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About the Author

Cliff Holste is Supply Chain Digest's Material Handling Editor. With more than 30 years experience in designing and implementing material handling and order picking systems in distribution, Holste has worked with dozens of large and smaller companies to improve distribution performance.

Logistics News

By Cliff Holste

August 29, 2012

Determining Actual CPM Rate Is First Step Toward Improving System Capacity

For DC Systems to Deliver Peak Performance they Must Be Capable of Operating at Maximum Capacity

While improving DC productivity gets a lot of attention, most distributors depend on shipments for revenues. Having the most efficient and productive operation is great! But, if the material handling system throughput is somehow constricted, then the system cannot produce at its full potential and the company is at risk of losing sales revenue. For a distribution system to yield peak performance it must be capable of operating at maximum capacity.

Enabling a distribution center material handling system to operate at or near its maximum capacity is a challenge requiring broad-based knowledge of system technology, creative thinking and ingenuity. And, once the improvements have been identified, implementing them without major disruption to on-going operations requires meticulous planning.

Holste Says:

Enabling a distribution center material handling system to operate at or near its maximum capacity is a challenge requiring broad-based knowledge of system technology, creative thinking and ingenuity.
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As it relates to conveyor equipment and sorting systems, the first step in the process is to understand what the current capacity is in terms of cases per minute (CPM). For this you will need a hand-held digital tachometer (instrument used for measuring the rotation speed of a shaft or disk) calibrated to measure in feet per minute (FPM). By placing the disk in contact with a moving belt or roller conveyor, the tachometer will provide the conveyors true FPM speed.

The maximum CPM rate of a conveyor system is dependent on the slowest speed mainline conveyor. In a typical batch order picking and sorting system, this is normally going to be the metering belt conveyor (also referred to as the induction conveyor) located in the mainline feeding the shipping sorter. This is a short (3 to 5 foot long) belt conveyor typically located just upstream of the sorting conveyor and is usually feed by one or more accumulation conveyor lines. Note that no other conveyor in the main transportation line should be running at a slower FPM than the sorter induction conveyor.

Cartons that are placed onto the conveyor system must flow through the induction conveyor prior to being sorted. The primary function of the metering/induction conveyor is to insure that there is sufficient space “gap” between cases so that the sorter can divert individual cases into the after-sort lines. The slow speed side of this conveyor determines the maximum case-feet-per-minute the system can produce.

If you want to know how many cases the system can sort per minute divide the metering belt speed (FPM) by the case length in feet. For example: if the metering belt speed is 60 FPM then it will handle (60) 1 foot long cases per minute; (40) 1.5 foot long case per minute; and (30) 2 foot long cases per minute. Because the CPM varies with case length, the aggregate system rate depends on the average length of the cases.

Once you determine the length of the average case you can then calculate the system CPM rate based on the speed of the metering belt. If that amount is more than what you need, the problem is not with the sorting system. You need to do a more in-depth analysis. However, if the metering belt speed is too slow, you need to contact your system provider to see what can be done to increase capacity.

Some problems appear to be obvious and easily fixed such as adding accumulation conveyor between picking and sorting operations to smooth out the surges without shutting down picking. However, while this relatively costly approach may provide some temporary relief, it may not fix the underlining system problem.

Digging deeper will sometimes reveal the hidden or less obvious problems that sap system performance.

A good example of this can often be found at the central merge. Here cases of product from multiple picking lines are buffered and automatically merged into a single conveyor line that feeds product to the induction conveyor and on to the sorter. Even when the merge is operating at peak performance, the gaps between slugs of cases being released from the accumulation lines during line switchover can reduce system throughput capacity by 10 to 15 percent. Installing new merge logic can reduce the gap between slugs from the typical 3 to 5 feet to just a few inches regardless of line release sequence. Your system provider can advise if this control upgrade would be beneficial.

For more examples of system modifications that enable higher shipping capacity see – “Conveyor, Sorter, and Operational Modifications Yield expanded System Capability & Performance.”

Final Thoughts

Capacity improvements, such as those discussed above, are low risk and can yield significant overall system performance benefits. It is never-the-less a good idea to first have an independent industry expert survey the operation and advise what the best approach is to accomplishing your objectives.

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