Line Balancing — Explanation of Total Idle Time
There were a couple of questions at the end of problem 5 on page 9. Let me give you further
explanation here so that you understand the logic. Keep that page with the solution in front of
you when you read the following:
Q1: Total idle time per day = (Available time / Bottleneck station time) x Idle time per cycle.
Total idle time in 1st arrangement = (480/12) x 6 = 240 min.
The question is: Why not 34 x 6 = 204?
As you know, 34 units is the required output, but the assembly line arrangement led to a 12-
minute bottleneck station time (instead of 14.1 min). With this cycle time of 12 minutes, we can
actually produce 40 units in 480 minutes, not just 34 units. If we produce only 34 units, the total
time it takes is only 408 minutes, not 480 minutes. So, we will have 72 minutes left in the day
either to do other things or to continue producing the same (6 more units) until 480 minutes.
Since we do not have any information about doing other things or wasting that time of 72
minutes, we assume that they continue producing until 480 minutes. So, they will actually
produce 40 units and the total idle time is 240 minutes.
Q2: If you have only 480 minutes in a day, how can you have 617 minutes of idle time in the 2nd
arrangement?
When you add idle time at each station, we got 18 minutes of idle time. If it takes only 14
minutes to produce one unit, how did we get 18 minutes of idle time? Well, these 18 minutes are
from six stations working simultaneously contributing on average 3 minutes of idle time per
station. Assume that you have one person at each station working 480 minutes per day. That is,
480 x 6 = 2880 minutes of time in a day. Out of that, the idle time is 617 minutes. So, % idle