Conveyor selection: It's all about throughput

Speed alone kills; maximum throughput for your line must acknowledge buffers and accumulators.

By Mike Pehanich, Plant Operations Editor

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A half-dozen changeovers in a day at 10 minutes per changeover translates into an hour of lost production time. In a climate of frequent changeover, reducing this planned downtime can boost production significantly.

Keep your lines well-buffed

Helm identifies three functions for conveyors: transport, buffering and accumulation. Transport conveyors move product from point A to point B. Effective buffering and accumulation can cut down or eliminate the "stops and goes" that gimp production and boost waste.

Buffering conveyors are meant to hold more product than can be used by the machine they precede. Accumulation conveyors are meant to accumulate product after a machine while a downstream machine is not running. Both are critical to dealing with constraints.

A constraint is a bottleneck or hang-up in the system. Its cause can be equipment breakdown, a slow or hampered machine or inadequate conveyors. Identifying constraints may be the most important step in process or packaging system improvement. Such analysis also provides critical insight into the selection and optimization of conveyors, buffers and accumulators.

Knowing the location of constraints will enable you to determine strategic buffer locations and the capacity of those buffers, and confirm needed rate capacity of "non-constraints," those line areas that run unencumbered, says Garvey. Understanding these constraints enables one to predict throughput and determine efficiency thresholds for each machine.

"As everyone pursues high speed in their lines, the thing that makes their system successful is buffering," says Hershey’s Woelfling. He also notes that food manufacturers have used simulation and modeling extensively to fine-tune buffering and accumulation requirements.

"In a close-coupled system, when a non-constraint [free-running machine or line portion] shuts down, it shuts down the constraint because the whole line is an interconnected system," says Garvey. "But you can protect the constraint by adding buffers in key locations. These buffers allow the constraint to run even when you have a malfunction of a non-constraint. If you do this right, you will increase throughput."

Buffers upstream of a constraint will be full, he explains. They can even run faster than the overall line and produce excess product to fill the buffer. During a malfunction of a non-constraint machine occurs upstream, the constraint can keep running with the product in the buffer. This provides time for the operator to fix the upstream problem and start again before the product in the buffer runs out.

Accumulators located downstream of the constraint will be empty most of the time by design. The downstream machines can even run faster to keep excess product from building up in the accumulator. Then when a downstream machine malfunctions, the constraint can still run. The excess product simply gets stored in the buffer.

In either case, the result is greater throughput.

Virtually all conveyance, buffering and accumulation needs can be accurately figured by determining throughput requirement and the instantaneous operating speed of the line, says Helm.

Hand in hand with proper equipment and conveyor selection is the communication between machines. Field devices must enable machines to communicate so that each machine is aware of the activity of the machines before and after it. The machines and/or their operators need to know when machines are running fast or slow or if they are running at all.

"The minimum signals required from one machine to another are: I am running; I have an emergency stop; I have a cycle stop," says Helm. This communication is absolutely critical to sustaining production through the inevitable constraints on a production line.

Where do most processors err when putting together packaging or processing lines?

"The two classic mistakes that people make are buying a machine that runs too slow or not buying a long-enough conveyor," says Helm. "You can make a conveyor a lot longer to protect your line against constraints, but this costs you more in equipment and also increases planned downtime because you have more bottles to move out when the line shuts down."

Though the principles of selecting conveyors for greater throughput are most easily followed in a packaging context, the model works equally well applied to a processing system.

CAPTIONS:

[spantech-print]

Three transport conveyors end-to-end: Chocolate chip cookies, hot out of the oven, are transferred from one conveyor to another using SpanTech’s MicroSpan transfer conveyor as an alternative to roller transfers.

[For one of the PPT slides:]

In this illustration of a packaging line, product flows from the process line (product delivery) to unit load handling, passing through primary, secondary and tertiary packaging stations and a unit load handling system before being stacked on pallets.

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