Chapter 2: Line Balancing

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Chapter 2 Line Balancing

• IE 5511 Human Factors
• Professor Hayes

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Line Balancing

• Goals
• To meet production goals,
• Maximize output.
• Common Approaches to Line Balancing
• Estimating the number of operators for a given
  number of stations,
• Work element sharing grouping activities pr
  work elements into stations or jobs performed
  by a single person (some times multiple people
  work in concert at a single station or machine)

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Estimating the number of operators

• In a perfectly balanced line, all operations at
  all station would take identical time.
• Efficiency would be 100
• However, this rarely happens!!
• 100 efficiency is rarely achievable,
• A more reasonable goal is 95 efficiency.
  (However, even that may not be achievable
  depending on the nature of the operations).

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Estimating the number of Operators

• To achieve a given rate of production, R,
• N operators are needed (total).
• (1) N R x S AM R x S SM
• E

Standard Minutes Time it actually takes to
complete an operation on average
Number of Operators Needed
Desired Rate of Production
Efficiency (expressed as fraction)
Allowed Minutes total time between pieces (e.g.
AM time of slowest operation)
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Procedure for Determining the Number of
Operators needed to meet production goals.

• Assumptions. You have already determined
• the number of workstations,
• their sequence
• the operations that will be performed at each
  one.
• Goals. To
• Meet production goals given to you by your
  management,
• Balance the workload between stations by putting
  more workers at the slower stations,
• Reduce idle time

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Procedure Estimating the Number of Operators

• Givens Production goal, operation sequence.
• Step 0 (Prior to the analysis) Perform time
  studies for each operation using experienced
  operators in order to obtain standard times (SM).
• Step 1 Convert the production rate, R, into the
  same time units as your standard times.
• Step 2 (optional) Estimate the total number of
  operators for the line using Equation (1) (see
  previous slides)
• Step 3 Estimate the number of operators needed
  for each operation,
• Step 4 Identify the slowest operation given the
  number of operators computed in previous step,
• Step 5 Test have you met the production goal?
• Step 6 Adjust. Add more operators, negotiate to
  reduce the production goal, or try additional
  methods.

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Example Estimating the Number of Operators

• Givens
• Production goal 700 units/day where day 8
  hours.
• Operation sequence Op1, Op2, Op3, Op4, Op5,
  Op6, Op7, Op8.
• Step 0 (Prior to the analysis) Perform time
  studies for each operation using experienced
  operators in order to obtain standard times in
  minutes (SM).

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Example Estimating the Number of Operators

• Step 1 Convert the production rate, R, into the
  same time units as your standard times.
• The standard times, SM, have been expressed in
  minutes, while R is in days, so
• R 700 units/day 1.458 units/min
• 480 min/day
• Also compute the desired cycle time (rate at
  which units exit line)
• cycle time 1 0.685 min/unit
• R

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Example Estimating the Number of Operators

• Step 2 (optional) Estimate the total number of
  operators, N, required to meet production goal,
  using Equation (1)

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Example Estimating the Number of Operators

• Step 3 Estimate the number of operators needed
  for each operation,
• Step 4 Identify the slowest operation given the
  number of operators computed in previous step,
• Step 5 Test have you met the production goal?

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Work Element Sharing

• A line can sometimes be balanced with less cost
  by rearranging the sub-work elements (e.g.
  activities composing a work element)
• For example, by giving activities from the
  busiest element to elements with idle time.

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Properties of Work Elements

• What is a work element?
• How big should a work element be?

Assemble items in box
Work Element
Load Styrofoam block
Load book
Sub-work elements
Grasp block
Move block to box
Orient Block
Release Block
Sub-sub work elements
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Work Element Properties

• Work elements can be represented at various
  levels of abstraction or detail
• Work elements can almost always be sub-divided
  into smaller elements.
• The appropriate representation depends on the
  task and situation.

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Work Element Sharing GEs Line Balancing A
Procedure for Assigning Work Elements to
Stations

• Given
• Precedence graph
• Production goal (e.g. 300 units per shift)
• Shift duration (e.g. 450 minutes)
• Number of workstations (e.g. 6 workstations)
• Decided how to assign elements to workstations so
  as to meet production goals without violating
  precedence constraints!

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Precedence relations 1 y is before x
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Compute positional weighs, Record immediate
predecessors,Sort from biggest positional weight
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The Final Assembly Line
A streamlined version
Station 3
Station 1
(05) (06)
(00) (02) (01) (03)
(07) (09)
(08)
(10)
Station 2
Station 6
Station 4
Station 5
(04)