Line balancing and heuristics

LINE BALANCING

Abrity BhattacharyaHariharan Ramanathan

LINE BALANCINGThis is an analysis process which tries to

equally divide work to be carried out in a production process among workstations.

In production line workstations and work centers are arranged in a sequence of task to be done along a straight or curved line.

GOAL AND OBJECTIVEGoal of production line analysis:Number of workstations needed.Type of task should be assigned to each

workstation.To know the minimum number of workers, tools

and machines that can be used to provide the required amount of capacity.

Objective of line balancing:Assigning task to each workstation in such a way

that there is only a little idle time.

TERMINOLOGIES RELATED TO PRODUCTION LINE ANALYSISTASKS: All the elements of work.TASK PRECEDENCE: The sequence in

which tasks are performed.TASK TIMES: Time required by a well-trained

worker or unattended machine to perform a task. This is usually expressed in minutes.

CYCLE TIME: The time expressed in minutes between two simultaneous products coming off the end of a production line.

Continued…PRODUCTIVE TIME PER HOUR: The average

number of minutes a workstation is working in an hour.WORKSTATION: A physical area where a worker

with tools/ one or more machines, or an unattended machines like a robot performs a particular set of task in a production line.

WORK CENTER: A small group of identical workstations, where each workstation performs the same set of task.

NUMBER OF WORKSTATIONS WORKING: The amount of work done at a work center expressed in number of workstations.

Continued…MINIMUM NUMBER OF

WORKSTATIONS: The least number of workstation that provides the required production.

ACTUAL NUMBER OF WORKSTATIONS: This is the total number of workstations required on the entire production line. It is calculated as the next higher integer of the number of workstations working.

UTILIZATION: The percentage of time a production line is working.

LINE BALANCING PROCEDURE1. Determination of tasks that must be performed to

complete one unit of a product.2. Determining the order or sequence of performing

the whole set of tasks.3. Drawing precedence diagram. In this flowchart

circles represents task and joining arrows represents precedence.

4. Estimation of task time.5. Calculation of cycle time.6. Determination of minimum number of workstation

required.7. Using one of the heuristics to assign tasks to

workstations for balancing production line.

COMPUTERIZED LINE BALANCINGLine balancing by hand becomes unwieldy as the problems

grow in size. There are software packages that will balance large lines

quickly. IBM's COMSOAL (Computer Method for Sequencing Operations for Assembly Lines) and GE's ASYBL (Assembly Line Configuration Program) can assign hundreds of work elements to workstations on an assembly line.

They use various heuristics, to balance the line at an acceptable level of efficiency.

The POM for Windows software lets the user select from five different heuristics: ranked positional weight, longest operation time, shortest operation time, most number of following tasks, and least number of following tasks. These heuristics specify the order in which work elements are considered for allocation to workstations.

Line Balancing HeuristicsTwo typesIncremental Utilization HeuristicLongest Task Time Heuristic

Incremental HeuristicTasks are added to each workstation in order

of task precedence one at a time until utilization is 100 percent or is observed to fall.

Used when one or more task time is equal to or greater than the cycle time.

Incremental Heuristics

ExampleTextech, a large electronics manufacturer, assembles

model AT7S handheld calculators at its Midland , Texas, plant. The assembly tasks that must be performed on each calculator are shown below.

The parts used in this assembly line are supplied by materials-handling personnel to parts bins used in each task. The assemblies are moved along by belt conveyors between workstations.

Textech wants this assembly line to produce 540 calculators per hour.

a. Compute the cycle time per calculator in minutes. b. Compute the minimum number of workstations. c. How would you combine the tasks into workstations

to minimize idle time? Evaluate your proposal.

SOLUTION

Precedence Diagram

Longest-Task-Time Heuristic Add tasks to a workstation one at a time in

the order of task precedence. If a choice must be made between two or

more tasks, the one with the longest task time is added.

This has the effect of assigning as quickly as possible the tasks that are the most difficult to fit into a station.

Tasks with shorter times are then saved for fine-tuning the solution.

Conditions1. It can be used only when each and every task

time is less than or equal to the cycle time.2. There can be no duplicate work stations.

If each and every task time is less than or equal to the cycle time, and if the primary focus of the analysis of production lines is minimizing the number of workstations and the amount of tools and equipment required, then this heuristic would be appropriate.

Steps in the Longest-Task-Time Heuristic1. Lei i = I. where i is the number of the workstation being formed.2. Make a list of all the tasks that are to be assigned to a workstation.

Have to satisfy the following conditions a. It cannot have been previously assigned to this or any previous

workstation. b. Its immediate predecessors must have been assigned to this or a

previous workstation. c. The sum of its task time and all other times of tasks are already

assigned to work station must be less than or equal to the cycle time . If no such tasks are found. Go to step 4.

3. Assign the from the list with the longest task time to the workstation. Go back to the step 2.

4. Close the assignment of tasks to Workstation i. This can occur in two ways. If there are no tasks on the list for the work station but there are still tasks to be assigned, set i=i+1 and go back to step 2. If there are no more tasks, the procedure is completed.

PROBLEM

Using the above information in the tablea. Draw a precedence diagramb. Assuming that 55 minutes per hour are productive,

compute the cycle time needed to obtain 50 units per hour.

SOLUTION

LINE BALANCING ISSUESEarlier conveyor belts are paced with the

speed of the employees work.Now they are paced with the machine speed.

APPLICATIONSThe classic example is Henry Ford’s auto

chassis line. • Before the “assembly line Balancing” was introduced in 1913, each chassis was assembled by one worker and required 12.5 hours. • Once the new technology was installed, this time was reduced to 93 minutes.