Line balancing explained

What line balancing is

Takt time is the pace customer demand sets: available time divided by demand. Each station's work is a list of small tasks called work elements, and their times add up to the station's cycle time. Line balancing moves elements between stations until every station's cycle time is at or under takt.

Teams draw this as a stacked bar chart with one bar per station, called a yamazumi chart (山積み).

M. E. Salveson first wrote about it as a math problem in 1955, in the Journal of Industrial Engineering.

Why it matters

The station with the most work sets the pace for the whole line. The stations after it wait, and the line builds fewer units than the customer needs. Plants often make up the shortfall with overtime.

The light stations cost money too. Their operators wait for part of every cycle.

A worked example

Here is one round of balancing on a five-station car assembly line.

How to balance a line

1. 1
   Work out takt

   Divide the available working time in a shift by the number of units the customer needs in that shift.
2. 2
   List and time the work elements

   Watch the work at each station, write down each element and time it over several cycles. The time study guide shows how.
3. 3
   Mark fixed and movable work

   Some elements can only happen at one station, because they use a machine, a fixture or a position on the line. The rest is hand work that another station could do. Note the order the elements must happen in, too. A part can't be torqued before it is fitted.
4. 4
   Work out how many stations you need

   Divide the total work content by takt and round up. 206 seconds of work at a 60-second takt needs at least 4 stations.
5. 5
   Stack each station against takt

   Draw one bar per station with its elements stacked, and a line across at takt.
6. 6
   Move work until every station fits

   Take movable elements off the stations over takt and give them to stations with spare time, in an order the work allows. Ask the operators which moves make sense. They know the work best.
7. 7
   Write it down as standard work

   Record the new split of work for each station and train every shift on it. Rebalance when takt changes.

Balance this line

Here is the same line before balancing. Each block is a work element. Drag a movable block to another station. Locked blocks are fixed. To go further, try to fit all the work into four stations.

When moving work is not enough

If one fixed element takes longer than takt by itself, no amount of moving fixes that station. Say a fixed element takes 66 seconds on a 60-second line. With nothing else at the station, it is still 6 seconds over.

The element itself has to change. You can split it into two shorter elements at two stations, shorten it with kaizen, add a second station that does the same element in parallel, or automate it.

When takt changes

When demand drops, takt gets longer. If this line's takt went from 60 to 80 seconds, its 206 seconds of work would need only 3 stations (206 ÷ 80 = 2.6, rounded up). When demand rises, takt gets shorter and the line needs more stations.

Either way the work has to be shared out again. Current element times and a clear list of what can move make that a short job.

Common mistakes

• Balancing with planned times. Times from a routing or a standard time system often differ from the real work. Time the work on the line.
• Ignoring the order of work. An element moved to an earlier station has to be possible there. Check that every part it needs is already on the unit.
• Balancing without the operators. The people doing the work know which elements can move and which moves will cause trouble.

How to start

Pick one line and time its work elements. Enter them in the free Yamazumi tool, set your takt and move elements between stations until every bar fits. The takt time guide shows how to set takt.