Buffers and Bears

The trip through our materials handling processes can be pretty rough without something to help cushion the ride.

Whether we realize it, or not, we continually insert small buffers into the process to smooth out the flow and improve the productivity of the operations before and after the buffer. However, these buffers are often too small to do the job, or so big as to be clumsy and expensive.

Sorters and spurs

For a time, I used to challenge my conveyor design colleagues by claiming that we could never make the accumulation lines in front of merges and sorters big enough, so maybe we should try to eliminate them all together. How, you might ask? By increasing the speed of the merge and/or sorter to accept the maximum feed rate to it. Or, by occasionally stopping the feed when it exceeds the merge/sort rate.

The other end of the sorter presents another interesting buffer challenge. The spurs, or chutes coming off the sorter serve two purposes; to absorb a surge of product arriving at that spur, and to accumulate a reasonable number of cartons so that the palletizing operator doesn’t have to jump from spur to spur too frequently. The first, wants the spur to be empty, and the second wants it to be full! Simulation, or some fancy arithmetic will help us determine the optimal length. But, I can assure you it will be longer than we have space for. The solution will always be a compromise.

Orderpicking

Similarly, as totes or order cartons enter a zone for picking, we want constant workload available for the picker. We also want space in the queue for a surge of arriving totes. However, every tote in the queue represents additional minutes added to the cycle time for getting the order out the door. This becomes especially critical as we approach carrier departure times.

In many systems, even single line orders are routed into each zone from a central induction point. Because these orders will be started and completed in one zone, it is much better to hold them in a “logical buffer” in the WMS and process them during lulls in the flow of other orders entering the zone. It shortens the queue and levels the workload at the same time.

Workstations

At individual workstations, packing stations for example, we may accumulate orders that require the same value added process, or the same packing material to achieve operational efficiency.

Large orders are often a problem. We usually find it best to create a special station for these, perhaps using flow rack to segregate individual orders, rather than trying to make all of the stations capable of accepting the largest order.

I contend that there is also a psychological factor at work here, too. If the queue of arriving work is short, the operator feels little pressure to perform at full capacity. On the other hand, if it is long, and constantly full, there seems to be little hope of catching up, and consequently, it is fruitless to hurry.

Getting them right

Just as Goldilocks sought in the bear’s cottage, we need to size the buffers in our operations to be “not too big and not too small, but just right.”

ABOUT THE AUTHOR

James M. Apple, Jr. is a Director in The Progress Group. Prior to co-founding The Progress Group in 1991, he was a Partner with Coopers & Lybrand's SysteCon division. During 1992-1995 he served as a Senior Systems Advisor with Vanderlande Industries, a major conveyor and systems provider in Europe.

Jim is an internationally recognized thought leader in the area of facility design and integrated distribution systems. His contributions to the improvement of distribution practices have been recognized by his receipt of the prestigious Reed-Apple Award, which is given for lifetime contributions to the advancement of the material handling profession. Jim has also received the Institute of Industrial Engineers' Facilities Planning and Design Award. He has written numerous articles and handbook chapters on warehousing and logistics operations and is a popular speaker on logistics seminar and conference programs.

Prior to SysteCon, Jim worked as an Industrial Engineer with IBM, was Supervisor of Facilities Planning for the Oldsmobile Division of General Motors and was Executive Vice President for an automotive aftermarket parts supplier. He holds B.S. and M.S. degrees in Industrial and Systems Engineering from the Georgia Institute of Technology.