[ Home ] [ Up ] [ Time ] [ Improve ] [ Flow-1 ] [ Flow-2 ] [ Delay Study ] [ Buffer mgt ] [ People & Mcs ] [ Set-Up Red'n ]

Flow The Operation - 1

LINE BALANCE PROCESS

So far we have been looking at the establishment of standards for individual operations. In the case of assembly lines several or many individual operations can be linked together. It is important to remember that an assembly line can only operate at the rate of the SLOWEST OPERATION. This operation is normally called the KEY OPERATION or CONSTRAINT OPERATION.
Example: -  

Op No Operation Description Actual Time Secs / Operation Wait Time Secs / Operation Total Time Secs / Operation No of People used Gross Parts / Hour

05

 Screw cap to shaft 15 5 20 1 240
10 Torque test & date stamp cap 10 10 20 1 360
15 Assemble 2 "O" rings to shaft 20 nil 20 1 180
20 Assemble 1st cover to shaft 14 6 20 1 257
25 Assemble 2nd cover to shaft 14 6 20 1 257
30 Inspect and pack assembly 10 10 20 1 360

Totals     

 83 37 120 6  

Effectiveness %      

 69% 31% 100%    
As you can see the slowest operation is op 15 at a 20 second cycle time and is designated the “Key Operation”.  The previous operations cannot operate at their potential speed because work would simple pile up in front of op 15.  The subsequent operations could not go faster than op 15 because they would be starved of work from it.  The line would run at 180 parts per hour.  Lets look at the effectiveness of the line as it is organized above.  We can see the line is poorly organized because 31% of the total time is enforced wait time due to poor balancing.  A well-balanced line seeks to minimize the enforced wait time.  To improve the effectiveness we need to re-organize the line, but before we do let’s take a graphical look at the line.  A convenient way of reviewing the line balance process is to look at the operations as a histogram in terms of throughput in gross parts per hour of each operation.  

There are a number of ways in which we can re-organize the line; perhaps the most obvious is to add an extra person at the slowest operation.  This would mean that operation 15 would run at 180 x 2 = 360 parts / hour.  The result of adding an additional person at operation 15 would be as follows -

To see if we have improved the situation we now need to revisit the original table and substitute the change that we have made.
Op No Operation Description Actual Time Secs / Operation Wait Time Secs / Operation Total Time Secs / Operation No of People used Gross Parts / Hour

05

 Screw cap to shaft 15 nil 15 1 240
10 Torque test & date stamp cap 10 5 15 1 360
15 Assemble 1 "O" rings to shaft 10 5 15 1 360
16 Assemble 1 "O" rings to shaft 10 5 15 1 360
20 Assemble 1st cover to shaft 14 1 15 1 257
25 Assemble 2nd cover to shaft 14 1 15 1 257
30 Inspect and pack assembly 10 5 15 1 360

Totals     

 83 22 105 7  

Effectiveness %      

 79% 21% 100%    
This is an improvement against the original line-up as the wasted time has been reduced from 31% to 21% of the total time taken.  We have also increased the line speed to 240 parts per hour.  Now lets look at combining operations and reducing the number of people on the line.  
Op No Operation Description Actual Time Secs / Operation Wait Time Secs / Operation Total Time Secs / Operation No of People used Gross Parts / Hour
05+10 Screw cap to shaft + Torque test & date stamp cap 25 nil 25 1 144
15 Assemble 2 "O" rings to shaft 20 5 25 1 180
20 Assemble 1st cover to shaft 14 11 25 1 257
25+30 Assemble 2nd cover to shaft + Inspect and pack assembly 24 1 25 1 150

Totals     

 83 17 100 4  

Effectiveness %      

 83% 17% 100%    
In this line-up we have combined operation 5 with operation 10 and operation 25 with operation 30.
In this case we have improved the effectiveness yet further, using less people but at the expense of output.  Under these circumstances the line can only produce at a rate of 144 parts per hour i.e. at the slowest operation rate.  

In the two previous revisions we have either added to an existing operation or combined adjacent operations.  Sometimes we have the opportunity to pick out parts of an operation and add it to another operation when the order of the operations is not important.  For example if a part can be assembled at the front of the track and can also be added at the completion of the assembly process then the operation could probably be placed anywhere along the line.  It may however be that the part can only be fitted at a certain point within the assembly process.  This latter condition is usually the norm and in most cases parts can only be assembled in a set order.  This factor can severely restrict the opportunity to effectively balance the old style conventional assembly line.

Another problem facing us when trying to balance an old style line is that to gain improvements we normally end up either increasing or decreasing the line speed.  The resultant line capacity may be acceptable but often it does not match demand.  If demand drops we have a number of options on how to run the line.

  • We could cut the overtime and reduce the number of shifts.

  • We could reduce the number of people and batch the line by building the first few operations, placing the part built assembly into containers and use the same people to complete the assembly at a later time.

  • We could physically re-layout the line to account for the new volume requirement.

  • We could totally disband the line and batch each operation.

  • We could adopt the lean manufacturing approach using “U” shaped cells and one piece flow attempts to overcome the above-mentioned difficulties.  Hey, we have not decided to go lean yet, so lets forget this option for now!!

Question:  The previous discussion on line balancing is OK but its all charts and tables, what’s the best way to get to grips with the process?
Answer:  By far the easiest way of understanding this process is to try it practically by using the good old “Lego” bricks.  Get a few of the folks to sit in a line and vary the number of bricks that each person is asked to add to the “Brick Assembly” as it passes down the line.  You will soon see what operation is the slowest (the constraint).  Time that slowest operation and multiply the time by the number of people in the line.  Then reduce the number of bricks assembled at the slowest operation and give them to someone else in the line that appears to have spare time.  Once again the constraint will be obvious, time it and multiply the time by the number of people used.  Add more people or reduce the number in the line but do not change the “Brick Assembly” design, just change the way you organize yourselves to build the assembly.