Line Balancing – How to do it

Line balancing is done to reduce or eliminate the waiting time in the workstations of an assembly line which runs in one piece flow. Thus we can utilize the resources such as manpower and machinery/equipment more effectively.

Line balancing definition

Line balancing is a method for eliminating idle time in assembly line and also for adapting the line to takt time.

Relationship between bottleneck and Line balancing in manufacturing industry

Before we go to how to do line balancing I recommend you to read the article on Bottleneck in a manufacturing industry. So that you can easily understand this line balancing. In the example explained in the article ‘Bottleneck in a manufacturing industry’, the operators at the work station A, B and D will be waiting, because the cycle time of these work station is lower than workstation C.

Aim of line balancing

So our aim of line balancing is to make the cycle time of all workstation almost same so that there is no waiting. Through line balancing we are eliminating one of the 8 wastes of lean manufacturing.

Also there is another very important aim of line balancing, which is aligning the assembly line according to the demand, by the concept of takt time. Thus we prepare the assembly line for delivering the right quantity according to the demand.

Ok lets start…

How to do line balancing of an assembly line (explained with example)

Let’s balance the assembly line of a toy.

Assembly process have following activities.

Activity 1 : Inspection and flash removal of plastic moulded part 1. Cycle time 1.48 minutes.

Activity 2 : Inspection and flash removal of plastic moulded part 2. Cycle time 1.70 minutes.

Activity 3 : Assembly of part 1 and part 2. Cycle time 0.98 minutes.

Activity 4 : Insertion of screw 1. Cycle time 0.48 minutes.

Activity 5 : Insertion of screw 2. Cycle time 0.50 minutes.

Activity 6 : Insertion of screw 3. Cycle time 0.48 minutes.

Activity 7 : Cleaning. Cycle time 1 minutes.

Activity 8 : Labelling. Cycle time 0.65 minutes.

Activity 9 : Inspection. Cycle time 0.75 minutes.

Activity 10 : Packing. Cycle time 0.9 minutes.

Demand of the toy per day is 240 nos. And the assembly line works in one shift per day. And working time per shift is 420 minutes.

This is the all details.

Now we shall start the calculation.

Takt time calculation

First thing to calculate is takt time. Here takt time is 420/240, which is 1.75 minutes.

And total cycle time is 8.92 minutes (sum of all activities).

Now we can calculate theoretical no of work station, which is total cycle time/takt time.

= 8.92/1.75

= 5.09

Say 6 work stations.

This theoretical work station is just for information. But we can fix it as the minimum no of work station required, which is 6 nos.

No we can allocate the activities to the work stations. But, before that we should know the precedence of each activity. This is usually communicated by precedence table or precedence diagram. Please refer the precedence table below.

Precedence table

Our situation is very simple and it’s good to start with the simple one.

By referring the precedence diagram or table we can know which are the activities to be done before and after an activity.

Ok, now we can allocate the activities to the work station. The most important thing to be noted while allocating activities to work station is, the total cycle time of the work station should not be more than the takt time, which is 1.75 minutes. If its not it will not deliver the desired output which is 240 nos per shift.

Work station 1 : Activity 1, Cycle time is 1.48 minutes.

Work station 2 : Activity 2, Cycle time is 1.70 minutes.

Work station 3 : Activity 3, Cycle time is 0.98 minutes.

Work station 4 : Activity 4 + Activity 5 + Activity 6, Cycle time is 1.46 minutes.

Work station 5 : Activity 7 + Activity 8, Cycle time is 1.65 minutes.

Work station 6 : Activity 9 + Activity 10, Cycle time is 1.65 minutes.

No we have finished the activities allocation to work station. We are lucky that we finished with the theoretical no of work station. This will not be the case all time, it depends on precedence table, it’s cycle time and takt time.

Line balancing formulas

Following are the formulas used while doing line balancing.

• Balance delay
• Line efficiency
• Smoothness index

These formulas are used to check the effectiveness of line balancing.

Please refer section below for knowing how to check the effectiveness of line balancing.

Checking the effectiveness of line balancing

Ok now we can check how good is our balanced assembly line. For that we can do some calculations for calculating balance delay, line efficiency and smoothness index.

Balance delay

Balance delay (%) = ((No of work station x bottleneck cycle time – total cycle time) / (No of work station x bottleneck cycle time)) x 100

= ((6 x 1.70 – 8.92) / (6 x 1.70)) x 100

= (1.28 / 10.2) x 100

= 12.55 %

Which means the workstations are idle for 12.55% of its working time.

Line efficiency

Line efficiency (%) = (total cycle time / (no of work station x bottle neck cycle time)) x 100

= 8.92 / (6 x 1.70)

= 8.92 / 10.5

= 87.45 %

Which means assembly line is active 87.45% of its working time.

Line efficiency can also be calculated by (1 – Balance delay)

Smoothness index

Smoothness index is calculated by the following formula;

Here smoothness index is 0.7933. Please do some maths and check the answer is correct or not.

Smoothness index indicates the smoothness of flow of product through the assembly line. If the index is close to zero its a perfect line.

Here we have done the line balancing. Here I don’t say this is a perfect one since the line efficiency is 87.45%. There is still a scope for improvement. This can be only achieved by method study and realignment of a activities. lets do it in another post of method study. But we have done a good work here.

Thanks for reading.

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