Making trade-offs in system design

You can't always get it all in a materials handling system so it's important to look at all the options to choose the right one.

By Jim Apple, Contributing Editor -- Modern Materials Handling, 5/1/2007

Sometimes trade-offs are inevitable. But, when my partners, and sometimes our clients, raise the issue of trade-offs, I normally ask: “Why can’t we figure out a way to have both?”

It’s rewarding when we can wrestle with the design and find a creative solution that ultimately eliminates the need for a trade-off. I will remember these work sessions as the most fun of my career.

But in the end, we can’t always have it all.

Sometimes we can substitute a little bit of operator intervention to forego a complex or expensive automatic function, however.

For example, I recall design sessions in Europe revolving around the length of the spurs on a shoe box sorter. The engineers argued, rightfully, that the similar size and weight of the boxes would give them the same trajectory coming off the sorter. As a result, the boxes would line up along one edge of the wide spur. This would require a 20-foot long spur to handle a full carton of shoe boxes—space consuming and very expensive!

Faced with that prospect, the client decided that the packing operators at the end of the spurs, assisted by photo cell alerts, could juggle the boxes in the spur to spread them over its width, cutting the length needed in half.

And other times, trade-offs require choosing between the lesser of two evils.

As another example, conveyors in a small system were designed to take away totes of product from several picking areas. To reduce picker walk distance to the conveyor, independent lines from several zones merged prior to flowing to the pack area.

The merges and controls represented more complexity than the budget could afford. A simpler single line running near a corner of each zone required that workers three or four times an hour walk an additional 30 to 50 feet to off-load each batch. This translated into about a 3% loss in productivity. Based on total staffing, it was judged a reasonable trade-off.

Further downstream in the same system, totes from several zones needed to be accumulated by batch and then released for sorting by order. The normal conveyor solution would have incorporated a sorter with simple 90 degree diverts into a bank of accumulation lanes—and then a slug release of a tote batch to the sorting stations.

The client questioned the logic related to lane selection and maintaining batch integrity, and then astutely asked whether or not the same logic could support manual sortation and release of the totes. Of course, the answer was yes. And, at current business levels, one operator could perform both tasks. Automation can follow when activity levels warrant.

It is important to remember when implementing a partial solution, the first phase needs to be designed to permit a smooth transition to the final design.

Trade-offs and phasing are vital steps in system design, but I encourage you to first spend some quality time in the exercise of going for it all!