mins
Each element is represented by a circle.
The circles are connected by arrows, which signify the ordering of the elements.
The precedence diagram, either using circles and arrows or transposed into tabular form, is the most common starting point for most balancing techniques.
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Worked example:
Balancing loss is that proportion of the time invested in processing the product or service which is not used productively.
When the work is equally allocated between the stages, the total time invested in each product or service is 4 x 2.5 = 10 minutes.
However, when work is unequally allocated, the time invested is 3.0 x 4 = 12 minutes, i.e. 2 minutes of time, 16.67% of the total is wasted.
Calculating balancing loss:Idle time every cycle =
Balancing loss =
0
0.5
1
1.5
2
2.5
3
1 2 3 4
Load
Stage
An ideal ‘balance’ where work is
allocated equally between the stages
Work allocated to stage
Idle time
0
0.5
1
1.5
2
2.5
3
3.5
1 2 3 4
Load
Stage
But if work is not equally allocated the cycle time
will increase and ‘balancing losses’ will
occur
Calculating balancing loss:Idle time every cycle =(3.0 - 2.3) +
(3.0 - 2.5) + (3.0 - 2.2) = 2.0 mins
Balancing loss = 2 4 x 3.0= 0.1667= 16.67%
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Balancing techniques
This general approach is to allocate elements from the precedence diagram to the first stage, starting from the left, in order of the columns until the work allocated to the stage is as close to, but less than, the cycle time. When that stage is as full of work as is possible without exceeding the cycle time, move on to the next stage, and so on, until all the work elements are allocated.
The key issue is how to select an element to be allocated to a stage when more than one element could be chosen. Two heuristic rules have been found to be particularly useful in deciding this:
● Simply choose the largest that will ‘fit’ into the time remaining at the stage.
● Choose the element with the most ‘followers’: that is the highest number of elements which can only be allocated when that element has been allocated.
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Worked Example (Karlstad kakes)mins 48.0
Worked ExampleConsider Karlstad Kakes, a manufacturer of specialty cakes, which has recently obtained contract to supply a major supermarket chain with a specialty cake in the shape of a space rocket. It has been decided that the volumes required by the supermarket warrant a special production line to perform the finishing, decorating and packing of the cake. This line would have to carry out the elements shown in the next slide, which also shows the precedence diagram for the total job. The initial order from the supermarket is for 5000 cakes a week and the number of hours worked by the factory is 40 per week. From this:
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Allocation of elements to stages and balancing loss (Case karlstad Kakes).
Element - - De-tin and trim 0.12 minsaElement - Reshape with off-cuts 0.30 minsbElement - Clad in almond fondant 0.36 minscElement - Clad in white fondant 0.25 minsdElement - Decorate, red icing 0.17 minseElement - Decorate, green icing 0.05 minsfElement - Decorate, blue icing 0.10 minsgElement - Affix transfers 0.08 minshElement - Transfer to base and pack 0.25 minsi
Total work content = 1.68 mins
a b c d
e
f g
h
i0.12 mins
0.30 mins
0.36 mins
0.25 mins 0.05 mins
0.17 mins
0.10 mins
0.08 mins
0.25 mins
Allocation of elements to stages and balancing loss (Case karlstad Kakes).
The required cycle time =40 hrs x 60 mins = 0.48 mins 5000
The required number of stages =1.68 mins (total work content) = 3.5 stages 0.48 mins (required cycle time)
This means 4 stages !!!
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Allocation of elements to stages and balancing loss (Case karlstad Kakes).
a b c d
e
f g
h
i0.12 mins
0.30 mins
0.36 mins
0.25 mins 0.05 mins
0.17 mins
0.10 mins
0.08 mins
0.25 mins
Stage 1 Stage 2 Stage 3 Stage 4
00.10.20.30.40.50.6
1 2 3 4
Cycle time = 0.48 mins
Idle time every cycle = (0.48 - 0.42) + (0.48 - 0.36) + (0.48 - 0.42) = 0.24 minsProportion of idle time per cycle = 0.24
4 x 0.48
= 12.5%
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Arranging the stages
Long thin: A process designed to have many sequential stages, each performing a relatively small part of the total task, the opposite of short fat processes.
Short fat: Processes designed with relatively few sequential stages, each of which performs a relatively large part of the total task, the opposite of long thin processes.
15 mins 15 mins 15 mins 15 mins
30 mins 30 mins
30 mins 30 mins
60 mins
60 mins
60 mins
60 mins
15 mins
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Arranging the stages
In any particular situation there are usually technical constraints which limit either how ‘long and thin’ or how ‘short and fat’ the layout can be, but there is usually a range of possible options within which a choice needs to be made.
The advantages of each extreme of the long thin to short fat spectrum are very different and help to explain why different arrangements are adopted.
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Arranging the stages
Controlled flow of material/customer easy to manage Simple material handling; especially for heavy, large
or difficult products Lower capital requirements (If a specialist piece of
equipment is needed for one element in the job, only one piece of equipment would need to be purchased; on short fat arrangements every stage would need one)
More efficient operation
Advantages of long thin arrangement:
Advantages of short fat arrangement: Higher mix flexibility: each stage could specialize in different types. Higher volume flexibility: as volume varies, stages can simply be
closed down or started up as required. (long thin arrangements would need rebalancing each time the cycle time changed)
Higher robustness: if one stage breaks down, the other parallel stages are unaffected. (a long thin arrangement would cease operating completely)
Less monotonous work
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Exercise:
An automobile company is pushed to manufacture 150 car per week, with working hour of 90 hours per week. If the total work content for manufacturing a car is 5 hours, determine:
1. The cycle time2. Number of the stages
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QUESTIONS
QUESTION 1 A DIY product manufacturing company wishes to create a ‘product’ focused layout for the manufacture of their new cordless drill. There order book requires them to produce 480 drills per eight hour day. The assembly operations for the drill and the associated information are given in the table below. Therefore the key tasks are to:
(a) calculate the maximum cycle time
(b) calculate the minimum number of work stations
(c) produce the precedence diagram
(d) produce the line design which achieves the required cycle time
(e) calculate the new cycle time and idle time/cycle.
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Assembly operation