Line Balancing (Designing Product Layout)
The process of assigning tasks to workstations in such a way
that the workstations have approximately equal time requirements
- The process of deciding how to assign tasks to workstations
- Minimizes idle time along line & results in a high utilization of labor & equipment
Perfectly balanced line will have a smooth flow of work
Cycle time
The maximum time allowed at each workstation to complete its
set of tasks on a unit
- Establishes the output rate of a line
The primary determinant of line balancing is what the line’s
cycle time will be
- Minimum cycle time is equal to the longest task time(1.0 min.) & the maximum cycle time is equal to the sum of the task times(0.1+0.7+1.0+0.5+0.2+2.5 min.)
- Minimum cycle time apply if there were five workstations
- Maximum cycle time apply if all tasks were performed at a single workstation
Assume, line will operate for eight hours per day (480 min.)
& cycle time is 1.0 min.
Cycle time also can be determined by the desired output rate
(suppose 480 units)
Theoretical minimum number of stations required to provide a
specified rate of output
2.5 stations is not feasible, so necessary to round up to
three stations (2.5 is minimum)
Precedence diagram (Balancing)
A diagram that shows elemental tasks & their precedence
requirements
- A very useful tool in line balancing
- Assign tasks in order of most following tasks
- Assign tasks in order of greatest positional weight (sum of each task’s time)
Assign tasks in order of most number of followers
- Begin
with task a; it has the most following tasks. Assign it to workstation 1.
- Next,
tasks b & c each have two following tasks, but only task c will fit in
the time remaining at workstation 1, so assign task c to workstation 1.
- Task b
now has the most followers, but it will not fit at workstation 1, so
assign it to workstation 2.
- There
is no time left at workstation 2, so we move on to workstation 3,
assigning task d & then task e to that workstation.
Balancing loss
The percentage of idle time of the line
(Nactual = Actual number of stations)
Parallel workstations (Balancing)
Network (precedence) analysis
Diagram of activities that shows sequential relationships by
use of arrows & nodes
- Graphical display of project activities
- Estimate of how long the project will take
- Indication of which activities are the most critical for timely completion
- Indication of how long any activity can be delayed without delaying the project
Two main methods:-
- Critical
Path Method (C.P.M.)
- Program
Evaluation & Review Technique (P.E.R.T.)
- Activities: Steps that consume resources &/or time.
- Events: The starting & finishing of activities, designated by nodes in the AOA
- Activity-on-arrow(AOA): Network diagram convention where arrows designate activities
- Activity-on-node(AON): Network diagram convention in which nodes designate activities
- Path: A sequence of activities, leads from the starting node to the finishing node
- AOA diagram, the sequence 1-2-4-5-6 & AON diagram, S-1-2-6-7 is a path
- Critical path: The longest path; determines expected project duration
- Critical activities: Activities on the critical path
- Slack (Float): Allowable slippage for a path; the difference between the length of a path & the length of the critical path
Activity on node format
- EFT: is the earliest possible time at which an activity can finish
- EST: is the earliest possible time at which an activity can start
- LFT: is the latest possible time at which an activity can finish without affecting the completion of project
- LST: is the latest time by which an activity can start without affecting the completion of project
- Total float: is the maximum leeway available to an activity (n) such that it will not delay the completion of the project
Total Float(n) = LSTn - ESTn
- Free Float: is the leeway available to an activity (n) such that it will not affect the float of succeeding activities
Free Float(n) = ESTn+1 - EFTn
A-O-N Calculations (CPM)
Total & free float Calculations & Critical path
Total & free float Calculations & Critical path
Critical path highlighted in red color (the sequence of
activities that takes longest time)
- Any delays of activities on the critical path will delay the entire project
- Only way to make project earlier is to decrease time from critical path operation
Critical Path Method (CPM) assumes
- The project networks defines a correct sequence of work in terms of technology & workflow
- Activities are assumed to be independent of one another with clearly start & finish time
- The activity time estimates are accurate & stable
- Once an activity is started it continues uninterrupted until it is completed
- There is infinite resource capacity, at least for the initial (baseline) project analysis
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